General Rule of Phase Decomposition in Zn-Al Based Alloys … · 2004-11-14 · General Rule of Phase Decomposition in Zn-Al Based Alloys ... of the phase transformations occurring

General Rule of Phase Decomposition in Zn-Al Based Alloys (II)

—On Effects of External Stresses on Phase Transformation—

Yao hua Zhu

Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University,Kowloon, Hong Kong, P.R. China

Microstructural changes and phase transformation of Zn-Al based alloys (ZA alloys) were systematically investigated during variousthermal and thermo-mechanical processes using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro-analysis(EMPA), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and differential scanning calorimeter (DSC) etc.techniques. Phase decompositions of the alloys were studied under various thermal and thermo-mechanical circumstances. General rule of phasedecomposition (II) (On effects of external stresses on phase transformation) was summarized with explanations from point view of Gibbs freeenergy.

(Received April 19, 2004; Accepted September 6, 2004)

Keywords: ageing, deformation, stress induced phase decomposition, zinc-aluminium based alloys

1. Introduction

Zn-Al based alloys have been commercially accepted formany years, since the first casting alloy was developed at theNew Jersey Zinc Company in 1922. The most popular diecasting alloy used, ZAMAK 4 was the 898 alloy discoveredin their series of alloy development. A new family of hyper-eutectic Zn-Al based alloys with high aluminum and coppercontents was developed based on the ZAMAK alloys inNorth America and China in the 1970’s. The copper contentwas up to 3% (in mass%) and aluminum content wereselected as about 8, 12, 22 and 27% (in mass%). Themechanical and physical properties of these alloys are muchimproved. This new family of the alloy has been candidatesas substitutions for traditional bushing alloys, such as bronzeand aluminum alloys. To meet the growing demands forapplication of these alloys in industry, extensive studies onmicrostructural changes and phase transformations whichoccur during various thermal and thermo-mechanical proc-esses are required.

2. General Rule of Phase Decomposition in Zn-Al BasedAlloy

Since the 1970’s, a systematic investigation of phaserelationships in Zn-Al based alloys (ZA alloys) has beencarried out. The studies began with the establishment ofphase diagrams of alloy systems of Zn-Al binary, Zn-Al-Cuand Zn-Al-Si ternary and Zn-Al-Cu-Si quaternary alloysystems.1–4) This was followed by studies of the phasetransformations which occurred in solution-treated alloys ofdifferent compositions, i.e., aluminum-rich, monotectoid andeutectoid alloys.5–20)

The phase diagram of Presnyakov et al. modified byGoldak and Parr has been adopted for representation of theZn-Al binary phase diagram,1) shown in Fig. 1. The phasediagrams of both Zn-Al-Cu and Zn-Al-Cu-Si systems areshown in Fig. 2.2–4)

The phase relationships in equilibrium state in Zn-Alalloys containing copper and/or silicon have been estab-lished as follows:2–4)

�þ T 0 ¼¼ �þ " at 285�C

�þ " ¼¼ �þ � at 276�C

�þ " ¼¼ T 0 þ � at 268�C

For alloys of composition in various ranges in Zn-Al, Zn-Al-Si, Zn-Al-Cu, and Zn-Al-Cu-Si systems, the mechanismsof the phase transformations occurring during post quench-aging have been investigated and previously published.12–20)

Fig. 1 Phase diagram of binary Zn-Al alloy.

350°C 280°C

270°C 250°C

Fig. 2 Isothermal sections of Zn-Al-Cu phase diagram.

Materials Transactions, Vol. 45, No. 11 (2004) pp. 3083 to 3097#2004 The Japan Institute of Metals OVERVIEW

The phases involved in the investigations are listed inTable 1.

Based on the studies of phase diagrams, i.e., equilibriumphase relationships, and phase transformations during vari-ous thermal holdings, an intrinsic co-relationship betweenequilibriums and non-equilibrium phase transformations, i.e.,a general rule of phase decomposition in Zn-Al based alloyswas summarized for the first time.3,21) It indicated thatequilibrium phase transformations were generally possibleduring post quench-aging, i.e., non-equilibrium processes.Among these possible equilibrium phase transformations,those at higher temperature occurred at early stage of aging,while those at relatively lower temperature always happenedduring prolonged aging.

3. Phase Transformations in Various Thermal andThermo-Mechanical Processes

3.1 Phase decomposition in isothermal holdingFurther studies extended the work to cover various

complex non-equilibrium and more practical processes inthe Zn-Al based alloys with various chemical compositions:eutectoid and monotectoid alloys.22–35) X-ray diffraction(XRD), scanning electron microscopy (SEM), electron probemicro-analysis (EPMA), transmission electron microscopy(TEM), electron back-scattered diffraction (EBSD) anddifferential scanning calorimeter (DSC) etc. techniques wereapplied in the studies.3.1.1 Decomposition of �0

s phase in solution treated ZAalloys

The Zn-rich � phase with face centered cubic (fcc)structure becomes an unstable �0

s phase at ambient temper-ature. Because more than 70% zinc-rich " and � phases areformed from decomposition of the �0

s phase,12) it is important

to understand well the mechanism of the decomposition ofthe �0

s phase. Shown in Figs. 3a and b are X-ray diffracto-grams of both as quenched and the aged specimen of theeutectoid Zn-Al based alloy, respectively. It was observedthat the �0

s phase decomposed at the early stage of aging. The

X-ray diffraction (XRD) intensity of (111) and (200) crystalplanes of the �0

s phase decreased, accompanying theformation of three phases �0

T, " and �, i.e., �0s ! �0

T þ"þ �, after quench-aging at room temperature for 16min.The �0

s phase decomposed at the grain boundaries duringquenching, and this discontinuous precipitation was devel-oped along the grain boundaries in the �0

s phase after agingat room temperature for 10 and 20min, as shown inback-scattered microscopy (BSEM) images, Figs. 4a, b andc.12,14,22,31)

3.1.2 Aging characteristics of furnace cooled (FC)eutectoid ZA alloy

Furnace cooling is a slow cooling process, which is ofteninvolved in various advanced metallurgical processes. It is ofconsiderable interests in studies of the structural evolution ofalloys during the metallurgical processes.

The eutectoid Zn-Al based alloy (ZnAl22Cu2) was solu-tion-treated at 350�C for 4 days, then cooled inside thefurnace chamber to ambient temperature. The FC alloyspecimens were aged at 100, 150 and 170�C. BSEMexamination was carried out on the FC and aged specimens.The BSEM images of the FC and 100�C-aged specimens areshown in Figs. 5a–c.29) Two phases, � (Zn-rich phase of fccstructure) and " (CuZn4 with hexagonal close packed, i.e.,hcp structure), were observed in the FC specimen. Duringfurnace cooling, the � phase became an unstable �0

s phaseand decomposed into various metastable phases: �, " and �0FC(Zn-rich hcp phase). The Al-rich and Zn-rich metastablephases decomposed further into fine and coarse lamellae dueto different diffusion rate of Al and Zn atoms. The " phase didnot decompose and appeared as the original particles in theFC alloy specimen. The resultant microstructure of the FCalloy specimen consisted of the coarse and fine lamellae, andthe light-contrast �0FC and the " phase particles, as shown inFig. 5a. X-ray diffractograms of the as FC alloy specimenshowed the co-existence of the three phases: �, " and �0FC,Fig. 6.29)

During aging at 100�C, more precipitates of � phase wereobserved in the �0FC phase, as arrows ‘‘!’’ indicated inFig. 5. The dark-contrast � phase precipitates grew at thephase boundaries after aging at 100�C for 10 h, and clearlydeveloped after aging at 100�C for 30 h. This was detected asa discontinuous precipitation in the X-ray diffractograms,Fig. 6. The (0002) diffraction peak of the �0FC phasedecreased in height at 36.8 degree, whilst a metastable �0Tphase appeared at a lower 2� ¼ 36:5 degree, accordinglyd-spacing values of the �0FC phase and the �0T phase were0.2437 nm and 0.2456 nm, respectively. After aging at 100�Cfor 20 h, the original (0002) peak of the �0FC phase decreasedgreatly, and that of the �0T phase remained apparently at thelower 2� side of the �0FC phase. This decomposition of the�0FC phase was followed by another phase transformation.After aging for 20 h, both X-ray diffraction peaks fromð10�110Þ and (0002) crystal planes of the " phase decreased inheight at 2� ¼ 37:6 degree and 2� ¼ 42:1 degree, relatively.Meanwhile the (110) diffraction peak of T0 phase increased,as shown in Fig. 6. It was recognized as a four-phasetransformation: �þ " ! T0 þ � or decomposition of the" phase, in previous research publications.12–27)

The relative XRD intensity changes of the various phases:

Table 1 The phases involved in the investigations.

�: Al rich fcc phase

�: Zn rich fcc phase

": hcp phase CuZn4�: Zn rich hcp phase

�: Si rich phase of bcc structure

T0: distorted bcc structure, Zn10Al35Cu55 (in mass%)

E: Rare earth containing Zn-rich phase

�0s: Supersaturated Al rich fcc phase

�0s: Supersaturated Zn rich fcc phase

�0T: Al rich eutectoid terminal fcc phase derived from �0s

and �0s phases

�00m: The first transition phase

�00: The Al rich matrix phase in equilibrium with �00m

�0m: The second transition phase

�0: The Al rich matrix phase in equilibrium with �0m

�f : Stable or final stable Al rich fcc phase

�0s: Supersaturated Zn rich hcp phase

�0E: Supersaturated Zn rich hcp phase in extruded Zn-Al alloys

�0FC: Supersaturated Zn rich hcp phase in furnace cooled Zn-Al alloy

�0T: Metastable Zn rich hcp phase

�f : Stable or final stable Zn rich hcp phase

3084 Y. h. Zhu

�, " and T0 at 100�C are plotted in Fig. 7.29) It is noticed thatthe relative XRD intensities of the � and " phases increased inthe early stage of ageing, accompanying the formation of themetastable phase �0T. This implied that the �0FC decomposeddiscontinuously into three phases, �0T, � and ", i.e.,�0FC ! �0T þ �þ ". Following the discontinuous decompo-sition of the �0FC phase, both the relative XRD intensities ofthe � and " phases decreased during prolonged ageing. Thelatter decreasing of XRD intensities of both the � and" phases was resulted from the four-phase transformation,�þ " ! T0 þ �, which is in agreement with the increasing ofthe T0 phases during the prolonged ageing. Finally the XRDintensity of the � phase decreased apparently, and this wassupposed to be related with the decomposition of the� phase.29)

It was clearly found that the different metastable phasesdominated the phase decomposition of the alloy at differentstages of ageing, as indicated by shadowed solid lines inFig. 7. The decomposition of the �0FC phase occurred duringthe whole ageing processes. The discontinuous decomposi-tion of the �0FC phase, �0FC ! �0T þ �þ ", dominated at theearly stage of ageing, both XRD intensities of the � and "phases increasing. Then the four-phase transformation,�þ " ! T0 þ �, started, accompanying the decreasing ofthe XRD intensities of the � and " phases and the increasingof the XRD intensity of the T0 phase. Further ageing thedecomposition of the metastable �0T phase became importantand the XRD of the � and " phases increased slightly. Finallythe decomposition of the � phase dominated the phasetransformation, while the other metastable phases had almostdecomposed completely, the XRD intensity of the � phasedecreasing clearly.

The above mentioned two stages of phase transformation:decomposition of �0FC phase and the four-phase transforma-tion, were also observed in the specimens during aging at 150and 170�C.29,44)

The dark-contrast � phase precipitates inside the �0T phasegrew clearly in the early stage of aging at 170�C, as shown inBSEM images (Fig. 8a).29) After aging at 170�C for 30 h, thegrey precipitates of the T0 phase were observed in the light-contrast " phase, as indicated by arrows ‘‘*!’’ in Fig. 8b.Both the precipitates of � phase and T0 phase were apparentlydeveloped after aging at 170�C for 52 h, as shown in Fig. 8c.In previous publications, the composition of the T0 phase wasanalyzed as Zn10Al35Cu55 (in mass%).3–11) As one of theproducts of the four-phase transformation, �þ " ! T0 þ �,the T0 phase appears as grey precipitates inside the " phase,being distinct from the dark-contrast Al rich � phaseprecipitates in the �0FC phase. Another product of the four-phase transformation was the � phase. Because both " phaseand � phase were Zn-rich hcp phases with light atomicimages, it was not possible to distinguish the � phase from the" phase using BSEM.

The X-ray diffractograms of the FC alloy specimens aftervarious periods of aging at 170�C are shown in Fig. 9. Thecharacteristic shifting of the (0002) diffraction peak of the�0FC phase was observed to the lower 2� angle. The changesof diffraction intensity of both " and T0 phases were alsoobserved during further aging. After 52 h aging at 170�C, theð10�110Þ and (0002) diffraction peaks of the " phase decreasedgreatly, whilst the (110) diffraction peak of the T0 phaseincreased in height, as shown in Fig. 9. Compared with theXRD results of 100 and 150�C,29,44) both two stages ofdecomposition of were apparently accelerated when aging at

Fig. 4 BSEM images of the eutectoid Zn-

Al based alloy specimens, as quenched (a)

and after aging at room temperature for

10min (b) and 20min (c).

Fig. 5 BSEM images of the furnace

cooled eutectoid Zn-Al based alloy speci-

men, FC state (a) and after aging at 100�C

for 10 h (b) and 30 h (c).

Fig. 3 X-ray diffractograms of the eutectoid Zn-Al based

alloy specimen, as quenched (a) and after aging at room

temperature for 16min (b).

General Rule of Phase Decomposition in Zn-Al Based Alloys 3085

Fig. 6 X-ray diffractograms of the furnace cooled

eutectoid Zn-Al based alloy after various periods

of aging at 100�C.

Fig. 7 Relative XRD intensity changes of

various phases in the FC eutectoid Zn-Al

based alloy aged at 100�C.

Fig. 8 BSEM images of furnace

cooled eutectoid Zn-Al based alloy

specimen, after aging at 170�C for 1 h

(a), 30 h (b), and 52 h (c).

Fig. 9 X-ray diffractograms of furnace cooled

eutectoid Zn-Al based alloy after various periods

of aging at 170�C.

Fig. 10 TEM bright fields (a), the SADP from [�1114]

zone of the [1�221�33] of the precipitate zone of the

eutectoid Zn-Al based alloy speicmens, to show the �phase (b) and the SADP from precipitates of � phase

inside � phase particle (c), and the dark field using

{10�110} (d) for the FC Zn-Al based alloy specimen

after 52 h ageing at 170�C.

Fig. 11 X-ray diffractograms of the extrud-

ed eutectoid Zn-Al based alloy after

various periods of aging at 170�C.

3086 Y. h. Zhu

170�C.Decomposition of the � phase was detected after pro-

longed aging using TEM technique. Shown in Fig. 10a is theTEM bright field of the FC alloy specimen after aging at170�C for 52 h.29) A diffraction pattern from the particle inFig. 10a is identified from the ½�1114� zone of the fcc � phase,as shown in Fig. 10b. The SADP of the precipitate inside the� phase is identified from the ½1�221�33� zone of the hcp � phase,as shown in Fig. 10c, and the dark field using f10�110g� isshown in Fig. 10d. In the TEM dark field, the light image ofthe � phase precipitates inside the � phase particle isdiffracted from f10�110g� in the SADP. It was found that theprecipitates �0FC phase of 40–50 nm in diameter inside the �phase in the FC specimen had grown to be the � phase of 80–100 nm in diameter after 52 h ageing at 170�C. It isreasonable because the � phase in the FC Zn-Al basedalloys, even after the four-phase transformation (the phasetransformation correlated with a phase equilibrium, �þ " ¼T0 þ � at 268�C) is unstable. The � phase at 268�C containsmore zinc in the Zn-Al based alloys compared with thecomposition of the � phase at 100 and 170�C, according tothe phase diagrams of Zn-Al binary, Zn-Al-Cu ternary andZn-Al-Cu-Si quaternary alloy systems.24–26) There must beZn-rich � phase precipitated from the unstable � phase at 100and 170�C.29)

3.1.3 Ageing characteristics of extruded eutectoid Zn-Albased alloy

Extrusion is normally applied for materials homogeniza-tion before any heat treatment. It is important to study agingcharacteristics of the extruded alloys. The eutectoid Zn-Albased alloy ingot was extruded at 250�C into rods of 20mmdiameter. The microstructural changes and the phase trans-formations were examined during aging.

It was observed that the extruded eutectoid alloy consistedof � phase and Zn-rich �0E phase and " phase, as shown in theX-ray diffractograms (Figs. 11a and b). The d-spacing of the(0002) crystal planes of the �0E phase was 0.2437 nm,accordingly 2� ¼ 36:8 degree before aging, as shown inFig. 11.37)

After aging at 170�C for 10min, the (0002) diffractionpeak of the �0E phase shifted to the lower 2� angle, and thed-spacing of the crystal planes increased to 0.2446 nm. Thisis characteristic of the decomposition of the �0E phase at theearly stage of aging. It was reported that the �0E phasedecomposed into three phases, i.e., �0E ! �þ �þ T0.37) Thedecomposition of the �0E phase was followed by decom-position of the " phase during prolonged aging. As shown inFig. 11a, the diffraction intensity of the " phase both fromð10�110Þ and (0002) planes decreased after aging at 170�C for3 h. With increasing aging time, the intensity of the " phasefurther decreased, whilst the diffraction intensity of theT0 phase from (110) crystal planes increased. After aging at170�C for 113 h, the diffraction peak of the " phase vanishedand the T0 phase was well developed.37)

The two stages of phase decomposition were clearlyobserved on BSEM images of the extruded alloy specimenafter various periods of aging at 170�C, as shown inFig. 12.37) The supersaturated Al-rich �0

s phase and Zn-rich�0

s phase decomposed during extrusion at 250�C andappeared as fine and coarse lamellar structures, as shown in

Fig. 12a. Particles of the �0E phase and " phase formed in theinter-dendritic regions of the original dendritic structure ofthe cast alloy specimen. Typical morphologies, i.e., the dark-imaged � phase precipitates and the gray-imaged T0 phaseprecipitates stand for the two stages of decomposition of the�0E and " phases, as indicated by arrows, ‘‘!’’ and ‘‘*!’’ inFig. 12.

A similar phase transformation sequence was observed inthe specimen aged at 140�C.37) However, the phase trans-formation rate at 140�C was apparently lower than that at170�C. The X-ray diffraction peak of the " phase vanishedafter aging at 170�C for 113 h, whilst the ð10�110Þ diffractionpeak of the " phase still existed after aging at 140�C for304 h.37)

3.1.4 Aging characteristics of cast Zn-Al based alloy(ZnAl7Cu3)

Study of ageing characteristics of the copper modified castZA8 is of practical importance. The analyzed composition ofthe alloy is Zn90.4Al6.7Cu2.9 (in mass%).

The cast alloy specimens were examined after variousperiods of ageing at 150�C. During solidification, �, � and" phases solidified first to form eutectic structure.25) Thesethree phases became supersaturated phases, �0S, �0

S and", and decomposed during casting and solidification. Theresultant metallographic microstructure of the three phasesappeared as tree-stem shaped eutectic structure together withflower-like cores of solidification, as shown in Fig. 13a. Thesubsequently solidified fcc � phase contained less aluminumand richer in zinc, and appeared layer structures, while littlechanges in composition of the hexagonal closed packed (hcp)� and " (Zn4Cu) phases, both were lightly contrasted inBSEM. The BSEM image of the cast alloy and the relatedX-ray mapping of Zn, Al and Cu elements are shown inFig. 14. It was reported that the �0

S phase decomposed in acellular reaction, �0

S ! �0T þ �0T þ ", where aluminum rich

�0T and zinc rich �

0T phase are both metastable phases. Due to

faster diffusion rate of zinc atoms than aluminum atoms inthe phase, the outer layer of the subsequent solidified � phasedecomposed rapidly, being different in shape from the innerlayer of the decomposed �0

S phase. The inner layer appeared

Fig. 12 BSEM images of the extruded eutectoid Zn-Al based alloy

specimen, as extruded (a) and after aging at 170�C for 1 h (b), 10 h (c),

and 50 h (d).


as fine lamellar structure. The analyzed composition of thedifferent layers is shown in Table 2.25)

The � and " phases and aluminum rich � phase solidifiedfinally to form plenty of eutectoid structure, as shown inFig. 13a.

The X-ray diffractogram of the as cast ZnAl7Cu3 showedthat three phases, �0

T, �0T, and ", existed in the as cast alloy,

as shown in Fig. 15, where the characteristic diffractionpeaks of the phases are separately indexed. The �0

S phase haddecomposed and did not appear on the X-ray diffractogram.After ageing at 150�C for 3min, the (0002) diffraction peak

of the �0T phase shifted to the lower 2� angle and d-spacing ofthe crystal plane increased to 0.2446 nm, accordingly. Afterageing at 150�C for 2 h, the (0002) was shifted further and thed-spacing increased to 0.2447 nm. The shifting of the (0002)diffraction peak was one of the characteristics of thedecomposition of the �0T phase. After ageing at 150�C for5 h, the decomposition of the �0T phase became apparent andthe diffraction intensity from ð10�110Þ and (0002) planes of the" phase started to decreased, as shown in Fig. 15. Withincreasing ageing time, the diffraction intensity of the " phasefurther decreased, while the diffraction intensity of (110)planes of the T0 phase increased. This implied that a fourphase transformation, �þ " ! T0 þ �, started to occur.After ageing at 150�C for 12 h, the four phase transformationbecame obvious, as shown in Fig. 15. The XRD intensities ofthe (0002) peaks of the " phase and (110) peak of the T0 phaseare plotted against time of ageing, shown in Fig. 16.

The two stages of phase decomposition were detectedusing BSEM.25) After ageing at 150�C for 20min, the darkimage precipitates of � phase developed at the boundaries of�0T and the decomposed �0

S phase, as shown in Fig. 13b. Thetypical layer structure of the decomposed �0

S phase is clearlyseen. After ageing at 150�C for 3 and 10 h, gray precipitateswere observed inside the light imaged " phase, as shown inFigs. 13c and d, respectively. This gray precipitates wererecognized to be the T0 precipitates, as one of the products ofthe four phase transformation, �þ " ! T0 þ �. The gray T0

phase precipitates were considerably increased after ageingat 150�C for 30 h, as shown in Fig. 13e. Both the dark- and

Fig. 13 BSEM images of the cast alloy (ZnAl7Cu3) after various periods

of ageing at 150�C: as cast (a), 20min (b), 3 h (c), 10 h (d), 30 h (e) and

126 h (f).

Fig. 14 BSEM image of the cast alloy (ZnAl7Cu3) and the related X-ray

mapping of Zn, Al and Cu elements.

Table 2 Compositions of various layers of the decomposed �0S phase

(in mass%).

Zn Al Cu

1st layer (inner layer) 80.8 17.0 2.2

2nd layer 82.8 14.1 3.4

3rd layer (outer layer) 88.9 9.5 1.6

Fig. 15 X-ray diffractogram of the as cast alloy ZnAl7Cu3 after various

periods of ageing at 150�C.

3088 Y. h. Zhu

gray-precipitates of � and T0 phases well developed afterprolonged ageing at 150�C for 126 h, as shown in Fig. 13f. Itis rather easier to determine the microstructure directly fromreflection of bulk specimens in SEM than thin foil specimensusing TEM. Electron back-scattered Kikuchi diffractionpattern (EBSDP) was obtained from a well polished bulkspecimens using EBSD technique, as shown in Fig. 17. Boththe �0T and " phases appeared light in contrast and hard todistinguish from each other in the BSEM. In performingEBSD, pre-determined lattice parameters of the �0T and" phases were applied. When electrons scanned on theselected area of the as cast specimens, EBSDP of both �0T and" phases were rapidly generated with automatic indexing.The previously determined lattice parameters of the �0T and "phases are listed in Table 3.36,59) Shown in Figs. 17a and bare the indexed EBSDP of �0T and " phases in the ZA alloyspecimen. EBSD mapping of �0T and " phases were realizedafter subtracting a background noise from each averageimage, separately. The �0T and " phases were clearlydistinguished from the different contrast on the EBSDmappings. Shown in Figs. 18a and b are the BSEM imagesof the alloy (ZnAl7Cu3) after ageing at 150�C for 8 and 700 hand the related EBSD mappings of the " phase, respectively.Because the resolution of the EBSD is about 5 mm, with care1 mm, the phase regions smaller than this resolution were

undetected, and appeared as dark images (Figs. 18a and b).The white and the light grey imaged regions were fromdifferent of the " phase particles with different crystalorientations. It was clear that the " phase decomposed almostcompletely after ageing at 150�C for 700 h.25)

3.2 Effects of external stress on phase transformationMaterials are subjected to one or more thermal processes,

in their manufacturing and in subsequent services. Mean-while, they suffer external stresses in various thermo-mechanical circumstances. Studies of the external stress-induced microstructural changes and phase transformationare vital to understanding of the reasons for determining thescope for further improvement of the properties of thematerials.

Various thermo-mechanical processes, such as extrusion,tensile, creep, fatigue, impact, damping, milling, rolling,cold-drawing, ultra-precision machining and mechanicalalloying etc. were correspondingly involved in the inves-tigation.36–58,60–62)

3.2.1 Effects of external stress on decomposition of the�0

s phaseThe cast ingot of the eutectoid Zn-Al based alloy

(ZnAl22Cu2) was extruded at 250�C for homogenizationand then machined into standard tensile specimens of 6mmdiameter with 20mm gage length. The specimens weresolution-treated at 350�C for 4 days, then quenched into icewater. Immediately after quenching, the specimens weretensile-tested on an Instron machine at a cross head speed of7:00� 10�3 mm/s at room temperature. The ultimate tensilestress and 0.2% proof stress of the quenched specimen werefound to be 276.0MPa and 130.5MPa respectively. Thespecimen broke as the strain reached 38.47% with a breakstress of 100.9MPa. The displacements at peak and at breakwere 3.68mm and 6.73mm respectively. The tensile test last16 min.45)

X-ray diffraction and BSEM examination were carried outbefore and immediately after tensile testing at room temper-ature to identify the phase transformation and microstructural

Fig. 16 Relative XDR intensity changes of (0002) peaks of the " phase and

(110) peak of the T0 phase of the cast alloy (ZnAl7Cu3) during ageing at

150�C.

Fig. 17 EBSDP of �0T (a) and " (b) phases in the cast alloy (ZnAl7Cu3).

Table 3 Lattice parameters of �0T and " phases.

a c c=a

�0T phase 0.2671 nm 0.4946 nm 1.852

" phase 0.2767 nm 0.4289 nm 1.550

a

b

Fig. 18 BSEM images of the alloy (ZnAl7Cu3) specimens after ageing at

150�C for 8 (a) and 700 (b) h and related EBSD mappings

of " phases.


change under tensile stress-deformation.45) The X-ray dif-fractograms of various parts of the specimen after tensiletesting at room temperature: the bulk part, the neck zone andthe rupture part, are shown in Fig. 19. Comparing XRD of thequench-aged specimen and the bulk part of the tensile testedspecimen, shown in Fig. 3a and Fig. 19a, the �0

s phasedecomposed in both the quench-aged specimen and the bulkpart of the tensile-tested specimen. And the XRD intensitiesof the decomposed �0

s phase and the three metastable phase,�0

T, " and �, were identical. Obviously the decomposition ofthe �0

s phase in the bulk part of the specimen was due to theaging at room temperature, but not the external stress.Discontinuous precipitation occurred along the grain boun-daries inside the �0

s phase, as shown in BSEM images,Fig. 20a.

In the neck zone and the rupture part of the specimen,plastic deformation occurred, accordingly the decompositionof the �0

s phase started to develop. As shown in Figs. 19b andc, the (111) and (200) XRD intensities of the �0

s phasedecreased, accompanying increases in the XRD intensities ofthe �0

T, " and � phases. As the distance from the rupturefrontier was decreased, the discontinuous precipitation of the�0

s phase apparently developed, as shown in BSEM images,Figs. 20b and c. Because of the high concentrated strain due

to necking, the decomposition of the �0s phase was greatly

accelerated in the rupture part of the specimen, as shown inXRD, Fig. 19d, and BESM, Fig. 20c. With additional straininduced by the plastic deformation, the probability of an atomreaching the activation state was increased greatly and thedecomposition of the �0

s phase was apparently accelerated inthe rupture part of the specimen.3.2.2 Effects of external stress on phase transformation

in FC Zn-Al based alloysThe same tensile testing procedure was performed on the

FC eutectoid alloy specimen at 150 and 100�C. Various partsof the FC alloy specimen, were examined after tensile testingat 150�C using XRD and BSEM.44) The decomposition of the�0FC phase was observed in the bulk part of the tensile testedspecimen. The X-ray diffractograms are shown in Fig. 21.The (0002) XRD peak of the �0FC phase in the bulk part of thespecimen had shifted from 2� ¼ 36:6 degree to a lower 2� ¼36:5 degree, accordingly the d-spacing of the �0FC phaseincreased from 0.2449 to 0.2456 nm. It has been known thatthe shifting of the (0002) XRD peak of the �0FC phase to thelower 2� angle was one of characteristics of the decom-position of the �0FC phase became �0T phase.29,30)

Another phase decomposition, i.e., the four-phase trans-formation: �þ " ! T0 þ �, occurred in the neck zone and

Fig. 19 X-ray diffractograms of various parts

of the solution treated eutectoid Zn-Al based

alloy specimen after tensile testing at

room temperature: the bulk part (a), the neck

zone (b)(c) and the rupture part (d).

Fig. 20 BSEM images of various parts of the

solution treated eutectoid Zn-Al based alloy

specimen after tensile testing at room temper-

ature: the bulk part (a), the neck zone (b) and

the rupture part (c).

Fig. 21 X-ray diffractograms of various parts

of the FC alloy specimen after tensile testing at

150�C: the bulk part (a), neck zone (b) and the

rupture part (c).

3090 Y. h. Zhu

t-

he rupture part of the tensile specimen. There was no T0 phasewhich was detected in the bulk part of the specimen.Approaching the rupture frontier, the XRD intensities of theð10�110Þ and (0002) planes of the " phase decreased, whilst thatof the (110) planes of the T0 phase increased at 2� ¼ 44:3degree, as clearly shown in Figs. 21b and c.

The BSEM observation provided metallographic eviden-ces for the two stages of the phase transformation. Shown inFig. 22 are the BSEM images of the various parts of thetensile tested specimen. Compared with the aged FC speci-men (Fig. 8), the dark-contrast Al-rich � phase precipitateswere observed inside the light-contrast Zn-rich � phase in thebulk part of the specimen, as indicated by the arrows ‘‘!’’ inFig. 22a. With increasing strain, the � phase precipitatesincreased in the neck zone, whilst the grey-contrast T0

precipitates started to form, as indicated by the arrows ‘‘*!’’in Figs. 22b and c. In the rupture part of the specimen, the T0

phase precipitates increased apparently.The tensile testing lasted for 16min at 150�C. In addition

to the external tensile deformation, the tensile specimenunderwent an aging process of 16min at 150�C. Incomparison, the decomposition of the �0FC phase lasted15 h at 150�C for the aging process (without external tensiledeformation), and the four-phase transformation: �þ " !T0 þ �, became apparent after ageing at 150�C for 30 h.44)

Obviously, because of the tensile deformation, the two stagesof phase transformation were accelerated in the tensile testedFC Zn-Al based alloy specimen.

The above detected two stages of phase transformationwere also examined in the FC specimen after tensiledeformation at 100�C.59) The BSEM images of the as FCspecimen (without external stress posted), and the neck zoneand the rupture part of the FC-tensile deformed specimen areshown in Fig. 23. The microstructure of the as FC specimenconsisted mainly of light Zn rich �0T phase particles of about6–10 mm in diameter, fine lamellar structure of about 0.2 mmin lamellar thickness and coarse lamellar structure of about0.8 mm in lamellar thickness. Some dark-contrast Al-rich �phase precipitates were observed in parts of the light �0Tphase particles, as shown in Fig. 23a.

In the neck zone and the rupture part of the FC-tensiledeformed specimen, the light-contrast Zn-rich particles wereelongated in alignment with the tensile direction, as shown inFigs. 23b and c. With increasing strain, the dark-contrast Al-rich � phase precipitates increased in the neck zone, whilstgray precipitates were observed inside the light �0T phaseparticles, as shown in Fig. 23b. The gray T0 phase precip-itates increased apparently in the rupture part of the speci-men, meanwhile particulate structure of about 0.5 mm indiameter was observed obviously in the original coarse


FC Zn-Al based alloy specimen after tensile

testing at 150�C: the bulk part (a), neck zone

(b) and the rupture part (c).

Fig. 23 BSEM images of the FC Zn-Al based

alloy specimen (without external stress posted)

(a), and the neck zone (b) and the rupture part

(c) of the specimen after tensile testing at

100�C.

Fig. 24 EBSD mappings of the as FC Zn-Al

based alloy specimen after tensile testing at

100�C, the neck zone, and the rupture part.


lamellar structure region, as shown in Fig. 23c.Approaching the rupture frontier, the dark-contrast pre-

cipitates of the � phase were developed, as shown inFigs. 23a and b.

The subsequent four-phase transformation, �þ " !T0 þ �, occurred in the FC-tensile deformed Zn-Al alloy.Inside the light-contrast " phase particles in the neck zone,small amount of the gray precipitates of the T0 phase wereobserved, and this gray precipitates increased in the rupturepart of the specimen, as shown in Figs. 23b and c. In otherwords, the external stress induced decomposition of the "phase became apparent in the rupture part of the specimen.

The EBSD technique was applied for determining thetensile deformation induced phase transformations and themicrostructural changes.60) Shown in Figs. 24a–c are theEBSD mappings of the as FC specimen, the neck zone, andthe rupture part of the FC-tensile deformed Zn-Al alloyspecimen, respectively. The dark grey imaged regions are the� phase and the black particles are the " phase. It was alsofound that the " phase particles were getting smaller when thedistance from the rupture frontier was decreased. Theundetected phases appeared as grey imaged regions inFig. 24. Also, this is shown in the (0001) inverse pole figuresof the " phase in Fig. 25. The amount of the project points ofthe " phase considerably decreased in the rupture part of thetested specimen.

The decomposition of the � phase was detected in the neckzone and the rupture part of the fatigue tested eutectoid Zn-Albased alloy specimen using TEM technique. Shown inFig. 26 is the TEM bright field of the alloy specimen after65MPa-fatigue testing at 100�C.53) Compared with theprecipitates of � phase inside the � phase after prolongedaging at 170�C (Fig. 10a), the precipitation of � phase wasconsiderably developed in the rupture part of the fatiguetested eutectoid Zn-Al based alloy specimen.3.2.3 Effects of external stress on phase transformation

in extruded Zn-Al based alloysVarious parts of the 250�C-extruded eutectoid Zn-Al based

alloy were examined after tensile testing at 100�C using XRDand BSEM techniques.41) Shown in Fig. 27 are the X-raydiffractogrmas. Two stages of phase transformation were alsodetected. The first was the decomposition of the metastable

Fig. 25 The (0001) inverse pole figures of the as

FC Zn-Al based alloy specimen, and the neck

zone, and the rupture part of the same specimen

after tensile testing at 100�C.

Fig. 26 TEM bright field of the FC Zn-Al based alloy specimen after

65MPa-fatigue testing at 100�C.

Fig. 27 X-ray diffractograms of various parts of

the extruded Zn-Al based alloy after tensile test

at 100�C: the bulk part (a), the neck zone (b)

and the rupture part (c).


extruded Zn-Al based alloy after tensile

testing at 100�C: the bulk part (a), the neck

zone (b) and the rupture part (c).

3092 Y. h. Zhu

�0E phase. In the bulk part of the specimen (without plasticdeformation), the �0E phase decomposed slightly after thetensile testing. The characteristic (0002) X-ray diffractionpeak of the �0E phase �0E phase �0E phase remained at about2� ¼ 36:9 degree after ageing at 100�C for a few minutes (thetime to tensile rupture). However, the material underwent athermal-mechanical treatment, i.e., ageing at 100�C andtensile deformation, in the neck zone and the rupture part ofthe specimen. The �0E phase decomposed obviously. The(0002) diffraction peak shifted from 2� ¼ 36:9 degree to alower 2� ¼ 36:6 degree, accordingly the d-spacing of the(0002) crystal planes increased from 0.2434 nm to0.2455 nm. This implied that the decomposition of the �0Ephase became apparent, as distance from the rupture frontierdecreased. It was reported that the discontinuous decom-position of the �0E phase occurred in the following form:�0E ! �0T þ �þ T0, which corresponded to equilibrium at268�C: �þ " ¼ T0 þ �.3,4,36)

The second is the phase decomposition of the " phase.In comparison of the X-ray diffractiograms of the bulk partand the rupture part of the specimen, the X-ray diffractionpeaks of the " phase decreased in intensity in the rupturepart of the specimen after tensile testing at 100�C, accom-panying an increase in intensity of the T0 phase, as shown inFigs. 27b and c. As recognized in previous investigation onthe ageing characteristics of the eutectoid and monotectoidZn-Al based alloys, this indicated decomposition of the "phase occurred via the four-phase transformation: �þ " !T0 þ �.3,12,13,18–20)

The two stages of decomposition of the �0E and " phaseswere observed as discontinuous precipitation inside these

light-contrast Zn-rich phases. Shown in Figs. 28a–c are theBSEM images of various parts of the tensile tested specimen.The precipitates of � phase in the �0E phase and the T0 phaseinside the " phase, were indicated by the arrows ‘‘!’’ and‘‘*!’’, respectively. In the rupture part of the specimen, thegrey rings of T0 precipitates had been apparently observed inthe light-contrast " phase.

It was obvious that this decomposition of the " phase, i.e.,four-phase transformation: �þ " ! T0 þ �, had been accel-erated in the neck zone and rupture part of the specimenunder tensile testing at 100�C, as the time to rupture of thetensile specimen was less than several minutes, while thisfour-phase transformation occurred only when this alloy wassubjected aging for dozens of hours at 100�C.12,39)

The decomposition of the �0T and " phases was observed aswell in the extruded Zn-Al based alloy specimens after creeptesting at 150�C. Shown in Fig. 29 are the BSEM images ofvarious parts of the extruded alloy specimen after creeptesting at 150�C.

Typical BSEM images of the dark-contrast � phaseprecipitates and the grey-contrast T0 phase precipitates wereas shown in Figs. 30a and b, respectively. Shown in Fig. 30ais BSEM image of the extruded alloy specimen after aging at170�C for 50 h, and Fig. 30b is the BSEM image of therupture part of the extruded alloy specimen after creep testingat 150�C.49)

3.2.4 Effects of external stress on phase transformationin cast Zn-Al based alloys (ZnAl7Cu3)

X-ray diffraction was carried out on various parts of thetensile tested specimen, bulk part, neck zone and the rupturepart of the alloy specimen.46) The X-ray diffratograms are


extruded Zn-Al based alloy specimen after

creep testing at 150�C: the bulk part (a), the

neck zone (b) and the rupture part (c).

Fig. 30 Typical BSEM images of the

dark-contrast � phase precipitates and

the gray-contrast T0 phase precipi-

tates in the light contrast �0T and "

phases, as indicated by arrows ‘‘!’’

and ‘‘*!’’, respectively: in the ex-

truded Zn-Al based alloy after aging

at 170�C for 50 h (a), and the rupture

part of the extruded alloy specimen

after creep testing at 150�C (b).

Fig. 31 X-ray diffractograms of various parts of the

cast alloy (ZnAl7Cu3) after tensile testing at

150�C: the cast specimen (a), the bulk part (b),

the neck zone (c) and the rupture part (d).


shown in Figs. 31b–d. The tensile tested specimen underwentactually both thermal and thermo-mechanical processes.During tensile testing at 150�C, (0002) X-ray diffraction peakof the �0S phase shifted to from (2�) 36.7� (d-spacing,0.2447 nm) to a lower 2� angle, as shown in Fig. 31b.Because there was no tensile induced plastic deformation inthe bulk part of the specimen, the shifting of (0002) peak ofthe �0S phase was supposed to be the phase transformationfrom �0S phase to �0T phase during ageing at 150�C.

When the distance from the rupture frontier decreased, the(0002) peak of the �0T phase shifted further to lower 2�diffraction angles. The (0002) peaks of the �0T phase were at36.6� (d-spacing, 0.2449 nm) and 36.5� (d-spacing, 0.2458nm) in the neck zone and the rupture part of the tensile testedspecimen, respectively, as shown in Fig. 27. This impliedthat with increasing tensile strength, the c-parameter of thehcp �0S phase and �

0T phase increased. The shifting of (0002)

diffraction peak of the �0S phase and �0T phase wascharacteristic of the decomposition of the hcp phases. Thedecomposition of the �0S phase and �0T phase was followedby another phase transformation, �þ " ! T0 þ �, i.e.,decomposition of the " phase. No T0 phase was observed inthe bulk part of the specimen. Approaching the rupturefrontier, the X-ray diffraction intensities of ð10�110Þ and (0002)diffraction of the " phase decreased, while that of the (110)diffraction peak of the T0 phase increased at (2�) 44.3�, asshown in the Figs. 31c and d. Obviously, The external tensilestress was responsible for the precipitation of the T0 phaseinside the " phase, i.e., decomposition the " phase.

The phase transformations that detected on X-ray diffrac-tion examination were observed using BSEM.46) Figure 32shows the BSEM images of various parts of the cast alloyspecimen after tensile testing at 150�C. In the bulk part of thespecimen, dark-contrast Al-rich � precipitates formed at the�0T phase boundaries, as shown in Fig. 32a. With increasingtensile strength, the T0 phase gray precipitates started to formin the light imaged " phase in the neck zone of the specimen,as shown in Fig. 32b. In the rupture part of the specimen, thegray imaged precipitates were apparently developed, andfracture occurred at the boundaries of the eutectoid structuresdue to the enhanced tensile strain, as shown in Fig. 32c. Withincrease of tensile strain, the precipitation of T0 phase wasconsiderably accelerated.

The tensile testing lasted for about 16min at 150�C. Bothtwo phase transformations, i.e., decomposition of the �0Sphase and �0T phase and the four phase transformation,�þ " ! T0 þ �, were obviously accelerated. In comparison,the decomposition of the zinc rich �0S phase and �0T phase ofthe alloy ZnAl7Cu3 lasted 5 h at 150�C, and the four phasetransformation, �þ " ! T0 þ �, became apparent after 15 hat 150�C.46)

The above examined tensile and creep stress induced phasetransformations and microstructural changes in the FCextruded and cast Zn-Al based alloys were detected invarious external stresses (such as fatigue, milling, ultra-precision machining and mechanical alloying etc.) deformedZn-Al based alloys. Further details were reported in refer-ences.36–62)

4. General Rule of Phase Decomposition in Zn-Al BasedAlloys (II) (On Effects of External Stress on PhaseTransformations)

4.1 External stresses accelerate decomposition of meta-stable phases

The extensive studies on effects of external stresses onphase transformation and microstructural change revealed afact, that is, the following decomposition of the metastablephases in Zn-Al based alloys were accelerated:1) Decomposition of the supersaturated �0

s phase:�0

s ! �0T þ "þ �.

2) Decomposition of the metastable �0T phase:�0T ! �þ �þ T0.

3) Decomposition of the metastable " phase:�þ " ! T0 þ �.

4) Precipitation of � phase in � phase.One of the reasons for the acceleration of the metastable

phases is that when the specimens are deformed, a highdensity of defects (dislocation and vacancies) are introduced,that enhance the diffusion and accelerate the decomposition.Also, it can be explained from point of view of Gibbs freeenergy, which is described in a separated part (4.4).

Fig. 32 BSEM images of various parts of the cast alloy (ZnAl7Cu3) after

tensile testing at 150�C: the bulk part (a), the neck zone (b) and the rupture

part (c).

3094 Y. h. Zhu

4.2 Intrinsic co-relationship of phase transformation inthe aged and the external stress-deformed Zn-Albased alloys specimens

During aging, two kinds of phase transformation, i.e., thedecomposition of the �0FC, �

0E, and �0S phases, and the four-

phase transformation: �þ " ! T0 þ �, occurred in the FC,extruded and cast eutectoid Zn-Al based alloys. Themetastable �0FC, �

0E, and �0S phases decomposed at the early

stage of aging, and this was followed by the subsequent four-phase transformation.

The same two kinds of phase transformation wereobserved in various external stresses (such as tensile, creep,fatigue, milling, ultra-precision machining, and mechanicalalloying etc.) deformed Zn-Al based alloy specimens. Thedecomposition of the �0T phase occurred in the bulk part ofthe specimen, while the four-phase transformation wasobserved in the neck zone, and well developed in the strainconcentrated rupture part of the specimen.

As a consequence, it was reasonable to elucidate that therewas an intrinsic co-relationship of phase transformations inthe aged and the external stress deformed alloy specimens.The phase transformation occurred in the less external strainpart of the specimen was related with the early stage of phasetransformation during aging (processes without externalstress), while the phase transformation occurred during theprolonged aging corresponded to that observed in theexternal strain concentrated parts of the specimen, such asthe neck zone and the rupture part of the specimen.

These co-relationships are schematically shown in Fig. 33.Solid lines — represent the correlation between the equili-briums and the phase transformations in the isothermalprocesses, the — �— lines represent the correlation betweenthe phase transformations in the isothermal processes, and theexternal stress induced phase transformations, and the dashedlines - - - imply the correlation between the phase equilibriumand the external stress induced phase transformations.

4.3 General rule of phase decomposition in Zn-Al basedalloys (II) (On effects of external stress on phasetransformation)

Based on the above discussions, the general rule of phasedecomposition in Zn-Al based alloys (II) (On effects ofexternal stress on phase transformations) is summarized asfollows:

a) External stresses accelerate decomposition of metasta-ble phases.

b) Phase transformation occurred in a less external strainpart of the external stress-deformed alloy specimen maycorrelated with the early stage of phase transformation duringageing (a process without external stress), while the phasetransformation during prolonged aging occur in the externalstrain concentrated parts of the alloy specimen.

According to the General rule of phase decomposition inZn-Al based alloys,21) the phase decomposition at the earlystage of aging may correlate with a higher temperature phaseequilibrium, whilst the phase transformation occurred duringthe prolonged aging corresponds to a lower temperatureequilibrium. In accordance, there is an inherent co-relation-ship between the external stress induced phase transforma-tion and phase equilibrium. In other words, the third point ofthe general rule of phase decomposition in Zn-Al basedalloys (II) is as followed:

c) Phase transformation occurred in a less external strainpart of the external stress deformed alloy specimen might becorrelated with a higher temperature phase equilibrium,whilst phase transformation occurred in an external strain-concentrated part of the alloy specimen might be correlatedwith a lower temperature phase equilibrium.

4.4 Free energy surfaceThe Gibbs free energy of any phase involved in the phase

transformation can be represented by a vertical distance fromthe points in the Gibbs triangle. For all possible compositionsthe points trace out the free energy surfaces for all the phases.The Gibbs free energy is particularly interesting because theequilibrium is characterized by minimum in this quantity andthe decrease in Gibbs free energy, �G, during a spontaneoustransformation, in case the volume changes are ignored.

The Gibbs free energies of all phases increase as temper-ature decreases, and the free energy surfaces of differentphases migrate up with different rate. As a result of thecompetition of the increase in free energy of phases, commontangential planes form and change their shape and position inspace. Fig. 34a shows the free energy surfaces movementcorrelating with the phase equilibrium on very slow cooling.In comparison, Fig. 34b presents the movement of the freeenergy surface correlating with phase transformation duringthe quench-ageing processes, based on the results ofexaminations of XRD and SEM.21) The curves are used toreplace the surfaces in the figures for simplifying thegraphical representation.

According to the examinations of XRD and SEM, there is afour-phase equilibrium, �þ " ¼ �þ � at 278�C, which isrepresented by a common tangential plane: �, ", � and �, asshown in Fig. 34a. This four-phase coexistence of �, �, " and� corresponds to the decomposition of the supersaturatedphase in the quench-aged alloy, i.e., �0

sðor �0sÞ ! �0

T þ

Fig. 33 Co-relationships between the phase equilibriums and both of the

phase decompositions, which occurred in the thermal and thermo-

mechanical processes: — the correlation between the equilibrium and

the phase transformations in the isothermal processes. — �— the

correlation between the phase transformations under external stresses

and without external stresses. - - - the correlation between the equilibriums

and phase transformations under external stresses.


"þ �. Below 278�C, the � phase becomes unstable and thefree energy surface migrates up from the common tangentialplane of �, ", � and � phases and disappears in themonotectoid and the eutectoid ZA alloys.3,12,13,21) Whentemperature decreases from 278 to 268�C, the T0 and � phasebecome more stable and their free energy surfaces migrate upmore slowly than those of the � and " phases. At 268�C,another common tangential plane, �, ", � and T0 forms. Onprolonged cooling, the " phase becomes unstable and its freeenergy surface moves up from the common tangential plane,�, ", � and T0. This is correlated with the four-phasetransformation, �þ " ! T0 þ �.

In case the external stress is added onto the alloys, thechanges of the Gibbs free energy includes three parts asfollows, �G ¼ �Gthermal þ�Gstrain þ�Gsurface. The exter-nal stresses results in additional increases of the free energyof the alloy system. In other words, the external stressesfasten the migration up of the free energy surfaces, i.e.,accelerating decomposition of the metastable phases. It isschematically shown in Fig. 34b. Because of the externalstress, the dislocation density becomes high both at grainboundaries and inside grains, which provides effective

nucleation sites for the formation of the new phases and thecoarse lamellar structure changes to fine grain structure. Thefree energy of the surface increases. That results in furtherincrease of the total Gibbs free energy of the alloy system.

Acknowledgements

The author would like to thank The Research committee ofthe Hong Kong Polytechnic University for the financialsupport (Project No. G-YY26).

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General Rule of Phase Decomposition in Zn-Al Based Alloys … · 2004-11-14 · General Rule of Phase Decomposition in Zn-Al Based Alloys ... of the phase transformations occurring