9The cause of high-angle boundaries formation at the severe plastic deformation of crystals...

© 2016 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 45 (2016) 9-15

Corresponding author: A.M. Glezer, e-mail: a.glezer@mail.ru

THE CAUSE OF HIGH-ANGLE BOUNDARIES FORMATIONAT THE SEVERE PLASTIC DEFORMATION

OF CRYSTALS: DEFORMATION FRAGMENTATIONOR DYNAMIC RECRYSTALLIZATION

A.M. Glezer1,2 and A.A. Tomchuk2

1National University of Science and Technology “MISiS”, Moscow, 119049 Russia2I.P. Bardin Central Scientific Research Institute of Ferrous Metallurgy, Moscow, 105005 Russia

Received: March 10, 2016

Abstract. By transmission electron microscopy and back-scattered electron diffraction the struc-tural parameters of deformation fragments and dynamically recrystallized grains in -Fe andFeNi alloy have been analyzed upon severe plastic deformation in a Bridgman camera at a roomtemperature. It was detected that the initiation of high-angle grain boundaries in the deformedstructure is associated with low-temperature dynamic recrystallization.

1. INTRODUCTION

Severe plastic deformations (SPD) is undoubtedlyassociated with extreme conditions [1]. This ap-proach to material treatment has been of growinginterest in recent years, since it allows consider-able improvement of the physical and mechanicalproperties of metallic materials [2]. This is largelydue to the creation of different fine-grained states,particularly ones that involve fragmentation pro-cesses. Four approaches to producing very highvalues of deformation are the ones most commontoday [3]: high pressure torsion (HPT) in a Bridgmancamera, equal channel angular extrusion, accumu-lated rolling, and screw extrusion. In the first ap-proach, a sample is placed between two anvils, oneof which slowly turns with the simultaneousproduc-tion of extremely high quasi-hydrostatic pres-sures (several GPa). In the second, a sample isextruded through two channels of equal diameterand inclined to each other at a certain angle (up to90°). Plastic deformations in this case are so greatthat ordinary values of the relative degrees of defor-

mation have no meaning, and we must switch totrue logarithmic strains e. For HPT, they are de-fined as [4]:

e r h h h0.52

0ln 1 / ln / , (1)

where r and h are the radius and height of a sampledisk treated in a Bridgman camera, and is theturning angle of the movable anvil. Number N of fullturns of the movable anvil corresponds to the defor-mation at which = 2nN.

Structural states created at such severe defor-mations are very complicated and difficult ot pre-dict. Unfortunately, most authors restrict the use ofHPT to only mechanical treatment and studying theresulting structures and corresponding propertiesof materials, without analyzing the physical pro-cesses that occur immediately at extremely highdegrees of plastic flow. In our opinion, classical dis-location and disclination approaches to the studyof structural processes upon severe plastic defor-mation are insufficiently effective and require pro-found rethinking.

10 A.M. Glezer and A.A. Tomchuk

Summarizing the many experimental studies ofthe structure of SPD-treated pure metals,, solidsolutions and intermetallic compounds we can pos-tulate the following feature of SPD process [5]:· Fragmentation· Existence of high-angle grain boundaries· Lack of work hardening· Low-temperature dynamical recrystallization· High diffusibility· Initiation or solving of non-equilibrium phases· Amorphization.

The most refined concept of structures formingupon severe plastic deformation was proposed byV.V Rybin [6]. He succeeded in correctly describ-ing phenomena that occur at significant degrees ofdefor-mation close to e ~ 1 based on ideas aboutthe dominant role of disclination mode and relatedfragmentation processes. Analysis of numerousexperimental data, especially for BCC crystals, ledRybin to the concept of a limit (critical) fragmentedstructure whose further evolution is impossible inthe disclination mode [6]. Breakdown sites form onthe boundaries of fragments that separate regionsnormally free of dislocations. The critical fragmentedstructure is an end product of plastic deformationthat yields to the growing effect of external and in-ternal stresses and should result in decay. This istrue only for the early stages of HPT (e < 2) andthus does not describe structural changes that oc-cur during later stages of deformation.

It was noted in [7] that changes to HPT in tech-nically pure iron were accompanied by jump-typechanges in the material’s structure that occurred atcritical value e

c ~ 1. The mechanical behavior of the

material also changed: the mechanical hardeningobeyed a linear instead of a parabolic law upon se-vere deformation. A copper wire was HPT-treated in[8] (e = 1.6 and 3.7); cyclic variations in the struc-ture were observed as deformation grew: fragmentedstructure (d = 0.2 m) recrystallized structure fragmented structure (d = 0.1 m, where d is wasthe mean size of fragments.

Energy aspects of a solid behavior under a largeload were considered in [9]. It was suggested thatSPD can be observed not only at special deforma-

Fig. 1. Stage principle of the plastic deformation ofsolids.

tion procedures as was commonly supposed be-fore [10], but at the any one deformation procedure(e.g., rolling) following after microplastic andmacroplastic stages with extremely high value ofdeformation (Fig. 1). If a finite-dimension solid canbe presented as a mechanical dissipative system[10], we suggest that some mechanical energy isintroduced into the solid under deformation action.A plastic flow is an obvious dissipation (relaxation)channel for this energy. When the energy’s dissipa-tive capabilities are exhausted, another dissipationchannel can be used, e.g., mechanical fracture.However, the high amount of accumulated elasticstrain energy can result in additional channels ofdissipation: low-temperature dynamic recrystalliza-tion, phase transformations, and the release of ther-mal energy. With SPD, the formation and growth ofcracks is partially or completely suppressed whenthe component of hydrostatic compression stressis high, and the progress of the fracture is consider-ably hindered. In the other words, using HPT or simi-lar loading procedures, we force the solid to deformwithout fracture.

If the limit structure concept is correct [6], thendislocation-disclination accommodation is effectiveup to a certain limit that corresponds to the forma-tion of the critical defect structure, and other fac-tors should act as additional channels of mechani-cal energy dissipation. Three possible structuralscenarios of SPD development were suggested in[11] on the basis of the energy approach. In materi-als where dislocation (disclination) transformationsmade much easier (e.g., in pure metals), low-tem-perature dynamic recrystallization occurs after frag-mentation. Local regions of the structure becomefree of defects and local stresses, and a plastic flowbegins again in new recrystallized grains as a re-sult of dislocation and disclination modes. Dynamicrecrystallization then acts like a high-power addi-tional channel of elastic strain energy dissipation. Ifthe mobility of plastic deformation carriers is rela-tively low (e.g., in solid solutions or in intermetalliccompounds), phase transitions serve as a high-power channel of dissipation, most often the crys-tal-to-amorphous state transition. As a result, theplastic flow localizes in an amorphous matrix with-out deformation hardening or the accumulation ofhigh internal stresses. The choice of one scenarioof structural transformation or another depends,among other things, on parameter T

SPD/T

m, where

TSPD

is the SPD temperature with allowance for theprobable effect of heat release and T

m is the mate-

rial melting temperature.

11The cause of high-angle boundaries formation at the severe plastic deformation of crystals...

Fig. 2. Distribution histograms of grain and fragment sizes and their division into two Gaussian distributionsafter HPT with different values of full turns N for FeNi alloy: general histogram (solid), deformation fragments(dashed) and recrystallized grains (dotted).

Assuming the first scenario of structural trans-formations during SPD, we may a priori state thatrecrystallization during SPD is possible even at roomtemperature. Recrystallization processes (includingdynamic ones) are purely diffusional [12]; we shouldtherefore say that the diffusion and self-diffusion pro-cesses of substitution atoms required for the for-mation of recrystallization nuclei and their furthergrowth can occur in iron, nickel, and other metals,along with their alloys at temperatures much lowerthan 0.3 T

m, where SPD experiments are usually

carried out. This statement might appear incorrectat first glance; however, there is a great deal of ex-perimental evidence that dynamic recrystallizationduring SPD actually occurs at room and even lowertemperatures than those given in the literature[13,14]. Dynamic recrystallization can occur bothwith the growth of new recrystallized grains (dis-continuous dynamic recrystallization) and withoutthe formation of nuclei of new grains (continuousdynamic recrystallization) [14].

One feature of the structure of a metal materialafter SPD is existence of large local misorientationsthat correspond to the high-angle boundaries ofgrains [15]. It is believed that these boundarieslargely determine unique properties of a material afterSPD. But what is the physical nature of high-angle

boundaries formation upon severe deformation?Unfortunately, this question has no correct answerwithin the current plastic deformation model, whichuses a structural kinetics approach and the con-cept of mesodefects induced during deformation atrelatively low true values of deformation (e ~ 1-2)[6,15]. This question remained essentially answeredin [8], a detailed survey of this problem publishedrelatively recently. The aim of this work is thereforeto suggest a realistic mechanism of high-angleboundary formation in pure metals and diluted solidsolutions during SPD.

2. EXPERIMENTAL

A Bridgman camera was used to produce HPT [4].Polycrystalline -Fe and single-phase polycrystal-line FeNi alloy with FCC crystal structure were usedas the material for our study. The mean size of grainswas ~50 after slow cooling from 850 °C, and therewas virtually no crystallographic texture. Thin (0.2mm) plates were subjected to torsion deformationin the Bridgman camera at room temperature, con-stant quasi-hydrostatic pressure (P = 4 GPa), at 1r/m rate of the movable anvil, and number N = 1/4,1/2, 1, 2, 3, and 4 of continuous full turns of themovable anvil.

12 A.M. Glezer and A.A. Tomchuk

The structure was studied after THP using a JEM-200CX transmission electron microscope with anaccelerating voltage of 160 kV in the dark field mode.A JSM-7500F scanning electron microscope with aresolution of 0.05 m and an attachment for EBSDanalysis were also used. All studies were conductedin regions corresponding to approximately one-halfthe radius of the sample disks.

3. RESULTS AND DISCUSSION

TEM histograms of the distribution of grain sizesand fragments after deformation were obtained fordifferent N (Fig. 2). Parameter D = 4S was selectedas the characteristic size of a structural element,where S is the image area of the grain or fragment.Analysis of all the distributions revealed two maxima(a bimodal distribution) whose parameters changedas functions of the degree of deformation, while thebimodal character of the distributions was main-tained.

We showed in [9] and in a number of other ex-periments [14] that within the proposed concept ofthe existence of additional channels for elastic strainenergy dissipation upon HPT, the SPD processesin metals and solid solutions at room temperatureare accompanied by dynamic recrystallization. Elec-tron microscope analysis of the structure of our puremetal and alloy sample after experiencing differentmodes of deformation in the Bridgman camera alsoshowed clear signs of similar processes (Fig. 3).Dark-field electron microscope images of the struc-ture clearly showed individual grains that were al-most regular hexahedrons and a low concentrationof defects (Fig. 3a), testifying to the formation ofsuch grains during dynamic recrystallization via themigration of high- angle boundaries formed accord-ing to one of the possible relaxation mechanisms ofdislocation structure transformation under the con-ditions of a high concentration of point defects and

Fig. 3. Electron-microscopic images of the structure for the FeNi alloy after HPT (N = 2); (200) dark fieldimages of recrystallized grain (a), and dislocation fragment (b).

high gradients of inner stresses [15]. At the sametime, some grains (fragments) are of irregular shapeand include high concentrations of defects and no-ticeable local distortions (Fig. 3b), indicating theirformation through deformation fragmentation [6].Each of the two grain types revealed in the struc-ture after HPT treatment is clearly characterized byits own size distribution. In other words, the histo-grams in Fig. 2 are in fact combinations of two his-tograms of distributions of grains and fragments thatdiffer in origin.

Fig. 2 also shows examples of the division ofthe experimentally obtained bimodal histograms fordifferent N into two Gaussian distributions. Analy-sis of electron microscope images shows that thedistributions shown by dashed curves correspondto fragments of deformation origin, while thoseshown by dotted curves correspond to recrystallizedgrains. The fractions of the volume occupied by de-formation fragments C

f and recrystallized grains C

rg

were then calculated for each mode of deformationby calculating the relative areas under the Gaussiandistributions (Fig. 4a) and the average size of re-gions corresponding to fragments D

f and recrystal-

lized grains Drg (Fig. 4b). It can be seen that at first,

Cf sharply increases with N; it then gradually de-

creases until it reaches a constant value of 0.6.Parameter C

rg obviously includes not only recrys-

tallized grains but the initial grains as well; it firstsharply falls to 0.2 and then rises to 0.4, where itremains almost constant. Parameter k = C

rg/C

f at

N > 1 is in our opinion an important structural con-stant and depends on the nature of the material andthe HPT parameters, which largely determine itsphysical and mechanical properties (particularly theratio between strength and plasticity). To confirmthis, we must emphasize that the measuredmicrohardness grew sharply when N < 1 and re-mained constant when N > 1. Dynamic equilibrium

(a) (b)

13The cause of high-angle boundaries formation at the severe plastic deformation of crystals...

Fig. 4. Variations in (a) volume fractions Crg and C

f

and (b) mean sizes Drg and D

f for recrystallized

grains and deformation fragments, respectively, asfunctions of the number of full turns N in FeNi alloy.

Fig. 5. Histograms of grain-boundary angle distribution in FeNi alloy for different numbers of full turns N(EBSD analysis).

between structural transformations F RG is thenobserved during HPT. Dependences D(N) aresimbate in Fig. 4; i.e., D

f and D

rg gradually fall to 50

and 100 nm, respectively, and then remain virtuallyconstant when N > 2.

EBSD analysis allowed us to determine the char-acters of the distributions of grain-boundarymisorientation angles for fragments and for grainsformed at different N (Fig. 5). The fraction of grainswith > 20° grew notably with N, while the grain-boundary angle distribution was invariable for all N:the misorientation maximum corresponded to =40° - 50°. This contradicts the severe plastic defor-mation model [6], in which the maximum growsmonotonally with N.

Let us now consider what we believe to be themost important result that follows from the above.We shall try to find a correlation between the frac-tion of dynamically recrystallized grains C

rg and the

fraction of high-angle grain boundaries a under thesame structural conditions of the alloy at differentstages of HPT (with different N values). The valuesof were calculated using the data in Fig. 5, withlimitations on minimum (

20 if > 20°;

30 if

> 30°; and so on). Parametric dependence (Crg)

were plotted for N = 1/2 - 4 and = 20

- 50

. Fig. 6shows the experimental results

40(C

rg), where each

point corresponds to a certain value of N. Linearcorrelation coefficient r was calculated using thestandard technique and was in our case 0.912. Inother words, a strict linear correlation was observed

14 A.M. Glezer and A.A. Tomchuk

between the increase in the fraction of grains formedduring dynamic recrystallization and that of the high-angle grain boundaries in the structure ( > 40°). Inour opinion, this clearly shows it is dynamic recrys-tallization, and not deformation fragmentation, thatis responsible for many of the high-angle grain bound-aries in the structure of the HPT-treated material.Let us note two important points:(1) The maximum of high-angle grain boundary dis-tribution is reached at angles = 40-50°, which cor-respond to the highest mobility of high-angle bound-aries, and thus to the most rapid growth of recrys-tallization nuclei [14].(2) The linear correlation coefficient k for parametricdependences (C

rg) was decreased when de-

creased the minimal value(50

20

), testifying tothe growing contribution from dynamic fragmentationto the formation of middle- and low-angle grainboundaries.

We may assume that the structural sensitivityand magnetic properties of single-phase HPT-treatedmaterials are determined by the ratio between thevolume fractions of deformation fragments and re-crystallized grains k = C

f /C

rg. The higher k, the

greater the strength and magnetic hardness of aferromagnetic. In our investigation for pure -Fe andfor FeNi alloy, k was close to unity. We may alsoassume that an increase in the volume fraction ofrecrystallized grains (a lower k) would correspondto an increase in the plasticity of the HPT- treatedmaterial; however, this hypothesis must be verifiedexperimentally.

Fig. 6. Linear correlation of parametric dependence

40 (C

rg) for FeNi alloy; C

rg - the volume fraction of

recrystallized grains and 40

- the fraction of grainboundaries with misorientation angle 40°. Thefigures show the number of full turns N; correlationcoefficient r = 0.912.

4. CONCLUSIONS

(1) The formation of structural regions correspond-ing to deformation fragments and dynamically re-crystallized grains in the structure of -Fe (BCC)and FeNi alloy (FCC) upon severe deformation in aBridgman camera at room temperature was experi-mentally ascertained using the energy concept ofadditional channels for elastic strain energy dissi-pation (relaxation) during HPT.(2) We showed via TEM and EBSD that the highconcentration of high-angle grain boundaries in thestructure of HPT-treated material is due mainly todynamic recrystallization process but not only de-formation fragmentation process, as was believedearlier.

ACKNOWLEDGMENTS

The authors are grateful to A. N. Belyakov for hishelp in the EBSD experiment. This work was sup-ported by the Russian Foundation for Basic Re-search, grants 13-02-12087.

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