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Metal Science and Heat Treatment Vol. 38, Nos. 9 - 10, 1996

UDC 669.55

EFFECT OF ALLOYING, HEAT TREATMENT, AND DEFORMATION ON THE STRUCTURE AND PROPERTIES OF DAMPING Zn-A I ALLOYS

A. I. Skvor tsov ~

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 27 -29 , September, 1996.

The damping capacity, mechanical properties, and structure of Zn - AI alloys are studied as a function of the regime of heat treatment, plastic deformation (hydroextrusion), and alloying with Fe, Be, Cu, and Mg. The present work continues previous investigations of damping Zn - AI alloys conducted by the author.

Intense development of alloys based on Zn-A I as a damping material began in foreign countries in the 1950s [ 1, 2]. In Russia these alloys have been mainly investigated as superplastic materials [3, 4]. The damping properties of Zn - AI alloys have been considered in [5, 6]. However, the authors o f [5] did not use heat treatment as an important factor in increasing the damping capacity, and the alloy sug- gested in [6] had an unsatisfactory damping capacity.

The author of the present work studied the damping, mechanical, and physical properties of binary Zn - AI alloys as a function of the heat treatment regime in [7]. The present pa- per is devoted to the effects of alloying, pressure treatment (hydroextrusion), and structure on the properties of binary Zn - AI alloys.

The methods for fabricating the alloys and the methods for their investigation (electron-microscopic, internal fric- tion) are described in [7]. An x-ray diffraction analysis was conducted on a DRON-2 diffractometer with the use of cobalt /Ca radiation. The alloys were deformed at 350C with a de- glee e = 12 -55% with cooling of the preforms in water. The specimens for the investigation were cut from hardened preforms.

Recent domestic research on the topic is mainly repre- sented by [6], which proposes a damping Zn- AI alloy con- taining 35-45% Zn, 0.02- 0.09% Ti, and the remainder AI and having a relatively low damping capacity characterized by a logarithmic decrement 6 = 0.012.

In foreign countries the investigation of damping Zn - Ai alloys is much more intense. The data on the effect of alloying on the damping and other properties are contained pre- dominantly in descriptions of patents. However, they are lim- ited and bear little information (for example, [8]).

It is known from [9] that alloying with magnesium and copper decreases the damping capacity ofZn - 22% AI alloys

I Vyatka State Engineer ing University, Vyatka, Russia.

but increases their strength and corrosion resistance. Russian specialists use Zn- AI alloys with Cu and Mg admixtures of grade TsA and TsAM, whose foreign counterparts are ZA = 8, ZA = 12, ZA = 27 [10].

The present work considers the effect of successive magnesium and copper alloying on the properties of alloys with an elevated content of Al relative to monotectoid alloys (4 - 6 in Table 1), i.e., lighter and stronger alloys retaining an elevated damping capacity [7], and the effect of beryllium and copper alloying on the properties of monotectoid alloys (1 -3 in Ta- ble 1), i.e., in the region of AI concentrations corresponding to the maximum damping capacity [7].

Alloys based on Zn-49 / , AI. The dependence of the damping capacity of these alloys on the aging temperature (hardening from the a'-region from 415C) has a maximum. The addition of Mg and Cu to the Zn-49% AI alloy decreases the height of the maximum 6 and shifts it to higher aging temperatures.

The cause of the appearance of the maxima in 6 has been established by means of an x-ray diffraction analysis. As a re- sult of supercooling of the ct'-phase to room temperature the proportion of the volumes of the ct'-, ct-, and 13-phases is 3 : 3 : 4. Alloyed alloys contain only the ct'-phase. This im-

395

TABLE I

Content o f elements, %* Al loy

AI Fe Be Cu Mg

/ 22.7 . . . .

2 22.9 1.2 0.1 - -

3 23.0 1.2 0.1 1.2 -

4 48.6 . . . .

5 48.7 - - - 0.13

6 48.6 - - 1.0 0.12

*The remainder is Zn.

0026.0673/96/0910-0395515.00 O 1997 Plenum Publishing Corl~rltion

396 A. I . Skvortsov

6,%

1.0

0.5

0 HV

150

100

50

6,%

6 6 q 3 ~""~0

100 150 200 0 " IO0 150 o,t, C

Fig. 1. Damping capacity (y = 10 -4) and hardness of alloys based on Zn- AI as a function of the aging temperature. The figures at the curves are the numbers of the alloys (see Table I): a) hardened from 415C; b) hardened from 370C.

plies that the ct'-phase possesses a low damping capacity and

its decomposition increases 6. This is observed in aging of the alloys (Fig. la). An increase in the aging temperature intensi-

fies the diffusion processes in the ~t'-phase, promotes its decomposition, and hence increases 6. Therefore, the magnitude of the maxima on the curves 6 =f(tag) depends on the degree of decomposition of the ct'-phase into or- and 13-phases.

The aging temperature at which the increase in 5 occurs at a maximum rate coincides with that of the fastest fall of HV in all the alloys (Fig. 1 a), which indicates that both effects are caused by the decomposition of the ct'-phase. A maximum in hardness in aging due to segregation of pre-precipitation zones is observed only in alloyed alloys (Fig. la).

Alloys based on Zn- 23*/0 AI. An increase in the content of alloying elements in such alloys is accompanied by a

6, % a

I 2 3

%.2, N/mm2 b

f 250

20 200 [-

o I 2 3

a, nlTl 0.404

0.404(

. %

B, mrad 15

G, vol, % 40

0 C, nm 0.494 0.493 a nm 0.2664' 0.2663

Fig. 2. Damping (y = 10 - 4) (a) and mechanical (b) propcrlies, widths Bi3 of the line (203)a ~ of

the 13-phase and B a of the line (331)c h of the a-phase, phase composition (G is the amount of the phase), and crystal lattice parameters (c) of alloys 1, 2, 3 based on Zn - 23*/, AI after hardening.

decrease in their damping capacity (Fig. l b). Due to the higher content of Zn the damping capacity of this group of alloyed alloys is higher than in alloyed alloys based on Zn - 49% AI (Fig. I a).

The dependence of the damping capacity on the aging temperature (hardening from 370C) has a slight maximum only for the most alloyed composition 3, which is explainable by monotectoid decomposition of the retained ct'-phase formed in hardening (Fig. 2c).

The dependence of the hardness of these alloys on the aging temperature has a minimum (see Fig. l b). Alloyed alloys exhibit it at higher temperatures. This seems to be a sign that in alloying the process of decomposition of the solid solution shifts to higher aging temperatures, and this factor plays a greater role up to the temperatures of minimum hardness than the factor of solid-solution strengthening caused by dissolu- tion of Zn in the a-phase in correspondence with the phase diagram ofZn - AI.

It can be seen from the data of Fig. 2 that alloying strengthens the hardened alloy, which follows from the higher degree of distortion of the crystal lattices of the ct- and 13-phases and the widths B a and Bp of the lines, and decreases its ductility, the phase composition and the parameters of the crystal structure of the fee and I~ phases with changing. The damping capacity is a function of the width of the lines of the or- and I~-phases, the degree of tetragonality of the 13-phase, and the characteristic a=, of solid-solution strengthening of the retained ct'-phase; a lower value of the latter corresponds to a higher content of Zn in the ct'-phase, i.e., to a higher degree of solid-solution strengthening.

It is well known that the damping capacity of Zn - Ai alloys increases after cold plastic deformation [9]. It is of inter- est to determine the effect of the deformation at a temperature exceeding that of the phase transformation (275C) on the damping capacity of Zn - AI alloys, A regime for heat treat-

ment of a Zn - 22% AI alloy that includes deformation in the ct' region, hardening, and natural aging is described in [ 11 ].

The properties of alloys 1 and 2 (Table 1) were studied 2 after a deformation at 350C with

= 12 - 55%. With increasing degree of deformation e the

damping capacity of the alloys tended to increase. In some cases this characteristic decreased. For example, the damping capacity of alloy 2 decreased by 0.2% when the degree of deformation was increased from 44 to 55%.

In alloy 1 an increase in e was accompanied by an increase in the fraction of lamellar structure (Fig. 3a, b). This allows us to assume that an increase in the damping capacity in these alloys is

Conducted by B. M. I~fros at the Donetsk Engineering Phys- ics Institute.

Effect o f A l loy ing , Heat T reatment , and Deformat ion on the Proper t ies o f Damping Zn - AI A l loys 397

Fig. 3. Microstructure of alloys 1 (a, b) and 2 (c, d) after a thermomechanical treatment with e = 12% (a, c) and c = 55% (b, d): a) x 23,000; b) x 27,000; c) x 29,000; d) x 53,000.

connected with an increase in the area of interphase and inter-

grain boundaries.

The 13-structure of hardened alloy 2 is characterized by a lamellar form of the phases, which is connected with its al-

loying. It can be seen from Fig. 3c, d that as e is increased from 12 to 55% the form and dispersity of the phases, as well as the damping capacity, change but little (Table 2). It should

be noted that the dispersity of the lamellar structure of alloy 2 is lower than that of alloy 1 (Fig. 3). This is one of the causes of the lower damping capacity of alloy 2 (Table 2).

The results of an x-ray diffraction analysis (Table 2) show the following.

1. An increase in e causes an increase in the amount of

retained a'-phase, which possesses a lower damping capacity than a monotectoid (after hardening a monotectoid alloy from

the ct' region to room temperature ~5---0.5 and 7%, respec- tively).

2. Depending on the degree of deformation the content

of Zn in the retained cd-phase can change considerably,

whereas in the or-phase of a monotectoid it can remain at the same level.

3. The degree of tetragonality of the 13-phase, which

changes as a function of e, corresponds to the damping capacity.

Thus, an increase in the degree of deformation can cause

an increase in the damping capacity of Zn - A1 alloys, when

the effect of the increase in the area of interphase and inter-

grain boundaries or in the degree of tetragonality of the

13-phase exceeds the effect of the increase in the amount of

retained ct'-phase.

CONCLUSIONS

1. The properties of Zn - A1 alloys are affected substan-

tially by the retained ct'-phase. It possesses an elevated

strength but its damping capacity and ductility are low. Its

stabilization in hardened Zn - AI alloys is promoted by an in-

TABLE 2

Alloy e.% 8,%(,/=10 -4) ct',vol.%Zna,,% Zna,% c/a

12 3.5 5 3.0 1.8558 1 55 4.3 20 3.0 1.8570

12 1.15 40 14 - 1.8553 2 55 1.20 55 19 - 1.8564

Notation: Zn a, and Zn a are the zinc contents in the ct' and ct phases, y is the deformation amplitude. Note. The content of zinc in the a' phase of alloy 2 was determined using a conventional coefficient that allows for the alloying.

398 A.I . Skvortsov

crease in the content of AI and in the degree of deformation; the Zn- 49 % AI alloy should be alloyed with Mg or Mg + Cu, and the Zn - 23% AI alloy should be alloyed with Cu.

2. Alloys based on Zn-A I with a structure of a monotectoid possess a high damping capacity and a good

combination of strength and ductility, namely, 8 = 2 - 7% at

),= 10 -4, 0"0. 2 = 310- 180 N/mm 2, and an elongation 85 that ranges between 2 and 42% depending on the heat treatment regime. Beryllium, copper, and iron alloying of alloys with a structure close to monotectoid increases their strength but decreases their damping capacity and ductility.

3. An increase in the dispersity of the monotectoid increases the damping capacity of the alloy, if the dispersity

factor is not accompanied by the action of retained ct'-phase. 4. When choosing the regime of heat treatment for

Zn - AI alloys the fact that hardening creates a structure that retains a nonequilibrium nature up to subcritical aging temperatures should be taken into account. The structure is more nonequilibrium than in the annealed state. Therefore, the damping capacity of a hardened and aged alloy is higher than that of an annealed alloy. The aim of aging can be different depending on the composition of the alloy, namely, (a)

monotectoid decomposition of the ct'-phase in order to increase the damping capacity and the ductility, when the aging temperature corresponding to a high damping capacity is determined by the composition of the high-temperature phase, (b) size stabilization, (c) increasing the strength by dispersion hardening.

REFERENCES

1. A. S. Nowick, "Anelastic effects arising from precipitation in aluminum-zinc alloys,"./. Appl. Phys., 22(7), 925 - 933 (1951).

2. "Vibroabsorbing A I - Zn alloy with elevated damping and a method of manufacturing," Application No. 59 - 10985, Japan, MKI C 22 C 18/04.

3. A. A. Bochvar and Z. A. Sviderskaya, "The phenomenon of superplasticity in zinc-aluminum alloys," lz~. Akad. Nauk SSSR. OTN, No. 9, 821 - 824 (1945).

4. A. A. Presnyakov, Superplasticity of Metals andAIloys [in Rus- sian], Nauka, Alma-Am (1969).

5. M. E. Drits, L. L. Rokhlin, and G. V. Ryuchina, "Damping properties of cast aluminum alloys," in: Physical Metallurgy of Non- ferrous Metals and AIloys [in Russian], Nauka, Moscow (1972), pp. 184- 187.

6. USSR Inventor's Certificate 430178, MKI C22 C21/00, "Alu- minum-base cast alloy," Byull. Otkryt. lzobret., No. 20 ( i 974).

7. A. I. Skvortsov, "Damping and mechanical properties of zinc- aluminum alloys," Izv. Vuzov. Tsvetnaya Metallurgiya, No. 1, 118- 122(1991).

8. "A method for fabricating a damping alloy," Patent No. 3032153 FRG, MKI C 22 C 18/04.

9. M. Tagami, T. Ohtani, and T. Usami, "Effects of heat trealanent, contents of Cu and Mg, and rolling reduction on the damping capacity and mechanical properties of Zn-22% AI alloys," J. Jpn. Inst. Light Metals, 38, 107- 113 (1988).

10. H. Goldenstein, DAJD Cunha Flores, P. Braga, and P. Fuoco. "As ligas zinko-aluminio: caracteristicas gcrais alguns resul- tados de msistencia ao desgaste," Metalurgia-ABM, 43(6), 97 - 102 (1987).

11. I. 1. Novikov, V. K. Portnoi, N. S. Zhuravleva, et al., "Ther- momechanical ~eatment for fabricating superplastic sheets from zinc-aluminum alloys," Izv. Vuzov. Tsvetnaya Metallurgiya, No. 4, 98 - 103 (1976).

Documents

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