The influence of Cd on the superplasticity of Zn-Al alloys

T H E I N F L U E N C E O F Cd O N T H E S U P E R P L A S T I C I T Y

O F Zn-A1 A L L O Y S

P. M~ilek, P. Luk~i6, J. SucMnek*)

Faculty of Mathernatics and Physics, Charles University, Ke Karlovu 5, 121 16 Praha 2, Czechoslovakia

In the paper the results of the superplastic deformation study in the fine grained Zn-1.1 wt. ~ A1 and Zn-0"35 wt. ~ A1 -- 0-25 wt. ~ Cd alloy are presented. The influence of the long-termed ageing at room temperature on the deformation characteristics is investigated and their changes are explained on the basis of the grain growth. The presence of Cd is found to increase the stability of the fine grained structure. The influence of strain rate is studied at 293 and 373 K. Both alloys exhibit superplastic properties with maximum ductilities A ~-- 600~ and maximum values of the parameter rn = 0-5. The region of the best superplastic properties is shifted to slower strain rates as a consequence of the Cd atoms presence. The flow stress corresponding to a given strain rate is found to be much higher in the Zn-A1-Cd alloy. The grain boundary segregation of Cd atoms is suggested as a possible reason for better stability of the five grained structure in the Zn-AI-Cd alloy as well as for the differences observed in the deformation behaviour of both alleys studied.

1. INTRODUCTION

Dilute Zn-A1 alloys are typical representatives o f materials exhibiting structural

superplasticity. The superplastic behaviour at r o o m temperature (maximum ducti-

lities Am, x > 400~ and maximum values o f the strain rate sensitivity parameter

m > 0.3) was observed in Zn-0"2 wt. ~ a l [1], Zn-0.36 wt. ~ a l [2], Zn-0-4 wt. ~o

A1 [3, 4] and Zn-l .1 wt. ~ A1 [5, 6] alloys. All these alloys were prepared f rom very

pure materials. The fine-grained structure condi t ioning the superplastic behaviour

is maintained with the aid o f small particles o f an Al-rich phase that are located

especially at grain boundaries o f the Zn-rich matrix phase. However, the investiga-

t ion o f the long-term stability o f the structure o f these alloys showed a grain growth taking place during the ageing at r o o m temperature and a loss o f superplastic proper-

ties [2, 3, 7]. The rate o f the grain growth was found to be decreasing with an increasing content o f A1.

Using starting materials o f commercial purity, bo th the superplastic behaviour

and the stability o f the fine-grained structure can be influenced by the presence

o f addit ional impurities either in the fo rm o f solute a toms or in the fo rm of particles

o f other phases. Such an influence is documented in our paper where the superplastic

behaviour o f two dilute Zn-based alloys is compared - one prepared f rom very pure

materials and another one prepared f rom commercial puri ty materials. The main at tent ion is paid to the influence of the long-term ageing at r o o m temperature. on the shape o f stress-strain curves, the flow stress o-, ductility A and the strain rate

*) Present address: ViTKO VICE, Research Institute, Pohrani~ni 31, 706 02 Ostrava 6, Czecho- slovakia.

Czech, J, Phys, B 3"7 [1987] '729

P. Mdlek et al.: Superplasticity of Zn-~tl alloys.. .

sensitivity parameter m introduced by the formula [8, 91

(1) a = KAY"

where K~ is an empirical constant and ~ a true strain rate. The second part of the present paper deals with the influence of strain rate on the same characteristics of superplasticity. The deformation behaviour of both alloys is compared and discussed from the point of view of chemical composition.

2. MATERIAL AND EXPERIMENTAL PROCEDURE

The alloys used in our experiments were prepared from materials of very different purity. The alloy Awas prepared from very pure metals (Zn 99"999~ and A199.9995~) and the alloy B from commercial purity materials (Zn 99.5~ and AI 99"9~). The chemical composition of both alloys in weight ~ is given in table 1.

Table 1.

A1 Cd Cu Fe Zn

Alloy A 1-1 . . . . Balance Alloy B 0-35 0"25 0"02 0"01 Balance

It follows from the table 1 that the alloy A can be considered as a binary Zn-A1 alloy. The amount of A1 exceeds highly the equilibrium solubility limit of A1 in Zn at temperatures used in our work [101 so that the alloy is a mixture of grains of the Zn-rich phase and of small particles of an Al-rich phase. The presence of such particles was experimentally confirmed by the method of small angle X-ray scattering [111. The alloy B has to be considered at least as a ternary Zn-A1-Cd alloy. At room temperature the amounts of AI and Cd exceed the equilibrium solubility limits of both elements in Zn [101 and these elements should be present, at least partially, in the form of particles of other phases. On the other hand the contents of Cu and Fe are lower or comparable with the equilibrium solubility limit of both elements in Zn [101 and these elements may be dissolved in the matrix phase.

Both alloys were prepared in a similar way. The castings were homogenized in vacuum at 573 K for 100 hours. Then they were roiled at 573 K with a reduction of 45~ and at 300 K with further reduction of 90~. Immediately after rolling, the sheets of the thickness of 0"95 mm were set into dry ice (T g 200 K) in order to prevent the structure of both alloys from coarsening at room temperature. The mean grain size d = (1.0 __+ 0.1) gm was determined in both alloys by the method of transmission electron microscopy of coal replicas. Some structure observations were also made using optical metallography.

730 Czech. J. Phys. B 37 [1987]

P. Mdlek et aL: Superplasticity of Zn-Al alloys...

The tensile specimens of the width of 8 mm and gauge length of 20 mm were spark machined with the tensile axis parallel to the rolling direction. Tensile tests were performed using the INSTRON 1195 testing machine in the range of strain rates between 17" x 10 .5 and 4.2 x 10 .2 s -1 at temperatures (294 __+ 1) K, i.e. 0"42Tm, and (374 + 1) K, i.e. 0"53Tm. An INSTRON temperature cabinet with a circulating air atmosphere was employed for performing the tests above room temperature

3. EXPERIMENTAL RESULTS

3.1. The i n f l u e n c e of the l o n g - t e r m age ing at r o o m t e m p e r a t u r e

The influence of the long-term ageing at room temperature on the deformation behaviour of the Zn-A1 alloy was described elsewhere [7]. It was found that the steady state character of stress-strain curves which is typical of superplastic deformation, is preserved only at ageing times t < l0 s s. At longer times the stress-strain curves are characterized by a strain hardening, the magnitude of which increases with increasing ageing time. Rather different picture was obtained in the Zn-A1-Cd alloy. Figure 1 shows the initial parts of stress-strain curves for three different ageing

Fig. 1. The influence of ageing on the stress- -strain curves of the Zn-A1-Cd alloy.

r-l I I f I Zn-At-Cd T=293K

%0

1'o

t r s ] 1 .. 36•

2- . 3.6• ~ 3 . . 38x107

i f

20 30 ~o ['%3

times. The starting material (curve 1) exhibits a remarkable strain softening at engineering strains 80 > 4%, i.e. behind the transient region at the onset of deformation. Such a course of stress-strain curves is preserved up to ageing times of the order of 106 s. Only at much longer ageing times ( ~ 107 s) a change in the character of stress-strain curves takes place and a strain hardening can be observed.

Czech. J. Phys, B 37 [1987] 7 3 ~

P. Mdlek et al.: Superplasticity of Zn-Al alloys.. .

250 ?-- T=)93 K r ~ , . . ZnL/~t_C d o j a_ ~,t 1.Tx10-3s -1 o.. Zn-At > ~ = 2 % x •

- - N " X x N

x

\ 150~-

100 L -o o ~ I I I I

103 104 105 106 107 t E s ]

Fig. 2. The influence of ageing on the flow stress at 8 o = 2%.

250 ~ r i i T= 293K x.. Zn-At-Cd

Ig I ~o = 1.7• "1 o.. Zn-A[ t_J

~= 30% 290 ~- x - - X

" X ~ x _ x _ _ • • x • x

i

150 i o, JO_O-o . . . .

loo)- -~ , o , ,

10 3 10 4 1 5 10 6 10 7 t E s ]

Fig. 3. The influence of ageing on the flow stress at ~o = 30%.

Figure 2 illustrates the influence of ageing time on the flow stress established in both alloys at the strain So = 2%. It follows f rom fig. 2 that the flow stress in the Zn-AI -Cd alloy is about twice higher than that in the Zn-A1 alloy. Whilst a slight

increase in the flow stress of the Zn-A1 alloy can be seen at ageing times higher than 105 s, a decrease in the flow stress of the Zn-A1-Cd alloy was observed only at much longer ageing times t > 106 s. Similar results can be drawn from fig. 3 where the

flow stress corresponding to the strain so = 30% is plotted against the ageing time

for both alloys. The influence of ageing on the ductility for both alloys is illustrated in fig. 4.

Two remarkable facts should be mentioned:

a) Ductility in the initial state is much higher in the Zn-A1 alloy than in the

Zn-AI -Cd alloy

7 3 2 C z e c h . J. Phys , B 37 [1987 j

P. Mdlek et al.: Superplasticity of Zn-Al alloys...

u.J i I

< 300 i

I

200 -

i

100

I

~ o ~ T* 293 K

~ o 6o" 12x10-3S-1 x-- Zn-AbCd ! 2o:

i I

o

j i 10 s 10 6 , , 7

t r s310 [

lO 4

Fig. 4. The influence of ageing on ductility.

O.4

m

03

t

10 3

J= 293 K ~ t

~g 1.7• -1 x., Zn-A[- Cd

~o~30% o.. Zn-A[ - - o ~

~ ~ ~ • o • ~-----x..~

los 66 t E s ]

Fig: 5. The influence of ageing on the parameter m.

b) In both alloys a decrease in ductility takes place during ageing. However, this decrease is much stronger and occurs at much shorter ageing times in the Zn-A1 alloy as compared with the Zn-A1-Cd alloy.

Similar trends can be also observed in fig. 5 where the strain rate sensitivity parameter m is plotted as a function of ageing time. The values of the parameter m were obtained from the strain rate changes performed between the strain rates ~1 = = 1"7 x 1 0 - 3 s - 1 and ~z = 8"3 x 10 -4 s -1 and evaluated according to the formula

lg (o,/~2) (8 m-- lg( i/ 2)

where ~r i are the flow stresses corresponding to the strain rates ~.

Czech. J. Phys. B 37 [1987] 733

P. Mflek et al.: Superplasticity o f Zn-Al alloys.. .

3.2. T h e i n f l u e n c e o f s t r a i n r a t e

The stress-strain curves for the Zn-A1 alloy deformed at various strain rates were published elsewhere [5, 6]. The curves established at 294 K had a nearly steady state

character, the curves established at 374 K revealed a slight hardening, especially at lower strain rates. The character of stress-strain curves for the Zn-A1-Cd alloy

is rather different. Figure 6 shows that the stress-strain curves established at 294 K exhibit a maximum in the flow stress at very small strains (% < 5%). Beyond this maximum a region of strain softening can be seen. The steady state character of stress-strain curves was observed only at very low strain rates (80 < 10 -4 s -1)

for engineering strains s o ~ 150%. The shape of stress-strain curves established a t 374 K is more complicated (Fig. 7). The maximum in the flow stress appearing

!r~ ' Zn-A[-Cd 3 0 0 ~ T=294 K

200~

o lbo 2oo

i &o[ s "I]

1-" 1.7• .5 2 " &2xl0 +s 3 " 8.3• -5 4 - 1.7• -L" 5 ' 4:2x10 -4 6 " 8.3~10 < 7 " 1.7x10 3 8 " 4.2• -3 9 ' 8.3 •

10 " 1.7*10 -2 11 "" 4.2"10 "2

__ 3 " 2 I

1:

300 Co[ % ] 400

Fig. 6. The influence of strain rate on the stress-strain curves of the Zn-AI-Cd alloy, T = 294 K.

200 - - - i - - Zn-A[-Cd

T=3'74K O

O_

%o~~_ ~ o o ~ " ~

' Lot_ s 4] [ 1" 1.7x10 -5 2-. 8340 -5 3.. 4.2,10 -4 4 1.7• -3 5 "z,2,.,10-~ 6.. 8.3~0- 1 7 "" 1.7,10 -2 I 8"" 42"10 .2 -__.~

" 7

0 100 200 300 Co[%3 400

Fig, 7. The influence of strain rate on the stress-strain curves of the Zn-A1-Cd alloy, T = 374 K.

7 3 4 Czech. J, Phys. B 37 [1987]

P. Mrlek et al.: Superplasticity of Zn-Al alloys...

at the beginning of deformation can be observed at almost all strain rates used but its magnitude decreases rapidly with decreasing strain rate. The softening character of stress-strain curves was observed only at the highest strain rates. At lower strain rates, the softening is replaced by strain hardening. The strain at which this strain hardening starts faUs from e0 ~ 1007o at ~o = 8"3 x 10 -a s -1 to zero at ~0 = 1.7 x x 10 -5 s - I . The shape of the stress-strain curve obtained at the lowest strain rate

is similar to that of usual non-superplastic polycrystalline materials.

800 I ] To 2 9 4 K x .. Zn-AI.-Cd -• o ". Zn-At

'- ' ~ x

~ x x o.....

t 8 0 0 j i T=374 K I I

u F

~ x ~ q i 0 10 ~ 10-3 10-2.

60[ s-']

Fig. 8. The influence of strain rate on ductility.

The strain rate dependence of ductility is given in fig. 8. The maximum values of ductility observed at room temperature are similar in both alloys (Amax ~ 60070). The strain rate at which this maximum occurs is much slower in the Zn-A1-Cd alloy. A similar, however rather smaller, shift of the maximum ductility to slower strain rates was observed in the Zn-A1-Cd alloy also at 374 K. Very low ductilities were observed in the Zn-A1-Cd alloy at those strain rates where the most intensive strain hardening took place (see fig. 7).

According to the empirical relation (1) the parameter m is defined by the expression

0 lg ~] ( 3 ) m =

~lg~ r,s

where symbols T and S characterize the condition of constant temperature and structure, and it can be evaluated, e.g., from the slope of the lg a vs. lg ~ dependence. The condition of a constant structure can be hardly fulfilled in our case and therefore

Czech. J. Phys. B 37 [19e7] 7 3 5

P. Mdlek et al.: Superplastlcity of Zn-Al alloys...

the lg a vs. lg ~ dependences were plotted for a constant strain. The upper part of fig. 9 gives such dependences for both alloys deformed at room temperature. Whilst the curve for the Zn-A1 alloy is nearly linear that for the Zn-A1-Cd alloy has a concave character in the whole range of strain rates used. Figure 9 also shows that the flow stress corresponding to a given strain rate is about twice higher in the Zn-A1-Cd alloy than in the Zn-A1 alloy. The lower part of fig. 9 shows much more

I i

2oo~

100 c~ o._

5O

106

5 {

2C

L i I x" ~

T=294K

x / • o- x / o /

, • o / . / x . Zn-At-Cd -

, , / F o. Zn-A[ I I I

T= 374 K &~= 30 % / ~

/x

/ / i

�9 " 0 / t I ~ - ~ J

~-5 10-4 10-3 & [S.I]IQ -2

Fig. 9. The lg cr vs. lg ~ dependences.

complicated lg a vs. lg ~ dependences obtained at 374 K. None of both alloys exhibits any linear parts in the dependences plotted. Even an increase in the flow stress with decreasing strain rate was observed in the Zn-A1-Cd alloy at very low strain rates. The difference between the flow stresses of both alloys is smaller than that at room temperature and diminishes with decreasing strain rate. The repeated increase in this difference occurs at very low strain rates where an intensive strain hardening takes place. The strain rate dependences of the parameter m obtained as a slope of the curves given in fig. 9 are presented in fig. 10. Despite high ductilities, the parameter m values obtained at room temperature are relatively small and do not reach (excluding the lowest strain rates in the Zn-A1-Cd alloy) the value m = 0"3 usually considered as the lower limit of superplasticity. The maximum values of rn > 0.3 corresponding to the superplastic behaviour were obtained at 374 K in both alloys at nearly the same strain rates as the maximum values of ductility. A significant decrease in the parameter m was observed at slow strain rates, especially

736 Czech. ]. Phys, B 37 [t987]

P. Mdlek et al.: Superplasticity of Zn-AI alloys...

0/"1, ' "i-=294 K ' m , - x ~ C~3O %

0 3 -

01'- i • Zn-AbCd

o . Zn-A[ 0 i - - I

03_ / f oG\ o2~ ~D...--o \

[ \ / ,,o% 1;4 1;3 csr

Fog. 10. The strain rate dependence of the parameter ra established as a slope of the Ig ~ vs. lg i dependence.

m 03 . . . . I Zn-AFCd t

T : 2 9 4 K o.4F 4

Q3

Q2

/ x . / • . 1 " )< - - •

/" x

_/d / / "

L 7 c"

0:1-

Q5

0.4,

&o[.s -I] • .. 1.7.10 -4

o .. 1.7• -3

~.. 1.7• -2

I i Zn-A[

T : 2 9 4 K

~, ~ . . ~ . - ~ ' ~ x ~ x

0.3 --~%• •

I a; ~o 2b ~o 4'00 soo

Co[%]

Fig. 11. The strain dependence of the parameter m established f rom strain rate changes at

room temperature.

m !

05 ~ - i ~ / ~-- .~._ ---o... o

or, /o/ ~ Lo[ _~ ' x.. 1.7•

'~ Z n - N - C d o.. 1.7• 0.3 T = 3 7 4 K = 1.7•

I I - - I . . . . . ! ', 0.6 Zn-AI.

T - 3 7 4 K

O.5

o m "- ->~ -

~ o n x i " o o 0.4 . ~ o ~ u

~ ~6o 2b~ sbo 4~o sb 5 0 [ % ]

Fig. 12. The strain dependence of the parameter m established f rom strain rate changes, T = 374K.

Czech. J. Phys. B 37 [1987] 7 3 7

P. Mrlek et al.: Superplasticity of Zn-Al alloys.. .

in the Zn-AI-Cd alloy where even negative values of the parameter m were found. The strain rate sensitivity parameter m may be also measured by the method of strain rate changes. The strain rate changes between only two strain rates (with a ratio 2 : 1) were performed during the tensile deformation of one specimen, always after about 15~ of engineering strain. Figure 11 shows the results obtained for both alloys at room temperature. The parameter m increases in all cases with increasing strain and reaches higher values in comparison with those obtained from the slope of the lg o- vs. lg ~ plots. Whilst the parameter m seems to be independent of strain rate (at least at higher strains) for the Zn-A1 alloy, it exhibits a significant strain rate dependence for the Zn-A1-Cd alloy. Comparing the behaviour of both alloys one can conclude that the parameter m is higher for the Zn-A1 alloy excluding the lowest strain rate used. The results of analogous experiments performed at 374 K are presented in fig. 12. The parameter m values exceed in both alloys the predicted lower limit of superplasticity. Tile increasing strain dependence of the parameter rn is preserved only in the Zn-A1 alloy. The dependences obtained in the Zn-A1-Cd alloy go through a maximum, the position of which shifts to lower strains with decreasing strain rate. Similarly to the results obtained at room temperature, the parameter m exhibits a marked strain rate dependence in the Zn-A1-Cd alloy only.

4. D I S C U S S I O N

4.1. The in f luence of the l ong- t e rm ageing at r o o m t e m p e r a t u r e

The main difference between both alloys investigated on the conditions of the long-term ageing at room temperature can be seen in the shift of the decrease in superplastic characteristics to longer ageing times in the Zn-A1-Cd alloy. In our previous work on the Zn-l.1 wt. ~o A1 alloy we postulated that the loss of superplastic properties during the ageing at room temperature was connected with an intensive grain growth [7]. The Al-content present in the alloy was not high enough to create the retarding force capable of complete suppressing the migration of high- angle grain boundaries. The retarding force was probably further reduced by the ripening of the M-rich phase particles during the ageing [11, 12].

The Al-content in the Zn-A1-Cd alloy is even lower and therefore a faster grain growth might be expected. However, the metallographic study revealed that the fine grained structure was retained up to ageing times of the order 106 s. Only in the specimen aged for 5 • 106 s some larger grains were observed within the fine grained structure. This ageing time correlates very well with the beginning of the decrease in ductility and the parameter rn (fig. 4 and 5). The reasons for a better structure stability in the Zn-A1-Cd alloy have to be sought in the presence of other alloying elements, especially of Cd.

According to the recent data on the equilibrium phase diagram of the binary Zn-Cd system [13] the Cd content in the Zn-A1-Cd alloy exceeds the equilibrium

738 Czech. J. Phys. B 37 [1987]

P. Mdlek et al.: Superplasticity o f Zn-Al alloys.. .

solubility limit at room temperature and the alloy should contain some particles of a Cd-based phase. However, the electrical resistivity measurements performed on the binary Zn-0.25 wt. ~ Cd alloy revealed no electrical resistivity changes due to a redistribution of Cd atoms in the whole temperature range between the room temperature and the melting point [14]. The presence of any Cd-based particles is very improbable and Cd atoms are probably dissolved in Zn. The discrepancy with the results reported in [10, 13] might be caused by a very difficult and slow decomposi- tion of the supersaturated solid solution of Cd in Zn, mentioned in [13]. Another reason which might contribute to the increase in the solubility limit of Cd in Zn is the presence of many grain boundaries in the fine grained Zn-A1-Cd alloy. As the

�9 distribution coefficient of Cd in Zn is only 0-2, a segregation of Cd atoms at grain boundaries may be expected. A very strong grain boundary segregation was really found in the Zn-0"l~ Cd alloy [15] where the layers of the pure Cd with a thickness of 6 nm had to be assumed for explaining the differences in the composition between the bulk material and the grain boundary regions.

A segregation of solute atoms at grain boundaries leads to a reduction of the grain boundary energy and, as a consequence of this, to the creation of a retarding force for grain boundary migration. Simultaneously, the segregated atoms can block the channels of easy diffusion tn grain boundaries. Making the diffusion movement of atoms in grain boundaries difficult, the grain boundary migration may be further restricted [16]. The better structure stability in the Zn-A1-Cd alloy might be explained as a result of inhibiting effects of the segregated Cd atoms on the grain boundary migration.

The presence of grain boundary segregation influences not only the grain boundary migration but modifies probably also the deformation mechanisms in the Zn-A1-Cd alloy. Then the deformation conditions chosen in our experiment correspond in initial materials to the superplastic region II in the Zn-A1 alloy but to the non- superplastic region III in the Zn-AI-Cd alloy. A possible explanation of these results will be discussed in a more detailed manner in the next section.

4.2. The in f luence of the s t ra in ra te

At high temperatures, processes leading both to the strain hardening and strain softening take part in the deformation. On the superplastic conditions, the hardening and restoration processes are usually in a dynamic equilibrium which results generally in a steady state character of deformation curves [8, 9]. In our previous work [6] we have found that small deviations from the steady state character observed in the Zn-l.1 wt. ~ A1 alloy might be caused by a decrease in the true strain rate and by the grain growth occurring during the deformation. Excluding these effects, the deformation curves can be considered as the steady state ones. Similar effects can be also expected in the Zn-AI-Cd alloy.

The mean grain size d = (1.8 _+ 0"2) gm was obtained in the Zn-AI-Cd alloy after

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P. Mdlek et al.: Superplasticity of Zn-Al alloys...

straining up to 240~ at 294 K at the initial strain rate e0 = 4.2 x 10 -s s -*. As the grain growth decreases in superplastic materials generally with increasing strain rate [6, 17] we can conclude that the grain growth occurring during the deformation at 294 K is negligible in the Zn-A1-Cd alloy. The possible influence of the decreasing true strain rate (during straining at a constant crosshead velocity) on the flow stress follows from the relation (1). Taking the maximum flow stress observed at the beginning of the deformation curve as a reference point the flow stresses corresponding to higher strains (i.e. lower true strain rates) may be calculated according to the expression (2). The parameter m values given in fig. 11 were used in these computa- tions. Table 2 shows the comparison of the flow stresses Crca1~ calculated in such a way with the flow stresses ~rmeas really measured.

Table 2

do [ s - l ] 8 o [~] Creale [MPa] ~rmeas [MPa]

1.7 • 10 - 4 100 106 98

1.7 • 10 -3 100 176 182

1.7 • 10 -2 50 289 280

The values of 0"eale and O'meas are in good accordance, which suggests that the softening observed at 294 K might be due to the decrease in the true strain rate during the deformation.

Table 3

~o [ s - l ] 1-7 • 10 - 6 4.2 X 10 - 6 1-7 • 10 - s 1-7 • 10 - 4 1.7 x 10 - 3

d[lxm] 18 -b 0'5 13 • 0-5 8 -4- 0"6 1"21 0-2 1'14- 0"2

Significant grain growth was observed during the deformation at 374 K. Table 3 gives the mean grain sizes established at strain about 22~ for samples deformed at various initial strain rates.

The apparent strain rate dependence of the grain growth can explain the change in the character of stress-strain curves with decreasing strain rate. A numerical estimate of the influence of the grain growt h on the flow stress can be made using the formula [18]

(4) a = KzdP"

where K2 and p are empirical constants. Using the measured values of the flow stress, grain size and parameter m, the exponent p can be calculated. The value p = 1.5

740 Czech. J. Phys. B 37 [1987]

P. Mdlek et al.: SuperpIastieity of Zn-Al alloys...

obtained at the strain rate ~o = 1-7 x 10 -s s -1 is in good agreement with the values obtained in the Zn-l.1 wt. % A1 alloy [6] or in other superplastic materials [19].

Thus we can conclude that the hardening observed in the Zn-AI-Cd alloy at 374 K especially at lower strain rates might be caused by an increase in the grain s ize occurring during the deformation.

Similarly to our previous work on the Zn-l.1 wt. % A1 alloy [6] the grain growth is probably responsible also for the decrease in the value of the strain rate sensitivity parameter m established at 374 K at low strain rates in the Zn-AI-Cd alloy (see fig. 10). The flow stresses obtained at different strain rates at a constant strain correspond generally to different structures and therefore the differences among them cannot truly describe the pure influence of the strain rate. The method of strain rate changes yields much higher and more reliable values of the parameter m. This is illustrated in fig. 13 which gives the stress-strain curves of the Zn-AI-Cd alloy obtained at 374K at the initial strain rates 8.3 x 10 -s and 1.7 x 10- r -1, and the stress-strain curve of the specimen subjected to the strain rate changes between both strain rates given. Figure 13 also shows that some transient phenomena after the strain rate changes take place, which makes the evaluation of the parameter m

50 t J F

I. /I l / T-374K

20~ / I / 1.,a3• -~ r\ / 2 1 . 7 • ~ =

+ V 3 ~ 1:~i6~83,,10 -5

0 100 200 ?_,o[ % ]

Fig. 13. The stress-strain curves obtained at constant strain rates and under the conditions of strain rate changes.

difficult. The strain rate dependence of the parameter m observed in the Zn-A1-Cd alloy at 294 K (fig. 11) reflects probably the fact that only the slowest strain rate falls into the superplastic region II whereas the other ones into the non-superplastic region III. A similar attitude can be used for explaining the difference between the m values obtained at go = 1.7 x 10 -2 s -1 and at other two strain rates at 374 K.

From our measurements one can conclude that both alloys studied exhibit superplastic characteristics in some ranges of deformation conditions. The main difference between both alloys is found in the shift of the superplastic region II to lower strain

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P. M6lek eta[.: Superplasticity of Zn-Al alloys...

rates and in an increase in the flow stress in the Zn-A1-Cd alloy. These results can be probably explained considering the influence of Cd on the deformation processes taking part in the superplastic deformation.

There is no doubt that grain boundary sliding is the most important deformation mechanism under the conditions of superplasticity (see, e.g., summarizing works [20, 21]). However, the lattice dislocation motion was also observed in many superplastic materials [22-25] . Kaibyshev et al. [26, 27] published recently a new model of structural superplasticity where they assume the occurrence of the following processes:

a) grain boundary sliding due to the motion of grain boundary dislocations, b) generation of lattice dislocations in grain boundaries, c) slip of lattice dislocations through grains, d) annihilation of lattice dislocations in grain boundaries.

The processes a) and c) can be considered as those bearing the deformation. Both motion of grain boundary and lattice dislocations can be impeded by Cd atoms situated either in grain boundaries or in lattice sites. Thus the presence of Cd can lead to the effect similar to the solid solution hardening.

The process b) and d) play a role of accommodation or restoration processes which prevent the generation of stress concentrations. These processes may be also influenced by the presence of Cd atoms, especially of those segregated at grain boundaries. The solute atoms should segregate predominantly at general grain boundaries and modify their structure in such a manner that their properties become closer to special grain boundaries [16]. It has been discovered that grain boundaries are the dominant sources of lattice dislocations (see, e.g., summarizing paper [28]). In very fine grained superplastic materials such dislocation sources are even more probable. Kurzydlowski et al. [29] established that the generation of lattice dislocations was influenced by the grain boundary structure and occurred at much lower stresses in general grain boundaries than in the special ones. The segregation of solute atoms at grain boundaries might then lead to an increase in the stress neccesary for the generation of lattice dislocations and thus impede the plastic accommodation of grain boundary sliding.

A low density of dislocations within grains is typical of superplastically deformed materials [20, 21]. The main reason for such a feature may be the absence of any obstacles for the dislocation motion. The grain boundaries, which are barriers for lattice dislocations at lower temperatures and contribute thus to the strengthening, play a completely different role under the conditions of superplasticity. The lattice dislocations can be relatively easily absorbed by grain boundaries and annihilate there [23]. The annihilation of lattice dislocations in grain boundaries occurs probably due to their transformation into the extrinsic grain boundary dislocations that disappear by the process of spreading [ 3 0 - 32]. The temperature at which spreading of extrinsic grain boundary dislocations occurs with a given rate depends on the:

742 Czech. J, Phys. B 37 [1987]

P. M6lek et aL: Superplasticity of Zn-A[ alloys.. .

material composition and very strongly on the grain boundary structure [28, 33]. Varin [33] established that the spreading occurred at much slower rate in special grain boundaries than in the general ones. Segregation of solute atoms at grain boundaries should therefore decelerate the process of spreading and enhance its temperature, which was really observed in [32].

On this general basis we can explain the differences between the deformation behaviour of the alloys studied in the present work. While in the Zn-A1 alloy all the processes mentioned above can occur at sufficiently high rate and the effect of superplasticity can develop, the assumed segregation of Cd atoms at grain boundaries in the Zn-A1-Cd alloy makes these processes slower and more difficult. There- fore the flow stress necessary for ensuring the deformation at a given strain rate should be higher in the Zn-A1-Cd alloy and the region of superplasticity should be shifted towards slower strain rates. It may be, moreover, expected that the lattice dislocations that were mobile during the preparation of alloys (by rolling) could more easily annihilate in grain boundaries of the Zn-AI alloy than in the Zn-A1-Cd alloy. A higher density of dislocations remaining from the process of preparation may be therefore expected in the Zn-A1-Cd alloy. This idea was supported by the analysis of X-ray diffraction line broadening [34] that showed a higher level of internal stresses in the Zn-A1-Cd alloy.

The different deformation behaviour of the Zn-A1-Cd alloy was explained under the presumption of the segregation of Cd atoms at grain boundaries. No attention was paid to the influence of the Al-rich phase particles and to their eventual inter- actions with Cd atoms. In order to solve this problem the experiments on a binary Zn-Cd alloy are necessary. Such experiments have already been performed and the results will be presented elsewhere.

5. CONCLUSIONS

1. A decrease in ductility and the parameter rn is observed in both alloys investigated during their long-term ageing at room temperature. The grain growth is found as the main reason.

2. The stability of the fine grained structure is much better in the Zn-A1-Cd alloy as compared with the Zn-A1 alloy. The segregation of Cd atoms at grain boundaries may create the necessary retarding force for grain boundary migration.

3. Both alloys exhibit superplastic properties ~it 293 and 373 K - maximum ductilies A =~ 600~ and maximum values of the parameter m ~ 0.5. The superplastic region is shifted to lower strain rates in the Zn-A1-Cd alloy.

4. The deviations from the steady state character of stress-strain curves may be explained considering a decrease in the true strain rate and the grain growth during the deformation. The flow stresses corresponding to a given strain are much higher in the Zn-A1-Cd alloy.

Czech. J. Phys. B 37 [1987] 743

P. Mdlek et al.: Superplasticity o f Z n - A l a l loys . . .

5. The superplas t ic behav iour o f bo th al loys can be expla ined on the basis o f the

mode l o f Ka ibyshev et al. [26, 27]. The mo t ion o f gra in b o u n d a r y d is loca t ions in

g ra in boundar ies and lat t ice d is loca t ion wi thin gra ins are suggested as the main

de fo rma t ion processes. The genera t ion and ann ih i l a t ion o f la t t ice d i s loca t ion is

assumed to occur in gra in boundar ies . Gra in b o u n d a r y segregat ion o f Cd a toms

makes these processes more difficult and can lead to the differences observed in the

de fo rma t ion behav iour o f bo th al loys invest igated.

Received 13. 3. 1986.

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