The effect of impurities on ductility and cavitation in the superplastic Zn-22%Al alloy

Materials Science and Engineering, A188 (1994) 59-67

59

The effect of impurities on ductility and cavitation in the superplastic

Zn-22%A1 alloy K.-T. Park, S. T. Yang, J. C. Earthman and F. A. Mohamed Materials Section, Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92717 (USA) (Received August 30, 1993; in revised form January 14, 1994)

Abstract An investigation of the effect of impurities on the ductility and cavitation behavior of a microduplex superplastic alloy was performed. Experiments were conducted in tension at 473 K on three grades of Zn-22%Al that contain different impurity contents. It is shown that a higher impurity content results in a drastic decrease in the ductility and an enhancement in the cavitation even when the deformation is considered superplastic. In addition, the orientation of the cavity stringers appears not to depend on the rolling direction but is always observed parallel to the tensile axis. The experimental observations are compared in terms of the strain rate sensitivity dependence of ductility. The results are also compared with the predictions of various theoretical models that attempt to describe cavity nucleation and growth. Finally, the mechanisms for the formation of cavity stringers parallel to the tensile axis are discussed.

1. Introduction

Superplasticity refers to the ability of some materials to exhibit, at elevated temperature, large neck-free elongation of several hundreds or even thousands of percent prior to failure. The two basic requirements for achieving structural superplasticity are: (a) a testing temperature T greater than about one half of the absolute melting temperature of the material, Tm (i.e. T~>0.5 Tin);and (b) a fine, stable equiaxed grain size d of less than about 10/~m (i.e. d<~ 10/zm). In addition to these two requirements, grain boundaries must be mobile and high-angled. It is well established that cavities can nucleate, grow and interlink in the process of superplastic deformation, leading to undesired premature failure. In general, the cavities nucleate at a grain boundary irregularity, such as ledges and triple junctions, and second-phase particles or precipitates on the boundaries. The presence of those cavities degrades the mechanical properties of superplastically deformed components and consequently results in serious constraints on the commercial use of the superplastic materials. For this reason, increasing attention has been devoted to studying the role of cavitation in limiting superplastic behavior [1 ]. Unlike cavitation in the creep of large-grained materials, in which cavities tend to lie on the boundaries perpendicular to the stress axis, the cavity 0921-5093/94/$7.00

SSD10921-5093(93)09527-4

alignment in superplastically deformed materials depends on the type of superplastic material considered. Different cavity alignments are observed in quasi-single phase materials, for example, Coronze CDA 638, when compared with that for microduplex materials, for example, Zn-22%A1. In quasi-single phase materials, cavities are aligned parallel to the rolling direction regardless of the direction of the tensile axis [2, 3], while in microduplex materials cavity alignment is parallel to the stress axis regardless of the rolling direction [4]. In the former, the rolling process causes the break-up of particles and the alignment of the resulting fragments along the rolling direction. The fact that cavity stringers in those materials are parallel to the rolling direction implies that particle fragments constitute the most favorable cavity nucleation site. However, for microduplex alloys which do not contain the second phase particles, the reason why the cavity stringers are parallel to the stress axis rather than the rolling direction is not clear. An investigation performed by Chokshi and Langdon [4] on the influence of rolling direction on mechanical behavior and cavitation in the superplastic Zn-22%A1 alloy has indicated that the cavities form stringers at all strain rates that are aligned along the stress axis regardless of the rolling direction. In addition, they have concluded that cavities in microduplex materials prefer to nucleate at triple junctions and/or ledges rather than at particles, impurities or © 1994 - Elsevier Sequoia. All rights reserved

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K.-T. Park et al.

/

Effect of impurities in Zn-22%AI

pre-existing voids. Although Miller and Langdon [5] reported that the extent of cavitation in moderately high purity Zn-22%A1 ( < 20 ppm) is comparable with that for Zn-22%A1 of commercial purity, recent theoretical treatments [6-10] suggest that impurities should significantly influence cavity nucleation and growth. In addition, experimental evidence [ 11, 12] has shown that the extent of cavitation increases with increasing impurity content. It was also observed that cavity stringers in Zn-22%A1 with a higher impurity level are always parallel to the stress axis. These are somewhat contradictory to the suggestion that the triple junctions and/or ledges are the primary sites for cavity nucleation; if cavities mainly nucleate at ledges and triple junctions in microduplex, materials, the extent of cavitation should not depend on the impurity content. Accordingly, the present investigation was performed to examine the effect of rolling direction on the formation of cavity stringers and the effect of impurities on ductility and cavitation in a microduplex superplastic material.

T A B L E 1. T h e characteristics of Z n - 2 2 % A l grades Grade A

Grade B

Purity (%) AI Zn Impurity content (ppm) Ag Cd Ca Cr Cu

0.5 1 0.1 -2

-15

Fe Pb Mg Mn Si

1 -0.2 -1

120 20 0.7 3 10

99.999 99.999

Grade C

99.99 99.9 7 3 0.2

99.99 99.99 -2 2 0.7 7 1460 15 0.2 15 2

present alloy. All tests were conducted at 473 K + 2 K by immersing the specimens in a silicone oil bath. After failure, a portion of the gage length of each specimen was cut, mounted and polished metallographically for an examination of the cavitation.

2. Experimental details 3. Results In the present investigation, three grades of the superplastic Zn-22% AI alloy were used. Grades A, B and C contain 6 ppm, 180 ppm and 1520 ppm impurities respectively (ppm in wt.). The purity of the starting materials and the impurity content in the final product are listed in Table 1. The major impurity element is Fe: 1 ppm, 120 ppm and 1460 ppm of Fe in Grades A, B and C respectively. Tensile specimens of shape and dimensions described elsewhere [13, 14] were machined from the three grades in two ways; the tensile axis is either parallel (hereafter, type L) or perpendicular (hereafter, type P) to the rolling direction. Before testing, the specimens were solution-treated in an argon atmosphere at 633 K for 15 h, quenched in an icewater bath to produce very fine grains of the two eutectoid phases by spontaneous decomposition, and then annealed at 533 K to give an equiaxed grain size of 2.5 /~m + 0.2/~m. Here, the grain size refers to the average spatial grain diameter d, defined as d = 1.74 L, where L is the mean linear intercept, and no distinction is made between phase and grain boundaries. The tensile specimens were tested on an Instron machine operating at a constant rate of cross-head displacement. The initial normal strain rate, go, was in the range from 6.67 × 1 0 - 4 s -1 to 3.33 × 1 0 - 6 s -1 which, according to the data reported for the alloy [13], covers from middle of Region II (the superplastic region) to the advent of Region I (the low stress region) for the

3.1. Ductility 3.1.1. Effect of impurity Figure 1 shows elongation to failure, ef, as a function of the initial strain rate go for a type L specimen. Ductility in Grade A (6 ppm impurities) exhibits a gradual decrease in ef with decreasing go while Grade B (180 ppm impurities) exhibits a much more drastic decrease in ef with decreasing go; i.e. Grade A retains high ductility, greater than 1100%, even at very low strain rates, while ductility in Grade B drops to about 700%. At go > 1.33 x 10 -4 S-1 (the superplastic region according to published data on Zn-22%AI [13]), ef in Grade A and Grade B reaches about 1200%-1400%. Grade C (1520 ppm impurities) also shows a gradual decrease in ef with decreasing go, which is similar to that of Grade A, but achieves a ductility of approximately half of that in Grade A for the entire range of go used in the present investigation. The above observation is consistent with the experimental results of a recent analysis reported for the impurity effect on superplastic flow [15, 16]. These results suggest that the high value of stress exponent n (equivalent to low strain rate sensitivity, rn (= l/n)) in the low stress region in the creep of Zn-22%A1 is attributed to the effect associated with the presence of excessive impurities. Accordingly, the ductility of materials with high impurity content is expected to be small in the low stress region since the higher n becomes, the lower the

K.-T. Park et al.

/

Effect of impurities in Zn-22%Al

61

(~y~-p") 18oo

14oo

1400

°

g

1000 /

/

6OO

600

,!

10"6

•

L

.

lOOO

200

T=473K d = 2.5 • 0.2Bm • Grade A • Grade C

1800

T = 473 K d = 2.5 + 0.2Bm o Grade A • Grade B n Grade C

........

,

,

,

10-5

.....

i

10-4 ~o"

200

. . . . . . .

10"3

,

,

10-6

,

,

,,,,i

,

,

, ....

|

10"4 ~0'

S'l

,

10"5

. . . . . . .

10"3

S'l

Fig. 1. E l o n g a t i o n to failure, el, in t h r e e g r a d e s o f Z n - 2 2 % A1 as

Fig. 2. E l o n g a t i o n to failure, el, in G r a d e A a n d G r a d e C of

a function of the initial tensile strain rate gofor type L specimens.

Zn-22%AI as a function of the initial tensile strain rate go for type P specimens.

ductility is for most superplastic materials [17-19]. (The effect of cavitation on ductility will be discussed later.)

c~

R.D.

3.1.2. Effect of rolling direction Figure 2 presents ef as a function of go for type P specimens of Grades A and C. For the purpose of comparison, ef of type L specimens taken from Fig. 1 is included as dashed lines. The general trend of the variation of er with go is comparable with that observed for type L specimens. The absolute values of ef appear slightly lower than that of type L specimens at the same

3.2. Cavitation 3.2.1. Effect of impurity In Fig. 3, polished gage sections for type L specimens of Grade A are shown for different values of go: 6.67 × 10 -4 s -I, 1.33 × 10 -4 s -1 and 1.33 × 10 -5 s -1. Regardless of g0, the cavities are rarely found in the gage section; extensive cavities are only found in the vicinity of fracture tip. Figures 4 and 5 illustrate the cavitation after fracture in type L specimens for Grade B and Grade C respectively. The cavitation in both grades is relatively extensive when compared with that observed in Grade A for all values of e0 used in the present investigation. Furthermore, the extent of cavitation becomes more severe with decreasing go. This finding is consistent with the result reported by Park and Mohamed [11]. Their work has shown that the extent of cavitation becomes more pronounced with increasing impurity level in Zn-22%AI which was tested under the experimental conditions identical with those imposed in the present investigation. A close comparison of the fracture tips for all three grades at

-133xlO-Ss-I lmm Fig. 3. C a v i t a t i o n in t y p e L s p e c i m e n s o f G r a d e A o f Z n - 2 2 % A I

(6 ppm impurities).

all go indicates that while the failure probably occurred by a combined effect of necking and cavity coalescence, the cavities are more evident in the alloys which contain higher impurity level.

62

K.- T. Park et al. / Effect of impurities in Zn-22%Al (Y

(I

R.D.

R.D.

A lmm Fig. 4. Cavitation in type L specimensof Grade B for Zn-22%A1 (t 80 ppm impurities).

v

lmm Fig. 5. Cavitation in type L specimensof Grade C of Zn-22%AI (1520 ppm impurities). a

3. 2. 2. Effect of rolling direction Figures 6 and 7 represent the cavitation after failure in type P specimens for Grade A and Grade C. Several features are noted by comparing Figs. 6 and 7 with Figs. 3-5. First, in Grade A, cavitation in type P specimens, like that in type L specimens, is not significant. Second, regardless of the rolling direction, the cavity stringers obviously tend to align along the tensile axis. This result is consistent with that reported by Chokshi and Langdon [4] for Zn-22%AI (200 ppm impurities), and confirms that microduplex superplastic materials behave differently from quasi-single phase materials in which cavity stringers are aligned parallel to the rolling direction. Third, the cavitation in specimen type L is less extensive than that in specimen type P for the same go in Grade C. This may, in part, explain the observation of a slightly larger elongation to failure for specimen type L (Fig. 2).

I R.D

lmm Fig. 6. Cavitationin type P specimensof Grade A of Zn-22%AI (6 ppm impurities).

4. Discussion

4.1. Effect of impurity on ductility One of the distinguishing characteristics of mechanical behavior in micrograin superplastic materials is the

observation of a sigmoidal trend when the steady-state creep rate is plotted against the applied stress on a logarithmic scale. Accordingly, the creep behavior can be divided into three distinct regions; the charac-

K. - T. Park et al.

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Effect of impurities in Zn-22%A I

(Y

63

TABLE 2. The characteristics of creep behavior in fine-grained superplastic materials Stress Region I

n

Low

3-4

Region II Intermediate Region III High

R.D

Q >

2-2.5 >3

Deformation mechanisma Qgbb

= Qgb

> Qgb

GBS~hindered by impurities GBS Dislocation creep

~'The most contributable mechanism; bactivation energy for grain boundary diffusion; Cgrainboundary sliding•

= 1.33 x 10 -4 s "1

10 -2

10-4

Zn - 22% AI T = 473 K d = 2.5 ± 0.21.tm Type"L" o Grade A • Grade B m GradeC

10 5

• •

~" o ffi 1.33 x 10 -5 s "1 103 lmm

Fig. 7. Cavitation in type P specimens of Grade C for Zn-22%AI (1520 ppm impurities).

ef = exp

- 1

( 1)

where c is a constant that depends on the shape and dimensions of samples and other structural factors. In order to examine the dependence of ductility on m, the present data obtained using tensile loading conditions are compared in Fig. 8 with the shear data reported previously [16] by taking o = 2 r and g = 2 ~/3 for the maximum values of o from the o - g curve. We note that data for Grade A (6 ppm impurities) fit the solid line taken from [16] whose slope is about 2.5 and which extends to strain rate as low as 3 x 10 .7 s -l. However, data for Grade B (180 ppm impurities) deviate from that line for ~ < 3 x 10 -4 s 1, i.e. n - ~ 3 .

..~A ~2"-

Type "P" •~

teristics of each region are listed in Table 2. Recent investigations [15, 16] have unambiguously shown that Region I originates from the existence of a threshold stress for grain boundary sliding that is caused by impurity segregation at boundaries. This implies that Region II may dominate both the intermediate and low stress regions if a very high purity material is tested; creep data reported for Grade A which contain 6 ppm impurities have shown the absence of Region I [15, 16]. The results on ductility obtained at low stresses in the present investigation lend support to the above possibility. In considering the values of strain rate sensitivity m ( m = i / n ) for each region (Table 2), the difference in ductility of each grade is anticipated to result from the different m value since ductility, in general, is related to m by an equation of the form [17-19]

A./2//

/ ¢ ~ ., z ~

Grade A GradeC

//(~

.~A

/1 2. 5 1

106

10-7 .1

....... 1

10

~:, MPa

Fig. 8. Comparison of the tensile data of three grades of Zn-22%A1 with the shear creep data in Zn-22%AI taken from [16] (solid line for Grade A and dashed line for Grade B).

The relative comparison of ef in terms of m between Grade A and Grade B is possible by using eqn. ( 1 ). The use of an average el(-- 1200%) and m = 0 . 4 for Grade A results in c--3.85 in eqn. (1). Then, by taking m = 0 . 3 3 for Grade B, eqn. (1) predicts e r = 6 0 0 % , a value which is in reasonable agreement with the experimental observation. Although cavitation is more significant in Grade B than that in Grade A, the failure in the alloy is likely to be more influenced by a value of m than damage mechanisms. This is consistent with the experimental observation reported on Zn-22%A1 previously [20] that localized necking is primarily responsible for failure at low strain rates. Grade C (1520 ppm impurities) reveals a different characteristic. Data for Grade C do not exhibit significant deviation from the line even at the lowest g0 ( = 1.33 x 10 -5 s -l, which is higher than the lowest g0 for Grade A and grade B), but ef is much smaller than that predicted from eqn. ( 1 ). A close inspection of cavitation for Grade C (Figs. 5 and 7) indicates that the fracture tip is more quasi-brittle in nature rather than

K.-T. Park et al. / Effect of impurities in Zn-22%Al

64

localized necking and that cavitation is relatively more extensive at the highest go (= 6.67 x 10 -5 s -1) than in Grade A and Grade B. Therefore, these observations imply that cavitation induced by introducing excessive impurities may cause premature failure in that alloy. It is worth mentioning that the deviation of the mechanical data in Grades B and C from the line fitted to Grade A data is expected to become more significant if the ductility tests are conducted at lower go than that used in the present investigation [15, 16]. In such a case, the ductility of Grades B and C would decrease drastically with decreasing go due to the combined effects of more extensive cavitation and smaller values of m while that of Grade A would remain high due to the continued presence of little cavitation and relatively large values of m.

4.2. Effect of impurities on cavitation On the basis of the results of numerous theoretical [6-10] and experimental studies [11, 12, 21, 22] on the effect of impurities on the various stage of cavitation during creep and superplastic deformation, the segregation of impurities may influence cavitation through one or a combination of the following processes: (1) a reduction of boundary cohesive strength; (2) a reduction of the surface energy of a vacancy cluster leading to a corresponding decrease in the critical cavity nucleation stress; (3) a reduction of the grain boundary diffusivity, Dgb; and (4) the formation of precipitates which serve as effective cavity nucleation sites. Although the addition of impurities was reported to increase Dgb in some cases at high temperature [7], it is assumed, on the basis of diffusion data in AI [23], which show that diffusivity in AI decreases with Fe addition, that Fe, a major impurity element in Zn-22%A1 in the present investigation, decreases Dgb. (Also, it is quite possible that, under the condition of segregation, Fe, being a smaller atom than AI, may occupy a significant number of sites in the boundary, a process which could block boundary diffusion paths.) In the following, these effects will be examined in view of theoretical treatments of cavity nucleation and growth. 4.2.1. Nucleation By considering classical nucleation theory, Yoo and Trinkaus [6] developed the expression for the cavity nucleation rate, C, under steady-state conditions as

where C O is the maximum number of potential cavity nucleation sites, F v is a geometrical factor depending on the ratio of the specific surface and interfacial energies, 7~ is the specific surface energy, on is the local normal stress at the nucleation site, 6gb is the boundary thickness and kT has its usual meaning. If impurities can form a precipitate of size and strength that are sufficient to produce a stress concentration, CO is expected to increase. Although a reduction of Dgb and ~ has an opposite effect on (~ (suppressing cavity nucleation by a reduction of Dgb and accelerating cavity nucleation by a reduction of 7s), examination of eqn. (2) indicates that the effect of 7s is more pronounced. In addition, a reduction of Dgb by impurities can be shown to delay the relaxation of stress concentrations. The characteristic time td for the relaxation of stress concentrations at particle apices is given by [24, 25] (1 - v2)r3kT to = 2 3E~gbDgbQ

(3)

where v is Poisson's ratio, r is the radius of the particle, E is Young's modulus, • is the atomic volume and the other terms have their usual meaning. Here, the relaxation of stresses at the ledges [25] is not considered for the following reasons. While Chokshi and Langdon [4] suggested that the most favorable cavity nucleation site is a grain boundary irregularity, such as ledges and triple points, the remarkable difference in the extent of cavitation between Grade A, Grade B and Grade C implies the possibility of the existence of another cavity nucleation site which becomes more active than the ledges and triple points during deformation; the density of ledges and triple points should be of the same order in all grades. However, one cannot rule out the possibility of annihilation of the cavities formed at ledges and triple junctions in Grade A by sintering in the initial stage of growth, i.e. diffusion-controlled growth stage, after nucleation [7]. The diffusion controlled cavity growth can occur when ml,/g b ( = -- if2 Tn) < A/,t s ( = - f27s r) where Apgb and A/~s are the atomic chemical potentials for grain boundary and a concave surface with an isotropic 7s respectively. Tn is the normal traction acting on the boundary and r is the curvature of the surface. In Grade A, the situation would be reversed since 7s is expected, due to the absence of excessive impurities, to be larger than that for the other two grades, and atomic flow occurs to negative chemical potential gradient, i.e. sintering due to A/./gb > A,U s.

4.2.2. Growth 7t

/ U2

C= Co 3Fv~s3kT]

4Fvy~ 3"

½onZDgbrgbexp

on2kT

(2)

In general, cavity growth during superplastic deformation occurs by three independent mechanisms depending on the cavity size: diffusion growth [26],

K.-T. Park et al.

/

Effect of impurities in Zn-22%Al

superplastic diffusion growth [27], and power law growth [28]. The change in the cavity size with strain [27] for each mechanism is briefly discussed here. For diffusion growth,

l, = °

r-kT"/L

(4a)

where a = {4 In (2~r)- [ 1 - ( ~ ) 2 ] I 3 - ( ~ f ) = ] } - '

(4b)

For superplastic diffusion growth

For power law growth

(dr / ~ e =r

~2o]

,6,

65

Later, Chokshi and Langdon [27] developed a superplastic diffusion growth theory and examined cavity growth in Zn-22%A1 in the case of two different grain sizes d (d < 3 /~m and d > 5 /~m). The estimation of cavity radius at which the transition from superplastic diffusion growth (eqn. (5)) to power law growth (eqn. (6)) occurs was shown to be larger than that measured experimentally when d < 3 /~m. They explained this discrepancy in terms of the occurrence of concurrent growth. However, because the Zn-22%A1 which was used in their analysis contains about 200 ppm impurities, the possibility cannot be ruled out that the combined effects of a reduction in D g b causing suppression of superplastic diffusional growth rate and reduction in )'~ causing enhancement of power law growth rate would reduce the critical cavity radius for the transition. This is depicted schematically in Fig. 9. When d < 3 /~m in Zn-22%AI, comparable with d in the present investigation, power law growth will dominate and the cavity growth rate will be enhanced by adding more impurities as discussed above.

4.3. Cavitystringers where e is the total strain, g is the imposed strain rate, a is the mean cavity spacing and the other terms have their usual meaning. Subscripts D, SD and P denote diffusion growth, superplastic diffusion growth and power law growth respectively. For diffusion growth, reduction of Dgb will suppress the growth rate while a reduction of 7~ will increase the growth rate. Also, during the constant cross-head displacement rate test, decreases and, therefore, the cavity growth rate should increase. The effect associated with decreasing is anticipated to be significant in superplastic deformation at large strains. Therefore, the addition of impurities can either increase or decrease the diffusional growth rate depending on whether the effect of impurities on 7s is larger or smaller than that of impurities on Dgb. The effect of impurities on superplastic diffusional growth would be similar with that for diffusional growth since D g b a n d 7s have the same opposite effect. However, it is clear that the power law growth rate increases with the reduction of 7s resulting in a different dependence on impurity content. In the literature, two sources are found regarding cavity growth in superplastic Zn-22%A1 [27, 29]. Miller and Langdon [29] analyzed cavity growth in terms of diffusion growth and power law growth and reported that power law growth dominates over diffusion growth except for extremely low strain rates for Zn-22%AI. Consequently, large cavities would be observed with more impurities in Zn-22%AI since the power law growth rate increases with a reduction of 7~ by Fe. This implication is consistent with the experimental results observed in the present investigation.

In the quasi-single phase superplastic materials, it is known that cavity stringers are formed parallel to the rolling direction since particles tend to align along the rolling direction. In the microduplex superplastic materials, stringers are formed parallel to the tensile axis even when tests are conducted with specimens

Zn -22% A1

POWER LAW GROWTH

d < 3 p.m

The increase in power law growth ~ rate due to a reduction of Ys / , , ~ / ~

1=

SUPERPLASTIC 7 -- -- ~ DIFFUSION ,'~ J / GROWTH ~ J i - - ,," ,." / i f

/

---~r.-- "7 I ~ T

i /

"~

The decreasein superplastic diffusion growth rate due to a reduction of Dgb

fl

v

~ I

I

GROWTH

f the I critical cavity radius

I I i log r, arbitrary units

Fig. 9. Schematic plot for the effect of Dgb and 7~ on cavity growth for Zn-22%AI with d < 3 pm. The critical cavity radius for the transition from superplastic diffusional growth to diffusional growth is decreased by a reduction of Dgb, suppressing superplastic diffusionalgrowth, and a reduction of y~, enhancing power law growth.

66

K.-T. Park et al.

/

Effect of impurities in Zn-22%Al

oriented such that tensile axis is perpendicular to the rolling direction. Several explanations for such a behavior in microduplex alloys have been suggested. First, the "cascade" mechanism proposed by Van Riet and DeMeester [30] suggests that the groups of grains slide independently and the sliding of one side of a group of grains requires accommodation on the opposite side. Consequently, cavities open up until the stress concentrations are relaxed leaving cavities lined up along the tensile axis. This model requires that cavity nucleation is aligned. However, some metallographic observations [31, 32] revealed that at low strains, cavities typically nucleate at random locations (for example, Fig. 2 in [31] and Fig. 5 in [32]). Second, Kashyap and Tangri [31] argued that the wavy nature of the interphase boundary due to tensile strain acts as an obstacle for grain boundary sliding, providing more favorable cavity nucleation sites. This suggestion does not seem valid since the two phases in the present microduplex alloy are homogeneously distributed and the phases or grains remain equiaxed up to large strains. Third, based on the observation of the cavity formation in the cluster of a phase grains in T i - 6 A I - 4 V alloy, Cope and Ridley [33] suggested that the break-up of a-phase clusters along the tensile direction during tensile deformation causes the cavity alignment along their direction. This suggestion cannot be applied to the Zn-22%A1 eutectoid since cavities in Zn-22%A1 are primarily nucleated in a - f l phase boundaries [4]. The preceding discussion suggests that cavities initially nucleate at random locations and stringers are formed during deformation, i.e. are strain-induced. According to the individual grain-rolling mechanism suggested by Paidar and Takeuchi [34], groups of grains can rotate toward the tensile axis by individual grain rolling. Therefore, it is possible that cavities nucleated randomly can migrate toward the tensile axis due to the activity of individual grains. Although this possibility, based on the individual grain-rolling model, may account for the occurrence of a random cavity nucleation and the initial stage of cavity migration, it appears that another mechanism is needed to help cavities align more perfectly.

5. Conclusions ( 1 ) The effect of impurities on the ductility of superplastic Zn-22% AI was studied by conducting a series of tensile tests and examining cavitation after failure. (2) In an ultrapure Zn-22% AI (6 ppm impurities), relatively high ductility and negligible cavitation are observed even at very low stresses. By contrast, lower ductilities and extensive cavitation are observed in the

superplastic region with the introduction of impurities in the alloy ( 180 ppm and 15 20 ppm). (3) Examination of the fracture tip of tensile specimens indicate that two grades of Zn-22%AI containing impurities fail by cavity coalescence although cavitation is more evident in the lower purity grade (1520 ppm impurities). (4) Cavity stringers always tend to align along the tensile axis in Zn-22%Al. (5) It is possible to predict qualitatively the effect of impurities on superplasticity in terms of the existing theoretical treatments for cavity nucleation and growth.

Acknowledgments This work was supported by the National Science Foundation under Grant No. D M R 902455.

References 1 J. Pilling and N. Ridley, in Superplasticity in Crystalline Solids, The Institute of Metals, 1989, p. 102. 2 C. H. Caceres and D. S. Wilkinson, Acta Metall., 32 (1984) 423. 3 A. H. Chokshi and T. G. Langdon, Acta Metall., 38 (1990) 867. 4 A. H. Chokshi and T. G. Langdon, Acta Metall., 37 (1989) 715. 5 D. A. Miller and T. G. Langdon, Metall. Trans., 9A (1978) 1688. 6 M.H. Yoo and H. Trinkaus, Metall. Trans., 14A (1983) 547. 7 W. D. Nix, K. S. Yu and J. S. Wang, Metall. Trans., 14A (1983)563. 8 I.W. Chen and M. H. Yoo, Acta Metall., 32 (1984) 1499. 9 M. H. Yoo and H. Trinkaus, Acta Metall., 34 (1986) 2381. 10 L.C. Lim, Acta Metall., 37(1989) 969. 11 K. T. Park and F. A. Mohamed, Metall. Trans., 21A (1990) 2605. 12 K. T. Park, J. C. Earthman and E A. Mohamed, in M. J. Mayo, M. Kobayashi and J. Wadsworth (eds.), Superplasticity in Metals, Ceramics and Intermetallics, MRS Symp. Proc., 1990, Vol. 196, p. 45. 13 F.A. Mohamed, M. M. I. Ahmed and T. G. Langdon, Metall. Trans., 8A (1977) 933. 14 H. Ishikawa, D. G. Bhat, F. A. Mohamed and T. G. Langdon, Metall. Trans., 8A (1977) 523. 15 P. K. Chaudhury, V. Sivaramakrishnan and E A. Mohamed, Metall. Trans., 19A (1988) 2741. 16 P. K. Chaudhury and E A. Mohamed, Acta Metall., 36 (1988) 1099. 17 E A. Nichols, A cta Metall., 28 (1980) 663. 18 M.J. Stowell, Met. Sci., 17(1983) 1. 19 E A. Mohamed, Scr. Metall., 13 ( 1979) 87. 20 M. M. I. Ahmed, F. A. Mohamed and T. G. Langdon, J. Mater. Sci., 14 (1979) 2913. 21 E.P. George, P. L. Li and D. P. Pope, Acta Metall., 35 (1987) 2471. 22 E.P. George, P. L. Li and D. P. Pope, ActaMetall., 35 (1987) 2487.

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Effect of impurities in Zn-22%Al

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