Development of Very High Strength and Ductile Dilute Magnesium Alloys by Dispersion of Quasicrystal Phase ALOK SINGH, YOSHIAKI OSAWA, HIDETOSHI SOMEKAWA, TOSHIJI MUKAI, CATHERINE J. PARRISH, and DONALD S. SHIH Very high strengths, with tensile yield strength from 377 to 405 MPa, combined with elongation to failure of over 12 pct, have been achieved in Mg-Zn-Y dilute alloys by direct extrusion. Alloys Mg-6xZn-xY, where x = 0.2, 0.35, and 0.5 (at. pct) were chill cast in a steel mold and direct extruded at a temperature in the range 508 K to 528 K (235 C to 255 C), which produced an average grain size of about 1 lm. Quasicrystalline i-phase particles were dispersed in the matrix with size ranging from 50 nm to 1 lm. In addition, high density of nano-precipitates of average size 15 nm was dispersed in matrix. Thus we have developed magnesium alloys of very high strength combined with ductility by a simple process using extrusion with very little addition of yttrium. DOI: 10.1007/s11661-013-2056-5  The Minerals, Metals & Materials Society and ASM International 2013

I.

INTRODUCTION

REDUCING weight of automobiles is a high priority for enormous savings in fuel consumption and global warming. It is therefore essential to use more of magnesium alloys, which are the lightest of all structural metals, and thus important to develop magnesium alloys with better mechanical properties. Common techniques for strengthening alloys are grain refinement, precipitation, and dispersion of hard particles. Magnesium alloys have a high slope of Hall–Petch plot, therefore grain size refinement is very effective in strengthening.[1] Grain refinement occurs during wrought processing, but which also leads to generation of a basal texture, resulting in anisotropy of deformation in tension and compression. Fine-grained alloys show better ductility and less anisotropy. Grain refinement during wrought processing can be enhanced by particle stimulated nucleation (PSN) during dynamic recrystallization by dispersion of hard intermetallic phase particles.[2] An alloy system of great interest is the ternary Mg-Zn-RE, where RE is Y or a rare earth element. Mg-rich alloys in this system contain a ternary intermetallic phase forming on grain boundaries which ALOK SINGH, Chief Researcher, is with the Microstructure Design Group, Structure Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. Contact e-mail: [email protected] YOSHIAKI OSAWA, Chief Researcher, is with the Structure Materials Unit, National Institute for Materials Science. HIDETOSHI SOMEKAWA, Senior Researcher, is with the Toughness Design Group, Structure Materials Unit, National Institute for Materials Science. TOSHIJI MUKAI, Professor, is with the Department of Mechanical Engineering, Kobe University, 1-1 Rokkadai, Kobe, Japan. CATHERINE J. PARRISH, Engineer, is with the Materials and Processes, Boeing Research and Technology, The Boeing Company, Huntington Beach, CA. DONALD S. SHIH, Technical Fellow, is with the Boeing Research and Technology, The Boeing Company, St. Louis, MO. Manuscript submitted June 27, 2013. Article published online October 16, 2013 3232—VOLUME 45A, JULY 2014

strengthen the grain boundaries, and strengthened by solid solution and precipitation of Mg-Zn phases.[3] The Mg-Zn-Y alloy system contains three ternary phases, each of which makes a two phase field with a-Mg.[4] One of them is a superstructure of magnesium, known as long-period superstructure-ordered phase (LPSO), whose dispersion strengthens the alloy. Very high strengths have been reported in combination with fine grain sizes, but with limited ductility.[5–7] Another ternary phase Mg3Zn6Y has a quasicrystalline structure with an icosahedral symmetry, or i-phase.[8,9] Good strengths with moderate ductility have been reported by dispersion of this phase by wrought processes.[10–18] Quasicrystal phases are known to have properties such as very high hardness and low surface energy.[19] They also show matching or epitaxial interfaces with crystalline phases, including magnesium matrix, on all facets, because of their high crystallographic symmetry and quasiperiodicity.[20,21] i-phase modifies the texture of the alloy[22] and results in fine grain-sized alloy by stimulating dynamic recrystallization during wrought processing. i-phase dispersed alloys are reported to have a good combination of strength and ductility, resulting in high fracture toughness.[23,24] Very high strengths in these alloys were achieved by further reduction in grain size by employing powder metallurgy.[25] However, it was subsequently shown shat such fine grain sizes are also possible in these alloys containing i-phase by a simple process of extrusion, without powder processing.[26,27] An alloy Mg-6 at. pct Zn-1 at. pct Y cast in a chill cast steel mold was processed by direct extrusion to obtain ultra-fine grain size with tensile and compressive yield strength (YS) of nearly 400 MPa accompanied by 16 pct elongation to failure.[26] Similar high strengths were shown in Mg-3 at. pct Zn-0.5 at. pct Y alloy.[27] Here we show that similar level of very high strengths with ductility can be obtained in more dilute alloys which contain quasicrystal phase by processing in a similar way, and study the METALLURGICAL AND MATERIALS TRANSACTIONS A

effect of composition on the microstructure and properties.

II.

EXPERIMENTAL PROCEDURE

Three alloys of compositions (in at. pct) Mg-6x Zn-x Y, where x = 0.2, 0.35, and 0.5, designated ZW31, ZW61, and ZW82, respectively, were prepared from commercial purity metals by melting in an electric furnace under a cover gas and cast into a chill cast steel mold of diameter 46 mm. Round billets of diameter 40 mm were machined out of the casting and extruded at temperatures between 508 K and 528 K (235 C and 255 C) into rods of 8 mm diameter with 25:1 reduction ratio. Microstructures were observed by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) on sections along the extrusion direction (longitudinal sections). Samples for TEM were ground down to 70 lm and then thinned by precision ion milling. SEM observations were made on JEOL JSM7001F and TEM observations were made on JEOL 2000FX microscopes. Grain sizes were determined by linear intercepts multiplied by a factor of 1.74 to compensate for stereological effects.[28] Tensile and compression test samples were machined from the extruded rods. The tensile samples had a gauge length of 15 mm and diameter 3 mm. Compression samples had a diameter of 4 mm and height 8 mm. These were tested on an Instron testing machine with an initial strain rate of 103/s. Three samples were tested in each condition.

III.

RESULTS

A. Cast Microstructures Optical micrographs of the as-cast alloys, Figure 1, show a dendritic structure within grains, i.e., each grain is a dendrite. From the grain contrasts, the grain sizes can roughly be estimated to be up to 2 mm in case of ZW31 alloy and about 600 lm in ZW61 and ZW82 alloys. The dendrite arms are defined by dark lines which are the ternary phase, mostly i-phase. In case of ZW82 as well as ZW61 alloy, the dendritic structure is well defined by continuous presence of the interdendritic i-phase. In both of these alloys, the thickness of the dendritic arms is estimated to be about 30 lm. In case of ZW31 alloy, the ternary i-phase is not continuous but appears as spots. These spots lie in rows, which define the dendritic arms boundary. In this case too, the dendritic arm thickness is observed to be about 30 lm. Figure 2 shows an SEM backscattered electron micrograph of ZW82 alloy. The interdendritic ternary phase is bright in contrast. Appearing as extensions of the interdendritic ternary phase are regions of higher alloy concentration, as compared to the matrix. Some of these regions are marked by arrows. The matrix composition was determined to be about 1.8 at. pct Zn and 0.2 at. pct Y. The supersaturated regions had a zinc concentration of about 2.8 at. pct and Y about 0.2 at. pct. METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 1—Optical micrographs of as-cast structure of alloys (a) ZW31, (b) ZW61, and (c) ZW82.

B. Extruded Microstructure On extrusion, the ternary phase was distributed in the extrusion direction, being broken into particles of a large size range, as shown in the SEM micrographs in Figure 3. In many places, the eutectic structure is not broken. From these micrographs, the volume fraction of the particles is estimated to be about 3.8, 8.1, and 9.1 in samples of ZW31, ZW61, and ZW82 alloy, respectively. Size frequency of particles is plotted in Figure 4. The plot for ZW61 shows smaller particle sizes than ZW31 VOLUME 45A, JULY 2014—3233

Fig. 2—A backscattered electron SEM image of the dendritic structure of ZW82 alloy showing composition contrast.

and ZW82, and therefore there is no clear trend with respect to the alloy composition. Most particles have a diameter of less than a micron, out of which most are with a diameter of less than half a micron. Very fine grains in the size range 500 nm to 1 lm are observed in TEM micrographs in Figure 5. The average grain sizes of the recrystallized fine grains were estimated to be 0.99 ± 0.17, 1.10 ± 0.41, and 1.00 ± 0.20 lm, for the alloys ZW31, ZW61, and ZW82, respectively. The dark particles in the micrographs are the i-phase. They are seen as clustered in ZW31 alloy (Figure 3(a)), but widely distributed in ZW82 alloy. The grain structure was not uniform everywhere. Regions of high strain unrecrystallized grains were also observed, as shown in Figure 6. These grains are elongated in the extrusion direction. A number of sharp low angle boundaries are observed within these grains, oriented along the extrusion direction. These large grains are surrounded by fine recrystallized grains. In Figure 6(b), the unrecrystallized grain is oriented along [0001] direction (perpendicular to the extrusion direction). The spot intensities in the inset diffraction pattern are uneven, indicative of the strain. In each of the alloys, very fine precipitation is observed. These can be observed clearly in Figure 5(a) and (b). These precipitates at higher magnifications in alloys ZW61 and ZW82 are shown in Figure 7. A rather large size range is observed, in which the larger size of about 50 nm are the i-phase dispersed particles (some of them are marked by arrows). The precipitate sizes are about 25 nm and less. A size distribution measured in these micrographs is given in Figure 7(c). The highest frequency (the mode) is about 10 nm for ZW61 alloy and 7 nm for ZW82 alloy. These values are close and we assume that the precipitate size is about the same in the three alloys. These precipitates are a mixture of the b1¢ Mg-Zn precipitates and i-phase.[17] At high magnifications, the b1¢ precipitates can be distinguished by fringelike contrast due to a layered structure while the i-phase precipitates have a dark uniform contrast. Precipitation 3234—VOLUME 45A, JULY 2014

Fig. 3—SEM micrographs of extruded alloys (a) ZW31, (b) ZW61, and (c) ZW82.

of the i-phase has been reported earlier.[14] These precipitates are assumed to have precipitated during extrusion from a supersaturated matrix. C. Mechanical Properties Stress–strain curves for tensile and compression tests are shown in Figure 8. The test values are listed in Table I. The alloys ZW31, ZW61, and ZW82 have tensile YS of 390.1 ± 1.8, 377.0 ± 1.1, and 404.6 ± 1.6 MPa, respectively, and the respective compressive METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 4—Particle size distribution measured from SEM micrographs for the alloys ZW31, ZW61, and ZW82.

YS are 266.0 ± 1.6, 275.1 ± 0.8, and 326.7 ± 3.4 MPa. The tensile YS are in the range of 377 to 405 MPa, and thus all the alloys show very high tensile strengths. At the same time, even though there is not much strain hardening, all the alloys show over 12 pct total elongation and about 10 pct of uniform elongation in tension. The stress maximum occurs at about 7 to 8 pct strain, with stresses rising (UTS) about 8 to 18 MPa above the YS. It is noted that the tensile YS of ZW31 is higher than that of ZW61 alloy. In this context, it can also be noted that the grain size of ZW31 alloy is finer than that of ZW61, and in a narrower size range. The elongations to failure in tension do not show any trend with the alloy composition, and are in range 12.3 to 13.4 pct. Similarly, the elongations to failure in compression were in the range 12.7 and 13.2 pct. Thus the elongations in tension and compression are similar.

IV.

DISCUSSION

We have obtained very high strengths in dilute alloys by a simple process which forms a very refined microstructure. Formation and features of the microstructure, and their strengthening contribution are discussed in the following. The major sources of strengthening are the dispersion of the i-phase, the fine grain size and fine precipitation formed during extrusion. A. Formation of Microstructure by Extrusion At first we would discuss about formation of very fine grain size in the alloys during extrusion. Very fine grain size has been achieved without using any special technique such as powder metallurgy used by Mora et al.[25] This is attributed to the presence of well distributed i-phase in the dendritic solidification structure and the relatively low temperature of extrusion. Even though the recrystallized grain size in all the alloys METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 5—TEM micrographs of alloys (a) ZW31, (b) ZW61, and (c) ZW82.

was quite similar, there were regions of unrecrystallized grains. The alloys with lower amounts of i-phase had larger unrecrystallized regions. Although due to compositional differences the volume fraction of the ternary phase is different in each alloy, the dendrite arm spacings are similar in all the alloys. Therefore, the ternary phase is well distributed in all the alloys. This would be effective for the i-phase acting as hard particles for stimulating recrystallization.[2] PSN is expected to result in weaker texture, especially in case of i-phase particles since they have higher symmetry which would give rise to a wider range of crystallographic orientations of grains nucleating on it. VOLUME 45A, JULY 2014—3235

Fig. 6—TEM micrograph showing unrecrystallized grains in alloys (a) ZW31 and (b) ZW61.

All the alloys are dilute alloys which contain over 95 pct magnesium. Therefore during solidification dendrites of a-Mg form first, making rest of the melt more concentrated, resulting in the formation of ternary phases in the interdendritic spaces. Even after formation of the ternary phase, a part of the melt remains supersaturated. The maximum solubility of zinc in magnesium is 2.4 at. pct, which occurs at the eutectic temperature.[29] Parts of the solidified alloy exhibit this composition. The supersaturation of the alloy in the as-cast condition leads to dynamic precipitation during extrusion process, resulting in very fine nano-sized roundshaped precipitates. These precipitates can pin down grain boundaries, thereby checking grain growth after recrystallization, resulting in very fine grain size after extrusion. B. Recrystallization It is observed that recrystallization decreases with decreasing concentration of alloying elements. Presence of hard i-phase can be considered to be the primary factor for stimulating recrystallization by PSN. However, presence of RE elements in solid solution itself has been reported to stimulate recrystallization.[30] Due to rapid cooling during solidification, some yttrium supersaturation is expected in the alloy, which can stimulate 3236—VOLUME 45A, JULY 2014

Fig. 7—TEM micrographs showing precipitates in alloys (a) ZW61 and (b) ZW82. Larger particles of about 50 nm in size are the i-phase, some of which are marked by arrows. (c) Size distribution of the fine precipitates in (a) and (b).

recrystallization. It should be noted that the recrystallized grain size are fine in all the alloys, and that the more dilute alloys have more volume fraction of unrecrystallized regions. These unrecrystallized grains, which are elongated parallel to the extrusion axis, are orientated with their c-axis perpendicular to the extrusion axis (such as shown in Figure 6(b)). This orientation is not favorable for twinning, and can accommodate extrusion strains in the basal slip system. Therefore, these grains do not have METALLURGICAL AND MATERIALS TRANSACTIONS A

can produce oriented columnar grains. Thus by controlling the solidification structure by composition of the alloy and undercooling, the degree of recrystallization by extrusion can be modified. C. Strengthening Due to Grain Size The Hall–Petch parameters of ZW82 alloy have been determined by extrusion at three different temperatures with similar processing as described here.[27] In case of tensile deformation, parameter constant r0 = 128.36 ± 6.75 MPa and slope k = 307.39 ± 9.61 MPa lm1/2. For an average grain size of 1 lm, this gives YS of 436 MPa. In case of compressive deformation the Hall– Petch parameters are reported to be r0 = 113.54 ± 4.44 MPa and k = 237 ± 6.19 MPa lm1/2, which gives a YS of about 350 MPa. Both the calculated values for tension and compression are 7 to 8 pct higher than the experimental values, which can be considered as reasonably close. The alloys ZW61 and ZW31 have average grain size similar to that of ZW82, but increasing unrecrystallized regions with decreasing alloy composition. These unrecrystallized regions are oriented with the basal texture of the alloys, which is not very amenable to basal slip during tension. Moreover, they contain a high degree of strain and low angle boundaries which resist deformation by slip. Therefore they do not reduce tensile strength significantly. However, situation is quite different in case of compressive deformation, in which twinning plays an important part. The stress required for twinning is lower in case of larger grains. Low angle boundaries do not resist twinning significantly. Yielding can occur by f1012g twinning in unrecrystallized grains and run across low angle boundaries or subgrain boundaries.[32] Lower compressive strengths in lower concentration alloys can be attributed to these unrecrystallized regions and difference in texture, whose effect is not insignificant. D. Strengthening Due to Precipitation

Fig. 8—Stress–strain curves in tension and compression for the alloys (a) ZW31, (b) ZW61, and (c) ZW82.

Precipitation strengthening by aging treatment in a ZW82 alloy has been dealt with in an earlier report,[33] where an extruded alloy was solutionized and aged under different conditions. With an average grain size of 17 lm, the YS in T6 condition (solutionized and peak aged) is reported to be 217 MPa (from the Hall–Petch parameters for ZW82 given above, calculated to be 202.5 MPa, about 7 pct lower than experimental), where the precipitates have a large aspect ratio. With finer precipitation obtained by T8 treatment (straining prior to aging), 287 MPa YS is achieved with a consequent loss in ductility. The well known equation for increase in YS (Dry) for Orowan looping mechanism is given as Dry ¼

enough stored plastic energy to undergo recrystallization.[31] This situation can be modified by controlling the grain orientations in solidification. Large undercoolings METALLURGICAL AND MATERIALS TRANSACTIONS A

Gb 1 dt pffiffiffiffiffiffiffiffiffiffiffi ln ; 2p 1  m k b

½1

where G is the shear modulus in GPa, b is the magnitude of the Burgers vector of the slip dislocations, m is VOLUME 45A, JULY 2014—3237

Table I.

Grain Size of the Extruded Alloys and Mechanical Properties in Tension and Compression Tensile

Compression

Alloy

Grain Size (lm)

YS (MPa)

UTS (MPa)

l (pct)

YS (MPa)

UCS (MPa)

l (Pct)

YAS Ratio

ZW31 ZW61 ZW82

0.99 ± 0.17 1.10 ± 0.41 1.00 ± 0.20

390.1 ± 1.8 377.0 ± 1.1 404.6 ± 1.6

397.8 ± 1.2 391.9 ± 1.7 418.6 ± 0.7

12.92 ± 0.35 13.44 ± 0.51 12.33 ± 0.86

266.0 ± 1.6 275.1 ± 0.8 326.7 ± 3.4

677.8 ± 31.2 659.9 ± 9.8 689.4 ± 5.0

12.70 ± 0.50 13.24 ± 0.35 13.06 ± 0.37

0.68 0.73 0.81

YS is yield stress, UTS is ultimate tensile strength, UCS is ultimate compressive strength, l is elongation to failure and YAS is yield asymmetry ratio.

Poisson’s ratio, dt the precipitate diameter, and k the precipitate spacing on the relevant slip plane. For spherical precipitates,[34]   0:779 p ffiffi k¼ ½2  0:785 dt f and for rod-shaped precipitates,   0:953 pffiffi  1 dt ; k¼ f

½3

where f is the volume fraction of precipitates. An extruded ZW82 alloy in T6 condition is reported to have 0.5 pct volume of precipitates (this could be increased to 2.3 pct by T8 treatment).[33] Assuming a volume fraction of 0.005 and dt of 7 nm, k is 72 nm. The increment in strength is calculated to be 40 MPa. In a ZW82 alloy with T6 treatment, precipitates with an average diameter of 20 nm and length 475 nm are reported, with k of 250 nm, which would account for 15 MPa increment in strength. Assuming rods of diameter 7 nm, k is 87 nm, and Dry is calculated to be 33 MPa. Thus discrete round-shaped precipitates (precipitated during extrusion) have an advantage over rod-shapes (precipitated by aging) in strengthening. Dispersion of fine sphericalshaped precipitates has been shown to have superior mechanical properties such as fracture toughness.[35,36] The fine round precipitates can pin dislocations more effectively. E. Yield Asymmetry Yield asymmetry arises out of strong basal texture, which favors f1012g type twinning in compression but not in tension. Yielding in tension is assumed to be caused by slip, but by twinning in compression along the extrusion axis. This effect is further modified by grain size, where twinning stress is more sensitive to grain size than slip. Even though the degree of texture is expected to vary with the composition in the three alloys studied here, we will mainly discuss the effect of unrecrystallized regions in the alloys. As is mentioned above, even though there are unrecrystallized regions in the extruded alloys, the accumulated strain and subgrain boundaries inside resist the dislocation slip. These subgrain boundaries, however, do not offer much resistance against twinning, resulting in a yield anisotropy. As observed in Table I, the yield asymmetry (YAS, ratio of compressive YS to tensile YS) ratio increases with increasing alloy composition. 3238—VOLUME 45A, JULY 2014

In an alloy of composition Mg-6 at. pct Zn-1 at. pct Y (ZW143) almost no YAS is observed[26] in the asextruded state. For a grain size of over 2 lm, the YAS ratio is over 1 (i.e., compressive YS is higher than the tensile).[17,26] This can be explained by the twin stresses becoming higher than the slip stress with the decrease in grain size. After precipitation, by dynamic precipitation during extrusion at a lower temperature, or by artificial aging, the YAS ratio is again less than 1, because the precipitates are effective against slip but not against twinning. F. Ductility Figure 8 and Table I show that all the alloys here exhibited total elongation to failure of over 12 pct, in tension as well as compression. Fine-grained alloys often show high ductility because fine grains show higher plasticity and because fine grained alloys have weaker texture due to high recrystallization rate which produces the fine grain size. In an earlier study, we had reported that in case of extruded Mg-6 at. pct Zn-1 at. pct Y alloy (ZW143), tensile strength as well as ductility increased with decreasing grain size.[26] Total elongation for alloy with grain size of about 1 lm was about 16 pct, while for grain size of about 6 lm was about 13 pct. After an aging treatment, the elongation to failure of the fine grained alloy decreased to 10.5 pct (while that of the coarser grained alloy increased to 17 pct). This reduction in ductility is attributed to nucleation of rod-shaped precipitates. In case of a ZW82 alloy,[27] the fine-grained samples show lower ductility than coarser-grained samples. Fine-grained (1 lm) samples showed total elongations of 13 pct (18 pct in case of coarser grains). In case of this alloy, however, no noticeable aging is observed after extrusion. Thus precipitation state is also an important factor for ductility. In case of coarser grains, the ductility seems to improve by precipitation which prevents sudden pile-ups on the grain boundaries. Precipitation in form of fine round precipitates does not appear to be detrimental for ductility, as also shown by improved fracture toughness.[23,24] In case of artificial aging (T6 and T8 conditions), the ductility is closely related to morphology and distribution of the precipitates.[33,37] Ductility of a precipitation strengthened alloy can be modeled by assuming that failure occurs when local dislocation density reaches a critical value[37] due to accumulation of geometrically necessary dislocations due to difference in elastic moduli between the precipitates and the matrix. The critical METALLURGICAL AND MATERIALS TRANSACTIONS A

stress is effectively proportional to the inter-precipitate spacing k on the slip plane. However, in this continuing discussion, it is seen that grain size and precipitate morphology also play an important role. In case of a T8 ZW82 alloy, an elongation of 12 pct is reported to correspond to k of 62 nm. This is close to a rough estimate of k of 72 nm in the present case which corresponds to about 12 pct elongation. The elongations to failure in compression remain nearly unchanged in all the alloys, irrespective of grain size and the precipitation/aging condition. In all cases, it is about 13 to 14 pct. Thus, even though the YS is closely dependent on the grain size, the ductility is not. G. Role of i-Phase The eutectic phases in the ternary alloys strengthen grain boundaries against pile-ups. Usually, dispersion of an intermetallic phase lowers the ductility of an alloy. However, alloys containing i-phase show a good combination of strength and ductility, as exhibited by their fracture toughness.[23,24] The i-phase particles do not become the source of crack formation and fracture.[24] In this series of alloys, the highest amount of i-phase is in the ZW143 alloy reported earlier.[26] In this alloy, the finest grain size exhibits the highest ductility in as-extruded condition, which is 16 pct, in combination with the highest strength. i-phase also provides ductility by modification of texture during extrusion.[17] A good combination of high strength and ductility, and the resulting high fracture toughness, impart properties for safe application of these alloys.

V.

CONCLUSIONS

In conclusion, 1. We have developed very high strength magnesium alloys with ductility using very little amount of yttrium (down to 0.2 at. pct or 0.7 mass pct), Mg-6x Zn-x Y alloys, where x = 0.2, 0.35, and 0.5 (designated ZW30, ZW61, and ZW82, respectively) 2. The alloys were prepared by chill casting and extrusion in the temperature range 508 K to 528 K (235 C to 255 C) to produce a very fine-grained microstructure with average size of recrystallized grains of about 1 lm. 3. Quasicrystalline i-phase was dispersed with sizes ranging from 1 lm down to about 50 nm. Very fine precipitates of average size about 15 nm existed in the grains. 4. Tensile yield strengths between 377 and 405 MPa were achieved in all the alloys. Compressive yield strengths ranged from 266 to 326 MPa. In all cases, total elongations to failure were over 12 pct. 5. YAS occurred due to texture, ranging from 0.68 (for ZW31 alloy) to 0.81 (for ZW82 alloy). Lower compressive strengths were attributed to unrecrystallized grains in more dilute alloys.

METALLURGICAL AND MATERIALS TRANSACTIONS A

ACKNOWLEDGMENTS The authors thank Ms. Reiko Komatsu for technical support.

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