Trans. Nonferrous Met. Soc. China 31(2021) 1700-1716

Hot deformation behavior and related microstructure evolution in Au-Sn eutectic multilayers

Yong MAO^1,2,3, Dan-li ZHU^1,3, Jun-jie HE^1,2,3, Chao DENG⁴,Ying-jie SUN^1,3, Guang-jie XUE^1,3, Heng-fei YU^1,3, Chen WANG^1,3

1. School of Materials and Energy, Yunnan University, Kunming 650091, China;

2. School of Physics and Astronomy, Yunnan University, Kunming 650091, China;

3. Materials Genome Institute, Yunnan University, Kunming 650091, China;

4. College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China

Received 5 July 2020; accepted 6 April 2021

Abstract: Hot compression was performed on a multilayered Au-20Sn eutectic alloy to investigate the deformation behavior and microstructure evolution. During hot compression, microstructural spheroidization was initiated from plastic instability regions, and it was preferentially activated in vertical lamellae with a growth direction parallel to the compressive direction. Continuous dynamic recrystallization associated with lattice dislocations was the mechanism in both AuSn and Au₅Sn multilayers. After spheroidization, strain accumulations were weakened in both of the equiaxed phases, and the deformation mechanism was substantially replaced by grain boundary sliding and migration. Based on these findings, hot rolling was conducted on an as-cast Au-20Sn alloy and a foil with a thickness of ~50 μm was successfully prepared. The present study can promote the development of Au-20Sn foils, and provide insights into the deformation behavior and microstructure evolution of multilayered eutectic alloys.

Key words: Au-20Sn eutectic alloy; intermetallics; hot compression; multilayered structure; deformation; spheroidization

1 Introduction

Research on lead-free solders has been a hot topic in the last decade due to the consideration of environmental and health concerns [1-3]. A large number of lead-free solders, such as Sn-Ag [4], Sn-Zn [5], Sn-In [6] and Au-Sn [7] systems, have been studied extensively as candidates for the replacement of traditional Sn-Pb solder. Among these lead-free solders, Au-20Sn eutectic alloy is widely used in high-power optoelectronics and hermetic sealing applications due to its excellent conductivity, high electromigration and corrosion resistance [8]. However, producing Au-20Sn foils with the desired size (thickness: 30-60 μm) and properties is still a technological challenge. The microstructure of Au-20Sn eutectic alloy consists of complex structured AuSn and Au₅Sn intermetallic compounds, which gives rise to the intrinsic brittleness of this alloy at room temperature [9,10].

In recent decades, Au-20Sn foils were commonly prepared using the cascading diffusion and electrodeposition processes [11,12]. However, these two processes both have obvious disadvantages. In the cascading diffusion, the thermodynamic condition in the diffusion process is hard to control, and incomplete alloying usually occurs. Regarding the electrodeposition process, it is costly and unable to support the large-scale production of Au-20Sn preforms. Most importantly, the lack of precise control of the eutectic composition over a large-scale is the common difficulty of the two processes. Once the composition of the Au-20Sn preform deviates from the eutectic composition point, the melting temperature of the alloy increases sharply, which significantly increases the welding temperature and reduces the welding quality [13,14]. Generally, the effective approaches used to fabricate Au-20Sn preforms are limited, and the development of new manufacturing processes is necessary.

In our earlier study, we found that the as-cast Au-20Sn alloy with fine lamellar colonies showed substantial plasticity during hot compression. This investigation shows that the Au-20Sn alloy may become malleable once the deformation temperature reaches a certain level. Hence, studying the deformation behavior and the microstructure evolution of the Au-20Sn alloy during hot deformation is indispensable. It can help us to understand the plastic behavior and further promote the formability of the alloy.

In this study, hot compression was conducted on the Au-20Sn alloy in a temperature range of 180-240 °C and a strain rate range of 0.001-0.6 s^-1. The evolution of the microstructure, including the plastic instability of the multilayers, spheroidization process, and deformation behavior after spheroidization were studied systematically. Based on the above findings, a hot rolling experiment was performed on an as-cast Au-20Sn alloy, and a foil with a thickness of ~50 μm was successfully prepared. The present study can promote the development of Au-20Sn alloy foils and provide insights into the deformation behavior of multilayered eutectic alloys.

2 Experimental

2.1 Preparation of as-cast Au-20Sn alloy

The Au-20Sn master alloy was prepared in an electromagnetic induction furnace in vacuum by melting a pure gold sheet (99.999% pure) and tin particles (99.99% pure) in a graphite crucible. The master alloy was then heat-treated at 350 °C for 10 min and cast to prepare rod-like specimens  with the dimensions of 5 mm × 5 mm × 50 mm by employing a graphite mould. A satisfactory surface quality of the as-cast specimens was obtained due to the excellent fluidity of the alloy at the heated temperature.

Additionally, an Au-20Sn ingot with dimensions of 20 mm (length) × 20 mm (width) × 4 mm (thickness) was prepared by employing a flat mold for the rolling process.

2.2 Hot compression

Samples with dimensions of 5 mm × 5 mm × 7 mm (height) were machined from the as-cast rod-like specimens for hot compression experiments. The hot compression tests were performed using an ETM205D electronic universal testing machine equipped with a high-temperature furnace. During hot compression, the temperatures were set as 180, 200, 220 and 240 °C, and the stain rates were 0.001, 0.1 and 0.6 s^-1, respectively, to systematically study the influence of temperature and strain rate on the microstructure evolution and deformation behavior. The accuracies of the electronic universal testing machine and the high-temperature furnace are ±0.5% and ±1.5 °C, respectively. Before compression, the samples were held in the furnace for 5 min to ensure that they reached the testing temperature. After compression, the samples were water quenched immediately to reserve the deformation structures.

2.3 Characterization

To clarify the microstructure evolution and deformation behavior during hot deformation, samples subjected to 20%, 60%, 120% and 200% compressive strains were analyzed. The samples used for microstructure observations were cut from the compressive direction-transverse direction (CD-TD) section and then prepared by a metallographic technique without etching. The backscattered electron (BSE) images were obtained by using a field emission scanning electron microscopy (FESEM, JEOL-JSM 7800F) system equipped with an energy-dispersive X-ray spectroscopy (EDS). The electron backscatter diffraction (EBSD) data were collected using an EBSD camera (HKL Channel 5 System installed on JEOL-JSM 7800F, Cambridge, MA). The samples used for EBSD measurements were prepared by argon ion polishing (Gatan Ilion II 697) to obtain a high-quality surface without residual stress.

2.4 Rolling

Based on the findings from the hot compression process, a tentative hot rolling experiment was performed on an as-cast Au-20Sn alloy. The rolling process was performed at 220 °C with a linear speed of 1 m/min and a pass reduction of 15%-20%. The principles used to select these parameters are discussed in detail in the Section 4.4. The rollers of the machine used in this study were heated to a given temperature to prevent the loss of heat (rollers were equipped with a resistive heater and a temperature measuring device). The temperature fluctuation during the hot rolling process was about ±3 °C, which was considered an acceptable error range.

3 Results

3.1 Nanolayered structure of as-cast Au-20Sn alloy

Figure 1(a) shows that the as-cast Au-20Sn eutectic alloy is composed of coarse columnar grains with a width of 20-150 μm, and plum blossom-like and punctiform particles with a size of 3-10 μm are randomly embedded in the coarse columnar grains. EDS confirms that these particles are the Au₅Sn primary phase. The high- magnification BSE images in Fig. 1(b) reveal that these columnar grains comprise a typical lamellar eutectic structure. The phases with dark and bright contrast regions are identified as AuSn and Au₅Sn intermetallic compounds in the high-resolution BSE images based on their distinct compositions. Phase identification can further refer to our previous TEM characterization and Au-Sn phase diagram [15,16]. Additionally, the eutectic colonies show random lamellar growth directions that can be classified into vertical, inclined, and horizontal lamellae according to the potential compressive direction (PCD). The lamellar boundary between the colonies is not straight, and many structural defects, such as disconnections and misconnections, are introduced at these locations, leading to the formation of disordered microstructures near the lamellar boundary. The measurements of lamellar thickness in the colonies with different growth directions indicate that there is no significant difference in the lamellar thicknesses of these colonies, and the average lamellar thickness of the AuSn and Au₅Sn layers can be given as ~107 nm and ~132 nm, respectively.

3.2 Hot compression behavior

Figure 2 shows the flow curves of Au-20Sn alloys subjected to hot compression at various temperatures (from 180 to 240 °C) and different strain rates (from 0.001 to 0.6 s^-1). Under these conditions, all the curves exhibit a stress peak during initial deformation and then drop rapidly because of strain softening caused by the dynamic recrystallization process (DRX) [17]. It can be seen that the peak stress is significantly influenced by the temperature and the strain rate, which monotonously increases with the decreasing temperature or increasing strain rate. At the lowest strain rate of 0.001 s^-1, the samples are successfully deformed at 200% strain without failure in the temperature range from 180 to 240 °C, as shown in Fig. 3. Nevertheless, serious wrinkles can be seen at the edge of the sample deformed at 180 °C.

Fig. 1 Microstructure of as-cast Au-20Sn eutectic alloy

Fig. 2 Flow curves of Au-20Sn alloys subjected to hot compression from 180 °C to 240 °C at different strain rates

Fig. 3 Au-20Sn samples after hot compression

According to Fig. 2(a), the peak stress of the sample deformed at 180 °C is twice as high as that of the sample deformed at 200 °C, and the rheological resistance is also much higher than that of the samples deformed at higher temperatures. The presence of serious wrinkles indicates that uneven deformation behavior occurs in the sample deformed at 180 °C. Regarding the Au-20Sn alloys with a nanolayered microstructure subjected to hot deformation, strain softening caused by DRX is closely related to deformation temperature. A low deformation temperature leads to localized strain concentration because the strain softening is inhibited, and uneven deformation behaviors, such as wrinkles, are introduced in the sample. In the compression process, 180 °C may be the minimum temperature for alloy forming at the strain rate of 0.001 s^-1. When the strain rate increases to 0.1 s^-1, brittle fracture occurs directly at the initial stage in deformation at 180 °C, and plastic deformation behavior can hardly occur in this condition due to the absence of strain softening. When the deformation temperature increases to 200 °C, the Au-20Sn alloy can be successfully deformed at 200% strain, but the edge of the deformed sample is rather rough. With increasing temperatures to 220 and 240 °C, the surface qualities of the deformed samples are improved significantly. When the strain rate further increases to 0.6 s^-1, the appropriate temperature for alloy forming increases. Edge cracks are intensely introduced, even to the sample deformed at 200 °C, and satisfactory surface-quality can only be obtained when the deformation temperature exceeds 220 °C. Clearly, besides the temperature, the formation of macroscopic defects during hot compression is related to the strain rate. At a given deformation temperature, a higher strain rate leads to more pronounced strain hardening (indicated by the much higher flow stress in Fig. 2), and a higher temperature is needed to achieve dynamic recovery, diffusion and recrystallization to suppress the localized strain concentration [18]. Overall, the formability of the Au-20Sn alloy is extremely sensitive to both the temperature and strain rate, these two parameters should be fully considered when formulating a hot forming process for this alloy.

3.3 Microstructure evolution

Figure 4 shows the microstructures of the Au-20Sn alloy deformed at 220 °C and 0.1 s^-1 with strains of 20% and 60%. Considering that the lamellar growth directions of the colonies are diverse, the analyses of microstructures focus on the locations that consist of colonies with different representative growth directions. For the sample with 20% compressive strain, spheroidization develops substantially in the vertical lamellae, while this phenomenon is hard to observe in the inclined and horizontal lamellae. Regarding the colonies with vertical lamellae, plastic instability, such as severe bending and “S” shear, can be seen clearly in the remaining lamellae. Additionally, little shear is also observed in the inclined lamellae, and spheroidized structures are gradually introduced in these sheared regions, as denoted in Fig. 4(a₃). As the compressive strain increases to 60%, severe bending and “S” shear become more pronounced in the inclined and horizontal lamellae. Simultaneously, spheroidization in the inclined and horizontal lamellae develops, which is gradually accompanied by the introduction of more plastic instability regions. However, the spheroidization rates in these two colonies are much lower than those in the vertical lamellae. The spheroidization rates in the colonies of the Au-20Sn alloy are in the following order: vertical > inclined > horizontal.

Fig. 4 Microstructures of Au-20Sn alloys deformed at 220 °C and 0.1 s^-1 to strain of ε=20% (a₁-a₃) and ε=60% (b₁-b₃)

As the compressive strain increases to 120%, microstructural spheroidization becomes more pronounced, and abundant equiaxed AuSn and Au₅Sn phases can be seen. In spite of this, quite a large fraction of phases still maintain a lamellar structure morphology, as seen in Fig. 5(a₁). Note that the vertical lamellae have been almost exhausted under such a strain condition, and microstructural spheroidization in the inclined lamellae becomes more intense. Generally, the closer the growth direction of the inclined colonies is to the vertical colony, the more easily the spheroidization can occur. However, there are still numerous horizontal or sub-horizontal lamellae remaining in the sample, as shown in Fig. 5(a₃). When the compressive strain further increases to 200%, the majority of the microstructure has completed the spheroidization process. AuSn and Au₅Sn phases in the sample become more equiaxed, and their phase sizes become larger than those with a strain of 120%. Nevertheless, some horizontal lamellae can still be observed in the sample, and the size of the equiaxed phases near these horizontal lamellae is much smaller than the phases elsewhere, as shown in Fig. 5(b₃). Evidently, it is further confirmed that the spheroidization rates of the colonies are rather different, which is strongly affected by the initial lamellar growth direction.

4 Discussion

4.1 Deformation behavior and spheroidization in multilayers

The Au-20Sn alloy shows intrinsic brittleness at room temperature because its microstructure consists of two brittle intermetallic compounds. However, the study has confirmed that this alloy shows superplasticity when the deformation temperature increases to 220 °C. Before spheroidization, deformation behaviors, such as bending and shear, can be observed clearly in the multilayers. When the alloy is subjected to hot compression, the plastic flow takes place in the AuSn and Au₅Sn layers to coordinate strain. Under such a mechanical model, bending instability is more likely to be primitively introduced in the vertical lamellae because the lamellar growth direction is exactly parallel to the compressive stress direction. With the increase in compressive strain, plastic flow in terms of bending in the two phases becomes more and more intense, and shears across the multilayers can be subsequently introduced, as shown in Fig. 6(a). Localized shear can lead to the effective initiation of spheroidized structures according to the microstructure  evolution. The distribution density of shear instability regions in the vertical lamellae will increase rapidly as the compressive strain further increases, leading to the formation of a knobby structure. Spheroidization then develops intensely based on this densely distributed shear and spreads rapidly in the colonies with vertical lamellae. As a result, a complete equiaxed microstructure can be introduced soon in these colonies. While for the colonies with inclined lamellae, the formation of bending and shear across the multilayers will slow due to the deviation angle between the lamellar growth direction and the compressive stress direction. The slower development of plastic instability regions leads to a slower spheroidization process in these colonies, as shown in Fig. 6(b). For the horizontal lamellae, compressive stress can also cause bending across the multilayers, but the radian of the bent lamellae is smooth and these bending instability regions can hardly transform into severe shear instability regions, as seen in Fig. 6(c). As a result, the colonies with horizontal lamellae show the lowest spheroidization rate during the hot compression process.

Fig. 5 Microstructures of samples subjected to different compressive strains of ε=120% (a₁-a₃) and ε=200% (b₁-b₃)

Fig. 6 Comparison of spheroidization processes in colonies with different lamellar growth directions

It is clear that the plastic behavior of the eutectic multilayers during hot compression is sensitive to the initial lamellar growth directions, and plasticity can be mediated by plastic instability. However, the distribution of localized shears is unable to steadily release the compressive strain, which will lead to local fracture during subsequent deformation [19]. Whereas for this alloy subjected to hot deformation, localized shear can be further mediated by spheroidization. The internal mechanism responsible for spheroidization favoring the shear instability needs to be clarified to further understand the microstructure evolution of the multilayers.

To explain this issue, EBSD is performed on the vertical lamellae that have been subjected to intense “S” shear, as shown in Fig. 7(a), where the dark and bright contrast regions are the AuSn and Au₅Sn phases. As shown in the band contrast map, numerous grain boundaries (>2°, GBs) gradually develop in both AuSn and Au₅Sn layers, with the formation of some equiaxed grains. Meanwhile, some GBs still appear discontinuous, and it can be predicted that spheroidization will take place here soon. Note that both the equiaxed phases and the GBs are all located near the shear instability regions. The misorientations that accumulated in two deformed AuSn layers and two deformed Au₅Sn layers are shown in Fig. 8. The typical point-to- origin misorientations parallel and vertical to the initial lamellar growth direction are analyzed near the newly developed grain boundaries. The results indicate that misorientations along the arrows A1, B1, C1 and D1 (along the initial lamellar growth direction) of the layers show the considerable misorientation accumulation, while misorientations along the arrows A2, B2, C2 and D2 (vertical to the initial lamellar growth direction) are barely changed.

Fig. 7 EBSD performed on deformed vertical multilayers in sample subjected to strain of 20% at 220 °C and 0.1 s^-1

Fig. 8 Misorientations accumulated in deformed AuSn and Au₅Sn layers (L-Lamellae)

Such misorientation accumulation in AuSn and Au₅Sn layers indicates that strain accumulation is more likely to distribute from the layer interior to the shear instability regions. The above findings suggest that the development of GBs in sheared regions is based on a gradual increase in the number of misorientations by the continuous absorption of dislocations in sub-GBs during straining, and this phenomenon is related to continuous dynamic recrystallization (CDRX) [20,21]. Kernel average misorientation (KAM) maps of the deformed AnSn and Au₅Sn phases shown in Figs. 7(b, c) can further confirm this mechanism. KAM can be interpreted in terms of the density of geometrically necessary dislocations to give local strain value. A higher KAM value means more strain accumulation [22,23]. Clearly, the shear instability regions in the two phases extremely favor the strain accumulation, which can provide sufficient energy for the formation and development of GBs. Thus, the spheroidization process in the nanolayered Au-20Sn alloy during hot compression is primarily based on CDRX in the shear instability regions. Additionally, strain accumulation in the AuSn phase seems to spread from the shear instability regions to the lamellar interior, but the majority of strain that accumulates in the Au₅Sn phase seems to be intensively distributed only near the shears. This phenomenon suggests that strain transfer is much easier in the AuSn matrix than that in the Au₅Sn phase. Furthermore, the average KAM value in the AuSn phase is higher than that in the Au₅Sn phase, even though they are subjected to the same compressive stress condition, indicating that the strain accumulation ability of the AuSn layer is much better than that of the Au₅Sn layer. The different deformation abilities in the two phases are probably attributed to their different crystal structures. This may be the same reason why the degree of microstructural spheroidization in the AuSn phase is more pronounced, with more equiaxed grains observed in the AuSn KAM map, as shown in Fig. 7 (b).

4.2 Deformation behaviors in equiaxed micro- structure

Microstructural spheroidization is a continuous process in the Au-20Sn alloy during hot compression, and its activity even lasted during the high-strain stage at strain of 200%. Nevertheless, the majority of the lamellae have transformed into equiaxed grains before being subjected to this strain condition, although a few horizontal lamellae with slow spheroidization rates still remain. The equiaxed microstructure has gradually become the primary carrier of compressive strain, and especially plays a dominant role in the latter compression process. Hence, study of deformation behaviors on the equiaxed microstructure is necessary.

Figure 9(a) shows the equiaxed microstructure of the sample subjected to strain of 200% at 220 °C and 0.1 s^-1. Misorientation analyses indicate that complete recrystallization has occurred in both AuSn and AuSn phases. Such a microstructure  can represent the general microstructure condition of this sample, according to the microstructure evolution in Fig. 5. The average grain sizes of the equiaxed AuSn and Au₅Sn phases in this sample are ~343.6 and ~452.7 nm, respectively.  The separated microstructures of the two phases  in terms of KAM maps are shown in Figs. 9(b, c), and the average KAM values are calculated to be 0.39° and 0.25°, which are obviously lower than those in the lamellar structures.

Fig. 9 EBSD performed on equiaxed microstructure in sample subjected to strain of 200% at 220 °C and 0.1 s^-1

Similar to the previous study on layers, two AuSn grains and two Au₅Sn grains are randomly selected to investigate the in-grain misorientation accumulations, as shown in Fig. 10. However, the misorientations parallel and vertical to the grain boundary in the four grains oscillate up and down but are all below 0.7°, which means that few misorientations accumulate inside both the equiaxed AuSn and Au₅Sn grains. These phenomena confirm that strain accumulation based on the lattice dislocations decreases significantly after the lamellae transformed into the equiaxed structure. This implies that the plastic deformation mechanism that mediates compressive strain may have changed from dislocation slip to another mechanism in the equiaxed phases.

To further investigate the deformation behavior of the equiaxed microstructure, EBSD is also conducted on the sample subjected to strain of 200% at 220 °C and 0.001 s^-1, as shown in Fig. 11. Similarly, misorientations also cannot accumulate effectively in the equiaxed phases; the number of accumulated in-grain misorientations is even less than that in the sample deformed at the strain rate of 0.1 s^-1, as determined by comparing the misorientation-distance curves. Figures 11(b, c) show that the ability of strain to accumulate (evaluated by the average KAM value) based on the lattice dislocations in this sample is also inferior to that in the sample deformed at 0.1 s^-1. Additionally, it is noted that the average grain sizes of the equiaxed AuSn and Au₅Sn phases in this sample are ~776.3 and ~984.2 nm, respectively, which are approximately 2.2 times larger than those in the sample deformed at 0.1 s^-1. Because grain boundary sliding and migration are sensitive to the strain rate, the introduction of a larger grain size in hot deformation at a lower strain rate is usually attributed to these deformation mechanisms, as reported in many studies [24-26]. Besides, grain boundary bulging is observed in the two deformed samples in Figs. 9 and 11 (seen in the KAM maps), which further confirms the occurrence of grain boundary sliding and migration in the equiaxed AuSn and Au₅Sn phases. Thus, it can be preliminarily concluded that grain boundary sliding and migration have become important deformation mechanisms that coordinate compressive strain after the multilayers is transformed into equiaxed grains, which leads to a rapid decrease in the extent of strain accumulation caused by the declining dislocation slip activity. The contribution of grain boundary sliding and migration to the strain coordination will be enhanced as the strain rate decreases.

Fig. 10 Misorientations accumulated in deformed AuSn and Au₅Sn equiaxed grains (G-Grain)

Fig. 11 EBSD performed on equiaxed microstructure of sample subjected to strain of 200% at 220 °C and 0.001 s^-1

4.3 Transition of deformation mechanisms

Regarding the Au-20Sn alloy during hot deformation, microstructure evolution is intricate due to the presence of two complicated structured intermetallic compounds (AuSn+Au₅Sn). The investigations on the deformation mechanisms of the two phases during hot compression are important because these mechanisms have a significant influence on microstructure evolution. As mentioned previously, CDRX, in terms of lattice dislocations, occurs intensely in both the AuSn and Au₅Sn layers, suggesting that slip occurs in these two lamellar phases. For the hexagonal-structure materials, analysis of the in-grain misorientation axis (IGMA) distribution is an effective method to identify what kinds of dislocations are activated in a deformed microstructure [27,28]. Figure 12(a) shows the IGMA analysis of the deformed AuSn layers. It shows that the IGMA is mostly along the <0001> direction, which suggests that lattice rotation of the matrix is primarily based on the c-axis and this grain reorientation behavior can directly correspond to prismatic slip [27]. It is well known that the c/a ratio usually plays a crucial role in influencing the selection of the deformation mechanisms in hexagonal-structure alloys [29,30]. It is reported that the dominant deformation mechanism in the hexagonal-structure metals, such as Mg (c/a: 1.623) and Zn (c/a:1.856) alloys with a higher c/a value, is basal slip, while the prismatic slip is the dominant mechanism in Zr (c/a: 1.592) and Ti (c/a: 1.588) alloys with a lower c/a ratio [31-34]. As the c/a ratio of the AuSn phase with a double HCP crystal structure is only 1.278,  prismatic planes are the most close-packed planes and the slip direction is likely to be the  direction, according to the crystal structure parameters in Fig. 12(b). Thus, it can be concluded that prismatic slip is the dominant deformation mechanism in the AuSn layers during the hot compression. Regarding the deformed Au₅Sn layers, the IGMA is mostly along the . Such an IGMA distribution can correspond to basal or pyramidal slip activities in hexagonal- structure alloys [35]. Figure 12(d) shows the crystal structure of the Au₅Sn phase, which possesses a trigonal structure. Because a majority of the crystal planes of the Au₅Sn phase exhibit 60° rotational symmetry (six-fold symmetric axis) about the c-axis, the trigonal structure (a=b≠c, α=β=90°, and γ=120°) can be transformed to a hexagonal structure. Note that the primary slip systems and their activities in the trigonal structure are in full accord with those in the hexagonal structure; discussions on the deformation mechanisms of the Au₅Sn phase made based on the hexagonal- structure alloys are fully applicable. Evidently, from crystallographic theory, the basal slip system probably activates easily in the Au₅Sn phase owing to its high c/a ratio and complicated crystal structure. Combined with the IGMA result, basal slip is the dominant deformation mechanism in the Au₅Sn layers during hot compression. Additionally, it is now clear that the difference in the strain accumulation abilities of the two phases is closely related to their crystal structures, and the higher crystal symmetry of the AuSn phase can give rise to a better deformability and thus more strain accumulation.

[27] CHUN Y, DAVIES C. Investigation of prism slip in warm-rolled AZ31 alloy [J]. Metallurgical and Materials Transactions A, 2011, 42: 4113-4125.