2430% Superplastic strain in a eutectic Au-Sn alloy with micrometer-sized grains maintained by spinodal-like decomposition

Polycrystalline metals with fine grains are highly possible to possess superplasticity via grain boundaries sliding (GBS). However, the mechanism of stabilizing the fine grains and releasing grain boundary strain incompatibilities to maintain GBS has puzzled the researchers for several decades. Here, a classic example is represented by the Au-Sn eutectic (ζ-Au ₅ Sn + δ-AuSn) alloy with micrometer-sized equiaxed grains, which achieved 2430% tensile strain at 473 K. Characterization at nanoscale reveals, unexpectedly, the universal occurrence of spinodal-like decomposition in one of the eutectic phases (δ-AuSn phase) in the Au-Sn alloy. The occurrence of the spinodal-like decomposition is energetically favorable. Therefore, the system`s free energy was minimized by the spinodal-like decomposition rather than grain growth. Micrometer-sized grains were thus maintained at relatively high temperatures for maintaining GBS. In addition, the stacking faults (SFs) were generated in the spinodal-like decomposed substructures during deformation. SFs coordinated the strain incompatibilities during GBS and contributed to the plastic flow during superplasticity. In the present Au-Sn alloy, the spinodal-like decomposition is the root cause to stabilize the fine grains, eventually leading to outstanding superplasticity. This study enriches our fundamental understanding of the relation between spinodal-like decomposition and mechanical performance. It also provides new insights into the design of polycrystalline metals to achieve superplasticity.