Formation mechanism of partial stacking faults by incomplete mixed-mode phase transformation: A case study of Fe-Ga alloys

• The faulted {111} distance of partial stacking faults is shorter than that of the typical SFs in deformed FCC materials. • The partial stacking faults have lower ordering degree than the well-stacked product phase. • The formation of partial stacking faults depends on both atomic ordering and lattice shear strain of the parent phase. Partial stacking faults (PSFs) formed by incomplete mixed-mode phase transformation have been found to exhibit unfixed slip distance of closely-packed planes unlike those of the deformation-induced stacking faults (SFs) with fixed distance. Though engineering PSFs can yield appealing properties, such as the enhanced damping capacity, understanding of the interaction between lattice distortion and atomic diffusion and their influences on forming PSFs is still far from being clear. Herein we performed a case study on aged Fe-Ga alloy that undergoes a mixed-mode phase transformation from body-centered cubic (BCC) to ordered face-centered cubic (FCC). The TEM investigations showed that the faulted {111}-FCC distance of the PSFs is shorter than a /6<112> of the typical {111}-<112> SFs in deformed FCC materials and the PSFs have disordered Fe and Ga arrangements. Further studies revealed that such PSFs will not be completely dissociated at FCC twin boundaries (TBs) even after long term isothermal aging. Consequently, the formation of PSFs can be ascribed to the transformation-dependent atomic ordering and lattice shear strain of the parent BCC lattice, where the diffusion-controlled glides of the PSFs-associated dislocations will accelerate atomic diffusions due to the dislocation-pipe effect along <112>-FCC direction, but may hinder the atomic diffusions across the {111}-FCC TBs due to the retarding effect. This study may add important insight into the defects process during mixed-mode phase transformation and broaden the knowledge of the interaction between concurrently-happened lattice distortion and atomic diffusion.