Stacking Fault Driven Phase Transformation in CrCoNi Medium Entropy Alloy.

Phase transformations are known to influence material properties, including ductility. The fcc-to-hcp transformation is observed in various alloys, but the mechanisms remain unclear. Some studies suggest nucleation occurs at grain boundaries, while others propose dislocation-based pathways. Prior research has shown that stacking faults may influence phase stability in alloys. However, the role of stacking faults in triggering phase transformations at the nanoscale remains uncertain. In CrCoNi, a medium-entropy alloy, theoretical predictions suggested a possible fcc-to-hcp transformation. Yet, experimental evidence for a bulk transformation under stress was lacking. This uncertainty motivated the need for in situ studies to clarify the nucleation mechanism. The absence of direct observations limited understanding of how stacking faults might initiate phase changes. This gap motivated the current investigation into the transformation process in CrCoNi.

The study aimed to investigate the nucleation mechanism of the fcc-to-hcp phase transformation in CrCoNi under tensile loading. The researchers sought to determine whether stacking faults could trigger the transformation at low temperatures. They focused on the deformation characteristics of the fcc phase and their correlation with hcp phase development. The goal was to confirm the role of stacking faults in phase nucleation. By using in situ neutron diffraction, the team aimed to observe the transformation process in real time. They also aimed to clarify whether the transformation occurred in bulk or only locally. The study aimed to address the lack of experimental evidence for stacking fault-driven transformations. This work aimed to shed light on the mechanisms behind CrCoNi’s exceptional low-temperature ductility.

The researchers employed in situ neutron diffraction to monitor phase changes during tensile loading. They conducted the experiments at 15 K to capture the transformation under controlled conditions. Neutron diffraction allowed them to track the development of the hcp phase in real time. They correlated the deformation of the fcc phase with the emergence of the hcp phase. The study used a bulk sample of CrCoNi to ensure the transformation could be observed macroscopically. They analyzed the diffraction patterns to identify the onset of the hcp phase. The team also examined the stacking fault density in the fcc phase before transformation. Their approach combined experimental observation with structural analysis to determine the nucleation mechanism.

The study revealed a bulk fcc-to-hcp phase transformation in CrCoNi at 15 K under tensile loading. Neutron diffraction confirmed the emergence of the hcp phase during deformation. The transformation was observed in real time through in situ loading experiments. The nucleation of the hcp phase was linked to intrinsic stacking faults in the fcc matrix. The researchers found that stacking faults acted as nucleation sites for the transformation. The correlation between deformation characteristics and phase development was strong. The transformation occurred in bulk, not just locally, as previously hypothesized. These findings support the role of stacking faults in triggering phase changes in CrCoNi.

The study confirms that stacking faults can trigger the fcc-to-hcp phase transformation in CrCoNi at low temperatures. The nucleation of the hcp phase was shown to occur in bulk, not only at grain boundaries. This finding supports the hypothesis that stacking faults act as nucleation sites. The transformation was observed under tensile loading at 15 K. The results suggest that stacking faults play a key role in the deformation mechanisms of CrCoNi. The confirmation of a bulk transformation adds to the known mechanisms contributing to ductility. The study provides direct evidence for stacking fault-driven phase transformations. These findings enhance understanding of the exceptional ductility observed in CrCoNi at low temperatures.