Localized phase transformation at stacking faults and mechanism-based alloy design

A new phenomenon has been observed in some Ni-based superalloys where phase transformations occur at and are confined to stacking faults, which we refer to as localized phase transformations (LPTs). Alloys with different types of LPTs are found to have drastically different creep properties. In this study, a thermodynamic model is proposed for the LPTs, and a new design strategy based on the relative stability of ordered and disordered stacking fault phases (SFPs) is established for the next-generation LPT-strengthened superalloys. The relative stability of an ordered SFP and its disordered counterpart determines the types of LPT (ordering vs. disordering) at each type of stacking faults and a thermodynamic parameter using the free energies of the bulk counterparts of these SFPs is introduced to evaluate the ordering tendency of SFP at each type of faults. The effect of LPT on shear resistance of subsequent dislocations is investigated using ab initio calculations and the results show that an ordered SFP increases the shearing resistance. Thus, the larger the ordering tendency is, the more significant the LPT hardening is, and the better the creep performance would be. The similarities and differences between LPT at stacking faults and segregation transition at grain boundaries are also discussed. The mechanisms described in this study settle fundamental questions about why LPT occurs, why LPT is confined at stacking faults and how LPT impacts the mechanical properties, which opens a new window for alloy design.