PhD Thesis Colloquium: Mr. Shubham Sisodia (22/04/26)
Thesis title:
Design strategies for enhancing the fatigue-resistance of Multi-Principal Element Alloys.
Faculty advisor(s):
Dr. Ankur Chauhan
When?
22nd April, 2026 (Wednesday), 03:00 PM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Multi-principal element alloys (MPEAs) have emerged as promising structural materials due to their vast compositional space and tunable properties. Within this class, face-centred cubic (FCC) alloys, such as equiatomic CrMnFeCoNi, are of particular interest because their low stacking fault energy (SFE) can be tailored to activate specific deformation mechanisms while leveraging solid-solution and grain-boundary strengthening. However, ensuring the structural integrity of these alloys requires a fundamental understanding of their fatigue behavior.
With respect to compositional design strategies, prior studies have predominantly explored the removal of one or two elements from equiatomic CrMnFeCoNi to obtain equiatomic subsets (e.g., CoCrNi and CrFeNi) with tailored intrinsic SFE. Alternatively, a systematic approach involves modulating the Cr/Ni ratio within the Cr–Mn–Fe–Co–Ni system to lower the SFE and enhance strengthening. By increasing this ratio from 0.54 in Cr14Mn20Fe20Co20Ni26 (HSFE) to 1.86 in Cr26Mn20Fe20Co20Ni14 (LSFE), the SFE drops from 69 ± 15 mJ/m² to 23 ± 3 mJ/m². This method allows for a controlled investigation of compositional effects on mechanical performance while keeping elemental constituents constant. Accordingly, the first objective of this study is to elucidate how the Cr/Ni ratio influences the room-temperature low-cycle fatigue (LCF) behavior of these single-phase MPEAs. Despite both alloys possessing similar ~60 µm grain sizes, the LSFE alloy demonstrates superior cyclic strength across all strain amplitudes (±0.3 % to ±0.7 %) with comparable or improved fatigue life. These results are discussed in terms of how SFE dictates the cyclic stress response and the resulting deformation and damage mechanisms.
Beyond compositional tuning, the CoCrFeMnNi system, specifically the LSFE composition, offers a unique pathway for enhancement through sigma-phase precipitation and concurrent grain refinement. While these features are known to improve tensile strength, their impact on cyclic loading is a critical area of investigation due to potential stress concentrations at precipitate–matrix interfaces. Consequently, the second objective of this work is to determine how sigma-phase-assisted grain refinement influences the LCF response and underlying deformation mechanisms. The results demonstrate that the dual-phase LSFE alloy, featuring a grain size reduced by an order of magnitude, achieves its peak cyclic strength without sacrificing fatigue life. This superior performance is driven by precipitates’ ability to deflect and arrest cracks, while extensive deformation twinning originating at the precipitate/matrix interface near fatigue cracks further reduces propagation rates, marking a significant advancement over single-phase MPEAs.
To broaden the applicability of MPEAs, single-phase alloys can be tailored to achieve fatigue strengths comparable to those of dual-phase systems. The CoCrNi system is ideal for this due to its low SFE and strong solid-solution strengthening. This can be further optimized by adding Mo, which increases lattice distortion and further reduces SFE due to its large atomic radius and high elastic modulus. A representative alloy for this strategy is Co35.4Cr22.9Ni35.5Mo6.2. Accordingly, the third objective is to evaluate the LCF performance of this Mo-alloyed system relative to baseline equiatomic CoCrNi and Co–Cr–Ni–Fe–Mn alloys. The results demonstrate that compared to these counterparts, the Mo-alloyed variant achieves higher peak stresses and superior fatigue life. These results are discussed in the context of how Mo addition governs lattice strength, grain boundary strengthening, SFE, and the resulting deformation and damage mechanisms. Specifically, the Mo-induced reduction in SFE promotes reversible planar slip and suppresses dislocation rearrangement into cells or walls. This synergy, amplified by twinning and shear banding, creates tortuous crack paths and slows crack growth, allowing the single-phase alloy to approach the fatigue resistance of dual-phase MPEAs.
While these strategies optimize fatigue resistance at ambient conditions, certain structural applictaions requires reliable performance at elevated temperatures, where thermally activated processes and environmental effects dominate. CoCrNi-based MPEAs, despite their excellent room-temperature response, may undergo distinct deformation and damage transitions under thermal stress. Therefore, the fourth objective is to evaluate the high-temperature LCF response of MPEAs, taking Co35.4Cr22.9Ni35.5Mo6.2 as a representative system at 550 °C. At this temperature, the alloy exhibits serrated flow (dynamic strain ageing), continuous cyclic hardening, and reduced fatigue life. Atomic-scale analysis reveals extensive solute-defect interaction including Suzuki-type segregation (Mo/Co enrichment and Cr/Ni depletion) at stacking faults (SFs), that lowers local SFE and intensifying planar slip. A critical shift occurs in the failure mode: while crack initiation is annealing twin-boundary-dominant at room temperature, it becomes grain-boundary-dominant at 550 °C. This transition is driven by grain-boundary segregation, which acts in synergy with DSA and oxidation. These insights provide essential guidelines for designing next-generation MPEAs capable of withstanding severe cyclic loading in demanding high-temperature environments.