PhD Thesis Defence: Mr. Akshat Godha (11/07/25)
Thesis title:
Exploring the atomic-scale solute-defect interactions for designing high-performance disordered Ni-Co base and ordered Ni base superalloys
Faculty advisor(s):
Prof. Surendra Kumar Makineni
When?
11th July, 2025 (Friday), 3:00 PM (India Standard Time)
Mode Online
KPA Auditorium, Materials Engineering
Abstract
For structural applications, a significant section of high-strength alloys has a base element of iron, nickel, cobalt, titanium, or their combinations. Such materials enable efficient, safe, and sustainable operations across various aerospace, automotive, energy, and defence industries. However, the increasing demands for high performance and efficiency in such applications necessitate further enhancements in the mechanical properties of these alloys. Increasing the strength of the alloy largely relies on imparting resistance to the motion of dislocations in their microstructure during external load. This is controlled by either solute additions and/or by mechanical working that induces large amounts of lattice defects. The most conventional practised strengthening strategies comprise solid-solution hardening, precipitation/dispersion hardening that depends on the type of added solutes, and grain boundary strengthening that can be tuned by thermomechanical processing. Recently, the nature of the interaction of solutes with defects has been seen as one of the future alloy design strategies for further improving the mechanical properties of alloys. The present thesis explores the role of atomic-level solute-defect interactions and their influence on 1) creep deformation of ordered γ/γ’ single-crystal CMSX-4 (Ni-base) superalloy and 2) tensile properties of disordered austenitic Ni-Co-Cr-Mo alloys.
Part 1 examines deformation behaviour and solute-defects interaction in a low-stacking-fault-energy (SFE) multi-component alloy Co-Ni-Cr-Mo alloy. At room temperature, the alloy exhibits remarkable tensile strength (~1.1 GPa), ~60% ductility, and high strain hardening. Post-deformation microstructure analysis revealed activation of multiple deformation mechanisms such as dislocation slip, stacking faults (SFs), twinning-induced plasticity (TWIP), and transformation-induced plasticity (TRIP). However, these alloys start yielding at very low-stress levels (250 MPa - 350 MPa), indicating their high tendency towards getting permanently deformed. To address this,
Part 2 explores strengthening by infusing solutes into deformation-induced structures via cold rolling and annealing. SFs and twins mainly dominate in the 45CR sample, while the 65CR sample exhibits a higher fraction of SFs/twins and an additional HCP(ε) martensite phase. Annealing at 600°C promotes Mo segregation into these defects, increasing yield strength to ~1.5 GPa for 45CR and ~2 GPa for 65CR, while maintaining ~6% ductility. The strengthening correlates with the fraction of deformationinduced structures, and mechanisms for their formation were also proposed.
Part 3 investigates the selective segregation of solutes to different types of defects in a multi-component single crystal γ/γ’ Ni-base superalloy (CMSX4) under different creep conditions. At 800°C (applied stress 800 MPa), SESFs and micro-twins form in γ’, while at 1000°C (applied stress 200 MPa), the interfacial dislocation network and APBs dominate. Atomic-scale compositional and structural analysis reveals that solute segregation depends on the local fault structure. During micro-twinning, Al variation along the twin boundaries and W/Ta enrichment at the Al-depleted portion is observed. Based on the segregation behaviour, an additional step of reordering in Kolbe’s mechanism of micro-twinning was introduced. At 1000°C, Mo/W and Re enrichments at dislocations and APBs suggest solute drag during γ’ shearing. This emphasizes the importance of the solutes Mo/W in contributing towards the “Re effect” during hightemperature creep. These new observations suggest, other than Re, the possible creep rate limiting solutes whose content can be optimized for future alloys with higher temperature capabilities for more energyefficient and cost-effective turbine engines with lower CO2 emissions.
The above experiments demonstrate that the solute-defect interactions can significantly influence the mechanical properties of alloys, leading the way for the design of next-generation structural materials with superior performance.