PhD Thesis Defense: Edwin Jiang

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"High-Temperature Creep Deformation Mechanism of Fe28.2Ni18.8Mn32.9Al14.1Cr6 High-Entropy Alloy and Its Modified Alloys"

Abstract

Fe28.2Ni18.8Mn32.9Al14.1Cr6 eutectic high entropy alloy exhibits a good combination of room-temperature and high temperature properties, including tensile strength, ductility, and corrosion resistance, making it a promising candidate for structural applications in extreme environments. However, the creep deformation behavior of this alloy—and high-entropy alloys (HEAs) in general—remains insufficiently understood. This dissertation systematically investigates the high-temperature creep deformation mechanisms of Fe28.2Ni18.8Mn32.9Al14.1Cr6 and its derivatives.

Creep mechanisms and associated microstructural changes were examined across a wide range of strain rates using strain-rate jump and constant-stress tests. Two dominant regimes were identified: dislocation glide/solute-drag at low strain rates, and dislocation climb at high strain rates. Stress exponent values averaged 2.7 at low strain rates (constant-stress tests) and 6.4 at high strain rates (strain-rate jumps). No evidence of precipitation was found. Fractography showed mixed intergranular and transgranular fracture, often initiated at FCC/B2 interfaces and grain boundaries due to dislocation pile-ups.

To clarify the role of individual phases, the effect of manganese variation on microstructure and creep was studied. Increasing Mn content led to a higher FCC phase fraction, grain refinement, and wider grain boundary phases. Although Mn8 exhibited higher strength, its creep life and resistance declined, likely due to increased grain boundary phases, reducing the inhibition of dislocation movement at high temperatures.

Zirconium additions were also explored. At low concentrations (<0.5 at.%), Zr dissolved into both FCC and B2 phases. At higher concentrations, Zr promoted formation of ZrAlNi intermetallics, observed along grain boundaries after creep at 1023 K. The Zr0.5 alloy showed the longest creep life and lowest secondary creep rate, though intermetallic-induced brittleness limited performance at lower temperatures.

Additionally, the effects of cold work and then annealing on both the microstructure and the high-temperature creep deformation were investigated. The treated alloys showed a transition to finer subgrain structures, improved ductility, and a change in fracture mode. The refined microstructure led to reduced creep resistance under low strain-rate conditions, which is likely due to finer sub-grains and a greater density of grain boundaries from the treatments, resulting in easier void formation and cracks propagation.

Thesis Committee

• Prof. Ian Baker (Chair)
• Prof. Jifeng Liu
• Prof. Geoffroy Hautier
• Prof. Yanfei Gao (external)

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.