Doctoral Dissertations

Date of Award

12-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter Liaw

Committee Members

Yanfei Gao, Hahn Choo, Hairong Qi

Abstract

Recently, exceptional properties that are continuously found in an intriguing new class of metallic structural materials, high-entropy alloys (HEAs), demonstrate their great potential for engineering applications particularly in extreme environments where conventional alloys reach their limits. The attractive properties in HEAs are mainly attributed to HEAs exhibited special effects, such as, vast compositional space, heterogeneous local atomic environments, tunable stacking-fault energy (SFE), ductile multicomponent intermetallic precipitates, and integrated various engineering concerns. Therefore, the concept of HEAs opens up new avenues to develop advanced high-performance alloys for overcoming some drawbacks appeared in traditional alloys. In this work, two main tasks are studied for developing advanced precipitation-strengthened alloys by the HEA concept through integrated experimental and theoretical methods.In the first task, to develop significantly cheaper and lighter high-temperature alloys than Ni-base superalloys, precipitation-strengthened body-centered-cubic (BCC) HEAs, which possess a microstructure analogous to that of the γ/γ′ Ni-base superalloys, are designed, using the CALPHAD-based high-throughput computational method (HTCM). The fundamental understanding of the phase stability, precipitation strengthening, and order-disorder transition in the newly-designed lightweight HEAs are revealed by in-situ neutron scattering, advanced microcopies, ab initio molecular dynamics (AIMD), and Monte-Carlo (MC) simulations. The insights obtained in this study offer a paradigm to develop high-performance lightweight HEAs via the high-throughput method. In the second task, in order to develop advanced fatigue-resistant HEAs, a B2 precipitation-strengthened Al0.5CoCrFeNi HEA is designed, which exhibits the outperforming fatigue life at low strain amplitudes compared with traditional materials. Its real-time cyclic-deformation mechanisms are revealed by in-situ neutron diffraction and advanced microscopy experiments. The present work indicates that various beneficial cyclic-deformation mechanisms, such as the multicomponent-precipitation strengthening, deformation twinning, and reversible stress-induced martensitic phase transformation can be synergistically integrated into HEAs together. Guided by this design idea, fatigue-resistant precipitation-strengthened HEAs can be developed, which can be generalized to other alloys systems.

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