MAO YUAN LUO

NATIONAL YANG MING CHIAO TUNG UNIVERSITY

Department of Materials Science and Engineering

Ph.D. Student

About

I am Mao-Yuan Luo, a Ph D. Candidate at the National Yang Ming Chiao Tung University (NYCU) majoring in Materials Science & Engineering. My research focus is to apply advanced photon sources, such as neutron and synchrotron X-rays, to investigate advanced metallic systems. I am investigating the advanced high entropy alloys focusing on fatigue behavior, which is the key issue for structural materials at multi-length levels. Lattice structures at Å are responsible for fundamental elastic behaviors originating from element distributions in the crystalline structure, directional defect structure at 10~1,000 nm, such as dislocation, stacking faults, and twins are responsible for the mechanical behavior, and grain sizes are responsible for the deformation modes of cyclic loading in the bulk systems.

Research Disciplines

Materials Science

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Title: Mixing-enthalpy modulation on phase transformation in the gradient chemical core/shell high-entropy shape-memory alloys

Keywords: High-entropy shape-memory alloys, Phase transformation, Mixing enthalpy, Gradient structure, Core/shell structure

Abstract: High-entropy alloys (HEAs) are of more chemical inhomogeneity than traditional alloys. How thermal effects perturb entropy and chemical fluctuations is a major scientific issue. In this study, selected benchmark phenomena revealed by in-situ synchrotron X-ray mapping demonstrates how local chemical gradients control temperature-dependent behaviors in HEAs. Specifically, the gradient chemical layers in the core/shell structure play an important role in the thermally-induced phase transformation of Cu15Ni35Ti50-x(HfZr)x high-entropy shape-memory alloys (HESMAs). The dendritic microstructure in Cu15Ni35Ti20(HfZr)30 exhibits the mixing enthalpy-driven pronounced composition inhomogeneity of an Ni-Hf-rich core and Cu-Zr-Ti-rich shell, demonstrated by thermodynamics calculations and second nearest-neighbor modified embedded atom method (2NN MEAM) formalism. The shell acts as an interfacial energy barrier for the stable martensitic transformation that occurred primarily in the effective Ni-Hf-rich core of the Cu15Ni35Ti20(HfZr)30. The temperature-dependent Gibbs-free energy agrees with the Cu movement, which widens the thermally-induced gradient layer of the elemental redistribution. Our findings are conducive to a potential design strategy of tailoring gradient chemical core/shell HEAs for stable high-temperature shape-memory applications.