8/13(二)演講公告

演講人廖楷輝   教授

服務單位Department of Materials Science and Engineering, The University of Tennessee, Knoxville, USA

演講題目Lattice Distortion in the NbTaTiV(Zr) Refractory High-entropy Alloys

演講時間108813(星期二)下午200

演講地點工綜館228會議室

演講摘要

The mixing entropy in high-entropy alloys (HEAs) can be maximized by forming a stable single phase with multiple principal elements. The resultant effects, such as lattice distortion, can contribute to excellent mechanical properties, which has motivated numerous efforts to develop and design single-phase HEAs. However, challenges still remain, particularly on quantifying the lattice distortion and relating it to materials properties. In this study, we have developed a NbTaTiV refractory HEA with a single body-centered-cubic (BCC) structure using integrated experimental and computational approaches. The alloy was subjected to the proper homogenization treatment to eliminate the structural inhomogeneity and chemical segregation. Importantly, results indicate that this HEA exhibits extraordinary mechanical properties at both room and elevated temperatures. Furthermore, the effects of the high mixing entropy on the mechanical properties are further discussed and quantified in terms of lattice distortions and interatomic interactions of the NbTaTiV(Zr) HEAs via first-principles calculations.

This comprehensive study provides a method for the design and development of the single-phase BCC solid-solution phase refractory HEA, using an integrated experimental and theoretical thermodynamic-calculation approach. The results of the atom probe tomography (APT) measurement and the neutron-diffraction patterns indicate that the structure is composed of a single-phase BCC solid solution as well as a homogeneous elemental distribution with equimolar ratios. Furthermore, the mechanical-test results of the homogenization-treated sample show the excellent yield strength and plasticity at room-temperature (RT) as well as elevated temperatures. The dominant strengthening mechanism was found to be solid-solution hardening, which stems from the distortion of the crystalline lattice during deformation. It is thought that this strengthening mechanism induces slow elemental diffusion at high temperatures, which, consequently, leads to the strong resistance of high-temperature softening. The distorted lattice for this alloy system was quantitatively measured and calculated by the mathematical computation, neutron/synchrotron diffractions, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and theoretical modeling, such as first-principles calculations. The results of the above modeling and analysis indicate that the local severe lattice distortions are induced, due to the local atomic interactions in the homogenization-treated NbTaTiV(Zr) refractory HEAs. These results provide (1) a novel alloy-design strategy, (2) a fundamental understanding of the lattice-distortion effect on mechanical properties, and (3) a road map to produce better materials for high-temperatures application.

20190813-3