報告題目：Effects of Time and Length Scales on Phase Transition Behavior of Solids

報告人：Prof. Qingping SUN

（Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong, China）

報告時間：2015年12月14日15：00

報告地點：   A9-506

主辦單位：國際交流合作處、科協、機械結構力學及控制國家重點實驗室、航空宇航學院

Abstract

We report recent advances in the experimental and theoretical study of effects of loading rate and grain size (GS) on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy (SMA). It is shown that not only the time scales of the physical processes strongly affects the phase transition responses, but also the internal length scales of material such as GS and interface thickness have great impact on the phase transition behavior of solids. When GS < 60 nm, the superelastic stress-strain hysteresis loop area (H) of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers. At the same time, the temperature dependence of the transition stress also decreases with GS and eventually approaches zero, leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship. Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization. It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originates from the large increase in the area-to-volume ratios of the nanometer-thick interfaces (grain boundary and Austenite-Martensite (A-M) interface) in the polycrystal. In particular, with GS reduction, interfacial energy terms will gradually become dominant over the bulk energy of the crystallite, eventually bring fundamental changes in the phase transition responses of the material.

Brief Bio: Qingping SUN is the Professor of the Department of Mechanical and Aerospace Engineering and the Director of the Institute of Integrated Microsystems at the Hong Kong University of Science and Technology. Prof. SUN received his PhD in solid mechanics from Tsinghua University (Beijing) in 1989. He joined HKUST in 1995. Prof. Sun’s primary research area is the mechanics of phase transitions in materials, with special interests in phase transition process in shape memory alloys, ceramics and nano- and biological materials/systems. His research work covers a broad spectrum of problems in the inter-disciplinary subject between solid mechanics, solid state physics, biology and materials science.

 

Professor SUN is an internationally renowned expert in mechanics of shape memory materials and is distinguished for his seminal contributions in the areas of solid-solid phase transition and mechanics of multi-scaled processes with multi-field coupling. He has published over 120 research papers in prestigious journals in the fields of mechanics, solid state physics and material science. He has received many national and international recognitions for his research and teaching, including the “State Natural Science Award of China” (1996); the “Best Engineering Teaching Excellence Award” (2002) of HKUST; the “Citation Classic Award” from ISI (2001); the “Distinguished Young Scholars Award” and the “Overseas Distinguished Young Scholars Award” both from the Natural Science Foundation of China (1998, 2011), and Natural Science Award of Minister of Education (2009), etc.. He gave over 30 invited Keynote Lectures and 6 Plenary Lectures in international conferences. He was the visiting professor in four universities/national labs (such as Ecole Polytechnique, Ecole Normal Superieur, etc.) in France. He served as the editorial boards for six International journals and as the Chairman and members of Scientific Committee for many international conferences.