Recent Developments in Small-Scale Shape Memory Oxides
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SPECIAL ISSUE: A TRIBUTE TO PROF. DR. GUNTHER EGGELER, INVITED PAPER
Recent Developments in Small-Scale Shape Memory Oxides Xiao Wang1 • Alfred Ludwig1
Published online: 26 August 2020 Ó The Author(s) 2020
Abstract This review presents an overview of the developments in small-scale shape memory materials: from alloys to oxides and ceramics. Shape memory oxides such as zirconia, different ferroelectric perovskites and VO2based materials have favorable characteristics of high strength, high operating temperature and chemical resistance, which make this class of shape memory materials interesting for special applications, e.g., in harsh environments or at the nanoscale. Because of the constraint and mismatch stress from neighboring grains in polycrystalline/ bulk oxides, the transformation strain of shape memory oxides is relatively small, and micro-cracks can appear after some cycles. However, recent progress in shape memory oxide research related to small-scale approaches such as decreasing the amounts of grain boundaries, strainengineering, and application in the form of nanoscale thin films shows that some oxides are capable to exhibit excellent shape memory effects and superelasticity at nano/ micro-scales. The materials systems ZrO2, BiFO3, and VO2 are discussed with respect to their shape memory performance in bulk and small-scale.
This invited article is part of a special issue of Shape Memory and Superelasticity to honor Prof. Dr.-Ing. Gunther Eggeler. This special issue was organized by Prof. Hu¨seyin Sehitoglu, University of Illinois at Urbana-Champaign, and Prof. Dr.-Ing. Hans Ju¨rgen Maier, Leibniz Universita¨t Hannover. & Alfred Ludwig [email protected] 1
Chair for Materials Discovery and Interfaces, Institute for Materials, Ruhr University Bochum, 44801 Bochum, Germany
Keywords Shape memory ceramics Shape memory films Mechanical behavior
Introduction Shape memory alloys (SMAs) undergo a reversible, diffusionless, solid-state phase transformation between martensitic and austenitic crystal structures in response to external stimuli (temperature, stress, electric field, magnetic field, or combinations thereof), accompanied by strains ranging up to and beyond 10%. The ability of returning to its original shape upon heating was defined as shape memory effect (SME) [1, 2]. At least two crystal structures are involved, i.e., austenite and martensite. Austenite is stable at high temperature (T [ AF, austenite finish temperature) and transforms to twinned martensite upon cooling (T \ MS, martensite start temperature). Upon loading, twinned martensite converts to detwinned martensite, with strain generated and remaining after unloading. Detwinned martensite transforms to austenite by heating, with the shape recovered. This cycle is called one-way SME. The shape can also be remembered in both states, when the material shows the two-way SME or reversible SME. In addition, a reversible transformation between austenite and martensite at T [ AF, by applying mechanical load is called superelasticity (SE). D
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