Phase Transformations of Nanocrystalline Martensitic Materials
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Transformations of Nanocrystalline Martensitic Materials T. Waitz, K. Tsuchiya, T. Antretter, and F.D. Fischer
Abstract The physical phenomena and engineering applications of martensitic phase transformations are the focus of intense ongoing research. The martensitic phase transformation and functional properties such as the shape-memory effect and superelasticity are strongly affected by the crystal size at the nanoscale. The current state of research on the impact of crystal size on the phase stability of the martensite is reviewed summarizing experimental results of various nanostructured martensitic materials and discussing the corresponding theoretical approaches. The review outlines the effects of crystal size on the complex morphology of the martensite, leading to interface structures not encountered in coarse-grained bulk materials. The unique shape-memory properties of martensitic materials can persist even at the nanoscale. Nanocrystalline martensitic materials can be processed to obtain tailored functional properties in combination with enhanced strength. Structural changes of metallic nanowires are similar to those arising by martensitic phase transformations and also can lead to shape-memory effects, as predicted by atomistic simulations.
Introduction Martensitic materials show a firstorder solid-to-solid phase transformation and unique functional properties that occur by cooperative movements of the atoms.1–3 The transformation from a hightemperature phase (the austenite) to a low-temperature phase (the martensite) can lead to shape-memory effects, since large reversible transformation strains are coupled to and thus controllable by temperature and external fields, such as stresses or magnetic fields. Martensitic materials can “remember” their initial shape and, once deformed, recover the deformation (e.g., upon heating). Shapememory materials are applied in many areas: medical devices,4,5 joints and damping devices,6,7 and, with increasing inter814
est, for example, from the automotive and aerospace industry, actuators, and sensors.8–10 The shape deformation of martensitic materials can yield extremely high work output per cycle and unit volume, which makes them favorable candidates for actuators for small-scale applications.11–14 Similarly, downscaling the grain size has the potential to obtain bulk martensitic materials with an optimized combination of the shape-memory effect and high mechanical strength.15 However, the shape-memory behavior of microcrystalline and nanocrystalline martensitic materials might differ substantially from that observed in their more coarsegrained counterparts. The transformation to martensite can even be completely sup-
pressed if the crystal size is less than a certain limit, typically in the range of several tens of nanometers. Therefore, knowledge of the impact of crystal size on martensitic phase transformations is crucial for engineering purposes. Studies of the physical mechanisms of the size dependence of martensitic phase transformations are challenging, sin
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