The martensitic phase transformation (MPT) is a lattice-distortive, virtually diffusionless and reversible structural change induced in certain metallic alloys either by a temperature variation or by applied external stresses. During the transformation, a new structural phase, called martensite, nucleates in the original austenitic matrix and grows. This MPT is the origin of technologically interesting mechanical properties, namely shape memory, superelasticity and high vibration damping capacity. Projects to integrate such Shape Memory Alloys (SMAs) in microsystems or functional materials for transport systems are the origin of extensive research in the development of SMA thin films and SMA-based composite materials.
Nickel-titanium shape memory metal is usually available in wire form, made by wire drawing. Because of the lack of ductility, it is much more difficult to fabricate the alloy in sheet form. On the other hand, both nickel and titanium are ductile in their pure states. An ingenious method of making large quantities of alloy-sheet is to make a multilayered composite of alternating plates of nickel and titanium, fabricate the composite into the final sheet form using conventional rolling technology, and then allow interdiffusion to create the nickel-titanium solid solution. The diffusion rates of nickel in titanium and titanium in nickel are different, so the process results in Kirkendall porosity. The details of the manufacturing process can be found in the section on the Kirkendall effect. The movie presented here shows the operation of a sheet of shape memory metal
MPG movie showing a shape-memory element in action.
Reference: D. Tomus, K. Tsuchiya, M. Inuzuka, M. Sasaki, D. Imai, T. Ohmori and M. Umemoto: Scripta Mater. Vol. 48 (2003) 489.
More information available on Materials Function Control Laboratory, Toyohashi University of Technology.
The movie has been provided for teaching purposes by Professor Koichi Tsuchiya of the Toyohashi University of Technology, Japan.
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