Materials Science Forum, vols. 386-388, pp. 597-602 (2002). 1 Bulk Shape Memory NiTi with Refined Grain Size Synthesized by Mechanical Alloying Wendy C. Crone 1 , Alief N. Yahya 1 , and John H. Perepezko 2 1 Department of Engineering Physics, University of Wisconsin, Madison, WI 53706, USA 2 Department of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706, USA Keywords: NiTi, shape memory, pseudoelasticity, metal-metal composite Abstract. Shape memory alloys (SMAs) constitute a unique class of materials that undergo a reversible phase transformation allowing the material to display dramatic stress-induced and temperature-induced deformations which are recoverable. Nickel Titanium (NiTi) SMA has be synthesized with refined grain size by a mechanical alloying process. The SMA is produced through a cold rolling fabrication strategy that yields nanocrystalline product structures. By starting with a stacked sandwich array of individual alloy components and subjecting this sample arrangement to repeated rolling and folding, a metal-metal multilayer composite is created. After the layer thickness is reduced to the nanoscale by multiple cold rolling and folding passes, an alloying and intermediate phase formation reaction is initiated. X-ray diffraction profiles confirm that the primary phase present is intermetallic NiTi phase, which is the phase that displays shape memory behavior. The presence of superelastic behavior and the shape memory effect in the product material is established by mechanical testing. At this refined grain size, the hysteresis stress observed is exceptionally small. Introduction During repeated plastic deformation as in ball milling or cold rolling, a concomitant refinement of grain size is observed. Recently, a new variation of the strategy has been developed which can yield nanocrystalline product structures [1-3]. By starting with a stacked sandwich array of elemental material and subjecting this sample arrangement to repeated rolling and folding, the resulting layers may be readily reduced to the nanometer size scale in bulk form before initiating an alloying and intermediate phase formation reaction [1-4]. One goal of the current research is to assess the suitability of a multilayer cold rolling effort for the preparation of Ni-Ti laminate composites with nanoscale layer thickness. If alloys with suitably refined layered thickness can be produced with this method, the composite material can be further processed to produce an alloyed material with refined grain structure. Similar preparation methods have been employed on elemental foils [5] and composite powders [6] of nickel and titanium, but the compositions investigated were well away from equiatomic NiTi and thus these materials did not display shape memory behavior. Shape memory alloys constitute a unique class of materials that have already proven to have wide ranging applications in industries ranging from aerospace to biomedical. These materials have a crystallographic structure that can change reversibly and reproducibly, allowing the material to display dramatic stress-induced and temperature-induced recoverable deformations. The behavior of NiTi SMA is governed by a phase transformation between austenite and martensite crystal structures. Transformation between the austenite (B2) and martensite (B19’) phases can be produced by temperature cycling between the high temperature austenite phase and the low temperature martensite phase (shape memory effect), or loading the material to favor the high strain martensite phase or unloading to favor the low strain austenite phase (superelasticity).