Society for Experimental Mechanics, 2002 SEM Annual Conference Proceedings, Milwaukee, WI, 2002. Mechanical Behavior of Nanostructured Melt Spun NiTi Shape Memory Alloy Dabin Wu æ , Wendy C. Crone ¥ § æ and John Perepezko F æ æ Materials Science Program, ¥ Department of Engineering Physics, § Engineering Mechanics Program, F Department of Materials Science and Engineering University of Wisconsin-Madison, WI 53706 USA Abstract As the size of grains in polycrystalline materials is reduced to nanoscale, the properties of these materials will be dominated by grain boundaries and surface effects. Nanostructured NiTi Shape memory alloys (SMAs) were fabricated by cold-rolling melt-spun near equatomic NiTi. SMAs represent a unique class of materials that undergo a reversible phase transformation allowing these materials to display dramatic stress-induced and temperature-induced deformation that is recoverable. Changes in shape memory effect and pseudoelastic behavior are expected as the grain size is reduced to the nanoscale regime. Mechanical behaviors of both as-received and cold-rolled melt-spun ribbons were investigated. Shape memory behavior was observed in the melt-spun ribbons, and pseudoelasitc behavior was observed after the melt-spun ribbons were subjected to cold rolling. 1. Introduction Nanostructured materials (NsM) have attracted considerable attention in recent years [1] because new materials behavior is expected as surface effects and grain boundaries play a more important role. A NsM may be categorized according to its morphology: layer-shaped crystallites, rod-shaped crystallines (with layer thickness or rod diameters in the order of nm), and equiaxed nm-sized crystallites. Generally, a NsM consists structurally of the following two components: the crystallites and the boundary regions. In the boundary regions, the average atomic density and the coordination between nearest neighbor atoms deviate from the ones in the crystallites. The presence of these two structural components (crystals and boundaries), with comparable volume fractions and having typical crystal size of a few nanometers, plays a crucial role in determining the properties of a NsM. Thus, the properties will depend on the size of the crystalline regions and on the atomic structure of the solid characterized by the average atomic density and the coordination between nearest neighbors. This research explores the fabrication of a NsM with shape memory behavior. Shape memory alloys (SMAs) represent a unique class of materials that undergo a reversible phase transformation allowing these materials to display dramatic stress-induced and temperature-induced deformation that is recoverable. These materials are still being explored as functional materials in a variety of aerospace, biomechanical, and microelectronics industries [2-4]. Among the known shape memory alloys, NiTi is the most commonly used because of its excellent mechanical properties, corrosion resistance and biocompatibility. A variety of methods have been attempted in order to create finer-grained shape memory alloys. These include ultrarapid melt quenching, ball milling followed by powder metallurgy, inclusion of microalloying elements, recrystallization of thin films deposited by pulsed laser deposition, melt spinning, and plastic deformation followed by recrystallization. The majority of these studies have been successful at producing micron scale grain sizes, however virtually all of the nanoscale grain size NiTi reported in the literature has been in the form of thin films prepared by physical vapor deposition. Unfortunately work on thin films has shown the correlation between shape memory behavior and grain size is confounded with the grain-size-to-sample thickness ratio [5]. Only a few reports concerning synthesis and characterization of bulk shape memory alloys having nanoscale grain size are available in the literature [6-7]. Xu and Thandhani [6] used ball milling to produce 44-58 nm grain size NiTi and showed that this decrease in grain size increased the temperature at which Martensitic transformation starts (Ms) as compared to micron scale powder. They proposed that this was a result of increased internal stresses arising from greater anisotropy in the material, rather than and increase in the local elastic energy between Martensites and grain boundaries. The research reported here explores new methods for producing nanostructured NiTi shape memory alloy material in bulk form and investigates the influence of the nanostructured nature of the material on its mechanical behavior. We have undertaken mechanical fabrication approaches to creating nanostructured shape memory alloys, including melt-spinning and cold rolling techniques. 2. Experimental Procedures 2.1 Melt-spinning Rapid solidification technique (RST) is an important method for producing metals with improved mechanical and/or physical properties. This technique employs a very high cooling rate (up to 10 6 ˚C/s). In general, such a high cooling rate has the advantage of refinement of grain sizes [8]. One