Investigating shape-memory alloys and composites with neutron-diffraction techniques

As the name suggests, the shape-memory effect refers to a phenomenon wherein a material when mechanically deformed and then heated “remembers” and returns to its preset shape. Associated with this behavior is the superelastic/ pseudoelastic effect whereby large and completely recoverable strains are generated (i.e., the strains are generated and recovered mechanically rather than thermally). Both of these effects are associated with “martensite” transformation, a first-order displacive transformation usually related to the hardening of steel. When a nickel-titanium (NiTi) shapememory alloy is mechanically loaded, a stress-induced cubic (austenite) phase to monoclinic (martensite) phase transformation can result in macroscopic strains as high as 8%. On unloading, the martensite becomes unstable and transforms back to austenite with a concomitant macroscopic strain recovery. This phenomenon is called the superelastic/pseudoelastic effect and finds application in, for example, mobile phone antennae, cardiovascular stents, and guidewires [11. By recording neutron-diffraction spectra during external loading, we can investigate this reversible stress-induced austenite-to-martensite transformation in situ. Such data can provide quantitative phase-specific information on the elastic strain, texture, and volume-fraction evolution. The basic principle of such measurements for strain determination involves using the lattice-plane spacing of grains within the crystal structure as internal strain gauges. For texture, the relative intensities of the diffraction peaks are considered, and for phase volume fraction, an integrated intensity of peaks corresponding to a specific phase is used.