B. S. Majumdar, New Mexico Institute of Mining and Technology, Socorro, NM; B. Ye, Northwestern University, Evanston, IL
We report on the near-reversible strain hysteresis during thermal cycling of a polycrystalline NiTi shape memory alloy at a constant stress that is below the yield strength of the martensite. Mechanical strain hysteresis loops are observed, whose range is proportional to the applied stress, and thereby may be utilized to fabricate novel intelligent structures. In situ neutron diffraction experiments are utilized to show that the strain hysteresis occurs due to the establishment of a stress-dependent crystallographic texture of the martensite during the first cool down from austenite, and thereafter repeated during thermal cycling under the same load. This texture is found to depend on the stress during thermal cycling experiments and may be considered as a texture memory basis for the mechanical strain behavior. A strain-pole map is derived from Bain transformation calculations, and is shown to explain the observed in situ texture during thermal cycling. The strain-pole methodology is shown to work with similar martensitic transformations in other material systems, and thereby provides a generic basis for bias texturing other intelligent material systems.
