A combination of microcompression experiments on single crystal micron-scaled pillars of NiTi of known orientations and in situ neutron diffraction during loading of the same NiTi but in bulk, polycrystalline form are carried out to understand the stress-induced transformation associated with superelasticity at reduced length scales. At the length scales investigated, there is evidence through this work of a fully reversible stress-induced transformation from B2 to B19′ NiTi that does not involve additional dislocation activity or irrecoverable strains. The orientation dependence of the elastic deformation of the B2 phase, the onset of its transformation to the B19′ phase, the gradient and the hysteresis in the stress-strain response during transformation, the elastic modulus of the stress-induced B19′ phase and the onset of plasticity are quantified and analyzed in these experiments by examining the crystallography of the B2 to B19′ transformation.
Bibliographical noteFunding Information:
This work was supported by NASA Fundamental Aeronautics Program, Supersonics Project (Grant No. NNX08AB51A), NSF (Grant No. CAREER DMR-0239512), and the DOE Center for Integrated Nanotechnologies (CINT). The authors are grateful to H. Li, P. Prakash, B. Clausen, D. W. Brown, and T. Sisneros for valuable experimental assistance. This work has benefited from the use of the Lujan Neutron Scattering Center at LANSCE, which is funded by the Office of Basic Energy Sciences (DOE). LANL is operated by Los Alamos National Security LLC under DOE Contract No. DE-AC52-06NA25396.