Dependence of functional degradation on crystallographic orientation in NiTi shape memory alloys aged under stress

Most shape memory applications require anisotropic properties; hence the materials used for sensors and actuators display properties like shape memory or superelastic strains that are usually optimized along a certain direction. It is well known that stress-free aging of near equiatomic NiTi single crystals improves microstructural stability under cyclic loading conditions. The present study shows that aging under compressive stress applied along the [111] orientation results in the formation of precipitates perpendicular to the loading axis, which further improves fatigue resistance. So far, the dependence of the functional degradation on the crystallographic orientation has only been studied after stress-free aging or after aging under compression followed by cyclic loading along the same crystallographic direction. Therefore, the scope of this study was to investigate the dependence of superelasticity on crystallographic orientation for different angles between the normal to the habit plane of the precipitates and the loading axis. It is shown that the functional properties can be controlled by aging of the single crystals in [111] orientation under stress. It was also observed that after aging under compression, functional degradation resistance is strongly dependent on the crystallographic orientation during cyclic loading. The effect of different superimposed stress modes and the influence of crystallographic orientation on the microstructural mechanisms that govern functional degradation as well as the ramifications on the design of NiTi nanocomposites with an optimized microstructure are discussed. (C) 2012 Elsevier B.V. All rights reserved.