Origin of zero and negative thermal expansion in severely-deformed superelastic NiTi alloy

We have investigated the physical origin of anomalous in-plane thermal expansion (TE) anisotropy leading to invar-like behavior and negative TE in nanostructured NiTi sheets manufactured via severe cold-rolling. The roles of grain size (GS), crystallographic texture, thermally-induced phase transformation, and intrinsic (lattice level) TE of austenite (B2) and martensite (B19') phases on the macroscopic TE behavior are addressed. It is shown that by controlling the cold-rolling thickness reduction and heat-treatment temperature the coefficient of thermal expansion (CTE) can be controlled in a wide range from positive (alpha similar to 2.1 x 10(-5) K-1) to negative (alpha similar to -1.1 x 10(-5) K-1) via in-plane anisotropy of TE. A very small CTE of alpha similar to -5.3 x 10(-7) K-1 (invar-like behavior) in a wide temperature window of 230 K (353-123 K) is obtained at an angle of 33.5 to the rolling direction (RD) of the severely cold-rolled sheet. TEM and XRD studies show that the microstructure underlying such anomalous TE behavior consists of a mixture of B2 nano-grains and retained/residual deformation-induced martensite and that the observed anomalous TE anisotropy is due to the intrinsic anisotropic TE of residual martensite. The invar-like behavior is the result of the cancellation of the positive TE of austenite phase with the negative TE of residual martensite along 33.5 to the RD. A simple rule of mixture model incorporating the intrinsic TE of B2 and B19' lattices and the texture coefficients of the sample is proposed which successfully captures the anomalous in-plane TE anisotropy. The discovery of high dimensional stability over a wide temperature window along with temperature insensitive non-hysteretic linear superelasticity of the severely-deformed NiTi opens up a new route for designing stable SMAs for applications in ragged environments. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.