Large tunable thermal expansion in ferroelastic alloys by stress

Metallic materials inherently possess small coefficients of thermal expansion (CTE). Engineering the CTEs of metals to achieve large values is demanded in many applications but has proved challenging. Here, we propose a strategy to obtain large CTEs in ferroelastically-transformable alloys through uniaxial stress control by utilizing the alloys' inherenty strong temperature dependence of Young's modulus (dE/dT). In nanostructured NiTi alloy with large elastic limit and large dE/dT, we tune the CTE from a large negative value of -53.2×10−6 K−1 to a large positive value of +40.4×10−6 K−1 by applying 1 GPa tensile stress. Such stress effect on CTE is 40 times those of non-ferroelastic metals and the created tunable CTE span is one order of magnitude larger than the typical span (10×10−6 K−1) obtained via the existing chemical modification and hydrostatic pressure control. Through in-situ neutron diffraction experiment and theoretical calculation based on Reuss approximation, we show that the observed large thermal expansion under stress mainly comes from the elastic strain change with temperature due to the large dE/dT of the NiTi and can be generally achieved in ferroelastic alloys. In contrast to the stress-insensitive thermal expansion of most metals, the unique stress-sensitive thermal expansion of ferroelastic alloys may enable new solutions to problems arising from thermal expansion problems.