Engineered colossal linear thermal expansion in nanocrystalline NiTi micropillars by stress

Materials with unusual large negative and positive thermal expansions are attractive for applications in high-precision actuation and thermal expansion compensation. So far, such unusual property has been reported in a variety of framework materials and polymers but it remains challenging to obtain in metals. Here, we report a colossal linear thermal expansion in single-phase nanocrystalline NiTi ferroelastic alloy under uniaxial compressive stress. By microscale isothermal severe plastic deformation, we fabricated fully austenitic and martensitic NiTi nanocrystalline micropillars that respectively possess high yield stresses of 2.65 GPa and 2.23 GPa and strong negative, and positive temperature dependence of Young′s modulus (dE/dT). We demonstrate that due to the large dE/dT, the coefficient of linear thermal expansion (CTE) of the austenitic and martensitic NiTi micropillars can be tailored to values of +106×10−6 K–1 and -88×10−6 K–1 by applying 2.5 GPa and 2.0 GPa compressive stress. Such tailored CTEs are approximately 10 times larger than the typical value (∼10×10−6 K–1) of metallic materials. Our work provides a mechanical route to obtain colossal positive and negative thermal expansion in single-phase nanostructured ferroelastic alloys at microscale. © 2023 Acta Materialia Inc.