Anisotropic Suppression of Martensitic Transformation in Precipitation-Hardened NiTiHf High-Temperature Shape Memory Alloys

Abstract

The reversible and diffusionless martensitic transformation of shape memory alloys (SMAs) has spurred their development for shape memory-based devices such as nano- and micro-electromechanical systems. However, when the size of a SMA is reduced to the scale of a few hundred nanometers, its shape memory effect becomes diminished and can eventually be suppressed. To investigate the microscopic origin of this behavior, we have characterized the thermally-induced martensitic transformation in precipitation-hardened NiTiHf high-temperature SMAs, using four-dimensional scanning transmission electron microscopy and in situ heating experiments. We show a distinct anisotropic suppression of the martensitic phase transformation, where B19′ martensite is successfully transformed into B2 austenite, while the reverse transformation is completely suppressed. Quantitative phase and strain analysis reveal strain accumulation in the austenite matrix, specifically a unique buildup of shear strain surrounding precipitates embedded in the matrix phase. This result indicates that the suppression of the martensitic transformation in precipitation-hardened SMAs is not solely due to extrinsic effects like the buildup of non-transforming oxides on the alloy surface, and that there is an additional intrinsic mechanism inhibiting the full shape memory transformation. Our findings encourage future research on the feasibility of scalable shape memory devices in the sub-hundred nanometer regime.