Nano-precipitation leading to linear zero thermal expansion over a wide temperature range in Ti22Nb

Abstract

Certain cold-worked martensitic Ti-alloys exhibit tunable linear thermal expansion along a specific sample direction. These intriguing properties are related to the anisotropic thermal expansion of the orthorhombic αʺ martensite. However, the tailored expansion obtained in this way is limited to the single-phase martensitic temperature field, below the reverse transformation of martensite into austenite. Here, we demonstrate that the orthorhombic αʺiso formed by diffusion can also regulate the thermal expansion using a prototypical Ti22Nb alloy. More importantly, the temperature range in which the tailored linear thermal expansion occurs is enormously broadened relative to the cold-worked martensitic alloy. We clarify the formation path of nano-sized αʺiso during thermal cycling and resolve the crystallographic relationship with the parent phase. These findings provide new routes for designing low/negative thermal expansion Ti-alloys with large operating temperature ranges.