Operation conditions in microelectronics, sustainable mobility, space exploration and high-precision optics increasingly involve large temperature swings and stricter dimensional tolerances, necessitating to master thermal expansion. However, customizing thermal expansion is difficult, especially for metals and alloys. As a result, common engineering alloys increase in size when heated. First, because thermal expansion of conventional alloys is largely insensitive to alloying. Second, because their lattice expansion changes only little or not at all with orientation, ruling out crystallographic texture to adjust their expansion behaviour. Even widely used Fe-Ni Invar alloys exhibit only a narrow range of positive (albeit small) expansion rates. Exceptions displaying negative thermal expansion exist, however most are non-metallic, often disappointingly brittle compounds, where introducing texture has no effect.
Recently, several groups demonstrated a new principle that enables tailoring of the macroscopic thermal expansion of a large class of alloys by simple thermomechanical processing. The materials’ common feature are martensitic phases exhibiting strongly anisotropic thermal expansion. For a single alloy composition, controlled deformation permits tailoring the macroscale polycrystal thermal expansion across a broad range of expansion coefficients by introducing texture. Among these materials, Ti alloys forming orthorhombic α'' phases stand out for their giant contraction and expansion rates, particularly good deformability, and the strong effect of alloying on their expansion behaviour.
In this presentation, we will review early and recent discoveries from the last two decades, and we will give an overview of current developments. A novel multi-pronged strategy, incorporating phase composition, texture, and volume fraction simultaneously, promises to tremendously enlarge the design space, providing ample opportunities for future research and innovative applications.