The mechanism of multi-directional Invar effect in a cross-rolling TiNb alloy

In this work, a binary metastable β-type Ti-41 Nb (wt. %) with a multi-directional Invar effect (i.e., near-zero thermal expansion) was fabricated through the cross-rolling (Cro-R), with an aim to elucidate the relationship between martensitic phase transformation and multi-directional Invar effect. It was found that the Cro-R TiNb alloy shows the Invar effect along the transverse direction (TD) and rolling direction (RD) across a wide temperature range (∼180 K), and the coefficient of thermal expansion (CTE) along the TD and RD are αTD = +0.64 × 10−6 K−1 and αRD = −0.016 × 10−6 K−1, respectively. Further experimental results show that the randomly oriented β parent phase and α″ martensitic phase are distributed in the tiny grains with a high density of dislocations. This high density of dislocations can effectively impede martensitic phase transformation from β to α″ that appears during the subsequent cooling, resulting in the martensitic phase transformation occurring over a wide temperature range. In such cases, the multi-directional Invar effect was achieved via the compensation between the negative thermal expansion (NTE) caused by martensitic phase transformation and the positive thermal expansion (PTE) due to the TiNb matrix lattice vibration during cooling. These experimental results act as a foundation for the design and development of the metastable β-type Ti alloy with a multi-directional Invar effect.