The influence of microstructural condition on the phase transformations in Ti-24Nb (at.%)

Abstract

Ti-Nb based alloys exhibit superelastic and shape memory properties over a wide range of temperatures, making them attractive for industrial applications. However, within the literature there are significant variations in the reported transformation behaviour for a given composition that remain unexplained. These variations are problematic to engineering design and limit the industrial uptake of these alloys. To investigate the source of these discrepancies, the transformation behaviour of Ti-24Nb (at.%) in two significantly different microstructural conditions, following cold rolling (CR) and following a subsequent solution heat treatment (ST), has been studied using in situ synchrotron diffraction data whilst undergoing a three step thermal cycle. When cooling from 350 to -196˚C, the CR condition material underwent both the β to αʺ and β to ω transformations, whilst only the β to ω transformation was observed in the ST condition material. These results show that the transformation behaviour of Ti-Nb alloys, in particular the β to αʺ transformation, is significantly influenced by prior processing and that theories of mutual exclusivity between αʺ and ω phase formation need revisiting. In addition, several features within the datasets were inconsistent with a solely thermally driven martensite formation mechanism and an alternative description that includes other factors is discussed.