Abstract:

Metastable β titanium alloys constitute a promising family of alloys due to their high specific strength, high damage tolerance, workability, hardenability as well as corrosion resistance that has attracted great attention worldwide. However, their deformation mechanisms still remain controversial, and vary from dislocation slip to mechanical twinning and deformation induced phase transformations, in a way that is strongly related to the stability of the β phase.

A Ti-7Mo-3Cr-3Nb-3Al (Ti-7333) metastable β titanium alloy, recently developed for high strength and toughness applications, was selected for this study. This alloy has a molybdenum equivalent of 9.64 located at the boundary of the Bo-Md phase map. Initially, dog-bone shaped specimens were cut off from Ti-7333 forging bar followed by heat treatment and water quenching to obtain a single β phase microstructure. Subsequently, after grinding and polishing, the specimens were subjected to electron backscatter diffraction (EBSD) analysis. Then, tensile tests were carried out up to different plastic strains: just after yielding, 3% and 6%, respectively. After deformation, post EBSD, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis as well as EBSD-assisted twin traces analysis were applied to reveal the operative deformation mechanisms.

Both deformation induced martensitic transformation and {332} <113> mechanical twinning have been observed during tension. Two types of stress induced martensitic (SIM) α″ laths were found in the deformed matrix, corresponding to either {110}α″ twinned structure or pure α″ band. The fraction and size of martensite, as well as {332}β twins, increase with applied strain, together with secondary martensitic transformation and secondary {112}<111> twinning. The {332}β twinning happens in a way either by the assist of SIM α″or by the reversion of {130}<310> α″ twins. The analysis of the twin traces reveals that most of the twins activated at low plastic strain follow Schmid law, while non-Schmid factors dominate the twin variant selection at large strains, presumably due to triaxial effects at grain boundaries.

Keywords: Metastable β titanium alloy, mechanical properties, deformation induced martensitic transformation, {332} <113> twinning, twin traces analysis