Refinement of α′ Martensite by Oxygen in Selective Laser Melted Ti–6Al–4V

Oxygen is crucial in influencing the microstructure evolution of selective laser melted (SLMed) Ti–6Al–4V, significantly impacting its applicability in various sectors. Therefore, this study investigates the influnce of oxygen on microstructure evolution, particularly α′ martensite transformation and refinement mechanisms. Four alloys, Ti–6Al–4V–xO (x = 0.11, 0.16, 0.21, and 0.25 wt%), were fabricated by the SLM process. The martensite start temperature (M_s) of Ti–6Al–4V, as evaluated by computation, is 656.8 °C, and oxygen was found to increase the M_s by about 10 °C per 0.1 wt%. The SLMed alloy samples exhibit [001]_β growth texture along the build direction. Crystallographic analysis of martensite morphology suggests internal twinning on \(\left\{ {10\overline{1}1} \right\}\) $\left\{10\overline{1}1\right\}$ planes as the lattice invariant strain, which becomes more predominant with increasing oxygen content. Refinement of α′ martensite plates by oxygen is due to increased lattice distortion, reduced prior β grain size, and oxygen segregation to β grain boundaries. Our findings contribute to improving the understanding of the effect of oxygen on the transformation mechanism of α′ martensite during SLM of Ti–6Al–4V.