Heterogeneous Microstructure in Laser-Processed Metastable Ti25Nb Alloy

Abstract The high thermal gradient and solidification velocity associated with the laser powder bed fusion process spurs formation of diverse microstructures in additively manufactured materials. This study focused on the phase composition observed in the microstructure of a laser-processed metastable titanium–niobium alloy. Through transmission electron microscopy experiments, we reveal the microstructures with several metastable phase, among which is a novel orthorhombic phase found in Nb-lean regions that is fundamentally different from the expected $${\alpha }^{{\prime}{\prime}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>′</mml:mo> <mml:mo>′</mml:mo> </mml:mrow> </mml:msup> </mml:math> orthorhombic phase. Second is the $${O}{\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>O</mml:mi> <mml:mo>′</mml:mo> </mml:mrow> </mml:math> phase and the $${O}{\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>O</mml:mi> <mml:mo>′</mml:mo> </mml:mrow> </mml:math> variant selection phenomenon in laser-processed metastable β-Ti alloys. Microstructural features were found to be highly sensitive to the processing history. We further examine the mechanisms behind these phase formations and discuss how these features can influence the properties of the alloy.