Accelerated martensite decomposition in additively manufactured TA15 titanium alloy under thermal cycling

Laser powder-bed fusion (L-PBF) enables the fabrication of complex metallic parts at reduced lead time and waste, however, the complex dynamic thermal profiles inherent to the L-PBF process makes it difficult to achieve high mechanical performance by real-time microstructure control during the fabrication of metallic components. In this study, we designed a series of specifically thermal cycling treatments imposed by laser scanning on fully martensitic L-PBF TA15 titanium alloy samples, which not only closely resemble the thermal history experienced by the samples during the L-PBF process, but also could accelerate martensitic decomposition in just a few minutes. Depending on the thermal environment developed during laser scanning, two transformation pathways are identified. These include accelerated direct decomposition of martensite via α′→α+β when thermal cycling largely in the α+β phase field below the β transus, and indirect transformation via α′→β→α+β while thermal cycling with peak temperatures well above the β transus for a prolonged duration and subsequent slow cooling. The accelerated martensite decomposition is proposed to stem from rapid accumulation of β-stabilizers at twin boundaries and the interfaces of α′ martensite laths.