Suppression and reactivation of transformation and twinning induced plasticity in laser powder bed fusion additively manufactured Ti-10V-2Fe-3Al

Abstract

Laser powder bed fusion (LBPF) was employed to fabricate a strain-transformable β-Ti alloy, Ti-10V-2Fe-3Al (wt%). While the alloy is known to exhibit transformation induced plasticity (TRIP), the as-fabricated alloy, under tensile loading, did not show the same TRIP effects, even though it exhibits the same β + ω microstructure. The repeated heating-cooling cycles experienced during the LBPF process leads to the early stages of rejection of solute elements (Fe, V, and Al), forming isothermal omega (ω) precipitates, which were captured via detailed investigations coupling transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). While these homogeneously distributed isothermal ω precipitates lead to a higher yield strength, the TRIP/TWIP effects within the β matrix were suppressed, leading to very low ductility and virtually no strain-hardenability. Interestingly, after a simple β-solution heat treatment followed by quenching, leading to a β + ω (athermal) microstructure, the TRIP/TWIP effects were reactivated in the same LPBF Ti-10 V-2Fe-3Al alloy. The alloy exhibited substantial recovery of tensile ductility and a very large strain hardening (tensile strength minus yield strength ~500 MPa), with a high average strain hardening rate ~15000. Such a very high strain hardening rate in case of LBPF processed Ti-10 V-2Fe-3Al, appears to arise from a rapid strain-induced transformation from β to α" at the early stages of plastic deformation, leading to a high-volume fraction of the martensitic phase, coupled with hierarchical twinning within the martensite plates.