Tailoring high-temperature mechanical properties of laser powder bed fusion Ti-6.5Al-2Zr-1Mo-1 V alloy via microstructure design

The microstructure of Ti-6.5Al-2Zr-1Mo-1 V (TA15) alloy fabricated by laser powder bed fusion (LPBF) is characterized by the ultrafine alpha ' martensite with a high density of dislocations and twins, which is distinct to that of conventionally forged TA15 alloy. The alpha ' martensite in the as-built alloy transforms to stable alpha phase when annealing below the beta transus, and meanwhile the dislocation density decreases and the alpha lamellae coarsen with increasing of annealing temperature. An excellent synergy of strength and ductility was achieved in the sample after designated heat treatments, which is better than that of the forged counterpart at both room temperature and 500 degrees C. In-situ tensile test at 500 degrees C demonstrated that the plasticity of as-built TA15 alloy was dominated by the formation of microscale shear bands (MSBs) between the alpha ' lamellae, which results in pronounced strain localization and limited tensile ductility. In the sample with coarsened alpha lamellae, it was found that the acti-vation of pyramidal < c + a > slip system at 500 degrees C effectively alleviated the strain localization and cracking tendency during mechanical loading, which is essential to achieve a desired balance of strength and ductility. These findings are helpful to promote the practical applications of LPBF near-alpha titanium alloys.