Achieving superior tensile strength of CoCrFeNiTi0.3 high-entropy alloy via in-situ laser powder bed fusion of CoCrFeNi and Ti

In this study, CoCrFeNiTi0.3 high-entropy alloy (HEA) was designed and prepared via the in-situ laser powder bed fusion (LPBF) of CoCrFeNi and Ti powders to improve the undesirable tensile strength of CoCrFeNi HEA. With the introduction of Ti, semi-elliptic melt pool characteristics became blurred with the concentration of Ti at melt pool boundaries, which was correlated with a Marangoni-convection-driven chaotic flow and insufficient diffusion of Ti during the rapid melting/solidification process of LPBF. During the solidification process, Ti acted as nucleation sites and triggered a columnar-to-equiaxed transition of grains in the printed CoCrFeNiTi0.3 HEA. Partial diffused Ti combined with Ni to form nanoscale Ni3Ti precipitates because Ti and Ni had the most negative mixing enthalpy compared with Co, Cr, and Fe. The nanoscale Ni3Ti precipitates were located at the sub-grains and showed a network structure. This was associated with the sub-grain boundary segregation driven by severe lattice distortion. The yield strength of the Ti-modified CoCrFeNi was enhanced from 509 MPa to 796 MPa, which suggested a 56.4% increase obtained by adding a small amount of Ti. The theoretical calculation was deduced to validate the strengthening mechanism mainly stemming from dislocation hardening and precipitation hardening. This study is anticipated to provide insights into the enhancement in the mechanical performance of HEA through composition modification via the in-situ alloying during LPBF.