Microstructure evolution in 3D-printed CoCrFeNi(AlTi)xwt% (x=0, 2.5, 5 and 7.5) high entropy alloys

Applying 3D printing technology to high entropy alloys (HEAs) offers new potential for materials design and optimization. However, there is a lack of deep understanding of the microstructure evolution with the element addition to HEAs in 3D printing. How the element addition influences the final microstructure of materials during 3D printing still remains a puzzle. In this study, we investigated the microstructure evolution of CoCrFeNi(AlTi)_xwt% (x = 0, 2.5, 5 and 7.5) HEAs fabricated by selective laser melting (SLM). We selected Al and Ti atoms as additions in the CoCrFeNi matrix based on their strong negative mixing enthalpy with Ni elements and larger atom radius than other matrix elements. It has been found that the Al and Ti addition leads to the formation of BCC and B2 precipitates, resulting in different mechanical properties. Yield strength (YS) of the HEAs exhibits a distinct increase from 512.64 MPa to 901.72 MPa at the cost of the ductility. The relationship between mechanical properties and microstructure evolution with Al and Ti additions has been elucidated. It has been found that the formation of a dislocation network in our HEAs serves as nucleation sites to benefit the precipitation. Besides, this network provides a rapid channel for atoms to diffuse during the thermal cycle of 3D printing, which also promotes the formation of precipitates. This research provides valuable insights into the modification of microstructure through SLM, contributing to the development of materials design and optimization in 3D printing.