Fe-36Ni Invar alloy with enhanced strength and low thermal expansion at cryogenic temperatures via laser powder bed fusion

Abstract

The microstructure, thermal expansion coefficient (CTE), and mechanical properties at cryogenic and room temperatures were investigated for a Fe-36Ni alloy fabricated by laser powder bed fusion (LPBF) additive manufacturing, and the effects of the LPBF-induced microstructure on the material properties and deformation behavior under ambient and cryogenic conditions were discussed. The LPBF Fe-36Ni alloy exhibited a high relative density of approximately 98.7 % and consisted of a single austenitic phase. The LPBF alloy showed finer grain size and higher dislocation density than those produced by a conventional process (hereinafter referred to as “wrought”). In addition, dislocation cells and nanoscale in-situ oxide precipitates were observed within the grains. The LPBF Fe–36Ni invar alloy exhibited a low CTE (∼1.0 ppm/K) and high strength. The yield strength and ultimate tensile strength were 445 MPa and 511 MPa, respectively, at room temperature (RT), and 637 MPa and 839 MPa, respectively, at cryogenic temperature (77 K). Microstructural observation results after cryogenic tensile testing revealed that the wrought specimen contained more than 16 % martensitic phase formed via deformation-induced martensitic transformation (DIMT). In contrast, the LPBF specimen exhibited significantly less transformation, with the martensitic phase fraction reduced to less than one-fourth of that in the wrought specimen. The high mechanical strength of the LPBF Fe-36Ni alloy was attributed to the presence of nanoscale precipitates, while the suppression of DIMT was suggested to result from the inhibition of shear band development by dislocation cells and fine grains.