Plastic deformation throughout strain-induced phase transformation in additively manufactured maraging steels

Abstract

A comprehensive study was intended to show the microstructural features of additively manufactured (AM) 18Ni-300 maraging steel. Uniaxial tensile tests were conducted on specimens built using laser powder bed fusion (LPBF) technique for two different powder layer thicknesses. The specimens were built to have the lowest possible porosity, and tensile tests showed two stages of strain hardening. In stage I, the dislocation density increased, leading to a positive strain hardening rate. A negative strain hardening rate due to the necking effect was then followed in stage II. X-ray diffraction (XRD) analyses revealed a phase transformation through the deformation. Various analyses via electron backscattered diffraction (EBSD) technique was then conducted with large scans over three different zones representing undeformed, deformed, and severely deformed close to the fracture area. The pole figures and orientation distribution functions (ODF) revealed a texture evolving through the deformation process in agreement with the kernel average misorientation (KAM) and grain boundary maps. Transmission electron microscopy (TEM) was used to detect the inclusions and segregated alloying elements adjacent to the fractured surfaces. Results indicated that the deformation led to diminishing the austenite (γ) phase, while the transformed austenite sourced the high dislocation density area at cell boundaries.