Enhanced dynamic-mechanical property and microstructural mechanism of a FeCoNiCrMo0.2 high-entropy alloy fabricated using powder bed fusion

Abstract

Powder bed fusion (PBF), along with subsequent heat treatment, plays a crucial role in enhancing the impact toughness of FeCoNiCrMo0.2 high-entropy alloys (HEAs) and expanding their potential applications in field for high-speed deformation. In this study, the dynamic-mechanical properties and microstructure of the as-built PBF–FeCoNiCrMo0.2 HEAs and quenched PBF–FeCoNiCrMo0.2 HEAs heated at 600–750 °C for 8 h were investigated. As the heating temperature increases, the columnar grains and cellular structures undergo coarsening and the dislocation density gradually decreases. Moreover, higher heating temperatures facilitate the precipitation of Mo-rich second phases. This occurrence can be primarily attributed to the segregation of Mo at the boundaries of the as-built specimens. Consequently, in the quenched specimens, the μ phases are predominantly distributed along the boundaries of the cellular structures. In the quenched Q700 specimen (heated at 700 °C for 8h), the size and volume fraction of the μ phases are measured to be 63.4 nm and 3.59%, respectively. Additionally, under the same condition, the Q700 specimens also exhibited a relatively slight increase in the size of the cellular structures. The impact energy absorption, dynamic yield strength, and dynamic compressive strength of the Q700 quenched specimens were found to be 210.1 MJ/m3, 1066 MPa, and 1649 MPa at a strain rate of 1840 s−1. These values represent a dramatic improvement of 38.1%, 52.7%, and 14.7% higher in comparison to those of the as-built specimens. Under impact deformation, the presence of the μ phases plays a significant role in impeding the movement of dislocations by acting as a pinning agent for the boundaries of the cellular structures, thereby enhancing the strength. Simultaneously, the cellular structures were significantly elongated to form the deformation bands and to coordinate the impact deformation, leading to good impact energy absorption. In combination, the synergistic strengthening and toughening effect of the μ phases and the elongated cellular structures contribute to a remarkable improvement in the impact toughness of the PBF-HEAs.