Microstructure evolution in the hypo-eutectic alloy Al0.75CrFeNi2.1 manufactured by laser powder bed fusion and subsequent annealing

Abstract

The hypo-eutectic medium entropy alloy Al0.75CrFeNi2.1 was processed by laser powder bed fusion (LPBF). The off-equilibrium solidification conditions prohibited coupled eutectic growth. Instead, the primary face centered cubic phase A1(FCC) solidified with a cellular morphology and the body centered, initially ordered B2(BCC) phase formed as a thin intercellular envelope. During post-build annealing an ultrafine quasi-lamellar pattern evolved following BCC growth and coarsening. The novel solid state transformation from cellular to lamellar morphology was attributed to a pronounced anisotropy of the FCC|BCC phase boundary energy. Microstructure evolution was also studied during continuous heating using in situ high-energy synchrotron X-ray diffraction (HEXRD) carried out at the beamline P07-HEMS of PETRA III (German Electron Synchrotron, DESY). The ultrafine and nano-scale features of the microstructure were quantitatively analyzed by atom probe tomography (APT) in the as-built condition and after isothermal annealing at 950 °C. The benefits of LPBF processing were discussed on the basis of mechanical properties measured by 3-point bending. The estimated tensile properties after annealing at 950 °C/6 h reached YS ≈ 860 MPa, UTS ≈1384 MPa with an elongation at fracture of ≈11%. Tensile properties in the as-built condition were comparable to martensitic steels.