Inhibiting cracking and improving strength for additive manufactured Al CoCrFeNi high entropy alloy via changing crystal structure from BCC-to-FCC

Abstract

Hot cracking in laser powder bed fused (LPBF) AlxCoCrFeNi high entropy alloys (HEAs) plague the production of high strength parts. Here we show that the body-centered cubic (BCC) phase with very high hardness (∼670 HV) in LPBF printed AlxCoCrFeNi(x = 1, 0.7) HEAs causes cracking due to the plastic incompatibility between grain boundaries. To mitigate this, we propose a BCC to face-centered cubic (FCC) transformation strategy in the AlxCoCrFeNi HEAs to inhibit cracking and found that the Al0.5CoCrFeNi alloy (i.e., when x = 0.5) with a single FCC crystal structure possess low cracking sensitivity and well intergranular plastic compatibility, accompanied with decreased residual stress confirmed by using X-ray diffraction testing. The mechanical properties of the LPBF printed Al0.5CoCrFeNi HEA can be further enhanced by precipitation heat treatment. Scanning electron microscope/ transmission electron microscopy analysis show that nano-sized B2, L12 and σ phases precipitated from FCC Al0.5CoCrFeNi HEA when aged at 700 °C for 1 h. The ultimate tensile strength (UTS) increases from 899 MPa (as-print) to 1552 MPa (aged) with the elongation decreasing from 35.6 % (as-print) to 7.1 % (aged) due to the Orowan bypass mechanism of nano-precipitates. An increased aging temperature to 800–1000 ℃, decreases the UTS while increases the elongation, accompanied by the L12-to-B2 transformation and B2 phase coarsening. The deformation mechanism is mainly dominated by dislocations and stacking faults in the Al0.5CoCrFeNi HEA sample.