Effect of cooling rate on microstructure and mechanical properties of AlCrFe2Ni2 medium entropy alloy fabricated by laser powder bed fusion

Abstract

The mechanical properties and serviceability of the alloy fabricated by laser powder bed fusion (L-PBF) additive manufacturing (AM) strongly depend on the phase composition and microstructure, which are affected by the thermal condition of the process, such us cooling rate. In this study, AlCrFe2Ni2 medium entropy alloy (MEA) was fabricated by L-PBF under varying cooling rate via changing process parameters. The phase transformation, microstructure and mechanical properties were experimentally investigated using X-ray diffraction (XRD), scanning transmission electron microscopy (TEM), scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The cooling rate was numerically calculated using a well-tested multi-physic thermal fluid model. The results showed that cooling rate had a significant effect on phase composition, microstructure and mechanical properties. The as-built MEA with lower cooling rate showed a nano-size structure composed of BCC and ordered B2 phases and a higher yield strength of 2055 MPa. While for the higher cooling rate, the as-built MEA showed a homogeneous structure consisted of single ordered B2 phase and a lower yield strength of 1544 MPa. The threshold value of cooling rate for phase transformation was between 4.9 × 106 K/s and 6.0 × 106 K/s. Through the strengthening mechanism analysis, it was crucial to achieve the nano-size dual-phase microstructure by manipulating cooling for obtaininng desirable mechanical properties in L-PBF AMed AlCrFe2Ni2 MEA.