Beneficial effects of deep cryogenic treatment on mechanical properties of additively manufactured high entropy alloy: cyclic vs single cryogenic cooling

Abstract

• Cyclic deep cryogenic treatment was utilized to homogenize the residual stress and tune the microstructure of a high entropy alloy fabricated by laser melting deposition, in order to improve its overall mechanical properties. • Cyclic deep cryogenic treatment can induce an increased compressive residual stress and introduce various types of reinforcing defect microstructures into a high entropy alloy, including dense dislocations, intersecting twins, transformed HCP phase and nanograins, thus effectively overcoming the strength-ductility trade-off. • Compared to single-step DCT, cyclic DCT can reduce the residual stress gradients in the LMD-built sample, relieving or removing the tensile residual stress at the top surface. • Transformation to HCP phase can occur during the cyclic DCT process due to the cyclic reverting of the thermal stress to a high level on each cryogenic cooling, which promotes twinning on secondary habit planes and more frequent faulting. Additively manufactured (AM) metallic materials commonly possess substantial tensile surface residual stress, which is detrimental to the load-bearing service behavior. Recently, we demonstrated that deep cryogenic treatment (DCT) is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy (HEA) samples fabricated by laser melting deposition (LMD), by introducing high compressive residual stress and deformation microstructures without destroying the AM shape. However, carrying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process, which are undesirable as the mechanical properties will not be homogeneous within the sample. In this work, we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated CoCrFeMnNi HEA, and improve tensile strength and ductility, compared with single-step DCT of the same cryogenic soaking duration. Under cyclic DCT, the thermal stress is re-elevated to a high value at each cryogenic cooling step, leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation, compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time. The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT. The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.