Intermediate temperature embrittlement in a precipitation-hardened high-entropy alloy : the role of heterogeneous strain distribution and environmentally assisted intergranular damage

Abstract

Severe intergranular embrittlement has been found in a wide range of polycrystalline metallic materials in the intermediate temperature regime, setting severe limits on their engineering applications. In this study, we have systematically investigated the origin of such temperature-dependent premature tensile failure in a precipitation-hardened high-entropy alloy. We highlight the role of heterogeneous strain distribution and environmental attack in facilitating intergranular crack initiation and propagation at intermediate temperatures. The dislocation accumulation has been found in the vicinity of grain boundaries to accommodate thermally activated grain-boundary sliding. Grain boundaries with extensive dislocation pile-ups served as preferential sites for the initiation of voids and associated cracks. The growth and linkage of microcracks lead to the destructive tensile failure. More importantly, environmentally assisted grain-boundary damage plays a vital role in exacerbating the embrittlement. In contrast to the laboratory air atmosphere, tensile testing in an inert argon atmosphere protected the specimens from the environmental damage and helped to recover the tensile ductility at intermediate temperatures. Electron microscopy analyses have uncovered unique deformation substructures, in which both Orowan looping and particle shearing took place. These findings provide a fundamental understanding of the temperature-dependent deformation behaviors of polycrystalline high-entropy alloys, and this understanding is of a great significance for developing high-performance structural materials for elevated temperature applications.