In-situ Neutron Diffraction Study to Understand the Microscopic Origin of Deformation in High-entropy Alloys

Description

High-entropy alloys refer to multi-element solid-solution material in equiatomic ratio.In spite of the complex chemistry, high-entropy alloy can crystallize into an incrediblysimple lattice, e.g., face centered cubic (fcc). High-entropy alloys exhibit large tensileductility. For example, for FeCoNiCrNi alloy, the room temperature ductility reaches 40-50%. Studies have shown that at small strains, the plasticity in high-entropy alloys ismediated by dislocations. At large strains, however, an unusual strain hardening isobserved: a rapidly decreasing hardening rate at small strains followed by an increase-decreasein hardening rate at large strains. Even more remarkably, the tensile ductilityfurther increases at low temperatures. For FeCoNiCrNi, when the test temperaturedecreases from 293 K to 77 K, the tensile ductility increases from about 40% to morethan 70%. Nanotwinning has been proposed as an additional deformation mechanism,with the main evidence coming from TEM analysis of samples taken from interruptedtests. Aside from twinning, other deformation mechanisms, e.g., stress-inducedmartensitic phase transformation, may also play a role. We propose to conduct in-situneutron diffraction study to elucidate the unusual hardening mechanisms in high-entropyalloys. In-situ neutron diffraction is a powerful tool for study of mechanicalbehaviors as well as phase transformation in materials. Neutrons are highlypenetrating, so it is particular well suited for study of bulk samples such as tensile testbars. In-situ loading experiments have made major contributions to our understandingof deformation in polycrystalline metals and in composite materials. The proposed studywill focus on measurements at cryogenic temperatures and at large plastic strains. Theneutron diffraction results will be corroborated with TEM analysis. Together, theseexperimental data will pinpoint the unusual deformation mechanisms in high-entropyalloys. Fundamental knowledge obtained from the proposed study will go a long waytowards the design of new structure materials with superior mechanical properties.?