Fundamental Investigation on High Entropy Alloy Deformation Mechanisms

Description

It is well known that trade-off between strength and ductility in metallic alloys is along-standing challenge for material scientists and solid physicists. In high entropyalloys, the simultaneous improvement of both properties at cryogenic temperaturesshows a promising prospect to conquer it. Deformation-induced nano-twin withunexpected low formation energy is apparently one major reason for the propertyimprovement. This counterintuitive phenomenon, that is, why introducing a defect suchas twins can in fact take a system down to a more stable state, remains an unknownpuzzle attracting a broad research interest at the present time. Our preliminary studybased on density functional theory (DFT) offers a clear physical explanation that thesurprisingly negative stacking fault energy is caused by three factors: (1) energypreference of the HCP phase and the meta-stability of FCC phase at low temperatures,as well as (2) atomic composition around the stacking fault and influence from (3) latticedistortion.This proposal will address four important issues in order to further understand theorigin of deformation mechanism. The first issue is to continue our preliminary staticstacking fault energy and work on dynamics aspect of it. Unstable stacking fault energy(USFE) and sluggish diffusion will be carefully studied through DFT calculations. Thesecond issue is to further analysis the atomic concentration around stacking faults byfocused ion beam and atom probe tomography (APT), since preliminary results alsostrongly hint that stacking fault deformation may start from cobalt-rich or nickel-leanlayers. The third issue is to investigate phase stability and precipitate concentration.Critical shear strain and atomic percentage to produce precipitate will be calculated byDFT and spatial atomic concentration will be examined by APT and transmissionelectron microscopy (TEM) energy-dispersive X-ray spectroscopy (EDS). The last issueis to develop the most common FeNiCrCo potential function for molecular dynamicsimulation purpose.In conclusion, our simulation and APT ability as well as support from collaboratorincluding sample preparation, mechanical test and TEM technique, outcome of thisresearch will bring in a whole new perspective to the understanding and design of HEAsfor structural applications in cryogenic temperatures. Beyond the common rule of thumbof choosing low stacking-fault-energy elements, this study will reveal a scientific baseto control the twin formability from thermodynamic stability in different latticestructures. Under this guideline, much previous trial and error efforts can besubstantially reduced, and greatly accelerate HEA alloy design in the future.?