Fundamental Investigation on High Entropy Alloy Deformation Mechanisms
DescriptionIt is well known that trade-off between strength and ductility in metallic alloys is a long-standing challenge for material scientists and solid physicists. In high entropy alloys, the simultaneous improvement of both properties at cryogenic temperatures shows a promising prospect to conquer it. Deformation-induced nano-twin with unexpected low formation energy is apparently one major reason for the property improvement. This counterintuitive phenomenon, that is, why introducing a defect such as twins can in fact take a system down to a more stable state, remains an unknown puzzle attracting a broad research interest at the present time. Our preliminary study based on density functional theory (DFT) offers a clear physical explanation that the surprisingly negative stacking fault energy is caused by three factors: (1) energy preference 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) lattice distortion.This proposal will address four important issues in order to further understand the origin of deformation mechanism. The first issue is to continue our preliminary static stacking fault energy and work on dynamics aspect of it. Unstable stacking fault energy (USFE) and sluggish diffusion will be carefully studied through DFT calculations. The second issue is to further analysis the atomic concentration around stacking faults by focused ion beam and atom probe tomography (APT), since preliminary results also strongly hint that stacking fault deformation may start from cobalt-rich or nickel-lean layers. The third issue is to investigate phase stability and precipitate concentration. Critical shear strain and atomic percentage to produce precipitate will be calculated by DFT and spatial atomic concentration will be examined by APT and transmission electron microscopy (TEM) energy-dispersive X-ray spectroscopy (EDS). The last issue is to develop the most common FeNiCrCo potential function for molecular dynamic simulation purpose.In conclusion, our simulation and APT ability as well as support from collaborator including sample preparation, mechanical test and TEM technique, outcome of this research will bring in a whole new perspective to the understanding and design of HEAs for structural applications in cryogenic temperatures. Beyond the common rule of thumb of choosing low stacking-fault-energy elements, this study will reveal a scientific base to control the twin formability from thermodynamic stability in different lattice structures. Under this guideline, much previous trial and error efforts can be substantially reduced, and greatly accelerate HEA alloy design in the future.?
|Effective start/end date||1/01/18 → …|