Transformation-assisted interstitial quinary high-entropy alloys

Transformation-assisted interstitial quinary high-entropy alloys

In this project, we aim at significantly enhancing the strength-ductility combination of quinary high-entropy alloys (HEAs) with five principal elements by simultaneously introducing interstitial C/N and the transformation induced plasticity (TRIP) effect. Thus, a new class of alloys, namely, interstitially alloyed TRIP-assisted quinary (five-component) HEAs is being developed.

Pursuing outstanding mechanical properties in HEAs have drawn great interest in the past several years due to immense compositional opportunities inherent to HEAs. Particularly, CoCrFeMnNi system, with a typical single FCC phase solid solution at room temperature, has excellent mechanical properties including exceptional cryogenic fracture toughness which can be made by conventional casting. Apart from single phase HEAs, a novel concept of metastable TRIP-assisted dual-phase HEAs has recently been proposed towards exceptional strength and ductility. The TRIP effect is mainly determined by the stacking fault energy (SFE), i.e. the energy carried by the interruption of the normal stacking sequence. Interstitially alloyed HEAs (iHEAs) exhibit even further significantly enhanced mechanical properties compared to the interstitial free HEAs. Thus, in this project, we focus on the mechanical properties of novel quinary TRIP-iHEAs and reveal the underlying deformation mechanism of the newly developed alloys.

By collaborating with theoretical scientists Dr. Fritz Körmann and colleagues, we combine state-of-the-art density functional theory (DFT) calculations and multiple experimental techniques such as rapid alloy prototyping (RAP), digital image correlation (DIC) assisted tensile testing, electron backscatter diffraction (EBSD), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atom probe tomography (APT). The DFT calculations include finite-temperature excitations and are used to screen the large compositional space of HEAs to identify promising candidate alloys. The key quantities linking the quantum-mechanical calculations and experiment are the intrinsic SFE and phase stabilities.

EBSD phase maps revealing deformation-induced martensitic transformation as a function of local strain (εloc) levels in a quinary dual-phase HEA.

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