Deformation Mechanism and Local Structure of CrCoNi-based High Entropy Alloys


Student thesis: Doctoral Thesis

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Award date24 Aug 2020


High entropy alloys (HEAs) with multi-principle elements have attracted increasing attention due to their high strength and ductility. Remarkably, the strength and ductility further increase upon cooling to cryogenic temperatures. The deformation mechanism of HEAs is mainly determined by stacking fault energy (SFE). Decreasing the SFE, the deformation mechanism can change from dislocation slip to twinning till phase transformation. Phase transformation is an effective means to increase the plasticity of a material. An fcc-to-hcp (face-centered-cubic to hexagonal-close-packed) phase transformation has long been anticipated in CrCoNi medium entropy alloy. However, no evidence of bulk phase transformation has been found. Although having complex composition, the structure of HEA is rather simple, like fcc or bcc (body-centered-cubic). First-principle calculations demonstrated that local chemical ordering can have a strong influence on the mechanical behavior of HEAs as it increases the SFE. Therefore, a thorough investigation of the deformation mechanism and local arrangement of different chemical species, i.e., the short-range chemical order, in HEAs would be highly desirable for fundamental understanding of the extraordinary mechanical behaviors.

Neutrons are highly penetrating, and thus neutron diffraction measurements are truly representative of the bulk. In addition, neutron diffraction measurements can be carried out in situ. Therefore, in situ loading measurements with neutron diffraction can not only identify the phase transformation but also provide microscopic insights on the deformation behaviors. Anomalous X-ray scattering and extended X-ray absorption fine structure (EXAFS) analysis are ideal methods to characterize the chemical ordering in compounds.

In this work, we studied the deformation mechanisms of CrCoNi alloy under tensile loading at cryogenic temperature (15 K) and the fatigue behavior of CrFeCoNiMo0.2 alloy at room temperature by in situ neutron measurement , and the short-range ordering of CrFeCoNiMox (x=0.11, 0.18 and 0.23) alloys by anomalous X-ray scattering and EXAFS.

A bulk fcc-to-hcp phase transformation was observed under tensile loading at 15 K. Quantitative analyses of the in situ lattice strain and texture data demonstrates that the nucleation of hcp phase was triggered by intrinsic stacking faults, a major deformation mechanism in multicomponent HEAs at low temperatures. It is shown that in CrCoNi, dislocation slip saturated early, and the late-stage deformation was dominated by stacking faults and phase transformation. The interaction between these different deformation mechanisms led to an increased strain hardening rate and hence the exceedingly large ductility at low temperatures.

The anomalous X-ray scattering data demonstrated that above the Mo K-absorption edge, the scattering intensities for all Bragg peaks show a proportional decrease, indicating that Mo atoms are randomly distributed in the CrFeCoNiMox fcc lattice. Detailed analysis of the EXAFS data provided evidence of a local chemical order in CrFeCoNiMox. It is shown that in Mo rich samples (e.g., x = 0.23), Mo atoms are surrounded by excessive lighter atoms, such as Cr.

The yield strength of CrFeCoNiMo0.2 alloy is around 290 MPa. The fatigue experiment was controlled by stress with 460 MPa and 500 MPa, which was low cycle fatigue. The total fatigue life was around 190000 cycles. The strain increased considerably during the last 10000 cycles before fracture. Texture development indicates that the deformation mechanism during fatigue was dominated by dislocation slip. The dislocation density was found proportional to the strain indicating that the deformation was driven by strain. No evidence of planar faults was found. The deformation substructure changed for the first 10000 cycles when fatigue stress applied on the sample and then became stable until the last 10000 cycles.