Thermal Stability and Hetero-deformation Induced Strengthening of Multi-principal Element Alloys


Student thesis: Doctoral Thesis

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Award date2 Jun 2022


There are overwhelming experimental observations indicating intermetallic (IM) phase formed in the intermediate temperatures in high entropy alloys. In the first part of this study, here proposed a model to show how kinetics could intervene the thermodynamic determination of phase formation. The model offers a good explanation for the prevalent formation of IM in the intermediate temperature range in high entropy alloys reported in the literature. To further demonstrate the kinetic effect, the equiatomic CrMnFeCoNi alloy (i.e., Cantor Alloy) was selected as a surrogate material and employed differential scanning calorimetry (DSC) at different heating rates to investigate IM formation in this alloy. In this case, we found the presence of four IM compounds, BCC-Cr, L10-NiMn, B2-FeCo, and Cr-rich σ phases, in the temperature ranging from about 200 to 900 ℃, above which the alloy was a complete solid solution. From the DSC measurements, we were also able to build a quasi-equilibrium TTT diagram for Cantor Alloy. It was the first time that such a schematic TTT diagram was established for a high entropy alloy.
Secondly, a single-phase lamellar grain structure was considered to develop another physical model to quantify hetero-deformation induced (HDI) strengthening at yield point, which cannot be simply predicted by conventional rule of mixture using Hall-Petch equation. Based on the classic theory of single-ended continuum dislocation pileup, the modified model gives solutions to the effective length of hetero-boundary-affected region (HBAR), previously known as interface affected zone (IAZ), as well as the contribution of HDI stress to 0.2% proof stress. Likewise, to further verify the model equations, the equimolar CoCrNi alloy was selected as a surrogate material. The heterogeneous grain structure (HGS) was introduced via thermal-mechanical treatment (TMT), and statistical analysis of microstructure was performed by means of electron backscatter diffraction (EBSD). By substituting the derived parameters, our model predicted a yield stress comparable to the experimental value from the tensile test. The reasonable fitting results not only proved the validity of the modified model, but also brought the physical explanations for the extra strengthening in materials with HGS.
Finally, heterostructure engineering was adopted to improve mechanical properties over a wide temperature range. Partial recrystallization and L12-precipitation were simultaneously introduced into a CoCrNi-based medium entropy alloy, resulting in a heterogeneous microstructure. The alloy achieves an ultrahigh tensile strength of 1.5 GPa at 500℃, and still maintains a tensile strength above 1.1 GPa at 600℃. All Elongations exceed 20% at temperatures ranging from -196 to 700℃, which indicates the breakthrough in temperature-dependent embrittlement encountered by many structural materials. This works demonstrates heterostructure engineering can be put at the forefront of the strengthening route to achieve high strengths without sacrificing elongations over an extended temperature range.