Study on the Microstructures and Mechanical Behaviors of Multi-principal Element Alloys Strengthened by Precipitations and Heterogeneous Grain Structures


Student thesis: Doctoral Thesis

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Award date14 Jul 2021


Alloys with ultra-high strength and sufficient ductility are highly desired for modern engineering applications. However, conventional strategies, such as precipitation hardening or working hardening alone, cannot provide enough strengthening effect or essentially cause a dramatic decrease of ductility. Recently, it has been reported that the heterogeneous grain structures (HGSs) or heterogeneous precipitates (HP) could also provide strengthening effects to multi-principal element alloys (MPEAs). But the strengthening effect of the heterogeneous matrix is modest while alloys with heterogeneous precipitation achieve superb strength with low ductility.

In this thesis, Al and Ti elements were added into CoCrNi alloys to introduce nano precipitates. The contents of the five elements were selected assisted by the CALPHAD (CALculation of PHAse Diagrams) technique. The microstructural evolutions, mechanical behaviors and deformation mechanisms were systematically investigated.

At first, we designed a Co34.46Cr32.12Ni27.42Al3Ti3 (at%) MPEA with a dual heterogeneous structure by controlling the alloy composition and thermomechanical process. The dual heterogeneous structures were characterized by the combination of a heterogeneous grain structure (HGS) with coarse recrystallized grains (10~30 μm) and ultra-fine recrystallized grains (0.5~2 μm) and hierarchical L12-structured nano precipitates (ranging from several nanometers to hundreds of nanometers). The HGS with face-centered cubic (FCC) structure enhances the yield strength of the matrix (by ultra-fine grains) while persists good ductility (by coarse grains). The hierarchical L12-type nano precipitates are fully coherent with the matrix, minimizing the elastic misfit strain of interfaces. So the interaction between dislocations and interfaces could be neglected, relieving the stress concentration during deformation and playing an active role on ductility. The heterogeneous nano precipitates strengthen the alloy mainly by ordered strengthening when dislocations shear through precipitates. Nevertheless, we also observed that the Cr-rich σ phase with hundreds of nanometers precipitated along grain boundaries. The non-equiatomic MPEA with dual-heterogeneous structure shows an ultra-high yield strength (YS) up to 2.0 GPa and ultimate tensile strength (UTS) up to 2.2 GPa still with remarkable uniform elongation (UE) up to 13.0% at ambient temperature, which has about 10% higher in YS, 15.4% higher in UTS and 44% higher in UE as compared to the ultra-strong high-entropy alloy (HEA) strengthened by high-content ductile coherent nanoprecipitates.

We further explored the mechanical behavior and the application prospect of CoCrNi based MPEA at cryogenic temperature. As the existence of σ phase could significantly degrade the cracking resistance of grain boundary at cryogenic temperature, we optimized the chemical composition to Co40Cr20Ni30Al5Ti5 and elaborately design the thermomechanical treatment process. Additionally, The HGS, comprising residual deformed grains (RDGs), micro-sized recrystallized grains (MRGs), and nano-sized recrystallized grains (NRGs), and heterogeneous precipitation (HP) of the L12 phase are simultaneously introduced into the alloy by the combination of cold rolling and annealing. The alloy exhibits an ultra-high YS of 1.73 GPa, a UTS of 1.89 GPa still with a remarkable total elongation (TE) up to 17.5% at ambient temperature as well as an ultra-high YS up to 2.05 GPa, a UTS up to 2.31 GPa with an excellent TE up to 21.2%.

We believe that our investigation on the combination of HGS and L12 precipitates opens up a new avenue on the design of high-performance alloys applied at both ambient and cryogenic temperatures.