Alloying Effects on the Precipitation Behaviors and Mechanical Properties of

L12-Strengthened Multi-Principle-Elements Alloys


合金化對L12強化多組元合金析出行為及力學性能的影響

Student thesis: Doctoral Thesis

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Award date28 Aug 2018

Abstract

Multi-principle-elements alloys (MPEAs) or High entropy alloys (HEAs) have greatly enlarged the number of alloy spectrum for achieving the unique microstructures and mechanical properties. Yet the early interested single-phase solid solutions are found to be insufficient to guarantee desired properties for advanced engineering applications. Specifically, the single face-centered-cubic (FCC) MPEAs usually exhibit relatively low yield strength, which limits their practical applications seriously. Whereas their high work-hardening capabilities make them an ideal candidate as an excellent base alloy. In this thesis, Al, Ti and Nb elements were introduced for nano-precipitation engineering. Through the combined merits of precipitation strengthening and good work-hardening capability, a serious of high-performance MPEAs were designed based on the CALPHAD (CALculation of PHAse Diagrams) technique. The microstructural evolutions, mechanical properties and microscopic deformation mechanisms were systematically evaluated.

In the first part of this thesis, a prototype equiatomic-FCC-base, i.e. CoCrNi base alloy, was selected as our starting material. A small amount of Al and Ti, strong gamma-prime phase (γ′, L12 structure) formers, was added into the CoCrNi matrix in order to introduce coherent precipitates and a dual-phase structure (FCC matrix and coherent γ′ phase) was successfully achieved. The room-temperature tensile strength was significantly enhanced due to the precipitation hardening, along with a good ductility of ~45%. Transmission electron microscope (TEM) study demonstrated that a stacking-fault-mediated deformation rather than mechanical twinning was dominated in this precipitation-hardened MPEA, which could be ascribed to the increasing critical twinning stress affected by the channel width of the matrix. In addition, effects of alloying addition and aging process on the phase stability and the corresponding mechanical properties were investigated. It was found that the growth rate of the nano-precipitates in the equiatomic CoCrNi-based MPEA is much slower than most of the Ni-based superalloys, suggesting a superior thermal stability of these nano-precipitates. Nevertheless, an excessive addition of the Al and Ti and/or the prolonged aging time appears to promote the formation of detrimental intermetallic phases, such as Cr-rich σ phase and body-centered-cubic (BCC) phase, and consequently greatly degrade the tensile ductility. This could be ascribed to the supersaturated Cr content in the matrix, especially for those with higher addition of Al and Ti. Thus, it can be concluded that the dual-phase structure can only be stabilized in the euqiatomic CoCrNi base with limited contents of Al and Ti.
Second, based on the above results, Fe was added to partially substitute Cr and non-equiatomic base alloys with minor addition of Al, Ti and Nb were designed. Experimental results show that the non-equiatomic CoCrFeNi-base alloy enables the stabilization of the dual-phase structure at higher contents of Al and Ti or Nb, whilst gigapascal yield strength can be achieved essentially by the high density of nanoscale γ′ precipitates. More interestingly, the γ′ phase can be retained even in the alloy with a Nb/Al ratio of 1.5, distinguished from that of the conventional Ni-based superalloy, where such a Nb/Al ratio is known to preferentially form γ′′ phase. We attributed the stabilization of γ′ phase to the high Co content in the precipitates. Besides, the deformation mechanisms were further systematically analyzed in terms of dislocation substructures, demonstrating the importance of stacking fault energy (SFE) in controlling the deformation behavior.
Third, a series of high-strength Co-free HEAs with different Ti/Al ratio were developed to further obtain a good combination of low cost, high strength and good ductility. The effect of Ti/Al ratio (from 0.6 to 1.7) on phase structures and mechanical properties were carefully investigated. Specifically, the alloy with a high Ti/Al ratio of 1.7 gives a high tensile strength of 1.25 GPa with a good ductility over 20%, obviously outperforming other previously developed Co-free HEAs. The strain-hardening behaviors and deformation micro-mechanisms were analyzed by TEM, which reveals the structural origins of their superior mechanical properties.
We believe that our present findings not only provide insight to in-deep understanding of the deformation micro-mechanisms of the precipitation-strengthened MPEAs, but also give a useful guidance for the future development of high-performance MPEAs via computational design for advanced structural applications.