Study on the formation mechanisms of core–shell structures and their influence on the properties of Ni-Co-Al-Ti-based high-entropy intermetallic

Abstract

The advancement of aerospace technology imposes increasingly stringent requirements on the mechanical properties of structural materials at both room and elevated temperatures. Intermetallic compounds have attracted considerable attention due to their excellent specific strength and high-temperature performance. However, their engineering applications are persistently constrained by issues such as room-temperature brittleness or insufficient high-temperature strength. Inspired by the concept of high-entropy design, this study employs a sublattice high-entropy alloying strategy to improve the mechanical properties of conventional intermetallic compounds, developing Ni-Co-Al-Ti-based high-entropy intermetallics with a core–shell structure that exhibit favorable room-temperature ductility. Through further compositional tuning, the core–shell structure evolves toward multi-scale characteristics, achieving synergistic enhancement of strength and ductility. This work reveals the formation mechanism of the multi-scale core–shell structure and its influence on room-temperature mechanical properties, while also elucidating the strengthening–ductilization mechanism of the multi-scale core–shell structured high-entropy intermetallics. In addition, the mechanical properties over a wide temperature range are investigated, revealing the embrittlement at intermediate temperature and an anomalous yield effect, along with the corresponding microscopic mechanisms. First, through thermodynamic and solidification analyses, the effect of Co addition on the Ni(75-x)CoxAl8.33Si8.33Ti8.34 series alloys is studied. The results show that Co addition favors the stable formation of an L12/FCC dual-phase structure. Further microstructural characterization reveals that, at a Co content of 25 at.%, a dual-phase core–shell structure consisting of ordered L12 cores and disordered FCC shells forms within the grains. Based on these observations, the mechanism by which Co addition promotes the formation of the core–shell structure is analyzed. On the one hand, Co addition induces the formation of the FCC phase and promotes the precipitation of the L12 phase, providing the phase-structural basis for the core–shell structure. On the other hand, it maintains a low L12/FCC interfacial energy, supplying an stable interfacial condition for core–shell formation. Mechanical testing indicates that the formation of this structure effectively improves the room-temperature ductility of the alloy. The Ni25Co25Al8.33Si8.33Ti8.34 alloy exhibits a yield strength of approximately 450 MPa, an ultimate tensile strength of about 650 MPa, and a plastic elongation of roughly 10%. Building upon the alloy in which the core–shell structure has already been established, compositional adjustments are performed to promote the development of the core–shell structure toward multi-scale features. The addition of Ta induces the formation of a dual-scale core–shell structure with micron- and submicron- scale in the Ni50Co25Al7Ti7Ta1.5Cr9.5 alloy, effectively enhancing the room-temperature strength. This alloy shows a yield strength of about 800 MPa, an ultimate tensile strength of approximately 1000 MPa, and a plastic strain of about 8%. Further increasing the Al/Ti ratio promotes the formation of a multi-scale core–shell structure comprising micron-scale, submicron-scale, and nanoscale in the Ni50Co25AlxTi(14-x)Ta1.5Cr9.5 alloys, achieving a synergistic improvement in room-temperature strength and ductility. Among them, the Ni50Co25Al9Ti5Ta1.5Cr9.5 alloy exhibits a yield strength of approximately 1000 MPa, an ultimate tensile strength of about 1250 MPa, and a plastic strain of roughly 12.5%. The formation mechanism of the multi-scale core–shell structure can be summarized as follows: Ta addition exacerbates compositional segregation at grain boundaries, inducing the formation of micron-scale core–shell structure. Meanwhile, an increased Al/Ti ratio enhances the sluggish diffusion effect, promoting the precipitation of submicron-scale and nanoscale core–shell structures. Furthermore, Ni50Co25Al9Ti5Ta1.5Cr9.5 alloy with multi-scale core–shell structure is selected as the research object. The evolution of dislocation substructures during room-temperature deformation was systematically characterized, revealing that the room-temperature deformation mechanism of this alloy is synergistically governed by superlattice intrinsic/extrinsic stacking faults, stacking faults, Lomer-Cottrell locks, dislocation forests, and dislocation walls.. On this basis, combined with theoretical calculations, the strengthening–ductilization mechanism is thoroughly elucidated. The results indicate that the high strength of the alloy mainly originates from precipitation strengthening arising from the high-volume-fraction ordered L12 cores and hetero-deformation-induced strengthening caused by the multi-scale core–shell structure, while the favorable ductility is attributed to continuous (FCC+nanoscale L12) dislocation motion channels and hetero-deformation-induced strain hardening. In addition, the highly coherent core–shell interfaces and serrated grain boundaries jointly promote cooperative deformation, enabling the material to achieve an excellent combination of strength and ductility at room temperature. Subsequently, the microstructural stability and mechanical properties of the Ni50Co25Al9Ti5Ta1.5Cr9.5 alloy at 800 ℃ are investigated, revealing a good thermal stability and relatively high yield strength, but with almost zero plastic elongation. Vacuum comparative tensile tests indicate that this intermediate-temperature embrittlement originates from grain-boundary weakening caused by oxygen penetration and stress concentration induced by grain-boundary sliding. By increasing the Cr content to 20 at.%, discontinuous α-Cr phases and small-scale core–shell structures are induced to form at grain boundaries, which effectively hinder oxygen diffusion and restore the 800 ℃ plastic elongation to about 5%, thereby mitigating the embrittlement issue at intermediate temperature. In addition, the mechanical properties of the high-Cr alloy over a wide temperature range from 25 to 1000 ℃ are evaluated. The results show that the yield strength first increases and then decreases with rising temperature, reaching a peak value of about 920 MPa at 800 ℃, exhibiting a clear anomalous yield effect. TEM characterization reveals that this effect is attributable to the transition of the deformation mechanism to anti-phase boundary shearing at elevated temperatures, as well as the formation of Kear-Wilsdorf locks and Lomer-Cottrell locks.