High-density Coherent Nano-precipitation and Mechanical Properties of Face-centered-cubic High-entropy Alloys


Student thesis: Doctoral Thesis

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Award date2 Nov 2018


Advanced alloys with superior strength-ductility combinations are highly desirable for modern engineering applications, and the design of which requires precise manipulation of their internal microstructures. Multicomponent high-entropy alloys (HEAs) greatly widen alloy design flexibility for tailoring the phase structures, defects and associated mechanical properties. The single-phase HEAs, especially those with the face-centered-cubic (FCC) structure, usually exhibit a relatively low yield strength, which limits their practical applications seriously. Nevertheless, their high work-hardening capabilities make them an excellent base alloy for introducing precipitation hardening. More importantly, improved compositional choices in HEA systems offer us great potentials for triggering various toughening mechanisms, such as the twinning-, transformation-, and microband-induced plasticity (TWIP, TRIP and MBIP, respectively), the combined operation of precipitation hardening and toughening mechanisms provides a promising approach towards superior mechanical properties. In this thesis, a series of HEAs strengthened by coherent L12-type nanoparticles were designed. The microstructural evolutions, mechanical properties and microscopic deformation mechanisms were systematically evaluated.

In the first part of this thesis, we demonstrated the feasibility to toughen strong-yet-brittle precipitation-hardened HEAs by tailoring the electron concentrations of precipitates for desirable phase transformation in the Ni-Co-Fe-Cr-Ti alloy system. It is clearly revealed that Co can strongly partition into the precipitates and substitute for Ni, and this substitution has effectively destabilized the brittle plate-like η phase (Ni3Ti-type) due to its decreasing electron concentration, leading to the formation of ductile γ′ phase ((Co, Ni)3Ti-type) with the ordered-FCC structure. The microstructures, tensile properties and fracture mechanisms with respect to different Co concentrations have been systematically investigated. Homogenous precipitation of these newly formed coherent γ′ nano-precipitates results in remarkable enhancements in both strength (up to 1.35 GPa) and ductility (above 35%).

Second, we reported a surprising strength-ductility synergy in the Ni-Co-Al-Fe-Cr-Ti HEAs designed by a computational-aided approach. Through co-alloying of Ti and Cr elements, high-density coherent L12-type nano-precipitates and stacking faults can be concurrently introduced, leading to a significant increase of tensile strength and ductility simultaneously. Of particularly, the resultant Ni30Co30Al6Fe13Cr15Ti6 alloy shows a yield strength of ∼1 GPa, a tensile strength over 1.3 GPa with a failure strain of ∼45% at 293 K. More interestingly, at the cryogenic temperature (77 K), its mechanical properties were found to be further improved, showing an ultra-high tensile strength of 1.7 GPa with a huge increase of the ductility up to 51 %. The underlying precipitation behaviors and deformation micro-mechanisms were systematically investigated using transmission electron microscope (TEM), atom probe tomography (APT) and first-principal calculations. The increased volume fraction of nanoparticles and associated ordering strengthening has been demonstrated as the main hardening mechanism. Upon yielding, the dynamic formation of nano-spacing stacking faults contributed to a substantially enhanced strain-hardening capacity of alloys, especially at 77 K, which delayed the early onset of plastic instability and contributed to a synergic enhancement of tensile ductility at high-strength levels.

Third, the equi-atomic FeCoNi alloy with a medium stacking fault energy was selected as the base alloy for precipitation hardening. Based on the thermodynamic predictions, high-concentrations of Ti and Al were added to introduce dense L12-type nano-precipitates. In difference from Ni-based superalloys hardening by Ni3Al particles, we have identified the complex long-range ordered L12 phase based on (Ni,Co,Fe)3(Ti,Al,Fe) as the multicomponent nanoparticles (MCNPs) for hardening FCC-based HEAs. We have demonstrated that a partial replacement of Ni with Co and Fe atoms will effectively lower the valence electron density (VEC) and stabilize the L12 ordered particles. Furthermore, a significant replacement of Al with Ti atoms eliminates the severe embrittlement induced by simple Ni3Al ordered particles. As compared to the particles-free base alloy, such HEAs exhibit a ~450% boost of the yield strength up to 1.1 GPa and a twofold increment of the tensile strength up to 1.5 GPa, while retaining an exceptional ductility as high as 50%. More surprisingly, the plastic instability, one critical issue for high-strength materials, is completely eliminated by generating a unique multistage work-hardening behavior, resulting from pronounced dislocation activities and deformation-induced micro-bands.

Our present discoveries are vital to the in-depth understanding of the deformation micro-mechanisms and plastic instability of high-strength precipitation-hardened HEAs. Furthermore, based on these results, we offer an important paradigm for the future development of high-performance HEAs for advanced structural applications.