快速凝固Ni-Ge合金的组织演化与力学性能研究

Structural evolution and mechanical performances of rapidly solidified Ni₉₅Ge₅ and Ni₃₃Ge₆₇ alloys were investigated under two conditions involving drop-tube containerless processing and copper-mold injection casting. As the cooling rate increased, the coarse dendritic structure of the primary (Ni) phase for Ni₉₅Ge₅ alloy changed to equiaxed grains structure, and the microstructure of Ni₃₃Ge₆₇ alloy showed a transition from coarse (NiGe + Ge) eutectic to refined structure plus some primary Ge phase. Containerless processing can produce finer microstructures than those of injection casting owing to higher cooling rate. Nanoindentation measurements indicated that both alloys showed a remarkable enhancement of Vickers hardness along with the microstructural evolution, and the hardening mechanism for a single-phase alloy was found to be grain-boundary strengthening, while that of the eutectic alloy was attributed to the multiphase coordinated strengthening. The strain hardening exponents of Ni₉₅Ge₅ and Ni₃₃Ge₆₇ alloys were calculated as 0.28 and 0.77, respectively. The special morphology evolution and lower strain hardening exponent of Ni₉₅Ge₅ alloy remarkably improved the hardening ability. Its hardness increased up to 56.6% that was much higher than the eutectic alloy. Moreover, these alloys displayed different relations between hardness and strength. The average ratio of hardness to strength for Ni₉₅Ge₅ alloy was 2.75, while that for Ni₃₃Ge₆₇ alloy was 9.09. Both the experimental evidence and theoretical analysis revealed that obvious enhancements of hardness and strength were achieved using a high cooling rate; thus, the drop-tube processed alloy particles showed better mechanical properties than those of the as-cast alloys owing to their higher cooling rates. It is obvious that rapid solidification conditions can improve the mechanical properties of both the alloys to various degrees.