Ni-Cr-W-Mo合金长时时效中的析出行为及对力学性能的影响

Precipitation Behavior During Long-Term Aging of Ni-Cr-W-Mo Alloy and Its Effect on Mechanical Properties

摘要:

利用SEM、TEM和相分析等手段，研究了Ni-Cr-W-Mo合金在800℃长时时效中的析出行为及对力学性能的影响。结果表明，时效中M23C6碳化物呈不同形态依次析出于晶界、孪晶界和晶内。晶间层片状M23C6是不连续反应的产物，而孪晶界M23C6呈颗粒状且沿{111}孪晶面生长，晶内纳米级M23C6可起到钉扎强化的作用；M6C碳化物在时效1000h~5000h发生了由外至内的退化，分解为M23C6、α-(W,Mo)相和贫W、Mo的基体。由于W、Mo平衡偏聚，时效5000h后又有α-(W,Mo)相在晶界M23C6处析出；力学性能的显著变化集中在时效前期。时效前200h的强度提升是碳化物剧增引起的，而塑韧性恶化归因于晶间M23C6的脆性及其对晶界结合力的降低。时效200h~5000h，性能缓慢退化。此阶段强度的降低与α相对固溶强度的削弱及纳米M23C6的粗化有关，塑韧性的退化与晶间M23C6粗化和M6C的分解相关。时效前后，断裂类型由穿晶韧性断裂转变为混合型断裂。

Abstract:

Using techniques such as SEM, TEM, and phase analysis, the precipitation behavior during long-term aging at 800°C and its effect on the mechanical properties of a Ni-Cr-W-Mo alloy were investigated. The results indicate that during aging, M23C6 carbides precipitated sequentially at grain boundaries, twin boundaries, and within grains in different morphologies. The intergranular lamellar M23C6 was a product of a discontinuous reaction, while the granular M23C6 at twin boundaries grew along the {111} twin planes. The intragranular nanoscale M23C6 contributed to pinning strengthening. M6C carbides underwent degeneration from the exterior to the interior between 1000h and 5000h of aging, decomposing into M23C6, α-(W,Mo), and W- and Mo-poor matrix. Due to the equilibrium segregation of W and Mo, α-(W,Mo) phase precipitated at grain boundary M23C6 after 5000h of aging. Changes in mechanical properties were mainly concentrated in the early aging stage. The increase in strength within the first 200h of aging was caused by a sharp rise in carbides, while the deterioration of ductility and toughness was attributed to the brittleness of intergranular M23C6 and its reduction of grain boundary cohesion. From 200h to 5000h of aging, the properties degraded gradually. The strength reduction in this stage was related to the weakening of solid solution strength by α phase and the coarsening of nanoscale M23C6, while the significant degradation of ductility and toughness was associated with the coarsening of intergranular M23C6 and the decomposition of M6C. The fracture mode transitioned from transgranular ductile fracture to a mixed mode after aging.