生长速率对定向凝固NiAl-43V过共晶合金组织演变和室温断裂韧性的影响

Effect of growth rates on microstructure evolution and room temperature fracture toughness of directionally solidi?ed NiAl-43V hypereutectic alloy

摘要:

本文采用高温度梯度液态金属冷却技术(LMC)制备了Ni-28.5Al-43V(at%)过共晶合金,利用光学显微镜(OM)、扫描电镜(SEM)和三点弯曲(3PB)测试对合金的组织演变和断裂韧性进行了研究。在温度梯度G_L=310K/cm,生长速率V=6～150μm/s的实验条件下, NiAl-43V的凝固组织均为初生V枝晶+共晶组织(NiAl片层+V片层)。断裂韧性测试表明NiAl-43V过共晶合金最大室温断裂韧性相比NiAl合金提高了四倍。随着生长速率的增大室温断裂韧性从6μm/s的22.679 MPa.m^_1/2逐渐下降到150μm/s的18.422 MPa.m^_1/2。这是因为生长速率增大产生的细晶强化效应弱于初生V枝晶和胞间区域的增加对断裂韧性产生的不良影响。合金的断口形貌分析表明合金断裂为准解理断裂,在裂纹扩展中裂纹钝化、裂纹再形核、裂纹偏转、界面剥离、裂纹桥接和微裂纹键合韧化机制对提高NiAl-43V合金的室温断裂韧性做出了贡献。初生V枝晶的析出一定程度降低了合金的断裂韧性。

Abstract:

In this paper, a Ni-28.5Al-43V (at%) hypereutectic alloy was prepared by liquid metal cooling technology (LMC), The microstructure evolution and mechanical properties of the alloy were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and three-point bending (3PB) test. Under the experimental conditions(Temperature gradient G_L=310K/cm, Growth rate V=6～150μm/s), the results show that the steady-state growth microstructure of NiAl-43V is composed of primary V dendrites and eutectic structure (NiAl lamellar +V lamellar). 3PB test shows that the maximum room temperature fracture toughness(RTFT) of NiAl-43V hypereutectic alloy is four times higher than that of NiAl alloy.The RTFT decreases from 22.679 MPa.m ^_1/2 at 6 μm/s to 18.422 MPa.m ^_1/2 at 150 μm/s as the growth rate increases.The reason is that the fine-grained strengthening effect caused by the increase of growth rate is weaker than the adverse effect of increased primary V dendrites and inter-cell regions on fracture toughness. Fracture morphology analysis of alloys indicates that alloy fracture is quasi-cleavage fracture.Crack blunting, crack renucleation, crack deflection, interfacial debonding, crack bridging and linkage of microcrack toughening mechanism contribute to the improvement of room temperature fracture toughness of NiAl-43V alloy in crack propagation. The precipitation of primary V dendrites reduces the fracture toughness of the alloy.