粉末冶金Cu-15Ni-8Sn合金时效组织的演变行为

对粉末冶金Cu-15Ni-8Sn合金进行热挤压、固溶处理后,研究400℃时效不同时间对合金力学性能及其组织的影响。结果表明:随时效时间延长,Cu-15Ni-8Sn合金的抗拉强度先升高后降低,伸长率先降低后升高,合金断裂方式表现为由沿晶断裂为主向穿晶断裂为主转变。合金抗拉强度在400℃时效1.5 h时获得最大值918MPa。综合考虑合金的强度和韧性,400℃的最佳时效时间为2 h。借助扫描电子显微镜和透射电子显微镜研究合金在400℃时效不同时间(1~3 h)的组织结构变化及其对合金强度和塑性的影响,结果表明:在欠时效阶段,合金组织在富Sn区出现了有序排列的介稳态DO22相,基体的FCC结构开始向DO22有序化结构转变,合金强度大幅提升。而在峰时效阶段,DO22向L12转变,合金强度继续增加,随片层状不连续析出组织开始由晶界向晶内生长,合金强度下降。在过时效阶段,DO22进一步向L12转变,并且片层状不连续析出组织开始大肆侵蚀基体调幅组织,导致合金强度降低,塑性提高。在时效后期,调幅组织被片层状组织大幅侵蚀,出现片层状组织粗化并断裂的现象,粒状γ相(DO3)不断生成,由于这一过程需要更大的浓度起伏而进行得非常缓慢,合金组织仍以大量片层结构为主,因而合金强度和塑性变化不明显。

Microstructure evolution of powder metallurgy Cu-15Ni-8Sn alloy at aging procedure

Effect of different aging time at 400℃ on microstructure and properties of powder metallurgy Cu-15Ni-8Sn alloy was investigated after hot extrusion and solution treatment. The results show that: the tensile strength increases initially and then decreases, the elongation decreases initially and then increases with increasing aging time. And the maximum tensile strength value of 918 MPa is obtained after aging at 400℃ for 1.5 h. Aging about 2 h at 400℃ is the best way to get a better performance in both strength and toughness. Scanning electron microscope (SEM) was employed to study the tensile fracture mode. The results show that: the tensile fracture mode evolves from predominance on intergranular fracture to trans-granular fracture after aging from 1h to 3 h at 400℃. The microstructural evolution of powder metallurgy Cu-15Ni-8Sn alloy with different aging time (1?3 h) at 400℃ and its effect on strength and plasticity were studied by transmission electron microscopy (TEM), and the results show that in the under aged condition, the Sn-rich regions form the metastable state (CuxNi1?x)3Sn particles of DO22 by continuous precipitation in the under-aged condition. The microstructure evolves from FCC to DO22 structure, the tensile strength increases widely and the plasticity decreases. In the peak-aged condition, DO22 evolves into L12, and the tensile strength increases furthermore. As the lamellar discontinuous precipitation structure grows from the grain boundary to the inside of grains, and the tensile strength decreases. What’s more, the lamellar structure grows from grain boundary to the inside of grain extensively in the over aged condition, and DO22 evolves into L12. The extension of lamellar structure is the main factor of influence on the strength, and the strength decreases. In the late stage of aging, the spinodal structure is consumed by the lamellar structure. The coarsening and breaking of the lamellar structure can be observed, and DO3is constantly formed. But the procedure is rather slow and requires more composition fluctuation, so that the lamellar structure is the main microstructure, and it is not obviously noticed the changes of the strength and plasticity.