显微组织对TC18合金裂纹扩展速率的影响

Crack Growth Rate of TC18 Alloy with Different Microstructure

对具有不同组织的TC18合金棒材和TC18锻件的L-T向裂纹扩展速率及其断口形貌进行了研究。结果表明：在应力比R相同时,TC18棒材的裂纹扩展速率要高于TC18锻件,表现为TC18棒材Paris公式中的C值约是TC18锻件Paris公式中C值的1.5～2.0倍;随着应力比R的增大,TC18棒材和TC18锻件的裂纹扩展速率均增加,而应力强度因子幅ΔK对裂纹扩展速率的影响程度降低;TC18锻件的裂纹扩展速率低于TC18棒材,这与其具有不同的显微组织密切相关,TC18锻件具有网篮组织,而网篮组织中裂纹的扩展路径曲折,裂纹扩展中需要消耗较多的能量,因此表现出较低的扩展速率。TC18棒材具有双态组织,而双态组织中裂纹的扩展路径比较平直,裂纹扩展中吸收的能量少,因此表现出较高的扩展速率。另外,TC18锻件的宏观断口中可观察到二次裂纹,二次裂纹的形成和扩展释放了裂纹尖端应力,消耗了更多的能量,从而使裂纹扩展速率降低。

The crack growth rates and fracture morphologies of L-T orientation of TC18 alloy bars and TC18 forgings with different microstructure were investigated. The results showed that the crack growth rate of TC18 bars was higher than that of TC18 forgings at the same stress ratio R,and the C value of Paris formula of TC18 bars was about 1. 5 ～ 2. 0 times that of TC18 forgings. The crack growth rates of TC18 bars and TC18 forgings increased and the impact of stress intensity factor range ΔK on crack growth rate decreased with the increase of stress ratio R. The crack growth rate of the TC18 forgings was lower than that of the TC18 bars,which was closely related to its microstructure. TC18 forgings showed basketweave microstructure,which led to crack propagation path twists and turns and more energy was needed in crack propagation. TC18 bars showed duplex microstructure,which led to crack propagation path flat and less energy was needed in crack propagation. In addition,secondary cracks could be observed in macroscopic fracture of TC18 forgings,and the formation and propagation of which could release the stress of crack tip and needed more energy. Therefore,the crack growth rate of TC18 forgings with basketweave microstructure was lower.