Fatigue Fracture Mechanisms of Titanium Alloy at Cryogenic Temperature

研究了Ti-6Al-4V合金77K下疲劳裂纹的萌生和扩展机制。与裂纹扩展有关的测量以及对试件表面、纵截面和断口的光学显微镜、扫描电子显微镜和高压电子显微镜的观察结果表明：1)77K时，α晶面开始开裂。在105次寿命应力作用下，未观察到亚表面裂纹萌生现象。2)77K时的疲劳裂纹扩展速率比293K时低1/2~1/4倍。在低生长速度区，生长速度的差异较大。从所测得的扩展速率来看，77K时的裂纹闭合速率高于300K时的。在77K和293K下的裂纹扩展速率作为有效DELTAK的参数相互吻合。3)在77K疲劳低扩展速率下观察到明显的裂纹分叉现象。4)低生长速率下的断口由细小的α-晶面组成，随着生长速率的增加，断口由平台型转变为平台型。77K时的断口与293K时的断口比较平坦。除条纹外，断口的另一个不同之处是，在77K的高生长速率区，观察到像腐蚀裂纹中观察到的“沟槽”一样沿生长方向伸长的小面。5)在远离裂纹尖端的低速扩展区域，观察到α-晶棱柱滑移面位错阵列。

The fatigue crack initiation and growth mechanisms of Ti-6Al-4V alloy at 77K were investigated. Measurement relevant to the crack growth as well as observations on the specimen surface, longitudinal section as well as fracture surface through optical microscope, SEM and high voltage TEM revealed the following results. 1)Crack was initiated in the surface alpha-grain at 77K. No subsurface crack initiation was observed in the 10^5 cycles life time stressing. 2)The fatigue crack growth rate at 77K is lower by factors of 1/2-1/4 as compared with that at 293K. The difference in the growth rate is larger at low growth rate region. The crack closure revel at 77K is higher than that at 300K through all the growth rates measured. The crack growth rate at 77K and 293K coincided to each other as a paramer of effective DELTAK. 3)Marked bifurcation of crack was observed at low growth rate in 77K fatigue. 4)The fracture surface in the low growth rate was consisted of facets of fine alpha-grain and it changed into plateaus as the growth rate increased. The fracture surface at 77K was rather flat as compared with that at 293K. Other than striations, as another difference in the fracture surface, facets elongated in the growth direction like "flute" observed in corrosion cracking was observed at 77K in the high growth rate region. 5)Arrays of dislocation in the prismatic slip plane in alpha-grain were observed in region remote from a crack tip which is growing at low rate.