Fatigue Fracture Mechanisms of Titanium Alloy at Cryogenic Temperature

研究了Ti-6Al-4V合金在77K高温下的疲劳裂纹萌生和扩展机制。通过光学显微镜、扫描电镜和高压透射电镜对试样表面、纵剖面和断口进行了裂纹扩展的测量和观察，结果如下：1)在77K时，表面α -晶粒开始开裂。在10^5次循环寿命时间应力下，未观察到地下裂纹萌生。2) 77K时的疲劳裂纹扩展速率比293K时低1/2 ~ 1/4倍。在低增长率区域，增长率差异更大。在所有测量的增长率中，77K时的裂纹闭合强度都高于300K时的裂纹闭合强度。作为有效DELTAK参数，77K和293K时的裂纹扩展速率一致。3)在77K低扩展速率下，裂纹出现了明显的分岔。(4)低生长速率时，断口由细小的α晶粒组成，随着生长速率的增加，断口变为高原。与293K时相比，77K时断口较平整。断口的另一个不同之处在于，在高生长速率区77K时，在腐蚀裂纹中观察到类似“凹槽”的生长方向上拉长的切面。(5)在远离裂纹尖端的区域，观察到α晶棱柱滑移面上的位错阵列，该区域的生长速率较低。

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.