微結構和應力比對Ti-6Al-4V高周和超高周疲勞行為的影響

微結構和應力比對Ti-6Al-4V高周和超高周疲勞行為的影響EFFECTS OF MICROSTRUCTURE AND STRESS RATIO ON HIGH-CYCLE AND VERY-HIGH-CYCLE FATIGUE BEHAVIOR OF Ti-6Al-4V ALLOY

采用旋轉彎曲和超聲疲勞實驗分別測試了全等軸和等軸雙態(tài)組織Ti-6Al-4V合金的高周和超高周疲勞性能, 并用SEM觀察了疲勞斷口特征. 結果表明: 2種組織Ti-6Al-4V合金的高周和超高周疲勞行為相似, 不同應力比下, 其S-N曲線均表現出單線形或雙線形的形式; 存在滑移機制和解理機制2種疲勞破壞機制. 隨應力比增加, 2種組織Ti-6Al-4V合金的高周和超高周疲勞破壞機制均從滑移機制向解理機制轉變. 基于疲勞壽命和疲勞強度建立模型分析了應力比對2種機制之間競爭行為的影響, 模型預測結果與實驗結果趨勢吻合.

Titanium alloys have been widely used as superior engineering materials because of their high specific strength, high temperature resistance and high corrosion resistance. In their engineering applications such as used in aircraft engines, titanium alloys may experience even 1010 fatigue cycles. Recently, faceted crack initiation was observed in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes of titanium alloys, which resulted in a sharp decrease in fatigue strength. Therefore, the HCF and VHCF of titanium alloys have both scientific significance and engineering requirement. In this work, the effects of microstructure and stress ratio (R) on HCF and VHCF of a Ti-6Al-4V alloy have been investigated. Fatigue tests were conducted on a rotating-bending fatigue machine and an ultrasonic fatigue machine. All the fatigue fracture surfaces were observed by SEM. The results show that the HCF and VHCF behaviors of the fully-equiaxed and the bimodal Ti-6Al-4V alloy are similar. The observations of fracture surface indicate that two crack initiation mechanisms prevail, i.e. slip mechanism and cleavage mechanism. With the increase of stress ratio, the crack initiation mechanism switches from slip to cleavage. The S-N curves present the single-line type or the bilinear type. For the cases of rotating-bending and ultrasonic axial cycling with R= -1.0, -0.5 and 0.5, the S-N curves are single-line type corresponding to the slip mechanism or cleavage mechanism. For the cases of R= -0.1 and 0.1, the S-N curves are bilinear type corresponding to both slip and cleavage mechanisms. A model based on fatigue life and fatigue limit is proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results.

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