Ti-6Al-4V在模拟体液中的超高周疲劳裂纹萌生与扩展

On the Very High Cycle Fatigue Crack Initiation and Growth of Ti-6Al-4V Alloy in Simulated Body Fluid Environment

采用超声疲劳方法,开展了医用Ti-6Al-4V在实验室大气和模拟体液两种环境条件下105~109周次范围内的疲劳实验,研究了Ti-6Al-4V的疲劳裂纹萌生与初始扩展行为。实验结果表明：与实验室大气相比,在模拟体液环境下,Ti-6Al-4V的疲劳强度降低了约200MPa。主要原因是疲劳裂纹扩展门槛值减小,促进了疲劳裂纹的萌生和扩展。在两种环境下都发现：高应力短寿命条件下,疲劳裂纹萌生源于微区破碎;而在低应力长寿命条件下,疲劳裂纹萌生源于微区开裂。理论分析表明,粒状亮面（Granular Bright Face,GBF）区的形成与裂纹尖端塑性区和Ti-6Al-4V晶粒尺寸大小有关。

Based on ultrasonic fatigue method and in a range of 10^5- 10^9 cycles, fatigue experiment was carried out for medical Ti-6Al-4V alloy in laboratory atmosphere and simulated body fluid （SBF）, respectively. Fatigue crack initiation and crack initial growth pattern of Ti-6Al-4V alloy was experimentally studied. Experimental results indicate that comparing with that obtained in laboratory atmosphere environment, the fatigue strength of Ti-6Al-4V alloy decreases about 200MPa in simulated body fluid environment. The main reason is the reduction of fatigue crack growth threshold, which promotes the initiation and propagation of fatigue crack. It was discovered in both environments that under high stress and short fatigue life condition, the fatigue crack initiation is due to micro area broken; while under low stress and long fatigue life condition, the fatigue crack initiation is due to micro area cracking. Theoretical analysis show that the formation of both granular bright face （GBF） area and plastic area at crack pointed end are related with the size of Ti-6Al-4V alloy crystalline grain.