SCC Characterisitics of Ti-6 Al-4V Alloy and Analysis of Cracking Process by Visual Observations and AE Technique

Abstract

SCC tests have been carried out in CH₃OH+0.5% I₂ solution for high strength Ti-6 Al-4V alloy solution treated at 926°C, 940°C, 954°C, and 968°C, and aged at 538°C or 550°C after (α+β) processing. Fatigue precracked CT specimens with a thickness of 7mm were tested under constant load. The specimens were prepared so that the cracking direction was parallel (specimen code: L) or normal (specimen code: T) to the prior rolling direction. Crack growth was monitored by measuring crack opening displacement and Acoustic Emission (AE). Both crack growth rate and AE count rate showed three stages on the curves plotted versus stress intensity factor K, demonstrating a good correlation between these two parameters. The measurement of the plateau crack growth rate at K=120kg·mm^-3/2 in stage II showed that the SCC susceptibility of the specimens-L was higher than that of the specimens-T. The higher SCC susceptibility was attributed to the large prior β-phase grains elongated in the direction of the prior hot rolling at high temperature above β transus. The cracking morphology was shown to be the mixture of intergranular cracking along the prior β-phase grain boundaries or the (α-β) phase boundaries and transgranular cracking. The detailed observations of microstructure along the cracking path and the analysis of AE detected discontinuously during the crack growth revealed that the cracking was obviously discontinuous and occurred in jumps of some α-grain diameters on a small scale and of a prior β-phase grain diameter on a large scale. In this case grain boundaries often arrested the cracks, although examples of cracks accelerating on crossing a boundary were observed.