Observation of tension-compression asymmetry in α and α/β titanium alloys

Titanium alloys are known to creep at room temperature at stresses below the macroscopic yield stress. In this study a two-phase alpha/beta alloy, Ti-6Al-2Sn-4Zr-2Mo (Ti-6242), and a single phase binary alloy, Ti-6Al, were observed to exhibit a dramatic asymmetry between tension and compression during room temperature creep. It was observed that the aero-engine alloy, Ti-6242, accumulated about five to six times higher creep strains in tension than in compression. A much smaller asymmetry was observed for the 0.2% yield strength such that it is higher in compression. It is shown that the present observations cannot be explained by the activation of predominantly slip. Further. CRSS measurements from single colony crystals of Ti-6242Si oriented for basal -type slip showed that there is a significant asymmetry between tension and compression, with the CRSS observed to be higher in compression. Studies on the single-phase alpha alloy Ti-6Al with low oxygen content suggest that the interstitial content could also play a significant role in determining the behaviour of these alloys. The single phase alloy also exhibited tension-compression asymmetry in creep, although it was observed to be less pronounced compared to the two phase alloy. HRTEM results suggest that the core structure of -type dislocations is altered by the addition of alloying elements, especially oxygen, and that the tension-compression asymmetry is primarily due to the behaviour of -type dislocations.