A study of the change in deformation mode under low cycle fatigue conditions of a commercial (?+?) titanium alloy

Abstract

A commercial (?+?) titanium alloy Ti-6Al-1.6Zr-3Mo-0.3Si (designated as VT9) of Russian origin is used for compressor discs and blades in jet engines up to temperatures of 773K. The ? transus for this alloy is ?1233K. The alloy blanks were subjected to ? solution treatment at ?1293K for 0.5 hours and cooled at different rates namely water quenching (WQ), air cooling (AC) and furnace cooling (FC). The blanks WQ, AC and FC were aged at 973K for 12 hours and these were designated as WQA, ACA and FCA respectively. Low cycle Fatigue (LCF) testing was carried out for all the conditions namely WQ, WQA, AC, ACA and FC, FCA at room temperature in the total strain amplitude range of 0.70% 1.7% at a frequency of 0.2Hz using triangular wave form. While Coffin-Manson (C-M) plots between plastic strain amplitude versus number of reversals to failure showed dual slopes in WQA, AC, ACA and FC conditions, single slope was observed in WQ and FCA conditions. Study of the slip activity on the surface close to the fracture end of the specimen failed in LCF for the AC and ACA conditions using scanning electron microscopy (SEM) clearly correlated with the results of TEM study. For other thermally treated conditions namely WQ, WQA and FC, FCA, SEM technique was used to study the slip band activity close to the fracture ends of the specimens failed under fatigue conditions from lower and higher strain amplitude regions. In the case of WQ and FCA where single slope is seen in C-M plots, there is no difference in the slip band activity and thus it indicates no change in deformation mode. However, distinct differences in slip band activity in specimens from lower and higher strain amplitude regions conform to the dual slope behavior in C-M plots in WQA and FC conditions.