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

A commercial (alpha+beta) titanium alloy Ti-6bAl-1.6Zr-3Mo-0.3Si (designate d as VT9) of Russian origin is used for compressor discs and blades in jet engines up to temperatures of 773K. The beta transus for this alloy is simi lar to 1233K. The alloy blanks were subjected to beta solution treatment at similar to 1293K for 0.5 hours and cooled at different rates namely water quenching (WQ), air cooling (AC) and furnace cooling (FC). The blanks WQ, A C and FC were aged at 973K for 12 hours and these were designated as WQA, A CA and FCA respectively. Low cycle Fatigue (LCF) testing was carried out fo r all the conditions namely WQ, WQA, AC, ACA and FC, FCA at room temperatur e in the total strain amplitude range of 0.70% +/- 1.7% at a frequency of 0 .2Hz using triangular wave form. White Coffin-Manson (C-M) plots between pl astic strain amplitude versus number of reversals to failure showed dual sl opes 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 fra cture 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, FC A, SEM technique was used to study the slip band activity close to the frac ture ends of the specimens failed under fatigue conditions from lower and h igher strain amplitude regions. In the case of WQ and FCA where single slop e is seen in C-M plots, there is no difference in the slip band activity an d thus it indicates no change in deformation mode. However, distinct differ ences in slip band activity in specimens from lower and higher strain ampli tude regions conform to the dual slope behavior in C-M plots in WQA and FC conditions.