On the Temperature Sensitivity of Dwell Fatigue of a Near Alpha Titanium Alloy: Role of Strain Hardening and Strain Rate
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In the present work, the temperature sensitivity of dwell fatigue of a near alpha titanium alloy IMI 834 is investigated. The time-dependent plastic strain accumulation during dwell fatigue loading is used to assess the temperature sensitivity of dwell fatigue. The plastic strain accumulation was considerably high at room temperature and very low at 360 C. This decades old phenomenon is given a revitalized perspective in the current study. Phenomenological modeling using the Kocks–Mecking approach is used to obtain microstructure-based explanations for this observed difference in dwell strain accumulation. Using this approach, dwell sensitivity is correlated with the monotonic strainhardening behavior and strain rate sensitivity for the first time. Insights into the role of solutes in the form of dynamic strain aging (DSA) on dwell fatigue are also thoroughly explained. A simple model considering the combined effects of strain hardening and strain rate sensitivity (SRS) on dwell fatigue is used to predict the dwell fatigue strain evolution at different temperatures. Consequently, a much simpler way of predicting the dwell fatigue response is also demonstrated. Based on these results, some recommendations for developing dwell fatigue-resistant materials are also made. The ambient temperature dwell fatigue in titanium alloys has been known since the 1970s. When a hold
K.U. YAZAR, SUMEET MISHRA, ANISH KARMAKAR, and SATYAM SUWAS are with the Department of Materials Engineering, Indian Institute of Science Bangalore, Bangalore, 560012, India. Contact e-mail: [email protected]. AMIT BHATTACHARJEE is with Titanium Alloys Group, Defence Metallurgical Research Laboratory, Hyderabad 500066, India. Manuscript submitted February 12, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
period is imposed at the peak stress of an otherwise normal load-controlled fatigue cycle, a reduction in fatigue life by an order of magnitude has been reported in the literature.[1] This reduction is called the ‘dwell fatigue debit.’ The time-dependent accumulation of plastic strain during the hold period in addition to the cyclic damage mechanisms lead to premature failure in dwell fatigue.[2] The phenomenon is very interesting in the sense that dwell sensitivity is very high at ambient temperatures (~ 25 C), remains pronounced at intermediate temperatures (~ 120 C) and vanishes at higher temperatures (> 200 C).[3] The insensitivity to dwell at temperatures > 200 C was attributed to dynamic strain aging (DSA) in a variety of a/b titanium alloys.[4] A decrease in the plastic anisotropy of a-titanium at temperatures > 200 C and the role of hydrogen have also been discussed in the literature.[1,5] Ambient temperature dwell sensitivity is attributed to the propensity of titanium alloys for low temperature creep. The contribution of creep to the overall damage in dwell fatigue is very high at higher load levels.[6,7] Therefore, the factors contributing to the low temperature creep (< 0.3 Tm) can be extended for dwell fatigue as well. The majo
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