A Hydrogen-Induced Decohesion Model for Treating Cold Dwell Fatigue in Titanium-Based Alloys
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TRODUCTION
NICKEL-BASED superalloys and Ti alloys in advanced turbopropulsion systems can experience multiple damage mechanisms that involve the interaction of cycle-dependent crack growth due to fatigue and time-dependent crack growth mechanisms such as creep, oxidation, and stress corrosion. To enable the design and reliability assessment of advanced turboengine systems, a probabilistic life-prediction methodology that treats concurrent cycle-dependent and time-dependent crack growth was recently developed[1] to address multiple damage modes that are pertinent in gas turbine alloys. In this investigation, we extend this probabilistic, time-dependent fracture life-prediction methodology to treat cold dwell fatigue in Ti-alloy disks. To reduce engine weight, there is a trend to use Ti alloys to replace the heavier Ni-based alloys in certain turbine sections by increasing the allowable service temperatures for Ti alloys.[2,3] There is also a desire to increase the strength of Ti alloys by controlling the texture in order to optimize the strength to match the KWAI S. CHAN, Institute Scientist, and JONATHAN MOODY, Senior Research Engineer, are with the Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238. Contact e-mail: [email protected] Manuscript submitted September 21, 2015. Article published online February 17, 2016 2058—VOLUME 47A, MAY 2016
in-service stresses.[4,5] Like Ni-based alloys, Ti-based alloys also exhibit cycle-dependent and time-dependent crack growth responses, which are often functions of temperature,[4,7] loading rate, and microstructural parameters such as grain size, phase morphology,[8,9] and crystallographic orientation.[10,11] There are considerable interactions between creep and fatigue, leading to dwell time effects[12,13] and hydrogen embrittlement.[14,15] Future engine designs, therefore, require consideration of interactions of multiple damage modes in Ti alloys that are operative over the entire range of service temperatures. For Ti alloys that are used in the compressor section of gas turbine engines, some near-a and a/b Ti alloys exhibit sensitivity to cold dwell fatigue.[12,13,16–23] Observed in Ti alloys such as Ti-6Al-2Sn-4Zr-6Mo (Ti-6246),[6,22] Ti-6Al-4V,[6,13,17–20] and IMI 685,[12,13,16–18,22] cold dwell fatigue is a time-dependent fracture process that involves fatigue in the presence of creep to produce low-energy fracture facets in hard alpha grains at ambient temperature. These low-energy fracture facets may be cleavage facets, decohered slip planes, hydride habit planes, or a/b interfaces. As indicated by Evans and co-workers,[12,13,16,21] the salient feature of cold dwell fatigue is facet formation with a low-energy fracture appearance. This fracture mode appears to exhibit a different stress-life (S-Nf) relation compared to fatigue cracking along slip bands, which is a strain-dominated fracture
METALLURGICAL AND MATERIALS TRANSACTIONS A
process.[16] Besides cold dwell fatigue, low-energy fracture facets were also observed in Ti alloys in conjunction with
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