Low-cycle dwell-time fatigue in Ti-6242
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I. INTRODUCTION Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) is a near-alpha titanium alloy that has been shown to be susceptible to reduced fatigue life when a dwell at maximum load is imposed on each loading cycle during ambient-temperature, load-controlled fatigue tests. Far fewer cycles to failure are observed when compared to tests without dwell. Much of the early and recent work was performed by Evans and Gostelow,[1] Evans,[2] Eylon and Hall,[3] and Woodfield et al.,[4] who studied low-cycle dwell-time fatigue (LCDF) in IMI 685 and Ti-6242. The addition of a several-minute dwell at ambient temperature sometimes resulted in over an order of magnitude fewer cycles to failure over the range of stresses tested. Later work by Evans[2] revealed that, at higher stresses, a 5-minute dwell decreased the cycles to failure by over an order of magnitude, but, as the stress decreased, the decrease in cycles to failure diminished to only a factor of 5, or so. Whether there is a “merging” of the behaviors at lower stresses is unclear. The LCDF mechanism is not fully understood. Evans[1,2] suggested the importance of time-dependent plasticity during the dwell, while others appear to have emphasized hydrogen.[5] Another factor that appears to be important is the microstructure. Evans and Gostelow,[1] Evans,[2,6] and Eylon and Hall[3] suggested that an “aligned” alpha microstructure in slower-cooled alloys can increase the vulnerability of near-alpha titanium alloys to dwell fatigue. Evans and Bache suggested that the prior beta grain size may also be important.[7] The objective of the present work was to confirm whether near-alpha Ti-6242 produced by alpha/beta processing is vulnerable to LCDF and what microstructural features affected the susceptibility. The microstructure can be manipulated by changing the solution-annealing temperature to below the beta transus (Tb). The alpha morphology is also influenced by the cooling rate. It is postulated that the microstructure may affect LCDF in a variety of ways. For example, M.E. KASSNER, Northwest Aluminum Professor, is with the Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331. Y. KOSAKA, Staff Process Metallurgist, and J.S. HALL, Director, Process Engineering and Development, are with OREMET WAH CHANG, an Allegheny Teledyne Co., Albany, OR 97321. Manuscript submitted February 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
the amount of primary alpha phase can affect strength, which would tend to alter plasticity and, perhaps, affect the susceptibility to LCDF, according to a time-dependent plasticity model. Some suggest that small cracks may propagate through the alpha phase and that groups of primary alpha phase within close proximity to one another and with similar crystallographic orientation could further enhance vulnerability to LCDF.[1–4,6–8] Early work by the authors[9] suggests that microtextured or aligned primary alpha phase becomes more evident with increased primary alpha content. Woodfield et al.[4] suggested that thermomechanical proc
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