Modeling thermomechanical fatigue life of high-temperature titanium alloy IMI 834
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I. INTRODUCTION
SERVICE conditions for many high-temperature components combine thermal transients during startup and shutdown with mechanical strain cycles. Such complex thermomechanical fatigue (TMF) cycles are often the lifelimiting factor in certain components of jet engines. As a result of the aerospace industries’ needs for new materials with higher strength-to-weight ratios at elevated temperatures, near-a titanium alloys with a maximum use temperature of about 600 8C became available commercially in recent years. These alloys were designed mainly to replace heavier nickel-base superalloys in the compressor section of jet engines. As high-temperature titanium alloys have low elastic modulus and high strength, thermally induced strains of the order of 1 pct can be totally elastic strains in these materials. The TMF was of no concern in earlier titanium alloys, which had a temperature potential of about 450 8C to 500 8C, thus, most previous TMF studies involving titanium alloys focused on continuous fiber reinforced titanium matrix composites, which have much higher temperature capabilities.[1] It is thus not surprising that the TMF behavior of monolithic titanium alloys has only been addressed recently.[2–4] With the advent of high-temperature titanium alloys that have a target operating temperature of 600 8C, cyclic plastic strains may develop during TMF loading, as the cyclic flow stress of titanium alloys drops significantly at around 600 8C.
¨ H.J. MAIER, Professor, is with the Lehrstuhl fur Werstoffkunde (Materi¨ als Science), Universitat Paderborn, D-33098 Paderborn, Federal Republic of Germany. R.G. TETERUK, Graduate Research Assistant, and H.-J. ¨ CHRIST, Professor, are with the Institut fur Werkstofftechnik (Materials ¨ Technology), Universitat Siegen, D-57068, Siegen, Federal Republic of Germany. Manuscript submitted March 5, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
So far, life models developed for high-temperature titanium alloys were intended mainly to predict fatigue behavior under isothermal low-cycle fatigue (LCF) or creep-fatigue loading situations. Thermomechanical fatigue life prediction models for high-temperature titanium alloys are still rare. Dai et al.[4] developed a life model based on fatigue crack growth studies performed on Ti-6Al-2Sn-4Zr-6Mo that includes TMF loading conditions. That work has demonstrated that oxygen-induced embrittlement is the dominant time-dependent damage mechanism in that alloy. The maximum temperature of the TMF cycles used in that study, however, was only 480 8C. Therefore, the objective of the present study was to develop a life model that covered a broader temperature range, and thus, would be applicable to more recent near-a titanium alloys with a maximum use temperature of about 600 8C. A variety of life prediction models have already been proposed for TMF loading conditions.[4–8] Note, however, that TMF tests require expensive test equipment and are often time-consuming. Hence, TMF tests conducted in the laboratory are short-term tests, and
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