Environmental interactions in high-temperature fatigue crack growth of Ti-1100
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I.
INTRODUCTION
R E S U L T S from air and vacuum c r a c k growth tests carried out on high strength alloys intended for use in elevated temperature applications have often shown a significant increase in fatigue c r a c k propagation resistance and low cycle fatigue strength with the removal o f the air environment. 1~-5] This improvement is generally accompanied by a change o r alteration in fracture m e c h anism observable in fracture surface morphology. N u merous studies have been carried out on these alloys in order to identify the nature o f crack tip damage occurring under varied temperature, loading frequency, load geometry, and environmental test conditions ( c . f . review in Reference 6). In these studies, the detrimental effect o f environment at elevated temperature is primarily associated with oxide-related reactions, causing changes in the intrinsic material behavior, such as grain boundary embrittlement, stress corrosion cracking phenomena, slip inhomogeneity, and adsorption effects. Extrinsic factors, such as closure levels, are also affected, although in a less systematic fashion (the effects not always degrading and dependent on a great n u m b e r o f test-specific variables), by altering the plasticity induced volume changes, degree o f surface roughness, and corrosion fretting building oxide debris. The role o f each o f these m e c h anisms is further complicated by the fact that within a single environment, many materials will experience ranges o f loading frequency in which other time-dependent responses, in particular creep, could exist, thus modifying o r accelerating environmental damage. In the present context, in high-temperature titanium alloys, such as Ti-6A1-4V, IMI-829, and Ti-6242-Si, which are susceptible to creep and reactive with oxygen, a great effect o f R. F O E R C H and A. MADSEN, Graduate S t u d e n t s , and H. GHONEM, Professor, are with the Mechanics of Solids Laboratory, Department of Mechanical Engineering and Applied Mechanics, University of Rhode Island, R I 0 2 8 8 1 . Manuscript submitted March 30, 1992. METALLURGICAL TRANSACTIONS A
combined creep-environment-fatigue damage can be expected. [7] Recognizing this f a c t , the new silicon bearing near alpha titanium a l l o y , T i - l l 0 0 , is studied specifically with regard to the high-temperature environmental effects on fatigue c r a c k growth rate (FCGR) in a multitude o f loading and environmental conditions. This alloy, nominally composed as Ti-6AI-2.8Sn-4.0Zr0.4Mo-0.45Si-0.070-
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EXPERIMENTAL RESULTS
Because the elevated temperature fracture process involves both oxidation and creep damage mechanisms, the experimental program was based on carrying out a series o f different loading frequency tests to be run in air and vacuum environments. Conclusions are made by comparing cases o f identical frequency loading between the two environments. It should be mentioned that this 1322--VOLUME
24A, JUNE 1993
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