Constitutive properties of hard-alpha titanium
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I. INTRODUCTION
PREMIUM -grade titanium alloys, formerly processed by double vacuum arc remelting (VAR) and now processed by triple VAR, are used for fan and compressor rotors and disks in aircraft jet engines. Occasional upsets during processing can result in the formation of nitrogen-rich alpha titanium (also referred to as hard-alpha in this article), which is brittle and often has microcracks and microvoids associated with it. The hard-alpha anomalies were measured to contain 3.5 to 14.8 wt pct N and up to 2.5 wt pct O.[1] A chemical or compositional definition of hard-alpha does not exist, but it is generally accepted that hard-alpha contains at least a few weight percentages (e.g., 3 to 4 wt pct) of nitrogen. The only defining characteristic of hard-alpha Ti is that it has a high hardness, which is influenced, in addition to N, by other interstitials and alloying elements also. Although rare, the hard-alpha anomalies have led to uncontained engine failures that resulted in fatal accidents such as the incident at Sioux City in 1989. As a result of the accident at Sioux City, the Federal Aviation Administration (FAA) requested in 1991 that industry, through the Aerospace Industries Association (AIA), review available techniques to see whether a damage-tolerance approach could be introduced to produce a reduction in the rate of uncontained rotor events. The industry working group concluded that additional enhancements to the conventional rotor-life-management methodology could be estab-lished that would explicitly address the hard-alpha anomalous conditions. In response to the AIA recommendations, the FAA “Titanium Rotating-Component Review Team Report,”[2] and the AGARD (Advisory Group for Aerospace Research and
K.S. CHAN, Institute Scientist, and G.R. LEVERANT, Program Director, are with the Southwest Research Institute, San Antonio, TX 78238-5166. L. PEROCCHI, Metallurgical Specialist, is with General Electric, CR&D Center, Schenectady, NY 12301. Manuscript submitted January 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Development),[3] National Aeronautics and Space Administration,[4] and United States Air Force[5,6] experience and recommendations on damage-tolerance concepts and operational experience for gas turbine disk alloys, the FAA in 1995 funded an industrial team, which consists of Southwest Research Institute, Allied Signal (which is now Honeywell), RollsRoyce Allison, General Electric (GE), and Pratt & Whitney, to develop a probabilistic-based, damage-tolerance design code to augment the current safe-life approach for life management of commercial aircraft gas turbine rotors/disks. The design code is not intended to replace existing design methods, but to provide an additional tool that the engine manufacturers can use for reliability assessment. Under the sponsorship of the FAA, the industrial team has developed a probabilistic design methodology for treating hard-alpha defects in Ti rotors. A supplementary effort has involved the development of a microcode that is incorporated into a c
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