Influence of microstructure on high-cycle fatigue of Ti-6Al-4V: Bimodal vs. lamellar structures
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INTRODUCTION
A1992 study conducted by the Scientific Advisory Board of the United States Air Force targeted high-cycle fatigue (HCF) as the single largest cause of turbine engine failures in military aircraft.[1] HCF can result in essentially unpredictable failures of engine components, particularly turbine blades, due to the premature initiation of fatigue cracks at small defects and their rapid propagation under high-frequency vibratory loading. To address this problem, a consortium of industrial, government, and academic programs was charged
with the overall task of modifying existing design methodologies for improved HCF reliability. These programs have largely focused on a single alloy, Ti-6Al-4V, in a single microstructural condition, namely, the solution-treated and overaged (STOA) or bimodal condition, which is typically used for blade and disk applications in the front (low-temperature) stages of the engine. The current study seeks to extend these observations for the standard bimodal Ti-6Al-4V microstructure to a fully lamellar  -annealed microstructure in the same alloy and to specifically compare the relative merits of these two microstructures for HCF applications. II. BACKGROUND
R.K. NALLA, Graduate Student, and R.O. RITCHIE, Professor, are with the Department of Materials Science and Engineering, University of California, Berkeley, CA 94720-1760. B.L. BOYCE, formerly Graduate Student with the Department of Materials Science and Engineering, University of California, is Senior Member, Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185-1411. J.P. CAMPBELL, formerly Graduate Student with the Department of Materials Science and Engineering, University of California, is Senior Manufacturing Engineer, Metals Fabrication Division, General Motors, Troy, MI 48084. J.O. PETERS, formerly Post-doctoral Researcher with the Department of Materials Science and Engineering, University of California, is Research Associate, Technische Universita¨t Hamburg-Harburg, D-21073 Hamburg, Germany. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
The problem of HCF in turbine components is typically associated with a variety of drivers,[2] which vary with the location of interest (Figure 1). In general, components such as blades and disks are subjected to HCF loading associated with the high-frequency (⬎1 kHz) vibrations in the engine, superimposed onto a low-cycle fatigue component associated with the start-to-stop cycles.[2,3] In the case of blades, this leads to a large variation in load ratios (the ratio (R) of minim
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