Microstructural influences on fatigue crack propagation in Ti-10V-2Fe-3Al
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I.
INTRODUCTION
THEnear threshold Fatigue Crack Propagation (FCP) rates of most structural metals are influenced by microstructure, but this influence is frequently suggested to be modest, or even nonexistent, in the Paris Law region, t In contrast, FCP in a and (c~ + /3) titanium alloys is strongly influenced by microstructure over the entire range of growth rates, and thus has been the subject of considerable attention and several reviews.2-7 The objective of this study was to determine the effects of microstructural variations on the rate of fatigue crack propagation in the /3-Ti alloy, Ti-10V-2Fe3 A l - - o f t e n abbreviated Ti-10-2-3. This particular alloy was chosen for two reasons. First, there has been a great deal of attention given to Ti- 10-2-3 and other/3-Ti alloys in fatigue critical applications, such as in aircraft structural components, 8'9'~~and despite this, very little has been done to try to correlate microstructures to FCP. That which has been done in this field has been either inconclusive,t~ or has involved idealized (model) alloys of little commercial interest. J2.13The second reason to study FCP in Ti-10-2-3 is that extremely high strengths can be developed through a variety of different microstructures, and thus one is able to test the influence of several markedly different microstructural features on the rate of FCP, and do so without having to cope with large differences in strength, ductility, and modulus. In this particular study, eleven different microstructural conditions were tested, all having yield strengths of roughly 1240 MPa. Some of the types of microstructural variables that were incorporated in the study were: the presence of fine a precipitates; the presence and volume fraction of coarse, globular, and soft primary a particles (ap); the presence of a soft, continuous grain boundary film; the size and volume fraction of fine precipitation; and the extent of recrystallization. One should note that the above microstructural features cannot be altered independently. For example, if one condition were to have a higher volume T.W. DUERIG, formerly with Carnegie-Mellon University, is now Manager of Alloy Development, Raychem Corporation, 300 Constitution Drive, Menlo Park, CA 94025. J.E. ALLISON, formerly with BrownBoveri in Switzerland, is now Senior Research Scientist at Ford Motor Company, Detroit, MI. J.C. WILLIAMS is Dean of Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted September 30, 1983.
METALLURGICALTRANSACTIONS A
fraction of soft aj,, it would also have to have a finer distribution of fine a precipitates to bring the strength back to the targeted value.
I1.
MICROSTRUCTURAL BACKGROUND
The same microstructural complexity and versatility that makes Ti-10-2-3 an interesting candidate for such a study, makes a full microstructural characterization somewhat l e n g t h y - - t o o lengthy to present in the context of this particular paper. Nevertheless, the nature of the investigation mandates that a terminology be defined and tha
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