Microstructural effects on ambient and elevated temperature fatigue crack growth in titanium aluminide intermetallics
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INTRODUCTION
INTERMETALLIC materials have been the focus of increasing research interest in recent years owing to their potential for use in elevated temperature structural applications. Ongoing research efforts on intermetallics are aimed at developing compositional and microstructural conditions which provide acceptable levels of strength, ductility, and damage tolerance at room temperature and at elevated temperatures typical of service conditions, tl-aj Titanium aluminides based on the composition Ti3AI, TiAI, and TiA13 form an integral part of these advanced intermetallic systems. The strong atomic bond between Ti and AI, in conjunction with an ordered D019 crystal structure, renders the Ti3AI alloy brittle at room temperature. Below 700 ~ Ti3A1 predominantly exhibits planar slip, with the slip vectors restricted to a/3 [1120] on the basal and prism planes. However, a different mode of deformation involving a high degree of cross slip emerges typically around 700 ~ t~,2,31 The addition of Nb to a Ti-AI alloy stabilizes the /3 phase, which is softer than the brittle t~2 phase in the intermetallic. Retention of a higher amount of/3 at lower temperatures thus offers one way of improving the ductility. Furthermore, there is evidence indicating that the inclusion of Nb in the intermetallic engenders nonbasal P.B. ASWATH, formerly Graduate Research Assistant, Division of Engineering, Brown University, is Assistant Professor, Department of Mechanical Engineering, The University of Texas at Arlington, Arlington, TX 76019. S. SURESH, Professor, is with the Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted August 23, 1990. METALLURGICAL TRANSACTIONS A
slip with Burgers vectors of the type a/3 [1123]. 16,7,8] The possibility of retaining higher amounts of/3 phase in the intermetallic by the incorporation of Nb also provides ways in which different composite microstructures made up of different mixes of a2 and/3 phases can be judiciously designed. Prior work in dual-phase ferrous alloys t9,1~ has shown that microstructures with a superior resistance to fatigue crack growth can be developed by a proper manipulation of the relative proportions of the ferrite and martensite phases. Similar approaches have also been reported in the case of lithium-containing aluminum alloys, where drastic changes in damage tolerance are observed with different mixtures of matrix and grain-boundary precipitation, tn'12j Based on this prior experience, it is of significant interest, for alloy design purposes, to develop a fundamental understanding of the effects of systematic variations in the relative proportions and distributions of the a2 and/3 phases in the intermetallic on the overall resistance to fatigue crack growth. Such detailed investigations have thus far not been reported in the open literature. Although aspects of damage tolerance in titanium aluminide intermetallics have become topics of considerable academic and practical interest, very little information is presently available on the ef
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