The effect of microstructure on the deformation modes and mechanical properties of Ti-6Al-2Nb-1Ta-0.8Mo: Part I. Widmans
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I.
INTRODUCTION
THEmicrostructure of alpha + beta titanium alloys can be greatly varied by modifications in primary processing procedures, and mechanical properties are sensitive to microstructural variations. ,.2 By suitable selection of the working and annealing temperature with respect to the beta-transus temperature, microstructures can be developed having equiaxed, platelet, or grain boundary alpha in a retained beta matrix; both phases can be fine, medium, or coarse, and continuous or noncontinuous. The plate-like alpha precipitates that nucleate and grow below the beta-transus produce a Widmanst~itten structure. The plates often precipitate in colonies of the same crystallographic orientation, presumably due to autocatalytic nucleation. 1 Martensite may form in quenched alloys with a plate-like or lath morphology. The individual plates tend to have different crystallographic orientations, in contrast to the colony microstructure formed by nucleation and growth, and they often have an internal structure. 3 The various microstructural features present in combination in alpha-beta alloys produce somewhat complicated strengthening effects. However, it is agreed that the strength results from a combination of solid solution, interface, and texture effects. Often certain microstructural features affect various mechanical properties differently, and, as a result, no "optimum microstructure" exists that improves all properties simultaneously. ~ In addition to microstructural parameters, solute elements are known to have a large effect on the slip behavior of the alpha phase, and the slip behavior is known to have a significant influence on fracture related properties. Consequently, the effect of alloy composition must be assessed even though microstructure-property reF.S. LIN is Research Scientist, Metallurgical Research, Reynolds Metals, Richmond, VA 23261; E.A. STARKE, Jr. is Earnest Oglesby Professor of Materials Science, Department of Materials Science, University of Virginia, Charlottesville, VA 22901; S.B. CHAKRABORTTY is Resident M. D., Norfolk General Hospital, Norfolk, VA; and A. GYSLER is Professor, Department of Metallurgy, Technische Universit~it Hamburg, Hamburg, West Germany. Manuscript submitted August 26, 1983.
METALLURGICALTRANSACTIONSA
lationships may be known for alloys of similar microstructure but different chemistry. The alloy Ti-6A1-2Nb-ITa-0.8Mo (Ti-6211) is an alpha + beta alloy having high specific strength, fracture toughness, and resistance to corrosion. Under certain processing conditions Ti-6211 has been shown to have a microstructure very similar to Ti-6A1-4V. 4 However, to the authors' knowledge there have been no studies or documentation on the processing-microstructure-deformation-mechanical property relationships of Ti-6211. We have recently completed such a study on Ti-6211 and the results are divided into two parts. Part I is concerned with modifications of the Widmanst~itten structure present in the as-received plate which are produced by thermal treatments, and Part II is c
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