On the Macroscopic Flow Behavior of the Phases Present in Ti-24Al-llNb
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ON THE MACROSCOPIC FLOW BEHAVIOR OF THE PHASES PRESENT IN Ti-24A1-llNb S. J. G1THS+ AND D. A. KOSS++ +General Electric Company, 1Neumann Way, M89, Cincinnati, OH 45215 ++Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 ABSTRACT The deformation behavior of two alloys whose compositions (Ti-25A1-9Nb and Ti-1 lAl23Nb in at.%) correspond to alpha-two and beta phases in the Ti-24Al-1 lNb alloy has been investigated. Results from both compression and tensile tests over a range of temperatures from 27" to 650"C indicate that the yield stress as well as the strain and strain-rate hardening characteristics of the Ti-24-11 alloy are controlled by the Ti 3Al-base alpha-two phase. In contrast to the Alpha-two alloy, the Beta alloy has a flow stress which is very sensitive to temperature and strain rate at 650"C, suggesting the onset of high temperature creep processes. INTRODUCTION A large amount of developmental effort has been directed at the use of the titanium aluminide Ti3AI (alpha-two) as a basis for a new generation of high temperature titanium alloys. In particular, alpha-two alloys containing small amounts of beta phase exhibit potential because of their ductility at low temperatures. As a result, a new generation of multiphase alpha-two base alloys is being developed. The Ti-24A1-11Nb (Ti-14Al-2lNb in wL%) alloy serves as a useful model system for these alloys. While several studies have recently been published (or are in print) on the deformation and fracture behavior of the multiphase Ti3AI-Nb alloys [1-4], relatively little is known of the individual deformation behavior of the alpha-two and beta phases present in these alloys. At best, deductions can be made from the behavior of stoichiometric Ti3 A1 [5-7]. This communication reports the results of a study of the macroscopic flow behavior of two alloys, Ti-25Al-9Nb and Ti-1 1A1-23Nb (in at.%), whose respective compositions correspond to the alpha-two and beta phases present in the Ti-24-11 alloy after it has been heat treated to produce a basketweave alpha-two plus beta microstructure. As will be shown, the macroscopic deformation behavior of the Alpha-two alloy indicates that flow within the multiphase Ti-24-1 1 alloy (at least in the basketweave microstructural condition) is controlled by the alpha-two phase. On the other hand, the thermally activated of flow behavior in the Beta alloy at 650*C suggests a potentially serious loss of creep strength in those microstructures in which the beta phase controls the deformation. The fracture behavior of these two alloys as well as the strong age-hardening response within the Beta alloy will be discussed elsewhere [8]. EXPERIMENTAL PROCEDURE The alloys examined in the present study have the compositions of the phases present in the Ti-24Al-11Nb after it was subjected to the following heat treatment: 1106°CA hr, He gas quench; 1038"C/1 hr, He gas quench; and finally 760"C/ hr, He gas quench. Utilizing STEM and energy dispersive x-ray analysis to indicate the co
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