Effect of creep strain on microstructural stability and creep resistance of a TiAi/Ti 3 ai lamellar alloy
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INTRODUCTION
THE present authors have previously reported that the minimum creep rate (emin)of a two-phase TiA1/Ti3AI alloy with a lamellar microstructure is about an order of magnitude lower than ~rnin of single-phase TiA1 and Ti3A1 alloys. t~j Transmission electron microscopy examination of the crept alloys revealed that the TiA1 and Ti3A1 phases within the lamellar microstructure possess significantly higher dislocation densities than single-phase TiA1 and Ti3Al alloys crept to the same strain. ~5: The higher dislocation density observed in the crept lamellar alloy is consistent with elevated temperature tensile test results from the work of Rao and Tangri,[6] which reveal that the workhardening rate of a lamellar TiA1/Ti3A1 alloy is substantially higher than the work-hardening rates of single-phase TiA1 and Ti3AI alloys over a wide temperature range. Recall that power-law creep can be regarded as a competition between recovery and work-hardening processes; t7,sJ quasi-steadystate creep in the secondary creep regime is attained when the rates of these two processes are approximately equal. Thus, both the lower ~min and the higher work-hardening rate of the lamellar TiA1/Ti3A1 alloy can be considered to reflect the higher dislocation density in the lamellar TiA1/ Ti3A1 alloy compared to single-phase TiA1 and Ti3A1 alloys. Several mechanisms have been proposed to account for the higher dislocation density in the TiA1/Ti3A1 alloy with a lamellar microstructure. The proposed mechanisms include storage of geometrically necessary dislocations as a result of deformation of a plastically inhomogeneous material,[ 9,~~ increased plasticity due to impurity segregation from the TiA1 phase to the Ti3A1 phase,VZ] and an increased dislocation source density associated with structural ledges on semicoherent TiA1/Ti3AI interfaces.t4,~3] In an effort to J.A. WERT, Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. M.F. BARTHOLOMEUSZ, Research Scientist, Corporate Research and Development Laboratory, Reynolds Metals Co., Richmond, VA 23219. Manuscript submitted December 21, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
evaluate the various proposed mechanisms, the present authors used annealing treatments to decrease the interphase interfacial area per unit volume (Sv), and subsequent creep experiments explored the effect of these changes on kmin.E4~ A strong correlation between Sv andEmin was found, lending support to the hypothesis that the high dislocation density of the lamellar TiA1/TiaA1 alloy results from a high dislocation source density at interphase interfaces. In addition to observing the effects of prior annealing (thermal exposure) on em~othe present authors have found that the secondary creep rate of the TiA1/TiaAl lamellar alloy is significantly increased as a result of prior creep strain (thermomechanical exposure) in compression creep tests, tq While this would be an ordinary observation for creep in tension where damage processes such a
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