Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing
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INTRODUCTION
CONTROL of the alpha morphology in cast and wrought (ingot metallurgy) alpha + beta titanium alloys through processing is of great importance in achieving desired combinations of static and dynamic mechanical properties. The presence of low aspect ratio alpha phase increases ductility at low temperature, as well as fatigue crack initiation resistance, and elevated temperature flow characteristics/-6 On the other hand, improved fracture toughness, fatigue crack propagation resistance, and creep strength are observed in high aspect ratio lamellar alpha structure. ~,7-~0 Thermomechanical processing is the most effective method to modify the alpha + beta morphology. Changes in alpha morphology from lamellar to equiaxed are a direct result of the amount of prior deformation, ~~working temperature, and mode of deformation applied." The first two parameters influence microstructure and texture, while the mode of deformation mainly affects texture development. "'~ It was previously observed that when material is cold or warm worked below the recrystallization temperature, equiaxed alpha grains are nucleated at the alpha/beta interfaces upon subsequent annealing. 13In addition, this study claimed that when the beta phase recrystallizes prior to the alpha phase, alpha penetrates the beta/beta interfaces producing new alpha/beta boundaries and a "scalloped" microstructure. ~3Peters et al. 14 reported that hot working of alpha + beta alloys at 925 ~ (1700 ~ followed by annealing at the same temperature resulted in simultaneous recrystallization of the alpha and the beta phase; however, the overall alpha and beta regions maintain I. WEISS is Associate Professor, Wright State University, Dayton, OH 45435. F.H. FROES is Technical Area Manager, Air Force Materials Laboratory, AFWAL/MLLS, Wright-Patterson Air Force Base, OH 45433-6533. D. EYLON, formerly with Metcut-Materials Research Group at Wright-Patterson Air Force Base, is a Professor with the University of Dayton, Dayton, OH 45469-0001. G.E. WELSCH is Associate Professor, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106. Manuscript submitted July 26, 1985. METALLURGICALTRANSACTIONS A
an elongated morphology. On the other hand, when the annealing temperature is below the deformation temperature, recrystallization and an increase in the amount of alpha phase occur concurrently. This takes place because the equilibrium proportion of alpha increases with decreasing annealing temperature. Generally, it has been observed that the rate at which lamellar alpha transforms to equiaxed alpha is a function of annealing temperature, time, and the amount of work the alpha phase has received. 14-~7Studies on isothermal forging and hot compression of Ti-6A1-4V 7'18-2~indicated the possibility of dynamic or metadynamic recrystallization of the alpha phase during deformation. Following quenching the microstructure contained fine equiaxed grains and a dislocation substructure. 2~ Dynamic or metadynamic recrystallization of the beta phase was also obs
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