Crystal plasticity modeling of deformation and creep in polycrystalline Ti-6242

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TWO-PHASE a/b titanium alloys are widely used in a number of high-performance aerospace, orthopaedic, dental, and sporting goods applications[1] on account of their desirable properties, such as high specific strength, elastic modulus, and fracture toughness. These alloys are created from allotropic transformation of pure titanium from hcp to bcc structures at around the phase transition temperature (;882 °C). Despite the desirable properties, the performance of Ti alloys is sometimes hindered due to creep at low temperatures (T/Tm , 0.2) and at a fraction of the yield strength.[2–5] Significant creep strains have been experimentally reported to accumulate at applied stresses as low as 60 pct of the yield strength.[4] Consequently, sufficient caution must be exercised in the application of these alloys where dimensional tolerance is a critical factor. The low-temperature strain rate sensitivity and creep behavior of Ti alloys are very different from most other metallic alloys.[6] Transmission electron microscopy (TEM) study has shown that deformation actually proceeds via dislocation glide, where the dislocations are inhomogeneously distributed into planar arrays. Planarity of slip has been attributed to the effect of short range order (SRO) of Ti and Al atoms on the hcp lattice.[7] The creep process is of a transient kind or ‘‘exhaustion’’ type where the creep rate continually decreases with time.[2–5] This ‘‘cold’’ creep characteristic has previously been attributed to rate sensitivity effects.[8] The creep in two-phase a/b Ti alloys has also been observed to be strongly dependent on the microstructure, DHYANJYOTI DEKA and DEEPU S. JOSEPH, Graduate Students, and SOMNATH GHOSH, Professor, are with the Department of Mechanical Engineering, The Ohio State University, Columbus, OH. Contact e-mail: [email protected] MICHAEL J. MILLS, Professor, is with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH. Manuscript submitted September 7, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS

with creep resistance decreasing as the colony size increases.[5] Plastic deformation in Ti alloys has considerable dependence on the grain orientation due to the low symmetry of the predominant hcp a phase. Differences in slip system deformation resistance of the individual slip systems result in a highly anisotropic behavior.[9] Grains with their [0001] crystal orientation close to the deformation axis (,c 1 a. oriented) are significantly stronger than other grains. This is because in the [0001] orientation, ,c 1 a. dislocation slip on pyramidal slip systems are activated that have a much higher critical resolved shear strength (CRSS) than the ,a. type slip on basal or prismatic planes. Large local stress concentrations arise in these ,c 1 a. oriented grains due to local load shedding from neighboring softer grains, and hence the local grain morphology is of considerable importance in crack initiation in Ti alloys. Recently, it has been revealed that crack initiation in Ti alloys is associated with gr