Observations of room-temperature creep recovery in titanium alloys
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TITANIUM alloys are attractive structural materials for a variety of aerospace, biomedical, and energy applications. However, it has been well established that these materials exhibit significant primary creep strains at ambient temperatures and at stresses well below their macroscopic yield stress.[1–4] The time dependence of this creep behavior for both single-phase ␣-Ti and two-phase ␣/ Ti alloys[5,6] has been observed to follow a power law behavior ⫽ Ata
[1]
where A is a pre-exponential constant that depends on both microstructure and stress, and the time exponent a indicates the level of exhaustion during the creep process. For single ␣-phase titanium alloys, the values of a are commonly 0.2.[6] This relatively slow exhaustion has been attributed to low observed work-hardening rates in these alloys, arising from the planar nature of the slip of a/3具1120典 (a-type) dislocations on basal and prism planes. The effects of repeated unloading during creep testing have been demonstrated by Shetty and Mechii,[7] who studied the creep response of high-purity polycrystalline and singlecrystal Al. It was shown that significant acceleration or
M.F. SAVAGE, formerly with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, is National Research Council Postdoctoral Fellow, Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8553. T. NEERAJ and M.J. MILLS are with the Department of Materials Science and Engineering. The Ohio State University, Columbus, OH 43210. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
retardation of the observed creep rate could occur in connection with numerous experimental factors, including applied stress, stress cycle amplitude, and temperature. Below a threshold stress, only creep retardation was observed. At elevated temperatures (T ⬎ 100 ⬚C), the effect of unloading abated, indicating the likelihood of spontaneous dislocation structure relaxation. The backward motion of dislocations due to internal stresses was cited as necessary for any effect on the macroscopic creep response. Grain boundary effects were discounted due to the observation of creep retardation and acceleration in single-crystal material. Similar observations of cyclic creep acceleration have been made in polycrystalline ␣/ Ti-6Al-2Nb-1Ta-0.8Mo by Miller et al.[8] This alloy was tested in a variety of microstructural conditions, ranging from coarse ␣/ colonies to a fine basketweave structure. Cyclic creep was performed with 2-minute dwells at a maximum str
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