The effect of grain size on the ambient temperature creep deformation behavior of a beta Ti-14.8 V alloy
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BETA-TITANIUM alloys are technologically important and find applications in various areas including aerospace, chemical, and biomedical industries. In some of these applications, they are subjected to stress over extended periods of time. Recently, ambient-temperature creep studies were carried out on ␣ -Ti alloys.[1] Creep deformation was observed and was found to increase with increasing grain size. Further, the mode of creep deformation was found to be due to timedependent twinning and slip. However, studies conducted on a  -Ti-13 wt pct Mn alloy showed no significant ambienttemperature creep.[2] In this regard, it is of interest to introduce the concept of  -titanium alloys. Traditionally, the stability of  alloys is defined in terms of c , where c is the critical amount of  stabilizer required to retain metastable  phase upon quenching without martensitic transformation. For example, c for molybdenum is 10 wt pct, that for manganese is 6.5 wt pct, and that for vanadium is 16 wt pct. Depending on the amount of  stabilizer, the alloys can be loosely classified as lower and higher stable alloys. For instance, a  alloy with 13 wt pct Mn is more stable than an alloy with 9.4 wt pct Mn. Stability can play a significant role in affecting creep behavior. Less stable alloys may contain intermittent metastable phases, such as phase, and are prone to twinning and stress-induced transformations on deformation. The question then arose as to whether a  alloy with lower stability would exhibit significant creep, and if so, what would the effect of grain size be on the creep behavior. The research described in this article is part of a study to determine the effect of stability of the  phase and grain size on the ambient-temperature tensile and creep behavior of  titanium alloys. An earlier communication[3] characterized the deformation products obtained on tensile deformation of a Ti-14.8 V alloy. This article describes the ambienttemperature creep behavior of a Ti-14.8 V alloy in terms of deformation modes, extent of deformation, and grain size. ANAND RAMESH, Graduate Student, and SREERAMAMURTHY ANKEM, Associate Professor, are with the Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 207422115. Manuscript submitted October 16, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Time-dependent twinning was observed in this alloy system. A quantitative relation between the instantaneous plastic and creep strain and time of testing was also sought. Further, the effect of grain size on the creep deformation was also studied. II. EXPERIMENTAL The chemical composition of the alloy used is given in Table I. The molybdenum equivalency, which is a measure of the stability of the beta phase, was found to be 9.9 pct.[4] The alloys were melted as 16.6-kg ingots and processed to 1.74-cm diameter bars at RMI Company (Niles, OH). The bars were then cut into individual pieces about 10-cm long and vacuum encapsulated in quartz tubes. This was then followed by a heat treatment spec
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