No abstract

Ambient (room) temperature studies have been carried out on an ␣-Ti-1.6 wt pct V alloy to determine the effects of stress level and grain size on ambient temperature creep behavior. Creep tests were performed at five different stress levels ranging from 75 to 95 pct of the yield stress value on specimens with an average grain size of 226 m. It has been found that the alloy exhibits appreciable creep at stress levels far below the yield stress, with creep occurring at values as low as 75 pct of the yield stress. The extent of creep strain was found to decrease with a decrease in stress level. Creep tests were also performed on this alloy with different grain sizes ranging from 38 to 226 m at a stress level of 90 pct of the yield stress. It was seen that the extent of creep strain decreased with a decrease in grain size. Fine slip and time-dependent twinning were found to be the creep deformation mechanisms. Based on the results of this investigation and earlier studies, it is suggested that time-dependent twinning is a major creep deformation mechanism in ␣-titanium alloys that contain small amounts of alloying elements. The time-dependent twinning phenomenon has been attributed to the diffusion of oxygen away from the twin-matrix interface, permitting the growth of twins.

Much interest has developed in the near-a titanium alloy Ti-5Al-1Sn-1V-1Zr-0.8Mo (Ti-5111) for naval applications. When gas tungsten arc welded with filler metal that has the same chemical composition as the base metal, however, the weld FZ tends to be harder and less ductile than the base metal, which may make the weld susceptible to failure. This behavior may be attributed to the presence of oxygen impurities and the large prior-b grain size in the weld. In this investigation, the addition of a small amount of yttrium to the weld filler metal can decrease hardening and increase the ductility of Ti-5111 welds, which is beneficial for weld performance. Microstructural and chemical analyses of unmodified and yttrium-modified Ti-5111 welds are presented along with results from mechanical testing of the welds.

No abstract

The U.S. Department of Energy (DOE) has included a drip shield (DS) as a principle component of the engineered barrier system (EBS) for the proposed high-level nuclear waste geologic repository at Yucca Mountain, Nevada. The current DS design consists of titanium grade 7 (Ti Gr 7) plates and Ti Gr 24 support beams and bulkheads. The intended functions of the DS are to divert dripping water around and prevent rockfall damage to the waste package (WP). Sustained static loading of the DS may occur as a result of rockfall or drift collapse. These static loads may cause residual stress that approaches the yield stresses of the different DS materials. This level of residual stress would enable various creep mechanisms to transpire. A preliminary assessment of the potential for DS creep after a dynamic rock block impact is presented in this paper by expressing the DS residual Von Mises stress levels as fractions of the Ti alloy yield stress (YS). It was determined, using creep data from similar alloys that the residual stress levels within a DS after a 2-tonne rock block impact per DS segment length could cause creep in both the Ti Gr 7 plates and Ti Gr 24 bulkheads and support beams. The results of this study will assist the U.S. Nuclear Regulatory Commission (NRC) in evaluating the risk significance of the expected DS performance characteristics under actual repository conditions.