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Deformation twinning occurs in various materials including metals, intermetallics, ceramics, superconductors, and even geological systems. The rate of twin growth depends on the material system, but there are no crystallographic models to understand the mechanisms of slow twin growth. The physics of twin growth is presented for bcc-Ti alloys where it is shown that octahedral interstitial sites are not conserved at the twin-matrix interface where oxygen resides. The predicted activation energy for twin growth correlated well with experimental values for the diffusion of oxygen. These models will have a broad application in understanding the twinning process and designing advanced materials.

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.