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The beta Titanium alloy Ti-23niobium-11aluminum (at. pct.) differs from other beta Ti alloys in that it is age-hardenable due to the formation of lath-like, alpha-two precipitates based on Ti3Al. This study reports the room temperature deformation and slip behavior of the Ti-23Nb-11Al alloy as a function of heat treatment. The results indicate that the formation of the alpha-two precipitates results in not only a large increase in yield stress but also can induce a change in slip mode from inhomogeneous to uniform slip.
The beta Ti alloy Ti-23Nb-11Al (at %) is unique in that it is age hardenable due to the formation of lath-like alpha 2-phase precipitates based on Ti3Al. Furthermore age-hardening occurs at temperatures significantly higher than most conventional beta Ti alloys. This suggests the possibility of the elevated temperature usage of alpha 2-strengthened beta Ti alloys such as Ti-23Nb-11Al. The study examines the compressive deformation behavior of the Ti-23Nb-11Al alloy in the -196 deg. C to 650 deg. C temperature range and demonstrates that very high strengths are possible even at 600 deg. C by precipitation hardening due to the (alpha 2 phase. The thermogravimetric oxidation behavior in laboratory air from 600 deg to 700 deg. C indicates parabolic behavior consistent with oxygen diffusion through a scale.
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The deformation characteristics of quenched and aged Ti-V alloys in the composition range 20 to 40% V have been examined by optical metallography and transmission electron microscopy. A coarse lenticular deformation product similar in appearance to previously reported strain induced 'martensites' was found to be associated with the occurrence of the omega phase. These features proved to be (112) 111 twins. Continued aging of the omega containing alloys resulted in a transition of the deformation mode from twinning to slip at a point which corresponded to the onset of embrittlement or alpha precipitation at the omega-beta interfaces. The formation of deformation twins in a two-phase omega-beta structure is discussed in terms of possible mechanisms.
Beta titanium alloys are one of the most rapidly developing areas of materials technology. As a group, beta alloys provide a wider range of processing, physical, chemical and mechanical properties than any other titanium alloys.
The temperature dependence of the flow stress behavior has been measured from 77 to 740 K for age-hardenable beta-phase (bcc) Ti-40V-1Si alloy single crystals. For solution-treated and aged crystals, an increase in the athermal yield stress at 740 K is accompanied by a decrease in the temperature dependence of the yield stress. Direct electron microscopy observations, slip character, and work hardening behavior indicate that the precipitate particles are predominantly sheared in the conditions tested. It is suggested that the reduced temperature dependence of the yield stress with aging results from: (a) the relative flow stress difference of screw vs. edge dislocations moving through penetrable obstacles and (b) solute association which may occur during aging.