Factors Influencing the Return to Equilibrium of Quenched Beta Ti Alloys and of Massively Transformed TiAl-Based Alloys
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beta Ti alloys can develop high strengths, of the order of 1000 MPa, by precipitation of alpha if they are cooled from the beta phase field at rates that result in retention of the beta phase and are then aged.[1] The microstructures are commonly heterogeneous because the alpha precipitates in bands within the grains and additionally coarse alpha precipitates are formed at the beta grain boundaries.[2] In earlier work, it had been found that grain boundary alpha precipitated during exposure at the operating temperature in a nominally fully beta stabilized alloy, Ti25V15Cr2Al, but that addition of 0.2 wt pct C prevented this precipitation.[3] The ductility of the alloy was reduced to zero by the presence of the grain boundary alpha, and the carbon addition prevented this embrittlement. Examples of the fracture surfaces of samples with and without addition of C are shown in Figure 1. Titanium carbide precipitates are formed at this level of addition of carbon; they pin grain boundaries during recrystallization and refine the grain size of forged samples. In addition, the carbides are Ti(CO) rather than TiC; thus, they reduce the oxygen content of the matrix and the grain boundaries. Since oxygen is a M.H. LORETTO, Professor, D. HU, Senior Research Fellow, and XINHUA WU, Professor in Aerospace Materials, are with the IRC in Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Contact e-mail: [email protected] A. HUANG, Material Technologist, is with the Rolls-Royce, Derby DE24 8BJ, United Kingdom. This article is based on a presentation given in the symposium entitled ‘‘Materials Behavior: Far from Equilibrium’’ as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India. Article published online September 26, 2007 1480—VOLUME 39A, JULY 2008
strong alpha stabilizer, the influence of the carbides on alpha precipitation appeared to be associated with the decrease in grain size and the reduction in oxygen content at grain boundaries. On the basis of the influence of C on the formation of grain boundary alpha in Ti25V15Cr2Al, work has been carried out on a number of metastable beta alloys in order to investigate whether the coarse grain boundary alpha commonly found in these alloys can be eliminated and the properties improved. Examples taken from this work are reviewed in this article.
II.
EXPERIMENTAL
Work on Ti-15-3 (Ti-15V-3Cr-3Al-3Sn), to which amounts of carbon between 0.05 and 0.2 wt pct have been added, will be reported here. The C content in samples to which no C was added (termed C-free) was typically about 0.02 wt pct. The Ti-15-3 was made by plasma melting elemental feedstock with appropriate master alloys and TiC to produce 1-kg buttons or ingots up to 100 kg. The oxygen content lay between 1100 and 1300 wt ppm. The TiAl alloy studied was Ti46Al8Nb (at. pct), and the buttons or ingots were again prepared using a plasma melter. The oxygen content was about 700 wt ppm. Samples of Ti-15-3 were quenched from 90
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