X-ray diffraction and resistivity studies of titanium-molybdenum alloys
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I.
INTRODUCTION
THE phase
transformation behavior of metastable beta (bcc) titanium alloys has been the subject of many recent investigations. 1-5 From this work it is clear that the stability of the beta phase under the alpha-beta transus is determined principally by two factors: the solution treatment temperature and the concentration of beta stabilizing elements. Most investigations have focused on alloys with less than 20 wt pct alloying elements where the primary transformation products, athermal and isothermal omega phase, are readily characterized by electron imaging and diffraction methods. 6'7's The extent of omega formation in the more concentrated alloys is less well known because of its diffuse nature in these systems. 9 In the concentrated alloys the beta phase may also transform by a diffusion controlled phase separation reaction (beta ~ beta' (bcc)) at temperatures between the beta transus and the upper bound of omega stability. ~0,H Both omega and beta' are considered to be nonequilibrium transition phases from which intragranular alpha nucleation takes place. ~ Diffuse omega has been studied by electron, ~2~3 X - r a y , ~4'j5'16 neutron, ~7`~s and M6ssbauer 14 scattering and is best described in terms of quasi-static heterophase domains resulting from a k,~ = Y3(lll)a~ -~ longitudinal modulation of the bcc lattice. Analysis of the M6ssbauer inelastic scattering indicates that part of the omega diffuse intensity has a dynamical origin with both omega lifetime and fluctuation size decreasing with increasing alloying concentration and temperature. Neutron scattering suggests that diffuse omega has some unusual aspects; for example, central phonon peaks for certain omega reciprocal lattice vectors were found to persist well into the beta phase region. This implies a static omega-like precursor at temperatures well beyond those determined by X-ray and TEM methods. ~9,20In addition, the elastic scattering was found to be incommensurate in the concentrated alloys. ~4 Several defect 2~ and stacking soliton 22 models have been proposed to account for the observed intensity displacement from the ideal ordering vector k~. Current metallurgical interest in diffuse omega phase derives from C. HAYMAN, Graduate Student, and W. W. GERBERICH, Professor, are with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. Manuscript submitted November 17, 1983. METALLURGICALTRANSACTIONS A
its possible roles in cyclic strain softening, 23'34 deformation mode changes] 5 and low temperature embrittlement) 6'27 The beta' phase forms as solute poor dispersed particles upon aging at 800 K or below. 1'1l Titanium atom clustering assisted by vacancies is apparently the mechanism of decomposition. 11 It has been suggested 28 that beta' and athermal omega phase may be coupled according to the transformation sequence: beta (age, 800 K) --~ beta . . . . 1 athermal omega, since the beta' zones are depleted in solute. Coherency stresses at the beta-beta' interface could promo
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