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[1] In our recent paper, the sensitivity of infrasound to the upper atmosphere is investigated using impulsive signals from the Tungurahua volcano in Ecuador. We reported on the coherent variability of thermospheric travel times, with periods equal to those of the tidal harmonics. Moreover, it was shown that the error in predicted thermospheric travel time is in accord with typical uncertainties in the upper atmospheric wind speed models. Given the observed response of the infrasound celerities to upper atmospheric tidal variability, it was suggested that infrasound observations may be used to reduce uncertainty in the knowledge of the atmospheric specifications in the upper atmosphere. In this paper, we discuss the estimation of upper atmospheric wind model updates from the infrasound data described in the aforementioned paper. The parameterization of the model space by empirical orthogonal functions is described; it is found that the wind model in the upper mesosphere and lower thermosphere can be described by a four-parameter model. Due to the small dimensionality of the model space, a grid search method can be used to solve the inverse problem. A Bayesian method is used to assess the uncertainty in the inverse solution given the a priori uncertainty in the data and model spaces and the nonlinearity of the inverse problem at hand. We believe that this is the first study in which such methods are applied to real infrasound data, allowing for a rigorous analysis of this inverse problem. It is found that the complexity of the a posteriori model distribution increases for a larger dimensional model space and larger uncertainties in the data. A case study is presented in which the nonlinear propagation from source to receiver is simulated using an updated wind model and nonlinear ray theory. As nonlinear propagation effects further constrain the propagation path, this is a way to check the physical self-consistency of the travel time inversion approach. We obtain excellent agreement between the simulated and observed waveforms.

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Deformation of Ti-6Al-4V alloy in the β phase field is generally considered to be trivial since the material is highly workable at these temperatures and does not normally pose problems during processing. In view of this, studies on the hot deformation behavior of β are scanty compared to those on the α−β deformation. This paper is focussed on understanding the β deformation characteristics in Ti-6Al-4V with a view to examine whether such studies help in optimizing the process design and achieving microstructural control. The emphasis has been on the two industrial grades, viz. commercial purity (CP) versus extra-low interstitial (ELI), and also on the effect of starting microstructure (transformed β versus equiaxed α+β). The stress-strain curves obtained in compression exhibited steady-state behavior at strain rates lower than 1 s−1 and oscillatory/softening behavior at higher strain rates. Kinetic analysis of the flow stress data obtained at different temperatures and strain rates has shown that the stress exponent is about 3.6-3.8 and the apparent activation energy is in the range 150-287 kJ/mole, which is comparable to that of self-diffusion in β phase (150 kJ/mol). Dynamic recrystallization (DRX) of β is identified as the microstructural mechanism in all the cases except in ELI grade with transformed β starting structure. The prior β grain size in the DRX region exhibits a good correlation with the Zener-Hollomon parameter. In case of ELI grade, a mechanism of large grained superplasticity involving sliding of prior colony boundaries has been identified. However, deformation close to the transus in ELI grade causes nucleation of voids which may grow during soaking at the deformation temperature under the influence of tensile residual stress. The results clearly show that a study of β deformation mechanisms holds the key during hot working of Ti-6-4. More importantly, the processing schedule used for CP grade Ti-6-4 should not be used for ELI grade to achieve microstructural control and avoid defects.