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Nb-Ti alloys are normally produced by vacuum arc remelting process. Due to inadequate processing parameters, freckles can be observed in macroetched ingots. In the present work, visual, chemical, metallographic, and X-ray are presented which establish the appearance, composition, microstructure differences between freckle regions and normal regions. It has been observed that in freckles parallel to the ingot axis, the Ti content is up to 53wt%, 7wt% higher than normal regions. It is also shown that a lot of precipitation phases appeared in freckle regions, because of the thermosolutal convection in the mushy zone. The Rayleigh number, which recommended as a criterion for freckle initiation, has been calculated that using a VAR melting software. Based on the experimental results and simulation results, it is concluded that freckles can be influenced by processing parameters, and freckles in Nb-Ti alloy can be eliminated by matching appropriate processing parameters.

A linear stability analysis is used to investigate the influence of mechanical vibration on the onset of thermosolutal convection in a horizontal porous layer heated and salted from above. Vibrations are considered with arbitrary amplitude and frequency. The Brinkman extended Darcy model is used to describe the flow and the Oberbeck-Boussinesq approximation is employed. Continued fraction method and Floquet theory are used to determine the convective instability threshold. It is found that the solutal Rayleigh number has the stabilizing effect. The existence of a closed disconnected loop of synchronous mode is predicted in the marginal curve for moderate values of solutal Rayleigh number and vibration amplitude.