EFFECT OF HEATING RATE AND GRAIN SIZE ON THE MELTING BEHAVIOR OF THE ALLOY Nb-47 MASS Ti IN PULSE-HEATING EXPERIMENTS

摘要

The effect of heating rate and grain size on the melting behavior of Nb-47 mass Ti is measured and modeled. The experimental method uses rapid resistive self heating of wire specimens at rates between ～102 and ～104 K/s and simultaneous measurement of radiance temperature and normal spectral emissivity as functions of time until specimen collapse, typically between 0.4 and 0.9 fraction melted. During heating, a sharp drop in emissivity is observed at a temperature that is independent of heat- ing rate and grain size. This drop is due to surface and grain boundary melting at the alloy solidus tem- perature even though there is very little deflection (limited melting) of the temperature-time curve from the imposed heating rate. Above the solidus temperature, the emissivity remains nearly constant with increas- ing temperature and the temperature vs time curve gradually reaches a sloped plateau over which the major fraction of the specimen melts. As the heating rate and/or grain size is increased, the onset tempera- ture of the sloped plateau approaches the alloy liquidus temperature and the slope of the plateau approaches zero. This interpretation of the shapes of the temperature--time curves is supported by a model that includes diffusion in the solid coupled with a heat balance during the melting process. There is no evi- dence of loss of local equilibrium at the melt front during meltiong in these experiments.