The alpha case thickness at the surface of a Ti-6Al-4V (wt pct) step wedge investment casting has been measured and successfully predicted. The prediction uses temperature-time results obtained from a heat flow simulation of the casting. The temperature-time results were coupled to a simple model for diffusion of oxygen into the beta phase during continuous cooling. Oxygen concentration and microhardness profiles were measured from the surface in contact with the ZrO 2 face coat of the shell mold into the interior of the casting. The oxygen content in the metal at the shell mold interface was between 5 and 9.5 wt pct in general agreement with a thermodynamic calculation for bcc Ti in contact with ZrO 2 . At the limit of the alpha case region, as determined by standard metallographic technique, the oxygen concentration was found to be no more than 0.02 wt pct above the level of oxygen in the bulk alloy. Using this information and one particular literature value for the activation energy for diffusion of oxygen, a nearly linear relationship was obtained between the measured and predicted alpha case thicknesses at various positions on the casting surface. Reduction of the prefactor of this diffusion coefficient by a factor of 7.5 produces excellent agreement between predicted and measured alpha case thicknesses. Such a reduction is not inconsistent with the scatter of literature values for the diffusion coefficient.

No abstract

The interdiffusion at 1293 °C between two multicomponent Ni-base superalloys, René-N4 and René-N5, was assessed by measuring the composition vs. distance curves and by comparing the measured curves to predictions obtained using a diffusion mobility database recently published by Campbell et al. (Acta Mat.50 (2002) 775-792). Although the diffusion database was constructed primarily from binary diffusion data, the extrapolation to the multicomponent systems gave good results in the prediction of the measured composition vs. distance curves. In addition, the location of the Kirkendall porosity on the René-N4 side of the diffusion couple was successfully predicted. This initial success points to the suitability of the general approach to the development of diffusion databases and to the desirability for additional database refinements including possible efforts from the first principles community.

The solidification behavior of Al–Mn alloy powders was studied as a function of cooling rate and manganese content. The phases present in the powders differed from those expecied at equilibrium due to competetive nucleation and growth kinetics. The equilibrium Al6Mn phase was absent due to its relatively sluggish nucleation kinetics, and in the more concentrated and rapidly cooled alloy powders, the metastable quasicrystalline phases were present. Nucleation temperatures measured in alloys cooled at 25°C/s are interpreted to represent formation of the icosahedral phase, which subsequently transforms to the decagonal phase and provides a lower bound estimate of 870°C for the melting point of the icosahedral phase in an Al-30 wt. % Mn alloy.