We have developed a model which is able to describe the formation of high temperature microstructures in β metastable titanium alloys. It is based on the classical theory of nucleation, and on growth laws obtained by assuming that the processes are controlled by the diffusion of the alloying chemical species only. Two morphologies are described: grain boundary allotriomorphs, and Widmanstätten plates gathered in colonies growing from the allotriomorphs. Moreover, a statistical description of the nucleation conditions on β grain boundaries has been introduced in order to obtain the global kinetics of the transformation. The model predictions proved to be in good agreement with experimental results.

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Experiments and thermodynamic calculations were performed on three iron-base alloys containing 30 wt.% of chromium and 3 to 6 wt.% of tantalum. Solidus temperatures, natures and surface fractions of all phases present after an exposure for 50 hours at 1000°C, 1100°C and 1200°C, were determined for each alloy. These results were compared to values calculated using Thermo-Calc. Two alloys display solidus temperatures above 1400°C, while all the liquidus temperatures are higher than 1500°C. Tantalum carbides are present with high fractions, compared to similar Co-base and Nibase alloys. Their observation and quantification using electron microscopy micrographs and image analysis may lead to overestimate the surface fractions of TaC. Calculations of carbide fractions from the chemical composition of matrix is to be preferred. A precipitation of coarse chromium carbides may occur for 1100°C and 1200°C.

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Experiments and thermodynamic calculations were performed for three alloys based on both iron and nickel in equal quantities, containing 30wt.% of chromium, 0.2 to 0.4wt.% of carbon and 3 to 6 wt.%Ta of tantalum. Solidus and liquidus temperatures were determined for the three alloys, and natures and fractions of the existing phases at 1000, 1100 and 1200°C were characterized. These experimental data were compared to values calculated by Thermo-Calc, as well as to such results obtained on previous similar alloys separately based on iron or on nickel. Generally there is a good agreement with calculations. Tantalum carbides in (Fe,Ni) base alloys are less present than in Fe-base alloys, but more present than in Ni-base alloys for the same carbon, chromium and tantalum contents.