One method of providing creep resistance is to precipitate a fine, homogeneous distribution of vanadium nitride (VN) particles in the matrix. Maximising this precipitation hardening effect requires a high nitrogen content, but this could cause gas bubble formation during conventional fabrication processes. It is therefore necessary to determine how much N can be added without encountering such problems.

Phase stability calculations, using ThermoCalc, were carried out to find high-N compositions which optimised the fraction of VN and the fabrication route for obtaining fine particles. Nine high-N alloys were fabricated as ingots; out of these, two exhibited porosity. ThermoCalc predicts that, in all nine alloys, nitrogen gas is an equilibrium phase as liquid transforms to solid. It is evident that porosity cannot simply be predicted from the equilibrium diagram. However, detailed analysis of the phases predicted in these alloys, and their variation with composition, allowed a porosity criterion, linking the phenomenon to the nature of the liquid-to-solid transformation, to be obtained. This transformation is not only affected by nitrogen content, but also by other alloying elements, with carbon playing an important role.

Thermodynamic calculations are a valuable tool for the design of these industrially important alloys. Input from experimental data has enabled the refinement of the initial design criteria, such that it should now be possible to propose compositions with high VN hardening but without the risk of porosity.