Effect of solidification velocity and nitrogen contamination on α formation in a Ti-54at%Al melt

We have shown that for zone melting conditions of slow growth solidification and minimal interstitial content, a Ti-54Al liquidus composition is beyond the alpha +L phase field. For arc-casting conditions of nominal interstitial impurities and non equilibrium growth, alpha is preferentially nucleated from the melt. The argument of possible nucleation barriers to gamma may not be a sufficient condition since, despite the presence of a gamma growth interface or heterogeneous nucleation agents, alpha is still preferentially nucleated. It is suggested that the alpha +L liquidus boundary is extended to higher aluminum concentrations for the Ti-Al-N ternary phase diagram. This is a subject for further study. Finally, while this paper reports only the effect of nitrogen, similar effects may be expected for oxygen since it, too, is a potent alpha stabilizer.