Prediction of sigma phase formation in stainless steels

Sigma phase is an intermetallic compound which may appear during prolonged ageing of stainless steels at nuclear plant temperatures. It is often associated with a dramatic fall-off in mechanical properties, especially ductility. The theoretical attempts to determine the onset of sigma phase formation are traced. In particular, the Phacomp approach, which utilizes electron vacancy numbers, is described together with the developments in its application to nickel-based alloys in the range 8500-12000C. The problems of extending the basic Phacomp equations to austenitic steels at typical service temperatures are examined. For this, the variation in the critical electron vacancy number with temperature and the average electron vacancy number of the matrix must be determined. To achieve the necessary increase in the temperature range data from ternary, as opposed to binary, phase diagrams have to be employed and the final expressions are substantially altered. In addition, it is shown that the type of intermetallic and the phase from which it nucleates have an important effect. The final predictive equations may be used for intermetallics, other than sigma, in the range 6000-12000C and they show an improved correlation with equilibrium diagram data than present models. (author)