Sigma phase formation in binary alloys of the transition elements

Abstract

The intermediate phase now designated the sigma phase was first brought to the attention of physicists and metallurgists by Bain and Griffiths (1). Since their discovery of a hard, brittle, and non-magnetic phase in iron-rhromium alloys of equiatomic composition, many investigations have been carried out in order to reveal the nature, occurrence, and effects produced by this phase. In particular, the possibility of the formation of sigma phase in austenitic stainless steels has been explored to show whether or not its presence was the cause of brittle failure in these alloys after prolonged service at elevated temperature. Until about ten years ago it was generally assumed that the formation of the sigma phase was limited only to alloys containing both iron and chromium. In 1918, however, it was first suggested by Sully and Heal (2) that a phase in the cobalt-chromium system was isomorphous with the sigma phase previously reported and that the range of the phase was 56.6 to 61 atomic per cent of chromium. This has been confirmed by other workers (3) (4) (5) (6). Since then the existence of the sigma phase has been proved in a number of binary and ternary alloy systems containing transition elements. Table I summarizes the binary sigma phases so far observed, together with the compositions and temperatures at which these phases occur. The same data was compiled graphically in Fig. 1 to show the relationship of the composition range of various alloy systems.