omega-Phase formation in a rapidly solidified Cr-40 at.% Al alloy

The diffuse omega structure has been identified by electron diffraction for the first time in a C11(b) matrix of the melt-spun Cr-40 at.% Al alloy rib bon. The C11(b) matrix consists of nanometre-scale ordered domains produced by a long-period chemical ordering in the precursor A2-B2 structure. Each C11(b) unit cell in the Cr-40 at.% Al alloy is based on three cubic B2 cell s along the c axis and contains two antiphase boundaries. The structure of the diffuse omega is consistent with that observed by De Fontaine. The rela tive stability of the crystalline omega structure has been predicted with r espect to bcc-type precursor phases as a function of the displacement param eter z from calculation of the cohesive energy of the omega phase using the full-potential linear muffin-tin orbital method. The results show that the three-dimensional crystalline omega structure is stable with respect to bc c-type precursor phases in the Cr-Al system and reveals the physical backgr ound as to why the omega structure in Cr-40 at% Al is diffuse in nature.