Natural formation of bcc Co: Initial stage of Co precipitation in supersaturated Au90Co10

Natural formation of the bcc modification for Co was demonstrated to occur upon precipitation of Co in supersaturated Au90Co10 alloy. The microstructural evolution was investigated by high-resolution transmission electron microscopy. Co precipitation starts with the formation of very thin coherent plates along {100}(Au). This initially precipitated Co was shown to be a hitherto unknown metastable bcc modification of Co exhibiting a Bain-type orientation relationship with the matrix: (100)(Au,fcc)parallel to(100)(Co,bcc) and [001](Au,fcc)parallel to[011](Co,bcc) (three variants). Prolonged annealing causes the bcc Co to transform according to a Bain-type transformation into face-centered-cubic (fcc) Co, exhibiting the (cube-on-cube) orientation relationship (100)(Au,fcc)parallel to(100)(Co,fcc) and [001](Au,fcc)parallel to[001](Co,fcc) (three variants), in association with loss of coherency and a morphological change from Co platelets to equiaxed Co particles. A delicate balance of interface energy and bulk energy upon precipitation and subsequent growth/coarsening governs the successive appearance of the bcc and fcc modifications of Co.