BORON SEGREGATION IN A (FE, V, B) TIAL BASED ALLOY

Primary boron containing dispersoids grown from the melt in a Ti-45.5at.%Al-1.6at.%Fe-1.1at.%V-O.7at.%B alloy, and then annealed at 1473K, have been investigated using optical microscopy, conventional transmission electron microscopy (CTEM) and analytical electron microscopy. The dispersoid morphology is in the form of high aspect ratio plates, hundreds of microns across and cross-sectional widths of just a few hundred nanometers, rather than a blocky or equiaxed morphology. These dispersoids are not monocrystalline, but have a layered structure parallel to the plane of the plates. The dispersoids are distributed with random orientations throughout the matrix and delineate the edges of lamellar domains, formed by the solid state transformation alpha--> gamma+ alpha(2)+ beta. Microchemical analysis by windowless energy dispersive x-ray analysis (EDX) and serial electron energy loss spectroscopy (EELS) show that the chemical structure of these zones is in fact a mixture of interleaved phases, rather than a single faulted boride crystal. Boron mapping across the zones edge on to the plates and quantitative EDX reveals boride plates down to a few nm wide have formed interleaved with ordered beta-phase (B2 CsCl structure). It is concluded that primary borides and beta-phase simultaneously nucleate within the melt, and the beta-phase is stabilized to room temperature by Fe and V segregation. Hence the borides can act as grain refiners by providing nucleation sites for beta-phase at high temperatures.