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.%Fel.lat.%V-0.7at.%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 a+ y + a2+ P. 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 $-phase (B2 CsCl structure). It is concluded that primary borides and P-phase simultaneously nucleate within the melt, and the $-phase is stabilized to room temperature by Fe and V segregation. Hence the borides can act as grain refiners by providing nucleation sites for P-phase at high temperatures. TiAl (y) and Ti3A1 alloys (a2) are currently under development as future aerospace alloys, having low density, good high temperature strength and oxidation resistance. It is well established that including even <lat.%B in two phase yla2 alloys can produce a dispersion of boride particles throughout the microstructure[l-41. The impact of these reinforcements on various alloy properties, such as modulus and high temperature creep strength, depends on the particulate morphology and dispersion throughout the matrix. These boride particles have been shown to refine grain size[4,5], possibly by providing additional nucleation sites at high temperatures in the melt for the primary TiAl phases P and a , and should improve the high temperature creep properties of the alloys. In this study we consider the effect of the addition of B to a Ti-A1-Fe-V system, which Nakagawa et a1 [6] have shown produces TiAl based alloys of good castability for turbine component manufacture. Within this alloy it has been established that Fe and V stabilize the P-phase [7] within the bulk microstructure, and the morphology of the boride particles is now investigated.