Morphology, crystallography, growth kinetics and mechanism of bainite nodules in a hypereutectoid Ti-25 wt% Cr alloy

Abstract

The formation of bainite nodules in a hypereutectoid Ti-25 wt% Cr alloy has been investigated with optical and transmission electron microscopy. These nodules are approximately hemispherical when formed in the grain boundary region and about spherical when developed intragranularly. They consist of a dispersion of eutectoid TiCr 2 in eutectoid α. The TiCr 2 crystals are observed to nucleate at eutectoid α-β boundaries. During subsequent growth they are eventually enveloped by the adjacent eutectoid α and must thus be repeatedly re-nucleated. Unlike pearlite, TiCr 2 crystals in a given nodule may have different orientations. Both eutectoid α-β and eutectoid TiCr 2-β boundaries are partially coherent, containing both misfit dislocations and ledges. Also unlike pearlite (vide current studies by Hackney and Shifiet in FeCMn), the ledge structures of eutectoid a and eutectoid TiCr 2 develop independently. Their growth directions are different and their height and average inter-ledge spacing may also differ in a given bainite nodule. Growth kinetics measured at four temperatures in the range 938–848 K are consistent with both interdiffusion between α and TiCr 2 during ledgewise growth, somewhat assisted (especially at lower temperatures) by interphase boundary diffusion, and independent, volume diffusion-controlled growth of eutectoid a by the ledge mechanism.