Advanced intermetallic g-TiAl-based alloys such as multiphase TNM alloys exhibiting a compositional range of Ti-(42-44)Al-(3-5)Nb-(0.1-2)Mo-(0.1-1)B (all compositions are given in at%, unless otherwise indicated) have attracted attention in the automotive and aircraft industries due to their potential as light-weight materials for high temperature-applications. [1][2][3][4][5] In the acronym TNM, T stands for TiAl, N and M stand for T. Klein et al./Morphological Evolution of a b-solidifying TiAl Alloy ADVANCED ENGINEERING MATERIALS 2015, 17, No. 6

Dendritic microstructures are most dominant patterns in solidified alloys. The microstructural features of these structures control the segregation profiles of solute elements in the interdendritic regions, thus determining the mechanical properties of cast structures. In this study, a 2D model of solid/liquid interface instability in a low carbon steel was introduced, using the multi-phase-field software code MICRESS ® combined with an in-situ study of solidification in a laser-scanning confocal microscope. The use of a moving-frame boundary condition and a linear temperature gradient within the simulation allows further optimization of the solidification studies in the laser-scanning confocal microscope. By analysing the shape of the delta-ferrite grain boundary at the solid/liquid interface, in-situ and at temperature, it was possible to experimentally determine the Gibbs-Thomson coefficient and the solid/liquid interfacial energy of the alloy. The interface mobility of the solid/liquid interface was calibrated in the model so as to reproduce the experimentally measured interface velocity at the onset of interface instability. The proposed model was used to describe the morphological transitions from planar to cellular to dendritic modes during solidification and solute segregation under a variety of processing conditions such as cooling rate and temperature gradient. The importance of this approach is that the verified model has been used to extend the prediction of microstructural development to cooling rates well beyond what can be achieved experimentally and into the regime pertinent to high-speed continuous casting. Significant microstructural differences that arise as a result of varying processing conditions are discussed.

SummarySolidification microstructure is a defining link between production techniques and the mechanical properties of metals and in particular steel. Due to the difficulty of conducting solidification studies at high temperature, knowledge of the development of solidification microstructure in steel is scarce. In this study, a laser-scanning confocal microscopy (LSCM) has been used to observe in situ and in real-time the planar to cellular to dendritic transition of the progressing solid/liquid interface in low carbon steel. Because the in situ observations in the laser-scanning confocal microscopy are restricted to the surface, the effect of sample thickness on surface observations was determined. Moreover, the effect of cooling rate and alloy composition on the planar to cellular interface transition was investigated. In the low-alloyed, low-carbon steel studied, the cooling rate does not seem to have an effect on the spacing of the cellular microstructure. However, in the presence of copper and manganese, the cell spacing decreased at higher cooling rates. Higher concentrations of copper in steel resulted on an increased cell spacing at the same cooling rates.

Solid-state phase transformations and grain growth in an intermetallic γ-TiAl based alloy were examined using a confocal laser-scanning microscope. This apparatus allows the in-situ visualization of the microstructural development at high temperatures. The sample is thus placed into a furnace and a predetermined temperature profile is followed, while the surface is scanned using a laser. The contrast is gained by the thermal etching.

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