Phase decomposition of liquid-quenched β-type Ti-Cr alloys
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Phase decomposition behaviour of liquid-quenched β (bcc) type Ti-Cr alloys was investigated by means of transmission electron microscopy and hardness measurements. It was found that decomposition of β to β1 (Ti-rich, bcc) + β2 (Ti-lean, bcc) takes place in the intermediate composition range of the Ti-Cr system. This experimental result proves the theoretical prediction made by Menon and Aaronson, but the observed β1 + β2 two-phase field expands towards higher temperatures than the predicted binodal line. The coherent β1 + β2 two-phase state exhibits the so-called 〈100〉 modulated structure and it was concluded that the formation of such a structure is a result of spinodal decomposition of the β-phase. We obtained time-temperature-transformation (TTT) diagrams of β-type Ti-30, 40 and 50 at % Cr alloys. A typical sequence of structural change is β → coherent β1 + β2 → incoherent β1 + β2 → incoherent β1 + β2 + grain boundary precipitates → stable state of β + TiCr2 or α + TiCr2. Not all the states in the above sequence appear, depending on alloy composition, liquid-quenching rate and ageing temperature.
KeywordsPolymer Transmission Electron Microscopy Structural Change Stable State Theoretical Prediction
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