Influence of carbon on martensitic phase transformations in NiTi shape memory alloys

In the present paper we show how carbon affects martensitic transformations in Ni-rich NiTi shape memory alloys. During vacuum induction melting in graphite crucibles, NiTi melts dissolve carbon and TiC particles form during solidification. Differential scanning calorimetry (DSC) shows that this is associated with a decrease in the phase transition temperatures. We provide new experimental evidence for increasing temperature intervals between the start and the end of the martensitic transformations (from B2 to B19') with increasing C content in as-cast and solution-annealed (850 degrees C) microstructures. The nucleation and growth of TiC particles in intercellular/interdendritic regions causes variations in the local Ni/Ti ratios. This results in wider transformation temperature intervals (DSC peak broadening) in as-cast and solution-annealed micro structures. Subsequent intense heat treatments (1000 degrees C) homogenize the alloy and re-establish sharp DSC peaks during martensitic transformations. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.