On the reaction between NiTi melts and crucible graphite during vacuum induction melting of NiTi shape memory alloys

The functional and structural properties of NiTi shape memory alloys (SMAs) which are often produced by vacuum induction melting (VIM) using electro graphite crucibles are strongly dependent on the alloy composition. It is known that NiTi melts react with electro graphite crucibles and the associated carbon pick up will affect the alloy properties. So far no effort has been made to study the thermodynamics and kinetics of this reaction. In the present work the reaction between NiTi melts and electro graphite crucibles during VIM processing was investigated. The reaction results in the growth of a TiC layer and a simultaneous increase of carbon concentration in the melt. A physical view of the underlying transport processes and reactions is given and a phenomenological rate equation for the increase of carbon concentration during melting is provided.