ON THE ROLE OF CARBON DURING PROCESSING OF NITI SHAPE MEMORY ALLOYS

Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on microstructural features. In the present work, we show how the microstructures of two Ni(50.7)Ti(49.3) SMAs with different carbon concentrations and related functional properties respond to thermomechanical treatments. Carbon is a typical impurity element in NiTi which can react with Ti to form TiC. We show that higher carbon concentrations result in higher volume fractions of carbides. This deteriorates workability. Recrystallization results in the formation of a fine B2 grain structure. It was found that higher volume fractions of carbides provoke the formation of a finer grain structure during thermomechanical processing. Carbides may well promote the nucleation of new grains in the early stages of recrystallization and subsequently pin grain boundaries. In as-cast NiTi, higher carbon levels are associated with the formation of Ti-depleted zones as indicated by wider phase transformation intervals on cooling and heating. We show that this type of micro scale heterogeneity can also be found in thin NiTi sheets.