Martensitic transformation in Ni-Ti-Co strip produced by twin roll casting

The process of the martensitic transformation was studied in a Ni47Ti50Co3 shape memory alloy produced by twin roll casting. Differential scanning calorimetry, X-ray diffraction and electron microscopy were used. The as-cast strip reveals a multi-step martensitic transformation. It proceeds from the parent phase through the R-phase after which two stages of the B19' martensite transformation are observed. Stabilization of the martensite was obtained directly after several thermal cycles carried out in a relatively low thermal range (-100 to 70 deg. C), without the commonly used annealing (350-700 deg. C). The thermal cycling leads to a single stage of the B19' martensitic transformation, however, the transformation enthalpy of the R → B19' transition does not change. The alloy transformation behavior was compared with that observed in a cycled melt-spun ribbon. The energy stored in the twin roll cast strip is higher than that stored in the melt-spun ribbon