| Popis: |
Rapid solidification techniques were first used to obtain amorphous or microcrystalline metallic materials, which required high quenching rates (10 4 -10 6 K.s -1 ). Applied to shape memory alloys, the twin roll casting technique allows semi-finished materials (strips) with reduced grain size to be obtained. This is particularly useful for certains shape memory alloys for which shaping is difficult because of their inherent lack of ductility. The melt is cast through a nozzle and solidified between the gap of two rollers rotating in opposite directions. Optimum conditions must be established to obtain thin foils with a smooth surface, not brittle and free of cracks and holes. It is only possible for a limited combination of the following experimental parameters: roll speed, melt temperature, melt flow, ejection pressure and roll gap. In this work, we have characterized (transformation temperatures, microstructures) copper based and Ni-Ti-Hf strips, with a thickness between 150 and 500 μm. For copper based alloys, it has been found that transformation temperatures were in a similar range to those of conventionnally cast alloys, which is not the case of melt spun ribbons whose transformation temperatures dramatically decrease. For Ni-Ti-Hf strips cast with different conditions, transformation temperatures vary: some are close to the conventionnally solidified material but some others are lower. |
