Microstructure and transformation temperatures in rapid solidified Ni-Ti alloys. Part I: The effect of cooling rate

摘要

Several issues are related to casting large ingots of Ni-Ti alloys, as well as to their post-processes (hot and cold working, machining). On the other hand, the effects and benefits of the rapid solidification on these materials are known. Very high solidification rates, such as 10~6-10~7 K/s, obtained by melt spinning and planar flow casting, were mainly explored up to now, but these processes have severe limitations. The aim of this research is to analyze the effects of the cooling rate in the lower regime of the rapid solidification (10~2-10~3 K/s), which is much more suitable for industrial processes. For this purpose, cone shaped samples of a near equiatomic Ni_(51)Ti_(49) alloy were obtained by rapid solidification in a water cooled copper mold. The conical shape of the samples allows the comparison among different cooling rates. The thermal and metallographic analyzes showed that a cooling rate within the explored range is effective to tailor the microstructure and the characteristic temperatures of the alloy. The thermal analysis revealed that the phase transformation temperatures are inclined to be lower as the cooling rate increases. On the other hand, a refinement of the microstructure, as well as a reduction in the amount of the precipitates can be obtained. Oxygen is present inside the Ni-Ti-O phase and these crystals are smaller if the cooling rate increases.