NANOCRYSTALLINE NiTi-Fe SMART ALLOYS PROCESSING BY COMBINED MECHANICAL ALLOYING-REACTIVE SYNTHESIS

摘要

Among smart metallic base materials, NiTi has the highest shape memory effect. However, its working temperature is limited to ～ 120 °C. Its increasing seems to be possible by additions of small proportions of alloying elements able to induce disorder in the NiTi B2 ordered crystalline lattice, delaying the martensite-austenite reversible transformation-responsible for the smart behavior. A similar effect is expected to be obtained by assuring a micro or nano cry higher structural un-homogeneity. However, the NiTi synthesis is difficult due to its small absolute value of Gibbs Free Energy of formation. Also, its obtaining in a bulk nanocrystalline state is difficult due to this state low thermodynamic stability. The present paper investigates the possibility to overcome these drawbacks by combining mechanical alloying - for powder activation and nanocrystalline structure obtaining, with reactive synthesis of NiTi with 5 at.% Fe addition, the preliminary results being very promising.