Effects of Si and Nb additions on CuAINiTi rapidily solidified alloys microstructures

Abstract
Intensive investigations have been carried out on CuAlNiTi shape memory alloys (SMAs) with 1wt.%Ti and Ni content from 5 to 6 wt%. In this present work, we used a rapid solidification technique to prepare alloys of two compositions: (wt%) Cu₇₉Al₁₃Ni₅Ti₂Si₁, Cu₇₉Al₁₃Ni₅Ti₁Nb₂ The ribbons obtained were characterised by means of XRD, DSC measurements and TEM observations. XRD patterns indicated that the first alloy has a two-phase structure with beta and martensite 18R phases, whereas the second one is only beta. DSC measurements gave characteristic transformations temperatures. The Si-containing alloy showed low temperature transformations with M_s about -88°C and A_f at -41°C in as-quenched state. The Nb-containing alloy exhibited high temperature transformations with A_f about 170°C in the as-quenched state, which is quite higher for this alloy family. The transformation temperatures are stabilised after only one cycle at ΔT_M=(M_s-M_f) constant for both alloys. This stabilisation is likely to be generated, in the case of Cu₇₉Al₁₃Ni₅Ti₁Nb₂ by nanocrystalline precipitates revealed on TEM micrographs. Narrow and faulted martensite plates have been observed in agreement with previous results on rapidly solidified CuAlNiTi SMAs.