Manufacture of shape memory alloys NiTi and NiTiCu by casting method and studying the effect of adding the copper element in different proportions to the binary alloy on the microstructure, phase transformations and conductivity

Abstract

Abstract Thanks to unique properties, such as light weight, good biocompatibility, high strength, and relatively low cost, shape memory alloys are of interest to researchers. In this work alloys NiTi and NiTiCu with different weight percents were made from the copper element using the casting method by (VAR) furnace. Where the elements were melted at (1350°C) without a crucible to obtain best alloys of high purity, and after that the manufactured ingots were cooled with ice water to obtain the best homogeneity. The melting process was followed by the annealing process by an electric furnace at (750°C). After completing the manufacturing process, specimens were prepared for microscopic and mechanical tests, and wires were drawn for the purpose of electrical testing. Several tests and examinations were carried out to ensure that the alloys manufactured are SMAs. The microscopic examinations that were performed via OM showed that the copper element is uniformly distributed within a NiTi matrix. The results showed FESEM that the microstructure is symmetric and homogeneous, and also showed phase (TI 002) and (Ti2Ni) and some defects. DSC examination showed the initial and final degrees of the austenite and martensite phase. In addition, the results of DSC showed that the best stable alloys are alloy NiTi and NiTiCu-3 and that the hysterical loop decreases with an increase in the percentage of copper. The results of the shape memory effect showed that the best SME was (88.98%) for alloy NiTiCu-3. As for the microhardness, it increased when the copper element was increased, so the maximum hardness of the specimen was at 2 wt.% Cu, and it seemed to decrease with the increase in the copper element. The electrical test that was performed using the 4T sensing method of wires showed that the conductivity value increases with the increase in ratio of Cu with the decrease in the resistance. The best conductivity ratio was for alloy NiTiCu-3 where the conduction ratio was 24096.4 (Ω.Cm)−1 and it had the lowest resistivity 41.5 (Ω.Cm)10−6. Strain rate was tested and the resistivity, so the increase in the strain rate was observed with the increase in the copper element and the decrease in the resistivity.