The effect of high melting point additive elements on characteristic transformation temperatures and structure of Cu-based shape memory alloys

In this thesis work, four types -one ternary and three quaternary- of Cu-based shape memory alloys (SMAs) with new different compositions of Cu-17.33Al-4.74Fe-2.08Mn (at.%),Cu-20.82Al-3.53Fe (at.%), Cu–18.73Al–21.06Be–0.13Mn (at. %) and Cu–12.9Al–22.73Be-0.37Mn (at.%) were produced by arc melting method. At first, the high purity (99.99 %) copper, aluminum, iron, beryllium, and manganese elements in powder forms were mixed and then formed into pellets by applying pressure and these pellets were melted in an arc melter to get as-cast ingots. The alloy samples obtained by cutting these ingots were heat-treated at 900 °C for 1h and right after quenched in traditional iced-brine water to form of β1' martensite phase in the alloy samples. In this way, the alloys were acquired shape memory effect property. Then the thermodynamic parameters and characteristic martensitic transformation temperatures of the alloys were detected by differential thermal analysis (DTA) measurements and differential scanning calorimetry (DSC) taken at different heating/cooling rates. The DSC results showed that the characteristic martensitic transformation temperatures of the alloy samples all became in the range of 100–400 °C by the effect of additive elements, so this means that each one of these alloys is to be defined as a high temperature shape memory alloy (HTSMA). The DTA results showed that the thermal behaviors of these alloys in high-temperature region became in accord with those of other Cu-based SMAs. Also, the other kinetic parameters related to the martensitic transformations of the alloy samples such as entropy, equilibrium temperature,enthalpy and activation energy were determined. As to structural measurements taken at room temperature, the formation of the martensite phases was detected in all of the alloy samples by XRD results and by optical micrographs.