The Influence of Heat Treatment on Microstructure and Phase Transformation Temperatures of Cu-Al-Ni Shape Memory Alloy

This paper presents the results of thermal and microstructural analysis of Cu-Al-Ni shape memory alloy before and after heat treatment. After casting, a bar of Cu-12.8 Al-4.1 Ni (wt.%) alloy, obtained by the vertical continuous casting technique, was subjected to a certain heat treatment procedure. Solution annealing was performed at 850 °C for 60 min, followed by water quenching. Tempering was then performed at four different temperatures (150 °C, 200 °C, 250 °C and 300 °C). The microstructural results were obtained by optical and scanning electron microscopy. Thermodynamic calculation of ternary Cu-Al-Ni system under equilibrium was performed using Thermo-Calc 5 software. Phase transformation temperatures were determined by differential scanning calorimetry (DSC). The DSC results show the highest values of transformation temperatures in as-cast (cid:79)(cid:80)(cid:61)(cid:80)(cid:65)(cid:10)(cid:2318)(cid:29)(cid:66)(cid:80)(cid:65)(cid:78)(cid:2318)(cid:79)(cid:75)(cid:72)(cid:81)(cid:80)(cid:69)(cid:75)(cid:74)(cid:2318)(cid:61)(cid:74)(cid:74)(cid:65)(cid:61)(cid:72)(cid:69)(cid:74)(cid:67)(cid:2318)(cid:61)(cid:74)(cid:64)(cid:2318)(cid:80)(cid:65)(cid:73)(cid:76)(cid:65)(cid:78)(cid:69)(cid:74)(cid:67)(cid:8)(cid:2318)(cid:80)(cid:68)(cid:65)(cid:2318)(cid:80)(cid:78)(cid:61)(cid:74)(cid:79)(cid:66)(cid:75)(cid:78)(cid:73)(cid:61)(cid:80)(cid:69)(cid:75)(cid:74)(cid:2318)(cid:80)(cid:65)(cid:73)(cid:76)(cid:65)(cid:78)(cid:61)(cid:80)(cid:81)(cid:78)(cid:65)(cid:79)(cid:2318)(cid:79)(cid:68)(cid:75)(cid:83)(cid:2318)(cid:72)(cid:75)(cid:83)(cid:65)(cid:78)(cid:2318)(cid:82)(cid:61)(cid:72)(cid:81)(cid:65)(cid:79)(cid:2318)(cid:83)(cid:69)(cid:80)(cid:68)(cid:2318)(cid:65)(cid:84)(cid:63)(cid:65)(cid:76)(cid:80)(cid:69)(cid:75)(cid:74)(cid:61)(cid:72)(cid:2318)(cid:79)(cid:80)(cid:61)(cid:62)(cid:69)(cid:72)(cid:69)(cid:80)(cid:85)(cid:2318)(cid:75)(cid:66)(cid:2318)


Introduction
Today there are a large number of known shape memory alloys (SMAs); nickel-based shape memory alloys, copper-based shape memory alloys, ferrous-based shape memory alloys, noble metal-based shape memory alloys, etc. The term shape memory alloys is applied to a group of metallic materials, which show the ability to return to treatment procedure. Shape change is a consequence of austenitic to martensitic transformation, which is characterised by the following temperatures: A s -austenite start temperatures, A f s -martensite start temperature and M f perature. [1][2][3][4][5][6][7] Compared to Ni-Ti SMAs that are generally considered to be superior to Cu-based alloys, Cu-Al-Ni alloys also offer some considerable advantages. Not only is the material cost 15-30 % of that for Ni-Ti, but the melting, composi-er Young's modulus, better machinability, and better work/ cost ratio. In addition, the stability of the two-way shape memory effect is better, which is very important when designing the actuators. 8 Phase diagrams are very important for designing and development of a material due to its functional properties, mi-Thermodynamic modelling offers valuable information for equilibrium thermodynamics, modelling of diffusion processes and grain growth. It represents the advantage acinvestigations. 9 Reliable thermodynamic databases, with optimised parameters, are crucial for thermodynamic cal-tigated system and accuracy of calculated phase equilibria. Thermodynamic descriptions of binary systems Cu-Al, Al-Mn, and Cu-Mn are given in a number of references, thermodynamic data for ternary Cu-Al-Ni alloy. 9,10,11 Considering the wide application of Cu-Al-Ni alloys, it seems very interesting to analyse the thermodynamic properties of this ternary system.
dure on microstructure and phase transformation temperatures of the alloy. The results obtained after heat treatment are compared with the results obtained on the sample in as-cast state.

Experimental
Thermodynamic calculation for equilibrate conditions was performed with Thermo-Calc 5 software, using database SSOL 6. Calculations of Gibbs energy were performed according to binary sub-systems Cu-Al, Cu-Ni and Al-Ni. [10][11][12][13] Cu-12.8 Al-4.1 Ni (wt.%) shape memory alloy was produced by vertical continuous casting procedure in a vacuum induction furnace connected to the device for vertical continuous casting. The alloy, in the shape of a bar of passing between two rolls rotating in opposite directions. The alloys' casting temperature was 1240 °C and casting speed was 320 mm min . After casting, the heat treatment procedure was performed as shown in Table 1.
For microstructural observation, the samples were metallographically prepared by grinding, polishing, and etching. The detailed metallographic preparation of the samples 14 The samples were investigated with optical microscope (OM) and scanning electron microscope (SEM). To determine phase transformation temperatures, differential scanning calorimetry (DSC) was performed. The samples were heated and cooled in one cycle from room temperature to 400 °C by heating/cooling speed of 10 K min . Figs. 1 and 2 show precipitation of austenite, parent -phase in B2 crystal structure at 1237 °C, under equilibrate conditions. Solidus temperature was observed at 1031 °C, and under this temperature, the as two crystal structures, B2 and A2. The -phase starts to precipitate at 611 °C. At 567 °C, the -phase in A2 crystal structure decomposes to -phase, and at room tempera-  -, -(B2) and -phase is obvious. Decomposition of parent -phase can be suppressed by fast cooling or quenching in water, which causes formation of martensitic structure.
During heat treatment, microstructural changes occur, affecting the phase transformation temperatures. Such behaviour of Cu-Al-Ni alloys, obtained by melt-spinning process, was observed by Morawiec et al. 16 The change in phase transformation temperatures can also be attributed to the effect of internal strains caused by different grain Pelegrina and Romero on Cu-Al-Zn SMA. 17 3. In order to avoid the possible forming of residual low temperature phases during casting, a heat treatment process to the literature 18 , copper-based SMAs are alloys in which heat treatment process cannot be avoided.
In the microstructure of the investigated Cu-Al-Ni alloy, (Figs. 3 and 4) was found. It can be noticed that the mi-crostructure consists of self-accommodating needle-like shaped martensite. In addition, the orientation of the crystal was different.
The Cu-Al-Ni alloy microstructure can change depending on the heat treatment procedure. Optical micrographs of solution annealing and tempering are presented in Figs. 3b-3d. The grain boundaries are clearly visible, and the microstructure after heat treatment was completely martensitic. Martensite needles had different orientation cleation of groups of martensitic plates in numerous places within the grain, and creation of local strain within the grain, which allowed the formation of several groups of differently oriented plates.
properties, which occur due to thermal processing. Solution annealing of the Cu-Al-Ni shape memory alloy must be performed in order to achieve a fully martensitic microstructure. Along with the martensitic phase which is heat treatment (heat treatment temperature, holding time at the selected temperature, or cooling agents). 19 ples by scanning electron microscopy (Fig. 4). The resulting martensite below M s temperature. Martensite originated primarily as a needle-like martensite. On some of the samples, after solution annealing and tempering (Figs. 4b-4d), the V-shape of the martensite can be noticed. The morphology of the resulting martensitic microstructure is a Higher aluminium content (>13 wt. %) follows the forma-1 ') martensite. If the chemical composition is on the boundary between both martensites, then both can the microstructure depends on the chemical composition, temperature conditions, and stress conditions. 18,21,23,24 The appearance of martensite in the microstructure can be described by the transformation mechanism of austenitic 1 '. Only after heat treatment, another type of martensite in the micro-1 ' martensite 1 ' martensite can be described by the following transformation 1 '. Also, not only is one type of martensite present in the microstructure, but both types of martensite, which can be described by the transformation 1 ' + 1 '. It was observed that changes in temperature of phase transformations occur due to changes in temperature of heat treatment process (Table 2 and Figs. 5-7). Figs. 5 and 6 present DSC curves for alloy in as-cast state and solution annealed, respectively. The phase transformation temperatures (M s , M f , A s , and A f ) were determined by tangent method and marked in Figs. 5 and 6. Fig. 7 shows the effect of heat treatment temperature on the phase transformation temperatures before and after heat treatment. It can be noticed that the temperature of the phase transformation is the highest for as-cast sample. The reason for this may be a large amount of internal strain and imperfection in the microstructure, as a result of cast-Since the heat treatment procedure (solution annealing and tempering) was carried out in order to achieve order in the alloy's structure and stabilisation of the phase transformation temperatures, the characteristic behaviour