The effect of heat treatment on the microstructure and mechanical properties of Cu- Al-Mn shape memory alloy

The 8 mm diameter bars of Cu-Al-Mn shape memory alloys were produced by continuous casting technique. The samples were characterised using optical microscopy and scanning electron microscopy along with EDX analysis. The continuous cast alloy revealed some martensitic phase which, after quenching, led to the microstructure that is completely martensite. Quenching of samples had an effect on several mechanical properties and change in morphology of fracture. After ageing at 200 °C and 300 °C the tensile strength increased and elongation is drastically decreased. Morphology of fracture surface was changed from primary ductile to a mixture of intergranular and ductile.


Introduction
Shape memory alloys (SMAs) are relatively new metallic materials, which are able to memorise and recover their original shape. These alloys show the ability to return to the appropriate treatment. It is a result of crystallographically reversible martensitic phase transformation. Such phase transformations can be obtained by mechanical (loading) or thermal treatment (both cooling and heating). SMAs are extremely sensitive to exact chemical compoloading conditions, etc. The main types of these alloys are nitinol (Ni-Ti), Cu-based, and Fe-based alloys. [1][2][3] SMAs are interesting in numerous commercial engineering applications. There is a high demand for SMAs with high strength and shape memory effect in technical applications. 2 The main advantages of Cu-based alloys are their low price, relatively simple fabrication procedure, and high electrical and thermal conductivity compared to other shape memory alloys. Among Cu-based SMAs, the Cu-Al-Ni and Cu-Al-Zn alloys are extensively investigated. [4][5][6] However, the Cu-Al-Ni and Cu-Al-Zn alloys are brittle and susceptible to intergranular fracture. For this reason, the Cu-Al-Mn shape memory alloy is proposed because it was found that the alloy shows better ductility and good strain recovery. The reason for higher ductility of Cu-Al-Mn shape memory alloys can be correlated to decreasing the degree 7,8 Addition of manganese to Cu-based SMAs stabilises the parent phase and improves ductility of the alloys. 9 The other advantages of Cu-Al-Mn alloys compared to other Cu-based SMAs are higher shape memory strain, larger recovery power, better ductility, and higher damping capacity. The addition of manganese increases the binding force between the constituent atoms leading to increased activation energy for diffusion and decreased diffusion rate of the atom for re-ordering. 10 Generally, SMAs are fabricated by the casting procedure followed by plastic working (rolling or drawing) including heat treatment. In recent years, the continuous casting technique has been one of many technologies for the production of SMAs due to the special competitive growth mechanism of crystal, and resulting cast products with a favourable texture formation. 11,12 In this paper, the mechanical properties of Cu-Al-Mn shape memory alloys obtained through the continuous casting process and after heat treatment (quenching and ageing) were compared.

Experimental
The Cu-Al-Mn alloy (Table 1) used in this research was prepared by melting pure elements (w(Cu) = 99.9 %, w(Mn) = 99.8 %, and w(Al) = 99.5 %) in a vacuum induction furnace under protective argon atmosphere. Chemical composition of the investigated alloy was estimated by Optical Emission Spectrometer ICP-OES AG-ILENT 700. Firstly, the ingot ( = 110 mm × 180 mm) was produced by graphite mould casting. The ingot was then remelted in the same furnace and continuously cast. The continuously cast strand (cylindrical bar with 8 mm diameter) of alloy was obtained using the device for vertical continuous casting which is connected to the vacuum induction furnace. Solid bars were produced directly from the 13. protective atmosphere was set around 500 mbar. Casting speed was 290 mm min . Heat treatment of samples was performed in a laboratory electro-resistance furnace. Solution annealing of samples was carried out at 900 °C for 30 min, followed by cooling in room-temperature water (quenching). After quenching, the ageing was carried out at temperatures of 100 °C, 200 °C, and 300 °C for 60 min, followed by cooling in water.  (Fig. 1a). After continuous casting, some martensite phase is observed (Figs. 1a After solution annealing at 900 °C and quenching in water, 1 rapid cooling in water, the alloy undergoes ordering transi-1 ), and then martensite trans-1 (L2 1 1 13 During rapid cooling from low the M s -temperature. The micrographs include grains and martensite plates (Figs. 1-3). The grains appear clearly, and martensite plates have different orientations into different grains. The martensite is formed primarily as the martensite in SMAs. The parallel bands in martensite can be considered twin-like martensite. As a typical example, Fig. 1 and Table 2 show the EDX results of Cu-Al-Mn alloy after quenching and aging at 300 °C.   Fig. 8 and Table 3.

Mechanical properties determination
The effect of heat treatment on mechanical properties of Cu-Al-Mn shape memory alloy was determined performing the standard tensile tests. Table 4 shows measured mechanical properties of the alloy. The values presented are average values obtained after four measurements. Results of mechanical properties presented in Table 4 show that while the plasticity of the alloy is very low (below 2 %). The highest tensile strength is obtained after aging at 300 °C (1002 MPa). This value is about 300 MPa higher compared to the alloy after continuous casting. The changes in mechanical properties (Table 4), especially the decrease in elongation and reduction in area of the alloy, can be caused by the change in microstructure or 1 phase and precipitation process in Cu-based shape memory alloys. 16 Microstructural change of cast, quenched, and aged samples can be explained by analysis of SEM micrographs with EDX spec-trums and fracture surfaces (Figs. 4-8). Optical micrographs of samples after ageing show no microstructural changes on the SEM micrographs these changes can be observed -1 matrix (Fig. 4). Ageing at 300 °C is accompanied by the precipitation of these phase particles and causes a chemical composition change in the matrix. EDX analysis of particles shows higher Cu-content and lower Al-content compared to the matrix (Fig. 4, Table 2).
The hardness of the alloy increased as the result of dispersion hardening, Fig. 9. Ageing in martensite phase is associated with diffusion and precipitation process. With the increase in ageing temperature, the increase in hardness corresponds to the precipitation of the particles. Obviously, the ageing is a problem for Cu-based shape memory alloys. Sutou et al. concluded that the increase in hardness by low temperature treating from 200 to 400 °C of Cu-Alwith plate-like bainite plates and the increase in the de-1 phase. 17 Kainuma suggests that the corresponding stable phases in Cu-Al-Mn alloys in the low-temperature range below 400 °C can be Cu 3 Mn 2 Al phase. 18 The metastable martensite and parent phase from pendent diffusion process during aging. 19 (Fig. 5), while after quenching, as well as after ageing, fracture surfaces are a mixture of intergranular and ductile fracture (Figs. 6 and 7). In some cases, it can be noticed that the crack occurs at three-fold node of grain ing in Cu-based shape memory alloys was caused due to a high degree of order, high elastic anisotropy, and large 21 Grain boundaries often provide the easiest propagation path. In addition, not only intergranular type of fracture was noticed in this sample. Some places at fracture surface show characteristically small and shallow dimples, possibly indicating that a certain plastic deformation occurred during the fracture. The fracture surface was also investigated with an EDX analysis. Fig. 8a shows a fracture surface of specimen after aging at 300 °C with dimples, in which the existence of small particles was observed. EDX analysis showed that the chemical composition of observed particle has higher Al-content in comparison to the matrix (Fig. 8, Table 3).
The results of hardness measurements are presented in Fig. 9. Hardness in cast and quenched state gives similar values (235 HV10 and 223 HV10). In ageing condition, the value of hardness in all samples increases. After ageing at 100 °C, hardness was 274 HV10, while after ageing at (432 HV10 and 332 HV10). Increase in hardness values can be caused by the second phase particle presence.

Conclusions
Continuous casting is revealed as a useful technique for production of Cu-Al-Mn SMAs bars (in our case with diameter of 8 mm). After continuous casting, the alloy shows tensile strength of 627 MPa and elongation of 8.5 %. Mi--After quenching the alloys microstructure completely becomes martensite, and mechanical properties are decreased in comparison to the cast state (tensile strength of ing temperature from 100 °C to 200 °C, and 300 °C, the of the alloy was very low (below 2 %). These changes in and microstructure. In addition, the morphology of fracture surface changed from primary ductile to a mixture of intergranular and ductile fracture types.