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Phase transformation temperatures of Cu-Al-Mn-Ag SMA alloys

Shape memory alloys can recover their shape as a consequence of shear-dominant and diffusion less martensitic transformation, which occur during fast cooling from -phase [1, 2]. Cu-Al-Mn SMA alloys with Al content lower than 18 %at. show very good shape memory properties, high ductility and ability for cold-rolling [3]. For further improving of target properties it is necessary alloying with one or more constituents. Recent investigations showed that start temperature of martensitic transformation, Ms, increases with addition of Zn, Si, Au and Fe, while decreases with addition of Ni, Ti and Co [4, 5]. In this paper, the influence of different content of silver to martensitic transformation temperatures of Cu-Al-Mn alloy was investigated. Specimens of ternary alloy Cu-9%wt.Al-10%wt.Mn and alloys with 6%wt. and 7%wt. of silver, were prepared by melting of pure elements in the electric-arc furnace, under argon atmosphere. Samples were re-melted for 4 times due to better homogenization and casted in the cylindrical moulds with dimensions  = 8 mm and length of 12 mm. Calculation of phase diagram under equilibrate conditions was performed by software Thermo-Calc, using optimized parameters for binary sub-systems and ternary phases from Miettinen, for Cu-rich corner (Fig. 1) [6]. Martensitic transformation temperatures were followed by Differential Scanning Calorimetry (DSC) under nitrogen atmosphere, in the temperature range from -100 to 200 °C, in 2 heating/cooling cycles. Ternary Cu-Al-Mn alloy, doesn`t show existence of martensitic transformation in DSC cooling curve in the as-cast state, but after thermal treatment at 900 °C and quenching, martensitic structure was formed, with start martensitic temperature, Ms=34°C and finish martensitic temperature, Mf=14°C. Unlike, addition of both compositions of Ag, 6%wt. and 7%wt., causes formation of martensitic structure already in the as-cast state, with significantly lower transition range and Mf temperatures. The existence of more exothermic peaks on the DSC cooling curves refers to formation of different martensitic structures in Cu-Al-Mn-Ag alloys. Cu-9Al-10Mn-7Ag alloy shows start martensitic temperature after 1st cooling cycle at Ms=65°C and finish at Mf= -45 °C, but after 2nd cooling cycle, transformation is in the range from 51°C till -23°C (Fig.2a). Addition of 6%wt. silver shifts Ms temperature after 1st cooling cycle to lower values (Fig.2b).

phase diagram, CALPHAD, Cu-Al-Mn-Ag alloys

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