Study on martensitic transformation, magnetic and compressive properties of spray casting Ni-Mn-Ga alloys

Abstract

The influence of spray casting technique on martensitic transformation temperature, magnetic properties and compressive properties of Ni50Mn25+xGa25–x (x = 0, 1.25, 2.5, 3.75 and 5) alloys was investigated. With the increase in Mn content, martensitic transformation temperature of the spray casting samples is increased, but slightly lower than that of the corresponding as-cast ones. The saturation magnetization test shows the opposite tendency and proves the spray casting technique can effectively enhance the magnetic properties. Compression experiments showed that the spray casting process did not change the fracture strain, but lead to the greatly increase in the compressive strength and improve the brittleness of the Ni-Mn-Ga alloys. This study may offer a new processing technique for developing high performance ferromagnetic shape memory alloys. Introduction Since large magnetic field induced strain (MFIS) was discovered in the ferromagnetic martensitic phase of near stoichiometric Ni2MnGa alloys , there has been an increasing interest in research and development of such alloys, due to its potential application as a magnetically driven shape memory alloy which is more efficient than temperature or stress driven shape memory devices . They are very attractive for applications as different kinds of actuators, sensors, magnetic micro-electromechanical systems. The most investigated Haussler alloy is Ni2MnGa single crystal since it possesses huge recoverable MFIS up to 9.5% . However, single crystal has some disadvantages, such as the preparation time is long, high cost and low repetition rate. This situation impedes the improvement of such material. Highly oriented polycrystalline alloy which has the similar performance to the single crystal could be a choice to solve this problem. Nevertheless, the high brittleness of Ni–Mn–Ga polycrystalline alloy restricts its practical applications. For now, one of the feasible method is using special processing technology to improve the high brittleness of the Ni-Mn-Ga alloy. In this paper, a new method called spray casting has been used for preparing a series of polycrystalline Ni-Mn-Ga samples. The martensitic transformation, the magnetic and compressive properties of the as-cast samples and the spray casting samples has been investigated. This study aims to offer a new processing technique which can improve the brittleness of the Ni-Mn-Ga alloys with suitable martensitic transformation temperature and good magnetic properties. International Conference on Materials, Environmental and Biological Engineering (MEBE 2015) © 2015. The authors Published by Atlantis Press 633 Experimental High purity metal elements (Ni~99.99%, Mn~97%, Ga~99.99%) are selected as the raw materials to synthesis the designed alloys. High vacuum non consumable arc melting furnace has been used to smelt and get a button ingot with the diameter of about 40mm and weight about 80g. the ingots have been homogenized in 900°C×24h to remove the composition segregation. Table. 1 shows the nominal composition of as-cast Ni50Mn25+xGa25-x(x=0, 1.25, 2.5, 3.75, 5) (at. %) alloys and the actual composition of the alloy measured by EDS. It can be seen that the difference between the nominal composition and the actual composition is reasonable. The content of Ni keeps almost invariant, the content of Mn increases and the content of Ga decreases gradually. It’s called MnGa series. To be convenient, the serial numbers MnGa-0, MnGa-1 ... MnGa-4 were used to replace the composition of the sample below. Table 1 the composition of Ni50Mn25+xGa25-x (x=0, 1.25, 2.5, 3.75, 5) alloy Serial number and Nominal composition (at %) Actual composition measured by EDS Ni (at %) Mn (at %) Ga (at %) Ni2MnGa Ni50Mn25Ga25 50.46 24.89 24.65 MnGa-1 Ni50Mn26.25Ga23.75 49.89 26.31 23.80 MnGa-2 Ni50Mn27.5Ga22.5 49.82 27.79 22.39 MnGa-3 Ni50Mn28.75Ga21.25 50.08 28.81 21.11 MnGa-4 Ni50Mn30Ga20 50.42 29.37 20.21 A special process called copper mound spray casting technique has been used in this study. Put the as-cast alloys placed in a quartz tube with a round hole at the bottom and the copper mound placed exactly under the quartz tube. The whole device is spaced in a vacuum system. Melt the ingot by high frequency induction heating, after the ingot molten completely, load argon with particular pressure into the quartz tube from the top. The molten alloy has been sprayed cast into the copper mound. Finally get a cylinder with the diameter of Φ8 mm. Fig.1 shows the copper mound spray-casting method and the alloy rod got by spray casting process. Spray casting alloy rod is brighter and much compact than traditional button as-cast ingot. Results and discussion Martensitic transformation. We investigated the effect of composition on the martensitic transformation temperature of the spray casting and as-cast Ni50Mn25+xGa25-x(x=0, 1.25, 2.5, 3.75, 5) alloys, the results have been shown in Fig. 2 (a). As seen from Fig. 2 (a), the martensitic transformation temperature increased gradually with the increase of Mn. Furthermore, the martensitic transformation temperature of the spray casting alloy is slightly lower than that of the as-cast alloy. Fig. 1. Processing diagram of copper mound spray-casting method and rod got by spray casting.