Powder elaboration

2.1.1 Arc melting furnace

An Edmund Buhler MAM-1 arc furnace was used in order to obtain polycrystalline samples from pure elements (figure 2.1). Arc-melting furnace is very common technique to melt the metals to obtain alloys. Ni, Mn, In, and Co high purity elements were melted to obtain polycrystalline samples with stoichiometry of Ni45Co5Mn36.7In13.3. The masses of the elements in the stoichiometric ratio are measured carefully and placed in the metal chamber. The melting is performed in a refrigerated copper crucible by introducing an arc discharge by inducing the voltage difference between a tip and Wolframium stick attached inside the chamber. The handle above the chamber controls the direction of the arc and it hits the elements right from the above.

The temperatures up to 4000K can be acquired that ensures the fusion of all the elements.

Constant water current is attached with the crucible in order to avoid the overheating of the constituents. The chamber is filled with Argon atmosphere by purging it for several times to ensure a pure Argon environment. This ensures the prevention from oxidation and a suitable environment for electric discharge to take place. The melting process has been performed for six to eight times to improve the homogeneity of the resulting ingot. This droplet shaped ingot is cleaned after each melting to ensure the oxide free bulk alloy. The ideal mass that can/should be produced is between 15-20gms. The higher masses are also feasible to produce up to 35-40gms, however, the quality and homogeneity of the ingot is reduced.

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Fig. 2.1. Arc furnace Edmund Bühler MAM-1.

2.1.2 Thermal treatments

A long thermal treatment under high temperature is necessary post-synthesis of the ingot to further ensure the homogenization. Since the crucible chamber is refrigerated by water, its temperature is lower than that of the melted ingot. As a result, the solidification of the ingot becomes directional and non-homogeneous (from the edges to the center). Therefore, post- annealing process at high temperature is very necessary to ensure the homogenization. The annealing temperature has to be selected, depending on the melting temperature of the alloy system. In the case of Ni45Co5Mn36.7In13.3 composition, the lowest melting point of the alloy is around 1200-1300K. Therefore, the annealing was performed at 1073K for 24hrs. This treatment improves the homogeneity by allowing the atomic diffusion provided by the thermal energy at high temperatures. The long-range atomic order at room temperature and hence the martensitic transformation can be modified just by selecting the cooling route of the ingot (previously explained in Chapter 1). Slow cooling from the high temperatures in a controlled atmosphere or a controlled quenching from high temperatures are usually performed thermal treatments in these kinds of alloys. The used vertical furnace is shown in figure 2.2. The alloy is placed hanging tied to a Kanthal Wire (a ferrite iron-chromium-aluminum (FeCrAl) alloy with excellent heat resistance), in a vertical quartz tube. This vertical tube runs through the furnace and is connected to argon supply and a rotary vacuum pump. An inert atmosphere is achieved by purging the tube multiple times and filling with the argon, in order to avoid any kind of oxidation. The sample is hanging by a pin kept horizontally in such a way that the sample falls once the pin is removed. This furnace was also used for annealing treatments (873K for 30 minutes) to remove deformation induced defects (see later) or change the atomic order.

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Fig. 2.2. Vertical furnace used for quenching and annealing treatments.

2.1.3 Mechanical treatments (milling)

Mechanical treatments, in particular hand crushing and ball milling, are used to obtain micro- particles which will be introduced in polymers (see later) to get functionalize and 3D printable composites. A key point is the understanding of the properties linked to the deformed microstructure. The first treatment the sample goes through is hand-crushing using agate mortar (figure 2.3a). The idea is to obtain smaller particles by soft crushing first to facilitate the ball milling process. Moreover, a controlled deformation needs to be introduced as an initial step.

The further decrease in the particle size is achieved by ball-milling. Ball milling is the most used grinding technique that is used to induce deformation, or to mix or grind or blend materials like ceramics, paints or metals. In the present case, Retsch planetary ball mill PM-100 has been used, which consists of a cylindrical container and the tungsten carbide milling balls. The planetary motion with 300rpm in which the cylindrical container rotates (with the collisions of the milling balls with the material and the balls itself) have a great impact on the material by changing the structure and physical properties of the material as well as reduction in the size of the particles. The number of the tungsten carbide balls (around 10mm diameter) to add while milling is decided by the volume of the powder (5:1- ball to powder ratio), which, in the end, controls the properties of the final product. The ball mill used in this research work does not have any chamber that allows any inert atmosphere or a temperature controller. Due to the multiple collisions between the balls, particles and the cylindrical chamber, the temperature of

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the system increases. This, consequently, leads to the risks of oxidation. To overcome this problem, the chamber is filled with nitrogen gas using glove box chamber. A few drops of methanol were added before closing the chamber tightly.

Fig. 2.3. a) Agate-mortar, b) Retsch planetary ball-mill PM 100.

In addition to this, the milling process is carried out in intervals, by interrupting the process every 5 minutes after each 5 minutes of milling. This helps to avoid the overheating of the chamber. The influence of the deformation on the microstructure and the magneto-structural properties has been studied in the hand-crushed and ball-milled powders in the next chapters.

The obtained powders are sieved in order to study the correlation between the particle size and the degree of deformation. The sieves ranging from 100μm to 10μm have been used to filter the different particle sizes. The maximum milling time used to obtain smaller particles is up to 15 minutes. The particle size up to 25μm has been achieved by milling time of 15 minutes.