# Applications of Tungsten Pseudo-Alloys in the Energy Sector
## 2 Materials and Experimental Works
### Materials Produced from New Raw Materials--TN and DS
#### 2.1.1 High-Strength Material with Lower Tungsten Content--TN  
The microstructure of the material is determined by the sintering mode of the initial powder mixture [15; 16; 17]. The gradual evolution of the sintered microstructure during the sintering process is shown in Figure 3. At sintering temperatures below the melting point of the impurities forming the future matrix, i.e., iron, nickel, and cobalt, a fine-grained structure of slowly growing tungsten grains is formed (Figure 3). A gradual uniform shrinkage of the material occurs without shape deformation. However, this fine-grained structure does not allow for subsequent metallurgical processing (annealing, quenching and forming) to achieve the desired mechanical properties (strength Rm and yield strength R\({}_{\text{p0.2}}\)) of the final TN product (R\({}_{\text{p0.2}}\) = R\({}_{\text{m}}\) > 1600 MPa while maintaining ductility A\({}_{\text{S}}\) > 6%). It is only at sintering temperatures above the melting point of the alloying elements that the growth of spherical tungsten agglomerates (grains) in the liquid matrix consisting of Ni/Co and Ni/Co/Fe melts accelerates (Figure 3b-d). The sinter becomes plastic, and depending on the tungsten content of the material, more or less dimensional deformation of the sinter occurs due to creep. In free sintering without a mould, significant shape deformation of the sinter occurs. The structure's evolution as a function of temperature and the amount of the maximum sintering temperature is shown in Figure 3. The desired final structure of the sinter (see Figure 3d) is composed of spherical tungsten aggregates with mean diameters in the range of 20-30 \(\upmu\)m, which are uniformly dispersed in the \"frozen\" matrix. In the next step, the sinter is annealed, followed by quenching in water. The basic quenching parameters are shown in Figure 2. The typical quenching temperature is 1230 \({}^{\circ}\)C with a temperature endurance of one hour. Quenching removes the internal stresses at the grain boundaries between the Ni/Co matrix and the tungsten grains, and stabilises the high ductility of the A5 material, which exceeds 30% for the quenched sinter (see Figure 4 and Table 3). The strength Rm and yield strength R\({}_{\text{p0.2}}\) remain unchanged after quenching. The strength characteristics are always the result of a tensile test on a breaking machine. Here, only one sinter sample was taken for each temperature in the graph; so, the spread of values is not visible in Figure 4. The quenching temperature has practically no effect on the values of R\({}_{\text{m}}\) and R\({}_{\text{p0.2}}\), which remain within the range of values valid for unquenched \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **R\({}_{\text{p0.2}}\)** & **R\({}_{\text{m}}\)** & **A\({}_{\text{S}}\)** & **Impact Test** \\  & **(MPa)** & **(MPa)** & **(\%)** & **(J/cm\({}^{2}\))** \\ \hline TN & 648–665 & 1015–1026 & 32.1–39.5 & \textgreater{}248 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of TN material samples in the hardened (quenched) sinter phase. sinter. Quenching will only significantly multiply and stabilise ductility, allowing for convenient subsequent strengthening through forming. No significant changes are evident up to a quenching temperature of 1000 \({}^{\circ}\)C. After that, there is a high increase in ductility A5 above 30%, and this increases ever so slightly with quenching temperature. For further strengthening, a 30% ductility is sufficient and there is no point in heating the material any further. Figure 4: Effect of quenching temperature on the strength (red curve), yield strength (purple curve) and ductility (black curve) of the quenched TN material. Figure 3: Structures of the TN-type semi-finished product depending on the sintering temperature (a) 1470 \({}^{\circ}\)C, (b) 1500 \({}^{\circ}\)C, (c) 1520 \({}^{\circ}\)C, (d) 1550 \({}^{\circ}\)C.  
The quenched sinter is subsequently hardened through forming.