## 3 Results and Discussion
A decrease in its microhardness occurs after annealing at above 300 \({}^{\circ}\)C. Below this temperature, the microhardness of the material changes slightly. The first peaks A and A\({}^{\prime}\) in the range from 100 to 250 \({}^{\circ}\)C in both DSC curves are usually attributed to recovery processes when the concentration of point defects formed as a result of MA decreases [67]. However, for the copper-containing material, this peak should also be associated with the formation of Al\({}_{2}\)Cu dispersoids from a supersaturated aluminum solid solution formed during milling [68]. It is possible that the processes of recovery and aging are superimposed and therefore the microhardness changes only slightly up to 300 \({}^{\circ}\)C. The second peak B (250-400 \({}^{\circ}\)C) on the DSC curve for the material with the addition of PCA is probably associated with the formation of dispersed Al\({}_{4}\)C particles which is responsible for the increase in microhardness. The formation of aluminum carbide is caused by the appearance of carbon due to the decomposition of stearic acid during milling and subsequent heating and its reaction with aluminum [69]. The third peak C, which lies in the range from 400 to 500 \({}^{\circ}\)C, and the second peak B\({}^{\prime}\) (380-480 \({}^{\circ}\)C) for the material with the addition of copper can be associated with grain growth (recrystallization) [67; 70; 71], which accounts for the decrease in the microhardness with increasing temperature for both materials (see Figure 3). Peak C\({}^{\prime}\) (566 \({}^{\circ}\)C) belonging to the latter material is probably related to the reduction reaction starting in the solid state. Figure 4 shows the XRD patterns of the studied materials after milling and hot pressing. It can be seen from this figure that there are two main phases in the materials: Al and TiO\({}_{2}\), which indicate the absence of a chemical reduction reaction. In addition, the Al\({}_{2}\)Cu phase is present in the hot-pressed sample with the addition of copper (Figure 4b), its formation being confirmed by DSC data. The absence of the Al\({}_{4}\)C phase in the material with the addition of stearic acid should be associated with its dispersion and small amount. Figure 4 also shows that the Al peaks, both after milling and after hot pressing, are significantly broadened, which may indicate the nanocrystalline nature of the aluminum matrix. Figure 3: The effect of annealing temperature on the microhardness of cold-pressed powders. A nanocrystalline structure is formed during MA as follows. At an early stage of MA, shear bands form due to high strain rates during MA [39]. These shear bands, which contain a high density of dislocations, have a typical width of about 0.5-1.0 mm. With further milling, the average deformation at an atomic level increases due to an increase in the dislocation density, and at a certain dislocation density in these highly stressed regions, the crystal breaks into subgrains separated by low-angle grain boundaries, causing a decrease in lattice deformation. During further processing, deformation occurs in shear bands located in previously undeformed parts of the material. The grain size decreases steadily, and the shear bands merge. Low-angle boundaries are replaced by high-angle grain boundaries, indicating grain rotation. As a result, dislocation-free nanocrystalline grains are formed. The minimum grain size achievable during milling is determined by the competition between plastic deformation due to the movement of dislocations and the behavior of the material during recovery and recrystallization. Crystallite size and microstrain were estimated using the Williamson-Hall equation [72]:  
\[\beta_{hkl}\cos\theta=\frac{K\lambda}{D}+4\epsilon\sin\theta \tag{1}\]  
where \(\beta\) is the integral broadening of the diffraction peak, \(\theta\) is the Bragg angle, \(K\) is the shape factor (0.9), \(\lambda\) is the wavelength of Cuk\({}_{\alpha}\) radiation (0.154 nm), \(D\) is the mean crystallite size, and \(\epsilon\) is the microstrain.