Another interesting feature viewed in the X-ray diffractograms shown in Fig. 6 is the increase in the intensity ofzro2 peaks after compos- te milling, which suggests a probable increase in the phase content of monoclinic and cubic structures. This proposes that ball milling of the composite promoted an additional phase transition in the reinforcement. Assuming that the tetragonal structure was further destabilized, a quan- titative Rietveld analysis was performed in the composite and the results are presented in Table 4. It can be seen that the monoclinic structure in creased its weight percentage upto 37% in the case of5 com- posite. It is important to mention that this feature was not found in the 2 wt% Zro2 composite due to a lower content of reinforcement. The dispersion of the ceramic particles was characterized by TEM. Fig. 7a shows the micrographs of the ceramic particles and their distri- bution in the matrix powders. It can be seen that there are agglomerates of the Zro2 particles inside the matrix powder in the AA 7075-2 wt.% Zro2 composite subjected to 22 h of milling. In some areas it is difficult to distinguish the size of the Zro2 particles because there was not a good resolution of their boundaries, due the dense clustering. On the other hand, the sample shown in Fig. 7b was subjected to 15hofballmill ing. It was found that the reinforcement deagglomeration occurred for this milling time, which is evident in the micrograph that clearly show the difference in contrast between the ceramic and metal particles. The appraisal of the reinforcement distribution indicates that 10 h of millin can be sufficient to avoid the agglomeration of them, a fact that could lead to harmful properties inthe consolidated composite, since these clus- ters can act as stress concentrators and crack initiation sites, characteris- tics which are undesirable in subsequent processing 12,11 and 15 h Fig. 8aand b shows the AA 7075-5 wt. Zro2 composite at2 can be of milling time, respectively. A denser clustering of Zroz particles is not seen on the surface of the matrix powders at 2 this figure it the possible to discern the boundaries of the reinforcement, and seems to be greater because the Zro2 content in this sample is higher. otherwise, the composite processed by of ball milling is shown in Fig. 8b, here the situation seems to be completely different, because there was no heavy agglomeration of the reinforce- istribution is more homogeneous