X-Ray Analysis
Figure 4.1 presents the typical XRD patterns of mechanically-mixed SiC10/Al90 powder after (a) 0 ks of ball-milling time (starting material mixture of elemental Al and SiC powders) and (b) 86 ks of ball-milling time (the end-product). Obviously, the starting materials contain coarse polycrystalline grains of SiC and Al, suggested by the sharp diffraction peaks of the powder mixture. On the contrary, the Bragg peaks of both the reinforcement (SiC) and metallic matrix (Al) powders become broad towards the end stage of milling [Fig. 4.1(b)], indicating the formation of nanocomposite SiCp/Al material. Remarkably, this end-product is still an intact mixture of polycrystalline SiC and Al and does not coexist with any reactive products, such as Al4C3 and Si, suggesting the absence of any undesirable reactions at the SiC/Al interfaces.
Morphology and Metallography
SEM technique was used to follow the characteristics of the powders during the several stages of ball milling. Figure 4.2 may summarize the morphology, topology, and size of mechanically mixed SiC10/Al90 (vol %) powder after selected ball-milling time. The powders at the starting stage of milling [Fig. 4.2(a)] are bulky with random shape and size. The soft Al particles in the initial starting mixture are affected by the cold working and the impact and shear forces generated by the milling media, and tend to agglomerate to form larger powder particles with an average diameter of nearly 1000 µm, after 3.6 ks of the milling [Fig. 4.2(b)]. At this stage of milling, the hard particles of SiC are randomly embedded into the deformed Al soft matrix (host matrix) to form coarse composite SiCp/Al powders, as shown in Fig. 4.3. Contrary to the Al powders, the SiC powders have been dramatically disintegrated to form finer particles with spherical-like morphology. During the subsequent disintegration stage (22 to 43 ks), the agglomerated powder particles are subjected to a continuous disintegration with fragmentation to form finer powders with extremely wide particle size distribution [Fig. 4.2(c)]. Towards the end of this stage (43 ks), the reinforcement SiC particles becoming finer (less than 1 µm in diameter) and more uniformly distributed in the Al matrix (Fig. 4.4). The powder of the end-product (86 ks of milling time) of SiC10/Al90 composite possesses excellent morphological appearance, characterized by the ultrafine (less than 0.5 µm in diameter), smooth.