Proceedings of SerbiaTrib '13

SiC particles are uniformly distributed in themetal matrix (Fig. 1a). A few inclusions can benoticed on the surface of the composite sampleand mechanical damages on the edge of thesample. Applied thermal treatment had noeffect on the particles of reinforcement andtheir distribution in the composite matrix.The microstructure of composites wasrevealed by etching (Figure 2a, b). It can beseen in Fig. 2a that SiC particles are distributedin the regions of η phase and regions of phasemixture α+η. There are no voids, due to thefallout of SiC particles from the compositebase (e.g. during machining or metallographicpreparation of samples). This indicates goodbonding between SiC particles and the matrixalloy.Figure 2. Microstructure of the thermally treatedZA27/3wt.%SiC p composite (OM, etched):a) before exposure, b) after 30-day exposure in 3.5 wt.%NaCl.Main micro-constituents in the compositematrix are also visible in Fig. 2a. Themicrostructure of the composite base is nondendriticand is characterized by the presenceof large primary particles [21, 23]. The primaryparticles are complex; they consist of a core(rich in aluminum) and a periphery (composedof the phase mixture α+η). Interdendritic ηphase, rich in zinc, is located between theprimary particles. The microstructure of thecomposite matrix and the microstructure ofthixocast ZA27 alloy are morphologically verysimilar [23]. The applied thermal treatment (T4regime) has caused changes in the structure ofthe composite matrix. The region of phasemixture α + η was expanded, while the regionsof individual phases (α and η) were reduced. Inaddition, the size of primary particles of αphase was decreased for about 30 vol. % [21,23]. All this resulted with finer microstructureof the composite matrix. However, the increasein number of micro-cracks on the phaseboundaries η/α+η was noticed in the compositematrix after the thermal treatment [23].Thermal stress at boundary surfacesmatrix/particle, due to quenching withinthermal treatment, was preceded by thermalstress during solidification of the compositemixture. The stress can cause a localdeformation of the metal matrix aroundparticles of reinforcement [26], appearance ofmicro-cracks or fracture of the particles.Electrolytes can be retained in the microcracks,causing local progress of corrosionprocesses into depth of the composite base.The number of boundary surfacesmatrix/particle was significantly increased inthe ZA27/SiC p composites due to the presenceof SiC particles.. It can be assumed thatdislocation density was also increased duringcooling of the composite mixtures, due todifferent coefficients of linear expansion of thematrix ZA27 alloy and SiC particles.Corrosion processes have influenced thesurface appearance and microstructure ofthermally treated ZA27/SiC p composites. Thesurface appearance of the composites with 3wt. % SiC p , after 30-day exposure in thesodium-chloride solution, is shown in Fig. 1b.Large primary particles of α phase are visiblein the central area of the composite sample. Itcan be seen that corrosion has started at placesof mechanical damages, voids, inclusions.Corrosion processes occurred in the compositebase, in the regions of phase mixture α+η andregions of η phase. The local progress ofcorrosion was noticed in the micro-cracks andpores. The micro-cracks were probably formedduring solidification of the composite mixtures,in hot pressing as well as during quenching13 th International Conference on Tribology – Serbiatrib’13 109