Examination of the Corrosion Process Impact on the Microstructure and Mechanical Properties of Zn27Al1.5Cu0.02Mg Alloy Castings Reinforced with Silicon Carbide Particles

Na osnovu izvršenih istraživanja ustanovljeno je da postoji međusobna povezanost mikrostrukture, mehaničkih svojstava i korozionog ponašanja čestičnih ZA27/SiC kompozita. Livena ZA27 legura predstavlja metalnu osnovu za dobijanje kompozita. Struktura legure je tipična dendritna. Trajna transformacija dendritne u ne-dendritnu strukturu izvršena je tiksoformiranjem, dok je poluočvrsli rastop legure bio izložen dejstvu sila smicanja. Kompoziti su dobijeni kompokasting postupkom, uz infiltraciju 1, 3 i 5 mas.% SiC čestica u metalnu osnovu. Otpornost prema koroziji matrične legure i ZA27/SiC kompozita određena je na osnovu vrednosti za brzinu korozije u rastvoru NaCl (pH=6,7). Određivanje brzine opšte korozije izvršeno je metodom gubitka mase i elektrohemijskim metodama. Otpornost matrične legure prema koroziji veća je od otpornosti kompozita. Uticaj korozije na mikrostrukturu ispitan je primenom svetlosne i skenirajuće elektronske mikroskopije. Posle izlaganja uzoraka u rastvoru NaCl, promena granice tečenja (pri ispitivanju pritiskivanjem), uzeta je kao pokazatelj uticaja korozije na mehanička svojstva matrične legure i kompozita. U matričnoj leguri procesi korozije zahvataju oblast η faze i oblast smeše faza α+η, a koroziona razaranja su površinskog karaktera. Zapaženo je da je u kompozitima sa većim sadržajem infiltriranih SiC čestica (3 i 5 mas.%) došlo do stvaranja nakupina SiC čestica, što je dovelo do obrazovanja mikroprslina i mikropora. Na ovim mestima proces korozije napreduje u dubinu materijala, što se negativno odrazilo na otpornost prema koroziji i mehanička svojstva čestičnih ZA27/SiC kompozita.On the basis of examination carried out, the mutual dependence of the microstructure, mechanical properties and corrosion behaviour of particulate ZA27/SiC composites was established. As-cast ZA27 alloy was the metal matrix for producing composites. The structure of as-cast ZA27 alloy is typically dendritic. A permanent transformation of the dendritic structure into nondendritic one was achieved through thixoforming during exposure of ZA27 alloy semi-solid melt to the influence of shear forces. Composites were obtained by compocasting process, through infiltration of 1, 3 and 5 wt.% SiC particles into the metal matrix. Corrosion resistance of the metal matrix and ZA27/SiC composites was evaluated on the basis of corrosion rate in NaCl solution (pH=6.7). Mass loss method and electrochemical polarization measurements were used to determine rate of general corrosion. Matrix alloy resistance to corrosion is higher than the resistance of the composites. The influence of corrosion on the microstructure was examined using light and scanning electron microscopy. After exposure of samples in NaCl slution, compressive yield strength was taken as an indicator of corrosion impact on the mechanical properties of the matrix alloy and the composites. Corrosion process in the matrix alloy takes place through η phase (interdendritic phase rich in zinc) and α+η phase mixture, while corrosion damage is limited to the surface of samples. Agglomeration of reinforcing particles was noticed within composites with higher content of SiC particles (3 and 5 mas.% SiC) which resulted in formation of microvoids and microcracks. These were places where progress of corrosion processes in the depth of material was observed, which had a negative impact on the corrosion resistance and mechanical properties of particulate ZA27/SiC composites