Microstructure and properties of ZrB2-reinforced aluminum matrix composites consolidated by ECAP process

Abstract

In this research work, the effects of mechanical alloying and subsequent compaction by ECAP on the microstructural, physical, and mechanical properties of Al-ZrB2 metal matrix composites (MMCs) were evaluated. To prepare composite powders, a mixture of Al and ZrB2 particles was mechanically alloyed at various times of 1, 6, 12, 18 and 24 h. SEM micrographs indicate that the size of the obtained particles decreases by increasing the time of mechanical alloying up to 18 h. However, after this time, increasing the time of mechanical alloying has led to an increase in the size of the particles. By means of mechanical alloying for 18 h, the average size of fine particles reached 823 nm, while coarse particles were 8 μm. The calculation via the use of XRD examination implies that the mean crystallite size of powder decreased from about 68 nm to 31 nm and 26.8 nm, for Al and Al-5%ZrB2 samples, respectively, while lattice strain increased from 0.25% to 0.64% and from 0.3% to 0.73% after 1 h to 24 h milling of same samples, respectively. However, the rate of crystallite size reduction after 12 h of mechanical alloying is gradually reduced, and that is reached its lowest level after 18 h. After that, increasing the time of mechanical alloying has led to a saturation in the lattice strain. In continue, the resulting composite powder was heated, consolidated and turned into bulk material by warm equal channel angular pressing (ECAP) at the temperature of 250 °C. The resultant optimum composite has been characterized by EDS elemental mapping. Microhardness measurements and shear punch tests were performed to evaluate the mechanical properties of the unreinforced and composite samples after the ECAP process. The Al-ZrB2 MMC bulk samples subjected to ECAP, with a ZrB2 amount of 5 wt%, had a relative density, hardness, and ultimate shear strength of 99.3%, 170 HV, and 151 MPa, respectively, using powders which have been mechanically alloyed up to 24 h.