Chemical reaction mechanism, microstructural characteristics and mechanical properties of in situ (α-Al2O3+ZrB2)/Al composites

Abstract

In-situ (α-Al2O3+ZrB2) particle reinforced aluminum matrix composites were fabricated from Al-ZrO2-B2O3 powder mixtures using chemical reaction method. The chemical reaction mechanism, microstructural characteristics and mechanical properties of the composite were investigated. The chemical reaction process was found to comprise three major steps: step 1: 2Al + B2O3→2[B]+Al2O3(amorphous), 2[B]+Al→AlB2,ZrO2+2B→Zr(O,B)2; step 2:Al + ZrO2→Al2O3(amorphous)+[Zr], [Zr]+Al→Al3Zr, [Zr]+Al→AlZr, 3Zr(O,B)2+4Al→2Al2O3+3ZrB2;and step3:Al3Zr + AlZr+2AlB2→6Al+2ZrB2, Al2O3(amorphous)→α-Al2O3. The transitional phases were found to be AlB2 and amorphous Al2O3. Notably, with increasing temperature, amorphous Al2O3 began to crystallize and eventually transformed to more stable α-Al2O3. The activation energies of the three step reactions were calculated and found to be 641, 330 and 192 kJ/mol, respectively. The reaction rate of the third step was observed to be much higher than that of the second step reaction. Reinforcement particles have a uniform distribution in the aluminum matrix. The α-Al2O3 particles exhibit a size ranging from several hundred nanometers to a few micrometers, while the size of ZrB2 particles is less than 3 μm. The tensile strength and elongation rate of the composites are determined to be 226 MPa and 4.6%, respectively.