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.