Effects of 1μm-Al
2O
3 content, the structure of the Watt's Ni matrix, and annealing on the tensile properties of 0-13.4vol.% Al
2O
3 composite were studied by scanning electron and optical microscopes. Under the optical microscope, the Al
2O
3 particles were observed to be dispersed uniformly in the as-deposited matrix, which had a columnar structure for 0-6.6vol.% Al
2O
3 composite, and a microgranular structure for 6.6-13.4vol.% Al
2O
3 composite. The tensile strength of the composite increased with increasing the amount of Al
2O
3 in the composite. Taking the grain refinement of the matrix into account, this effect could be explained by the Orowan mechanism. Tensile fracture of the composite initiated at microvoids that had been formed at the interface of the matrix and the particle. On annealing at 1000°C, however, recrystallization of the Ni matrix and coagulation of the Al
2O
3 particles were found within 2 hours, and after an induction periode of 10 hours, a firm bonding was found to have formed between the matrix and the particles, and the tensile strength of the composite increased with annealing time.
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