MECHANICAL-PROPERTIES OF STRONTIUM FERRITES WITH DUPLEX STRUCTURE

The microstructures and the mechanical properties of the strontium ferrites (Sr-ferrites) with duplex structure were examined. As-received Sr-ferrites had a homogeneous microstructure and their average grain size was 1.58 mu m. Some grains increased up to 106 mu m and 228 mu m in size after annealing at 1250 degrees C for 1 h and 6 h, respectively. The volume fractions of the large grains were 4% after 1 h-annealing and 45% after 6 h-annealing. According to the microstructural evolution, the bending strength decreased from 148 MPa (as-received) to 140 MPa (3 h-annealing) and 93 MPa (6 h-annealing), but the fracture toughness increased from 2.14 MPa . m(1/2) (as-received) to 2.20 MPa . m(1/2) (3 h-annealing) and 2.36 MPa. m(1/2) (6 h-annealing). These results indicated that the large grains in the duplex structure acted as both fracture origins and reinforcements. The Weibull modulus of materials was improved from 7 (as-received materials) to 17 (material after 3 h-annealing) and 9.5 (material after 6 h-annealing).