High Temperature Behavior of Grain Boundary of Polycrystaline Ni-Zn Ferrite under Bending Stress

Abstract

Polycrystalline Ni_0.5Zn_0.5Fe₂O₄ was deformed under the bending stress of 11kg/cm²⋅min from room temperature to 1400°C. Below 600°C, the fracture of polycystals was completely brittle and the fractured surface was mostly transgranular. In the temperature range between 800°C and 1300°C, the relative contribution of intergranular failure increased with increasing temperature. The temperature at which the intergranular failure increased coincided with that at which internal friction of grain boundary increased. The brittle-ductile transition temperature under bending stress was about 1200°C, the same as the transition temperature under compressing stress. Above 1350°C, the fracture stress decreased rapidly with increasing temperature, and the fracture stress under bending stress was the same as that under compressing stress, and the fracture was entirely intergranular.
Grain boundary slip was not large enough to be detected by the off-set of the marker line. Grain boundary was too brittle to slip at low temperatures, and was so plastic above 1350°C that it easily migrated and corrugated preventing the boundary slip. Only in the intermediate temperature range around 1300°C, slight grain boundary slip was observed by the off-sef of the marker line.