Microstructural processes during subcritical crack growth in hard metals and cermets under cyclic loads

The occurence of fatigue in hard metals and cermets under cyclic loads has been demonstrated recently by the present authors. This work focusses on the investigations of the microstructural processes during subcritical crack growth of small natural cracks in fatigued hard metals and cermets. The mechanical tests were performed under static, monotonically increasing, and cyclic loads (22 Hz) in air at toom temperature. After conducting the experiments under the mentioned loads, the fracture surfaces of the samples were investigated by SEM and the microstructure in the volume around the grown cracks was studied by TEM. The following results were obtained. bcritical crack growth during cyclic loads occurs predominantly in the ductile binder phase. The binder phase undergoes a strong plastic deformation, especially in the ligaments formed behind the crack tip. The ligament zone has a great influence on the mechanical behaviour of the materials. TEM investigations show that in samples subjected to cyclic loading, a martensitic phase transformation from the f.c.c. to the h.c.p. structure occurs in the cobalt binder ligaments. This transformation plays an important role in the fatigue of the hard metals. he microstructural features observed on the fracture surfaces and in the volume of the materials under the different loads and from the results in the mechanical tests conclusions can be drawn about the damage processes, which govern the lifetimes and reliabilities of the materials. This knowledge is essential for well-designed improvements of the materials with regard to the fatigue under cyclic loads in their application in cutting tools.