MICROSTRUCTURAL PROCESSES DURING SUBCRITICAL CRACK-GROWTH IN HARD METALS AND CERMETS UNDER CYCLIC LOADS

The occurence of fatigue in hard metal and cermets under cyclic loads has been demonstrated recently by the present authors. This work focus ses on the investigations of the microstructural processes during subc ritical crack growth of small natural cracks in fatigued hard metals a nd cermets. The mechanical tests were performed under static, monotoni cally increasing, and cyclic loads (22 Hz) in air at toom temperature. After conducting the experiments under the mentioned loads, the fract ure surfaces of the samples were investigated by SEM and thr: microstr ucture in the volume around the grown cracks was studied by TEM. The F ollowing results were obtained. The subcritical crack growth during cy clic loads occurs predominantly in the ductile binder phase. The binde r phase undergoes a strong plastic deformation, especially in the liga ments formed behind the crack tip. The ligament zone has a great influ ence 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 c obalt binder ligaments. This transformation plays an important role in the fatigue of the hard metals. From the microstrucural features obse rved on the fracture surfaces and in the volume of the materials under the different loads and from the results in the mechanical tests conc lusions can be drawn about the damage processes, which govern the life times and reliabilities of the materials. This knowledge is essential for well-designed improvements of the materials with regard to the fat igue under cyclic loads in their application in cutting tools.