INTRINSIC STAGE-I CRACK-GROWTH OF DIRECTIONALLY SOLIDIFIED NI-BASE SUPERALLOYS DURING LOW-CYCLE FATIGUE AT ELEVATED-TEMPERATURE

Strain-controlled low-cycle fatigue tests of cylindrical smooth specimens of two kinds of directionally solidified Ni-base superalloys, RENÉ 80+Hf and CM 247LC, were carried out at a temperature of 873 K, and the successive process from the crack initiation to small crack propagation was investigated by employing a replication technique. Both materials exhibited typical features of stage I fatigue fracture; that is, the fracture occurred on the crystallographic 111 planes, the most important slip planes in face-centered cubic (fcc) materials. It was found that the rate of stage I crack growth, when not influenced by a nearby grain boundary, proportionally increased with the crack length. However, as the crack tip neared a grain boundary, the rate rapidly decreased. It was also shown that the crack growth rate fell when the crack deflection occurred due to secondary slip. Comparison was also made between the stage I crack growth rate and the long crack growth rate in polycrystalline Ni-base superalloys. © 1990 The Metallurgical of Society of AIME.