A Study on the Factors Affecting the Transition of Fatigue Cracking under High Temperature Low Cycle Fatigue in the Nb-added A286 Superalloy

摘要

Continuous low cycle fatigue tests have been conducted on Nb-A286 Fe-base superalloy in an air atmosphere at the temperature range of 25 to 650 deg C to investigate the transition of fatigue cracking with test temperature and applied strain range. The intergranular fatigue cracking at high temperatures was due to the precipitation of the #eta# phase at the grain boundary assisted by the applied stress during low cycle fatigue. It is investigated whether the precipitate at the grain boundary provides the site for the grain boundary cavitation, which induces the intergranular cracking in low cycle fatigue. The fatigue cracking mode changes from the transgranular mode to the predominantly intergranular mode with the applied strain range. The change of the fatigue cracking with strain range is induced by the combination of mutually competing factors involving the grain boundary precipitate, the density of the slip band, and the concentration of stress at points where the slip bands impinge upon the grain boundaries.