AFM, SEM AND TEM study of damage mechanisms in cyclically strained mar-M247 at room temperature and high temperatures

Abstract

Cylindrical specimens of γ́ precipitate strengthened superalloy MAR-M247 were cyclically strained under total strain control conditions using a symmetrical push–pull cycle at 23 °C, 700 °C and 800 °C in air. The fatigue hardening/softening curves and low cycle fatigue life curves are presented. Special attention is paid to the effect of temperature on plastic strain localisation and fatigue crack initiation at 23 °C, 700 °C and 800 °C. Plastic strain localisation into dislocation-rich slabs parallel to the 111slip planes was found at 23 °C and 700 °C. Plastic strain localisation manifested itself by the formation of pronounced surface relief, known as persistent slip markings (PSMs), at locations where dislocation-rich bands emerged on the surface. PSMs consisted of extrusions accompanied by parallel intrusions, where the intrusions were identified as primary crack nucleation sites at 23 °C. With an increase in the testing temperature to 700 °C, the impact of PSMs on fatigue crack initiation was reduced. At 800 °C, the appearance of both PSMs and dislocation arrangement changed significantly. The short PSMs with a wavy character appeared and bands of superlattice stacking faults (SSFs) on 111 planes with Burgers vector b of 〈112〉 type were identified. At 800 °C, fatigue cracks were initiated at the defects and carbides.