MECHANICAL BEHAVIOR AND DAMAGE PROCESSES OF UDIMET 720Li: INFLUENCE OF LOCALIZED PLASTICITY AT GRAIN BOUDARIES

The creep-fatigue properties at 700 degrees C of cast and wrought Udimet 720Li superalloy have been investigated. As expected, the introduction of a dwell time as low as Is at maximum stress induces a creep-fatigue life debit compared to pure fatigue conditions. It is demonstrated that the mechanical behavior (plastic strain rate, ductility) is greatly affected by the grain size and precipitation distribution. Moreover, the creep-fatigue life seems to be hardly affected by the alloy microstructure, due to the development of localized damage. Whatever the environmental conditions (i.e. in air or under vacuum), failure has been identified to be intergranular. To get a better understanding of the contribution from grain boundaries to the first stage of plasticity and damage development, in-situ tensile tests were performed in a scanning electron microscope (SEM). In a first step to get a full understanding of the role of grain boundaries in controlling the mechanical properties and alloy durability, we first paid attention to the contribution of the local grain crystallography to the plasticity heterogeneity near grain boundaries. This was performed at room temperature where, compared to creep-fatigue loading at 700 degrees C, the role of viscoplasticity can be neglected. First evidences of slip activity were shown to depend only on the local Schmid factor and not on the grain size. In addition, these experiments were performed using samples with a homogeneous grain size and a homogeneous intragranular gamma' particles distribution. By using the recently developed electron backscattered diffraction (EBSD) pattern cross-correlation technique, it has been shown that critical grain configurations exist, which lead to the activation of micro-volumes (- 1-2 mu m(3) at maximum) where intense plastic activity develop near grain boundaries. These volumes nucleate at grain boundaries separating two grains having a slip activity contrast: they develop in the grain less favorably oriented for slip, ahead of slip bands which had developed in the grain with easier slip. It is observed that the very first stages of development of intense plasticity micro-volumes are quite insensitive to a contrast in Schmid factors between neighboring grains. The local von Mises stress magnitude in these volumes can be as large as several GPa, sufficient enough to develop micro-cracking.