Dwell Fatigue of a Fully Lamellar Ti6242 Alloy: Deformation Mechanisms at Different Scales

Abstract

Ti6242 alloy is used for high pressure compressor disks in jet engines due to its good fatigue resistance at low and moderate temperatures. These parts undergo complex in-service thermomechanical loading that can be simulated on laboratory samples by introducing a dwell time at maximum stress in cyclic loading tests. The use of such trapezoidal signal appears more realistic than pure fatigue to simulate flights. Under these conditions, this alloy is well-known to present an important decrease of its lifetime when compared to standard fatigue. This phenomenon is commonly observed in metallic materials for high temperatures when viscoplasticity mechanisms are active. The fact that this decrease of the fatigue life resistance appears at room temperature for the Ti6242 has led the scientific community to name it "cold dwell effect". The present work aims at studying the microstructure configurations favoring micro-plasticity mechanisms during dwell cycling. Instrumented micro-samples are tested in situ in a SEM under tensile, creep and dwell conditions. Microstructure and deformation are analyzed at the scale of lamellae and colonies taking into account the local crystalline orientation (EBSD) and the morphological orientation (image processing). The appearance of micro-relief at large scale is also considered and quantified using white light interferometry. Relationships between microstructure, micro-texture and mechanical deformation are established.