３１６ステンレス鋼の高温低サイクル疲労変形特性と転位構造の関係

Abstract

Low cycle fatigue tests with constant plastic strain range Δep were carried out on 316 stainless steel at room and elevated temperatures (650°C) in order to investigate a relationship between macroscopic cyclic deformation and microscopic dislocation structures. Macroscopically, cyclic work hardening behaviors under constant temperature and two-step temperature change tests (a room to high temperature test and a high to room temperature test), and cyclic stress-strain curves by means of three different methods (a companion specimens, an increasing step and a multiple step methods) were obtained. Microscopically, the quantitative variation of dislocation structures was measured by transmission electron microscopy observation. The results obtained were summarized as follows, (1) The cyclic work hardening properties under two-step temperature change tests were explained from the dislocation structures at the time of temperature change and the dislocation rearrangement after the change during the fatigue process.(2) The cyclic stress-strain curves by the three different methods at 650°C were almost the same in the range of Δep>0.3%.(3) The area fraction occupied by cell Sc/S0 increased with the number of cycles N and the Δep, and the increasing tendency during the fatigue process at 650°C was very different from that at room temperature. The Sc/S0 at 650°C increased at a very early stage of fatigue and the value was remarkably larger than that at room temperature.(4) The relations between the normalized stress amplitude (Δσ/2G) and the average cell size lc, and between the (Δσ/2G) and the total dislocation density ρt were obtained as follows, (Δσ/2G)∝lc-0.5, (Δσ/2G)∝ρt0.5where G is a shear modulus.