13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Materials and experimental details 2.1. Materials The austenitic stainless steels, AISI 316L and the nickel-base alloys, Alloy 617 were used in this study. Both types were solution treated before testing. Table 1 shows the chemical composition of the materials. Table 1. Nominal composition of the investigated materials in wt%. Material C Si Mn Cr Ni W Co Cu Nb N Mo Fe AISI 316L 0.04 0.4 1.7 17 12 - - - - - 2.6 Bal. Alloy 617 0.1 - - 22.5 53.8 - 12 - - 0.5 9 1.1 2.2. Tensile deformation For the slow strain rate tensile testing (SSRT) a Roell-Korthaus and an Instron 5982 tensile test machines were employed. The tensile test machines were equipped with a MTS 653 furnace and Magtec PMA-12/2/VV7-1 extensometer and ElectroHeat Sweden furnace and Epsilon extensometer respectively, both active in air environments. Round-bar specimens with 5 mm in diameter and 50 mm gauge length were used. The tensile tests were carried out at a strain rate of 2*10-3s-1 down to 10-6s-1. Three temperatures: room temperature (RT), 650°C and 700°C, were used. Alloy 617 was tested using strain rates 10-2s-1, 2*10-3s-1, 10-3s-1, 10-4s-1, 10-5s-1 and 10-6s-1, AISI 316L was tested using strain rates 2*10-3s-1 and 10-6s-1. 2.3. Scanning electron microscopy investigation For the microstructural investigation of deformation and fracture behaviour the scanning electron microscopy (SEM) techniques electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) were used. ECCI uses the interaction between backscattered electrons and the crystal planes to generate contrast resulting in an image where local orientation, defects and strain fields are shown as contrast variations [10]. 3. Results 3.1. Strain rate and temperature influence on tensile deformation and damage Low strain rates and elevated temperatures are increasing the probability of dynamic strain ageing (DSA), because the diffusion rate of the solute atoms increases and rate of dislocation movement decreases. During DSA the deformation mechanisms is shifting towards planar deformation [7]. None of the tested materials showed DSA at room temperature, but both materials showed DSA at elevated temperatures for all tested strain rates. For a strain rate of 10-6/s at both elevated temperatures, AISI 316L and Alloy 617 show different types of PLC effects. AISI 316L display Btype and Alloy 617 shows C-type serrated yielding, see fig.1. B-type serrations means that the stress oscillates from a origin stress level and C-type of serration is characterized by a sudden drop in stress and then raises to the normal stress level [8, 9]. Alloy 617 show serrated yielding from low strain values until fracture for all tested strain rates at both elevated temperature except of strain rates 10-3/s and 10-4/s. At both elevated temperatures and strain rate 10-3/s serrated yielding is present at low strain levels up to around 20% but DSA also appears just before fracture, see fig. 1a). For 10-4/s at 650°C serrated yielding disappears after approximately 4% strain and after around 12%
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