PSI - Issue 23

Joakim Nordström et al. / Procedia Structural Integrity 23 (2019) 457–462 Joakim Nordström / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Twinning in a crystalline material occurs when two or more individual crystals of the same phase share some of the same crystal lattice points in a symmetrical manner (Friedel G. 1904). The formation of deformation twins (DT) and twin boundaries in metallic materials can effectively strengthen the material by impeding mobile dislocations and at the same time increase the work-hardening capability and ductility of the same (Christian, J. W. and S. Mahajan, 1995) With that background, deformation twinning in steel has attracted a great deal of attention and resources during the last decades as it can improve both strength and plasticity simultaneously (Grassel O et. al, 2000; Cooman B et. al, 2012). However, whether twinning is involved in the fracture or not remains a matter of debate, particularly for austenitic materials. Nevertheless, in nickel base alloys deformation twinning is a rare phenomenon, at least under typical conditions (tensile stress and air at atmospheric pressure and room temperature) partly due to the high stacking fault energy. Though, at cryogenic temperatures, deformation twinning in nickel base alloys has been found, which can contribute to improvement of the plasticity (Nordström, J. et. al, 2018). 2. Experimental procedures The material used was a hot extruded tube with the dimensions 63x7 mm, from which the longitudinal tensile test samples were cut and lathed. The composition of the nickel base material was (mass%): 0.015 C, 0.11 Si, 0.10 Mn, 0.005 P, 0.0004 S, 21.61 Cr, 61.8 Ni, 8.27 Mo, 0.53 Co, 3.23 Nb, 0.07 Ti, 0.17 Al, 0.04 Cu and balance Fe (3.99). The tensile testing was carried out in two different tensile test machines: a Roell-Korthaus and an Instron 5982 equipped with a Magtec PMA-12/2/VV7-1 (air). Cylindrical specimens with a diameter of 5 mm and a gauge length of 25 mm were used in accordance with ASTM E8. The tensile testing was carried out with an initial strain rate of 8 ∙ 10 −4 [1/s] and changed to 5 ∙ 10 −3 [1/s] at a strain corresponding to 0.044. The tensile tests were performed at two different temperatures: room temperature (RT) (25 °C ) and at cryogenic temperature (CT), liquid nitrogen (- 196°C ). Three replicates were run at both temperatures. Fracture surfaces were investigated in a Zeiss Sigma VP field emission gun (FEG) scanning electron microscope (SEM) with secondary electrons and an accelerating voltage of 10 kV after ultrasonic cleaning in acetone for 4 min and subsequent cleaning with Deconex (2 min) and later flooded with distilled water. The water was later evaporated with hot air. The microstructure was investigated in the above mentioned FEG-SEM that is also equipped with a Nordlys electron backscatter diffraction (EBSD) detector. The microstructure was first investigated with SEM-EBSD and later features were studied within the previously EBSD-scanned areas and now in specifically interesting grains, this time in channeling mode. The SEM-EBSD investigation was performed at 20 kV and with an aperture size corresponding to 120 µm. The threshold values for low angle grain boundaries were set between 2 and 10°, above 10° for high angle grain boundaries and to a maximum deviation of 5° for twin boundaries. The longitudinal sections were marked in the center at three positions using a Vickers 0.5 kg indenter, 0.5, 2.5 and 10 mm from the fracture surface as recognized on the polished mount. The indentations served as orientation marks to be able to find the specific EBSD-scanned area to resolve twins in channelling mode, see for instance fig. 3. 3. Results and discussion Before the start of the tensile test the texture should be considered as nearly isotropic, see pole figure map in fig. 2a. 3.1 Mechanical behavior during tensile deformation Figure 1 shows the tensile deformation and fracture behaviour, and Table 1 shows the tensile properties of Alloy 625 at RT and at CT. Clearly, the material shows higher strength and elongation simultaneously at CT in contrast to at RT. The 0.2% proof stress increases from 370 MPa at RT to 550 MPA at CT, the ultimate tensile stress from 792 MPa to 1134 MP and the elongation from 69% to 89.4%. However, it was qualitatively found that the necking