Surface relief adjacent to the persistent slip bands (PSBs) in polycrystalline 316L stainless steel cycled with constant plastic strain amplitude up to 60% of fatigue life was studied using atomic ...force microscopy with respect to the crystallographic orientation (determined by electron backscattering diffraction—EBSD) and the size of individual grains. Surface relief is formed mostly by ribbon-like extrusions whose height in a grain was found to be proportional to the thickness of the corresponding PSB. The extrusions grow predominantly in the direction of the active Burgers vector. The height of an extrusion in the direction of the active Burgers vector is proportional to the grain size. No systematic dependence of the extrusion height in the active slip direction on the grain orientation was observed. The experimental results are discussed in terms of the recent vacancy models of surface relief evolution.
Das Oberflächenrelief persistenter Gleitbänder (PGB) in vielkristallinem rostfreiem Stahl (316L) nach zyklischer Verformung mit konstanter plastischer Dehnungsamplitude bis zu 60% der Lebensdauer wurde mit Hilfe der Atomkraft-Mikroskopie (AFM) in Abhängigkeit von der kristallografischen Orientierung (bestimmt mit Hilfe der Rückstreu-Elektronenbeugung—EBSD) und der Größe einzelner Körner studiert. Das Oberflächenrelief besteht hauptsächlich aus bandförmigen Extrusionen, deren Höhe senkrecht zur Kornoberfläche sich als proportional zur Dicke der entsprechenden PGB erwies. Die Extrusionen wachsen vorzugsweise in Richtung des aktiven Burgers-Vektors. Die in Richtung des aktiven Burgers-Vektors gemessene Höhe ist der Korngröße proportional. Es wurde keine systematische Abhängigkeit der in aktiver Gleitrichtung gemessenen Extrusionshöhe von der Kornorientierung beobachtet. Die experimentellen Ergebnisse werden hinsichtlich aktueller Leerstellenmodelle zur Oberflächenreliefentwicklung diskutiert.
The influence of the grain orientation on short crack growth was studied in fatigued austenitic stainless steel (316L). Global and mesotexture were analysed using the electron backscattering ...diffraction (EBSD) technique in the scanning electron microscope (SEM). For two specimen types with and without preferred 〈1
0
0〉 oriented grains, respectively, the propagation rates of naturally grown short cracks were estimated. In both cases the damage evolution starts at grain boundaries, especially at twin boundaries, but is dominated later by transcrystalline propagating “TC”-cracks. Shorter fatigue life is correlated with a greater portion of TC-crack segments showing relatively high propagation rates. The expected acceleration of the crack propagation by plastic blunting/resharpening via double slip in a global texture with many 〈1
0
0〉 oriented grains of high slip symmetry will be overcompensated by small axial stresses within the elastically “weak” 〈1
0
0〉 grains. This agrees with the larger fatigue life of such specimens.
It is well-known that crack initiation in fatigued austenitic steel (316L) specimens is dominated at lower deformation amplitudes by twin boundaries (TBs). For medium plastic strain amplitudes, it is ...shown here that the propagation of short cracks starting at TBs can be explained when both the surface tractions caused by elastic anisotropy as well as the related slip processes are considered. This conclusion has been obtained from grain orientation measurements along damaged TBs using the electron backscatter diffraction technique in the scanning electron microscope. The frequency of the damaged TBs strongly depends on the meso-texture given by the distribution of 60° 〈111〉 rotation axes of the twins in the pole figure. The texture was determined by automatic orientation mapping. Specimens, which were machined transverse to the rolling direction of the plate, show more damaged TBs than those machined parallel. Consequently, the risk of TB cracks can be reduced by favorable alignment of the specimens with respect to the rolling direction.
Twin boundaries as crack nucleation sites Blochwitz, C.; Tirschler, W.
Crystal research and technology (1979),
January 2005, Letnik:
40, Številka:
1-2
Journal Article
The development of slip steps and extrusions on well-polished specimen surfaces was investigated for nickel polycrystals. The specimen was cyclically pre-deformed up to 60% of its lifetime. Further ...deformation was carried out in situ in a scanning electron microscope. Two main aspects were investigated. First, the formation of slip steps during one loading cycle was studied. Second, the development of extrusions was observed in dependence on the number of cycles of post-deformation. Slip steps at active PSBs with a minimum step height of 2
nm were imaged using a four-quadrant backscatter (BSE)-detector in combination with special imaging conditions. It is shown that during one loading cycle the slip steps do not develop proportionally to the applied plastic strain. More than 60% of the applied plastic strain is necessary to produce slip steps at the polished surface. Extrusions begin to develop within PSBs after a distinct number of cycles of post-deformation. Moreover, it is shown, that nearly all grains containing cumulated PSBs were reactivated during 1000 cycles of post-deformation. The reactivation of the cumulated PSB volume is different for individual grains and evolves more slowly than the reactivation of grains.
The appearance of glide localizations after cyclic deformation in the saturation stage was investigated for polycrystalline nickel. It was shown that persistent slip bands (PSBs) are formed in a wide ...range of grain orientations. Concerning the grain size it was found, that the probability for the appearance of PSBs is higher for larger grains.
The local slip activity of the formed PSBs was studied after half-cycle deformation using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The fraction of grains with glide-active PSBs and the glide-active PSB volume itself is very small after the half-cycle loading. The obtained local shear strain amplitudes are quite high and vary in the range of 0.2–5%. They are comparable with those found in nickel single crystals at the same loading procedure.
The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as ...well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.
The dependence of fatigue crack initiation and propagation of microstructurally short cracks on plastic strain amplitude and grain orientation was investigated for cyclically deformed nickel and ...austenitic stainless (316L) steel. The damage potential of different crack types was studied. It is shown that grain orientation influences crack path and propagation rate of growing microstructurally short cracks in polycrystalline face-centred cubic (f.c.c.) metals. Transcrystalline crack paths follow amplitude-dependently low-indexed crystal plane traces, but not always slip plane traces as sometimes supposed in literature.
The influence of different overstraining events on the crack-opening displacement of microstructurally short cracks in polycrystalline nickel was investigated. Both symmetric and asymmetric ...overstraining experiments were carried out and the influence of increasing overstraining ratio and number of overstraining cycles was analysed. Symmetric and asymmetric overstraining events lead to characteristic changes in the plastic part of the crack-opening displacement, which can be attributed to strain hardening and/or residual stresses. By applying asymmetric deformation modes in tension or compression, it is possible to separate the effects of strain hardening and residual stresses on crack-opening displacement. Additionally, some results for austenitic stainless steel are given.
Crack opening displacement (COD) along the edges of microstructurally short fatigue cracks in austenitic stainless steel was investigated in situ in the SEM. The measured plastic part of the COD ...range between maximum and minimum load was extrapolated to the near crack tip region, assuming plastic blunting and resharpening by cooperative slip as well known for long cracks. In accordance with the blunting model, the measured crack propagation per cycle is about the half of the extrapolated plastic COD range.
