Half-cycle slip activity of persistent slip bands at different stages of fatigue life of polycrystalline nickel (original) (raw)
Related papers
The Half-Cycle Slip Activity of Persistent Slip Bands in Polycrystals
Materials Science Forum, 2008
The development of the volume fraction of cumulated persistent slip bands (PSBs) in cyclically deformed nickel polycrystals was investigated in dependence on the number of cycles using scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was shown that there is a large scatter of the volume fraction of PSBs from grain to grain. Three different tendencies for the development of the volume fraction with increasing number of cycles were distinguished. It was shown that there is a correlation of the orientation of the primary slip systems with the volume fraction of cumulated PSBs and the activation of PSBs during half-cycle deformation.
Materials Science and Engineering: A, 1996
By comparison of the observed trace angles of active slip planes with the expected traces in plastically deformed metal polycrystals, conclusions for the local stress state within the grains of polycrystalline aggregates can be drawn. The expected slip systems can be calculated when the local stress tensor and the orientation of the crystallites in the specimen space are known. In fatigued nickel polycrystals, the crystal orientation was determined by the EBSD (electron backscattering diffraction) method in the scanning electron microscope. It was shown that at the relatively small plastic strains under fatigue conditions the crystalline interactions do not essentially influence the local stress state in the grains, but the external uniaxial stress tensor remains valid in good approximation.
Cyclic stress–strain behavior of polycrystalline nickel
Materials Science and Engineering A, 2004
The influence of strain amplitude and fatigue cycles on cyclic deformation of polycrystalline specimens of commercially 99.53% purity nickel was investigated using fully reversed strain-controlled fatigue tests. The fatigue tests were performed under constant strain amplitude, in air and at room temperature, at a constant strain rate of 0.0001 s −1. The objective was to study cyclic deformation behavior and characterize cyclic hardening response by microstructural observations using transmission electron microscopy. The cyclic stress-strain (CSS) curve of cyclically deformed polycrystalline nickel exhibited three distinct regions with a short quasi-plateau region in the intermediate amplitude range and a slight increase of saturation stress with plastic strain amplitude. The plastic strain amplitude at which the test was conducted influences the cyclic hardening rate. Dislocation structures in fatigued polycrystalline nickel are amplitude dependent and are classified into three types of dislocation structures corresponding to the three regions in the CSS curve. Vein structures consisting of loop patches were observed at low strain amplitudes. These structures become mixed with labyrinth structures at intermediate amplitudes. In addition, persistent slip bands (PSBs) were observed in the quasi-plateau regions of CSS curve. Cellular structures were observed at higher amplitudes which become increasingly equiaxed and smaller with fatigue cycles. A correlation between stress-strain response and development of dislocation structures in deformed polycrystalline nickel revealed that the saturation stress is linearly related to the inverse wall spacing. This relationship is equivalent to the mesh-length theory of work hardening over the strain range used in this study.
Crystals
The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in th...
Slip Activity of Persistent Slip Bands in early Stages of Fatigue Life of Austenitic 316L Steel
Key Engineering Materials, 2013
Flat specimen of 316L steel was cyclically pre-deformed with constant plastic strain amplitude to early stage of fatigue life relevant to the period of cyclic strain localization and fatigue crack initiation. To document slip activity and reversibility/irreversibility of persistent slip bands (PSBs) in situ experiments in the high-resolution SEMFEG under special imaging conditions were performed. The half-and full-cycle slip activity and distribution of plastic strain within PSBs in individual grains were investigated via slip steps generated in half-and full-cycle deformation after intermediate vibration polishing. After completion of in situ tests the surface topography in identical locations was quantitatively documented using atomic force microscopy (AFM).
The effects of grain size on the cyclic deformation behaviour of polycrystalline nickel
Materials Science and Engineering: A, 2004
The present paper reviews experimental results about the effect of grain size on cyclic stress-strain (Σ as -ε pa ) behaviour of high purity (99.99%) nickel material. The effect of grain size (d) on cyclic stress-strain behaviour can be described using the classical Hall-Petch relationship: Σ as = Σ 0 (ε pa ) + k(ε pa )d −1/2 . The parameters k and Σ 0 both depend on the plastic strain amplitude ε pa . k increases rapidly for small plastic strains, decreases for high plastic strains and stays lower than the values obtained under tensile loading. This aspect is discussed with guidance from qualitative and quantitative transmission electron microscopy (TEM) observations and stress partitioning in terms of back stress (X) and effective stress (Σ ef ). More precisely, the back stress is partitioned into intra-granular and inter-granular long-range interactions. According to TEM observations, the interpretation of a quasi-plateau in the cyclic stress-strain curve, generally associated with the occurrence of ladder structures is questionable for the smaller grain size (18 m). The quasi-plateau seems to be associated more with a competition between inter-granular and intra-granular back stresses than with the formation of ladder structures in polycrystalline nickel.
2013
A study of microstructural characterization of copper polycrystals during strain control fatigue has been studied for three decades. However, the reported results are quite controversial in regard to the plateau behavior, which has been well established for single copper crystals. The absence of complete and reliable results on the effects of the copper grain size when polycrystals are subjected to strain control fatigue testing was the primary motivation for the present study, with the goal being to address the cause of a clear answer in the existing literature. Saturation stresses measured in strain control fatigue tests were plotted as a function of corresponding plastic strain amplitudes to obtain a cyclic stress strain curve (CSSC) for three different grain sizes. After testing, cycled specimens were sectioned and cut longitudinally in the gauge length section of the specimens to produce samples for etching and microscopic investigation. Scanning electron microscopy (SEM) was conducted on the etched samples to determine the presence of persistent slip bands (PSBs) in each grain size.
Nucleation of Slip Bands near Twin Boundary in High-Cycle Fatigue
JSME International Journal Series A, 2004
In order to investigate the criterion for the persistent slip band (PSB) formation near a Σ3(111) coherent twin boundary, where is a preferential site for formation of PSB, a highcycle fatigue test is carried out at room temperature using a copper bicrystal specimen with the twin boundary. PSBs are observed near the boundary in the connection with the evolution of extrusion/intrusion on the specimen surface. The slip behavior is different from that predicted by the Schmid factor. A finite element method (FEM) analysis is conducted for the bicrystal, and it reveals that the increase of resolved shear stress on the specific slip system due to the constraint of deformation between the crystals is the main cause for promoting the nucleation of PSB near the twin boundary. By taking into account the interaction between the primary and the secondary slip systems, it is possible to specify the formation behavior of PSB near the twin boundary.
The role of cyclic slip localization in fatigue damage of materials
Le Journal de Physique IV, 1993
Cyclic slip localization is demonstrated in several classes of materials as copper single crystals, polycrystals and in a duplex austenitic-ferritic stainless steel, in measuring the cyclic stress-strain response, studying the surface relief evolution and the internal dislocation structures. The characteristic structure of the persistent slip bands accommodating high local plastic strain amplitude is correlated with the extrusion/intrusion formation on the surface, which results in fatigue crack nucleation, short crack growth, dominant crack formation, its growth and fracture. These stages represent the physical manifestation of the fatigue damage.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1987
Shallow surface layers containing supersaturated solid solutions of aluminum in nickel, or ~'-T' dual-phase structures, were produced on fatigue specimens by ion beam mixing. 0.5 MeV K.r ions were used to ballistically mix multiple nickel and aluminum layers deposited by vacuum evaporation. Such surface modifications inhibited the penetration of persistent slip bands and suppressed the formation of notch-peak morphologies in the PSBs which eventually penetrated the surface layer.