C. Robertson

Effect of grain disorientation on early fatigue crack propagation in face-centred-cubic polycrystals: A three-dimensional dislocation dynamics investigation

Abstract Three-dimensional dislocation dynamics simulations are used to study micro-crack interaction with the first micro-structural barrier, in face-centred-cubic bi-crystals loaded in high-cycle fatigue conditions. In the examined configuration, we assumed that micro-crack transmission occurs due to surface relief growth in the secondary grain ahead of the primary crack. This indirect transmission mechanism is shown to strongly depend on grain-1/grain-2 disorientation. For instance, small grain disorientation induces plastic strain localization ahead of the crack and therefore, faster transmission through the first barrier. Conversely, large grain-1/grain-2 disorientation induces plastic strain spreading similar to crack tip blunting, yielding slower indirect transmission. A semi-analytical micro-model is then proposed based on the present simulation results and complementary experimental observations, highlighting the original notion of first-barrier compliance. The model captures well-known experimental trends, including the effects of grain-size, grain disorientation and micro-crack retardation at the first barrier.

An extensive 3D dislocation dynamics investigation of stage-I fatigue crack propagation

Stage-I fatigue crack propagation is investigated using 3D discrete dislocation dynamics (DD) simulations. Slip-based propagation mechanisms and the role of the pre-existing slip band on the crack path are emphasized. Stage-I crack growth is found to be compatible with successive decohesion of the persistent slip band/matrix interface rather than a mere effect of plastic irreversibility. Corresponding crack tip slip displacement magnitude and the associated crack growth rate are evaluated quantitatively at various tip distances from the grain boundary. This shows that grain boundaries systematically amplify slip dispersion ahead of the crack tip and consequently, slow down the stage-I crack growth rate. The results help in developing an original crack propagation model, accounting for the boundary effects relevant to polycrystals. The crack growth trend is then evaluated from calculations of the energy changes due to crack length increments. It is shown that the crack necessarily propagates by increments smaller than 10 nm.

Internal stress evolution in Fe laths deformed at low temperature analysed by dislocation dynamics simulations

Stress evolution in Fe laths undergoing plastic deformation is investigated using three-dimensional dislocation dynamics simulations adapted to body centred cubic crystals, in the ductile to brittle transition temperature range. The selected boundary conditions, applied stress tensor and initial dislocation structures account for the realistic microstructure observed in bainitic steels. The effective stress field projected in the three different {1?0?0} cleavage planes is calculated for two different temperatures (50 and 200?K) and presented quantitatively, in the form of stress/frequency diagrams. It is shown that plastic activity tends to relax the stress acting in certain cleavage planes (the (0?1?0) and (0?0?1) planes) while, at the same time, amplifying the stress acting in other cleavage planes (the (1?0?0) planes). The selective stress amplification in the latter planes depends on the applied load direction, in combination with the limited set of available slip systems and the lath geometry. In the examined configuration, this selection effect is more pronounced with decreasing temperature, emphasizing the role of thermally activated plasticity on deformation-induced stress concentrations.

Influence of post-irradiation thermal annealing on the mechanical properties of ion irradiated layers in 316L stainless steel

Irradiation-induced modification of the mechanical properties of metals is an important but poorly understood technological problem, involving microscopic phenomenon upon which the present study concentrates. In an attempt to take advantage of charged particles irradiation, we applied a combination of transmission electron microscopy (TEM) and sub-micron indentation technique to ion-irradiated layers of 316L steel specimens, irradiated with krypton ions up to 3 dpa at 350°C. It was found that hardness as measured on the ion-irradiated zone is modified by post-irradiation thermal annealing and that the amplitude of this modification is important at 600°C.

Effect of Grain Disorientation on Early Fatigue Crack Propagation in FCC Polycrystals: Dislocation Dynamics Simulations and Corresponding Experimental Validation

Abstract3-dimensional dislocation dynamics (DD) simulations are performed, in face-centred cubic bi-crystals, to study the microcrack interaction with first microstructural barrier under high cycle fatigue loading conditions. Based on experimental observations, we presumed that microcracks are blocked by grain boundaries and that subsequent propagation/transmission occurs by the growth of surface relief in a secondary grain adjoining the primary crack. This mechanism is herein called indirect transmission and is found to strongly depend on grain-to-grain disorientation. A semi-analytical model proposed earlier is discussed with the DD simulation results in the context of first-barrier compliance. The proposed model describes the documented experimental results related to the effect of grain size, grain misorientation and microcrack propagation kinetics in fatigued 316L steel polycrystals.

3D Discrete Dislocation Dynamics Investigations of Fatigue Crack Initiation and Propagation

Both nucleation and propagation of fatigue cracks in fcc metals are investigated, using 3D discrete dislocation dynamics (DDD) simulations. Firstly, DDD simulations explain the mechanisms leading to the formation of persistent slip bands in surface grains loaded in fatigue. Extrusions are evidenced where the bands intercept the free surface. The extrusion growth rate is estimated for different material parameters and loading conditions. Energy and stress calculations performed inside the simulated grain lead to a possible scenario for the crack initiation at the interface between the band and the matrix, as reported in the literature. Secondly, a crack is inserted at the persistent slip band interface and the crack tip slip displacement evolutions are evaluated. It is shown that the crack growth rate is strongly related to the grain size and to the distance to the grain boundary; the smaller the grain, the faster the crack growth. Finally, the crack propagation to the next grain is investigated by conducting DDD fatigue simulations in a surface grain next to a cracked grain. It is shown that the development of the persistent slip band is modified by the presence of the crack. The crack orientation affects the orientation of the persistent slip band, as well as the extrusion rate, and consequently the crack propagation in the next grain.

Dislocation structures in 16MND5 pressure vessel steel strained in uniaxial tension at −196°C

Abstract Cylindrical specimens of bainitic pressure vessel steel 16MND5 were strained in uniaxial tension at a temperature of −196°C. Dislocation structure and surface relief were studied at 2.5 and 8% of plastic deformation and their evolution with increasing plastic strain is documented. Great heterogeneities were observed in the dislocation arrangements of individual laths. The dislocation substructure is dominated by short straight dislocation segments of the screw type and by dislocation debris. Dislocation clusters and walls can be seen with increasing deformation. Slip markings are wavy, and some of them become pronounced with increasing plastic strain.