Spall fracture and twinning in laser shock-loaded single-crystal magnesium

As a major failure process in materials subjected to dynamic loading, spall fracture is one of the most widely studied issues in shock physics. To investigate its dependence on the microstructure, including both initial and shock-induced features, laser shock experiments were performed on single crystal magnesium. Shock loading was applied in directions parallel and perpendicular to the c-axis of the crystals. Both the spall strength and the fracture surface morphology are found to depend on the direction of the shock application with respect to crystal orientations. The results complement data obtained previously over ranges of lower strain rates. A detailed analysis of the residual microstructure and crack patterns in the recovered samples shows strong correlations between damage localization and twins, both pre-existing and shock-induced. Thus, cracks match specific twinning directions, which is discussed on the basis of deformation mechanisms reported under quasi-static loading conditions, either prismatic slip or twinning depending on local orientations

Out-of-plane magnetic patterning on austenitic stainless steels using plasma nitriding

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.A correlation between the grain orientation and the out-of-plane magnetic properties of nitrogen-enriched polycrystalline austenitic stainless steel surface is performed. Due to the competition between the magnetocrystalline anisotropy, the exchange and dipolar interactions, and the residual stresses induced by nitriding, the resulting effective magnetic easy-axis can lay along unusual directions. It is also demonstrated that, by choosing an appropriate stainless steel texturing, arrays of ferromagnetic structures with out-of-plane magnetization, embedded in a paramagnetic matrix, can be produced by local plasma nitriding through shadow masks

In situ EBSD investigation of deformation processes and strain partitioning in bi-modal Ti-6Al-4V using lattice rotations

International audienceIdentification of operating deformation processes and assessment of the resulting strain partitioning are critical concerns for mechanical properties prediction and microstructure optimization in complex alloys such as α/β titanium alloys. Lattice rotation relative to the initial orientation was presently used as a marker of slip activity. A Ti-6Al-4V specimen with a bi-modal microstructure was tested in tension in a scanning electron microscope. Crystallographic orientations were characterized in situ using electron back-scattered diffraction (EBSD). A successful prediction of activated slip systems was achieved using the rotation axis associated with plastic activity. The combination of this procedure and slip traces analysis offers an insight into the determination of both slip plane and slip direction of active slip systems. Based on classical crystal plasticity formulations, the magnitude of the rotation relative to the initial orientation was interpreted in terms of plastic shear magnitude. A quantitative assessment of plastic strain at the microstructure scale was then carried out using lattice rotation data. This approach enabled to discuss strain partitioning in Ti-6Al-4V considering the influence of microstructural features and active slip modes

In situ SEM investigation of slip transfer in Ti-6Al-4V: Effect of applied stress

International audienceSlip transfer is a major concern to understand fatigue behavior and damage, especially in titanium alloys. The presently reported investigation focused on the influence of the applied stress through the use of in situ tensile tests carried out inside a scanning electron microscope. Slip transfer between primary α nodules in Ti-6Al-4V was studied using slip trace analysis combined with the electron backscattered diffraction technique to consider crystallographic aspects. The influence of the resolved shear stress and the alignment between slip systems on slip transmission was thoroughly characterized considering a large number of blocked or transferred slip bands. In particular, the evolution of alignment and stress related indicators was assessed with respect to the applied stress

Application of dynamical ion mixing techniques to the improvement of the fatigue resistance of a 316L stainless steel

In the last years new techniques involving ion implantation combined with a simultaneous deposition method in a sputtering evaporator have been developed at the University of Poitiers. These techniques have been employed to elaborate NiTi amorphous coatings in order to improve the fatigue resistance of a 316 L austenitic stainless steel at room temperature and 573 K. It has been shown that a significant improvement of the fatigue life is obtained by this way at 293 K and at 573 K due to a considerable modification of surface damage leading to an important delay for crack initiation

Electron backscattered diffraction and atomic force microscopy analysis of slip bands induced by fatigue in 316L austenitic stainless steel

The techniques of Electron Backscattered Diffraction (EBSD) and Atomic Force Microscopy (AFM) have been associated to study surface slip features on 316L austenitic stainless steel polycrystals tested in the Low Cycle Fatigue (LCF) range. EBSD investigations allow the identification of activated slip planes for each grain, the determination of slip directions based on Schmid factors calculations as the local inclination of the slip plane according to the surface. AFM allows the measurement of steps height induced at the surface along slip bands and also the characterization of the local morphology of extrusions at a nanometric scale. In this study both techniques are used on the same surface of interest in order to combine crystallographic and topographic information. Consequently a schematic model of the slip band emergence is proposed

Deformation processes near a crack initiation site under dwell-fatigue loading of Ti-6Al-4V

The present article reports an investigation of the mechanism of surface crack initiation of a dwell fatigue tested Ti-6Al-4V alloy with a bi-modal microstructure. Interactions between slip bands and grain boundaries were characterized in order to obtain insights into the crack initiation process and discuss the similarities with models described in the literature. Twinning and local lattice rotation occurred as a result of the slip band blocking at the interface and suggests high local stress concentrations. Nevertheless, crack initiation happened to be intergranular and not transgranular. The crack opened up the basal plane that was located at the interface between two nodules poorly oriented for slip and having a common c axis of the hexagonal unit cell

Analysis of texture effects on fatigue behaviour of 316L stainless steel

The influence of crystallographic texture on cyclic behaviour, fatigue crack initiation and fatigue life of a 316L austenitic stainless steel was investigated in the low cycle fatigue range at room temperature. Specimens of five different orientations of the rolled sheet were compared to study different textures. It is shown that texture influences the stress-strain response of the material and the fatigue damage (crack density). Moreover, to study the influence of the environment on the fatigue behaviour and the coupling effects with crystallographic aspects, fatigue tests were performed in air and in vacuum. It is shown that the environment modifies both the total crack density and the ratio of transgranular to intergranular cracks. All results are discussed with regard to the relative fraction of and grains oriented for multiple slip or to the fraction of grains favouring single slip behaviour

Concurrent operation of ⟨c + a⟩ slip and twinning under cyclic loading of Ti-6Al-4V

International audienceDeformation processes operating in hard/soft grain pairs were characterized in Ti-6Al-4V specimens with a bimodal microstructure submitted to fatigue and dwell fatigue loadings. Concurrent operation of 1st order ⟨c + a⟩ pyramidal slip and slip-stimulated deformation twinning was observed in hard grains, resulting from multi-scale strain incompatibilities. The former constitutes a direct experimental evidence of stress redistribution under cyclic loadings while the latter, which is triggered by local stress concentrations due to dislocations pile ups, is detected for the first time. Insights into the crack initiation processes are then discussed