1,721,016 research outputs found
A study on slip activation for a coarse-grained and single crystalline CoCrNi medium entropy alloy
Full text linkThe new CoCrNi medium entropy alloy (MEA) has emerged to be one of the most promising systems which provide extraordinary ductility and strength at cryogenic temperatures. In this study, utilizing both polycrystalline and single crystal specimens, as well as advanced optical strain measurements, the deformation mechanisms dictating the mechanical behavior at the onset of plasticity were detected and precisely quantified. Independent of deformation temperature, the accumulation of permanent strains at the microstructural level was attributed to plastic slip at the onset of yielding and at low levels of deformation (<10%). The resolved shear stress for slip activation was measured to be 78 MPa at 298 K and between 140 MPa and 160 MPa at 77 K. These unique measurements were used to provide an estimate of the temperature dependent resolved shear stress for slip at temperatures ranging from 77 K to 575 K. In addition, strain heterogeneities at the grain-scale were measured to study the nucleation of slip at the micro-scale in polycrystalline specimens. In summary, the present study aims to quantitatively assess the accumulation of plastic strain and reveal the underlying deformation mechanisms (i.e., slip or/twinning) leading to the buildup of plastic strains at the microstructural level
Sub-grain Plastic Strain Localization in CoCrNi Medium Entropy Alloy at Cryogenic Temperatures
No full textHigh and medium entropy alloys are currently attracting significant research interest due to their potential to achieve superior mechanical properties compared to traditional alloys systems. The CoCrNi alloy has been of particular interest owing to the simple single phase structure, superior fracture toughness, and exceptional strength and ductility at cryogenic temperatures. Previous works have been primarily focused on identifying the operative microstructural mechanisms responsible for improved ductility. The activation of deformation twining at low deformation temperatures and high strains has been identified as a primary source for the improved ductility. However, detailed quantitative analysis focused on the deformation heterogeneities in the vicinity of grain boundaries, in particular at cryogenic temperatures, remains limited. Strain heterogeneities across grain boundaries reveal the micro-mechanisms responsible for the alloy strengthening and fracture properties, thus their measurements is of fundamental importance. The current work is dedicated to study the local strain accumulation in the vicinity of grains boundaries of plastically deforming CoCrNi. High resolution digital image correlation was used to measure and quantify the deformation heterogeneities at room temperature (298 K) and cryogenic temperature (77 K). The work aims to further elucidate the role of grain boundaries in improving the strength and ductility at cryogenic deformation temperatures
A comparison of DIC-based techniques to measure crack closure in LCF
No full textCrack closure is one of the most important phenomenon for the comprehension of fatigue behavior of metallic alloys. The effect of mean stress, overloads and variable amplitude loadings can be predicted modeling the crack closure in terms of opening and closing stress levels. Experimentally, the characterization of the crack closure in high and low cycle fatigue has gained large attention in the last fifty years. In fact, the proper detection of the opening and closing levels enhances the definition of a proper crack driving force and its modeling. This work primarily focuses on the measurement of the opening and closing stresses for cracks propagating in low cycle fatigue conditions. High-resolution full-field digital image correlation technique was adopted to track the crack profiles during cyclic loading and different approaches were adopted to analyze the displacement fields extracted with the virtual extensometers. The closure measurements were performed for three metallic alloys, different strain ranges (from elastic to dominant-plastic behaviors) and strain ratio to enlarge the field of applicability of the techniques presented
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Analysis of strain and stress concentrations in micro-lattice structures manufactured by SLM
No full textPurpose: Additive manufacturing (AM) enables the production of lightweight parts with complex shapes and small dimensions. Recent improvements in AM techniques have allowed a significant growth of AM for industrial applications. In particular, AM is suitable for the production of materials shaped in lattice, which are very attractive for their lightweight design and their multi-functional properties. AM parts are often characterised by geometrical imperfections, residual porosity, high surface roughness which typically lead to stress/strain localisations and decreasing the resistance of the structure. This paper aims to focus on the study of the effects of geometrical irregularities and stress concentrations derived from them. Design/methodology/approach: In this paper, several technique were combined: 3D tomography, experimental tests, digital image correlation and finite elements (FE) models based on both the as-designed and the as-manufactured geometries of lattice materials. The Digital Image Correlation technique allowed to measure local deformations in the specimen during the experimental test. The micro-computed tomography allowed to reconstruct the as-manufactured geometries of the specimens, from which the geometrical quality of the micro-structure is evaluated to run FE analyses. Findings: Experimental and numerical results were compared by means of a stress concentration factor. This factor was calculated in three different specimens obtained from three-different printing processes to compare and understand their mechanical properties. Considering the as-designed geometry, it is not possible to model geometrical imperfections, and a FE model based on an as-manufactured geometry is needed. The results show that the mechanical properties of the printed samples are directly related to the statistical distribution of the stress concentration factor. Originality/value: In this work, several techniques were combined to study the mechanical behaviour of lattice micro-structures. Lattice materials obtained by different selective laser melting printing parameters show different mechanical behaviours. A stress concentration factor can be assumed as a measure of the quality of these mechanical properties
Strain localizations in notches for a coarse-grained Ni-based superalloy: Simulations and experiments
Full text linkAlloys used for turbine blades have to safely sustain severe thermomechanical loadings during service such as, for example, centrifugal loadings, creep and high temperature gradients. For these applications, cast Ni-based superalloys characterized by a coarse-grained microstructure are widely adopted. This microstructure dictates a strong anisotropic mechanical behaviour and, concurrently, a large scatter in the fatigue properties is observed. In this work, Crystal Plasticity Finite Element (CPFE) simulations and strain measurements performed by means of Digital Image Correlations (DIC) were adopted to study the variability introduced by the coarse-grained microstructure. In particular, the CPFE simulations were calibrated and used to simulate the effect of the grain cluster orientations in proximity to notches, which reproduce the cooling air ducts of the turbine blades. The numerical simulations were experimentally validated by the DIC measurements. This study aims to predict the statistical variability of the strain concentration factors and support component design
Pseudoelasticity in FeMnNiAl shape memory alloy lattice structures produced by Laser Powder Bed Fusion
Full text linkThis work focusses on microstructure and mechanical properties of FeMnAlNi bulk and micro-lattice structures produced by Laser Powder Bed Fusion, an Additive Manufacturing technology. Microstructure investigation was conducted by means of Electron Back Scattered Electrons analysis, Optical Microscopy and X-Ray Diffraction. The mechanical behavior of the material was evaluated by compression tests and strains were measured by Digital Image Correlation. The material is characterized by coarse grains and by the presence of both austenitic (α-bcc) and martensitic (γ′-fcc) phases. The bulk material recovered an overall and local pseudoelastic strain of 1.3% and 2.9%, respectively. The lattice specimen exhibited ~2% strain recovery
- …
