Influence of Surface Preparation on Cracking Phenomena in TIG-Welded High and Medium Entropy Alloys

Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.

An Accurate Cubic Law for the Upscaling of Discrete Natural Fractures

Modeling fluid flow in fractured reservoirs requires an accurate evaluation of the hydraulic properties of discrete fractures. Full Navier-Stokes simulations provide an accurate approximation of the flow within fractures, including fracture upscaling. However, its excessive computational cost makes it impractical. The traditionally used cubic law (CL) is known to overshoot the fracture hydraulic properties significantly. In this work, we propose an alternative method based on the cubic law. We first develop geometric rules based on the fracture topography data, by which we subdivide the fracture into segments and local cells. We then modify the aperture field by incorporating the effects of flow direction, flow tortuosity, normal aperture, and local roughness. The approach is applicable for fractures in 2D and 3D spaces. This paper presented almost all existing CL-based models in the literature, which include more than twenty models. We benchmarked all these models, including our proposed model, for thousands of fracture cases. High-resolution simulations solving the full-physics Navier-Stokes (NS) equations were used to compute the reference solutions. We highlight the behavior of accuracy and limitations of all tested models as a function of fracture geometric characteristics, such as roughness. The obtained accuracy of the proposed model showed the highest for more than 2000 fracture cases with a wide range of tortuosity, roughness, and mechanical aperture variations. None of the existing methods in the literature provide this level of accuracy and applicability. The proposed model retains the simplicity and efficiency of the cubic law and can be easily implemented in workflows for reservoir characterization and modeling.

Bedding Effect of Carbonaceous Slate Failure Behavior at Direct Shear Tests

In this study, the carbonaceous slate of Muzhailing tunnel was selected to explore the deformation and failure mechanism of tunnel layered slate under direct shear conditions. Five sets of direct shear tests were carried out on slate with different bedding dip angles (β: 0°, 30°, 45°, 60°, and 90°). The strength parameters, failure mode, fracture topography, dissipated energy, and acoustic emission (AE) characteristics were analyzed in detail to study the bedding effect of slate failure. The results show that the cohesion and internal friction angle of slate increases and decreases linearly with β from 0° to 90°, respectively, and the shear strength of bedding plane is lower than that of matrix. Additionally, the relative roughness (δ) of gray-scale image was defined, and the linear relationship between δ and root-mean-square height (Sq) was established to improve the efficiency of evolution of fracture topography. Furthermore, the shear energy consumption per unit area (us) was calculated, less sensitive to the variation of β. Moreover, a larger AE count and energy release rate were observed when the shear stress decreased, and a relative quiet period was observed before the peak loading. Additionally, both the count and energy release rate linearly decreased with the increase in β, and the proportion of tensile microcracks for vertical bedding is slightly higher than that for horizontal bedding. The main frequency of vertical bedding slate is smaller than that of horizontal bedding, i.e., the size of microcracks of slate with β of 90° is relatively larger. Generally, the frequency bandwidth of slate became narrower when β was varied from 30° to 60°, and the main frequency increased, indicating that the degree of shear failure of matrix is weakened. In contrast, the degree of cracking along the bedding plane increased.

Fractography Study of Explanted Intramedullary Nails

This study comprises the studies performed on six in vivo failed centromedullary fixation devices, three made of stainless steel 316L and three of titanium alloy Ti-6Al-4V. Fractographic studies were performed using stereomicroscopy and scanning electron microscopy while metallographic studies were performed by light microscopy. Loading mode which caused failure was inferred based upon surface fracture topography and light microscopy studies were performed to observe if abnormal microstructures or inclusions could be the cause of the failure. In all cases the chemical composition was according to standard specifications and the microstructures appeared normal with low inclusionary content. The mode of failure was found to be fatigue and the crack initiation site was on the implant surface on roughness features and surface induced defects.

Microstructure, Strength, and Fracture Topography Relations in AISI 316L Stainless Steel, as Seen through a Fractal Approach and the Hall-Petch Law

The influence of the fracture surface fractal dimension DF and the fractal dimension of grain microstructure DM on the strength of AISI 316L type austenitic stainless steel through the Hall-Petch relation has been studied. The change in complexity experimented by the net of grains, as measured by DM, is translated into the respective fracture surface irregularity through DF, in such a way that the higher the grain size (lower DM values) the lower the fracture surface roughness (lower values of DF) and the shallower the dimples on the fractured surfaces. The material was heat-treated at 904, 1010, 1095, and 1194°C, in order to develop equiaxed grain microstructures and then fractured by tension at room temperature. The fracture surfaces were analyzed with a scanning electron microscope, DF was determined using the slit-island method, and the values of DM were taken from the literature. The relation between grain size, DM, mechanical properties, and DF, developed for AISI 316L steel, could be generalized and therefore applied to most of the common micrograined metal alloys currently used in many key engineering areas.

The Effect of Rock Mechanical Properties on Fracture Conductivity for Shale Formations

Hydraulic fracture treatments are used in low permeability shale reservoirs in order to provide conductive pathways from the reservoir to the wellbore. The success of these treatments is highly reliant on the created fracture conductivity. Optimizing fracture designs to improve well performance requires knowledge of how fracture conductivity is affected by rock and proppant characteristics. This study investigates the relationship between rock characteristics and laboratory measurements of propped and unpropped fracture conductivity of outcrop sample from the Eagle Ford shale and the Fayetteville shale. Triaxial compression tests were performed on core specimens in order to determine the Young's Modulus and Poisson's Ratio of the outcrop samples. Profilometer surface scans were also performed to characterize the fracture topography. The results from this study show that the main factors affecting fracture conductivity are closure stress and proppant characteristics (concentration, size, and strength). For unpropped fractures, the fracture topography is the main factor in determining fracture conductivity. The topography interaction of the two surfaces determines the fracture width. Higher Young's Modulus helps maintain fracture width by resisting deformation as closure stress increases compared with lower Young's Modulus. For propped fractures, the more influential factor in determining fracture conductivity is proppant characteristics (concentration, size, and strength). At a proppant monolayer placement, the major mechanism for conductivity loss is proppant embedment, leading to decreased fracture width. A higher Young's Modulus reduces the proppant embedment and better maintains fracture conductivity as closure stress increases. For a multilayer proppant pack concentration, the effect of rock characteristics is negligible compared to the effect of proppant pack characteristics.

The Research on Microstructure of Catalyst and Growth Mechanism of Boron-Doped Diamond

The microstructure of catalyst discs and fracture topography of boron-doped diamond blocks are analyzed in this paper. The results show that boron can promote or inhibit the growth of diamond crystal. The battens of metal carbides in catalyst are the main carbon source in the nucleation and growth of diamond. Crystal nucleus can absorb the carbon atoms from metal carbide in catalyst disc, and continually grow. It is resulted that the carbon atoms around the diamond grains in catalyst disc reduce, and the battens of metal carbides are thin. The more diamond grains are produced, the thinner battens of metal carbides in catalyst disc are formed.