Papers by Keyword: Grain Size

Abstract: Abrasive properties of selected rocks and their influence on button bit wear were investigated. Rock samples were obtained from three locations in Southwest, Nigeria. These samples were tested in the laboratory for grain size and circularity factor. Also, porosity, Equivalent Quartz content (EQC), Rock Hardness Number (RHN), Rock Abrasivity Index (RAI) and Uniaxial Compressive Strength were determined. Bit button lengths consumed were measured at regular interval as drilling operation progresses. Results of grain size revealed that medium feldspar granite has least average grain size varying from 0.12 – 0.14 mm. There is possibility that rate of drilling may be low while drilling through this rock. The values of circularity factor varied from 0.624 for medium feldspar granite to 0.786 for coarse muscovite granite. The mean values of porosity varied from between 0.85 and 1.33% for medium feldspar granite and coarse muscovite granite respectively. Biotite hornblende granite has the highest mean uniaxial compressive strength value of 119.48 MPa while coarse muscovite granite has the least mean value of 90.65 MPa. The values of free silica, EQC, RHN, and RAI varied from 35.93 – 51.35%, 45.60 – 67.21%, 575.52 – 821.64 kg/mm³ and 4133.64 – 7003.71 respectively. It was observed that grain size affects the void size in these rocks, which accounted for least porosity value obtained in medium feldspar granite. Average bit button length consumed varied from 15.56 – 24.54 mm for gauge button while length of centre button varied from 13.00 – 19.61 mm. Higher gauge button length consumption could be inferred that it effects the cutting, crushing and wedging of rock mineral components at rock-bit interface. The understanding of grain size, circularity factor and abrasive properties will provide adequate information for quarry to select appropriate bit and drilling machine.

Abstract: The effect of grain size on the initiation behavior of exfoliation corrosion in cold-rolled Mg-14mass%Li-3mass%Al alloy was examined. Exfoliation corrosion initiated after 30 minutes in the coarse-grained structure (279μm), whereas it was delayed to 60 minutes in the fine-grained structure (75μm) and further to 75 minutes in the ultrafine-grained structure (39μm). This delay is attributed to the suppression of shear band formation and localization during cold rolling with decreasing grain size, which enhances the uniformity of surface reactions in the corrosive environment and promotes faster and denser formation of the protective film at the early stage. Accordingly, the improved condition of the initial corrosion film is considered the primary factor responsible for the delayed onset of exfoliation corrosion.

Abstract: Magnesium alloys are promising for bioabsorbable stents due to their biocompatibility and degradability. Unlike conventional stainless steel stents that remain in the body and may cause complications, magnesium stents gradually degrade, reducing risks like restenosis and thrombosis. However, magnesium has low corrosion resistance, and its corrosion resistance needs to be improved. The crystal structure is one factor affecting the corrosion properties of metallic materials. Several studies have been conducted on the relationship between crystal structure and corrosion properties to improve magnesium's corrosion resistance. It is essential to elucidate the relationship between crystallographic factors and corrosion mechanisms, in the case of stents, plastic deformation during expansion results in the formation of fine crystal grains and twinning deformation. Therefore, the purpose of this study is to investigate the influence of refined grains and twinning on the corrosion properties of magnesium. Hot rolling and compression are used to refine the crystal grains and form twinning in experiments. The crystal structure can be observed by optical microscopy and SEM-EBSD. Following the evaluation of the crystal structure, immersion tests in brine are conducted to measure the mass loss and observe the corrosion behaviour.

Abstract: We investigated the electrical and structural effects of silicon (Si), yttrium (Y) and lanthanum (La) doping in 10-45 nm thick hafnium dioxide (HfO₂) films on silicon carbide (SiC) and Si substrates. We show that the introduction of Si dopants leads to a significant enhancement of the electric breakdown field and a reduction of the leakage current density by elevating the crystallization temperature. This effect becomes stronger with higher Si content. In contrast, Y and La doping does not raise T_C but increases the tetragonal and orthorhombic phase portion within the crystalline films and therefore enhances the dielectric constant k. Furthermore, we show that larger grains in crystalline films are associated with a higher leakage current density.

Abstract: Lightweight metallic alloys in the transport sector are the essential choice to reduce carbon monoxide emissions. Magnesium (Mg) can serve this purpose appreciably because it has a low density compared to other metallic metals and a high strength in a small portion of metals. The reason behind this is having very low weight. Notwithstanding the alloys exhibit high susceptibility to corrosion especially galvanic corrosion, which impedes it from its various applications. The corrosion resistance of magnesium alloy depends largely on the surface film whether it can protect well and the corrosion due to galvanic effect between the second phase particles or microstructures and the magnesium matrix. Role of second phase particles eventually improves the corrosion property by enhancing its resistance to corrosion. Mg-4Zn being a promising alloy, 3 wt% Gd has been added further to investigate the corrosion resistant properties of Mg-4Zn-3Gd alloy. After preparing the alloys by casting method in induction furnace followed by homogenization at 410°C, the sample was hot rolled at 400°C. Preparation of the samples has been verified by EDS, XRF and XRD analysis. Corrosion study has been done for 1 hour, 24 hours and 72 hours. Microstructures have been taken for as cast, homogenized, and as rolled condition before corrosion test. The analysis shows a large difference in the grain size and phase distribution. Due to dynamic recrystallization during rolling hardness also shows differences compared to as cast and homogenized sample. The corrosion test is performed by weight loss test, electrochemical measurement, and immersion test. In the results, it has been seen an increase in corrosion rate at the initial stage, however it came to a constant rate after some time. After corrosion test, optical micrographs (OM) and scanning electron microstructures (SEM) images show typical morphology of corroded surface with some micro cracks. The presence of Gd in Mg-4Zn alloy enhanced the corrosion performance when it is done for longer time.

Abstract: Titanium and its alloys have a wide range of applications in various industries, including medicine. However, the low strength and high friction coefficient hinder their development in contact friction due to fretting fatigue. Among many factors, structure refinement, temperature and amplitude are the most responsible for fretting wear of structural materials. The purpose of the article is to investigate the effect of displacement amplitude, size of grain and test temperature on the fretting wear of the pure titanium in coarse-grained and ultrafine-grained states. It is shown that an increase in the test temperature for both structural states leads to a multiple increase in wear. Structural refinement of titanium to hundreds of nanometers helps to reduce wear at room and elevated temperatures.

Abstract: The main objective of this work is to optimize welding parameters of AISI 430 FSS welds, focused to minimization of ferrite grains size using Taguchi’s design. Two input parameters of speed and welding current; were chosen to select the minimum grain size and to ascertain their effects on ferrite grain size. ANOVA method was used to evaluate the influence of varying factors on the overall quality of welds. Optimal combination of the parameters were be predicted by S/N analyses, it was accessed on employing an 80 A with 6mm/s. Experimental characterizations of optimum weld joint were performed by using tensile test assisted by image correlations, optical and electronic microscopy. As a result, welding speed had the main influence on grain size by 84.30%. Optimum welding parameter offered finest microstructure with low rate of martensite precipitates in both fusion zone and heat affected zone, and best combination of strength and ductility, it presented a homogeneous distribution of tensile stresses that caused a ductile fracture in base material. ,it is found that that optimized welding parameters permit to give greater resistance to corrosion, which exhibit a lower corrosion current, indicating that coarse ferrite grains are more susceptible to corrosion compared to fine grains.

Abstract: Due to the operating conditions of weld and calibrating rolls used in the production processes of large electric-welded pipes, their material is subject to stringent wear and abrasion resistance requirements at high temperatures. The limited capabilities of conventional Cr-Mn-Ni tool steels and open die forging technologies with heat treatment processes do not provide the required performance properties for large welded rolls. Therefore, the material of the product was replaced with Cr12MoV high-chromium steel. This required identifying the formation patterns of the fine grain structure of high-chromium steel in order to adjust the production technology with adaptation to the unique conditions and equipment (12.5 MN hydraulic forging press, heating and thermal furnaces) of the forging shop. The technology was offered, which included the development of modes from heating to heat treatment with intermediate two-stage forging from Cr12MoV steel ingots in two sets of combined dies. At the first stage, deformation with a low reduction ratio ε = 5% and a relative feed rate of 0.4 per pass was provided to break and refine the carbide mesh, and at the second stage, intense deformation with a reduction ratio ε = 15% was performed. Further practical application has shown that the durability of weld rolls made from the new material increases by 20–30%.

Abstract: The combination of advection and migration of grain boundaries is analyzed on the basis of a simple mesoscale model, where parallelepipedic grains are considered under uniaxial compression straining. Strain hardening and dynamic recovery are described by the classical Yoshie-Laasraoui-Jonas equation. Grain-boundary migration is driven by the difference in dislocation densities between one representative grain and the average over the material. Finally, nucleation is assumed to occur at grain boundaries. Special attention is paid to the aspect ratio, which starts from unity (infinitely small cubic nucleus) and tends to zero when the grain disappears. In spite of the role of migration, the average shape of the grains is determined as a first approximation by their lifetimes.

Abstract: In present work, a high Zn-containing Al-Zn-Mg-Cu alloy with different grain sizes was fabricated by extrusion and related precipitation characteristics and mechanical property were investigated after uniform heat treatments. The results showed that precipitation characteristics for the three alloys were almost the same. Matrix precipitates were GPII zone and η' phase and possessed small size and dense distribution while grain boundary precipitates exhibited discontinuous distribution. The rank of strength and fracture toughness for the three alloys are SG>MG>LG. Tearing ridges had been found on all the fracture surface while only LG alloy possess obvious dimple characteristics. The a-N curve showed that crack length list is MG>LG >SG under a same cycle number. The da/dN-ΔK curve also proved that fatigue crack propagation (FCP) rate of MG alloy is slightly larger than that of LG alloy, both were apparently larger than that of SG alloy. The width of fatigue striations on FCP fracture surface also backed it. Besides, obvious transgranular cracking characteristics and apparent secondary cracks were found on the FCP fracture surface.