Effect of Grain Size on Carburization Characteristics of the High-Entropy Equiatomic CoCrFeMnNi Alloy

In this study, the carburization characteristics of cast and cold-rolled CoCrFeMnNi high-entropy alloys (HEAs) with various grain sizes were investigated. All specimens were prepared by vacuum carburization at 940 °C for 8 h. The carburized/diffused layer was mainly composed of face-centered cubic structures and Cr7C3 carbide precipitates. The carburized/diffused layer of the cold-rolled specimen with a fine grain size (~1 μm) was thicker (~400 μm) than that of the carburized cast specimen (~200 μm) with a coarse grain size (~1.1 mm). In all specimens, the carbides were formed primarily through grain boundaries, and their distribution varied with the grain sizes of the specimens. However, the carbide precipitates of the cast specimen were formed primarily at the grain boundaries and were unequally distributed in the specific grains. Owing to the non-uniform formation of carbides in the carburized cast specimen, the areas in the diffused layer exhibited various carbide densities and hardness distributions. Therefore, to improve the carburization efficiency of equiatomic CoCrFeMnNi HEAs, it is necessary to refine the grain sizes.

Effect of heat treatment and electric discharge alloying on the tribological performance of Selective Laser Melted AlSi10Mg

Selective Laser Melting (SLM) is an emerging additive manufacturing technology for fabrication of complex light-weight components along with improved mechanical properties. The present work investigates influence of various post processing methods such as Heat Treatment and Electric Discharge Alloying (EDA) on ambient and elevated temperature wear behavior of Selective Laser Melted (SLM) AlSi10Mg alloy and compared with its tribological behavior with cast AlSi10Mg. The dry wear tests were conducted using a Pin On Disk (POD) tribometer with EN-31 as counter body. The EDA treated SLM AlSi10Mg showed the least wear rate and coefficient of friction (COF) at both ambient and elevated temperatures (1.05 × 10−4 mm31/Nm and 0.434 & 3.12 × 10−5 mm3/Nm and 0.531 respectively) due to its higher hardness (189.8 HV) as compared to other samples. The wear rate and COF of cast specimen are found to be highest among all specimens at both ambient and elevated temperatures (1.34 × 10−4 mm3/Nm and 0.528 and 4.49 × 10−5 mm3/Nm and 0.724 respectively). Lower wear rate and higher COF are observed at elevated temperature due to the excessive formation of wear-resistant oxides (Al2O3, SiO2 and MgO) and glaze layers for all samples compared to ambient temperature wear behaviors of its counter parts. Abrasive wear, adhesive wear, oxidation wear and surface delamination are the prominent wear mechanisms observed for ambient and elevated temperatures for all the specimens.

Negative Strain Rate Sensitivity Induced by Structure Heterogeneity in Zr64.13Cu15.75Ni10.12Al10 Bulk Metallic Glass

The negative strain rate sensitivity (SRS) of metallic glasses is frequently observed. However, the physical essence involved is still not well understood. In the present work, small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM) reveal the strong structure heterogeneity at nanometer and tens of nanometer scales, respectively, in bulk metallic glass (BMG) Zr64.13Cu15.75Ni10.12Al10 subjected to fully confined compression processing. A transition of SRS of stress, from 0.012 in the as-cast specimen to −0.005 in compression processed specimen, was observed through nanoindentation. A qualitative formulation clarifies the critical role of internal stress induced by structural heterogeneity in this transition. It reveals the physical origin of this negative SRS frequently reported in structurally heterogeneous BMG alloys and its composites.

Improving Corrosion and Corrosion-Fatigue Resistance of AZ31B Cast Mg Alloy Using Combined Cold Spray and Top Coatings

In this paper, we report the application of zinc phosphate electrostatic-painting top coating on cold sprayed AA7075 leading to a significant improvement in corrosion-fatigue performance. High strength AA7075 powder was sprayed on AZ31B substrate, followed by the application of the top coating. The electrochemical corrosion and corrosion-fatigue tests of the coated and uncoated specimens were performed in 3.5% NaCl solution. Transmission electron microscopy (TEM) analysis showed that a continuous nanolayered mixture of Mg/Al was formed at the cold spray coating/substrate interface leading to high bonding strength. The results showed that the combined coatings improved the corrosion resistance remarkably, and significantly increased the fatigue life, with a fatigue strength of 80 MPa at 107 cycles, as compared to the as-cast specimen. Surface topographic analysis of the corrosion-fatigue-tested specimens demonstrated the presence of deep macro-pits on the cold sprayed AA7075 coating after 3.7 million cycles, while there were no such pits on the top-coated specimens, even after 107 cycles when tested at 30 Hz. The fractographic analysis of the fatigue-fractured specimens showed that the formation of pits allowed the NaCl solution to penetrate in the AZ31B substrate, creating localized corrosion pits resulting in premature failure, which eventually reduced the fatigue life.

Regional Differences of Dental Microwear on the Occlusal Surface of an M2 from Neolithic Japan: A Case Study

Regional differences of dental microwear among four small areas on the heavily worn occlusal surface of a mandibular M2 of an adult male from Neolithic Japan were investigated using a scanning electron microscope (SEM). The M2 specimen was cast using a high-resolution epoxy resin under low pressure for SEM, and the cast specimen was sputter-coated with gold. Among the four regions of the M2, two (facets 3 and 9) showed higher proportions of pits (78.6% and 75.0%, respectively), and the two others (lingual marginal facet 7n, and the inner side of facet 7n) showed lower proportions of pits (5.6% and 33.3%, respectively). The two pitted regions seem to reflect the processing of hard foods, and the two other regions with higher frequencies of striations might reflect exposures to less gritty, softer foods. The variation of these pits and striations suggests that the Jomon subsisted on stone-processed hard foods, with coarse grain sizes of sand in foods that included tuberous roots, animal meats with bones, and clams. The analyses of regional differences of dental microwear will develop important ways to study tooth use and past diets.

Wear and Corrosion of Wrought A6061 Aluminium Alloy in DOT3 Brake Fluid

The twofold impact of wear and corrosion on wrought A6061 alloy in hydraulic DOT3 brake fluid environment was studied. The wear studies were performed on the samples using a developed wear-jig. Weight loss corrosion test method was used to determine the corrosion rate of the wrought A6061 alloy samples immersed in the brake fluid for a total of 1680 hours. From the results of wear tests carried out on the A6061 alloy sample with brake oil, the highest wear value of 5.24x10-7 mg/mm2/cycle (approx.) was obtained from 6 N (approx) force after 130 minutes. The wrought A6061 alloy material demonstrated the highest corrosion rates nearly 3.0 x10-2 mg/mm2/yr within the early 168 hours of immersion in brake fluid. The result is practically lower than the corrosion rate of cast specimen in DOT3 brake oil or some other alloys immersed in other corrosive media that were previously reported in the literature. The results show that small amount of chemical corrosion is sufficient to cause and accelerate mechanical wear of the material in usage.

Tribological behavior of heat treated CuCrZr alloy

Optimal combination of strength, ductility and conductivity in CuCrZr alloy can be achieved by suitable heat treatment involving solution annealing at high temperature to dissolve alloying elements, water quenching to produce a super-saturated solid solution, and an aging treatment at an intermediate temperature to produce fine precipitates giving rise to high strength. In this study, Cu-1Cr-0.1Zr alloy was manufactured as cast material and following hot forging, solution annealing and aging were applied conventionally. In order to enhance the precipitation kinetics, solution annealed and quenched alloy was cold deformed and then aged at the same condition. Specimens obtained after (i) casting, (ii) conventional solution annealing and aging and (iii) aging after cold deformation were investigated in order to determine microstructural features, hardness and tribological properties. The results showed that while heat treatment process increased hardness the of cast specimen, it was enhanced further with deformation before aging. Besides hardness, tribological properties of the cast specimen were improved further by deformation before aging.

Fabrication of High-Strength Gray Cast Iron Using Permanent Magnet Scrap

In this study, we have developed the manufacturing technology for high strength gray cast irons by using the spent permanent magnet scraps. The cast specimen inoculated by using a spent magnet scraps showed the excellent tensile strength up to 306MPa. This tensile strength value is 50MPa higher than that of the specimen cast without inoculation, and is similar to that of the specimen inoculated by using the expensive misch-metal. These superior mechanical properties are attributed to complex sulfides created during solidification that promote the formation and growth of Type-A graphite. It is therefore concluded that spent magnets scrap can provide an efficient and cost-effective inoculation agent for the fabrication of high-performance gray cast iron.

Precision investment casting: A state of art review and future trends

Investment casting process is known to its capability of producing clear net shape, high-dimensional accuracy and intricate design. Consistent research effort has been made by various researchers with an objective to explore the world of investment casting. Literature review revealed the effect of processing parameters on output parameters of cast specimen. This article highlights the advancements made and proposed at each step of investment casting and its hybridization with other process. Besides, investment casting has always been known to manufacture parts such as weapons, jewellery item, idols and statues of god and goddess since 3000 BC; this article reviews the present applications and trends in combination of rapid prototyping technique as integrated investment casting to serve in medical science. Advancements in shell moulding with incorporation of fibre and polymer, development of alternative feedstock filament to fused deposition modelling are duly discussed. The aim of this review article is to present state of art review of investment casting since 3200 BC. This article is organized as follows: in section ‘Introduction’, introduction to investment casting steps is given along with researches undertaken at each step; in section ‘Rapid prototyping technique’, background is given on the concept of rapid prototyping technique by examining the various approaches taken in the literature for defining rapid prototyping technique; section ‘Biomedical applications of RPT’ presents the medicine or biomedical applications of investment casting and rapid prototyping technique; section ‘Future trends’ provides some perspectives on future research and section ‘Conclusion’ closes the article by offering conclusions.