Use of Thermally Modified Jarosite for the Removal of Hexavalent Chromium by Adsorption

Jarosites are residues generated during the purification of zinc and are composed mainly of iron sulfates ((Na, K)Fe3(SO4)2(OH)6). Due to the large volume of jarosite generated during the process, these residues tend to be deposited in large land areas and are not used. In the present work, jarosite was used without heat treatment (JST) as an adsorbent of hexavalent chromium contained in a sample of wastewater from a chrome plating industry under the following conditions: C0 = 200 mg/L of Cr, T = 25 °C, and pH = 3. It was only possible to remove 34% of Cr (VI). Subsequently, a thermal treatment of a jarosite sample (JTT) was carried out at 600 °C. The heat-treated sample was later used as an adsorbent in the same conditions as those for JST. The maximum chromium removal was 53%, and the adsorption capacity was 10.99 mg/g. The experimental data were fitted to the Langmuir model and to the pseudo-second-order kinetic model. It was determined that the adsorption process involved electrostatic attractions between the surface of the positively charged adsorbent and the chromium anions contained in industrial wastewater.

Functional properties of NiTi nanofilm/Kapton composite

The aim of the paper was to study the one-way and two-way shape memory effects in the NiTi nanofilm/Kapton composite. 500 nm film of the Ni50Ti50 alloy was deposited to Kapton by flash evaporation. After deposition, the NiTi layer was amorphous and the sample was held at a temperature of 350 - 400 °C for two hours in vacuum to crystallize the NiTi layer. As deposited sample as well as samples after heat treatment were bent around the mandrel with various diameters at room temperature and subjected to heating – cooling – heating through a temperature range of the martensitic transformations. It was shown that as-deposited sample did not demonstrate the recoverable stain variation. At the same time, the heat treated sample demonstrated the one-way shape memory effect on heating and a maximum recoverable strain was found to be 2 %. The two-way shape memory effect was not observed on further cooling and heating.

Microstructural Evolution, Hardness, and Strengthening Mechanisms in SLM AlSi10Mg Alloy Subjected to Equal-Channel Angular Pressing (ECAP)

The AlSi10Mg alloy is characterized by a high strength-to-weight ratio, good formability, and satisfying corrosion resistance; thus, it is very often used in automotive and aerospace applications. However, the main limitation of using this alloy is its low yield strength and ductility. The equal-channel angular pressing is a processing tool that allows one to obtain ultrafine-grained or nanomaterials, with exceptional mechanical and physical properties. The purpose of the paper was to analyze the influence of the ECAP process on the structure and hardness of the AlSi10Mg alloy, obtained by the selective laser melting process. Four types of samples were examined: as-fabricated, heat-treated, and subjected to one and two ECAP passes. The microstructure analysis was performed using light and electron microscope systems (scanning electron microscope and transmission electron microscope). To evaluate the effect of ECAP on the mechanical properties, hardness measurements were performed. We found that the samples that underwent the ECAP process were characterized by a higher hardness than the heat-treated sample. It was also found that the ECAP processing promoted the formation of structures with semicircular patterns and multiple melt pool boundaries with a mean grain size of 0.24 μm.

Influence of post-heat treatment on microstructure, mechanical, and wear properties of maraging steel fabricated using direct metal laser sintering technique

The post-heat treatment of direct metal laser sintered parts is expected to have superior mechanical properties. Therefore, the purpose of the present study is to investigate the post-heat treatment effect on the microstructure, mechanical and wear properties of direct metal laser sintering processed maraging steel. Hence, a systematic methodology for microstructural characterization, mechanical properties, and tribological performance evaluation was performed. The microstructural examinations were performed using optical and scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction technique. The micro-hardness and tensile properties were determined. The unidirectional sliding wear test was performed using a pin on disc wear testing machine for three different sliding velocities (0.8, 1.2, and 1.6 m/s) and three different normal loads (5, 10, and 15 N). The present study’s findings establish that the post-heat treatment techniques significantly altered the microstructural morphology and features. The results showed that the heat-treated sample had finer and non-continuous microstructure and more complex intermetallic precipitate phases, leading to higher hardness (∼64%) and higher tensile strength properties (70–80%) compared to the as-printed sample. The unidirectional sliding wear test results showed that the sliding velocity significantly affected frictional and wear characteristics of direct metal laser sintering processed maraging steel. The wear resistance of the heat-treated sample was three times higher than the as-printed sample, particularly at higher sliding velocities. In addition, the lower coefficient of friction values (∼24%) was observed for heat-treated sample compared to as-printed sample at higher sliding velocities. The post-heat treatment aids as an effective method to enhance mechanical properties of direct metal laser sintered parts and qualify them for tribological applications. The results endorse the suitability of the heat-treated direct metal laser sintered maraging steel in practical tool and die applications involving extreme tribological operating conditions such as higher sliding velocities and contact stresses.

Effect of Electric Current Heat Treatment on Commercially Pure Titanium Sheets

Rapid electric current heat treatment has been successfully applied to a cold-rolled sheet of commercially pure titanium (CP Ti). The electric current heat treatment was conducted at various temperatures (400, 500, 600 and 700 ∘C) by altering the current density (A/mm2). The detailed microstructure and texture evolution was studied using electron backscatter and X-ray diffraction analysis. For comparison, conventional heat treatment at 400, 500 and 600 ∘C were also applied to the cold-rolled sheets. The electrically heat-treated sample showed a much smaller and uniform grain size with a relatively weak texture than the conventionally heat-treated one. As a result, the electrically heat-treated samples exhibited better tensile properties than conventionally heat-treated samples. Furthermore, the electric current treatment produced minimum sheet distortion and good oxidation resistance compared with the conventional heat treatment.

Effect of Heat Treatment on the Properties of (Fe0.6Co0.4)71Si5Nb4B20 Bulk Metallic Glass

Bulk metallic glasses (BMGs) are an important class of materials with unique set of properties. A bulk metallic glass with composition of (Fe0.6Co0.4)71Nb4Si5B20 was cast in the form of a 1 mm thick strip in a water cooled copper mold. The BMG produced was characterized for structure, thermal and mechanical properties. The X-ray diffraction performed on the as cast alloy has shown completely amorphous structure. The glass transition and crystallization peak temperatures obtained through differential scanning calorimetry scan were 542 °C and 588.4 °C, respectively. Some cast amorphous alloy sample was annealed below glass transition (450 °C for 30 mi93nutes) and others above glass transition (580 °C for 5 minutes) temperatures. Nano- indentation hardness of 13.3 GPa was obtained for as cast alloy while a hardness values of 12.8 and 15.84 GPa were measured for heat treated alloys at temperature of 450 °C and 580 °C, respectively. Increase in hardness was attributed to formation of crystals in an amorphous matrix whereas decrease in hardness was due to relaxation of quenching residual stresses. The maximum value of elastic modulus obtained through indentation was 255 GPa for 580 °C heat treated sample.

Multistage heat treatment and the development of a protective oxide-scale layer on the surface of FeCrAl sintered-metal-fibers

This study investigates the effects of Multistage Heat Treatment (MSHT) on the development of an oxide-scale layer on the surface of FeCrAl sintered-metal-fibers. The oxide-scale layer was developed using an MSHT cycle at 930 °C for 1 h, followed by 960 °C for 1 h, and finally at 990 °C for 2 h. In this study, three samples were considered: Sample 1 was kept without thermal oxidation, while Samples 2 and 3 were exposed to one and eighteen MSHT cycles. Thermo-gravimetric analyses show that the weight gain of the heat-treated sample slows with time, confirming the growth of the protective oxide-scale layer. Scanning electron microscope images, after one MSHT cycle, reveal nonuniform oxide-scale growth with platelet-like on the surface. After eighteen MSHT cycles, however, clumped particles formed on the surface of the fibers. Atomic force microscopy was utilized to study the surface topography of the fibers. The results show that MSHT increases the surface roughness, where the surface roughness of one and eighteen MSHT cycles are the same. The x-ray diffraction analyses of the baseline sample and the sample with one MSHT cycle show pattern peaks of crystalline Fe2CrAl. In contrast, the results of eighteen MSHT cycles displayed diffraction pattern peaks of crystalline Cr and stable α-Al2O3. In summary, the results of this study reveal the changing nature of the oxide-scale layer. The findings of this study form the foundation for new techniques to protect and prepare the FeCrAl fibers as a support for catalysts.

Transparent ferroelectric glass–ceramics for wastewater treatment by piezocatalysis

In piezocatalysis the polarization field found in piezoelectric materials enables and enhances catalytic redox reactions. Here, we explore piezocatalytic dye degradation through transparent glass–ceramics containing piezoelectric crystals. 30SiO2–35Li2O–35Nb2O5 (in mol%) glass–ceramics containing varying amounts of LiNbO3 crystallites were fabricated by melt-quenching, followed by heat-treatment at a crystallization temperature of 650 °C for 2, 3 and 6 hours. During piezocatalysis, the 2 hour heat-treated sample showed up to 90% degradation of methylene blue dye within 150 min of ultrasonication, with no significant change in performance after three piezocatalysis cycles. This sample showed promising activity for degrading cationic and neutral dyes, and is optically transparent. This work demonstrates that transparent ferroelectric glass–ceramics are promising for water-cleaning applications by piezocatalysis.

Effect of Uniaxial Tension-Induced Plastic Strain on the Microstructure and Corrosion Behavior of 13Cr Martensitic Stainless Steel

This study aims to understand the correlation between the manufacturing process-induced plastic deformation, microstructure, and corrosion behavior of a 13Cr martensitic stainless steel tubing material (UNS S42000). Comparisons were made between the microstructure, crystallographic orientation, and corrosion performance of a texture-free, heat-treated sample and uniaxially tensioned samples to the elongations of 5% and 22%. Cyclic potentiodynamic polarization tests and electrochemical impedance spectroscopy were performed on all samples in aerated 3.5 wt% NaCl electrolyte at room temperature. Overall, the corrosion resistance of the samples was found to decrease with increasing deformation level. A more stable and higher corrosion potential and pitting potential values with a better stability of the passive film were derived for the nondeformed sample, whereas the 5% and 22% elongated samples exhibited lower corrosion and pitting potential values and were characterized by having a less stable passive layer. All samples consistently revealed micropit formation on the lath boundaries where a high concentration of chromium carbide precipitates was detected. Increasing the level of plastic strain in 13Cr stainless steel was found to enlarge the size of sensitized regions along the matrix/coarse chromium carbide precipitates interface, leading to more regions susceptible to initiation and propagation of pitting.

Effects of Intermetallic Microstructure on Degradation of Mg-5Nd Alloy

The influence of intermetallic microstructure on the degradation of Mg-5Nd alloy with different heat treatments was investigated via immersion testing in DMEM + 10 pct FBS under cell culture conditions and subsequent microstructural characterizations. It was found that T4 heat-treated sample had the poorest corrosion resistance due to the lack of finely dispersed precipitates inside grains, continuous lamellar particles along grain boundaries and outer Ca-P layer, and to the formation of a loose corrosion product layer. In contrast, the aged samples exhibited a better corrosion resistance due to their presence and to the formation of a compact corrosion layer. Their degradation behavior largely depended on the intermetallic microstructure. Corrosion was initiated in the matrix around stable globular particles Mg41Nd5 at grain boundaries. In the sample aged at high temperature 245 °C, the coexistence of lamellar Mg41Nd5 particles and their nearby Nd-poor regions enhanced the corrosion. The corrosion first started in such regions. It was shown that those finely dispersed precipitates formed during aging had no influence on the corrosion initiation. However, they indeed affected the subsequent corrosion propagation with the immersion proceeding. They supplied barriers for corrosion propagation and hence were beneficial for improving the corrosion resistance. The continuously distributed lamellar Mg41Nd5 precipitates formed at grain boundaries during aging at 245 °C supplied an additional effective obstacle to corrosion propagation. This was especially beneficial for hindering the corrosion propagation at the later stage of corrosion.