CaO Alloy Research Articles

Effect of MWCNT (Multi-Walled Carbon Nano-Tube) Doping on Hydrogenation Kinetics of Mg-CaO Alloys.

Magnesium hydride has a high hydrogen storage capacity (7.6 wt.%), and is cheap and lightweight, thus advantageous as a hydrogen storage alloy. However, Mg-based hydrides undergo hydrogenation/ dehydrogenation at high temperature and pressure due to their thermodynamic stability and high oxidation reactivity. Various attempts have been made to lower the reaction rate and dehydrogenation temperature by adding transition elements (e.g., Ti, Fe, Co, Ni, Ce), metal oxides, and intermetallic compounds to overcome these shortcomings. On the other hand, carbon materials have been mainly studied in the field of hydrogen storage with high specific surface area and lightweight properties; however, results show that they cannot store a large amount of hydrogen. Recently, it has been theoretically reported that carbon materials act as adsorbents in hydrogen storage. This study focuses on the hydrogenation behavior of MgHx-CaO and MgHx-CaO-MWCNT composites prepared by hydrogen-induced mechanical alloying, and investigates the properties of these composite materials.

Effect of Yttria Mould Coating on the Investment Casting of AZ91D-1 wt% CaO Magnesium Alloy

The main objective of the present research is to develop a technique to inhibit the mould–metal interaction in the investment casting of magnesium (Mg) alloys. The use of yttria layers (Y2O3) as an interface mould coating to manufacture thin-walled samples of AZ91D-1 wt% CaO alloy was investigated and the reaction mechanisms were exploited. Additive manufacturing models with thin walls (2 mm thick) were designed to study the casting behaviour of the melt in ceramic moulds (plaster), for both coated and uncoated models. Additionally, the optimal processing parameters were taken into account using a vacuum induction furnace under a flowing argon atmosphere to prevent the ignition of the molten Mg. Regarding uncoated models tests, a considerable content of reaction products was revealed, resulting in large defects all over the castings. In these samples, new phases have been formed around the surface and the matrix due to the mould–metal reactions. On the other hand, the interaction between the liquid metal and the plaster was prevented in those ceramic moulds where yttria was used as an interface protective refractory, which led to a cleaner surface and a more uniform microstructure, improving the effectiveness of this process in the manufacture of Mg structures.

Microstructures, Mechanical and Corrosion Properties of the Extruded AZ31-xCaO Alloys

The effects of the extrusion process and CaO addition amount on microstructure, mechanical, and corrosion properties of AZ31 alloys were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), standard tensile testing, and so on. The grain size of AZ31 or AZ31-1%CaO alloy becomes larger with increasing extrusion temperature. The grain size of AZ31-1%CaO alloy is much smaller than that of AZ31 alloy at the same extrusion temperature. In addition, the formation of the Al2Ca phase caused by CaO addition refines the grain size, and the recrystallization of AZ31-1%CaO alloy is improved significantly. The recrystallization grains distribute more uniformly as the increase of extrusion ratio, and the completely recrystallized grains distribute uniformly in the form of equiaxed crystals with an extrusion ratio of 9. Tensile testing results show that extruded AZ31-1%CaO alloy at the extrusion temperature of 300 °C and an extrusion ratio of 9 exhibits the best mechanical properties. While corrosion properties of AZ31 alloys decreases due to the addition of CaO.

Corrosion behaviour of as-cast ZK40 with CaO and Y additions

Abstract The microstructures of as-cast ZK40, ZK40 with 2% (mass fraction) CaO and ZK40 with 1% (mass fraction) Y were investigated, and the intermetallic phase morphology and the distribution were characterised. By having discrete intermetallic particles at the grain boundaries for the ZK40, the microstructure was modified to a semi-continuous network of intermetallic compounds along the grain boundaries for the ZK40 with CaO or Y additions. The CaO was not found in the microstructure. However, Ca was present in Ca2Mg6Zn3 intermetallic compounds which were formed during casting. Hydrogen evolution and electrochemical impedance spectroscopy tests revealed that the addition of CaO slightly enhanced the corrosion resistance whereas Y had a negative effect on the corrosion resistance of ZK40. Immersion tests showed that severe localised corrosion as well as corrosion along the intermetallic compounds played an important role in the corrosion process of ZK40–Y whereas the localised corrosion was not pronounced for ZK40 or ZK40–CaO alloys. Micro-segregation in the α-Mg matrix was notably higher for the ZK40 alloy compared with the modified alloys. The combination of this effect with a possible formation of a more stable corrosion layer for the ZK40–CaO was attributed as the main reason for an improved corrosion resistance for the ZK40–CaO alloy.

CaO 첨가에 의한 AZ31 합금 미세조직의 열적 안정성 변화

Grain growth behaviors of hot-rolled AZ31 (Mg-3%Al-1%Zn) and AZ31-0.3%CaO alloys at elevated temperatures have been investigated in order to clarify the effect of CaO addition on grain stability of Mg-Al-based wrought alloy. The grain size of CaO-free alloy increased steeply from 673 K with an increase in annealing temperature from 573 to 773 K, whereas the grains of CaO-containing alloy were relatively stable up to 723 K. The activation energies for grain growth (<TEX>$E_g$</TEX>) were 12.2 and 18.3 kJ/mole between 573 and 673 K and 119.2 and 126.9 kJ/mole between 673 and 773 K in the AZ31 and AZ31-0.3%CaO alloys, respectively. This result indicates that grains in the CaO-added alloy possess higher thermal stability than CaO-free alloy. SEM observations on the annealed alloy samples revealed that higher grain stability resulting from CaO addition would be associated with the suppression of grain growth by Ca-related precipitate particles distributed in the microstructure.

AZ31 마그네슘 합금의 고온산화에 미치는 CaO 첨가 영향

Magnesium alloys of AZ31 containing (0.5, 1, 1.5) wt.% of initially added CaO particles were cast in air, and their oxidation behavior was studied at <TEX>$450-650^{\circ}C$</TEX> in air. The initially added CaO particles either decomposed to dissolve in the <TEX>${\alpha}$</TEX>-Mg matrix or precipitated as <TEX>$Al_2Ca$</TEX> along the grain boundaries of the matrix during casting. The ignition temperatures were <TEX>$565.4^{\circ}C$</TEX> for AZ31, <TEX>$608.6^{\circ}C$</TEX> for AZ31+0.5 wt.%CaO, and <TEX>$689.7^{\circ}C$</TEX> for AZ31+1 wt.%CaO. No ignition occurred for AZ31+1.5 wt.%CaO up to <TEX>$700^{\circ}C$</TEX>, displaying good oxidation resistance. The CaO-rich oxide scales that formed on the surface of the AZ31+(0.5, 1, 1.5) wt.%CaO alloys improved the oxidation resistance of AZ31 alloys.

Calculation of structural characteristics of Cd1–xCaxO (x = 0–1)

The structural characteristics of isovalent and isostructural CdO–CaO alloy system are studied from the viewpoint of crystallography. The relative atomic positions in the Cd1–xCaxO alloy lattice can be determined by the Cd(Ca)–O bond lengths, which slightly decrease with increasing Ca concentration. Equivalent cation sites and Cd(Ca)–O bond lengths demonstrate that Cd1–xCaxO (x = 0−1) alloys crystallise in cubic Fm–3m space group (rocksalt structure). Each cation site in the Cd1–xCaxO alloy lattice has the identical probability of occupying such site by Cd and Ca, leading to the equivalent cation sites in alloy structure. Our calculated lattice parameters, volume of unit cell and 2θ angles of (200) and (111) peaks in the X-ray diffraction pattern of Cd1–xCaxO (x = 0−1) alloys agree with experimental measurements, validating our current crystallographic viewpoint.

Microstructure and Mechanical Properties at Room and Elevated Temperatures in AM50-0.3 wt%CaO Alloy

The present study is intended to comparatively investigate the changes in microstructure and tensile properties at room and elevated temperatures in commercial AM50(Mg-5%Al-0.3%Mn) and 0.3 wt%CaO added ECO-AM50 alloys produced by permanent mould casting. The typical microstructure of AM50 alloy was distinctively characterized using two intermetallic compounds, <TEX>${\beta}(Mg_{17}Al_{12})$</TEX> and <TEX>$Al_8Mn_5$</TEX>, along with <TEX>${\alpha}$</TEX>-(Mg) matrix in an as-cast state. The addition of a small amount of CaO played a role in reducing dendrite cell size and quantity of the <TEX>${\beta}$</TEX> phase in the AM50 alloy. It is interesting to note that the added CaO introduced a small amount of <TEX>$Al_2Ca$</TEX> adjacent to the <TEX>${\beta}$</TEX> compounds, and that inhomogeneous enrichment of elemental Ca was observed within the <TEX>${\beta}$</TEX> phase. The ECO-AM50 alloy showed higher hardness and better YS and UTS at room temperature than did the AM50 alloy, which characteristics can be mainly ascribed to the finer-grained microstructure that originated from the CaO addition. At <TEX>$175^{\circ}C$</TEX>, higher levels of YS and UTS and higher elongation were obtained for the ECO-AM50 alloy, demonstrating that even 0.3 wt%CaO addition can be beneficial in promoting the heat resistance of the AM50 alloy. The combinational contributions of enhanced thermal stability of the Ca-containing <TEX>${\beta}$</TEX> phase and the introduction of a stable <TEX>$Al_2Ca$</TEX> phase with high melting point are thought to be responsible for the improvement of the high temperature tensile properties in the ECO-AM50 alloy.