Abstract: Empirical Electron Theory in Solids and Molecules (EET) was used to analyze the valence electron structure of ZrTiHfVNb, ZrTiHfVTa and ZrTiHfNbMo high-entropy alloys. The parameters characterizing the valence electron structure of high-entropy alloys were calculated, which were used to discuss the hardness and melting temperature of high-entropy alloys. The results show that the hardness of high-entropy alloys is positively correlated to the shared electron pair number in valence electron structure. The theoretical melting temperatures of high-entropy alloys were predicted by the parameters characterizing the valence electron structure.
Abstract: Water droplets formed by the deliquescence of pre-deposited NaCl on 316L stainless steel were investigated. Different total weights of NaCl particles between 0.0005 g and 0.01 g were deposited on 316L stainless steel surface, which were exposed to an atmospheric condition at 80oC and relative humidity of 80% until NaCl droplets were stabilized. The volume of NaCl droplet was linearly proportional to the total weights of pre-deposited NaCl and consequently the chloride concentration in droplets, ranging from 3 to 6 M, did not depend on the NaCl weights. After exposed to the NaCl droplets at 80oC and relative humidity of 80% over 5, 10, and 15 days, all 316L stainless steel samples suffered from pitting. The Cl- ions in the surface films of samples were measured by ToF SIMS. The pit densities on samples were well correlated with the Cl- intensities in the surface films. These results suggest that the pit initiation under NaCl droplets by deliquescence is caused by the Cl- ions either incorporated into surface films or penetrated through them.
Abstract: The crystal structure of GdFeMn alloy has been investigated by using X-ray powder diffraction (XRD) followed by Rietveld refinement technique, differential thermal analysis (DTA) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) techniques. XRD results showed that annealed GdFeMn alloy was a single phase of Gd6(Fe0.5Mn0.5)23 compound, with Th6Mn23-structure. SEM/EDS results and Rietveld refinement revealed that GdFeMn alloy was not really a new ternary compound as reported, but a solid solution Gd6(Fe0.5Mn0.5)23 which was only a point between Gd6Fe23 and Gd6Mn23. It was also found from DTA measurement that a reaction, Gd (Fe0.5Mn0.5)2 → liquid (rich Gd) + Gd6(Fe0.5Mn0.5)23, had taken place above 650.81 oC, and the educed Gd existed in educed part of GdFeMn sample. The results demonstrated the real structure of the GdFeMn compound as reported was Gd6(Fe0.5Mn0.5)23 compound.
Abstract: In this investigation, the characteristics of bulky retained austenite in an austempered ductile iron are evaluated in two tempered conditions. which were intially tempered at 200oC for 1h before cooling to room temperature, and then tempered at 350oC for 1h. The result shows that the hardness within retained austenite areas is distributed unevenly with a range from 423 HV to 897 HV, which is attributed to the transformation from austenite to martensite during austempering. Also, the mechanism regarding the quenched marteniste formation is discussed. The poor fatigue resistance of ADI is hypothesized to be due to the amount of austenite transformed to martensite.
Abstract: IN718 alloy possesses excellent mechanical properties at high temperatures, good process ability, therefore, it has been widely used in aero engine turbine disks, compressor disks, and power turbine shafts (i.e., rotating components). The fatigue properties of the alloy are a key factor that determines the safety and reliability of the engine. In this paper, the fatigue properties of IN718 alloy are investigated under low-and middle-frequency conditions at 600 °C and 455 °C, the initiation of fatigue cracks, and the relation between fatigue life and grain size are discussed. The results show that the carbides response as a crack initiation site at low-frequency fatigue condition (1 Hz), and string-type or heap-type carbides distribution promotes crack propagation and shortens fatigue life, the twin boundaries in large grains are act as a crack initiation site at middle-frequency fatigue condition (10 Hz). The grain size is smaller, and the low cycle fatigue properties of the alloy are better.
Abstract: To improve medical pure magnesium corrosion and wear resistance, the advanced plasma implantation technology were used to implanted medical pure magnesium with nitrogen ions under certain conditions, obtaining a certain depth of nitrogen ion implantation layer, and to study the surface properties of the implantation layer. The sample after ion implanted, the surface morphology, phase composition were analyzed, and have electro chemical corrosion tests, friction and wear tests, the results showed that: pure magnesium by nitrogen ion implantation, can be obtained a surface organizations which whole flat, compact, no surface cracks and holes; the surface implantation layer mainly composed of Mg and MgO, also found a small amount of Mg3N2, which is also the main reason for corrosion and wear resistance improved; compared to pure magnesium base, nitrogen ion implantation (process conditions: implantation energy: 40KeV, implantation dose: 3×1017ions/cm2, control temperature: 200°C) improved the corrosion resistance of the sample, but not obvious, about 1.2%; however, the friction coefficient decreased significantly, approximately 61%, the amount of wear also reduced significantly, about 74%, this means that, its wear resistance has been improved significantly. This study provides a reference to improve the surface properties of pure magnesium and be learned to develop a more reasonable parameters for further study of medical pure magnesium by ion implantation of nitrogen.
Abstract: Type-I clathrates have been considered as very promising thermoelectric (TE) materials thus attracting attention widely. Here we report new clathrates Ba8CuxSiyGe46-x-y (4≤ x ≤ 6.5, y = 0 and 5.15 ≤ x ≤ 6.425, 2.05≤ y ≤ 36.9), focusing on their phase purity and TE properties. Our results show that samples prepared by arc melting followed by annealing are multi-phases alloys. The composition of the clathrate phase is also inhomogeneous. This indicates that the kinetic factor dominates the reaction of forming the clathrate phase during element-melting and sample-annealing. We select three compositions in these two series of samples, which have less impurity and better composition homogeneity for the clathrate phase, and the annealed alloys are furthered processed by ball milling (BM) and powder-solidification (either by hot pressing (HP) or by spark plasma sintering (SPS)) for TE properties investigations. The BM and HP/SPS processes can improve the phase purity and homogeneity. The TE measurements show that the Si-substituted samples have better performance than the Ge-based sample, mainly by decreasing the electrical resistivity. This indicates that the elemental substitution may be still an effective way to improve the TE performance of clathrates.
Abstract: In this work, crystal structure, magnetic properties and magnetocaloric effect of Nd0.7Gd0.3Mn2Si2 alloy were studied by X-ray diffraction (XRD), Physical Property Measurement System (PPMS) and Differential Scanning Calorimetry (DSC). Nd0.7Gd0.3Mn2Si2 crystallizes in ThCr2Si2-typed structure with space group I4/mmm, in which the Nd, Gd, Mn and Si atoms occupy 2a (0, 0, 0), 2a (0, 0, 0), 4d (0, 1/2, 1/4) and 4e (0, 0, 0.38261) position, respectively. The Curie temperature (Tc) of Nd0.7Gd0.3Mn2Si2 alloy is 42 K, while the spin reorientation temperature (TSR) is 26 K and the Nel temperature (TN) is 410 K. The Tc and TN were determined using PPMS and DSC, respectively. The maximum value of the magnetic entropy change (-Smax) in the field change of 0-5 T is 11.862 J/kg K, while the value of relative cooling power (RCP) in Nd0.7Gd0.3Mn2Si2 alloy is 69.666 J/kg under the field change of 5 T.
Abstract: In this work, the effects of substitution of Nd by Gd on the magnetic properties of Nd-Fe-B magnets were investigated. The results demonstrate that the size of grain decreases with the addition of Gd element. The Curie temperatures (Tc) are found to improve with the increase in Gd content, which means that the thermal stability of the magnets is enhanced. The coercivity and saturation magnetization decrease with the increase in Gd content due to the low magnetocrystalline anisotropy and saturation magnetization of Gd2Fe14B.