Band 107, Heft 6

The article deals with the development of a theoretical model to predict the nature (amplitude and shape) of deformation-induced electromagnetic radiation (EMR) in metals and alloys during progressive plastic deformation. The model explicitly embraces the effect of Peierls stress and strain hardening to envisage the nature of EMR in materials with relatively large Peierls stress. The theoretical results were evaluated for 0.15 % plain carbon steel and compared with experimental results. The signals obtained from the model were found to be exponential in shape and are in good agreement with the experimental observations. The results suggest that inclusion of strain hardening parameters in the earlier model is necessary for determining deformation induced EMR in bcc metals and alloys during progressive plastic deformation.

Using molecular dynamics simulations based on the embedded atom method, we present the calculations of adsorption and activation energies for the diffusion of adatoms (Cu, Ag or Au) on Cu (100), Ag (100) or Au (100) surfaces with steps. We have also examined the relaxation trends and bond lengths of the adatoms for both fourfold and bridge sites. We note that the vertical distance of the adatom relaxation to the first nearest neighbors is the largest (1.56 %) for Ag on Cu (100) and the shortest (–14.58 %) for Cu on Au (100) as compared to other systems. On the other hand, for Cu on the Au (100) system, we find the adatom barrier for hopping along the step edges to be 0.44 eV, which is the highest for this process among the systems studied here, but the lowest barrier is found for Ag on Cu (100) compared to other systems and costs only 0.20 eV. Attention has also been focused on the evaluations of the adsorption and activation energies for the nine systems in the presence of step edges. The diffusion barriers over and along step edges are interpreted in terms of the cohesive energies of the adatoms and substrates. Moreover, these results can offer some basic rules for forecasting precise atomic surface morphologies in homo- and hetero-epitaxial growth.

We carried out global transient simulation of impurity distribution in multi-crystalline silicon grown by directional solidification using appropriate software. Non-metallic impurities such as carbon, nitrogen and oxygen originate from the furnace parts and each impurity atom was segregated into the mc-Si ingot based on their solubility in the crystal. During the solidification process the temperature distribution causes melt fluctuation in the Si melt, which is controlled by applying an external body force that leads to a uniform distribution of impurity atoms. Here the uniform distribution of impurity atoms was obtained by rotating the crucible at 10 rpm and the investigations were also carried out without crucible rotation and with a crucible rotation rate of 3 rpm. In this paper we have analyzed the segregation of non-metallic impurity atoms and chemical reactions between the impurity atoms, wherein three types of simulations were carried out and their results analyzed.

Both experimental investigation and thermodynamic calculation were performed for the Mg–Sr–Zr system. Four decisive alloys were firstly selected and prepared using a powder metallurgy method to measure the isothermal section at 400 °C via a combination of X-ray diffraction and electron probe microanalysis. No ternary compound has been observed for this ternary system. Four three-phase regions, (Mg) + (αZr) + Mg 17 Sr 2 , Mg 17 Sr 2 + (αZr) + Mg 38 Sr 9 , Mg 38 Sr 9 + (αZr) + Mg 23 Sr 6 , and Mg 23 Sr 6 + (αZr) + Mg 2 Sr, have been identified at 400 °C. No appreciable ternary solubility has been detected in the binary Mg–Sr compounds. Phase transition temperatures of the Mg–Sr–Zr alloys were measured by means of differential scanning calorimetry. The thermodynamic calculations match well with the experimental data in the present work, indicating that no ternary thermodynamic parameters are needed for the thermodynamic description of this ternary system. In order to verify the reliability of the current thermodynamic calculations of the Mg–Sr–Zr system, eight as-cast alloys in the Mg-rich corner were also prepared. The calculated liquidus projection is consistent with the observed primary phase regions. The present thermodynamic calculations are reliable and can be used in the development of Mg alloys.

Cr–Cu/W–Cu bi-layer coatings with composite structures were fabricated by means of mechanical alloying. The Cr–Cu layer and the W–Cu layer were deposited successively and the as-synthesized bi-layer coating was made up of an inner Cr–Cu layer and an outer W–Cu layer. Microstructures, chemical and phase compositions of the as-prepared coatings were characterized. The results indicated that the bonding between the inner coating and the substrate was improved with the increase of Cu in the raw powder. The annealing treatment of the inner Cr–Cu layer was beneficial to the bonding between the inner Cr–Cu coating and the outer W–Cu coating layer. Mechanical properties such as microhardness, friction and wear resistance were tested. The as-synthesized coating could effectively improve the hardness and wear resistance of the Cu substrate.

Mg foams produced by powder metallurgy are considered as a new class of materials with unique properties. However, the high reactivity of Mg powder during sintering has a detrimental effect on their properties. The study aims at evaluating the possibility of using an insignificant amount of sulfur hexafluoride in the form of N 2 +0.2 %SF 6 atmosphere for protecting EW62 Mg alloy during sintering at high temperature. The results obtained clearly demonstrate the relative advantage of N 2 +0.2 %SF 6 as a protective atmosphere and cost effective environment compared to pure Ar. This was related to the active role of fluorine during sintering which improves the oxidation resistance of the Mg powder and consequently the properties of Mg foams in terms of structural integrity and strength.

Friction stir welding was successfully performed on B 4 C-reinforced foamable AlSi7 materials produced via a powder metallurgy route. Starting materials of 1 wt.% TiH 2 , 7 wt.% Si and 6 wt.% B 4 C were mixed with aluminum matrix powder. Mixed powders were then pressed, extruded and rolled for the production of foamable materials. Microstructural and macrostructural examination along with microhardness and tensile tests were performed to evaluate the weld zone characteristics of foamable materials welded by friction stir welding. Tunnels or needle-shaped gaps were observed in the macrostructures of the weld zones of samples joined at 1 250 rpm and 2 000 rpm rotational speeds. Moreover, the rough surface in the welding zone of materials decreased with increasing rotational speed. Defect-free joints were observed for samples at 1 600 rpm rotational and 40 mm min −1 traverse speeds. There was no significant difference between the microstructures of the weld zone and the base material in terms of particle size and distribution. It was observed that the B 4 C particles in the nugget zone were partly directed or clustered as a result of the physical mixing. Ultimate tensile strength values of B 4 C-reinforced foamable AlSi7 materials joined by friction stir welding were found to be lower compared to the base material. The maximum ultimate tensile strength of 135 MPa was obtained at a 1 600 rpm rotational speed and 40 mm min −1 traverse speed. The minimum tensile strength in the samples of 95 MPa was obtained at a 1 250 rpm rotational speed and 80 mm min −1 traverse speed. In all welding conditions, the hardness in the nugget zone increased compared to the base material.

The solid-state nature of the friction stir welding (FSW) process leads to several advantages over fusion welding methods as problems associated with cooling from the liquid phase are avoided. This process eliminates many of the defects associated with fusion welding techniques such as shrinkage, solidification cracking and porosity. In general, FSW has been found to produce a low number of defects and is very tolerant of variations in parameters and materials. Nevertheless, physical and chemical properties of the weld zone developed during FSW are a primary concern. In this study, the mechanical properties and corrosion behavior of the weld zone resulting after FSW are investigated and improved by addition of SiC particles. AZ31 magnesium alloy sheets were FS welded and simultaneously processed by incorporation of SiC particles. Mechanical properties and corrosion behavior of FS welded specimens were compared with those of FS welded and processed specimens. The results showed that the addition of SiC particles during FSW considerably improved the mechanical properties and corrosion resistance of the weld zone. Based on the obtained results, incorporation of SiC hard particles in the microstructure of weld zone produced by FSW is highly recommended.

Piezoceramic fibers as the active deforming component in piezoelectric fiber composites have the largest influence on the overall strain of the composites. The results of this study show that the fabricated composites exhibited clearly anisotropic actuation properties. The free strain of piezoelectric fiber composites with 70 % by volume fibers in the longitudinal direction was 2.9 times of that in the transverse direction at the excitation voltage of 2 000 V and excitation frequency of 0.1 Hz. Larger fiber volume fraction can increase the free strain of the composites, e. g. in this case the free strain increased by about 24.6 % and 33.6 % in the longitudinal and transverse directions, respectively, as the fiber volume fraction increased from 70 % to 78 %. The piezoelectric strain coefficient d 33 of piezoelectric fiber composites exhibits nonlinear dependency on the actuation voltage and linear dependency on the logarithm of frequency.

This study investigated the effect of use of a cooling slope on the microstructure, hardness and wear behavior of Al-7Si-Mg alloy. The Al-7Si-Mg alloy was cast with and without a cooling slope at a pouring temperature of 640 °C. Examination with optical and electron microscopes showed that the microstructure of cast samples using the cooling slope comprised fine and globular primary α-Al phase with homogeneous distribution of eutectic phase, while conventionally cast samples featured coarse and dendritic primary α-Al phase. The wear resistance of the cast sample was significantly better with cooling slope casting. The wear mechanism was found to be a combination of adhesion, delamination, oxidation and abrasive wear for both cooling slope- and conventionally cast samples. The wear mechanisms of cast samples both with and without cooling slope are similar and follow Archard's law. The cooling slope-cast samples with fine and globular α-Al phase, high hardness and low specific wear rate ( K′ ) showed the highest wear resistance of 10.08 × 10 −5 mm 3 N −1 m −1 .

NiSe was prepared in-situ on nickel foam (NiSe/NF), using a solvothermal process. The NiSe/NF consisted of interwoven nanorods and nanowires. The interwoven NiSe/NF was of good crystallinity and contained no impurities, and consisted of many nanorods and a few nanowires. The interwoven structure uniformly covered the NF substrate, which can potentially yield a large surface area with abundant active sites, and thus high catalytic activity. The NiSe/NF was used as a working electrode for hydrazine oxidation, in 1.0 mol L −1 KOH containing 20 mmol L −1 N 2 H 4 · H 2 O. Excellent electrocatalytic activity and stability of NiSe/NF for hydrazine oxidation was observed, with a current density of 22 mA cm −2 at −0.53 V. The interwoven structure and good conductivity of NiSe/NF promoted its electrocatalytic activity.