Band 108, Heft 12

The Ni–W binary system was assessed using critically evaluated experimental data with assistance from first principles analysis and the CALPHAD method. The solution phases (liquid, fcc-A1 and bcc-A2) were modeled using the substitutional regular solution model. The recently discovered Ni 8 W metastable phase was evaluated as Fe 16 C 2 -like martensite with three sublattices, and shown to be possibly stable according to first principles calculations. Ni 8 W was also modeled as an interstitial compound, but the model is not good because the solubility of tungsten in nickel is very low, especially at low temperatures. There is no experimental evidence for such low solubility. The other binary compounds Ni 4 W and Ni 3 W were assessed as stoichiometric ones. Compared independent experimental and first principles data agree well with the calculated phase diagram using updated thermodynamic parameters.

In this research, for the first time, forming limit diagrams (FLDs) of ultra-fine grained Al/Al samples produced by accumulative roll bonding (ARB) process were evaluated. The samples were successfully prepared through ARB up to the 7 th pass. Mechanical properties, tensile fracture surfaces, and crystallite size of samples were investigated. The level of the FLD and elongation at first decreased and then increased, continuously. The results of tensile testing revealed that by increasing the number of ARB passes, the tensile strength increased continuously. Microhardness of Al strips increased and reached a value of 51 VHN after the seventh pass. Results of X-ray diffraction demonstrated that crystallite size decreased from 1 341 nm for annealed condition and it reached to 175 nm for ARBed samples after the 7 th pass. SEM images showed that the fracture mode in the ARBed samples was a shear ductile fracture with elongated sophomoric shear dimples.

Metal-matrix composites produced via spark plasma sintering are characterized by enhanced mechanical and microstructural properties. Obviously, the material behavior is strongly related to the reinforcement properties, in particular the reinforcement shape. The present paper is aimed at showing the friction stir welding of Al-based composites reinforced with different percentages of nano (2%) and microsized (8%) alumina particles. The joints, microstructural and mechanical properties were evaluated as a function of the different processing parameters such as rotating and advancing speeds of the tool. The joint evolution was related to the heat input during welding. The welded material fracture behavior showed different types of rupture compared to the spark plasma sintered material.

As a solution to the low convection strength of the Bridgman method and the convection control difficulties of the electromagnetic induction melting method, a novel, large directional solidification device with annular heaters arranged above the crucible is designed. The inhomogeneous heating field causes differences in melt density, intensifies controllable natural convection and accelerates the moving of impurities from the solid–liquid interface to the surface of the melt, thereby improving purification efficiency and reducing energy consumption. Although the temperature field is inhomogeneous, vertical crystal growth can still be achieved. Mathematical analytic modelling is used to explain the principle, and the feasibility is verified by experiments. The results show that high-quality and large bulk silicon ingot (1 m × 1 m × 0.45 m) can be produced at an average solidification rate of 3.68 μm s −1 .

In this paper, a transient numerical simulation method was used to analyze the direct chill casting process of 7A04 alloy under electromagnetic pulse treatment. The distribution and evolution of the electromagnetic force, fluid and thermal fields were studied to determine their effect on the solidification characteristics of alloy ingots. A modified nucleation theory for the refinement of grain size was presented considering the effect of electromagnetic energy on the critical Gibbs free energy ΔG*$\Delta {G^*}$ of the unity, which was verified by the corresponding experiments. The results showed that the swirl rings of the axial section in the melt were restrained and a lower temperature gradient was formed in the radial section when the magnetic flux intensity was increased. Under these conditions for nucleation, the grain refinement could be explained by the electromagnetic pulse energy that reduced the energy barrier and improved the nucleation rate, either heterogeneous nucleation or homogeneous nucleation.

It is well-known that the strain induced melt activation process results in a globular microstructure which improves the hardness and the ultimate tensile stress. However, its disadvantage is degrading some other properties such as elongation and formability due to the presence of continuous and brittle intermetallic compounds along with the formation of eutectic structures at grain boundaries. Hence, the aim of this research is to introduce a new process called two-step strain induced melt activation to overcome these drawbacks and to improve the microstructure and the formability of Al 7075 alloy. This new process yielded a globular structure and modified the microstructure. Also, the resultant precipitates were discontinuous with a more homogeneous distribution and the eutectic mixture was eliminated. Moreover, the alloy formability and the hardness were effectively enhanced by performing a rolling process.

Al-4.5Mg-0.5Mn alloy powders were mechanically alloyed by the addition of 1–3 wt.% cerium oxide (CeO 2 ) nanoparticles using a planetary ball mill under a dry argon atmosphere. A milling speed of 300 rpm and ball to powder ratio of 20:1 was used for all samples. An optimum milling time of 20 h was chosen considering the morphological and structural changes in the milled powders. The composite powders were consolidated via vacuum hot pressing at 515°C. Mechanical properties were evaluated by hardness and compression tests. The evolution of the structure and microstructure during the milling process was studied using X-ray diffraction and field emission scanning electron microscopy, respectively. The corrosion behavior of the samples was investigated by potentiodynamic polarization testing in a standard 3.5% chloride salt solution and assessed in terms of pitting potential and passivation range. CeO 2 reinforced mechanically alloyed Al-4.5Mg exhibited a major improvement in its hardness and compressive strength compared to the un-reinforced base alloy, i.e. 40 and 50% increment respectively. Nevertheless, the most important effect of CeO 2 incorporation was observed in the pitting resistance of the alloy.

Manufacturing tailored materials is commonly faced with the challenge of shrinkage mismatch between layers resulting in delamination. The effects of sintering temperature and carbon variation on the densification and interfacial bond strength of bilayer cemented tungsten carbide and steel processed through powder metallurgy are analyzed. It is revealed through field-emission scanning electron microscopy images that inter-layer diffusion induced by liquid-phase sintering plays a major role in the densification and bonding of layers. Through dimensional analysis of sintered bilayer specimens, the strain rate of cemented tungsten carbide is observed to surpass that of steel. An enhanced densification rate of 6.1 % and M 6 C (eta carbide) reduction with increased carbon level results in strong interfacial bonding in specimens sintered at 1 280 °C. At 1 295 °C, diffusion accelerates and the axial and radial shrinkage increase by 14.05 % and 13.35 %, respectively, in 93.8 wt.% WC – 6 wt.% Fe – 0.2 wt.% C and 93.2 wt.% Fe – 6 wt.% WC – 0.8 wt.% C, thereby increasing the tendency for complete delamination.

The effect of sub-zero treatment on 17-4 PH stainless steels is analysed based on ultrasonic cavitation experiments conducted on a vibratory machine with piezoceramic crystals. The results prove that the proposed treatment (solution treatment + sub-zero cooling + tempering), favours a reduction, by about 40%, of the mean depth of erosion, and mean depth erosion rate after 165 min of erosion by vibratory cavitation, compared to the solution treatment. The improvement of the cavitation erosion behaviour is explained based on hardness, metallographic and X-ray diffraction analysis.

Cu–ZrO 2 nanocomposites were prepared by an in-situ reactive synthesis of copper nitrate Cu(NO 3 ) 2 and zirconium oxychloride ZrOCl 2 . Zirconia (ZrO 2 ) was added by 2.5, 5 and 10 wt.% to the Cu matrix to assess its effect on thermal conductivity and thermal expansion behavior. The results showed that ZrO 2 nanoparticles (30–50 nm) were homogeneously distributed in the copper matrix. The measured thermal conductivity for the Cu–ZrO 2 nanocomposites decreased from 372.8 to 94.4 W m −1 K −1 with increasing ZrO 2 content from 0 to 10 wt.%. Cu-10 wt.% ZrO 2 nanocomposite yields a low thermal conductivity of 94.4 W · m −1 K −1 along with a low coefficient of thermal expansion, 11.47 × 10 −6 K −1 .