Metals2015, 5(2), 504-529; doi:10.3390/met5020504 - published 25 March 2015 Show/Hide Abstract
Abstract: A review of the new approach to the understanding of the structural relaxation of metallic glasses based on the interstitialcy theory has been presented. The key hypothesis of this theory proposed by Granato consists of the statement that the thermodynamic properties of crystalline, liquid and glassy states are closely related to the interstitial defects in the dumbbell (split) configuration, called also interstitialcies. It has been argued that structural relaxation of metallic glasses takes place through a change of the concentration of interstitialcy defects frozen-in from the melt upon glass production. Because of a strong interstitialcy-induced shear softening, the defect concentration can be precisely monitored by measurements of the unrelaxed shear modulus. Depending on the relation between the current interstitialcy concentration c and interstitialcy concentration in the metastable equilibrium, different types of structural relaxation (decreasing or increasing c) can be observed. It has been shown that this approach leads to a correct description of the relaxation kinetics at different testing conditions, heat effects occurring upon annealing, shear softening and a number of other structural relaxation-induced phenomena in metallic glasses. An intrinsic relation of these phenomena with the anharmonicity of the interatomic interaction has been outlined. A generalized form of the interstitialcy approach has been reviewed.
Metals2015, 5(2), 484-503; doi:10.3390/met5020484 - published 25 March 2015 Show/Hide Abstract
Abstract: We report on the effects of annealing on the martensitic phase transformation in the Ni-based Heusler system: Mn50Ni40Sn10 and Mn50Ni41Sn9 powder and Co50Ni21Ga32 nanoparticles. For the powdered Mn50Ni40Sn10 and Mn50Ni41Sn9 alloys, structural and magnetic measurements reveal that post-annealing decreases the martensitic transformation temperatures and increases the transition hysteresis. This might be associated with a release of stress in the Mn50Ni40Sn10 and Mn50Ni41Sn9 alloys during the annealing process. However, in the case of Co50Ni21Ga32 nanoparticles, a reverse phenomenon is observed. X-ray diffraction analysis results reveal that the as-prepared Co50Ni21Ga32 nanoparticles do not show a martensitic phase at room temperature. Post-annealing followed by ice quenching, however, is found to trigger the formation of the martensitic phase. The presence of the martensitic transition is attributed to annealing-induced particle growth and the stress introduced during quenching.
Metals2015, 5(1), 471-483; doi:10.3390/met5010471 - published 16 March 2015 Show/Hide Abstract
Abstract: Among all recognized severe plastic deformation techniques, a new method, called the equal channel forward extrusion process, has been experimentally studied. It has been shown that this method has similar characteristics to other severe plastic deformation methods, and the potential of this new method was examined on the mechanical properties of commercial pure aluminum. The results indicate that approximate 121%, 56%, and 84% enhancements, at the yield strength, ultimate tensile strength, and Vickers micro-hardness measurement are, respectively, achieved after the fourth pass, in comparison with the annealed condition. The results of drop weight impact test showed that the increment of 26% at the impact force, and also decreases of 32%, 15%, and 4% at the deflection, impulse, and absorbed energy, are respectively attained for the fourth pass when compared to the annealed condition. Furthermore, the electron backscatter diffraction examination revealed that the average grain size of the final pass is about 480 nm.
Metals2015, 5(1), 457-470; doi:10.3390/met5010457 - published 13 March 2015 Show/Hide Abstract
Abstract: The microstructure and tensile properties of the thixoforged in situ Mg2Sip/AM60B composite were characterized in comparison with the as-cast composite and thixoforged AM60B. The results indicate that the morphology of α-Mg phases, the distribution and amount of β phases and the distribution and morphology of Mg2Si particles in thixoforged composite are completely different from those in as-cast composite. The Mg2Si particles block heat transfer and prevent the α-Mg particles from rotation or migration during reheating. Both the thixoforged composite and thixoforged AM60B alloy exhibit virtually no porosity in the microstructure. The thixoforged composite has the highest comprehensive tensile properties (ultimate tensile strength (UTS)) of 209 MPa and an elongation of 10.2%. The strengthening mechanism of the Mg2Si particle is the additive or synergetic effect of combining the load transfer mechanism, the Orowan looping mechanism and the dislocation strengthening mechanism. Among them, the load transfer mechanism is the main mechanism, and the latter two are minor. The particle splitting and interfacial debonding are the main damage patterns of the composite.
Metals2015, 5(1), 439-456; doi:10.3390/met5010439 - published 12 March 2015 Show/Hide Abstract
Abstract: Calcareous deposits were formed on steel under conditions of cathodic protection in artificial seawater at applied constant current densities ranging from 50 to 400 mA·m−2. The calcareous layers were characterized using a Field Emission Gun Scanning Electron Microscope (FEG SEM) in conjunction with Energy Dispersive X-Ray Analysis (EDX), and Electrochemical Impedance Spectroscopy (EIS). At cathodic current densities of 50–100 mA·m−2 where corrosion was still occurring, a clear correlation existed between the iron containing corrosion product and the overlying magnesium hydroxide layer. This revealed that the mapping of magnesium rich areas on a steel surface can be used in the identification of local corrosion sites. At current densities of 150–200 mA·m−2, a layered deposit was shown to occur consisting of an inner magnesium-containing layer and an outer calcium-containing layer. At current densities of 300–400 mA·m−2, intense hydrogen bubbling through macroscopic pores in the deposits gave rise to cracking of the deposited film. Under such conditions deposits do not have a well-defined double layer structure. There is also preferential formation of magnesium-rich compounds near the steel surface at the early stages of polarisation and within the developing pores and cracks of calcareous deposits later on. Based on SEM/EDX investigation of calcareous depositions the impedance model was proposed and used to monitor in situ variations in steel corrosion resistance, and to calculate the thickness of formed deposits using the length of oxygen diffusion paths.
Metals2015, 5(1), 428-438; doi:10.3390/met5010428 - published 12 March 2015 Show/Hide Abstract
Abstract: A new phenomenon is described in this paper: the formation of macroscopic channel structures on the bottom of copper ingots which were used as the target for the synthesis of copper nanoparticles by high-power electron beam evaporation and condensation. In the synthesis experiment, the cylindrical copper ingot is melted and partially evaporated in a graphite crucible. The channel structures were originally observed after a series of nanoparticle synthesis experiments in varying conditions. In the present work, various process conditions are varied in order to recreate the structures and identify their mechanism of formation. Conditions in which the channel structures form and do not form are identified and interesting microstructures are observed near the channel structures.