Electron microscopy and diffraction of icosahedral twins in an aluminium—manganese alloy

The existence of icosahedral twins has been established in Al-10at.% Mn alloy. By a stereographic approach a close resemblance to the decagonal phase is pointed out. The simulation of twin diffraction patterns has been done based on the projection formalism. The physical significance of twinning in terms of hyperdimensional projection is discussed.

Formation and crystallization of hydrogen-induced amorphous SmFe2H3.6 alloy

On hydrogenation of the Laves phase SmFe2, an amorphous SmFe2H3.6 (a-SmFe2H3.6) alloy was formed between 400 K and 500 K. The amorphous nature of the alloy was confirmed by X-ray diffraction, transmission electron microscopy and thermal analysis. However, SmFe2 absorbed hydrogen in the crystal state below 350 K and decomposed into SmH2 and α-Fe above 550 K. The crystallization behaviour of a-SmFe2H3.6 was investigated by differential scanning calorimetry in combination with electron microscopy. Even after considerable hydrogen desorption (Image ) by an endothermic reaction on heating, the amorphous state was retained. Crystallization of a-SmFe2H3.6 took place in two stages. The first stage involved the precipitation of α-Fe in the amorphous matrix. The second stage involved the decomposition of the remaining amorphous phase into the equilibrium phases SmH2 and SmFe2.

Competing magnetic interactions in a dinuclear Ni(II) complex: Antiferromagnetic O-H center dot center dot center dot O moiety and ferromagnetic N-3(-) ligand

Synthesis, structural characteristics, magnetic studies and DFT calculations in Ni(II) dinuclear complexes containing two bridging N-3(-) and an O-(HO)-O-... linkage reveal the existence of ferromagnetic interactions between Ni(II) centers via N-3(-) ligands and antiferromagnetic interactions through the H-bonded moiety. The overall magnetic behavior of the system depends on the delicate balance between these two competing interactions.

Creep Behaviour of AE42 Magnesium Alloy and Its Composites Using Impression Creep Technique

The creep behaviour of a creep-resistant AE42 magnesium alloy has been examined in the temperature range of 150 to 240 degrees C at the stress levels ranging from 40 to 120 MPa using impression creep technique. A normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the temperatures and stresses employed The stress exponent varies from 5.1 to 5.7 and the apparent activation energy varies from 130 to 140 kJ/mol, which suggests the high temperature climb of dislocation controlled by lattice self-diffusion being the dominant creep mechanism in the stress and temperature range employed The creep behaviour of the AE42 alloy has also been compared with its composites reinforced with Saffil short fibres and SiC particles in four combinations. All the composites exhibited a lower creep rate than the monolithic AE42 alloy tested at the same temperature and stress levels and the decrease in creep rate was greater in the longitudinal direction than in the transverse direction, as expected. All the hybrid composites, i.e., the composites reinforced with a combination of Saffil short fibres and SiC particles, exhibited creep rates comparable to the composite reinforced with 20% Saffil short fibres alone at all the temperature and stress levels employed, which is beneficial from the commercial point of view.

Laser Surface Alloying of a Creep Resistant Magnesium Alloy MRI 230D with Al and Al2O3

A creep resistant permanent mould cast Mg alloy MRI 230D was laser surface alloyed with Al and a mixture of Al and Al2O3 using pulsed Nd:YAG laser irradiation at four different scan speeds in order to improve the corrosion and wear resistance. The microstructure, corrosion and wear behavior of the laser surface alloyed material is reported in this manuscript. The coating comprised of a featureless microstructure with cellular-dendritic microstructure near the interface and exhibited good interfacial bonding. A few solidification cracks reaching down to substrate were also observed. The two step coating with Al followed by a mixture of Al and Al2O3 exhibited a slightly better corrosion resistance than the single step coating with Al. In the long run, however, corrosion resistance of both the coatings became comparable to the as-cast alloy. The corroded surface of the laser surface alloyed specimens revealed a highly localized corrosion. The laser surface alloyed specimens exhibited an improvement in wear resistance. The laser scan speed did not exhibit a monotonic trend either in corrosion or wear resistance.

Microstructure and Properties of the Spray-Formed and Extruded 7075 Al Alloy

In the present investigation, a very good combination of strength and ductility, 630MPa 0.2% proof stress and 14.8% elongation to fracture in tensile test, has been obtained for the 7075 Al alloy after optimizing the processing parameters for spray forming, hot extruding the spray deposit, and peak aging the samples taken from the extruded rod. The spray deposits contained some porosity but it was almost eliminated on hot extrusion. Electron probe microanalysis revealed that even though spray forming was carried out in an open atmosphere, it did not affect the oxygen content and its distribution in the material on spray forming, because the atomizing argon gas provided a protective cover to molten droplets and prevented their oxidation. The chemical composition of the spray-formed material was found to be almost the same as the raw material, and the major alloying elements were found to be uniformly distributed in the extruded rod.

Spinodal decomposition of a Co-Ti-Nb alloy

The microstructural evolution on aging a Co-3 wt pct Ti-2 wt pct Nb alloy has been followed by transmission electron microscopy and diffraction to show that the solid solution decomposed by the spinodal mode. The strengthening observed has been correlated with the differences in lattice parameters of the coexisting phases. The several stages of coarsening have been documented to yield information about their kinetics and morphological changes.Formerly Visiting Assistant Professor, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, is with .

Thermodynamic measurements of b.c.c. Cr-Mn solid solutions at 1373 K

Thermodynamics of Cr-Mn alloys have been studied by Eremenko et al (l) using a fused salt e.m.f.technique. Their results indicate positive deviations from ideality at 1023 K. Kaufman (2) has independently estimated negative enthaipy and excess entropy for the b.c.c. Cr-Mn alloys, such that at high temperatures, the entropy term predominates over the enthalpy term giving positive deviations from ideality. Recently the thermodynamic properties of the alloys have been measured by 3acob (3) using a Knudsen cell technique in the temperature range of 1200 to 1500 K. The results indicate mild negative deviations from ideality over the entire composition range. Because of the differences in the reported results and Mn being a volatile component in the alloys which leads to surface depletion under a dynamic set up, an isopiestic technique is used to measure the properties of the alloys.

Use of metastable equilibria for determination of Gibbs energy of solids

Attempts are made to measure activities of both components of a binary alloy (A�B) at 650 K using a solid-state galvanic cell incorporating a new composite solid electrolyte. Since the ionic conductivity of the composite solid electrolyte is three orders of magnitude higher than that of pure CaF2, the cell can be operated at lower temperatures. The alloy phase is equilibrated in separate experiments with flourides of each component and fluorine potential is measured. The mixture of the alloy (A�B) and the fluoride of the more reactive component (BF2) is stable, while (A�B) + AF2 mixture is metastable, Factors governing the possible use of metastable equilibria have been elucidated in this study. In the Co�Ni system, where the difference in Gibbs energies of formation of the fluorides is 21.4 kJ/mol, emf of the cell with metastable phases at the electrode is constant for periods ranging from 90 to 160 ks depending on alloy composition. Subsequently, the emf decreases because of the onset of the displacement reaction. In the Ni�Mn system, measurement of the activity of Ni using metastable equilibria is not fully successful at 650 K because of the large driving force for the displacement reaction (208.8 kJ/mol). Critical factors in the application of metastable equilibria are the driving force for displacement reaction and diffusion coefficients in both the alloy and fluoride solid solution.

Strain-rate sensitivity and microstructural evolution in a Mg-Al-Zn alloy

Strain rate sensitivity measurements are used to identify twinning and changes in deformation mechanisms in a Mg AZ31 alloy over a wide range of temperatures and grain sizes. At low temperatures, there is significant twinning at low strains with strain-rate insensitivity; at large strains, strain rate sensitivity is noted, corresponding to deformation by multiple slip. At high temperatures, there is very little twinning and this leads to a significant strain rate sensitivity from the early stages of deformation. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of grain size and texture on the mechanical properties of warm worked cadmium-1.5 Pct zinc alloy

The structural changes occurring during warm working of Cd-1.5 pct Zn alloy and their effect on the subsequent mechanical properties are studied. It is observed that changes in grain size and preferred orientation are important to a large extent in controlling the mechanical strength. The Hall-Petch slope,R decreases in the warm worked material while the friction stress, σo increases. The lowerR values are attributed to the development of a (101l) texture and the higher σo values are interpreted on the basis of changes in the basal texture.