Enhancement of workability in AZ31 alloy-processing maps: Part I,castmaterial

Processing maps developed on the basis of the Dynamic Materials Model provide valuable information that might help the metal working industry in solving problems related to workability and microstructural control in commercial alloys. In this research, the processing maps for an as-cast AZ31 magnesium alloy are presented. The results are validated via microstructural observations, clearly delineating safe and unsafe regimes for further process design of this alloy.

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.

Experimental and Computational Characterization of Alloy--Spinel--Corundum Equilibrium in the System Fe--Co--Al--O at 1873K

The three-phase equilibrium between alloy, spinel solid solution and alpha -Al sub 2 O sub 3 in the Fe--Co--Al--O system at 1873k was fully characterized as a function of alloy composition using both experimental and computational methods. The equilibrium oxygen content of the liquid alloy was measured by suction sampling and inert gas fusion analysis. The O potential corresponding to the three-phase equilibrium was determined by emf measurements on a solid state galvanic cell incorporating (Y sub 2 O sub 3 )ThO sub 2 as the solid electrolyte and Cr + Cr sub 2 O sub 3 as the reference electrode. The equilibrium composition of the spinel phase formed at the interface between the alloy and alumina crucible was measured by electron probe microanalysis (EPMA). The experimental results were compared with the values computed using a thermodynamic model. The model used values for standard Gibbs energies of formation of pure end-member spinels and Gibbs energies of solution of gaseous O in liquid Fe and cobalt available in the literature. The activity--composition relationship in the spinel solid solution was computed using a cation distribution model. The variation of the activity coefficient of O with alloy composition in the Fe--Co--O system was estimated using both the quasichemical model of Jacob and Alcock and Wagner's model along with the correlations of Chiang and Chang and Kuo and Chang. The computed results of spinel composition and O potential are in excellent agreement with the experimental data. Graphs. 29 ref.--AA

Dephosphorisation of Iron–Chrome Alloy with Ca–CaF2 Melt during Electro Slag Refining

Calcium-calcium fluoride melt was used to remove phosphorus from the ferro-chrome alloy (64.5 wt% Cr, 0.15 wt% P) during electro slag refining process. The effect of atmosphere and deoxidisers, viz. Al, Fe–Mo and misch metal were also studied during dephosphorisation reaction. The thermodynamic properties of Ca–CaF2 melt is calculated from a known phase diagram and these results are discussed in relation with the dephosphorisation reaction.

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.

Characterization of Alloy-Spinel-Corundum Equilibrium in the System Fe-Ni-Al-O

The three phase equilibrium between alloy, spinel solid solution and α-alumina in the Fe-Ni-Al-O system has been fully characterized at 1823K as a function of alloy composition using both experimental and computational methods. The oxygen potential was measured using a solid state cell incorporating yttria-doped thoria as the electrolyte and Cr+ Cr2O3 as the reference electrode. Oxygen concentration of the alloy was determined by an inert gas fusion technique. The composition of the spinel solid solution, formed at the interface between the alloy and an alumina crucible, was determined by EPMA. The variation of the oxygen concentration and potential and composition of the spinel solid solution with mole fraction of nickel in the alloy have been computed using activities in binary Fe-Ni system, free energies of formation of end member spinels FeO•(1+x)Al2O3 and NiO•(1+x)Al2O3 and free energies of solution of oxygen in liquid iron and nickel, available in the literature. Activities in the spinel solid solution were computed using a cation distribution model. The variation of the activity coefficient of oxygen with alloy composition in Fe-Ni-O system was calculated using both the quasichemical model of Jacob and Alcock and the Wagner's model, with the correlation of Chiang and Chang. The computed results for the oxygen potential and the composition of the spinel solid solution are in good agreement with the measurements. The measured oxygen concentration lies between the values computed using models of Wagner and Jacob and Alcock. The results of the study indicate that the deoxidation hyper-surface in multicomponent systems can be computed with useful accuracy using data for end member systems and thermodynamic models.

Photo induced optical changes in Sb/As2S3 multilayered film and (As2S3)(0.93)Sb-0.07 film of equal thickness

The increase in optical band gap (photo bleaching) due to light illumination was studied at room temperature as well as at low (4.2 K) temperature for Sb/As2S3 multilayered film of 640 nm thickness by Fourier Transform Infrared Technique. The interdiffusion of Sb into As2S3 matrix results the formation of Sb-As2S3 ternary solid solutions which is explained by the change in optical band gap (E-g), absorption coefficient (alpha), Tauc parameter (B-1/2), Urbach edge (E-e). At the same time, photo darkening phenomena was observed in (As2S3)(0.93)Sb-0.07 film of same thickness both at low and room temperatures. From our X-ray Photoelectron Spectroscopy measurements,we are able to show that some of the As-As, S-S and Sb-Sb bonds are converted into As-S and S-Sb bonds in case of multilayers. We found that the energetically favoured heteropolar bond formation take place by a phonon-assisted mechanism using the lone pair pi electrons of S-2(0). But in case of (As2S3)(0.93)Sb-0.02 film, the homopolar bonds are playing a major role. (C) 2010 Elsevier B.V. All rights reserved.

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.

A computer study of the miscibility limits of Sb-Te-Bi

The quaternary system Sb1bTe1bBi1bSe with small amounts of suitable dopants is of interest for the manufacture of thermoelectric modules which exhibit the Peltier and Seebeck effects. This property could be useful in the production of energy from the thermoelectric effect. Other substances are bismuth telluride (Bi2Te3) and Sb1bTe1bBi and compounds such as ZnIn2Se4. In the present paper the application of computer programs such as MIGAP of Kaufman is used to indicate the stability of the ternary limits of Sb1bTe1bBi within the temperature ranges of interest, namely 273 K to 300 K.

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.

Optical absorption and photoluminescence studies on heavily doped (Ga,Mn)Sb crystals

Ga1-xMnxSb crystals are grown with different Mn doping concentrations by the horizontal Bridgman method (x = 0 - 0.04). Optical absorption and photoluminescence studies are carried out in the temperature range 3-300 K. Optical absorption studies reveal that the inter-valence band transition from the spin-orbit split-off band to the light/heavy hole bands is dominant over the fundamental valence band to conduction band absorption. In higher doped crystals, the fundamental absorption peak is merged with the inter-valence band transition and could not be resolved. Photoluminescence measurements in heavily doped crystals reveal the band gap narrowing and band filling effects due to the Fermi level shifting into the valence band.