First principles study of structural stability and site preference in Co-3 (W,X)

Since the discovery 1] of gamma' precipitate (L1(2) - Co-3 (Al, W)) in the Co-Al-W ternary system, there has been an increased interest in Co-based superalloys. Since these alloys have two phase microstructures (gamma + gamma') similar to Ni-based superalloys 2], they are viable candidates in high temperature applications, particularly in land-based turbines. The role of alloying on stability of the gamma' phase has been an active area of research. In this study, electronic structure calculations were done to probe the effect of alloying in Co3W with L1(2) structure. Compositions of type Co-3(W, X), (where X/Y = Mn, Fe, Ni, Pt, Cr, Al, Si, V, W, Ta, Ti, Nb, Hf, Zr and Mo) were studied. Effect of alloying on equilibrium lattice parameters and ground state energies was used to calculate Vegard's coefficients and site preference related data. The effect of alloying on the stability of the L1(2) structure vis a vis other geometrically close packed ordered structures was also studied for a range of Co3X compounds. Results suggest that the penchant of element for the W sublattice can be predicted by comparing heats of formation of Co3X in different structures.

High-strength bulk Al-based bimodal ultrafine eutectic composite with enhanced plasticity

An in situ bulk ultrafine bimodal eutectic Al-Cu-Si composite was synthesized by solidification. This heterostructured composite with microstructural length scale hierarchy in the eutectic microstructure, which combines an ultrafine-scale binary cellular eutectic (alpha-Al + Al2Cu) and a nanometer-sized anomalous ternary eutectic (alpha-Al + Al2Cu + Si), exhibits high fracture strength (1.1 +/- 0.1 GPa) and large compressive plastic strain (11 +/- 2%) at room temperature. The improved compressive plasticity of the bimodal-nanoeutectic composite originates from homogeneous and uniform distribution of inhomogeneous plastic deformation (localized shear bands), together with strong interaction between shear bands in the spatially heterogeneous structure.

An Investigation Of Alkali Aluminosilicate Glasses By Si-29 And Al-27 Magic-Angle-Spinning Nmr-Spectroscopy

Alkali aluminosilicate glasses prepared by the gel and the melt routes have been investigated by Si-29 and Al-27 MAS NMR spectroscopy. It is found that Al has a tetrahedral coordination in the gel glasses modified with equivalent proportions of alkalis unlike in a pure aluminosilicate glass where Al has both four and six coordinations. Silicon is present as Q4 units in all the 5M2O 5Al2O3 9OSiO2 ( M = Li, Na and K) gel glasses studied whereas it is present in Q2 or Q3 species in the lithium aluminosilicate glasses of compositions 40Li2O x Al2O3 (1-x)SiO2 (1 less-than-or-equal-to x less-than-or-equal-to 15) and xLi2O 10Al2O3 (1-x)SiO2 (20 less-than-or-equal-to x less-than-or-equal-to 40). The combination of Q2 and Q3 is also found in certain sodium aluminosilicate glasses, but they change to Q2 and Q1 as the concentration of SiO2 decreases.

Quasicrystals, crystals and multiple twins in rapidly solidified Al-Cr-Si, Al-Mn-Si and Al-Mn-Cr-Si alloys

The coexistence of quasicrystals and rational approximant structures (RAS) has been observed in melt-spun Al80Cr14Si6, Al80Mn14Si6 and Al75Mn10Cr5Si10 alloys. The presence of a b.c.c. alpha-AlMnSi phase in Al-Mn-Si and alpha-AlMnSi(Cr) phase in Al-Mn-Cr-Si has been seen. A multiple twinning around an irrational axis of the RAS has been reported in an aggregate of fine size cubic crystallites in all three alloys. Selected area diffraction patterns show that the crystalline aggregate symmetry is linked to the icosahedral point group symmetry (m35). Various ways of expressing the twin relationship in the cubic crystalline aggregates have been discussed. The thermal stability of the icosahedral phase at high temperatures reveals that the icosahedral phase in Al-Mn-Si and Al-Mn-Cr-Si alloys transforms to alpha-AlMnSi at temperatures of 690 and 670 K, respectively. In Al-Cr-Si alloy, heating to a high temperature (615 K) leads to the transformation of the icosahedral phase into a new metastable phase having an ordered cubic structure equivalent to alpha-AlMnSi. The occurrence of multiple twinning leading to icosahedral symmetry in the as-spun Al-Cr-Si alloy is presumably due to this metastable phase. Copyright (C) 1996 Acta Metallurgica Inc.

Optimization of hot workability of an Al-Mg-Si alloy using processing maps

The hot workability of an Al-Mg-Si alloy has been studied by conducting constant strain-rate compression tests. The temperature range and strain-rate regime selected for the present study were 300-550 degrees C and 0.001-1 s(-1), respectively. On the basis of true stress data, the strain-rate sensitivity values were calculated and used for establishing processing maps following the dynamic materials model. These maps delineate characteristic domains of different dissipative mechanisms. Two domains of dynamic recrystallization (DRX) have been identified which are associated with the peak efficiency of power dissipation (34%) and complete reconstitution of as-cast microstructure. As a result, optimum hot ductility is achieved in the DRX domains. The strain rates at which DRX domains occur are determined by the second-phase particles such as Mg2Si precipitates and intermetallic compounds. The alloy also exhibits microstructural instability in the form of localized plastic deformation in the temperature range 300-350 degrees C and at strain rate 1 s(-1).

Nanoscale embedded alloy particles through rapid solidification: the case of Al-Pb-In and Cu-Fe-Si

Rapid solidification techniques can be used to produce the embedded nanoparticles in a desired matrix. The origin and morphology of these small particles and their transformation behaviour are still not fully understood. In this paper, we discuss the issues involved and present some interesting results in Al-Pb-In and Cu-Fe-Si systems.

A Single-Stage Water-Gas Shift Reaction over Highly Active and Stable Si- and Al-Substituted Pt/CeO2 Catalysts

Ce0.88Si0.1Pt0.02O2-d and Ce0.88Al0.1Pt0.02O2-d catalysts were synthesized by using a low-temperature sonochemical method and characterized by using XRD, TEM, XPS, FTIR, and BET surface analyzer. The catalytic activities of these compounds were investigated for the watergas shift reaction in the temperature range of 140-440 degrees C. The substitution of Si in Ce0.98Pt0.02O2-d increased the releasing capacity of lattice oxygen, whereas the substitution of Al decreased the reducibility of Ce0.98Pt0.02O2-d, as evidenced by hydrogen temperature-programmed reduction studies. However, both the catalysts showed a considerable improvement in terms of activity and stability compared to Ce0.98Pt0.02O2-d. The combined activity measurement and characterization results suggest that the increase in the oxygen vacancy, which acts as a dissociation center for water, is the primary reason for the improvement in the activity of modified Ce0.98Pt0.02O2-d. Both the catalysts are 100?% selective toward H2 production, and approximately 99?% conversion of CO to CO2 was observed at 260 and 270 degrees C for Ce0.88Si0.1Pt0.02O2-d and Ce0.88Al0.1Pt0.02O2-d, respectively. These catalysts do not deactivate during the daily startup/shutdown operations and are sustainable even after prolonged reaction. Notably, these catalysts do not require any pretreatment or activation during startup/shutdown operations.

Structural, electrical and dielectric properties of sputtered TiO2 films for Al/TiO2/Si capacitors

Metal-oxide semiconductor capacitors based on titanium dioxide (TiO2) gate dielectrics were prepared by RF magnetron sputtering technique. The deposited films were post-annealed at temperatures in the range 773-1173 K in air for 1 hour. The effect of annealing temperature on the structural properties of TiO2 films was investigated by X-ray diffraction and Raman spectroscopy, the surface morphology was studied by atomic force microscopy (AFM) and the electrical properties of Al/TiO2/p-Si structure were measured recording capacitance-voltage and current-voltage characteristics. The as-deposited films and the films annealed at temperatures lower than 773 K formed in the anatase phase, while those annealed at temperatures higher than 973 K were made of mixtures of the rutile and anatase phases. FTIR analysis revealed that, in the case of films annealed at 1173 K, an interfacial layer had formed, thereby reducing the dielectric constant. The dielectric constant of the as-deposited films was 14 and increased from 25 to 50 with increases in the annealing temperature from 773 to 973 K. The leakage current density of as-deposited films was 1.7 x 10(-5) and decreased from 4.7 X 10(-6) to 3.5 x 10(-9) A/cm(2) with increases in the annealing temperature from 773 to 1173 K. The electrical conduction in the Al/TiO2/p-Si structures was studied on the basis of the plots of Schottky emission, Poole-Frenkel emission and Fowler-Nordheim tunnelling. The effect of structural changes on the current-voltage and capacitance-voltage characteristics of Al/TiO2/p-Si capacitors was also discussed.

Post-deposition annealing influenced structural and electrical properties of Al/TiO2/Si gate capacitors

Titanium dioxide (TiO2) thin films are deposited on unheated p-Si (100) and quartz substrates by employing DC reactive magnetron sputtering technique. The effect of post-deposition annealing in air at temperatures in the range 673-973 K on the structural, electrical, and dielectric properties of the films was investigated. The chemical composition of the TiO2 films was analyzed with X-ray photoelectron spectroscopy. The surface morphology of the films was studied by atomic force microscope. The optical band gap of the as-deposited film was 3.50 eV, and it increased to 3.55 eV with the increase in annealing temperature to 773 K. The films annealed at higher temperature of 973 K showed the optical band gap of 3.43 eV. Thin film capacitors were fabricated with the MOS configuration of Al/TiO2/p-Si. The leakage current density of the as-deposited films was 1.2 x 10(-6) A/cm(2), and it decreased to 5.9 x 10(-9) A/cm(2) with the increase in annealing temperature to 973 K. These films showed high dielectric constant value of 36. (C) 2013 Elsevier Ltd. All rights reserved.

Influence of sputter power on structural and electrical properties of TiO2 films for Al/TiO2/Si gate capacitors

Titanium dioxide (TiO2) thin films were deposited onto p-Si substrates held at room temperature by reactive Direct Current (DC) magnetron sputtering at various sputter powers in the range 80-200W. The as-deposited TiO2 films were annealed at a temperature of 1023K. The post-annealed films were characterized for crystallographic structure, chemical binding configuration, surface morphology and optical absorption. The electrical and dielectric properties of Al/TiO2/p-Si structure were determined from the capacitance-voltage and current-voltage characteristics. X-ray diffraction studies confirmed that the as-deposited films were amorphous in nature. After post-annealing at 1023K, the films formed at lower powers exhibited anatase phase, where as those deposited at sputter powers >160W showed the mixed anatase and rutile phases of TiO2. The surface morphology of the films varied significantly with the increase of sputter power. The electrical and dielectric properties on the air-annealed Al/TiO2/p-Si structures were studied. The effect of sputter power on the electrical and dielectric characteristics of the structure of Al/TiO2/p-Si (metal-insulator-semiconductor) was systematically investigated. Copyright (c) 2014 John Wiley & Sons, Ltd.

Development of alloys with high strength at elevated temperatures by tuning the bimodal microstructure in the Al-Cu-Ni eutectic system

The development of high-strength aluminum alloys that can operate at 250 degrees C and beyond remains a challenge to the materials community. In this paper we report preliminary development of nanostructural Al-Cu-Ni ternary alloys containing alpha-Al, binary Al2Cu and ternary Al2Cu4Ni intermetallics. The alloys exhibits fracture strength of similar to 1 GPa with similar to 9% fracture strain at room temperature. At 300 degrees C, the alloy retains the high strength. The reasons for such significant mechanical properties are rationalized by unraveling the roles and response of various microstructural features. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Intermetallic eutectic alloys in the Ni-Al-Zr system with attractive high temperature properties

We describe a group of alloys with ultrahigh strength of about 2 GPa at 700 degrees C and exceptional oxidation resistance to 1100 degrees C. These alloys exploit intermetallic phases with stable oxide forming elements that combine to form fine nanometric scale structures through eutectic transformations in ternary systems. The alloys offer engineering tensile plasticity of about 4% at room temperature though both conventional dislocation mechanisms and twinning in the more complex intermetallic constituent, along with slip lengths that are restricted by the interphase boundaries in the eutectics.

Phase transformations and metastable phase equilibria in Al-Ge alloy

In this paper the results of a detailed investigation on the metastable phase relations in undercooled as well as rapidly solidified Al---Ge alloys containing 2–50 at.% Ge are reported. Data obtained on the structure and morphology of phases enable us to arrive at the phase relations and transformation processes occurring in undercooled and rapidly quenched melts of this system. These results are explained with the help of a metastable phase diagram consisting of a peritectic and eutectic reaction involving metastable phases.

Deformation processing of an aluminum alloy containing particles: Studies on AI-5 pct Si alloy 4043

Al-5 wt pct Si alloy is processed by upset forging in the temperature range 300 K to 800 K and in the strain rate range 0.02 to 200 s−1. The hardness and tensile properties of the product have been studied. A “safe” window in the strain rate-temperature field has been identified for processing of this alloy to obtain maximum tensile ductility in the product. For the above strain rate range, the temperature range of processing is 550 K to 700 K for obtaining high ductility in the product. On the basis of microstructure and the ductility of the product, the temperature-strain rate regimes of damage due to cavity formation at particles and wedge cracking have been isolated for this alloy. The tensile fracture features recorded on the product specimens are in conformity with the above damage mechanisms. A high temperature treatment above ≈600 K followed by fairly fast cooling gives solid solution strengthening in the alloy at room temperature.