Preparation of Ni-Ti-Mo and Ni-Ti-Mo-Zr Alloys by High-Energy Ball Milling and Subsequent Hot Pressing

This work discusses on the preparation of Ni-45Ti-5Mo, Ni-40Ti-10Mo and Ni-46Ti-2Mo-2Zr (at-%) alloys by high-energy ball milling and hot pressing, which are potentially attractive for dental and medical applications. The milling process was performed in stainless steel balls (19mm diameter) and vials (225 mL) using a rotary speed of 300rpm and a ball-to-powder weight ratio of 10:1. Hot pressing under vacuum was performed in a BN-coated graphite crucible at 900 degrees C for 1 h using a load of 20 MPa. The milled and hot-pressed materials were characterized by X-ray diffraction, electron scanning microscopy, and electron dispersive spectrometry. Peaks of B2-NiTi and Ni4Ti3 were identified in XRD patterns of Ni-45Ti-5Mo, Ni-40Ti-10Mo and Ni-46Ti-2Mo-2Zr powders milled for 1h. The NiTi compound dissolved small Mo amounts lower than 4 at%, which were measured by EDS analysis. Moreover, it was identified the existence of an unknown Mo-rich phase in microstructures of the hot-pressed Ni-Ti-Mo alloys.

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

Recently, a new ternary phase was discovered in the Ti-Si-B system, located near the Ti6Si2B composition. The present study concerns the preparation of titanium alloys that contain such phase mixed with α-titanium and other intermetallic phases. High-purity powders were initially processed in a planetary ball-mill under argon atmosphere with Ti-18Si-6B and Ti-7.5Si-22.5B at. (%) initial compositions. Variation of parameters such as rotary speed, time, and ball diameters were adopted. The as-milled powders were pressureless sintered and hot pressed. Both the as-milled and sintered materials were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometry. Sintered samples have presented equilibrium structures formed mainly by the α-Ti+Ti6Si2B+Ti5Si3+TiB phases. Silicon and boron peaks disappear throughout the milling processes, as observed in the powder diffraction data. Furthermore, an iron contamination of up to 10 at. (%) is measured by X-ray spectroscopy analysis on some regions of the sintered samples. Density, hardness and tribological results for these two compositions are also presented here.

The effect of Cu addition and milling contaminations on the microstructure evolution of ball milled Al-Pb alloy during sintering

Al-10 wt.%Pb and Al-10 wt.%Pb-x wt.%Cu (x = 0-7.0) bulk alloys were prepared by sintering the mechanically alloyed powders at various temperatures. The microstructure changes of the as consolidated powders in the course of sintering were analyzed by differential scanning calorimetry, scanning electron microscopy, X-ray diffraction analysis and transmission electron microscopy. It has been found that, with respect to the Al-10 wt.%Pb-x wt.%Cu alloy, CuAl2 and Cu9Al4 phases formed in the milling process, and the amount of CuAl2 phase increased while the Cu9Al4 phase disappeared gradually in the sintering process. In both Al-10 wt.%Pb and Al-10 wt.%Pb-x wt.%Cu alloys, the sintering process results in the coarsening of Pb phase and the growth rate of Pb phase fulfills the Lifshitz-Slyozov-Wagner equation even though the size of the Pb phase was in nanometer range. The Pb particle exhibits cuboctahedral morphology and has a cubic to cubic orientation relationship with the Al matrix. The addition of Cu strongly depressed the growth rate of Pb. Contamination induced by milling has apparent influence on the microstructure of the sintered alloys. Al7Cu2Fe and aluminium oxide phases were identified in the sintered alloys. The cuboctahedral morphology of Pb particles was broken up by the presence of the oxide phase. (c) 2006 Elsevier B.V. All rights reserved.

Effect of solute segregation on the strength of nanocrystalline alloys:Inverse Hall-Petch relation

We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe90Ni10, Fe85Al4Si11, Ni99Fe1 and Ni90Fe10 powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T < 0.4 Tm) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0 at.% impurities).

Species Fractionation In Atomic Chains From Mechanically Stretched Alloys.

Bettini et al (2006 Nat. Nanotechnol. 1 182-5) reported the first experimental realization of linear atomic chains (LACs) composed of different atoms (Au and Ag). The different contents of Au and Ag were observed in the chains from what was found in the bulk alloys, which raises the question of what the wire composition is, if it is in equilibrium with a bulk alloy. In this work we address the thermodynamic driving force for species fractionation in LACs under tension, and we present the density-functional theory results for Ag-Au chain alloys. A pronounced stabilization of the wires with an alternating Ag-Au sequence is observed, which could be behind the experimentally observed Au enrichment in LACs from alloys with high Ag content.

Effect of casting atmosphere on the shear bond strength of a ceramic to Ni-Cr and Ni-Cr-Be alloys

The success of metal-ceramic restorations depends on an optimal bond between metal and ceramic. This study evaluated the effect of 3 casting atmospheres on the metal-ceramic bond strength (MCBS) of 2 Ni-Cr alloys, with beryllium (Fit Cast V) and without beryllium (Fit Cast SB). Sixty acrylic resin patterns (8 mm long and 5 mm diameter) were obtained using a fluorocarbon resin matrix. Wax was used to refine the surface of acrylic resin patterns that were invested and cast in an induction casting machine under normal, vacuum, and argon atmospheres at a temperature of 1340ºC. The castings were divested manually and airborne-particle abraded with 100-µm aluminum-oxide. Ten castings were obtained for each group. The IPS Classic V ceramic was applied (2 mm high and 5 mm diameter). The shear bond strength was tested in a mechanical testing machine with a crosshead speed of 2.0 mm/min. The MCBS data (MPa) were subjected to 2-way analysis of variance (α=0.05). There was no statistically significant difference (p>0.05) between the alloys or among the casting atmospheres. Within the limitations of this study, it may be concluded that the presence of beryllium and the casting atmosphere did not interfere in the MCBS of the evaluated metal-ceramic combinations

Ethanol electrooxidation on Pt-Sn and Pt-Sn-W bulk alloys

Ethanol oxidation has been studied on Pt-Sn and Pt-Sn-W electrodes prepared in an arc-melting furnace. Different electrochemical techniques like cyclic voltammetry and chronoamperometry were used to evaluate the catalytic activity of these materials. The electro-oxidation process was also investigated by in situ infrared reflectance spectroscopy in order to determine adsorbed intermediates and reaction products. Experimental results indicated that Pt-Sn and Pt-Sn-W alloys are able to oxidize ethanol mainly to acetaldehyde and acetic acid. Adsorbed CO was also detected, demonstrating the viability of splitting the C-C bond in the ethanol molecule during the oxidation process. The adsorbed CO was further oxidized to CO2.This reaction product was clearly detected by SNIFTIRS. Pt-Sn-W catalyst showed a better electrochemical performance than Pt-Sn that, in it turn, is better than Pt-alone.

Evaluation of Ti(3)Si Phase Stability from Heat-Treated, Rapidly Solidified Ti-Si Alloys

Ti-base alloys containing significant amounts of silicon have been considered for high temperature structural applications. Thus, information concerning phase stability on the Ti-Si system is fundamental and there are not many investigations covering the phase stability of the Ti(3)Si phase, specially its dependence on oxygen/nitrogen contamination. In this work the stability of this phase has been evaluated through heat-treatment of rapidly solidified Ti-rich Ti-Si alloys at 700 A degrees C and 1000 A degrees C. The rapidly solidified splats presented nanometric scale microstructures which facilitated the attainment of equilibrium conditions. The destabilization of Ti(3)Si due to oxygen/nitrogen contamination has been noted.

Anomalous lattice parameter of magnetic semiconductor alloys

The addition of transition metals to III-V semiconductors radically changes their electronic, magnetic, and structural properties. We show by ab initio calculations that in contrast to the conventional semiconductor alloys, the lattice parameter in magnetic semiconductor alloys, including those with diluted concentration, strongly deviates from Vegard's law. We find a direct correlation between the magnetic moment and the anion-transition metal bond lengths and derive a simple and general formula that determines the lattice parameter of a particular magnetic semiconductor by considering both the composition and magnetic moment. This dependence can explain some experimentally observed anomalies and stimulate other kind of investigations.

Antilocalization of hole carriers in Pb(1-x)Eu(x)Te alloys in the metallic regime

Magnetoresistance measurements in p-type Pb(1-x)Eu(x)Te alloys, for x varying from 0% up to 5%, have been used to investigate localization and antilocalization effects. These are attributed to both the spin-orbit scattering and to the large Zeeman splitting present in these alloys due to the large values of the effective Lande g factor. The magnetoresistance curves are analyzed using the model of Fukuyama and Hoshino, which takes into account the spin-orbit and Zeeman scattering mechanisms. The spin-orbit scattering time is found to be independent of the temperature, while the inelastic-scattering time increases with decreasing temperature suggesting the electron-phonon interaction as the main scattering mechanism.

Accurate band gaps of AlGaN, InGaN, and AlInN alloys calculations based on LDA-1/2 approach

We present parameter-free calculations of electronic properties of InGaN, InAlN, and AlGaN alloys. The calculations are based on a generalized quasichemical approach, to account for disorder and composition effects, and first-principles calculations within the density functional theory with the LDA-1/2 approach, to accurately determine the band gaps. We provide precise results for AlGaN, InGaN, and AlInN band gaps for the entire range of compositions, and their respective bowing parameters. (C) 2011 American Institute of Physics. [doi:10.1063/1.3576570]

Magnetostriction of polycrystalline Fe-Ge alloys

The longitudinal and transverse magnetostriction and microstructure of polycrystalline Fe(100-x)Ge(x) (x= 8, 12, 15, 20) alloys were investigated in order to correlate the magnetostriction with microstructure. In order to obtain different microstructures in the Fe(100-x)Ge(x) alloys, the samples were annealed at 600 degrees C during 2 h and at 1150 degrees C for half hour and then quenched in cold water. For Ge concentrations lower than 14 at.%, the longitudinal magnetostriction is positive and increases positively up to 22 ppm at 12 at.% Ge then decreases and vanishes at about 14 at.% Ge. For further Ge concentration increase the longitudinal magnetostriction is negative and reaches -30 ppm for Fe(80)Ge(20). This behavior, that is very similar to that reported for Fe-Si alloys, is explained by the structural changes caused by different thermal histories of the alloys. (C) 2008 Published by Elsevier B. V.

Production, microstructural characterization and mechanical properties of as-cast Ti-10Mo-xNb alloys

Beta titanium (Ti) alloys are one of the most promising groups of Ti alloys for biomedical applications. This work presents the production, microstructural characterization, and mechanical properties of as-cast Ti-10Mo-xNb (x = 0, 3, 6, 9, 20, and 30) alloys. They were produced via arc melting and characterized by scanning electron microscopy and X-ray diffraction. The density of each alloy was measured by the Archimedes method and the mechanical characterization was carried out by using the Vickers microhardness test and Young`s modulus measurements. The results show a near beta microstructure for niobium (Nb) contents lower or equal to 9 wt% while beta single-phase microstructure was obtained for higher Nb additions. The microhardness values decreased with the increase of Nb content in the alloys. The elastic modulus values of Ti-10Mo-3Nb and Ti-10Mo-20Nb alloys were lower than those of cp Ti and Ti-6Al-4V.

Microstructural characterization of as-cast Cr-B alloys

Accurate knowledge of several Me-B (Me - Metal) phase diagrams are important to evaluate higher order systems such as Me-Si-B ternaries. This work presents results of microstructural characterization of as-cast Cr-B alloys which are significant to assess the liquid compositions associated to most of the invariant reactions of this system. Alloys of different compositions were prepared by arc melting pure Cr and B pressed powder mixtures under argon atmosphere in a water-cooled copper crucible with non-consumable tungsten electrode and titanium getter. The phases were identified by scanning electron microscopy (SEM), using back-scattered electron (BSE) image mode and X-ray diffraction (XRD). In general, a good agreement was found between our data and those from the currently accepted Cr-B phase diagram. (c) 2006 Elsevier Inc. All rights reserved.