Effect of post-weld-annealing on the tensile deformation characteristics of laser-welded NiTi thin foil

Laser welding is an important process for fabricating complex components involving NiTi shape memory
alloy. As welding is a thermal process, the amount of heat input and the rate of cooling have significant
impact on the microstructure and hence the resultant characteristics of NiTi. In this study, the effect of
laser welding and post-weld-annealing from 573 K to 1173 K on the thermal phase transformation behaviors,
tensile deformation and micro-hardness characteristics of the laser-welded NiTi thin foils were investigated.
It was found that the as-welded sample exhibited inferior super-elasticity compared to the base
material, and the super-elasticity could be partially restored by annealing at 573 K. On the other hand,
annealing of the weldment above the recrystallization temperature would lower the super-elasticity.

Susceptibility to stress corrosion cracking of NiTi laser weldment in Hanks’ solution

In this study, the susceptibility to stress corrosion cracking (SCC) of laser-welded NiTi wires in Hanks’ solution at 37.5 °C was studied by the slow strain-rate test (SSRT) at open-circuit potential and at different applied anodic potentials. The weldment shows high susceptibility to SCC when the applied potential is near to the pitting potential of the heat-affected zone (HAZ). The pits formed in the HAZ become sites of crack initiation when stress is applied, and cracks propagate in an intergranular mode under the combined effect of corrosion and stress. In contrast, the base-metal is immune to SCC under similar conditions. The increase in susceptibility to SCC in the weldment could be attributed to the poor corrosion resistance in the coarse-grained HAZ.

Effect of post-weld heat-treatment on the oxide film and corrosion behaviour of laser-welded shape memory NiTi wires

Post-weld heat-treatment (PWHT) was applied to NiTi weldments to improve the corrosion behaviour by modifying the microstructure and surface composition. The surface oxide film on the weldments is principally TiO2, together with some Ti, TiO, and Ti2O3. The surface Ti/Ni ratio of the weldments after PWHT is increased. The oxide film formed in Hanks’ solution is thicker on the weldments after PWHT. The pitting resistance of the weldments is increased by PWHT. The galvanic effect in the weldments is very small. The weldment with PWHT at 350 °C shows the best corrosion resistance among other heat-treated weldments in this study.

Preparation and surface modification of submicron YAl2 particles by mixed milling with magnesium for fabricating YAl2p/MgLiAl composites

A new process for the preparation and surface modification of submicron YAl2 intermetallic particles was proposed to control the agglomeration of ultrafine YAl2 particles and interface in the fabrication of YAl2p/MgLiAl composites. The morphological and structural evolution during mechanical milling of YAl2 powders (< 30 μm) with magnesium particles (~ 100 μm) has been characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that YAl2 particles are refined to submicron scale and separately cladded in magnesium coatings after mixed milling with magnesium particles for 20 h. Mechanical and metallurgical bonds have been found in YAl2/Mg interfaces without any interface reactions. Both the refining and mechanical activation efficiencies for YAl2 particles are enhanced, which may be related to the addition of magnesium particles leading to atomic solid solution and playing a role as “dispersion stabilizer”.

Microstructure, properties and application of YAl2 intermetallic compound as particle reinforcements

An yttrium aluminum (YAl2) intermetallic compound ingot was prepared in an induction furnace under vacuum. The microstructure of YAl2 ingot was characterized by optical microscopy, scanning electron microscopy, and X-ray diffraction. The load bearing response of YAl2 intermetallic was investigated and compared with SiC ceramic by indentation combined with optical microscopy and scanning electron microscopy. Additionally, the tensile properties of the Mg–Li matrix composites reinforced with ultrafine YAl2 particles fabricated by planet ball milling were tested. The results show that the intermetallic compound ingot in this experiment is composed of a main face-centered-cubic structure YAl2 phase, a small amount of YAl phase, and minor Y and Al-rich phases. YAl2 intermetallic compound has excellent stability and shows better capability in crack resistance than SiC ceramic. The YAl2 intermetallic compound has better deformation compatibility with the Mg–14Li–3Al matrix than SiC reinforcement with the matrix, which leads to the superior resistance to crack for YAl2p/Mg–14Li–3Al composite compared to SiCp/Mg–14Li–3Al composite.

Aging Behavior of YAl2p/Mg-14Li-1Al Composite

YAl2p/Mg-14Li-1Al composite was made by a stir-casting technique. The aging behavior of the composite was investigated using hardness test, differential scanning calorimetry, and X-ray diffraction. The results show that the microhardness variations of both the matrix alloy and its composite are related to the precipitation and decomposition of θ-MgLi2Al phase. The strengthening of the composite is from the addition of YAl2 particulates and the precipitation in matrix alloy. The former contribution is stable, but the latter is unstable and depends on the aging behavior. The addition of YAl2 particulates delays occurring of the aging behavior of composite.

Studies on grain refinement and alloying additions on the microstructure and mechanical properties of Mg-8Zn-4Al alloy

MAGNESIUM ALLOYS have strong potential for weight reduction in a wide range of technical applications because of their low density compared to other structural metallic materials. Therefore, an extensive growth of magnesium alloys usage in the automobile sector is expected in the coming years to enhance the fuel efficiency through mass reduction. The drawback associated with the use of commercially cheaper Mg-Al based alloys, such as AZ91, AM60 and AM50 are their inferior creep properties above 100ºC due to the presence of discontinuous Mg17A112 phases at the grain boundaries. Although rare earth-based magnesium alloys show better mechanical properties, it is not economically viable to use these alloys in auto industries. Recently, many new Mg-Al based alloy systems have been developed for high temperature applications, which do not contain the Mg17Al12 phase. It has been proved that the addition of a high percentage of zinc (which depends upon the percentage of Al) to binary Mg-Al alloys also ensures the complete removal of the Mg17Al12 phase and hence exhibits superior high temperature properties.ZA84 alloy is one such system, which has 8%Zn in it (Mg-8Zn-4Al-0.2Mn, all are in wt %) and shows superior creep resistance compared to AZ and AM series alloys. These alloys are mostly used in die casting industries. However, there are certain large and heavy components, made up of this alloy by sand castings that show lower mechanical properties because of their coarse microstructure. Moreover, further improvement in their high temperature behaviour through microstructural modification is also an essential task to make this alloy suitable for the replacement of high strength aluminium alloys used in automobile industry. Grain refinement is an effective way to improve the tensile behaviour of engineering alloys. In fact, grain refinement of Mg-Al based alloys is well documented in literature. However, there is no grain refiner commercially available in the market for Mg-Al alloys. It is also reported in the literature that the microstructure of AZ91 alloy is modified through the minor elemental additions such as Sb, Si, Sr, Ca, etc., which enhance its high temperature properties because of the formation of new stable intermetallics. The same strategy can be used with the ZA84 alloy system to improve its high temperature properties further without sacrificing the other properties. The primary objective of the present research work, “Studies on grain refinement and alloying additions on the microstructure and mechanical properties of Mg-8Zn-4Al alloy” is twofold: 1. To investigate the role of individual and combined additions of Sb and Ca on the microstructure and mechanical properties of ZA84 alloy. 2. To synthesis a novel Mg-1wt%Al4C3 master alloy for grain refinement of ZA84 alloy and investigate its effects on mechanical properties.

Influence of Si, Sb and Sr Additions on the Microstructure, Mechanical Properties and Corrosion Behavior of AZ91 Magnesium Alloy

All the exciting work on developing new and better alloys has led older alloys, such as AZ9l , being abandoned by researchers. lt is believed that the full potential of AZ9l in automotive design has not been realized. Whatever works have been carried out on AZ9lalloy to improve its mechanical properties are insufficient in terms of its potential usage in auto industries. Due to the fact that AZ91 offers high room temperature mechanical properties and good castability, still this alloy is a primary choice for the auto component manufactures. Small improvement in its creep properties will have a huge impact in the transportation industries. Hence, in the present work, “Influence of Si, Sb and Sr Additions on the Microstructure, Mechanical Properties and Corrosion Behavior of AZ91 Magnesium Alloy”, an attempt has been made to improve the creep properties of AZ9l alloy through minor alloying elemental additions and to understand its strengthening mechanisms. The effect of alloying additions on the ageing and tensile properties of AZ9l is also studied. In addition to that, role of various intermetallics formed due to the alloying additions on the corrosion properties of AZ9l alloy is investigated.

Preparation and characterization of ordered intermetallic platinum phases for electrocatalytic applications

Ordered intermetallic phases of Pt with several transition metals have been prepared and their electrocatalytic properties studied. In light of these tests it is proposed that these catalysts could be used as electrodes in fuel cells, as they combine an excellent capacity to adsorb organic fuels at the Pt sites with low susceptibility to being poisoned by intermediates and reaction products at the transition-metal sites. An experimental procedure used to obtain the four intermetallic phases Pt-M (M = Mn, Pb, Sb and Sn) is described. The phases thus produced were characterized by X-ray diffraction, scanning electron microscopy with surface analysis by energy-dispersive X-ray spectrometry, scanning tunneling microscopy and X-ray photoelectron spectroscopy. The data thus obtained support the conclusion that the method described here is highly effective for the preparation of Pt-M phases featuring a range of structural and electronic modifications that will allow a useful relation to be established between their physicochemical properties and predicted electrocatalytic activity. (C) 2007 Elsevier Ltd. All rights reserved.

Defeitos pontuais nos compostos intermetálicos ZrNi e Zr/sub 2/Ni estudados por dinâmica molecular

Empregamos a técnica de Dinâmica Molecular para estudar propriedades de defeitos pontuais nos compostos intermetálicos ZrNi e Zr2Ni. Descrevemos as configurações estáveis de defeitos e mecanismos de migração, assim como as energias envolvidas. Os potenciais interatômicos foram derivados do Embedded Atom Model. No intuito de levar em conta a variação de estequiometria causada pela presença de alguns tipos de defeitos em intermetálicos, apresentamos um método numérico que fornece a energia efetiva de formação de defeitos e aplicamos o método ao ZrNi e Zr2Ni. Os resultados mostraram que vacâncias são mais estáveis na sub—rede do Ni, com energia de formação de 0,83 e-0,61 eV em ZrNi e Zr2Ni, respectivamente. Vacâncias de Zr são instáveis em ambos compostos; elas decaem espontaneamente em pares anti—sítio e vacância de Ni. Configurações e energias de formação de intersticiais também foram calculadas e mostraram comportamentos similares. Em ZrNi, a migração de vacâncias ocorre preferencialmente nas direções [025] e [100], com as respectivas energias de migração 0,67 e 0,73 eV, e é um processo essencialmente bidimensional no plano (001). Em Zr,Ni, a migração de vacâncias é unidimensional, ocorrendo na direção [001], com energia de migração de 0,67 eV. Em ambos compostos a presença de defeitos de anti—sítio de Ni diminui a energia de migração da vacância de Ni em até 3 vezes e facilita a movimentação em três dimensões. Mecanismos de anel não são energeticamente eficientes em comparação com saltos diretos. As configurações estáveis de intersticiais em ambos compostos consistem em um átomo de Ni sobre o plano (001) entre dois vizinhos de Zr fora do plano. Intersticiais de Zr são instáveis e tendem a deslocar um átomo de Ni, ocupando seu sítio. Energias de deslocamento foram estudadas através de simulações de irradiação de ambos compostos. Durante o processo de colisão binária, um potencial universal ZBL foi usado para colisões a curta distância. Para distâncias intermediárias usamos um potencial de união arbitrário. Zr mostrou—se mais difícil de ser arrancado de seu sítio do que Ni. Encontramos valores de energia de deslocamento no intervalo de aproximadamente 29 eV até 546 eV. Alguns resultados experimentais são mostrados e apresentam boa concordância com os cálculos.

Interface microstructure of alumina mechanically metallized with Ti brazed to Fe–Ni–Co using different fillers

NASCIMENTO,R.M. et al.Interface microstructure of alumina mechanically metallized with Ti brazed to Fe–Ni–Co using different fillers. Materials Science and Engineering A, v.466, n.1/2, p. 195-200, 2007.

Synthesis and Characterization of Ordered Intermetallic Nanostructured PtSn/C and PtSb/C and Evaluation as Electrodes for Alcohol Oxidation

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Preparation and characterization of ordered intermetallic platinum phases for electrocatalytic applications

Ordered intermetallic phases of Pt with several transition metals have been prepared and their electrocatalytic properties studied. In light of these tests it is proposed that these catalysts could be used as electrodes in fuel cells, as they combine an excellent capacity to adsorb organic fuels at the Pt sites with low susceptibility to being poisoned by intermediates and reaction products at the transition-metal sites. An experimental procedure used to obtain the four intermetallic phases Pt-M (M = Mn, Pb, Sb and Sn) is described. The phases thus produced were characterized by X-ray diffraction, scanning electron microscopy with surface analysis by energy-dispersive X-ray spectrometry, scanning tunneling microscopy and X-ray photoelectron spectroscopy. The data thus obtained support the conclusion that the method described here is highly effective for the preparation of Pt-M phases featuring a range of structural and electronic modifications that will allow a useful relation to be established between their physicochemical properties and predicted electrocatalytic activity. (C) 2007 Elsevier Ltd. All rights reserved.

Hydrogen oxidation on ordered intermetallic electrodes covered with CO

This describes an experimental evaluation of the electrocatalytic activity of the hydrogen oxidation reaction on electrodes of platinum and the ordered intermetallic phases PtSb and PtSn, on which CO has previously been deposited. The experiments were carried out in perchloric acid solution and the analysis based on steady-state polarization curves and Tafel plots derived from chronoamperometric data. Both intermetallics, PtSb and PtSn, performed better than Pt towards the HOR, when their surface was deliberately covered with CO. It is suggested that the intermetallic surfaces have a lower affinity for CO molecules, causing a lower CO coverage on these surfaces, and/or a weaker surface-CO interaction, compared to Pt under the same conditions. (C) 2007 Elsevier B.V. All rights reserved.