Comparative Study Of The Influence Of Natural Convection On Directional Solidification Of Al-3.5 Wt% Ni And Al-7 Wt% Si Alloys

We present numerical simulations of thermosolutal convection for directional solidification of Al-3.5 wt% Ni and Al-7 wt% Si. Numerical results predict that fragmentation of dendrite arms resulting from dissolution could be favored in Al-7 wt% Si, but not in Al-3.5 wt% Ni. Corresponding experiments are in qualitative agreement with the numerical predictions. Distinguishing the two fragmentation mechanisms, namely dissolution and remelting, is critical during experiments on earth, when fluid flow is dominant. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

The ^(26)Al(p,γ)^(27)Si reaction : stellar origins of galactic ^(26)Al

To explain the ^(26)Mg isotopic anomaly seen in meteorites (^(26)Al daughter) as well as the observation of 1809-keV γ rays in the interstellar medium (live decay of 26Al) one must know, among other things, the destruction rate of ^(26)Al. Properties of states in ^(27)Si just above the ^(26)Al + p mass were investigated to determine the destruction rate of ^(26)Al via the ^(26)Al(p,γ)^(27)Si reaction at astrophysical temperatures.

Twenty micrograms of ^(26)Al were used to produce two types of Al_2O_3 targets by evaporation of the oxide. One was onto a thick platinum backing suitable for (p,γ) work, and the other onto a thin carbon foil for the (^3He,d) reaction.

The ^(26)Al(p,γ)^(27)Si excitation function, obtained using a germanium detector and voltage-ramped target, confirmed known resonances and revealed new ones at 770, 847, 876, 917, and 928 keV. Possible resonances below the lowest observed one at E_p = 286 keV were investigated using the ^(26)Al(^3He,d)^(27)Si proton-transfer reaction. States in 27Si corresponding to 196- and 286-keV proton resonances were observed. A possible resonance at 130 keV (postulated in prior work) was shown to have a strength of wγ less than 0.02 µeV.

By arranging four large Nal detector as a 47π calorimeter, the 196-keV proton resonance, and one at 247 keV, were observed directly, having wγ = 55± 9 and 10 ± 5 µeV, respectively.

Large uncertainties in the reaction rate have been reduced. At novae temperatures, the rate is about 100 times faster than that used in recent model calculations, casting some doubt on novae production of galactic ^(26)Al.

Acidity and catalytic activity of rare earth modified Al/Zr pillared clays

The pillared montmorillonite has been prepared by exchanging Na+ in the interlayer of montmorillonite with Al hydroxy cation followed by calcination. Pillared clays are also prepared after exchanging Na' ions with Ce, La, Sm-ions and then pillarcd with aluminium oxides, The surface characterisation of the prepared catalysts has been done using XRD and surface area measurements. To probe the acidic property of the system, temperature programmed desorption (TPD) of NH, has been done. Toluene alkylation by benzyl chloride has been carried out for the evaluation of catalytic activity. The most active system is found to be mixed Al/Zr pillarcd montrnorillonite.

Ag precipitation and dissolution reactions in the Cu-3 wt.% Al-4 wt.% Ag alloy

The Ag precipitation and dissolution reactions in the Cu-3 wt.% Al-4 wt.% Ag alloy were studied using isothermal and non-isothermal analyses. The activation energy values, obtained for the Ag precipitation reaction indicated that, when the Kissinger, Ozawa and Johnson-MehI-Avrami methods are compared, the Kissinger method is the most appropriate. Although the Johnson-Mehl-Avrami equation often does not fit precipitation data, the energy values obtained for Ag precipitation kinetics are in agreement with what was experimentally observed. For the dissolution reaction of Ag precipitates the activation energy values obtained from the Kissinger and Ozawa methods are higher than that found in the literature for the Ag dissolution in Cu. This discrepancy seems to be related to the fact that the activation energy is influenced by the heating rate. (c) 2006 Elsevier B.V. All rights reserved.

Reverse martensitic transformation in the Cu-10 wt% Al-6 wt%Ag alloy

The reverse martensitic transformation in the Cu-10 wt%Al-6 wt%Ag alloy was studied by classical differential thermal analysis (IDTA), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results indicated that the presence of Ag in the Cu-10%Al alloy is responsible for the separation of the competitive reactions that occur during the reverse martensitic transformation and is also associated to an increase in the disordering degree at high temperatures, when compared with alloys without Ag addition. (c) 2005 Springer Science + Business Media, Inc.

Evaluation of Lubricating Oil Preparation Procedures for the Determination of Al, Ba, Mo, Si and V by High-Resolution Continuum Source FAAS

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

Influence of thermal parameters on the dendritic arm spacing and the microhardness of Al-5.5wt.%Sn alloy directionally solidified

As ligas Al-Sn são amplamente utilizados em aplicações tribológicas. Nesse estudo, análises térmica, microestrutural e dureza (HV) foram realizadas ao longo de um lingote da liga Al-5,5%Sn, obtido por solidificação direcional horizontal transitória. Os principais parâmetros analisados incluem a velocidade de deslocamento da isoterma liquidus (V_L) e a taxa de resfriamento (T_R). Esses parâmetros térmicos desempenham um papel fundamental na formação da microestrutura. A microestrutura dendrítica foi caracterizada através dos espaçamentos dentríticos primários (λ₁), os quais foram determinados, experimentalmente, e correlacionados com V_L, e T_R. O comportamento apresentado pela liga Al- 5,5% Sn, durante a solidificação,é semelhante ao de outras ligas de alumínio, isto é, observa-se rede dendrítica mais grosseira com a diminuição da taxa de resfriamento, indicando que a imiscibilidade entre o alumínio e estanho não tem um efeito significativo sobre o relação entre o espaçamento dendrítico primário e taxa de resfriamento. A dependência da microdureza em V_L, T_R e no λ₁ foi também analisada. Verificaram-se menores valores de HV para maiores T_R. Por outro lado, os valores HV aumentam com valores crescentes de λ₁.

Homogeneity characterization of NBS spectrometric standards IV; Preparation and microprobe characterization of W-20 percent Mo alloy fabricated by powder metallurgical methods

Mode of access: Internet.

Stress corrosion cracking and hydrogen embrittlement of an Al-Zn-Mg-Cu alloy

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an Al-Zn-Mg-Cu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat treat 7175 to high strength and simultaneously to have high SCC resistance. The SCC susceptibility increased with increasing Mg segregation at the grain boundaries. Hydrogen embrittlement (HE) increased with increased hydrogen charging and decreased with increasing ageing time for the same hydrogen charging conditions. Computer simulations were carried out of (a) the Mg grain boundary segregation using the embedded atom method and (b) the effect of Mg and H segregation on the grain boundary strength using a quasi-chemical approach. The simulations showed that (a) Mg grain boundary segregation in Al-Zn-Mg-Cu alloys is spontaneous, (b) Mg segregation decreases the grain boundary strength, and (c) H embrittles the grain boundary more seriously than does Mg. Therefore, the SCC mechanism of Al-Zn-Mg Cu alloys is attributed to the combination of HE and Mg segregation induced grain boundary embrittlement. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.