Barothermal analysis of phase transformations of an Al-15 at % Ni alloy and its structure and magnetic properties

Phase transformations of the 15Ni-85Al alloy have been studied by differential barothermal analysis at temperatures of up to 900°C and pressures of up to ? 100 MPa. We have determined the pressure coefficients of the liquidus and solidus temperatures and the temperatures of solid-state dissolution and precipitation of the intermetallic phase Al₃Ni in the Al matrix. Melting and crystallization in compressed argon have been shown to increase the micropore concentration in the material by about one order of magnitude relative to the as-prepared alloy. Crystallization of the alloy at 100 MPa has a significant effect on its microstructure and the particle size and morphology of the intermetallic phase in the aluminum matrix and increases the unit-cell volumes of the Al and Al₃Ni. We have compared the magnetic properties of the alloy before and after barothermal analysis.     

The occurrence of a new metastable phase in an Al-6.8 at % Zn alloy

Evidence for the occurrence of a hitherto unknown metastable phase ( _m) is found by lattice fringe imaging an Al-6.8 at. % Zn alloy aged at 120° C for about 4 days. Additional information is obtained by the analysis of Moiré fringes related to the precipitates of the _m phase. It is suggested that the novel phase has an h c p structure witha ^hex = 0.274 nm andc ^hex = 0.450 nm as the lattice parameters. The ratioc ^hex/a ^hex = 1.64, which corresponds to an almost ideal h c p structure. The possibility of whether _m precipitates can form directly from large ellipsoidal Guinier-Preston zones or small precipitates of the rhombohedral _R phase is discussed.[/p]     

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis. The plate-like primary silicon crystals grow by the same mechanism as that for germanium dendrites, i.e. the TPRE (twin plane re-entrant edges) mechanism. The spherical primary silicon crystal in sodium treated melts is composed of several pyramidal grains with tops at the centre of the sphere. Many of these grains have a twin relation to each other. The sodium enriched regions are found at the boundaries of these pyramidal silicon grains. The external surfaces of the spherical primary crystals exhibit regular crystal facets. The surface facets are most frequently parallel to {111} plane but there are also some facets parallel to other less densely packed planes such as {100}, {211} and so on.     

Microstructural characterization of a rapidly-solidified Al-12 wt% Si alloy

Optical, electron metallography and X-ray diffraction techniques were employed to characterize the microstructure of a rapidly solidified Al-12 wt% Si alloy. Ribbons produced with the planar flow casting process at a cooling rate of 10⁶ Ks^–1, had a very fine cellular structure of Al and nanosize Si particles which are distributed inside and at the boundaries of these cells. Thinner sections (t<30 m) have apparently experienced higher cooling rates and show, in addition to a cellular structure, a distinct zone which is featureless at optical microscope magnifications.     

Pressure infiltration of Al and Al-12 wt% Si alloy into compacts of SiC and oxidized SiC particles

Oxidizing SiC particles is a common practice in the fabrication of SiC/Al composites through direct mixing of the liquid metal and the ceramic particles. This practice is addressed to reduce the reaction between the two materials. In a previous work it was concluded that oxidizing SiC particles scarcely affected the threshold pressure for infiltration of pure Al into compacts made of those particles. In this work we present a detailed study of this problem for infiltration of pure Al and the eutectic Al-Si alloy. Our results indicate that oxidizing of SiC particles does not affect significantly either the threshold pressure or the infiltration kinetics. This conclusion is equally valid for both pure Al and the eutectic alloy.     

Electron microscopic studies on the effect of Sn addition on the ageing characteristics of Al-0.7% Mg2Si alloy

The effect of trace addition of Sn on the ageing characteristics of Al-0.7% Mg₂Si has been studied by hardness measurements and transmission electron microscopy. The changes observed in the microstructural features have been explained on the basis of the strong interaction existing between Sn atoms and quenched-in vacancies. At ageing temperatures below 200°C Sn addition brings about a ratardation in the kinetics of G.P. zone growth whereas at higher ageing temperatures, it aided the precipitate nucleation and growth. The hardness data agree well with the deductions based on observations made on the thin foils.     

The effect of an addition of molybdenum on the quenched and aged structure of a Ti-1 wt % Si alloy

Titanium-1% silicon alloys with the addition of 0.5 and 1 wt % Mo, when water quenched from the field, contain small regions of the -phase lying between the martensitic crystals indicating that some diffusional changes occur during quenching. On ageing the 1% Mo alloy at 650° C titanium suicide precipitation occurs together with spheroidization of the -phase.Formerly at Imperial College.     

Serrated flow and related microstructures in an Al-8.4 at.% Li alloy

Serrated flow in as-quenched and aged Al-8.4 at.% Li alloys has been investigated between 253 K and 353 K at strain rates ranging from 8.9 × 10^–5 s^–1 to 1.2 × 10^–2 s^–1. Size and volume fraction of precipitates were determined by small angle scattering and transmission electron microscopy. Growth and coarsening of the precipitates induces different trends of critical strain of serrated flow changing with temperature and strain rate. The stress drop of serrations increases to some extent with increasing ageing time, increasing deformation temperature and decreasing strain rate. The volume fraction of precipitates decreases as deformation proceeds. The characteristics of serrated flow are related to the changes in microstructures during deformation.     

On the various stages of spinodal decomposition in an Al-38 at % Zn alloy

Spinodal decomposition of an Al-38 at % Zn alloy in the temperature range 293 to 573 K has been studied by transmission electron microscopy. Microstructural evidences for the existence of various stages of spinodal decomposition, namely, the initial, the corsening and the particulate stages have been established. The morphological characteristics and the different diffusion regimes under which these stages are operative are explained.     

Ti-4.4Al-3.8Mo合金的亚稳相变及其对硬度的影响

对锻态 Ti-4Al-3.8Mo合金分别进行了β单相区和α β两相区(相变点之下45℃)淬火处理,研究了亚稳相变对硬度的影响.结果表明,β单相区淬火处理的样品硬度比α β两相区处理的样品高约10%.β单相区淬火后的相组成为全α'马氏体,自910℃的α β两相区淬火后的相组成为等轴初生α相 α'马氏体 α"马氏体.硬度出现差异的主要原因是后者中存在硬度较低的初生α相和α"马氏体.本文修正了Ti-β稳定元素二元系亚稳相图(示意图),指出在α α'和α α"相区之间存在α α' α"相区,并使用修正后的相图解释了Ti-4.4Al-3.8Mo合金的相组成与硬度的关系.     

Undercooling and supersaturation of alloying elements in rapidly solidified Al-8.5% Fe-1.2% V-1.7% Si alloy

Dispersion-strengthened Al-8.5% Fe-1.2% V-1.7% Si alloy was produced by inert gas atomization and atomized melt deposition processes. Differential scanning calorimetry was used to estimate the extent of undercooling in the alloy powders as a function of powder size and in the atomized melt-deposited alloy as a function of process parameters. The estimated undercooling was found to be a strong function of powder size and processing conditions and varied from 380–200 °C. Alloy powders of diameter greater than 180 jam did not experience any undercooling during solidification. X-ray diffraction analysis was performed to study the dependence of supersaturation of alloying elements and metastable phase formation on the extent of undercooling. When the undercooled alloy was heated to about 400 dgC, formation of Al₁₂(Fe, V)₃Si phase with b c c crystal structure from the supersaturated matrix was observed.     

Hydrogen trapping in an Al-2.1 wt % Li alloy

Hydrogen trapping in an Al-2.1 wt % Li alloy aged up to typical stages in the age-hardening curve, has been studied by measuring the tritium release rate after charging. The distribution of hydrogen in the aged alloy has been studied by tritium electron microautoradiography. It has been found that the coherent precipitate and the incoherent precipitate act as a trapping site for hydrogen, while the semi-coherent precipitate does not trap hydrogen. A dislocation has been found to be capable of trapping hydrogen, while hydrogen trapping by the grain boundary has not been observed.     

Tensile behaviour of two DO3-ordered alloys: Fe3Si and Fe-20 at % Al-5 at % Si

The tensile behaviour, including fracture modes and deformation substructures, of two powder-produced DO₃-ordered alloys having compositions Fe-25 at % Si (Fe₃Si) and Fe-20 at % Al-5 at % Si, has been investigated from room temperature to 800° C. The brittle-to-ductile transition temperature for the Fe₃Si alloy occurred at a temperature between 500 and 550° C, while that of the Fe-20 at % Al-5 at % Si alloy was approximately room temperature. In both alloys fracture occurred by transgranular cleavage at room temperature, with the occurrence of an increasing proportion of intergranular cavitation with increasing temperature. At low strains plastic deformation occurred chiefly by movement of perfect superlattice dislocations which, with increasing strain, dissociated to produce next-nearest-neighbour antiphase boundary trails.     

Grain-boundary segregation in an Al-8 wt % Mg alloy

The grain-boundary segregation of Mg atoms in a high-purity Al-8wt% Mg alloy, water quenched from the solution heat-treatment temperature, has been investigated by energy dispersive X-ray microanalysis in a TEM/STEM electron microscope. Many grain boundaries showed a segregation of Mg atoms to a level 2 to 3 times higher than the alloy composition. In the as-quenched state, Mg was uniformly distributed along the grain boundaries, but formed clusters during ageing at room temperature. Hydrogen bubbles were often associated with these Mg-rich regions. Mg-depleted zones were observed adjacent to most boundaries, but in most cases on only one side of the boundary.     

Development of diamond coated tool and its performance in machining Al-11% Si alloy

An attempt has been made to deposit CVD diamond coating on conventional carbide tool using hot filament CVD process. ISO grade K10 turning inserts with SPGN 120308 geometry were used to deposit diamond coating. This diamond coating well covering the rake surface, cutting edges and flank surfaces could be successfully deposited. The coatings were characterized by SEM, XRD and Raman spectroscopy for coating quality, morphology etc. Performance of diamond coated tool relative to that of uncoated carbide tool was evaluated in turning Al-11% Si alloy under dry environment. The diamond coated tool outperformed the uncoated carbide tool which severely suffered from sizeable built-up edge formation leading not only to escalation of cutting forces but also poorer surface finish. In contrast, the diamond coated tool, owing to chemical inertness of diamond coating towards the work material, did not show any trace of edge built-up even in dry environment and could maintain low level of cutting forces and remarkably improved surface finish. It has been further revealed that success of the diamond coated tool depends primarily on adhesion of the diamond coating with the carbide substrate and this is strongly influenced by the pre-treatment of the carbide substrate surface before coating.     

Effect of matrix precipitation on cellular growth kinetics in an Al-28 at. % Zn alloy

The reported existence of two metastable miscibility gaps in the aluminium-zinc (Al-Zn) system is confirmed by an electron microscopic study of precipitate reversion in an Al-28 at. % Zn alloy. The metastable Guinier-Preston (G.P.) zones are replaced above ～125° C within minutes by a metastable rhombohedrally distorted f c c phase (R-phase). Consequently, the recently published calculations for the growth rate of the cellular reaction in the 100 to 250° C temperature range are revised in accordance with the nature of the continuous precipitate ahead of the advancing cell boundary. The observed growth rates are now shown to agree most satisfactorily with current theories of cellular decomposition as controlled by cell boundary diffusion and influenced by continuous precipitation in the matrix.     

The phase transformations and structure of Cu83.34Pt16.66 alloy

The structure and phase transformation of Cu_83.34Pt_16.66 alloy have been investigated by X-ray diffraction, TEM, differential thermal analysis, and measurements of electrical resistance and hardness. It was found that this alloy underwent a spinodal order decomposition. During heat treatment, superstructure with an f c c structure was formed rapidly and then progressively transformed to the stable ordered phase PtCu₅ with an fcr structure.