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.     

Hydrogen trapping and repelling in an Al-6 wt % Zn-2 wt % Mg alloy

Hydrogen trapping in an Al-6 wt % Zn-2 wt % Mg alloy aged up to typical stages in the agehardening 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 Guinier-Preston zones in the alloy do not act as trapping sites but as a repeller for hydrogen, and that precipitate does not trap hydrogen, but the interface between the matrix and precipitate acts as a trapping site for hydrogen. Dislocation has been found to be capable of trapping hydrogen, while trapped hydrogen by the grain boundary has not been observed.     

Micro-yielding in a precipitation hardening Cu 1.81 wt % Be 0.28 wt % Co alloy

The microstrain characteristics of a polycrystalline Cu 1.81 wt % Be 0.28 wt % Co precipitation hardening alloy have been determined for various precipitate conditions. The friction stress derived from measurements of closed hysteresis loops was found to remain constant ( 3 MN m^–2) for all the conditions investigated. In contrast, the microscopic yield stress (MYS) remained constant (18 to 24 MN m^–2) for most ageing conditions, but increased significantly (to 48 to 64 MN m^–2) for conditions associated with a high G.P. zone or precipitate density.     

Microstructural and mechanical stability of Cu-6 wt. % Ag alloy

The microstructural and mechanical stability of Cu-6 wt. % Ag alloy obtained by cold rolling combined with intermediate heat treatments have been investigated. The stress-strain responses and fracture behavior of Cu-6 wt. % Ag alloy were examined and correlated with the microstructural change caused by thermo-mechanical treatments. The deformation bands stabilized by silver precipitates were observed in heavily rolled Cu-6 wt. % Ag alloy. The highly deformed microstructure stabilized by silver filament was observed to be unstable at temperatures above 200 °C. The strength of Cu-6wt.%Ag alloys were found to decrease remarkably if they were heat-treated above 300°C. The fracture surfaces of Cu-Ag two phase alloys showed typical ductile type fracture. The electrical conductivity did not change appreciably up to the aging temperature of 200°C and increased rapidly at temperatures above 300°C. The increase of the conductivity and the decrease of the strength can be associated with the microstructural coarsening of heavily deformed linear band structure. The difference of the UTS and the conductivity between the rolling direction and the direction perpendicular to the rolling direction (on the rolling plane) were found to be relatively small.     

Ion-nitriding of an Fe-19 wt % Cr alloy

The ion-nitriding behaviour of an Fe-18.75wt% Cr alloy was investigated at 803 K under constant plasma conditions. Both a thin surface layer of-Fe₄N and an internal-nitriding layer were observed. The nitride formed in the internal-nitriding layer was found to be CrN, rather than Cr₂N. The hardness of the nitriding layer rises to Hv=1200 due to small CrN precipitates. The growth rate of the internal nitriding layer, in the present alloy is controlled by a nitrogen diffusion process in the matrix metal,-iron. Because such ion-nitriding behaviour is analogous to that of internal-oxidation, the growth rate of nitriding was discussed according to the rate equation to that of internal-oxidation. The nitrogen diffusion in the present alloy is scarcely affected by the CrN precipitates.     

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.     

The effect of direct electric current on precipitation in a bulk Al-4 wt % Cu alloy

The effect of the stress of direct electric current on precipitation in a quenched Al-4.15 wt % Cu alloy is investigated by measurement of the change in electrical resistance of a bulk specimen as a function of current density up to about 3000 A cm^–2 at a given annealing time. A similar effect of d.c. stress is observed at 50, 75 and 100 C. For a lower current density, a constant or slightly increasing resistance is observed, while for a current density higher than a critical value, the resistance decreases with an increase in density. On the assumption that the temperature increase of a specimen due to the Joule heating is proportional to the square of the current density, it is concluded that there exists some effect of d.c. stress per se to prevent the precipitation reaction, aside from the effect of the temperature increase due to the Joule heating, and it is suggested that this retarding effect is related to the sweeping out of quenched-in excess vacancies into grain boundaries by electromigration.     

The growth of germanium precipitates in an Al-4.0 wt % Ge alloy

The growth of germanium precipitates in an Al-4.0 wt % Ge alloy aged at 145, 160 and 190° C has been studied by small-angle X-ray diffraction. The intensity curves show that the precipitates are not spherical. By assuming that the precipitates are long cylindrical rods the experimental and theoretical intensity distributions are in a good agreement. The radius of the rods was determined as a function of ageing time.     

The effect of 0.5 wt.% Ce additions on the electromigration of Sn9Zn BGA solder packages with Au/Ni(P)/Cu and Ag/Cu pads

It has previously been established that Sn-9Zn-0.5Ce alloy possesses mechanical properties superior to those of undoped Sn-9Zn alloy, and is free of the problem of rapid whisker growth. However, no detailed studies have been conducted on the electromigration behavior of Sn-9Zn-0.5Ce alloy. In this research, Sn-9Zn and Sn-9Zn-0.5Ce solder joints with Au/Ni(P)/Cu and Ag/Cu pads were stressed under a current density of 3.1 × 10⁴ A/cm² at room temperature for various periods of time. Due to finer grain sizes, the electromigration effects were more severe in Sn-9Zn-0.5Ce solder joints than in Sn-9Zn solder joints when joint temperature was around 80 °C. In addition, both solder joints (Sn-9Zn and Sn-9Zn-0.5Ce) with Au/Ni(P)/Cu pads possess longer current-stressing lifetimes than those with Ag/Cu pads because Ni is more resistant than Cu to migration driven by electron flow.     

Characterization of microstructural changes in an Al-6.8 wt.% Mg alloy by electrical resistivity measurements

Microstructural changes in an AlMg6.8 alloy after different thermo mechanical (TMT) and sensitization treatments were investigated by electrical resistivity measurements. The electrical resistivity was most affected by the content of Mg solute atoms in the α-Al matrix, due to β-phase precipitation, while contribution of the dislocation density to the resistivity of the AlMg6.8 alloy was less pronounced. The amount and distribution of the β-phase precipitated during sensitization were found to be strongly affected by the microstructure developed under the previously applied TMTs, i.e. by the dislocation density and the primary β-phase particles in the dual (α + β) phase structure. During sensitization of the specimens with a recovered/recrystallized dual (α + β) phase structure, precipitation of randomly distributed, globular β-phase particles occurred. Sensitization of cold deformed and recrystallized single α-Al structures induced β-phase precipitation in the form of a continuous layer along the shear bands/grain boundaries.     

Correlation of grain-boundary precipitates parameters with fracture toughness in an Al–Cu–Mg–Ag alloy subjected to long-term thermal exposure

The effect of thermal exposure on grain-boundary precipitation in Al–Cu–Mg–Ag alloy was studied using quantitative transmission electron microscopy. Grain-boundary precipitate parameters, such as average size, number density and precipitate-free zone width, were measured. The effective diameter of precipitates, number of precipitates per grain-boundary area and area fraction of precipitates on the grain boundary were calculated. These data were applied to a grain-boundary fracture model to calculate grain-boundary fracture strain. The calculated fracture strains, in turn, were used to check the validity of two existing models of fracture toughness, which are based on grain-boundary nucleation of cracks and their propagation through precipitate-free zones. The fracture toughness model of Hornbogen and Graf closely agrees with the experimental results.     

The effect of hot-rolling on chill-cast Al,Al-2 wt % Ni and Al-4 wt % Ni alloys

The effect of hot-rolling on the mechanical properties and microstructure of directionallysolidified hypoeutectic Al-Al₃Ni alloys has been studied. Chill-cast hypoeutectic alloys were produced by casting into pre-heated mild-steel moulds placed on copper chills. The chill-cast Al-2 wt% Ni and Al-4 wt% Ni hypoeutectic alloys can be hot-rolled at 500 C to reductions of greater than 95%. Deformation is achieved by deforming the aluminium-rich dendrites in the rolling direction, followed by interpenetration of the Al₃Ni fibres into the dendrites resulting in a homogeneous microstructure. The variations of room-temperature tensile properties for the chill-cast hypoeutectic alloys were measured as a function of reduction of thickness during hot-rolling. The ultimate tensile strength and yield strength increase during rolling because of increasing Al₃Ni fibre alignment, homogeneous dispersion of the Al₃Ni fibres throughout the Al matrix, and work hardening in the Al matrix. The as-chill-cast alloys have strengths which agree with the composite law of mixtures for a combination of Al dendrites and Al-Al₃Ni eutectic. After hot-rolling, the alloy strengths can be predicted from discontinuous fibre reinforcement theory.     

Effect of Ca additions on microstructure and microhardness of an as-cast Mg-5.0 wt.% Al alloy

The effects of Ca additions (0.5-2.0 wt.%) on the microstructure and the microhardness of an as-cast Mg-5.0 wt.% Al alloy have been investigated. The coarse microstructure of the base alloy can be refined through adding Ca. DSC and TEM results showed that, as Ca additions increased up to 1.5 wt.% Ca, the β-Mg₁₇Al₁₂ phase was completely replaced by a (Al, Mg)₂Ca phase. The Vickers microhardness of the as-cast Mg-Al-Ca alloys increased with increasing Ca content. Tests on the Mg-5.0Al-2.0Ca (wt.%) alloy showed an indentation size effect, which was well described by Meyer's Law.     

Precipitation in an Al-1.78 wt % Hf alloy after rapid solidification

A metallographic study has been made of an Al-1.78 wt % Hf alloy after rapid solidification, and after high temperature annealing of the as-cast alloy. After solidification, Hf stays in supersaturated solid solution with a rather inhomogeneous distribution. On annealing, spheres and dendrites of an intermediate Al₃Hf phase withLI ₂ structure, and perfectly coherent with the matrix, are formed. The spheres form by a continuous, the dendrite by a discontinuous precipitation reaction. After continued annealing theLI ₂ structure is replaced by a laminated structure ofLI ₂ andDO₂₂, whereDO₂₂ is the equilibrium structure of Al₃Hf. The precipitation reactions in the systems Al-Hf and Al-Zr are compared.     

Quasicrystal–crystal structural transformation in Al–5 wt.% Mn alloy

The atomic structure of Al–5 wt.%Mn (Al–5Mn) alloy, prepared by rapid solidification, and pre-annealed at 623 and 773 K for 5 and 1 h, respectively, were characterized by X-ray powder diffraction (XRD) and extended X-ray absorption fine structure (XAFS) techniques. The sample in as-quenched stage was found crystalline, consisting of metastable α-Al (Al–Mn solid solution) and icosahedral quasicrystalline I-Al₆Mn phases. Five hours annealing at 623 K proved thermal stability of both the phases. Pre-annealing at 773 K/1 h on the other hand leads to α-Al phase decomposition and structural transformation of metastable I-Al₆Mn to stable orthorhombic Al₆Mn phase. The EXAFS results indicate that Mn atoms are located preferably on the outer shell of icosahedrons. During the I-Al₆Mn→o-Al₆Mn transformation the total Al atoms coordinating one Mn were found to be constant (∼10). Based on the results, only distance/symmetry changes in atomic arrangement around Mn atoms were suggested.     

The deformational instability of an Al-4% Cu alloy

A dynamic structural instability due to strain affects the deformation texture and work hardening characteristics of an Al-4% Cu alloy. It is hypothesized that this structural instability is composed of at least two parts: (1) that due to a redistribution of solute, (2) that due to a change in distribution of line defects.