Effect of grain size upon flow and fracture in a precipitation-strengthened Ti-8 wt % Al-0.25 wt % Si alloy

The interaction of grain size and precipitation strengthening has been studied in a Ti-8 wt % Al-0.25 wt %Si alloy. Grain sizes varying from 9 to 90m were produced by warm-working and annealing the alloy in the single--phase field. A uniform distribution of the coherent ₂ particles in the matrix was produced by ageing the alloy in the two-phase ( + ₂) field. The yield strength Hall-Petch slopes of the alloys with and without the ₂ precipitates were found to be nearly equal, indicating that the precipitation and grain-boundary strengthening are linearly additive. While specimens containing no precipitates exhibited a high ductility for all grain sizes, the ductility of the specimens with the ₂ particles decreased drastically with increasing grain size. TEM examination of the specimens containing the precipitates revealed a highly planar, localized slip and SEM examination of the fracture surfaces of these specimens revealed a transition in fracture behaviour from highly dimpled to mixed cleavage and intergranular with increasing grain size.     

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.     

Internal friction due to G-P zones in pure Al-16 wt % Ag and Al-16 wt % Ag-0.2 wt % Fe-0.1 wt % Si alloys

A study has been made of an internal friction peak which occurs due to the precipitation of Guinier-Preston zones from two supersaturated Al-Ag alloys. The peak observed was interpreted as being due to relaxation around the zones of different elastic constants from that of the Al matrix. Differences in internal friction characteristics between specimens heated at 100 and 200 °C were attributed to the reversion process occurring around 200 °C in pure alloy. The height of the relaxation peak and the level of internal friction background were found to be highly affected by the presence of Fe and Si impurities in the doped alloy.     

Interface and fracture of carbon fibre reinforced Al-7 wt% Si alloy

The interface structure in an aluminium-7 wt% silicon alloy reinforced with carbon fibres has been investigated using analytical electron microscopy. Crystals of aluminium carbide (Al₄C₃) have been identified in interface regions and their structure and growth are discussed. Mechanical properties of the composite have been measured and fracture behaviour studied using acoustic emission analysis in parallel with microstructural examination. The results indicated that the aluminium carbide interfacial reaction had produced a strong fibre matrix bond, but reduced the fibre strength and embrittled the matrix. Consequently, whole fibre bundles failed in a brittle manner in the longitudinal direction with limited pull-out of individual fibres. The findings are discussed in relation to the method used to manufacture the composite.     

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.     

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 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.     

Studies on carbon fibre reinforced aluminium composite processed using pre-treated carbon fibres

Carbon fibre reinforced Al-12% Si alloy composite has been fabricated by pre-treating the fibres with K₂ZrF₆ followed by molten alloy infiltration and subsequent hot pressing of the preforms. The infiltration conditions were arrived at based on the measurement of tensile strength of the fibres extracted from the preforms. The fibre volume per cent of 20 was found to result in composite tensile strength of about 240 MPa as compared to tensile strength of 100 MPa for the unreinforced matrix. Characterization of the interface revealed the formation of ZrSi₂ and diffusion of potassium and aluminium into the fibre. The interfacial bonding was strong as is evinced by the absence of fibre pull-out on to the fracture surface.     

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.     

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 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.     

The structure and properties of a cold-rolled and annealed Al-0.8 wt % Zr alloy

The effect of annealing at 400? C on the microstructure of a cold-worked Al-0.8 wt % Zr alloy is reported. It is shown that the initially high dislocation density in the cold-rolled material is progressively reduced, although the grains and subgrains were exceptionally resistant to coarsening. Precipitation of the metastable cubic Al₃Zr phase occurred, both discontinuously in the form of fan shaped precipitates and also on the grain boundaries and within the grains as small, nearly spherical particles. The mechanical properties of the alloy at 400? C are consistent with a major dislocation contribution to the overall deformation process, in contrast with most other fine grained materials which are superplastic.     

Chromate conversion coating on Al-0.2 wt.% Fe alloy

SEM and TEM investigations revealed that the chromate coating developed rapidly over the macroscopic alloy surface. The coating thickness increased over the immersion period employed in this study and was about 50 nm and 200 nm for coatings formed after 30 s and 120 s respectively. The coating is composed of chromium compounds with aluminium compounds probably concentrated at the alloy/coating interface.     

Interfacial analysis between Mg matrix and carbon nanotubes in Mg-6 wt.% Al alloy matrix composites reinforced with carbon nanotubes

Magnesium containing 6 wt.% aluminum alloy composites reinforced with carbon nanotubes were fabricated with powder metallurgy based wet-processing. Yield stress and tensile strength were successfully improved by the addition of carbon nanotubes. Field emission-transmission electron microscopy microstructural analysis clarified that needle-like ternary carbides of Al₂MgC₂ were synthesized at some interfaces between magnesium matrix and carbon nanotubes, and the other interfaces were clean without any other materials or defects. Tensile loading transfer from magnesium matrix to carbon nanotubes was effectively strengthened by both the production of Al₂MgC₂ compounds and the clean interface between magnesium matrix and carbon nanotubes.     

Investigation of the annealing kinetics of quenched-in vacancies in the Al-0.1 wt % Mn alloy

Isochronal and isothermal annealing characteristics of Al-0.1 wt % Mn alloy quenched from the temperature range 400 to 600 C are investigated by electrical resistivity measurements. The two recovery stages in the alloy are similar to those found in pure aluminium excepting that the temperatures at which these stages occur in the alloy are higher than those in the pure metal. The isothermal annealing of vacancies in pure aluminium and the alloy quenched from 445 C obey first order kinetics. By comparing the rate constants for annealing of vacancies in pure aluminium and the alloy, the manganese-vacancy binding energy is calculated to be 0.12 eV. Deviation from the first order kinetics is observed in the alloy when the quenching temperature is 500 C. The annealing of vacancies in this case is analysed by computer simulation on the basis of a model where the migration of both divacancies and single vacancies to sinks are considered. This analysis also indicates that the manganese-vacancy binding energy is of the order of 0.1 eV. It is concluded that the value of binding energy obtained by the kinetic method is more reliable than that obtained by the equilibrium method.