P. Niessen

Off-eutectic composite solidification and properties in Al-Ni and Al-Co alloys

To investigate the range of “coupled” eutectic growth in Al-Co alloys from 1 to 4 wt% Co and Al-Ni alloys from 5 to 10 wt% Ni directional solidification, using rates from 0.8×10−2 cm/sec to 10.6×10−2 cm/sec, was employed. Both alloy systems exhibited coupled zones skewed towards the hypereutectic compositions. Fully eutectic structures were obtained in the ranges Al-1 wt% Co to Al-3 wt% Co and Al-5.7 wt% Ni to Al-9.2 wt% Ni.The off-eutectic alloys which exhibit a fully eutectic structure behave as reinforcing composite materials, with the tensile strength and microhardness increasing as the volume fraction of the strengthening phase increases.

Halo formation during unidirectional solidification of off-eutectic binary alloys

Halos of the non-faceted aluminum-rich phase have been observed around primary intermetallic dendrites in unidirectionally solidified Al-Al3Ni and Al-Al9Co2 hypereutectic alloys, but no halos were observed in hypoeutectic alloys. Details of the influence of the growth rate on the halo fractions and morphologies have been determined. The model for halo formation from previous nucleation experiments is not satisfactory for the unidirectional growth experiments. A new model is proposed which shows the influence of details of the relevant phase diagram, as well as allowing for the effects of growth rate on the growth undercoolings of the two individual phases and of the coupled growth.

A model of vacancy flux induced segregation to grain-boundaries during cooling

A recently developed method of analysis provided a direct measure of grain-boundary solute enrichment in slowly cooled Zn-Cd alloys as a function of holding temperature. These experimental data are here compared with the change of boundary solute enrichment computed from a model based on the vacancy flux generated uphill diffusion of solute to boundaries. The agreement between experiment and theory is satisfactory. This agreement is considered to be a direct proof that previously reported grain-boundary hardening of slowly cooled dilute alloys is a consequence of non-equilibrium solute adsorption.

Recrystallization in internally oxidized Cu-Ag-Al alloys

The isothermal recrystallization kinetics of rolled Cu-Ag alloys containing up to 0.9 vol % alumina have been followed through hardness measurements; and nucleation rates have been obtained by X-ray back-reflection techniques. Electron microscopic studies have been carried out to characterize the as-deformed structures and the structural changes occurring during the annealing process. The recrystallization of the alloys was either accelerated or retarded as the alumina content increased. This effect has been found to depend both on the dispersion parameters and the degree of deformation. The recrystallization behaviour of these alloys is controlled primarily by the nucleation process. Nucleation occurred after an incubation period and involved the formation of subgrains from the cell structure and their subsequent growth to a viable size. The difficulty of the formation of viable nuclei has been attributed to the particle restraint on sub-boundary migration and to the lack of heavily distorted regions in the defect structure. The rate of nucleation was a function of time and found to depend on the dispersion, initial deformation and the temperature of anneal. The process of grain boundary migration during recrystallization was anisotropic.

The constitution of Cu-Ni-Mg alloys

The location of the monovariant eutectic trough extending from the binary eutectic Cu-Cu2Mg into the Cu-Mg-Ni ternary alloy system has been established for alloys containing up to 25 at.% Ni. Liquidus and solidus temperatures along this eutectic trough were determined by thermal analysis. Microscopic and electron microprobe analyses confirm the existence of a peritectic reaction in the Cu2Mg-Ni2Mg quasibinary system and establish its participation in the ternary system. The foregoing, together with data from earlier literature, allow the equilibrium phase relationships for the entire Cu-Mg-Ni ternary system to be estimated.

An electron microprobe study of internal oxidation in a silver-manganese alloy

A quantitative study of internal oxidation in a silver-10 at. % manganese alloy was performed. This alloy does not exhibit any external oxide layer after oxidation in air at high temperatures and its high manganese content allowed accurate microprobe analysis. The experimental results are in very good agreement with the predictions of Wagners theory and yield values for the diffusion coefficient of manganese in silver consistent with previous determinations. Experiments performed on silver/silver-10 at. % manganese diffusion couples have shown that this is true over a wide range of concentrations.

The effect of silicon on the eutectic grain size of gravity cast Zn-12% Al

For gravity castings made from Zn-12% Al alloys wide variations of impact strength have been observed that could not be explained in terms of casting soundness. Previous unpublished research isolated trace impurities of silicon as the cause of a large increase in eutectic grain size and subsequent decrease in impact strength. In the present study the effect of silicon on the as-cast eutectic grain size and impact strength of Zn-12Al is confirmed. Thermal analysis and interface equilibration experiments indicate that silicon impedes nucleation of the eutecticβ phase. Finding that the presence of silicon lowers the primaryα′/eutectic liquid interfacial energy, it is concluded that silicon, rejected by the pro-eutecticα′phase during primary solidification, poisons the interface causing the difficulty with the nucleation of eutecticβ on the primaryα′ surface leading to the increase in eutectic grain size.

Unidirectional solidification of monovariant CuNiMg eutectic alloys II. Microstructures and properties

Abstract Examination of solidification microstructures showed that the lamellar morphology of the CuCu 2 Mg binary eutectic is maintained over much of the composition range studied when solidification proceeds with a planar solid/liquid interface. The thermal stability of lamellar monovariant alloys was shown by annealing tests to be unaffected by the presence of nickel. The growth characteristics of the alloys were observed to undergo a gradual transition with added nickel, becoming faceted/nonfaceted in alloys containing 20 at%Ni. Microstructural variations as a function of composition and solidification conditions were determined and are discussed in relation to the tendency toward a faceted/non-faceted growth morphology. Results of room temperature compression tests on lamellar samples containing up to 4 at% Ni showed that, with the exception of fracture strength, no significant variation in the composite properties occurs over this range of composition. The fracture strength of monovariant alloys containing as little as 0.5 at% Ni was significantly greater than that of the binary alloy.

The deformation behaviour of a dispersion strengthened superplastic zinc alloy

The deformation behaviour of a new dispersion strengthened superplastic zinc alloy was investigated. A significant long range internal stress was observed at all strain-rates. The activation volume of deformation decreased very rapidly with a decrease in the true effective stress. The maximum strain-rate sensitivity corresponds to a region of change from this high stress dependence of the activation volume to a much lower stress dependence. The observation of a metallographic halo effect shows that apart from dislocation movement, diffusive creep plays a role during superplastic deformation. It is stipulated that both these processes aid boundary sliding which accounts for the largest part of the strain.

Recrystallization studies in solid-solution alloys of Cu-Ag-Al

The isothermal recrystallization kinetics of rolled Cu-Ag alloys containing up to 1.2 at.% Al have been followed metallographically and by hardness measurements and the nucleation rates measured by X-ray back-reflection techniques. Electron-microscopic studies have been carried out to characterize the as-deformed structures and the structural changes occurring during the annealing process. The recrystallization of the alloys was retarded as the aluminium content increased. Nucleation occurred both in the bulk and along the existing grain boundaries and the rates of nucleation varied with time. These effects have been found to be dependent on the deformation structure, alloy composition and the temperature of anneal. The process of grain-boundary migration during recrystallization was very anisotropic and this has been shown to be caused primarily by the nature of the deformation structure.