Mutsumi Abe

Nucleation of Precipitates in AlZnMg alloys

Abstract Mechanism of nucleation of precipitates in Al-6 wt.% Zn-1.2 wt.% Mg and Al-6 wt.% Zn-3 wt.% Mg alloys has been examined by means of transmission electron microscopy and electrical resistivity measurement. Bimodal nucleation of precipitates has been observed; nucleation takes place homogeneously in the matrix when no excess vacancy is present, and homogeneously on vacancy clusters when vacancies are quenched-in. No evidence has been obtained for G. P. zones to act as nuclei for the precipitates. The critical temperature proposed in the current nucleation theory has been shown to correspond to the temperature at which C-curves for η′ and η phases intersect with each other and not to the G. P. zone solvus temperature. On the basis of the observations, the Lorimer and Nicholson theory has been discussed and it has been shown that there is a difficulty in applying the theory to the AlZnMg system.

Influence of the precipitate-free zone width on the tensile properties of an Al-6 Wt pct Zn-1.2 Wt pct Mg alloy

The mechanical properties of an Al-6 wt pct Zn-1.2 wt pct Mg alloy with various width of precipitate-free zones have been investigated. The width of the precipitate-free zone (PFZ) has been changed by the quench interruption technique without any appreciable change in the size and distribution of precipitates. An important relationship has been observed between the width of the PFZ and the quench-interruption period;i.e., the width of the PFZ increases in proportion to the square root of the holding time at 200°C. From the analysis of stress-strain curves as well as the observation of dislocation arrangements in slightly deformed specimens, the plastic deformation has been found to occur preferentially in the PFZ. The initial stage of deformation is much affected by the change in the width of the PFZ, but in the later stage, the work-hardening rate seems to be almost independent of the PFZ width. Tensile tests show that the ultimate tensile strength and the 0.2 pct proof stress decrease very little with increasing width of the PFZ, while the uniform elongation is practically constant regardless of the reduction in the nonuniform elongation. The work-hardening rate at the initial stage of deformation is found to decrease in proportion to the reciprocal of the PFZ width. This relationship can be explained from the dislocation model for work hardening.

Plastic instability related to grain-boundary failure in an Al-Zn-Mg alloy at low temperatures

The strength and ductility of an Al-6 wt % Zn-1.2 wt % Mg alloy at low temperatures have been investigated by varying the grain size, the amount of grain-boundary precipitate and the nature of the precipitates in the matrix. Pronounced grain-boundary embrittlement has been observed at temperatures below — 100° C in alloys age-hardened by GP zones and especially in coarse-grained alloy specimens, but was not observed in over-aged specimens. From the condition for plastic instability related to the grain-boundary micro-cracking, the temperature and structure dependences of the ultimate tensile properties are expressed by; (σi − σo)/σi=εu/n + C, where σi is the instability stress (true ultimate tensile strength), σ0 the 0.2% proof stress, εu the uniform elongation, n a constant found to be about 1/2 and C a constant related to the work-hardening rate of the matrix.