Abstract
In a previous paper the author suggested that there are three hardening phenomena in Duralumin alloys, namely:
(1) Primary hardening (Natural age-hardening)
(2) Secondary hardening (Natural age-hardening after “Rückbildung”)
(3) Temper-hardening (Artificial age-hardening)
The primary hardening may be explained by the theory of congregation (or aggregation) of solute atoms at certain selected positions on the Aluminium lattice, and the sudden reduction of the hardness on short period tempering at somewhat higher temperatures (“Rückbildung”, “Retrogression, ” “Intermediate softening” or “De-hardening”) may be attributed to the dispersion of the aggregated solute atoms into solid solution, and the subsequent recovery of hardness at room temperature (The secondary hardening) may be attributed to reaggregation of solute atoms by the quite same process as that occurring in primary hardening. The temper hardening takes place on the basis of precipitation of so-called “intermediate” phase or of compounds, but the maximum hardness generally is attained prior to the appearance of lamellar structure under the microscope.
According to the latest investigations by Preston, the formation of Cu-rich thin platelike aggregates are presumed when the Cu-Al alloy is aged at room temperature and there are diminution of aggregates when “Rückbildung” takes place by short period tempering at 200°, but if the tempering is continued for several hours, the thin platelike aggregates are formed again, The linear dimensions of thin platelike aggregates which were formed at room temperature are far more smaller than that of aggregates which were formed at 200°. (about 40 Å for the former, and about 4000 Å for the latter).
In the present paper the author has made a characteristic differentiation between the natural age hardening and the temper-hardening by the experimental evidence, and deduced the theory of aging.
In the above mentioned explanations there are not any differences in the opinions of the present author and that of other authors to presume the formations of thin platelike aggregates which are vary in dimensions at different temperatures (the one formed at room temperature and has smaller dimensions, the other formed at higher temperature and has larger dimension). But the presentt author suggests the opinion that there are differences not only in the dimensions of the two kinds of aggregates, but also in their coaguration of atoms in the thin platelike regions of these respective cases.
As the coaguration of solute atoms in the thin platelike region which was formed by natural ageing is far more weaker, so the sudden dispersion of the atoms would be expected by short period tempering, and this phenomenon corresponds to the “Rückbildung” The thin Plate which was formed by tempering is also an aggregate of solute-atoms, but it requires an activation energy to bring about aggregation, and the “state of aggregation” is much stable than that of the former
In the above explanation, the author deduced the theory of aging by applying somewhat same idea of adsorption theory, because the movements of solute in the, solid solution (i.e. the congregation and the dispersion etc.) are analogous to the case of adsorption.
The relationship between the two forms of aggregations is similar to that of two forms of adsorption, namely; the van der Waals adsorption (the weaker state of adsorption) and the activated adsorption (the stronger state of adsorption). The van der Waals adsorption takes place at low temperature and removed more or less completely at somewhat higher temperatures. The activated adsorption recommences at still higher temperatures, but in a different form, as the stronger state of adsorption
