Aluminium aging

When the phase diagram for an alloy has the shape shown in Fig. 10.3 (a solid solubility that decreases markedly as the temperature falls), then the potential for age (or precipitation) hardening exists. The classic example is the Duralumins, or 2000 series aluminium alloys, which contain about 4% copper. 

Finally, Table 10.4 shows that copper is not the only alloying element that can age-harden aluminium. Magnesium and titanium can be age hardened too, but not as much as aluminium. 

A batch of 7000 series aluminium alloy rivets for an aircraft wing was inadvertently over-aged. What steps can be taken to reclaim this batch of rivets ... 

The effect of thermal aging on polyethylene and isotactic polypropylene have been studied by Konar et al. [49]. They used contact angle, contact angle hysteresis, and XPS to characterize the modified surfaces of the polymers. Hysteresis increased with aging temperature. In the case of polyethylene, thermal aging led to a significant increase in adhesion strength of polyethylene with aluminium, but the increase in the case of polypropylene was much less marked. 

Aluminium coatings exposed as sprayed show remarkable protective properties and in many cases the only evidence of age is the appearance of small hard nodules of aluminium oxide, and these do not appear to be deleterious. However, the dead white of the sprayed surface soon becomes discoloured by dirt and so a paint covering is usual. A chromate inhibitor is not essential and two coats of a simple vinyl-based sealing paint have given remarkable results in the tests of the American Welding Society over periods of 12 and 15 years. 

The discussion so far has been limited to the structure of pure metals, and to the defects which exist in crysteds comprised of atoms of one element only. In fact, of course, pure metals are comparatively rare and all commercial materials contain impurities and, in many cases also, deliberate alloying additions. In the production of commercially pure metals and of alloys, impurities are inevitably introduced into the metal, e.g. manganese, silicon and phosphorus in mild steel, and iron and silicon in aluminium alloys. However, most commercial materials are not even nominally pure metals but are alloys in which deliberate additions of one or more elements have been made, usually to improve some property of the metal examples are the addition of carbon or nickel and chromium to iron to give, respectively, carbon and stainless steels and the addition of copper to aluminium to give a high-strength age-hardenable alloy. 

Fig, 20.54 Curves of strength against the logarithm of the ageing time for a precipitationhardening aluminium alloy aged at different temperatures... 

As the cement ages, absorption of water and loss of aluminium ions ceases (after 7 days). Other species - sodium and fluoride ions and silicic acid - continue to be eluted. The release of fluoride is important, for the glass polyalkenoate cement can be seen as a device for its sustained release. 

Aluminium overload Allergic encephalomyelitis (demyelinating diseases) Ageing... 

The aetiopathogenesis of Alzheimer s disease is evidendy related to an iU-defined and complex interplay of age-related, genetic and environmental neurotoxic fiictors, the most widely mooted of putative agents being aluminium. 

Another feature of meteorites that proves to be important is the calcium-aluminium inclusions (CAIs), which, as the name suggests, show regions of enhanced Ca and Al. These micron- to centimetre-sized particles are some of the oldest objects known and have a similar temperature history. They probably formed at temperatures in the region 1700-2400 K and so are close to the centre line of the solar nebula. Although it is hard to be sure about the origin of these objects, there is agreement on their age based on radioisotope dating. 

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