Duplex ageing

7.2.5 Duplex ageing

In non-ferrous heat treatment there is considerable interest in double (or duplex) ageing treatments to obtain the best microstructure consistent with optimum properties. It is now realized that it is unlikely that the optimum properties will be produced in alloys of the precipitation-hardening type by a single quench and ageing treatment. For example, while the interior of grains may develop an acceptable precipitate size and density, in the neighborhood of efficient vacancy sinks, such as grain boundaries,

a precipitate-free zone (PFZ) is formed which is often associated with overageing in the boundary itself. This heterogeneous structure gives rise to poor properties, particularly under stress corrosion conditions.

Duplex ageing treatments have been used to overcome this difficulty. In Al–Zn–Mg, for example, it was found that storage at room temperature before heating to the ageing temperature leads to the formation of finer precipitate structure and better properties. This is just one special example of two-step or multiple ageing treatments which have commercial advantages and have been found to

be applicable to several alloys. Duplex ageing gives better competitive mechanical properties in Al alloys (e.g. Al–Zn–Mg alloys) with much enhanced corrosion resistance, since the grain boundary zone is removed. It is possible to obtain strengths of 267–308 MN/m −2 in Mg–Zn–Mn alloys, which have very good strength/weight ratio applications, and nickel alloys also develop better properties with multiple ageing treatments.

The basic idea of all heat treatments is to ‘seed’ a uniform distribution of stable nuclei at the low temperature which can then be grown to optimum size at the higher temperature. In most alloys, there is a critical temperature T c above which homogeneous nucleation of precipitate does not take place, and in some instances has been identified with the GP zone solvus. On ageing above T c there is a certain critical zone size above which the zones are able to act as nuclei for precipitates and below which the zones dissolve.

In general, the ageing behavior of Al–Zn–Mg alloys can be divided into three classes, which can

be defined by the temperature ranges involved:

1. Alloys quenched and aged above the GP zone solvus (i.e. the temperature above which the zones dissolve, which is above ∼155 ◦

C in a typical Al–Zn–Mg alloy). Then, since no GP zones are ever formed during heat treatment, there are no easy nuclei for subsequent precipitation and a very coarse dispersion of precipitates results, with nucleation principally on dislocations.

2. Alloys quenched and aged below the GP zone solvus. GP zones form continuously and grow to

a size at which they are able to transform to precipitates. The transformation will occur rather more slowly in the grain boundary regions due to the lower vacancy concentration there, but since ageing will always be below the GP zone solvus, no PFZ is formed other than a very small ( ∼30 nm) solute-denuded zone due to precipitation in the grain boundary.

3. Alloys quenched below the GP zone solvus and aged above it (e.g. quenched to room temperature and aged at 180 ◦

C for Al–Zn–Mg). This is the most common practical situation. The final dispersion of precipitates and the PFZ width are controlled by the nucleation treatment below 155 ◦

C, where GP zone size distribution is determined. A long nucleation treatment gives a fine dispersion of precipitates and a narrow PFZ.

It is possible to stabilize GP zones by addition of trace elements. These have the same effect as raising T c , so that alloys are effectively aged below T c . One example is Ag to Al–Zn–Mg, which raises T c from 155 to 185 ◦

C, another is Si to Al–Cu–Mg, another Cu to Al–Mg–Si and yet another Cd or Sn to Al–Cu alloys. It is then possible to get uniform distribution and optimum properties by single ageing, and is an example of achieving by chemistry what can similarly be done with physics during multiple ageing. Whether it is best to alter the chemistry or to change the physics for a given alloy usually depends on other factors (e.g. economics).

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