Precipitation hardening of Al–Ag alloys

7.2.2 Precipitation hardening of Al–Ag alloys

Investigations using X-ray diffraction and electron microscopy have shown the existence of three distinct stages in the age-hardening process, which may be summarized: silver-rich clusters →

392 Physical Metallurgy and Advanced Materials

As quenched

Figure 7.5 Small-angle scattering of CuKα radiation by polycrystalline Al–Ag. (a) After quenching from 520 ◦

C (after Guinier and Walker, 1953). (b) The change in ring intensity and ring radius on ageing at 120 ◦

C (after Smallman and Westmacott, unpublished). (c) After ageing at 140 ◦

C for 10 days (after Guinier and Walker, 1953).

intermediate hexagonal γ ′ → equilibrium hexagonal γ. The hardening is associated with the first two stages in which the precipitate is coherent and partially coherent with the matrix, respectively.

During the quench and in the early stages of ageing, silver atoms cluster into small spherical aggregates and a typical small-angle X-ray picture of this stage, shown in Figure 7.5a, has a diffuse ring surrounding the trace of the direct beam. The absence of intensity in the center of the ring (i.e. at (0 0 0)) is attributed to the fact that clustering takes place so rapidly that there is left a shell-like region surrounding each cluster which is low in silver content. On ageing, the clusters grow in size and decrease in number, and this is characterized by the X-ray pattern showing a gradual decrease in ring diameter. The concentration and size of clusters can be followed very accurately by measuring the intensity distribution across the ring as a function of ageing time. This intensity may be represented (see Chapter 4) by an equation of the form:

1 / 3λ 2 )] 2 (7.1) and for values of ε greater than that corresponding to the maximum intensity, the contribution of

l(ε) = Mn 2 [exp(

−2π 2 R 2 ε 2 / 3λ 2 )

2 R 2 ε − exp(−2π 2

the second term, which represents the denuded region surrounding the cluster, can be neglected.

Mechanical properties II – Strengthening and toughening 393

0.1 ␮ (a)

0.5 ␮ (b)

Figure 7.6 Electron micrographs from Al–Ag alloy: (a) aged 5 hours at 160 ◦

C showing spherical zones and (b) aged 5 days at 160 ◦

C showing γ ′ precipitate (after Nicholson, Thomas and Nutting, 1958–9).

Figure 7.5b shows the variation in the X-ray intensity, scattered at small angles (SAS) with cluster growth, on ageing an aluminum–silver alloy at 120 ◦

C. An analysis of this intensity distribution, using equation (7.1), indicates that the size of the zones increases from 2 to 5 nm in just a few hours at 120 ◦

C. These zones may, of course, be seen in the electron microscope and Figure 7.6a is an electron micrograph showing spherical zones in an aluminum–silver alloy aged for 5 hours at 160 ◦ C; the diameter of the zones is about 10 nm, in good agreement with that deduced by X-ray analysis. The zone shape is dependent upon the relative diameters of solute and solvent atoms. Thus, solute atoms such as silver and zinc which have atomic sizes similar to aluminum give rise to spherical zones, whereas solute atoms such as copper which have a high misfit in the solvent lattice form plate-like zones.

With prolonged annealing, the formation and growth of platelets of a new phase, γ ′ , occur. This is characterized by the appearance in the X-ray pattern of short streaks passing through the trace of the direct beam (Figure 7.5c). The γ ′ platelet lies parallel to the {1 1 1} planes of the matrix and its structure has lattice parameters very close to that of aluminum. However, the structure is hexagonal and, consequently, the precipitates are easily recognizable in the electron microscope by the stacking- fault contrast within them, as shown in Figure 7.6b. Clearly, these precipitates are never fully coherent with the matrix, but nevertheless, in this alloy system, where the zones are spherical and have little or

394 Physical Metallurgy and Advanced Materials no coherency strain associated with them, and where no coherent intermediate precipitate is formed,

the partially coherent γ ′ precipitates do provide a greater resistance to dislocation movement than zones and a second stage of hardening results.

The same principles apply to the constitutionally more complex ternary and quaternary alloys as to the binary alloys. Spherical zones are found in aluminum–magnesium–zinc alloys as in aluminum– zinc, although the magnesium atom is some 12% larger than the aluminum atom. The intermediate precipitate forms on the {1 1 1} Al planes and is partially coherent with the matrix, with little or no strain field associated with it. Hence, the strength of the alloy is due purely to dispersion hardening and the alloy softens as the precipitate becomes coarser. In nickel-based alloys the hardening phase

is the ordered γ ′ -Ni 3 Al; this γ ′ is an equilibrium phase in the Ni–Al and Ni–Cr–Al systems and

a metastable phase in Ni–Ti and Ni–Cr–Ti. These systems form the basis of the ‘superalloys’ (see Chapter 8), which owe their properties to the close matching of the γ ′ and the fcc matrix. The two phases have very similar lattice parameters (∼ < 0.25%, depending on composition) and the coherency

(interfacial energy γ 1 ≈ 10–20 mJ m −2 ) confers a very low coarsening rate on the precipitate, so that the alloy overages extremely slowly, even at 0.7T m .