Solute hardening

Both sohd-solution hardening and precipitation hardening can be accounted for by internal strains generated by inserting either solute atoms or particles in an elastic matrix (11). The degree of elastic misfit, 5, produced by the difference, Ai , between the lattice parameter, of the pure matrix and a, the lattice parameter of the solute atom is given by... 

The Zn—A1 system permits manipulation of the mechanical properties by suitable heat treatment. The aluminum-rich alpha phase is especially suitable for solution hardening since it can be supersaturated by as much as 30 wt % zinc. Furthermore, both alpha and beta phases can be strengthened by precipitation because of decreasing solute solubiUty with decreasing temperature. 

Elements that can dissolve in copper, such as zinc, tin, and nickel for example, increase annealed strength by varying amounts depending on the element and the quantity in solution. The effect of selected solution hardening elements on tensile properties of annealed copper aUoys is iUustrated by the data in Table 4, where the yield strength is the stress at 0.2% offset strain in a tensile test. 

Zinc, traditionally less expensive than copper, does not too gready impair conductivity and ductihty as it solution hardens copper. 

When other elements dissolve in a metal to form a solid solution they make the metal harder. The solute atoms differ in size, stiffness and charge from the solvent atoms. Because of this the randomly distributed solute atoms interact with dislocations and make it harder for them to move. The theory of solution hardening is rather complicated, but it predicts the following result for the yield strength... 

Of the generic aluminium alloys (see Chapter 1, Table 1.4), the 5000 series derives most of its strength from solution hardening. The Al-Mg phase diagram (Fig. 10.1) shows why at room temperature aluminium can dissolve up to 1.8 wt% magnesium at equilibrium. In practice, Al-Mg alloys can contain as much as 5.5 wt% Mg in solid solution at room temperature - a supersaturation of 5.5 - 1.8 = 3.7 wt%. In order to get this supersaturation the alloy is given the following schedule of heat treatments. 

Solution hardening is not confined to 5000 series aluminium alloys. The other alloy series all have elements dissolved in solid solution and they are all solution strengthened to some degree. But most aluminium alloys owe their strength to fine precipitates of intermetallic compounds, and solution strengthening is not dominant... 

Commercially pure aluminium (1000 series) and the non-heat-treatable aluminium alloys (3000 and 5000 series) are usually work hardened. The work hardening superimposes on any solution hardening, to give considerable extra strength (Table 10.5). 

Bragt R. C., 1967, Chromium Oxide Solid Solution Hardening of Aluminium Oxide, J. Am. Cer. Soc., 50-54. 

Iron-niekcl base superalloys were developed primarily from the stainless steels. In the United Slates, these alloys included 19-9 DL. lb-25-6, and A-286, Luler, higher nickel contents were employed to take advantage of the superior oxidation resistance of nickel and the beneficial effects of y -forming elements. All iron-nickel base superalloys rely on solid solution hardening lo some extent. 

Wrought heat-treatable Al alloys contain Mg, Zn, and Cu on the level of several percent as precipitation and solid solution hardening ingredients. The precipitation process forms nanometer size Guinier Preston (GP) zones that are coherent with the matrix and produce the age-hardening effect... 

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