Aluminium precipitation hardening

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. 

Fig. 1.10 Grain structure of a wrought high-strength precipitation-hardening aluminium alloy showing potential crack growth paths...

Fig. I.I2 Curves showing the relationship between strength, stress-corrosion susceptibility and heat treatment for a high-strength precipitation-hardening aluminium alloy...

Other more highly alloyed types, of which a typical example is given in Table 3.11, have the designation of precipitation hardening martensitic. Relative to the simple 13% chromium types they have a substantial nickel content and low carbon with additions from molybdenum, copper, aluminium, titanium and niobium. These offer improved corrosion resistance, strength, toughness, weldability and fabrication properties, but not always together. 

Quenching (front the solid state). Metastable alloys have been very familiar to metallurgists for a long time now. Several alloys employed in everyday applications contain metastable phases. Typical examples are quenched steels and precipitation hardened aluminium alloys. Until the 1960s, metastable alloys were always obtained by quenching (rapid cooling) from the solid state. 

Strengthening of materials by fine particles is frequently obtained by precipitation hardening. This process will now be explained, using the example of the alloy system aluminium-copper. 

Figure 6.46(a) shows part of the phase diagram of the Al-Cu system. One prerequisite for precipitation hardening is the existence of a two-phase region where the matrix phase (in the example, aluminium with copper in solid solution) is in equilibrium with the precipitation phase (a copper-rich phase in the example), a so-called miscibility gap (see section C.3). 

Precipitation-hardened alloys can be very strong, but their service temperature is limited. For example, long-time application of precipitation-hardened aluminium alloys at temperatures above 200°C is impossible due to excessive coarsening of the precipitates. To use high-strength materials at high temperatures, another method of particle strengthening can be achieved by... 

The yield strength of an aluminium-copper alloy is to be increased by precipitation hardening by Ai po,2 = 600 MPa. 

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... 

Figure 5.11 (Crisp Wilson, 1974b) shows the time-dependent variation of the concentration of soluble ions in setting and hardening cements. Note that the concentrations of aluminium, calcium and fluoride rise to maxima as they are released from the glass. After the maximum is reached the concentration of soluble ions decreases as they are precipitated. Note that this process is much more rapid for calcium than for aluminium and the sharp decline in soluble calcium corresponds to gelation. This indication is supported by information from infrared spectroscopy which showed that gelation (initial set) was caused by the precipitation of calcium polyacrylate. This finding was later confirmed by Nicholson et al. (1988b) who, using Fourier transform infrared spectroscopy (FTIR), found that calcium polyacrylate could be detected in the cement paste within one minute of mixing the cement. There was no evidence for the formation of any aluminium polyacrylate within nine minutes and substantial amounts are not formed for about one hour (Crisp et al, 1974).

In the subsequent hardening phase, precipitation and hydration continue. The set cement consists, essentially, of partly-reacted glass particles embedded in an aluminium phosphate gel. The morphology of the filler particles is one where a glass core is sheathed by silica gel. 

ASTRINGENTS precipitate proteins and are used in lotions to harden and protect skin where there are minor abrasions. They can also be used in lozenges, mouthwashes, eye-drops and antiperspirants. Examples include zinc oxide, and salts of aluminium (aluminium acetate, aluminium hydroxide). 

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