Supersaturated alloys

C. W. White, Segregation, Solute Trapping, and Supersaturated Alloys... 

Most systems used in material science are nonequilibrium ones aging (supersaturated) alloys dissociate by initiation and coarsening of decay products. Grains start growing in nano- and polycrystaUine materials, amorphous alloys crystallize, interdiffusion takes place in protective coatings and powder alloys, metals oxidize in the atmosphere irreversibly, and so on. All materials listed above are considered to be either metastable or absolutely unstable ones and it is just a matter of the time period required for relaxation to equilibrium, or, more commonly, to a less nonequilibrium state. The production of those materials following the chemical reactions, thermal treatment or mechanical operation is accompanied thus, by irreversible nonquasistatic processes. 

The precipitation develops in nanomettic volumes, which can be reahzed either in nanomettic isolated particles or in small spherical regions (of radius R) around nucleation sites in case of simultaneous nucleation at many sites such as in a highly imperfect supersaturated alloy or fast multiple homogeneous nucleation in bulk metallic glasses. Even if we assume that the formation of a new phase has a symmetric form, in principle, different possibUities must be considered. They are represented in Figure 13.10 a a + 1, a a" + 2, Q O " + 1 + 2. The detailed analysis of competitive nucleation is presented in [58, 59, 66]. 

The supersaturated alloys formed by laser irradiation indicate clearly that the solidification process occurs far from thermodyna-... 

Most of the work has been done in silicon, in a comparison [ 23 ] with metals one must note that usually interface velocities are higher due to the greater thermal conductivity. Supersaturated alloys have been obtained by ion implantation in combination with electron or laser beam pulsed heating. In some cases the coupling between thermal and matter transport, i.e. the Soret effect, has been evidenced [24] Precipitation in the liquid phase has been shown in Sb-implanted A1 system together with the measurement of submicrosecond nucleation times. Amorphous phase formation requires usually alloys. 

Precipitation Heat Treatment. The supersaturated solution produced by the quench from the solution temperature is unstable, and the alloys tend to approach equiUbrium by precipitation of solute. Because the activation energies required to form equiUbrium precipitate phases are higher than those to form metastable phases, the soHd solution decomposes to form G-P zones at room temperature (natural aging). Metastable precursors to the equihbrium phases are formed at the temperatures employed for commercial precipitation heat treatments (artificial aging). 

If small specimens are prepared in which the austenite can be cooled to 250—500°C sufficiendy rapidly to avoid the above microconstituents, and transformed at temperatures in this range, the formation of a completely different phase, a bcc a-phase supersaturated with carbon and containing small cementite particles (bainite), which is both strong and tough, occurs. Bainite is rarely found in plain carbon steels, but it can be obtained in commercial practice by judicious alloying and is increasing in importance. 

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. 

Aluminium and magnesium melt at just over 900 K. Room temperature is 0.3 T and 100°C is 0.4 T, . Substantial diffusion can take place in these alloys if they are used for long periods at temperatures approaching 80-100°C. Several processes can occur to reduce the yield strength loss of solutes from supersaturated solid solution, overageing of precipitates and recrystallisation of cold-worked microstructures. 

Application of Eqs. (21)-(27) to the calculations of the nucleation rates J for various alloy models revealed a number of interesting results, in particular, sharp dependence of J and embryo characteristics on the supersaturation, temperature, interaction radius, etc. These results will be described elsewhere. 

In the case of the aluminium-magnesium system, most commercial alloys are usually supersaturated, so that elevated service temperatures and inexpert heat treatment are inadvisable, since any resultant grain boundary precipitation may induce susceptibility to intercrystalline attack. The extent of this susceptibility may be approximately deduced from the continuity of... 

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