Supersaturated solid solution

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. 

Figure 11.9 shows that the hardness of martensite increases rapidly with carbon content. This, again, is what we would expect. We saw in Chapter 8 that martensite is a supersaturated solid solution of C in Fe. Pure iron at room temperature would be b.c.c., but the supersaturated carbon distorts the lattice. 

Austenitic steels of the 304S15 type are normally heat treated at 1 050°C and cooled at a fairly rapid rate to remove the effects of cold or hot working, and in this state much of the carbon is in supersaturated solid solution. Reheating to temperatures below the solution treatment temperature leads to the formation of chromium-rich MjjCj precipitates predominantly at the grain boundaries with the production of chromium gradients and reduced corrosion resistance as is the case with the martensitic steels. Any attack is... 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Li et al. (2000) have employed nanometer scale analysis in a FEG-TEM operating at 200 kV to distinguish between true GP zones in an Al-Zn-Mg-Cu alloy and GP zone-like defects caused by electron beam irradiation in the TEM. They studied an Al-6.58Zn-2.33Mg-2.40Cu (wt%) alloy, in which it is well known that the decomposition of supersaturated solid solutions takes place via the formation of GP zones, using conventional techniques to produce thin foil specimens of aged material. 

I.M. Lifshitz and V.V. Slyozov Kinetics of Diffuse Decomposition of Supersaturated Solid Solutions. Soviet Physics JETP35, 331 (1959). 

Using specific metal combinations, electrodeposited alloys can be made to exhibit hardening as a result of heat treatment subsequent to deposition. This, it should be noted, causes solid precipitation. When alloys such as Cu-Ag, Cu-Pb, and Cu-Ni are coelectrodeposited within the limits of diffusion currents, equilibrium solutions or supersaturated solid solutions are in evidence, as observed by x-rays. The actual type of deposit can, for instance, be determined by the work value of nucleus formation under the overpotential conditions of the more electronegative metal. When the metals are codeposited at low polarization values, formation of solid solutions or of supersaturated solid solutions results. This is so even when the metals are not mutually soluble in the solid state according to the phase diagram. Codeposition at high polarization values, on the other hand, results, as a rule, in two-phase alloys even with systems capable of forming a continuous series of solid solutions. 

Lifshitz I.M. and Slyozoc V.V. (1961) The kinetics of precipitation from supersaturated solid solutions. /. Phys. Chem. Solids 19, 35-50. 

In the case of Al-Co-Ag, rapid solidification was effective to make a supersaturated solid solution (13). In this system, the magnetization also increased with increase of temperature and the diffraction lines of fee Co appeared upon annealing at 1073K for 3.8ks as shown in Fig. 11 (14). 

In Ni—P electroless deposits, there can be as much as 10% by weight of phosphorus. The amount depends on the added complexing agents and the pH. The Ni—P deposits are fine-grained supersaturated solid solutions, which may be precipitation hardened by heat treatment to form dispersed Ni3P particles in a nickel matrix. 

By a change of temperature or pressure, it is often possible to cross the phase limits of a homogeneous crystal. It supersaturates with respect to one or several of its components, and the supersaturated components eventually precipitate. This is an additive reaction. It occurs either externally at the surfaces, or in the crystal bulk by nucleation and growth. Reactions of this kind from initially homogeneous and supersaturated solid solutions will be discussed in Chapter 12 on phase transformations. Internal reactions in the sense of the present chapter occur after crystal A has been brought into contact with reactant B, and the product AB forms isothermally in the interior of A or B. Point defect fluxes are responsible for the matter transport during internal reactions, and local equilibrium is often established throughout. 

Cocco et al. [54] discuss the preparation of metallic glass, while copper-titanium, aluminum-titanium, and palladium-titanium systems in particular are prepared under a controlled atmosphere with hydrogen and argon. Components of Nb-Ni and Nb-Y have also been described [55]. Amorphous Ni-Ti alloys have been prepared by Schwarz et al. [56], while Ni-Ga, Ni-Ge, Ni-In, and Ni-Sn has been synthesized in supersaturated solid solutions [57]. Fe, Co, Ni-Ta-alloys are described by Lee and Yang [58], while FeSi2 doped with Co or Al for thermoelectric material is also mentioned [59]. 

Mo are single phase, supersaturated solid solutions having an fee structure very similar to that of pure Al. Broad reflection indicative of an amorphous phase appears in deposits containing more than 6.5 atom% Mo. As the Mo content of the deposits is increased, the amount of fee phase in the alloy decreases whereas that of the amorphous phase increases. When the Mo content is more than 10 atom%, the deposits are completely amorphous. As the Mo atom has a smaller lattice volume than Al, the lattice parameter for the deposits decreases with increasing Mo content. Potentiodynamic anodic polarization experiments in deaerated aqueous NaCl revealed that increasing the Mo content for the Al-Mo alloy increases the pitting potential. It appears that the Al-Mo deposits show better corrosion resistance than most other aluminum-transition metal alloys prepared from chloroaluminate ionic liquids. 

---