Aging of Metals

the change in the mechanical, physical, and chemical properties of metals and alloys resulting from the lack of thermodynamic equilibrium in the original state and the gradual approach of the structure to the equilibrium state under conditions that permit a sufficient diffusion rate for the atoms. Upon rapid cooling from high temperatures (in hardening or after crystallization and hot working), metals and alloys retain either partially or completely the atomic structure characteristic of the high-temperature state. In pure metals, the irregularity of this structure consists in an excess concentration of vacancies (for low temperatures) and the presence of other defects of the crystal structure. In alloys, the disequilibrium of the structure may be related to the retention of phases that are unstable at low temperatures. Of greatest importance is the aging of alloys, which is caused by the decomposition of a supersaturated solid solution. The state of supersaturation in a solid solution arises after the cooling of alloys from high temperatures inasmuch as the solubility of admixtures or of specially introduced alloying elements usually increases with temperature.

There are many alloys for which aging is carried out as a special operation of heat treatment that ensures an aggregate of important mechanical and physical properties. Aging, or precipitation hardening, is the principal method for the strengthening heat treatment of alloys based on Al, Mg, Cu, and Ni. In addition to high strength, alloys may acquire other valuable properties while aging, for example, high coercivity.

At a sufficiently high degree of supersaturation, a solid solution is completely unstable and a separation into layers occurs throughout the body of the solution with the formation first of an inhomogeneous solid solution with a continuously changing composition and then of particles arranged in periodic fashion with a sharp separation of boundaries. Decomposition of this type is described as spinodal and is observed in a number of technically important alloys (Cunife-type alloys for permanent magnets). More common for alloys undergoing aging is the metastable state of the solid solution, the decomposition of which must proceed through the formation and growth of embryos of a new phase. This process of nucleation, however, requires that an energy barrier be overcome. The barrier is lowered significantly upon the formation of coherent particles, that is, particles whose crystal lattice is elastically conjugated with the lattice of the original solid solution. At relatively low temperatures, the decomposition of solid solutions often stops at the stage in which zones are formed. The zones, which are highly dispersed regions enriched in the excess component and which retain the crystal structure of the original solution, were first detected by the diffuse scattering of X-ray beams (Guinier-Preston zones). With the aid of electron microscopy, Guinier-Preston zones have been observed in Al-Ag alloys in the form of spherical particles with diameters of approximately 10 angstroms and in Al-Cu alloys in the form of plates with a thickness of the order of a lattice spacing (less than 10 angstroms). The formation of zones is characteristic of natural aging, which proceeds at room temperatures in the case of Al-base alloys, as well as of low-carbon steel or iron, in which there is a solid solution (ferrite) supersaturated with carbon or nitrogen. In some cases, the zones may be considered as embryos of a separation phase.

The concept of natural aging is contrasted to artificial aging, which in the case of aluminum alloys (the first materials to be hardened by aging) is carried out at elevated temperatures (above 100°C). In the contemporary literature, these terms are frequently replaced, respectively, by the terms “low-temperature aging” and “high-temperature aging.” In view of the differences in the decomposition process in different temperature ranges for various alloys, the optimal set of properties is attained after a complex aging process carried out in a definite sequence and at low and high temperatures.

Two principal mechanisms are distinguished for the decomposition of a supersaturated solution. One is the continuous mechanism, which proceeds by the formation and growth of separate embryos—particles of a phase containing the excess component of the solid solution; the other is the discontinuous (or cellular) mechanism, in which cells or colonies appear and begin growing. The cells or colonies usually consist of the equilibrium phases, namely, the new phase enriched in the excess component and the depleted (equilibrium) solid solution. With the continuous mechanism, the particles are formed throughout the entire volume, and their growth is accompanied by the gradual and continuous depletion of the matrix solid solution. With the discontinuous mechanism, there is movement of the separation boundary between the colonies and the unconverted region of the solid solution. The colonies usually have a platelike structure and form on the grain boundary. Their moving front is a mobile, high-angle boundary with a grain of the original solid solution.

Especially high strength values are obtained in the decomposition of a solid solution whose crystal structure contains a high concentration of defects, for example, dislocations, caused by intense prior cold working. Decomposition processes of solid solutions may also lead to undesirable changes in alloy properties. In low-carbon boiler steel, for example, these changes include era-brittlement and a reduction in ductility; in Armco iron there is often a loss upon magnetic reversal and an increase in coercivity. Some alloys tend to undergo what is called strain aging. Relatively weak cold working, which by itself does not appreciably alter the properties of a material, greatly accelerates demarcation processes between the components of a solid solution. These processes lead first to segregation and then to separation, which occurs near the dislocations. Deformation and aging greatly lessen the resilience and ductility of alloys, which is usually undesirable in materials subjected to deep stamping, for example, the sheet steel used in the manufacture of automotive vehicles. The adverse effects of aging may be reduced considerably by special alloying and heat treatment.