Precipitation kinetics in metallic alloys

For structural applications requiring both mechanical strength and capacity for plastic deformation or resistance to fracture, metal alloys are essential. The nature of their bonds favors plastic deformation, and hence ductility, resistance to brittle fracture, energy absorption capacity, toughness and so on. However, preserving these properties while ensuring high yield strength requires "dotting" the simple crystallographic structure of pure metals with defects. These may be simply crystallographic, such as dislocations, but it is very effective to disperse on a very fine scale (of the nanometer order of magnitude) one or more crystallographic phases, known as precipitates, formed from the alloying elements during heat treatment. These phases form obstacles to the movement of dislocations, and help control the alloy's yield strength. Their presence also affects all the alloy's other properties: plastic deformation, of course (work-hardening, ductility, toughness, fatigue strength, formability), as well as corrosion resistance, conductivity, irradiation behavior and more.

A number of points need to be considered to achieve the formation of these phases in the desired form. At the nanoscale, the formation of one phase within another requires the creation of a large amount of surface energy, which often makes germination very difficult. Matter finds non-equilibrium ways of lowering the germination barrier, resulting in many so-called metastable states. Precipitating a secondary phase usually requires the diffusion of additive elements, which controls the kinetics. In alloys with several addition elements, these can diffuse at very different rates, and precipitation paths can pass through transient states. When structural defects, such as grain boundaries or dislocations, are present, they strongly modify the characteristics of the precipitates formed.

This article describes the various steps controlling the precipitation kinetics of secondary phases in metals. After a brief description of the characterization and modeling techniques used to characterize this phenomenon, it lists the different types of precipitation kinetics, starting with the simplest, and progressively introducing levels of increasing complexity to explain the effects mentioned in the previous paragraph.