Strengthening of metallic alloys Microstructure influence on plastic deformation

Metal alloys are widely used in mechanical engineering. Metal structures must often remain dimensionally stable under the stresses of use. It is therefore important to increase the yield strength, in order to improve the structure's performance or to reduce the weight of the various elements for the same performance.

At the same time, processors need metal that can be easily shaped. In addition to yield strength, work hardening must also be controlled.

It is therefore essential to study in detail the physical mechanisms of deformation that enable the best possible adjustment of mechanical characteristics for use. In this article, we limit ourselves to the case where the deformation and use temperature is well below the melting temperature, i.e. diffusion mechanisms are not taken into account.

The main mechanisms of plastic deformation originate in the displacement, under stress, of dislocations, which are linear defects in crystals. The hardening of a metal alloy, i.e. the increase in its yield strength, is the result of obstacles to dislocation movement that do not completely prevent it, in order to avoid unacceptable brittleness. These main obstacles are :

• other dislocations that intercept the sliding plane of mobile dislocations (strain-hardening);

• foreign atoms inserted or substituted in the crystal lattice (solute hardening);

• precipitates of second-phase particles dispersed in the grains (structural hardening);

• grain boundaries and interfaces between the major components of the microstructure.

These mechanisms are general for all metals. Depending on their composition, some alloys may exhibit complementary hardening mechanisms. This is particularly true of multi-phase or duplex steels, which are composed of a deformable phase, generally ferrite, and a harder component such as martensite. Finally, plastic deformation can also occur through maculation or induce the transformation of metastable phases (e.g. austenite in the case of steels).

The action of these obstacles, alone or in combination, leads to a range of hardening mechanisms, the mastery of which has developed as knowledge has become more refined. In the following, we describe these main hardening mechanisms in metal alloys and study their impact on yield strength and work hardening.

These different mechanisms are widely used for steel hardening and the development of new grades combining high hardening with high ductility. Articles