Mechanical properties of metals.

The properties of a material are altered by coldworking, i.e. deformation at a low temperature relative to its melting point, but not all the properties are improved, for although the tensile strength, yield strength and hardness are increased, the plasticity and general ability to deform decreases. Moreover, the physical properties such as electrical conductivity, density and others are all lowered. Of these many changes in properties, perhaps the most outstanding are those that occur in the mechanical properties; the yield stress of mild steel, for example, may be raised by cold work from 170 up to 1050 MN/m2.

Such changes in mechanical properties are, of course, of interest theoretically, but they are also of great importance in industrial practice. This is because the rate at which the material hardens during deformation influences both the power required and the method of working in the various shaping operations, while the magnitude of the hardness introduced governs the frequency with which the component must be annealed (always an expensive operation) to enable further working to be continued.

Since plastic flow occurs by a dislocation mechanism the fact that work-hardening occurs means that it becomes difficult for dislocations to move as the strain increases. All theories of work-hardening depend on this assumption, and the basic idea of hardening, put forward by Taylor in 1934, is that some dislocations become “stuck” inside the crystal and act as sources of internal stress which oppose the motion of other gliding dislocations. One simple way in which two dislocations could become stuck is by elastic interaction. Thus, two parallel edge dislocations of opposite sign moving on parallel slip planes in any sub-grain may become stuck, as a result of the interaction. G. I. Taylor assumed that dislocations become stuck after travelling an average distance, L, while the density of dislocations reaches p, i.e. work-hardening is due to the dislocations getting in each other’s way. The flow stress is then the stress necessary to move a dislocation in the stress field of those dislocations surrounding it.

Taylor’s assumption that during cold work the density of dislocations increases has been amply verified, and indeed the parabolic relationship between stress and strain is obeyed, to a first approximation, in many polycrystalline aggregates where deformation in all grains takes place by multiple slip. Experimental work on single crystals shows, however, that the work- or strain-hardening curve may deviate considerably from parabolic behaviour, and depends not only on crystal structure but also on other variables such as crystal orientation, purity and surface conditions (see Figures 1 and 2).

The crystal structure is important (see Figure 2) in that single crystals of some hexagonal metals slip only on one family of slip planes, those parallel to the basal plane, and these metals show a low rate of work-hardening. The plastic part of the stress–strain curve is also more nearly linear than parabolic with a slope which is extremely small: this slope (dτ/dү) becomes even smaller with increasing temperature of deformation. Cubic crystals, on the other hand, are capable of deforming in a complex manner on more than one slip system, and these metals normally show a strong work-hardening behaviour. The influence of temperature depends on the stress level reached during deformation and on other factors which must be considered in greater detail. However, even in cubic crystals the rate of work-hardening may be extremely small if the crystal is restricted to slip on a single slip system. Such behaviour points to the conclusion that strong work-hardening is caused by the mutual interference of dislocations gliding on intersecting slip planes.

Figure 1 Stress–strain curves of single crystals (afterHirsch and Mitchell, 1967; courtesy of the NationalResearch Council of Canada).

Figure 2 Stress–strain curve showing the three stages of work hardening.

Many theories of work-hardening similar to that of Taylor exist but all are oversimplified, since workhardening depends not so much on individual dislocations as on the group behaviour of large numbers of them. It is clear, therefore, that a theoretical treatment which would describe the complete stress–strain relationship is difficult, and consequently the present-day approach is to examine the various stages of hardening and then attempt to explain the mechanisms likely to give rise to the different stages. The work-hardening behaviour in metals with a cubic structure is more complex than in most other structures because of the variety of slip systems available, and it is for this reason that much of the experimental evidence is related to these metals, particularly those with fcc structures.

2. Look through the text and find cases of:

1) Present Simple (Passive Voice);

2) Modal Verbs;

3) Gerund.