Nucleation and growth of twins

6.5.2 Nucleation and growth of twins

During the development of mechanical twins, thin lamellae appear very quickly ( ≈ speed of sound) and these thicken with increasing stress by the steady movement of the twin interface. New twins are usually formed in bursts and are sometimes accompanied by a sharp audible click, which coincides with the appearance of irregularities in the stress–strain curve, as shown in Figure 6.33. The rapid production of clicks is responsible for the so-called twinning cry (e.g. in tin).

Although most metals show a general reluctance to twin, when tested under suitable conditions they can usually be made to do so. As mentioned in Section 6.3.1, the shear process involved in

328 Physical Metallurgy and Advanced Materials twinning must occur by the movement of partial dislocations and, consequently, the stress to cause

twinning will depend not only on the line tension of the source dislocation, as in the case of slip, but also on the surface tension of the twin boundary. The stress to cause twinning is therefore usually greater than that required for slip, and at room temperature deformation will nearly always occur by slip in preference to twinning. As the deformation temperature is lowered the critical shear stress for slip increases and then, because the general stress level will be high, the process of deformation twinning is more likely.

Twinning is most easily achieved in metals of cph structure where, because of the limited number of slip systems, twinning is an essential and unavoidable mechanism of deformation in polycrystalline specimens (see Section 6.4.11), but in single crystals the orientation of the specimen, the stress level and the temperature of deformation are all important factors in the twinning process. In metals of the bcc structure twinning may be induced by impact at room temperature or with more normal strain rates at low temperature, where the critical shear stress for slip is very high. In contrast, only a few fcc metals have been made to twin, even at low temperatures.

In zinc single crystals it is observed that there is no well-defined critical resolved shear stress for twinning such as exists for slip, and that a very high stress indeed is necessary to nucleate twins. In most crystals, slip usually occurs first and twin nuclei are then created by means of the very high stress concentration which exists at dislocation pile-ups. Once formed, the twins can propagate provided the resolved shear stress is higher than a critical value, because the stress to propagate a twin is much lower than that to nucleate it. This can be demonstrated by deforming a crystal oriented in such a way that basal slip is excluded, i.e. when the basal planes are nearly parallel to the specimen axis. Even in such an oriented crystal it is found that the stress to cause twinning is higher than that for slip on non-basal planes. In this case, non-basal slip occurs first, so that when a dislocation pile-up arises and a twin is formed, the applied stress is so high that an avalanche or burst of twins results.

It is also believed that in the bcc metals twin nucleation is more difficult than twin propagation. One possible mechanism is that nucleation is brought about by the stress concentration at the head of a piled-up array of dislocations produced by a burst of slip as a Frank–Read source operates. Such behavior is favored by impact loading, and it is well known that twin lamellae known as Neumann bands are produced this way in α-iron at room temperature. At normal strain rates, however, it should be easier to produce a slip burst suitable for twin nucleation in a material with strongly locked dislocations, i.e. one with a large k-value (as defined by equation (6.19)), than one in which the dislocation locking is relatively slight (small k-values). In this context it is interesting to note that both niobium and tantalum have a small k-value and, although they can be made to twin, do so with reluctance compared, for example, with α-iron.

In all the bcc metals the flow stress increases so rapidly with decreasing temperature (see Fig- ure 6.29), that even with moderate strain rates (10 −4 s −1 ) α-iron will twin at 77 K, while niobium with its smaller value of k twins at 20 K. The type of stress–strain behavior for niobium is shown in Figure 6.33a. The pattern of behavior is characterized by small amounts of slip interspersed between extensive bursts of twinning in the early stages of deformation. Twins, once formed, may themselves act as barriers, allowing further dislocation pile-up and further twin nucleation. The action of twins as barriers to slip dislocations could presumably account for the rapid work hardening observed at 20 K.

Fcc metals do not readily deform by twinning, but it can occur at low temperatures, and even at

C, in favorably oriented crystals. The apparent restriction of twinning to certain orientations and low temperatures may be ascribed to the high shear stress attained in tests on crystals with these orientations, since the stress necessary to produce twinning is high. Twinning has been confirmed in heavily rolled copper. The exact mechanism for this twinning is not known, except that it must occur by the propagation of a half-dislocation and its associated stacking fault across each plane of a set of parallel (1 1 1) planes. For this process the half-dislocation must climb onto successive twin planes, as below for bcc iron.

Mechanical properties I 329