Volume defects

3.5 Volume defects

3.5.1 Void formation and annealing

Defects which occupy a volume within the crystal may take the form of voids, gas bubbles and cavities. These defects may form by heat treatment, irradiation or deformation and their energy is derived largely from the surface energy (1–3 J m −2 ). In some materials with low stacking-fault energy

a special type of three-dimensional defect is formed, namely the defect tetrahedron. This consists of a tetrahedron made up from stacking faults on the four {1 1 1} planes joined together by six low-energy stair-rod dislocations. This defect is discussed more fully in Section 3.6.2.3.

The growth of the original vacancy cluster into a three-dimensional aggregate, i.e. void, or a col- lapsed vacancy disk, i.e. dislocation loop, should, in principle, depend on the relative surface to strain energy values for the respective defects. The energy of a three-dimensional void is mainly surface energy, whereas that of a Frank loop is mainly strain energy at small sizes. However, without a detailed knowledge of the surface energy of small voids and the core energy of dislocations it is impossible to calculate, with any degree of confidence, the relative stability of these clustered vacancy defects.

The clustering of vacancies to form voids has now been observed in a number of metals with either fcc or cph structure. In as-quenched specimens the voids are not spherical but bounded by crystallographic faces (see Figure 3.54) and usually are about 50 nm radius in size. In fcc metals they are octahedral in shape with sides along metals bounded by prism and pyramidal planes. Void formation is favored by slow quenching rates and high ageing temperatures, and the density of voids increases when gas is present in solid solution (e.g. hydrogen in copper, and either hydrogen or oxygen in silver). In aluminum and magnesium, void formation is favored by quenching from a wet atmosphere, probably as a result of hydrogen production due to the oxidation reactions. It has been postulated that small clustered vacancy groups are stabilized by the presence of gas atoms and prevented from collapsing to a planar disk, so that some critical size for collapse can be exceeded. The voids are not conventional gas bubbles, however, since only a few gas atoms are required to nucleate the void, after which it grows by vacancy adsorption.

3.5.2 Irradiation and voiding

Irradiation produces both interstitials and vacancies in excess of the equilibrium concentration. Both species initially cluster to form dislocation loops, but it is the interstitial loops formed from clustering of interstitials which eventually develop into a dislocation structure.

In general, interstitial loops grow during irradiation because the large elastic misfit associated with an interstitial causes dislocations to attract interstitials more strongly than vacancies (see Chapter 6). Interstitial loops are therefore intrinsically stable defects, whereas vacancy loops are basically unstable defects during irradiation. Thus, interstitials attracted to a vacancy loop, i.e. a loop formed by clustering vacancies, will cause it to shrink as the interstitials are annihilated. Increasing the irradiation temperature results in vacancies aggregating to form voids. Voids are formed in an intermediate temperature range ≈0.3–0.6T m , above that for long-range single-vacancy migration and below that for thermal vacancy emission from voids. To create the excess vacancy concentration it is also necessary to build up a critical dislocation density from loop growth to bias the interstitial flow.

There are two important factors contributing to void formation. The first is the degree of bias the dislocation density (developed from the growth of interstitial loops) has for attracting interstitials, which suppresses the interstitial content compared to vacancies. The second factor is the important role played in void nucleation by gases, both surface-active gases such as oxygen, nitrogen and hydrogen frequently present as residual impurities, and inert gases such as helium, which may be generated continuously during irradiation due to transmutation reactions. The surface-active gases

Crystal defects 129

0.25␮m

Figure 3.33 Scanning electron micrograph of a medium-carbon (0.4%) steel with a quenched and tempered martensite structure, showing large dimples associated with oxide inclusions and small dimples associated with small carbide precipitates (courtesy Dr L. Sidjanin).

such as oxygen in copper can migrate to embryo vacancy clusters and reduce the surface energy. The inert gas atoms can acquire vacancies to become gas molecules inside voids (when the gas pressure is not in equilibrium with the void surface tension) or gas bubbles when the gas pressure is considerable (P ˜> 2γ s / r). Voids and bubbles can give rise to irradiation swelling and embrittlement of materials.

3.5.3 Voiding and fracture

The formation of voids is an important feature in the ductile failure of materials. The fracture process involves three stages. First, small holes or cavities nucleate, usually at weak internal interfaces (e.g. particle/matrix interfaces). These cavities then expand by plastic deformation and finally coalesce by localized necking of the metal between adjacent cavities to form a fibrous fracture. A scanning electron micrograph illustrating the characteristics of a typical ductile failure is shown in Figure 3.33. This type of fracture may be regarded as taking place by the nucleation of an internal plastic cavity, rather than a crack, which grows outwards to meet the external neck that is growing inwards. Experimental evidence suggests that nucleation occurs at foreign particles. For example, oxygen-free high conductivity (OFHC) copper necks down to over 90% reduction in area, whereas tough-pitch copper shows only 70% reduction in area; similar behavior is noted for super-pure and commercial purity aluminum.

Thus, if no inclusions were present, failure should occur by the specimen pulling apart entirely by the inward growth of the external neck, giving nearly 100% reduction in area. Dispersion-hardened materials in general fail with a ductile fracture, the fibrous region often consisting of many dimples arising from the dispersed particles nucleating holes and causing local ductile failure. Ductile failure is discussed further in Chapter 7.