Basic mechanism

The most simple view of the growth mechanism is that it is due to interstitial <a> loops (with displacement vectors or Burgers vectors pointing in the a-direction) lying on prism planes, and vacancy <c> loops (with displace­ment vectors or Burgers vectors pointing in the c-direction) lying on basal planes (Buckley 1961). Figure 4.67 gives a schematic illustration.

The simple vision is that planar arrays of vacancies cause shrinkage in the direction normal to the plane, and planar arrays of interstitials cause expan­sion. However, all vacancies and interstitials do not end up as loops. As dis­cussed in earlier sections on irradiation creep, they can also be deposited at grain boundaries, solute atoms and network dislocations (that is, dislocations introduced by deformation rather than irradiation) having <a> or <c+a> Burgers vectors (i. e., line dislocations having a net strain in the <a> or <c+a> lattice directions). In the final analysis, irradiation growth is due to aniso­tropic deposition of vacancies and interstitials at these sinks and is strongly influenced by the anisotropic diffusion of self interstitial atoms (SIAs) in the basal plane, or more exactly, in the directions normal to the c-axis.

The mechanisms for irradiation growth parallel those for irradiation creep, with the notable exception of any applied stress.

In general, growth is now believed to occur when there is an anisotropic distri­bution of sinks receiving a net flux of vacancies and this anisotropy is different from that of the distribution of sinks receiving a net flux of the self intersti­tial atoms (SIAs). The dilations associated with the addition of lattice planes accompanying the precipitation of SIAs then do not cancel the contractions associated with the removal of lattice planes associated with the condensation of vacancies. For a complete understanding of irradiation growth one must identify the possible sinks and explain their evolution, identify the source

of the anisotropy and explain the partitioning of the PDs amongst the sinks. (Holt, 1988)

The key unique mechanistic feature of irradiation growth is the initiation and growth of <c> component dislocations (either as irradiation-produced loops or deformation-induced networks). Without them breakaway growth does not occur. It also appears to be essential that diffusion of SIAs be anisotropic and favour the directions in the basal plane. More details are given in Holt et al. (1996), Woo (1988), Holt etal. (2000), Holt (2008) and Christien and Barbu (2009).

Irradiation growth will occur to some extent in all zirconium alloys. To minimize the effects of growth several actions are important:

• minimize residual stresses,

• plan for the effects of cold work,

• minimize hydrogen absorption,

• plan for the effects of temperature,

• understand and control alloy chemistry,

• control texture.