Due to the complex deformation behaviour of highly-anisotropic Zirconium alloys, these materials have been studied extensively by neutron diffraction, in combination with polycrystalline deformation models, the elasto-plastic and visco-plastic self­consistent models [18], to understand the deformation and texture development.

The difference in tensile and compressive behaviour in Zircaloy-2 was demon­strated by MacEwen et al. [19]. The strains were measured for each lattice plane on a time-of-flight (TOF) instrument (the General Purpose Powder Diffractometer at the intense-pulsed neutron source (IPNS), which is no longer operational). Neutron time-of-flight instruments can measure multiple lattice planes without reorienting the sample, as is required on a constant-wavelength instrument.

The residual stresses, stress tensor, and inter-granular stresses were characterized after 5 % strain by Pang et al. [20]. The measured lattice strains were in good agreement with those predicted by elasto-plastic self-consistent models, which predict deformation modes such as slip and twinning.

Zirconium and its alloys have a hcp structure, and there are too few slip systems for standard plasticity so twinning is an important contributor to plastic deforma­tion. Rangaswamy et al. [21] compared changes in texture and twin volume-frac­tions to predictions from a visco-plastic self-consistent polycrystal model, which described both slip and twinning.

Balogh et al. [22] examined the deformation behaviour of Zr-2.5Nb samples by full-pattern diffraction line-profile analysis (DLPA) to determine the evolution of the density and type of the dislocation-structure induced by irradiation and plas­ticity. Control samples were compared to samples removed from a CANDU nuclear reactor pressure-tube to determine the evolution of microstructure and plasticity characteristics during deformation (27 % cold work during manufacture). The pressure tube was in service for 7 years at *250 °C with a neutron fluence of

1.6 x 1024 m-2 (E > 1 MeV). Results show that fast-neutron irradiation signifi­cantly increases the overall dislocation density, accomplished entirely by an increase in the (a) Burgers vector dislocations.