Micromechanisms and micromechanics of Zircaloy-4
Creators: name: Evans, Christabel
Publisher: name: Imperial College London
Year: 2014

description: The micromechanisms of Zircaloy-4 are investigated in relation to texture evolution, hydride formation and fatigue. The Zircaloy-4 plate used throughout this thesis was provided by Rolls- Royce plc, Derby, and was annealed post unidirectional rolling. The effect of strain rate on the texture evolution of Zircaloy-4 was investigated to understand how different processing methods would effect the final texture. Texture evolution during high temperature (550◦C) compression and tension tests were investigated using synchrotron X- ray diffraction in the transverse and rolling directions (TD and RD) at strain rates ranging from 10−4s−1 to 10−1s−1. The post deformation microstructures showed the presence of twins at the higher strain rates (10−1s−1 to 10−2s−1 ), with minimal twinning seen at the slower rates. The pole figures obtained throughout testing showed no texture evolution during tensile testing, regardless of strain rate and the basal poles remained orientated ±30◦between the normal direction (ND) and the transverse direction (TD), which is the original texture for the as received material. During the compression tests specimens tested in the RD showed an evolution in the pole figures as strain rate was increased. At a strain rate of 10−1s−1 a reorientation of the basal poles to lie almost solely in the RD was seen, indicative of twinning. As the strain rate was reduced, this effect diminished and at a strain rate of 10−4s−1only a slight rotation of the basal poles was observed. The Kearns’ factor evolution with strain confirmed this result. These results were then used in an elasto-plastic self-consistent model to simulate the slip and twin levels during deformation. The computational results were consistent with the notion that increasing the strain rate increased twin density, as shown in the post deformation microstructures. To understand the micromechanical effects hydride precipitates have on the alloy, a section of the alloy was charged with hydrogen in a vacuum furnace to 375 ppm ± 50 ppm. Microstructural characterisation of the material indicated that high levels of hydrides forming predominantly at grain boundaries. Nanoindentation tests were carried out at room temperature on individual hydride packets, the surrounding matrix and the as received material to characterise the me- chanical properties. The results obtained from these tests were used in computational modelling scenarios to determine more accurate mechanical properties. The nano-hardness of the matrix was found to be highest (4.64 GPa), followed by the matrix and the as received material (3.62 GPa and 2.74 GPa respectively). As part of the initial scope of this thesis it was the author’s original intention to understand how the presence of hydrides affects dislocation propagation and micro-deformation mechanisms. However, since carrying out the experimental procedures and results analysis, a number of papers have come to the author’s attention which outline the importance of the final processing steps prior to testing. It has been found that mechanical polishing as a method for material preparation induces work hardening into the surface of the material. Although this does not have an affect in macro and indeed micro scale hardness testing, where the tested layer is in the scale of a few microns, this work hardened layer does have a major effect in nano-hardness tests, where the testing layer is in the region of nanometers. As a result of this no dislocation analysis was carried out as it would be impossible to distinguish between dislocations present from mechanical polishing and those induced by the presence of hydrides. In spite of the work hardened layer rendering the absolute hardness values invalid, the relative values in relation to the matrix, hydride and as received material are still of interest. High cycle fatigue tests were carried out on samples taken from the rolling and the transverse direction of the material. Fractographic examination of the samples showed facets in the area immediately surrounding the initiation site. There were only found to be between 10-20 faceted grains, which were confined to this region. These features showed feather-like characteristics, indicative of plastic deformation. Site specific transmission electron microscopy (TEM) was carried out on the initiation facets, showing mostly dislocations, although and imperfect dislocations segments were also found to be present. The low dislocation density in these features compared to that of titanium suggests that these features may be quite brittle in nature. Crack propagation was found to occur via striated crack growth. The direction of the striations appear to be affected by grain orientation. TEM analysis of the underlying grain did not show the presence of any dislocations. It is thought that this may be a result of image stresses causing the dislocation to evaporate out the TEM specimen once it is removed from the fracture surface, although further work needs to be done to confirm this.