Papers by Keyword: α-Zirconium

Abstract: Hydrogen in excess of solid solubility precipitates as hydride phase of plate shaped morphology in hcp α-Zr with the broad face of the hydride plate coinciding with certain crystallographic plane of α-Zr crystal called habit plane. The objective of the present investigation is to predict the habit plane of δ-hydride precipitating in α-Zr at 298 K using strain energy minimization technique. The δ-hydride phase is modeled to undergo isotropic elasto-plastic deformation. The α-Zr phase was modeled to undergo transverse isotropic elastic deformation but isotropic plastic deformation. Accommodation strain energy of δ-hydride forming in α-Zr crystal was computed using initial strain method as a function of hydride nuclei orientation. Hydride was modeled as disk with round edge. Contrary to several habit planes reported in literature for δ- hydrides precipitating in α-Zr crystal, the total accommodation energy minima at 298 K suggests only basal plane i.e. (0001) as the habit plane.

Abstract: Zirconium alloys are widely used for different applications in nuclear industry. Precise knowledge of their texture is of great relevance since this hcp metal exhibits a strong crystal anisotropy. Despite that, the mechanisms of texture change during its deformation and subsequent annealing are still not precisely known. Thus, there is a need for a better understanding of the fundamental mechanisms of recrystallisation. Earlier works on Zr702 [1-3] suggested that the kinetics and local mechanisms of recrystallisation after cold-rolling was controlled by the heterogeneity of the deformed microstructure and that, at the end of recrystallisation (corresponding to the disappearance of the deformed matrix), the position of the major texture components remained almost unaffected. The aim of the present work is to confirm whether these statements can be generalized for various deformation conditions or not.

Abstract: The mechanisms governing the very first stage of static recrystallization in two hexagonal alloys (commercially pure titanium and low alloyed zirconium) are investigated in this paper. Initially fully recrystallized and equiaxed materials were cold-rolled to 80% thickness reduction and subsequently recrystallized at 500°C for short times. High resolution EBSD maps were acquired in a FEG-SEM before and after annealing in order to see where and how the new grains appear. Nonoriented nucleation mechanisms are involved in both materials, and there is a strong correlation between the local deformation substructures and the recrystallization kinetics. Recrystallization is extremely fast in the areas where the deformation cells are small and highly misoriented, i.e. in the areas which underwent severe grain fragmentation. Twinning plays an important role for that purpose in the studied titanium sheet.

Abstract: The micron-size grain refinement of pure a-zirconium obtained with elevated temperature tensile deformation was investigated. The development of low-misorientation subboundaries caused the serration of the original grain boundaries at low strains. The final microstructure (e.g. strains > 3) was predominantly composed of fine, equiaxed “crystallites” with ⅔ of the boundaries being of very low misorientations (< 3°) and the remaining ⅓ being high angle boundaries (θ > 8°, and typically 25-35°). Discontinuous dynamic recrystallization was excluded as a possible mechanism due to the absence of newly formed grain nuclei. The bimodal distribution of the crystallite or (sub)grain boundary misorientations is inconsistent with the occurrence of continuous dynamic recrystallization and rotational recrystallization. The continual thinning of the original grains, the serration of the high angle boundaries, the bimodal misorientation distribution of misorientations, ⅔ of boundaries of very low misorientations at high strains all strongly suggest geometric dynamic recrystallization and dynamic recovery as the grain refinement and restoration mechanisms.

Abstract: The microstructure and crystallographic texture in zirconium (Zr702) sheets, initially deformed by 80% cold rolling, are investigated at different stages of the primary recrystallization. Inhomogeneities were observed in the deformed microstructure at different scales down to the submicrometer range. The influence of these inhomogeneities on the local recrystallization mechanisms is discussed. The measurement of the orientation of the new grains shows that the nucleation is definitely not oriented. Since the global texture change is very slight, recrystallization by subgrain growth is probably one of the most important mechanism during the recrystallization process in zirconium.

Abstract: Primary recrystallization of a 80% cold–rolled T40 or Zr702 sheets leads to equiaxed microstructures. Subsequently, only normal grain growth takes place in T40 while a few grains can grow abnormally after sufficient time at high annealing temperature (close to the transus) in Zr702. The grain sizes reached after extended grain growth at moderate temperatures in Zr702 are smaller than in T40. The presence of precipitates in Zr702 is probably responsible for this and also for the abnormal phenomena observed at high temperature in this material. The texture changes occurring in both materials under normal grain growth conditions (often roughly described as “30° rotation around c axes”) are due to the development of the largest grains produced by the primary recrystallization. These large grains are preferentially oriented around {j1=0°, F=30°, j2=30°} for T40 and around {j1=0°, F=25°, j2=30°} for Zr702, orientations which become predominant after extended grain growth.