In Situ Observation of Dynamic Recrystallization in the Bulk of Zirconium Alloy

Abstract

Dynamic recrystallization broadly occurs in thermomechanical processes, examples of which include metal forming, [1] the formation of the Earth's crust, [2,3] and the flow of glaciers. [4] This process impacts a correspondingly broad range of areas including the mechanical properties of industrial products, [5] the size limitation of nano-crystalline materials, [6] the occurrence of earthquakes, [7] and the simulation of polar ice sheets in climatic change. [8] Dynamic recrystallization results from a competition between thermally driven nucleation and growth processes, as well as grain breakage and refinement during plastic deformation. Although this phenomenon is well understood in a general sense, predictions for a specific system are enormously difficult due to dependence on a number of variables including chemical composition, temperature, strain, strain rate, and thermo-mechanical history. Here, we present unprecedented in situ observations of dynamic recrystallization in individual bulk crystallites of a macroscopic Zircaloy-4 specimen under thermo-mechanical load. The microstructural kinetics, grain statistics, and crystallographic correlations inherent to this process are revealed. Utilizing synchrotron high-energy X-ray diffraction [9] allows direct observation of dynamic recrystallization and related effects in real time and at high temperature. Conventionally, such effects were either studied retrospectively after quenching, during which additional phase transitions may occur, or indirectly through changes in physical properties, relying on empirical experience or on the surface only using electron microscopy. Even more potential of this work may be raised by future material scientists, engineers, and industry on tailoring their metal products by thermo-mechanical simulation directly in a synchrotron beam.In classical powder X-ray diffraction, individual crystallites of the sample are ideally randomly oriented and their number is high, in order to find statistically enough crystallites to diffract radiation into any direction of observation, under the condition that Bragg's law is fulfilled. This ideal condition leads to Debye-Scherrer cones which are projected into concentric rings when scattered onto a two-dimensional detector oriented perpendicular to the primary beam. The condition of isotropy is no longer fulfilled if the illuminated volume of the sample is small compared to its grain size, leading to individual illuminated spots on the Debye-Scherrer rings as seen in Figure 1. These reflections stem from a small number of crystallites, which ought to be oriented on the Ewald sphere, in order to reflect to the observed positions. The number of distribution of reflections, their orientation, and their correlations give insight into grain size, texture, and orientation relationships of the crystallites. This has been elaborated earlier [10] and will be employed in the present study.High-energy X-rays (as used here at 90 keV) have small diffraction angles and therefore scatter in the forward directi...

In Situ Characterization Techniques Based on Synchrotron Radiation and Neutrons Applied for the Development of an Engineering Intermetallic Titanium Aluminide Alloy

“…In contrast to this, the broadening of a reflection indicates an increasing defect density within the grain [113]. Sharp dots, which relate to one original reflection and occur within a small angular range, are caused by subgrains stemming from a dynamic recovery process [77,111]. As peak broadening followed by the formation of new reflection spots can be observed in the insert of Figure 12b, fast dynamic recovery processes are suggested to dominate the β phase.…”

“…Yan et al [110] used this form of data representation to illustrate the formation of texture during the deformation of an initially coarse-grained Cu sample. In a similar experimental setup, recovery and recrystallization processes were studied in a Zr alloy during hot compression [111]. Mosaicity was derived from the azimuthal angle spread of single reflection spots.…”

In situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy

“…1͑a͒ and 1͑b͒. 15,26 Additionally, the box in Fig. 1͑a͒ shows the orientation correlations between all three phases where the ␥-002/ 200 spot aligns in a triangle with ␣-101, ␤-110, and ␣-002.…”

“…106, 113526 ͑2009͒ which are then separated by a subgrain boundary representing a small-angle boundary. 15 Soon after subgrain formation, the intensity is spread all over the rings, and, hence, grains and subgrains cannot be distinguished any longer. Dynamic recrystallization, a process in which new grains, separated by large-angle boundaries, are formed from a highly distorted microstructure, is experimentally observed by the popping up and disappearance of reflections.…”

“…9,10,13 The aim of the present study is to obtain in situ and in real time information from the bulk of the Ti-43.5Al-4Nb-1Mo-0.1B alloy during hot compression. The starting poly- crystalline microstructure consists of lamellar ␥ / ␣ 2 colonies with globular ␥-grains and ordered ␤ 0 -grains containing ␥-precipitates with a grain size of about 100 m. The recently developed high-energy synchrotron x-ray diffraction method 14 combined with the evaluation of the morphology of the diffraction Debye-Scherrer rings 15,16 is applied to a multiphase alloy undergoing hot plastic deformation. This allows the evaluation of grain size statistics 17 such as growth and refinement, subgrain formation, 18 lattice strain, 19 grain orientations, orientation relationships between grains and phases, 20 grain rotations, 21 texture evolution, 16 mechanisms of phase transformations, 22 dynamic recovery, and dynamic recrystallization.…”