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 Synchrotron X-Ray Diffraction of Ti-6Al-4V During Thermomechanical Treatment in the Beta Field

“…Likewise, as a consequence of the increase of misorientation within the grains, Debye-Scherrer rings become more homogeneous ( Figure 5). The evolution of the microstructure due to dDRX must result in more inhomogeneous intensity distribution of the Debye-Scherrer rings that would be formed mainly by spots [22,32,33]. This characteristic was not observed in this work.…”

Load partition and microstructural evolution during in situ hot deformation of Ti–6Al–6V–2Sn alloys

“…The grain rotation observed during the in situ HEXRD experiments in the α-phase is a combination of the geometrical reorientation of the lamellae orthogonally to the load direction [42,43] and crystallographic rotation within the lamellae [42,44] in order to fulfil geometrical requirements of the imposed strain [42]. No evidence of dynamic recrystallization was revealed by the AS-plots for the colony-α microstructure since no popping up and disappearance of reflections is observed according to Liss et al [45]. This agrees with dynamic processes observed for a similar alloy during compression tests at strain rate of 0.001 s À 1 and strains larger than 0.7 [44].…”

In Situ Characterization of Lattice Structure Evolution during Phase Transformation of Zr‐2.5Nb

“…The diffraction peaks of the retained b-Zr(Nb) phase, such as the b-200 peak in Figure 3, initially appear at higher scattering vectors and shift to the position of the expected b solid solution peaks during the phase transformation. Distinctive from previous studies of a-b transformations in Zr and Zircaloy-4, [6] which are martensitic in nature and have strict crystallographic transformation relations, the phase transformation process in the present case includes the migration and dissolution of Nb atoms by the transforming a-Zr into a high temperature, Zr-rich b-Zr(Nb) solid solution.…”