Non-uniform Recrystallization Research Articles

Effect of Successive Heat Treatment on the Performance of Superconducting Radio Frequency Niobium Cavities

One of the primary sources of radio frequency residual losses leading to lower quality factor is trapped residual magnetic field during the cooldown of superconducting radio frequency cavities. It has been reported that non-uniform recrystallization of niobium cavities after the post fabrication heat treatment leads to higher flux trapping during the cooldown, and hence the lower quality factor. Here, we fabricated two 1.3 GHz single cell cavities from high purity fine grain niobium and processed with successive heat treatments in the range 800-1000 °C to measure the flux expulsion and flux trapping sensitivity. The result indicates that although flux expulsion improves with increased heat treatments, there is a noticeable difference between the flux trapping sensitivity depending on the cavity. Evaluation of microstructure maybe crucial to understand the impact of flux trapping sensitivity on cavity performance.

Enhancing mechanical properties of wire + arc additively manufactured INCONEL 718 superalloy through in-process thermomechanical processing

Wire + arc additive manufacture (WAAM) was applied to produce INCONEL 718 superalloy (IN718) components in a layer by layer manner; further, interpass cold rolling was introduced to generate in-process thermomechanical processing effect during the deposition process. Mechanical testing showed that with rolling applied, the strength of the solution plus aging treated WAAM IN718 was improved from 1056 MPa (unrolled) to 1351 MPa (rolled) which met the wrought standard (1276 MPa), and the material anisotropy was eliminated. The unrolled IN718 featured large columnar grains developing along the building direction, with the length and width as large as 11 mm and 0.8 mm respectively; rolling induced plastic deformation triggered a non-uniform recrystallization upon successive depositions, which produced a recrystallized core with small columnar grains and numerous finely equiaxed grains with the grain size of 12.7 μm. The overall strengthening produced by interpass rolling was attributed mostly (76%) to the rolling induced recrystallization which produced grain size reduction strengthening and created larger grain boundary area to allow more precipitation at the grain boundaries, and partially (24%) due to the improved aging response of the recrystallized grain structure.

Nonuniform Recrystallization and Growth Behavior of β Grains Dominated by Grain Misorientation and Interfacial Energy in Metastable β Titanium Alloy

Metastable β titanium alloys usually exhibit nonuniform β grain growth behavior under β solution treatment, resulting in “black spots” with dimension of millimeter or centimeter. In this paper, the nonuniform recrystallization and growth behavior of β grains were studied in the Ti-5Al-5Mo-5V-1Cr-1Fe metastable β titanium alloy. Electron backscatter diffraction technique was used to characterize the crystallographic orientation after β solution in both the normal and abnormal (black spot) macroscopic regions, containing grains with high misorientation angles (> 10 deg) and subgrains with low misorientation angles (< 10 deg), respectively. The nonuniform growth behavior of β grains in the abnormal regions was clarified based on the following two aspects: (1) β grain recrystallization and growth inside the abnormal region were delayed due to the low-accumulated plastic strain and low-angle subgrain boundaries; and the growth of some subgrains with favorable neighboring region was accelerated with the transition from low-angle subgrain boundary to high-angle grain boundary; (2) with enough holding time at the β phase region, the abnormal region was consumed via the normal grains growing inside. The migration of grain boundaries with various orientations depends on the interfacial energy, which is relevant to the misorientation angle and the interfacial plane between the neighboring grains.

Effects of asymmetric rolling process on ridging resistance of ultra-purified 17%Cr ferritic stainless steel

In ferritic stainless steels, a significant non-uniform recrystallization orientation and a substantial texture gradient usually occur, which can degrade the ridging resistance of the final sheets. To improve the homogeneity of the recrystallization orientation and reduce the texture gradient in ultra-purified 17%Cr ferritic stainless steel, in this work, we performed conventional and asymmetric rolling processes and conducted macro and micro-texture analyses to investigate texture evolution under different cold-rolling conditions. In the conventional rolling specimens, we observed that the deformation was not uniform in the thickness direction, whereas there was homogeneous shear deformation in the asymmetric rolling specimens as well as the formation of uniform recrystallized grains and random orientation grains in the final annealing sheets. As such, the ridging resistance of the final sheets was significantly improved by employing the asymmetric rolling process. This result indicates with certainty that the texture gradient and orientation inhomogeneity can be attributed to non-uniform deformation, whereas the uniform orientation gradient in the thickness direction is explained by the increased number of shear bands obtained in the asymmetric rolling process.

Acquisition of recrystallization information using optical metallography in a metastable beta titanium alloy

The precipitates of α phase cooled from recrystallization annealing temperature were rather inhomogeneous from grain to grain in a metastable β titanium alloy. Electron backscatter diffraction technique (EBSD) had confirmed that inhomogeneous distribution of α precipitates was directly related to nonuniform recrystallization (or recovery). The accelerated precipitation kinetics in the deformed regions may be explained in terms of the increased number of nucleation sites provided by the substructure. Thus, the recrystallized and deformed regions can be easily distinguished by optical metallography.

Process–microstructure–property relationships in AA3003 expanded metal periodic cellular truss cores

Despite their significant potential as ultra-lightweight composite materials, comparatively little attention has been given to the microstructure of deformation formed periodic cellular metals (PCMs). Because shape plays such a determining role in their performance, it is the spatial distribution of microstructure which controls both the useful range of cellular architectures and their resultant mechanical properties. The present study examines the effects of non-uniform recrystallization in expanded AA3003 sheet precursors on the PCM composite formability and mechanical performance. Because fabrication induced work-hardening and crack formation is localized to the hinge region, the pre-annealing temperature prior to fabrication provides a trade-off between the resultant compressive modulus and inelastic buckling strength of the PCM.

Numerical Simulation of Austenite Recrystallization in CSP Hot Rolled C-Mn Steel Strip

Abstract An integrated mathematical model is developed to predict the microstructure evolution of C-Mn steel during multipass hot rolling on the CSP production line, and the thermal evolution, the temperature distribution, the deformation, and the austenite recrystallization are simulated. The characteristics of austenite recrystallization of hot rolled C-Mn steel in the CSP process are also discussed. The simulation of the microstructure evolution of C-Mn steel ZJ510L during CSP multipass hot rolling indicates that dynamic recrystallization and metadynamic recrystallization may easily occur in the first few passes, where nonuniform recrystallization and inhomogeneous grain size microstructure may readily occur; during the last few passes, static recrystallization may occur dominantly, and the microstructure will become more homogeneous and partial recrystallization may occur at relatively low temperature.

Recrystallization and Related Phenomena. Effect of Lubrication Condition on Recrystallization Texture of Ultra-low C Sheet Steel Hot-rolled in Ferrite Region.

The application of the EBSD technique with an SEM provides a new approach to the analysis of variations of the recrystallization texture in the thickness direction of ultra-low carbon sheet steels rolled in the ferrite region. A nonuniform recrystallization texture through the thickness is formed due to the additional shear strain introduced by frictional force between the rolls and the material in sheet steel rolled with a friction coefficient over 0.15, <110>//ND recrystallized grains being formed near the surface, and <111>//ND and <100>//ND recrystallized grains being formed at the mid plane. On the other hand, <110>//ND recrystallized grains are not formed preferentially near the surface, and <111>//ND and <100>//ND recrystallized grains are formed uniformly throughout the thickness in sheet steel rolled with a friction coefficient under 0.15. Orientation imaging also reveals differences related to the tolerance of <110>//ND recrystallized grains around the <110>//ND axis in sheet steel rolled without lubrication, in that <110>//ND grains with a small tolerance form at the 1/10 plane from the surface and <110>//ND grains with larger tolerances form at thickness locations from the 1/10 plane toward the surface and center. The r-value depends on the friction coefficient and increases significantly as the friction coefficient decreases under 0.15, resulting in high r-values over 1.2. This change in the r-value with the friction coefficient corresponds to the distribution of the recrystallization texture through the thickness. Orientation imaging allows a more visual and intuitive understanding of the nonuniformity of the recrystallization texture than conventional methods such as optical microscopy and X-ray diffraction measurement.

The effects of die angle on texture and annealing response of ETP copper wire

As-annealed, 2.47 mm ETP copper wire was drawn in five passes to 1.45 mm using carbide dies with nominal included angles of 8, 16 and 24 degrees. A single die, low speed drawing block was used, and the lubricant was fat-based and water soluble. Annealing was undertaken in a box furnace, for six minutes at temperatures ranging from 185 to 230°C. Annealing response and texture evolution were assessed by way of tensile tests, microhardness measurements and x-ray diffraction. Redundant strain accumulation was projected by way of deformation zone geometry, or Δ, analysis. The as-drawn wire reveals volumes of (111) and (200) fiber texture, as well as a significant volume of essentially randomly oriented grain structure. The ratio of (111) to (200) texture decreases as die angle increases. Increases in die angle and related redundant strain promote the development of random orientation and reduce the sharpness of the (111) and (200) texture components. Increases in die angle and related redundant strain also lower the annealing temperature range and cause non-uniform recrystallization, presumably due to redundant strain and texture gradients. The as-annealed 1.45 mm wire displays a large volume of randomly oriented grain structure for all three die angles. In the case of the as-annealed wire, increases in die angle are associable with a decrease in the volume percent of (111) fiber texture, a decrease in the sharpness of the (200) fiber texture, and an increase in randomly oriented grain structure.

Nonuniform recrystallization in a mechanically alloyed nickel-base superalloy

The recrystallization behavior of mechanically alloyed and extruded MA6000 oxide dispersion strengthened (ODS) nickel-base superalloy has been studied as a function of the position of the sample in the extruded bar. It was found that there are profound differences across the cross section of the bar, the edge regions recrystallizing more easily relative to the core. Furthermore, the recrystallized grains tend to be much more anisotropic at the edges. It appears that these differences are caused by inhomogeneous deformation. The oxide particles are more aligned along the extrusion direction at the regions near the surface, presumably because of the more extensive deformation in those regions. This explains the greater anisotropy in the recrystallized grains and the relative case of recrystallization (since the boundary mobility along the extrusion direction is high for aligned particles). Stored energy measurements confirm that the observed effects cannot be explained by differences in the driving force for recrystallization.