Deformation of NaCoF₃ perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism

Texture, plastic deformation, and phase transformation mechanisms in perovskite and post-perovskite are of general interest for our understanding of the Earth's mantle. Here, the perovskite analogue NaCoF3 is deformed in a resistive-heated diamond anvil cell (DAC) up to 30 GPa and 1013 K. The in situ state of the sample, including crystal structure, stress, and texture, is monitored using X-ray diffraction. A phase transformation from a perovskite to a post-perovskite structure is observed between 20.1 and 26.1 GPa. Normalized stress drops by a factor of 3 during transformation as a result of transient weakening during the transformation. The perovskite phase initially develops a texture with a maximum at 100 and a strong 010 minimum in the inverse pole figure of the compression direction. Additionally, a secondary weaker 001 maximum is observed later during compression. Texture simulations indicate that the initial deformation of perovskite requires slip along (100) planes with significant contributions of {110} twins. Following the phase transition to post-perovskite, we observe a 010 maximum, which later evolves with compression. The transformation follows orientation relationships previously suggested where the c axis is preserved between phases and hh0 vectors in reciprocal space of post-perovskite are parallel to [010] in perovskite, which indicates a martensitic-like transition mechanism. A comparison between past experiments on bridgmanite and current results indicates that NaCoF3 is a good analogue to understand the development of microstructures within the Earth's mantle.

Effect of β Phase Decomposition on Recrystallization in α-Zr Region in Zr-2.5%Nb Pressure Tube Material

The effects of β phase decomposition on recrystallization and texture variation in Zr-2.5% Nb alloy pressure tube material were investigated. Isothermal annealing was conducted at 350 to 550 ^oC for 240 hours, and isothermal annealing was performed at 500 ^oC for 240 to 3,000 hours. The recrystallization and texture variation were analyzed by inverse pole figure variation using the XRD and EBSD methods. Annealing in α-Zr region at below 610 ^oC induced recrystallization and texture variation in the α-Zr. These results differ from those from a previous annealing study of the α+β region at 750-830 ^oC. Annealing above 400 ^oC for 240 hours caused β-Zr decomposition into β-Nb. The decomposition of the β-phase by annealing above 475 ^oC caused a contraction of 7.5% in the d(110) spacing in the β-phase, and a reduction in the volume fraction of the β phase by about 80%. It seems that the stress internally formed by the lattice contraction of the β-phase provides the driving force for recrystallization. In addition, it suggests that the newly formed α-Zr produced by β phase decomposition provides new nucleation sites for recrystallization and causes texture variation in the α-Zr. The reason why the recrystallization and the texture variation occurs only in the α-Zr stable region at below 610 ^oC is discussed.

Defining the hematite topotaxial crystal growth in magnetite–hematite phase transformation

Magnetite and hematite iron oxides are minerals of great economic and scientific importance. The oxidation of magnetite to hematite is characterized as a topotaxial reaction in which the crystallographic orientations of the hematite crystals are determined by the orientation of the magnetite crystals. Thus, the transformation between these minerals is described by specific orientation relationships, called topotaxial relationships. This study presents electron-backscatter diffraction analyses conducted on natural octahedral crystals of magnetite partially transformed into hematite. Inverse pole figure maps and pole figures were used to establish the topotaxial relationships between these phases. Transformation matrices were also applied to Euler angles to assess the diffraction patterns obtained and confirm the identified relationships. A new orientation condition resulting from the magnetite–hematite transformation was characterized, defined by the parallelism between the octahedral planes {111} of magnetite and rhombohedral planes \{10\bar {1}1\} of hematite. Moreover, there was a coincidence between one of the octahedral planes of magnetite and the basal {0001} plane of hematite, and between dodecahedral planes {110} of magnetite and prismatic planes \{11\bar {2}0\} of hematite. All these three orientation conditions are necessary and define a growth model for hematite crystals from a magnetite crystal. A new topotaxial relationship is also proposed: (111)Mag || (0001)Hem and (\bar {1}\bar {1}1)_{\rm Mag} || (10\bar {1}1)_{\rm Hem}.

Direct Observation via In Situ Heated Stage EBSD Analysis of Recrystallization of Phosphorous Deoxidised Copper in Unstrained and Strained Conditions

Recrystallization of phosphorous deoxidised copper used for strength critical applications at elevated temperatures was investigated by means of in situ heated stage EBSD analysis using a Gatan Murano heated stage mounted within a Carl Zeiss Sigma FEGSEM electron microscope. The influence of applied strain as the result of deformation within a Nakajima test as an analogue for industrial forming on the recrystallization temperature was investigated, the impact of increased heating rates on microstructural evolution was also investigated. Inverse pole figure plots combined with regions of reduction in local misorientations and variations in geometrically necessary dislocations were used to establish the point of recrystallization and the recrystallized fraction of the material. Recrystallization was observed to occur at temperatures as low as 130 °C in highly strained samples compared to around 300 °C within the annealed samples dependent upon heating rate. Increased heating rates were observed to produce a finer final grain structure but had little effect on presence of <111> 60° grain twins, which was influenced more by initial material condition. Graphic Abstract

Investigation of Ferroelectric Grain Sizes and Orientations in Pt/CaxSr1–xBi2Ta2O9/Hf–Al–O/Si High Performance Ferroelectric-Gate Field-Effect-Transistors

Electron backscatter diffraction (EBSD) was applied to investigate the grain size and orientation of polycrystalline CaxSr1–xBi2Ta2O9 (CxS1–xBT) films in ferroelectric-gate field-effect transistors (FeFETs). The CxS1–xBT FeFETs with x = 0, 0.1, 0.2, 0.5, and 1 were characterized by the EBSD inverse pole figure map. The maps of x = 0, 0.1, and 0.2 showed more uniform and smaller grains with more inclusion of the a-axis component along the film normal than the maps of x = 0.5 and 1. Since spontaneous polarization of CxS1–xBT is expected to exist along the a-axis, inclusion of the film normal a-axis component is necessary to obtain polarization versus electric field (P–E) hysteresis curves of the CxS1–xBT when the E is applied across the film. Since memory windows of FeFETs originate from P–E hysteresis curves, the EBSD results were consistent with the electrical performance of the FeFETs, where the FeFETs with x = 0, 0.1, and 0.2 had wider memory windows than those with x = 0.5 and 1. The influence of annealing temperature for C0.1S0.9BT poly-crystallization was also investigated using the EBSD method.

Notched Creep Damage Assessment Based on EBSD Observation for Austenitic Stainless Steel

Creep damage processes for smooth and notched specimen of austenitic stainless steel through interrupted creep tests using multiple specimens. The material used was 18-8 stainless steel for boiler tube use. The mid-sections of interrupted creep test specimens were observed through SEM(Scanning Electron Microscope) instrumented with EBSD(Electron BackScatter Diffraction patter) equipment. IPF(Inverse Pole Figure) maps, Phase maps and GOS(Grain Orientation Spread) maps were used for investigating creep damage process. For smooth specimen, the relationship between macroscopic creep time fraction and GOS averaged for all pixels showed linearity, while the relationship between creep strain and the averaged GOS showed non-linearity regressed by Green function successfully. For notched specimen, the EBSD maps became noisy possibly due to extensive phase transformation under highly concentrated notch stress. Obtained GOS data for gamma phase only showed non-monotonic change with time and nominal strain. The evaluated local strains in the vicinity of the notch showed relatively small amount, which might cause the very long creep life compared with smooth specimen under the same nominal stress condition.

Texture of nanocrystalline solids: atomic scale characterization and applications

A methodology is presented to characterize the crystallographic texture of atomic configurations on the basis of Euler angles. Texture information characterized by orientation map, orientation distribution function, texture index, pole figure and inverse pole figure is obtained. The paper reports the construction and characterization of the texture of nanocrystalline configurations with different grain numbers, grain sizes and percentages of preferred orientation. The minimum grain number for texture-free configurations is ∼2500. The effect of texture on deducing grain size from simulated X-ray diffraction curves is also explored as an application case of texture analysis. In addition, molecular dynamics simulations are performed on initially texture-free nanocrystalline Ta under shock-wave loading, which shows a 〈001〉 + 〈111〉 double fiber texture after shock-wave compression.

Inverse pole figure mapping of bulk crystalline grains in a polycrystalline steel plate by pulsed neutron Bragg-dip transmission imaging

A new mapping procedure for polycrystals using neutron Bragg-dip transmission is presented. This is expected to be useful as a new materials characterization tool which can simultaneously map the crystallographic direction of grains parallel to the incident beam. The method potentially has a higher spatial resolution than neutron diffraction imaging. As a demonstration, a Bragg-dip neutron transmission experiment was conducted at J-PARC on beamline MLF BL10 NOBORU. A large-grained Si–steel plate was used. Since this specimen included multiple grains along the neutron beam transmission path, it was a challenging task for existing methods to analyse the direction of the crystal lattice of each grain. A new data-analysis method for Bragg-dip transmission measurements was developed based on database matching. As a result, the number of grains and their crystallographic direction along the neutron transmission path have been determined.