Differential Aperture X-ray Microscopy Research Articles

Low temperature recovery of OFF-state stress induced degradation of AlGaN/GaN high electron mobility transistors

Thermal annealing is a widely used strategy to enhance semiconductor device performance. However, the process is complex for multi-material multi-layered semiconductor devices, where thermoelastic stresses from lattice constant and thermal expansion coefficient mismatch may create more defects than those annealed. We propose an alternate low temperature annealing technique, which utilizes the electron wind force (EWF) induced by small duty cycle high density pulsed current. To demonstrate its effectiveness, we intentionally degrade AlGaN/GaN high electron mobility transistors (HEMTs) with accelerated OFF-state stressing to increase ON-resistance ∼182.08% and reduce drain saturation current ∼85.82% of pristine condition at a gate voltage of 0 V. We then performed the EWF annealing to recover the corresponding values back to ∼122.21% and ∼93.10%, respectively. The peak transconductance, degraded to ∼76.58% of pristine at the drain voltage of 3 V, was also recovered back to ∼92.38%. This recovery of previously degraded transport properties is attributed to approximately 80% recovery of carrier mobility, which occurs during EWF annealing. We performed synchrotron differential aperture x-ray microscopy measurements to correlate these annealing effects with the lattice structural changes. We found a reduction of lattice plane spacing of (001) planes and stress within the GaN layer under the gate region after EWF annealing, suggesting a corresponding decrease in defect density. Application of this low-temperature annealing technique for in-operando recovery of degraded electronic devices is discussed.

Recovery of deformation microstructure in the bulk interior revealed by synchrotron X-ray micro-diffraction

A synchrotron X-ray micro-diffraction based technique, namely differential aperture X-ray microscopy (DAXM), is used to investigate the subgrain evolution in the bulk interior during ex-situ recovery annealing of a lightly deformed Al sample. This is the first 4D DAXM study of recovery. Supplementary TEM observations reveal that DAXM enables proper reconstruction of the microstructure, with the important addition that it is in 3D. The DAXM result provides a direct observation of the 3D microstructural evolution during annealing and confirms that subgrain boundary migration is an important mechanism for subgrain growth, preferentially removing small subgrains. Moreover, it gives new evidence of subgrain coalescence as an alternative mechanism for subgrain growth—some neighboring subgrains are observed to merge due to the decrease of the misorientation angle across certain subgrain boundaries. A possible interaction of the two recovery mechanisms is discussed, as well as the advantage and limitation of the DAXM technique for recovery studies.

Non-destructive depth-resolved characterization of residual strain fields in high electron mobility transistors using differential aperture x-ray microscopy

Localized residual stress and elastic strain concentrations in microelectronic devices often affect the electronic performance, resistance to thermomechanical damage, and, likely, radiation tolerance. A primary challenge for the characterization of these concentrations is that they exist over sub-μm length-scales, precluding their characterization by more traditional residual stress measurement techniques. Here, we demonstrate the use of synchrotron x-ray-based differential aperture x-ray microscopy (DAXM) as a viable, non-destructive means to characterize these stress and strain concentrations in a depth-resolved manner. DAXM is used to map two-dimensional strain fields between the source and the drain in a gallium nitride (GaN) layer within high electron mobility transistors (HEMTs) with sub-μm spatial resolution. Strain fields at various positions in both pristine and irradiated HEMT specimens are presented in addition to a preliminary stress analysis to estimate the distribution of various stress components within the GaN layer. γ-irradiation is found to significantly reduce the lattice plane spacing in the GaN along the sample normal direction, which is attributed to radiation damage in transistor components bonded to the GaN during irradiation.

Recovery of small orientation gradients in a recrystallized grain observed in 3D during ex-situ annealing

We report the observation of a small orientation gradient (~0.5° over a few micrometers) in a recrystallized grain formed at the sample surface in pure Al and the elimination of this orientation gradient during subsequential ex-situ annealing. The observation is in 3D and done using synchrotron white-beam differential-aperture X-ray microscopy with high angular resolution of 0.01°. The elimination of the orientation gradient during subsequent annealing are analyzed and discussed.

Processing Laue Microdiffraction Raster Scanning Patterns with Machine Learning Algorithms: A Case Study with a Fatigued Polycrystalline Sample.

The massive amount of diffraction images collected in a raster scan of Laue microdiffraction calls for a fast treatment with little if any human intervention. The conventional method that has to index diffraction patterns one-by-one is laborious and can hardly give real-time feedback. In this work, a data mining protocol based on unsupervised machine learning algorithm was proposed to have a fast segmentation of the scanning grid from the diffraction patterns without indexation. The sole parameter that had to be set was the so-called “distance threshold” that determined the number of segments. A statistics-oriented criterion was proposed to set the “distance threshold”. The protocol was applied to the scanning images of a fatigued polycrystalline sample and identified several regions that deserved further study with, for instance, differential aperture X-ray microscopy. The proposed data mining protocol is promising to help economize the limited beamtime.

Revealing the effect of local stresses on twin growth mechanisms in titanium using synchrotron X-ray diffraction

Deformation twinning has a significant impact on the evolution of microstructure and mechanical response in hexagonal close packed (hcp) metals. Understanding the physical mechanisms associated with twin nucleation and growth processes is important to enable the broad use of hcp metals. While both nucleation and growth are conditioned by the local stress state, few studies have quantified it in bulk deformed samples. In this study, we reveal the effects of local stresses on twin thickening using in-situ synchrotron experiments with differential aperture X-ray microscopy. High purity Ti is deformed under four-point bending to activate {1012} tensile twins. 3D stress fields with a spatial resolution of 0.5μm are mapped in the vicinity of low and high macroscopic Schmid factor (MSF) twins growing inside a grain. Reconciling this experimental analysis with recent twin growth models reveals that the growth of low MSF twins is limited by the nucleation of twin growth defects in the high-purity Ti sample. While growth-inducing defect nucleation may occur in many places on the interface of the high resolved shear stress (RSS)/MSF twin, the nucleation rate of twinning disconnections for the low MSF twin is high only at the end of the twin near a stress concentration. Once nucleated, these defects easily propagate into lower RSS regions of the grain resulting in twin growth. This work highlights the importance of stress concentrations not only for twin (embryo) nucleation, but also to the growth process of twins, particularly in low MSF twins.

Recrystallization boundary migration in the 3D heterogeneous microstructure near a hardness indent

Boundary migration during recrystallization in the heterogeneous microstructure near a hardness indent in lightly rolled pure Al was followed in 3D. The microstructure after multiple steps of ex-situ annealing was examined using synchrotron white-beam differential-aperture X-ray microscopy, supplemented by scanning electron microscopy. Heterogeneous recrystallization boundary migration was observed and analyzed in terms of driving force and boundary characteristics. The results reveal very similar local stored energies and boundary misorientations for the migrating and stationary boundary segments, whereas the grain boundary normals differ significantly. Effects of grain boundary mobility and deformation microstructure morphology on the migration are discussed.

In Situ Synchrotron X-ray Micro-Diffraction Investigation of Elastic Strains in Laminated Ti-Al Composites

Spatially resolved elastic strains in the bulk interior of a laminated Ti-Al metal composite were studied during in situ tensile loading at strains up to 1.66% by a synchrotron-based micro-diffraction technique, namely differential aperture X-ray microscopy (DAXM). For both Al and Ti grains, deviatoric elastic strains were estimated based on polychromatic X-ray microbeam diffraction, while lattice strains along the normal direction of the tensile sample were directly measured using monochromatic X-ray microbeam diffraction. The estimated deviatoric strains show large spatial variations, and the mean values are consistent with the external loading conditions, i.e., increasing tensile strain along the tensile direction and increasing compressive strain along the sample normal with increasing loading. The directly measured lattice strains also show large spatial variations, although the magnitude of this variation is smaller than that for the estimated deviatoric strain. The directly measured lattice strains in Ti grains are largely consistent with the external loading, whereas those in Al grains are in contradiction with the external loading. The causes of the experimental results are discussed and related to both the laminated microstructure of the composite material and the limitations of the techniques.

In-situ synchrotron X-ray micro-diffraction investigation of ultra-low-strain deformation microstructure in laminated Ti-Al composites

An ultra-low-strain deformation microstructure was revealed for the first time non-destructively in the bulk interior of an annealed laminated Ti-Al composite—in the “fully recrystallized” Al layer—by a synchrotron-based micro-diffraction technique, namely differential aperture X-ray microscopy (DAXM), through real space mapping with a very high angular resolution (0.01°). This ultra-low-strain deformation microstructure was found to result from the thermal stress, induced during cooling after annealing, due to the different coefficients of thermal expansion for the Ti and Al layers. The annealed sample was further tensile deformed to a strain of 1.66% and followed by in situ DAXM and analyzed by various misorientation mapping methods. The results pointed to the important effects of the initial microstructure and the interface constraint, as well as the grain size and crystal orientation, on the plastic deformation. A gradient in dislocation density from the layer interface to the center was found in the Al layer of the annealed sample, and this gradient increased slightly during tensile deformation. The variation of the dislocation density was further discussed based on the activation and interaction of dislocations in grains of different sizes and orientations during plastic deformation. The findings of this study provided valuable insights in understanding the constraint effect of the laminated metal composite and the design of novel composite materials.

Study of the dislocation activity in a [sbnd]Mg–Y alloy by differential aperture X-ray microscopy

Differential aperture X-ray microscopy (DAXM) was employed in this study to reveal the development of geometrically necessary dislocations (GNDs) when a Mg–5 wt% Y alloy is subjected to bending. The characters of the GNDs were determined from the streak direction of the diffraction peaks. Five grains were analyzed voxel by voxel for the GND content. Among those voxels with peak streaking, 77% contain basal or non-basal 〈a〉 GNDs, while the other 23% contain 〈c + a〉 GNDs. Those 〈c + a〉 dislocations were nucleated to accommodate local stress concentration. Even though 〈c + a〉 dislocations are less mobile, they make important contribution to the high ductility of Mg–Y alloys.

Deformation twinning and grain partitioning in a hexagonal close-packed magnesium alloy

Pervasive deformation twinning in magnesium greatly affects its strength and formability. The local stress fields associated with twinning play a key role on deformation behavior and fracture but are extremely difficult to characterize experimentally. In this study, we perform synchrotron experiments with differential-aperture X-ray microscopy to measure the 3D stress fields in the vicinity of a twin with a spatial resolution of 0.5 micrometer. The measured local stress field aids to identify the sequence of events involved with twinning. We find that the selected grain deforms elastically before twinning, and the twin formation splits the grain into two non-interacting domains. Under further straining one domain of the grain continued to deform elastically, whereas the other domain deforms plastically by prismatic slip. This heterogeneous deformation behavior may be mediated by the surrounding medium and it is likely to lead to asymmetric twin growth.

Comparison of dislocation content measured with transmission electron microscopy and micro-Laue diffraction based streak analysis

The subsurface dislocation content in a Ti-5Al-2.5Sn (wt%) uniaxial tension sample deformed at ambient temperature was characterized by peak streak analysis of micro-Laue diffraction patterns collected non-destructively by differential aperture X-ray microscopy, and with focused ion beam transmission electron microscopy of material in the same volume. This comparison reveals that micro-Laue diffraction streak analysis based on an edge dislocation assumption can accurately identify the dominant dislocation slip system history (Burgers vector and plane observed by TEM), despite the fact that dislocations have predominantly screw character. Other dislocations identified by TEM were not convincingly discernible from the peak streak analysis.

Evidence of 3D strain gradients associated with tin whisker growth

We have used Differential Aperture X-ray Microscopy (DAXM) to measure grain orientations and deviatoric elastic strains in 3D around a tin whisker. The results show strain gradients through the depth of the tin coating, revealing a higher strain deeper in the Sn layer. These higher strains are explained by the volume change occurring during growth of the intermetallic phase Cu6Sn5 at the interface between the Cu substrate and the Sn coating and at grain boundaries between Sn grains.

Subsurface Grain Morphology Reconstruction by Differential Aperture X-ray Microscopy

A multistep, non-destructive grain morphology reconstruction methodology that is applicable to near-surface volumes is developed and tested on synthetic grain structures. This approach probes the subsurface crystal orientation using differential aperture x-ray microscopy on a sparse grid across the microstructure volume of interest. Resulting orientation data are clustered according to proximity in physical and orientation space and used as seed points for an initial Voronoi tessellation to (crudely) approximate the grain morphology. Curvature-driven grain boundary relaxation, simulated by means of the Voronoi implicit interface method, progressively improves the reconstruction accuracy. The similarity between bulk and readily accessible surface reconstruction error provides an objective termination criterion for boundary relaxation.

Direct observation of nucleation in the bulk of an opaque sample

Remarkably little is known about the physical phenomena leading to nucleation of new perfect crystals within deformed metals during annealing, in particular how and where volumes with nearly perfect lattices evolve from structures filled with dislocations, and how local variations at the micrometer length scale affect this nucleation process. We present here the first experimental measurements that relate directly nucleation of recrystallization to the local deformation microstructure in the bulk of a sample of cold rolled aluminum, further deformed locally by a hardness indentation. White beam differential aperture X-ray microscopy is used for the measurements, allowing us to map a selected gauge volume in the bulk of the sample in the deformed state, then anneal the sample and map the exact same gauge volume in the annealed state. It is found that nuclei develop at sites of high stored energy and they have crystallographic orientations from those present in the deformed state. Accordingly we suggest that for each nucleus the embryonic volume arises from a structural element contained within the voxels identified with the same orientation. Possible nucleation mechanisms are discussed and the growth potentials of the nuclei are also analyzed and discussed.

Three-dimensional local residual stress and orientation gradients near graphite nodules in ductile cast iron

A synchrotron technique, differential aperture X-ray microscopy (DAXM), has been applied to characterize the microstructure and analyze the local mesoscale residual elastic strain fields around graphite nodules embedded in ferrite matrix grains in ductile cast iron. Compressive residual elastic strains are measured with a maximum strain of ∼6.5–8 × 10−4 near the graphite nodules extending into the matrix about 20 μm, where the elastic strain is near zero. The experimental data are compared with a strain gradient calculated by a finite element model, and good accord has been found but with a significant overprediction of the maximum strain. This is discussed in terms of stress relaxation during cooling or during storage by plastic deformation of the nodule, the matrix or both. Relaxation by plastic deformation of the ferrite is demonstrated by the formation of low energy dislocation cell structure also quantified by the DAXM technique.

Effect of realistic 3D microstructure in crystal plasticity finite element analysis of polycrystalline Ti-5Al-2.5Sn

The effect of constitutive parameters and microstructure on the kinematic and constitutive responses within grains in a crystal plasticity finite element (CPFE) simulation of a polycrystalline titanium alloy are compared with experimental results. The simulation of a Ti-5Al-2.5Sn sample deformed in uniaxial tension at room temperature used a phenomenological power-law based CPFE model, which includes four families of slip systems commonly observed in structural metals with a hexagonal lattice structure. The experimentally characterized microstructure patch was approximated by a quasi-3D columnar grain structure and by a more realistic 3D representation. The quasi-3D microstructure was generated by extending the EBSD characterized surface microstructure in the depth direction, while the 3D microstructure was built based on subsurface orientation information acquired using differential-aperture X-ray microscopy (DAXM). The effect of grain morphology and constitutive parameters on simulation results are compared in terms of stress–strain responses and lattice reorientation.

Microstructural characterization of polycrystalline materials by synchrotron X-rays

Third generation synchrotron X-rays provide an unprecedented opportunity for microstructural characterization of many engineering materials as well as natural materials. This article demonstrates the usage of three techniques for the study of structural materials: differential-aperture X-ray microscopy (DAXM), three-dimensional X-ray diffraction (3DXRD), and simultaneous wide angle/small angle X-ray scattering (WAXS/SAXS). DAXM is able to measure the 3D grain structure in polycrystalline materials with high spatial and angular resolution. In a deformed material, streaked diffraction peaks can be used to analyze local dislocation content in individual grains. Compared to DAXM, 3DXRD is able to map grains in bulk materials more quickly at the expense of spatial resolution. It is very useful for studying evolving microstructures when the materials are under deformation. WAXS/SAXS is suitable for studying materials with inhomogeneous structure, such as precipitate strengthened alloys. Structural information revealed by WAXS and SAXS can be combined for a deeper insight into material behavior. Future development and applications of these three techniques will also be discussed.

Spatially resolved in situ strain measurements from an interior twinned grain in bulk polycrystalline AZ31 alloy

In this paper, we report for the first time, to our knowledge, spatially resolved measurements of strain gradients across a grain containing twins, located in the bulk of a polycrystalline Mg AZ31 sample. We also report orientation mapping on three parallel sections from the bulk of the sample. We use for such purpose the technique of differential-aperture X-ray microscopy (DAXM) based on synchrotron X-rays. The DAXM technique allows us to map crystallographic strains with sub-micron-sized spatial resolution. The results of this experiment confirm indirect evidence from previous experiments having less spatial resolution that important stress gradients exist in the vicinity of twin boundaries. Such a result is relevant to understanding twin growth and de-twinning, since both mechanisms are affected by stress-driven twin dislocations at the interface.

Diffraction Across the Length Scale

This JOM special topic concerns advances in neutron and x-ray diffraction directed toward the study of the structural properties of materials at different length scales. This year, the scientific community is celebrating 100 years since the discovery of x-ray diffraction by Laue, Bragg, and the first experimental diffraction pattern obtained from a crystal by Friedrich and Knipping. After their experiment, it was immediately understood that x-rays could become one of the most powerful tools for material characterization, medicine, etc. During the last century, the importance of diffraction methods utilizing x-rays and later electrons, neutrons, and Mossbauer photons increased enormously, while their area of applicability significantly broadened. For example, compare the first laboratory x-ray tubes of 100 years ago with the ultrabrilliant synchrotron x-ray sources of today that have opened fantastic possibilities for the analysis of structure in a broad spectrum of materials. In the first decades of x-ray science, the studies of regular Laue–Bragg reflections, specifically their positions and intensities, were the primary focus of research. This allowed the characterization of crystallographic structure starting from simple lattices and moving toward complicated organic structures. Diffraction studies became the basis for the development of crystallography, physics, chemistry, etc. Later, the focus of x-ray science moved to the understanding of both static and dynamic lattice defects. In real crystals, thermal vibrations and other defects disturb periodic atomic arrangements and alter the diffracted intensity. Diffraction lines and reflections may broaden, their intensity weakens, and sometimes the so-called diffuse scattering appears around and between the Bragg peaks. The character of the scattered intensity relates to the shortand long-range order, type, and symmetry of defects, the interaction energy between different atoms, the Fermi surface, and phonons, and critical fluctuations near phase transitions, etc. The selected manuscripts present a slice of several x-ray and neutron diffraction applications that are currently the focus of research in different laboratories worldwide. These manuscripts demonstrate the possibilities of materials studies utilizing small local probes providing information about submicron volumes in those materials and larger beams probing statistically averaged properties of those materials. First, three manuscripts titled ‘‘Microscopic Deformation in Individual Grains in an Advanced High-Strength Steel’’ by Zhenzhen Yu, Roziliya Barabash, Oleg Barabash, Wenjun Liu, and Zhili Feng, ‘‘Acquisition, Sharing and Processing of Large Datasets for Strain Imaging: An Example of an Indented Ni3Al/Mo Composite’’ by N.S. McIntyre, R.I. Barabash, J. Qin, M. Kunz, N. Tamura, and H. Bei and ‘‘In Situ Observation of the Dislocation Structure Evolution During a Strain Path Change in Copper’’ by Christian Wejdemann, Henning Friis Poulsen, Ulrich Lienert, and Wolfgang Pantleon present example applications of different versions of x-ray microdiffraction with a beamsize 0.5 lm for in-depth understanding of the mechanical behavior at the mesoscale. Several versions of microdiffraction were developed: Polychromatic x-ray microdiffraction (PXM) and differential aperture x-ray microscopy (DAXM) were developed by Oak Ridge National Laboratory at the microdiffraction beamline 34-IDE at the Advanced Photon Source at Argonne National Laboratory. This method is directly based on Laue diffraction experiments. Modernization of the beam line apparatus and the high brilliance of synchrotron emissions create unusual opportunities to extract the mesoscale information about the materials. A slightly different version of this technique is described by McIntyre et al. where it is employed at the microdiffraction beamline at the Advanced Light Source. The PXM technique utilizes both a monochromatic beam and a white (pink) beam with an energy range of 5–25 keV. It is best suited to study defects in polycrystalline or multiphase materials Rozaliya I. Barabash is the guest editor for the Mechanical Behavior of Materials Committee of the TMS Structural Materials Division, and coordinated the topic Diffraction Studies across the Length Scales in this issue. JOM, Vol. 65, No. 1, 2013