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All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Surface plasmon polaritons in a composite system of porous silicon and gold

A composite system of silicon quantum dots and gold particles with properties periodically changing along the surface (i.e., a system exhibiting the properties of a diffraction grating) is obtained by a one-step metal-assisted chemical etching. The spectral and angular dependences of the photoresponse for the composite system on single-crystal silicon are studied. The photoresponse peaks were observed, which behavior (the dependence on the parameters of the diffraction grating, wavelength and incidence angles of light) is attributed to the excitation of plasmon-polariton modes at the surface of the composite system with the diffraction grating. At the same time, the obtained values of the wave vectors for these modes are smaller than those calculated for plasmon polaritons excited at the interface between air and metal (gold) diffraction grating.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Neutron and x-ray scattering study of phonon dispersion and diffuse scattering in ( Na , Bi ) Ti O 3 - x BaTi O 3 single crystals near the morphotropic phase boundary

Neutron and x-ray scattering measurements were performed on (Na 1/2 Bi 1/2 )TiO 3-x at % BaTiO 3 (NBT-xBT) single crystals (x = 4, 5, 6.5, and 7.5) across the morphotropic phase boundary (MPB), as a function of both composition and temperature, and probing both structural and dynamical aspects. In addition to the known diffuse scattering pattern near the gamma points, our measurements revealed new, faint superlattice peaks, as well as an extensive diffuse scattering network, revealing a short-range ordering of polar nanoregions (PNR) with a static stacking morphology. Furthermore, in samples with compositions closest to the MPB, our inelastic neutron scattering investigations of the phonon dynamics showed two unusual features in the acoustic phonon branches, between the superlattice points, and between the superlattice points and gamma points, respectively. Finally, these critical elements are not present in the other compositions away from the MPB, which suggests that these features may be related to the tilt modes coupling behavior near the MPB.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synthesis of new Ga{sub 7}Ge{sub 3}Te{sub 11}—One-dimensional phase in Ga–Ge–Te ternary system: Crystal structure and physical properties

Ga{sub 7}Ge{sub 3}Te{sub 11} single crystals were grown using transport in a temperature gradient. The prepared single crystals have a needle-like appearance, 1 to 40 μm in diameter and 2 to 20 mm in length. The Ga{sub 7}Ge{sub 3}Te{sub 11} single crystals were characterized by both single crystalline and powder X-ray diffraction. The structure consists of one-dimensional molecular strings bonded together through tellurium atoms by a weak bond of the van der Waals type. A large unit cell volume indicates the complexity of the structure. The room temperature theoretical density is 5.73 g cm{sup −3}. The compound shows semiconducting properties and possesses peculiar magnetic properties. - Graphical abstract: 1D Ga{sub 7}Ge{sub 3}Te{sub 11} - cross-view of the molecular string. - Highlights: • We prepared and characterized novel compound Ga{sub 7}Ge{sub 3}Te{sub 11}in single crystalline form. • Ga{sub 7}Ge{sub 3}Te{sub 11} is one-dimensional structure; orthorhombic lattice; space group Pnma. • Ga{sub 7}Ge{sub 3}Te{sub 11} is an intrinsic semiconductor; E{sub g}≅0.7 eV. • Ga{sub 7}Ge{sub 3}Te{sub 11} is paramagnetic with an effective magnetic moment of p{sub eff}=0.06 μ{sub B} per atom. • Crossover from para – to diamagnetism is observed in field dependence of susceptibility.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Deeper Understanding of Mononuclear Manganese(IV)–Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)–Hydroxide Complex

Binding of Lewis acidic metal ions and Brønsted acid at the metal–oxo group of high-valent metal–oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal–oxo group has been questioned in several cases and remains to be clarified. In this work, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)–oxo complex binding triflic acid, {[(dpaq)Mn IV (O)]–HOTf} + (1–HOTf). First, 1–HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn–O stretching frequencies of 1–HOTf (e.g., ν Mn–O at ~793 cm –1 ) and 1–M n+ (Mn + = Ca 2+ , Zn 2+ , Lu 3+ , Al 3+ , or Sc 3+ ) and the Lewis acidities of H + and M n+ ions, suggesting that H+ and Mn+ bind at the metal–oxo moiety of [(dpaq)Mn IV (O)] + . Interestingly, a single-crystal structure of 1–HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)–oxo complex but a Mn(IV)–hydroxide complex, [(dpaq)Mn IV (OH)](OTf) 2 (4), with a Mn–O bond distance of 1.8043(19) Å and a Mn–O stretch at 660 cm –1 . More interestingly, 4 reverted to 1–HOTf upon dissolution, demonstrating that 1–HOTf and 4 are interconvertible depending on the physical states, such as 1–HOTf in solution and 4 in isolated solid. The reactivity of 1–HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1–Mn+ complexes; an interesting correlation between the Mn–O stretching frequencies of 1–HOTf and 1–M n+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

α-Particle Detection and Charge Transport Characteristics in the A 3 M 2 I 9 Defect Perovskites (A = Cs, Rb; M = Bi, Sb)

Here, we have investigated the defect perovskites A 3 M 2 I 9 (A = Cs, Rb; M = Bi, Sb) as materials for radiation detection. The phase purity of Bridgman-grown A 3 M 2 I 9 single crystals was confirmed via high-resolution synchrotron X-ray diffraction, while density functional theory calculations (DFT) show surprisingly dispersive bands in the out-of-plane direction for these layered materials, with low effective masses for both holes and electrons. Accordingly, each of the four A 3 M 2 I 9 defect perovskites showed response to 241 Am α-particle irradiation for hole and electron electrode configurations, a remarkable ambipolar response that resembles the 3D halide perovskites. The electron response spectra were used to estimate the mobility–lifetime product (μτ) e for electrons in these materials, with Rb 3 Bi 2 I 9 showing the lowest (μτ) e value of 1.7 × 10 – 6 cm 2 V –1 and Cs 3 Bi 2 I 9 the highest (μτ) e of 5.4 × 10 –5 cm 2 V –1 . The rise time of the α-particle-generated pulse was used to estimate the electron mobility μ e of the A 3 M 2 I 9 defect perovskites, which ranged from 0.32 cm 2 V –1 s–1 for Rb 3 Sb 2 I 9 to 4.3 cm 2 V –1 s –1 in Cs 3 Bi 2 I 9 . Similar analysis of the hole response spectra yielded (μτ) h values for each A 3 M 2 I 9 compound, with Cs 3 Bi 2 I 9 again showing the highest (μτ) h value of 1.8 × 10 –5 cm 2 V –1 , while Rb 3 Bi 2 I 9 showed the lowest (μτ) h with 2.0 × 10 –6 cm 2 V –1 . Rise time analysis gave hole mobilities ranging from 1.7 cm 2 V –1 s –1 for Cs 3 Bi 2 I 9 to 0.14 cm 2 V –1 s –1 for Cs 3 Sb 2 I 9 . Comparing the experimental electron and hole mobilities to the effective masses obtained from DFT calculations revealed sizable discrepancies, possibly indicating self-trapping of charge carriers due to electron–phonon interactions. The α-particle response of the A 3 M 2 I 9 defect perovskites demonstrates their potential as semiconductor radiation detectors, with Cs 3 Bi 2 I 9 and Cs 3 Sb 2 I 9 showing the most promise.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Long term stable deep red light-emitting electrochemical cells based on an emissive, rigid cationic Ir( iii ) complex

Here, the new cationic iridium complex [Ir(bzq) 2 (biq)][PF 6 ] (bzq = benzo[ h ]quinolinato and biq = 2,2'-biquinoline) has been synthesized for application as an emitter in light emitting electrochemical cells (LECs). The molecular structure and crystal packing of this complex were established by single X-ray diffraction (SXRD). The electrochemical and photophysical properties of the complex were examined to determine the frontier orbital energies as well as the optical transitions that led to photoemission. The complex was found to emit at 644 nm and 662 nm for powder and thin films, respectively. A high powder photoluminescence quantum yield of 25% was determined, which is attributed to a reduction in vibrational modes of the complex due to the use of the rigid cyclometalated (C^N) bzq ligand. A LEC with [Ir(bzq) 2 (biq)][PF 6 ] as the emitter was fabricated which showed a deep red emission (662 nm) with a luminance of 33.65 cd m –2 , yielding a current efficiency of 0.33 cd A –1 and a power efficiency of 0.2 lm W –1 . Most importantly, the LEC based on [Ir(bzq) 2 (biq)][PF 6 ] demonstrated a lifetime of 280 hours which is among the longest device lifetimes reported for any deep red light emitting LEC.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Shape-controlled electrodeposition of single Pt nanocrystals onto carbon nanoelectrodes

In this paper, we report the electrosynthesis and characterization of individual, shape-controlled Pt nanocrystals (NCs) electrodeposited on carbon nanoelectrodes (CNEs). Single Pt NCs were deposited onto the CNEs using an empirically developed square-wave potential program. Characterization by scanning electron microscopy indicates that the sizes of Pt NCs are remarkably reproducible (relative standard deviation = 6%). Electrochemically active surface areas, determined by Cu underpotential deposition and H adsorption/desorption analyses, are also reproducible. Selected area electron diffraction indicates that each Pt NC is comprised of just one single crystal (no grain boundaries). Although different square-wave potential programs lead to different types of crystals, the Pt NCs discussed here have a concave hexoctahedral geometry bound primarily by {13 6 2} surface facets. The results in this report represent a first step toward our ultimate goal of studying electrocatalysis at individual, shape-controlled, single-crystal nanoparticles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Nickel(ii) complexes based on dithiolate–polyamine binary ligand systems: crystal structures, hirshfeld surface analysis, theoretical study, and catalytic activity study on photocatalytic hydrogen generation

To ascertain the influence of binary ligand systems [1,1-dicyanoethylene-2,2-dithiolate (i-mnt -2 ) and polyamine {tetraen = tris(2-aminoethyl)amine, tren = diethylene triamine and opda = o-phenylenediamine}] on the coordination modes of the Ni(II) metal center and resulting supramolecular architectures, a series of nickel(II) thiolate complexes [Ni(tetraen)(i-mnt)](DMSO) (1), [Ni 2 (tren) 2 (i-mnt) 2 ] (2), and [Ni 2 (i-mnt) 2 (opda) 2 ] n (3) have been synthesized in high yield in one step in water and structurally characterized by single crystal X-ray crystallography and spectroscopic techniques. X-ray diffraction studies disclose the diverse i-mnt -2 coordination to the Ni +2 center in the presence of active polyamine ligands, forming a slightly distorted octahedral geometry (NiN 4 S 2 ) in 1, square planar (NiS 4 ) and distorted octahedral geometries (NiN 6 ) in the bimetallic co-crystallized aggregate of cationic [Ni(tren) 2 ] +2 and anionic [Ni(i-mnt) 2 ] -2 in 2, and a one dimensional (1D) polymeric chain along the [100] axis in 3, having consecutive square planar (NiS 4 ) and octahedral (NiN 6 ) coordination kernels. The N–H···O, N–H···S, N–H···N, N–H···S, N–H···N, and N–H···O type hydrogen bonds stabilize the supramolecular assemblies in 1, 2, and 3 respectively imparting interesting graph-set-motifs. The molecular Hirshfeld surface analyses (HS) and 2D fingerprint plots were utilized for decoding all types of non-covalent contacts in the crystal networks. Atomic HS analysis of the Ni +2 centers reveals significant Ni–N metal–ligand interactions compared to Ni–S interactions. We have also studied the unorthodox interactions observed in the solid state structures of 1–3 by QTAIM and NBO analyses. Moreover, all the complexes proved to be highly active water reduction co-catalysts (WRC) in a photo-catalytic hydrogen evolution process involving iridium photosensitizers, wherein 2 and 3 having a square planar arrangement around the nickel center(s) – were found to be the most active ones, achieving 1000 and 1119 turnover numbers (TON), respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Enhanced magnetic hysteresis in Ni-Mn-Ga single crystal and its influence on magnetic shape memory effect

Enhanced magnetic hysteresis due to boron doping in combination with magnetic shape memory effect in Ni-Mn-Ga single crystal results in new interesting functionality of magnetic shape memory (MSM) alloys such as mechanical demagnetization. In Ni{sub 50.0}Mn{sub 28.5}Ga{sub 21.5} single crystal, the boron doping increased magnetic coercivity from few Oe to 270 Oe while not affecting the transformation behavior and 10 M martensite structure. However, the magnetic field needed for MSM effect also increased in doped sample. The magnetic behavior is compared to undoped single crystal of similar composition. The evidence from the X-ray diffraction, magnetic domain structure, magnetization loops, and temperature evolution of the magnetic coercivity points out that the enhanced hysteresis is caused by stress-induced anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Pulsed laser deposition of single layer, hexagonal boron nitride (white graphene, h-BN) on fiber-oriented Ag(111)/SrTiO{sub 3}(001)

We report on the growth of 1–10 ML films of hexagonal boron nitride (h-BN), also known as white graphene, on fiber-oriented Ag buffer films on SrTiO{sub 3}(001) by pulsed laser deposition. The Ag buffer films of 40 nm thickness were used as substitutes for expensive single crystal metallic substrates. In-situ, reflection high-energy electron diffraction was used to monitor the surface structure of the Ag films and to observe the formation of the characteristic h-BN diffraction pattern. Further evidence of the growth of h-BN was provided by attenuated total reflectance spectroscopy, which showed the characteristic h-BN peaks at ∼780 cm{sup −1} and 1367.4 cm{sup −1}. Ex-situ photoelectron spectroscopy showed that the surface of the h-BN films is stoichiometric. The physical structure of the films was confirmed by scanning electron microscopy. The h-BN films grew as large, sub-millimeter sheets with nano- and micro-sheets scattered on the surface. The h-BN sheets can be exfoliated by the micromechanical adhesive tape method. Spectral analysis was performed by energy dispersive spectroscopy in order to identify the h-BN sheets after exfoliation. The use of thin film Ag allows for reduced use of Ag and makes it possible to adjust the surface morphology of the thin film prior to h-BN growth.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Magnetic order effects on the electronic structure of K M MnS 2 ( M = Cu , Li ) with the ThCr 2 Si 2 -type structure

We study the relationship between antiferromagnetic order and the electronic properties of KCuMnS 2 with the ThCr 2 Si 2 -type structure. We propose two magnetic structures for KCuMnS 2 with the ThCr 2 Si 2 -type structure. Powder samples of KCuMnS 2 and KLiMnS 2 were prepared for structural studies and magnetization measurements. In both compounds, the Mn 2+ site is alloyed by either Cu + or Li + . We also prepared single crystals of KCuMnS 2 for x-ray and neutron-diffraction studies and resistivity measurements. We relate these properties to the electronic structure calculated with density functional theory. Neutron-diffraction studies reveal that KCuMnS 2 exhibits long-range magnetic ordering with a Néel temperature near 160 K and a moment of 0.92(2) μ B /Mn 2+ at 6 K. In contrast, KLiMnS 2 never exhibits long-range magnetic ordering down to 3.5 K. Here, both sulfides never display a crystallographic phase transition from our temperature-dependent x-ray and neutron-diffraction studies. We discuss the magnetic phases in detail and how they relate to isostructural phases such as iron-based superconductors and related chalcogenides. Electrical resistance measurements indicate that while KCuMnS 2 is semiconducting, there is an anomaly around the Néel temperature, which indicates that long-range magnetism influences its electronic structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Growth from Solution, Structure, and Optical Properties of Single-Crystal para-Quaterphenyl Films

Samples of single-crystal para-quaterphenyl films up to 8 mm long and 50 μm thick have been obtained for the first time by the solvent‒antisolvent growth method. The films have been grown both on the liquid–air interface and in the solution bulk. The morphology, surface quality, and thickness of crystal films have been investigated by the optical, laser confocal, and atomic force microscopies. The structure of the para-quaterphenyl single crystal at room temperature has been refined by X-ray diffraction analysis. The formation and growth mechanisms of crystal films on the liquid–air interface have been discussed. The opticalabsorption and photoluminescence spectra of the solutions and crystal samples have been investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Accurate Refinement of the Crystal Structure of Ba{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14} from Langasite Family and Analysis of Structural Transformations Due to Isomorphous Cation Substitutions

An accurate structure analysis of a Ba{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14} single crystal from langasite family was performed using four X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector (sp. gr. P321, Z = 1, sinθ/λ ≤ 1.35 Å{sup –1}; at 295 K a = 8.516(1) Å, c = 5.1910(6) Å, R/wR = 0.58/0.56%, Δρ{sub min}/Δρ{sub max} =–0.73/0.42 e/Å{sup 3}, 4414 independent reflections; at 106 K a = 8.5109(9) Å, c = 5.1861(9) Å, R/wR = 0.75/0.86%, Δρ{sub min}/Δρ{sub max} =–0.81/1.06 e/Å{sup 3}, 4382 reflections). The distinguishing feature of the Ba{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14} structure is a strong disorder of the Ga atom at the 3f site. Structural transformations in the series of Сa{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14}–Sr{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14}–Ba{sub 3}TaGa{sub 3}Si{sub 2}O{sub 14}–Ba{sub 3}TaFe{sub 3}Si{sub 2}O{sub 14} crystals were analyzed.

36 MATERIALS SCIENCE↗
NASA NTRS1986

Abrasion and deformed layer formation of manganese-zinc ferrite in sliding contact with lapping tapes

Wear experiments were conducted using replication electron microscopy and reflection electron diffraction to study abrasion and the deformed layers produced in single-crystal Mn-Zn ferrite simulated heads during contact with lapping tapes. The crystaline state of the head is changed drastically during the abrasion process. Crystalline states ranging from nearly amorphous to highly textured polycrystalline can be produced on the wear surface of a single-crystal Mn-Zn ferrite head. The total thickness of the deformed layer was approximately 0.8 microns. This thickness increased as the load and abrasive grit size increased. The anisotropic wear of the ferrite was found to be inversely proportional to the hardness of the wear surface. The wear was lower in the order 211 111 10 0110. The wear of the ferrite increased markedly with an increase in sliding velocity and abrasive grit size.

Miyoshi, K.↗
DOE OSTI2017

Crystal defects in solar cells produced by the method of thermomigration

The results of studying the crystal structure of regions in silicon, recrystallized during the course of thermomigration of the liquid Si–Al zone in the volume of the silicon substrate, are reported (similar regions doped with an acceptor impurity are used to obtain high-voltage solar cells). X-ray methods (including measurements of both diffraction-reflection curves and topograms) and also high-resolution electron microscopy indicate that single-crystal regions in the form of a series of thin strips or rectangular grids are formed as a result of the thermomigration of liquid zones. Dislocation half-loops are detected in the surface layers of the front and back surfaces of the substrate. (311)-type defects are observed in the recrystallized regions.

14 SOLAR ENERGY↗
DOE OSTI2020

Atomic structures determined from digitally defined nanocrystalline regions

Nanocrystallography has transformed our ability to interrogate the atomic structures of proteins, peptides, organic molecules and materials. By probing atomic level details in ordered sub-10 nm regions of nanocrystals, scanning nanobeam electron diffraction extends the reach of nanocrystallography and in principle obviates the need for diffraction from large portions of one or more crystals. Scanning nanobeam electron diffraction is now applied to determine atomic structures from digitally defined regions of beam-sensitive peptide nanocrystals. Using a direct electron detector, thousands of sparse diffraction patterns over multiple orientations of a given crystal are recorded. Each pattern is assigned to a specific location on a single nanocrystal with axial, lateral and angular coordinates. This approach yields a collection of patterns that represent a tilt series across an angular wedge of reciprocal space: a scanning nanobeam diffraction tomogram. Using this diffraction tomogram, intensities can be digitally extracted from any desired region of a scan in real or diffraction space, exclusive of all other scanned points. Intensities from multiple regions of a crystal or from multiple crystals can be merged to increase data completeness and mitigate missing wedges. It is demonstrated that merged intensities from digitally defined regions of two crystals of a segment from the OsPYL/RCAR5 protein produce fragment-based ab initio solutions that can be refined to atomic resolution, analogous to structures determined by selected-area electron diffraction. In allowing atomic structures to now be determined from digitally outlined regions of a nanocrystal, scanning nanobeam diffraction tomography breaks new ground in nanocrystallography.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Optimization of the crystal growth of the superconductor CaKFe 4 As 4 from solution in the FeAs - CaFe 2 As 2 - KFe 2 As 2 system

Measurements of the anisotropic properties of single crystals play a crucial role in probing the physics of new materials. Determining a growth protocol that yields suitable high-quality single crystals can be particularly challenging for multicomponent compounds. Here we present a case study of how we refined a procedure to grow single crystals of CaKFe 4 As 4 from a high temperature, quaternary liquid solution rich in iron and arsenic (“FeAs self-flux”). Temperature dependent resistance and magnetization measurements are emphasized, in addition to the x-ray diffraction, to detect intergrown CaKFe 4 As 4 , CaFe 2 As 2 , and KFe 2 As 2 within what appear to be single crystals. Guided by the rules of phase equilibria and these data, we adjusted growth parameters to suppress formation of the impurity phases. The resulting optimized procedure yielded phase-pure single crystals of CaKFe 4 As 4 . In conclusion, this optimization process offers insight into the growth of quaternary compounds and a glimpse of the four-component phase diagram in the pseudoternary FeAs–CaFe 2 As 2 –KFe 2 As 2 system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature-dependent elastic anisotropy and mesoscale deformation in a nanostructured ferritic alloy

Nanostructured ferritic alloys (NFA) are a new class of ultrafine-grained oxide dispersion-strengthened steels, promising for service in extreme environments of high temperature and high irradiation in the next-generation of nuclear reactors. This is owing to the remarkable stability of their complex microstructures containing a high density of Y-Ti-O nanoclusters within grains and along the grain boundaries. While nanoclusters have been recognized to be the primary contributor to the exceptional resistance to irradiation and high-temperature creep, very little is known about the mechanical roles of the polycrystalline grains that constitute the bulk ferritic matrix. Here we report the mesoscale characterization of anisotropic responses of the ultrafine NFA grains to tensile stresses at various temperatures using the state-of-the-art in situ neutron diffraction. We show the first experimental determination of temperature-dependent single-crystal elastic constants for the NFA, and reveal a strong temperature-dependent elastic anisotropy due to a sharp decrease in the shear stiffness constant [c'=(c_11-c_12)/2] when a critical temperature ( T_c ) is approached, indicative of elastic softening and instability of the ferritic matrix. We also show, from anisotropy-induced intergranular strain/stress accumulations, that a common dislocation slip mechanism operates at the onset of yielding for low temperatures, while there is a deformation crossover from low-temperature lattice hardening to high temperature lattice softening in response to extensive plastic deformation.

36 MATERIALS SCIENCE↗