Residual strain gradient determination in metal matrix composites by synchrotron X-ray energy dispersive diffraction

An X-ray technique for the measurement of internal residual strain gradients near the continuous reinforcements of metal matrix composites has been investigated. The technique utilizes high intensity white X-ray radiation from a synchrotron radiation source to obtain energy spectra from small (10^-3 mm³) volumes deep within composite samples. The energy peak positions satisfy Bragg’s law and allow determination of the lattice parameter. As the probe volume is translated, the peaks of the spectra shift and are used to infer lattice spacing changes and thus strains with a precision of 10^-3 to 10^-4 (depending on the sample grain size/probe volume ratio). The viability of the technique has first been tested using a model system with 800 μm A1₂O₃ fibers and a commercial purity titanium matrix. For this system (which remained elastic on cooling), good agreement was observed between the measured residual radial and hoop strain gradients and those estimated from a simple elastic concentric cylinders model. The technique was then used to assess the strains near (SCS-6) silicon carbide fibers in a Ti-14Al-21Nb matrix after consolidation processing. Reasonable agreement between measured and calculated strains was seen provided the probe volume was located 50 μm or more from the fiber/matrix interface. Close to the interface, the measured elastic strains were smaller than anticipated, due to relaxation of the residual stress by plasticity and radial cracking during sample cooling. Formerly Graduate Student, Department of Materials Science, University of Virginia, Charlottesville     

Determination of long range order in Ni-Base ternary alloys by X-ray anomalous diffraction using synchrotron radiation

Long range order (LRO) parameters in three ternary Ll₂ alloys [Ni₇₅Al₁₅Ti₁₀, Ni₇₀Al₂₀Cr₁₀ and (Ni₃Fe)_96.4Cr_3.6] have been measured using a new method. This method takes advantage of the variation in atomic scattering factors for X-rays near absorption threshold and opens a new area of research with the development of synchrotron facilities. The results show that Ti substitutes mostly, but not exclusively, for Al in NiAlTi. In NiAlCr, Cr occupies both Ni and Al sites. In NiFeCr, the order is not perfect; the Cr atoms are distributed on both types of sites, with a slight preference for Fe sites.     

Mechanochemical mechanism for peptidyl free radical generation by amyloid fibrils

beta-Amyloid peptides (A beta) form the core of Alzheimer's disease (AD) senile plaques, and are implicated in AD neurotoxicity. A beta and some derivatives generate free radicals upon fibrilogenesis. A mechanism for free radical generation is proposed, based upon fibril cross beta-sheet structure: (1) During fibrilogenesis there is a small probability of mispacking of A beta monomers, resulting in abnormal fibril packing. (2) Continued fibrilogenesis traps a packing defect within the beta-sheet. Surrounding beta-sheet resists distortion, and the abnormally packed polypeptide(s) is strained. (3) Thermal processes cause homolytic bond scission and radical production from strained polypeptide through mechanically activated thermal decomposition. (4) Reaction with oxygen produces peroxy radicals, prevents unproductive radical recombination, and promotes observed cross-linking, production of reactive oxygen species and peptide fragmentation. Adiabatic mapping suggested significant strain would be generated by beta-sheet misalignment. The mechanism relates the common structure of fibrils to radical production, and may be relevant to cytotoxicity in prion and other amyloidoses.     

X-ray diffraction study of the structure of Al-Ni melts

The structures of Al-Ni melts with 0, 10, 18, 25, 28, 32, 40, 75, 90, and 100 at % Ni are studied by X-ray diffraction. The structural models of the melts, which were analyzed by the Voronoi-Delaunay transformation method, are reconstructed by a procedure that is inverse to the Monte Carlo method using experimental structure-factor curves. The parameters characterizing atomic ordering at short and longer distances are determined to change nonmonotonically in the range 10–40 at % Ni with an extreme in the vicinity of the melt containing 28 at % Ni. In this concentration range, nickel atoms are predominantly located in the pentagonal rings of polytetrahedral clusters of the icosahedral type; these atoms are ～4.1 Å from each other. Such clusters generate a prepeak in the structure-factor curves of the melts in the range 10–40 at % Ni.     

X-ray diffraction study of one-dimensionally disordered structure formation in Co-based alloys

Formation of one-dimensionally disordered (ODD) structures with non-random distribution of growth stacking faults (SF's) in Co-based alloys has been investigated by X-ray diffraction. It has been found that the usual process of growth of coarse crystals with fairly perfect h.c.p. lattice in pure Co is substituted for the accumulation of numerous thin non-randomly distributed lamellae with a h.c.p. structure in doped Co. This results in the growth SF's formation as isolated cubic type planes, clamped by neighbouring lamellae. The character of the SF's distribution in the final ODD structure is determined by ordering of the lamellae in the initial stages of the transformation.     

Ultrastructural organization of hemodialysis-associated beta 2-microglobulin amyloid fibrils

Fibrils of hemodialysis-associated beta 2-microglobulin amyloid were examined by high resolution electron microscopy and immunohistochemical labeling. The amyloid containing tissues obtained through autopsy were prepared for thin section observations. In contrast to other forms of amyloid, the most conspicuous feature of these fibrils were their curved conformations. The fibril core showed ultrastructural and immunohistochemical features in common with the core of connective tissue microfibrils and of previously observed fibrils of experimental murine AA amyloidosis and familial amyloid polyneuropathy (FAP). The core was wrapped in a layer of 3 nm wide ribbon-like "double tracked" structures identified as chondroitin sulfate proteoglycan (CSPG) with immunogold labeling as well as from the results of previous in vitro experiments. Finally, the outer surface of the fibril was associated with a loose assembly of 1 nm wide filaments immunohistochemically identified as beta 2-microglobulin. This is similar to the manner in which AA protein and transthyretin filaments are associated with their respective fibrils. The results of this study provide an additional example for the concept that amyloid fibrils in general are microfibril-like structures externally associated with amyloid protein filaments. An unusual feature of the fibrils of hemodialysis-associated amyloid, however, is the presence of a peripheral layer composed of CSPG rather than of heparan sulfate proteoglycan (HSPG) as in the case of the other two amyloids above. These chondroitin sulfate chains in the outer CSPG layer may be less effective in providing rigidity to the fibril core, thus allowing for the curved conformations of beta 2-microglobulin amyloid fibrils.     

X-ray diffraction study of myelin structure in immature and mutant mice

X-ray diffraction patterns were obtained from freshly dissected central and peripheral nerves of quaking, myelin synthesis deficiency (msd), and trembler mutants, as well as immature and adult normal mice. The patterns were compared with respect to strength of myelin diffraction, background scatter level, repeat period, and intensity and linewidth of Bragg reflections. The deficiency of myelin in optic nerves was found to be (in decreasing severity): quaking greater immature greater trembler approximately normal adult; and in sciatic nerves: trembler greater immature greater quaking greater msd approximately normal adult. Repeat periods about 3 A less than that for normal adult sciatic myelin were detected in corresponding nerves from immature, quaking, and trembler mice. In some trembler sciatic nerves a second phase having a 190-200 A period and accounting for about 60% of the total ordered myelin was also evident. Comparison of electron density profiles of membrane units calculated from the repeat periods and diffracted intensities for sciatic myelins indicate structural differences at the molecular level. The main findings are: (1) quaking myelin shows a significant elevation of density in the external protein-water layer between membrane bilayers; (2) the membrane bilayer of immature myelin is approximately equal to 2 A thinner than that for normal adult; (3) the membrane bilayer of the more compact phase in trembler myelin is approximately equal to 5 A thinner than for normal; and (4) the difference in repeat periods for the two phases present in some of the trembler nerves can be accounted for predominantly by distinct membrane bilayer separations at the external boundary.     

Texture distributions imaged by energy dispersive X-ray diffraction

An X-ray scanning apparatus has been developed for the study of texture topography and element distributions on bulk sample surfaces. The set-up consists of a white X-ray source, a collimator system to produce a narrow primary beam, an x-y sample stage with stepper motors, and an EDX detecting system. The apparatus is controlled by a microcomputer. The spectrum of secondary X-rays is composed of broad diffraction peaks, sharp characteristic fluorescence lines, and a low background of scattered radiation. Peak separation and intensity measurement is performed by energy dispersive X-ray spectroscopy (EDX). The density distributions of selected crystallographic directions or elements in the sample surface are acquired spot by spot, and represented by false-colour images. Several texture distributions as well as element composition images can be obtained simultaneously. Local resolution is presently limited to 0.2 mm, due to the low intensity of a collimated primary X-ray beam.     

Determination of subboundary misorientation in aluminum by X-ray diffraction

Abstract

A method has been developed for determining the average misorientation of subboundaries in both melt grown and polygonized single crystals of aluminum. Symmetrical tilt subboundaries were the predominant type of substructure found in both types of single crystals.

Résumé

Les auteurs ont mis au point une technique pour déterminer la désorientation moyenne des sous-joints dans des monocristaux d'aluminium obtenus soit à partir d'un bain liquide soit par polygonisation. Les sous-joints de flexion symétriques sont prédominants pour la sous-structure observées dans les deux types de monocristaux.     

Serum amyloid P component prevents proteolysis of the amyloid fibrils of Alzheimer disease and systemic amyloidosis

Extracellular deposition of amyloid fibrils is responsible for the pathology in the systemic amyloidoses and probably also in Alzheimer disease [Haass, C. & Selkoe, D. J. (1993) Cell 75, 1039-1042] and type II diabetes mellitus [Lorenzo, A., Razzaboni, B., Weir, G. C. & Yankner, B. A. (1994) Nature (London) 368, 756-760]. The fibrils themselves are relatively resistant to proteolysis in vitro but amyloid deposits do regress in vivo, usually with clinical benefit, if new amyloid fibril formation can be halted. Serum amyloid P component (SAP) binds to all types of amyloid fibrils and is a universal constituent of amyloid deposits, including the plaques, amorphous amyloid beta protein deposits and neurofibrillary tangles of Alzheimer disease [Coria, F., Castano, E., Prelli, F., Larrondo-Lillo, M., van Duinen, S., Shelanski, M. L. & Frangione, B. (1988) Lab. Invest. 58, 454-458; Duong, T., Pommier, E. C. & Scheibel, A. B. (1989) Acta Neuropathol. 78, 429-437]. Here we show that SAP prevents proteolysis of the amyloid fibrils of Alzheimer disease, of systemic amyloid A amyloidosis and of systemic monoclonal light chain amyloidosis and may thereby contribute to their persistence in vivo. SAP is not an enzyme inhibitor and is protective only when bound to the fibrils. Interference with binding of SAP to amyloid fibrils in vivo is thus an attractive therapeutic objective, achievement of which should promote regression of the deposits.     

Formation of the myosin.ADP.gallium fluoride complex and its solution structure by small-angle synchrotron X-ray scattering

The products of the SOS-regulated umuDC operon are required for most UV and chemical mutagenesis in Escherichia coli, a process that results from a translesion synthesis mechanism. The UmuD protein is activated for its role in mutagenesis by a RecA-facilitated autodigestion that removes the N-terminal 24 amino acids. A previous genetic screen for nonmutable umuD mutants had resulted in the isolation of a set of missense mutants that produced UmuD proteins that were deficient in RecA-mediated cleavage (J. R. Battista, T. Ohta, T. Nohmi, W. Sun, and G. C. Walker, Proc. Natl. Acad. Sci. USA 87:7190-7194, 1990). To identify elements of the UmuD' protein necessary for its role in translesion synthesis, we began with umuD', a modified form of the umuD gene that directly encodes the UmuD' protein, and obtained missense umuD' mutants deficient in UV and methyl methanesulfonate mutagenesis. The D39G, L40R, and T51I mutations affect residues located at the UmuD'2 homodimer interface and interfere with homodimer formation in vivo. The D75A mutation affects a highly conserved residue located at one end of the central strand in a three-stranded beta-sheet and appears to interfere with UmuD'2 homodimer formation indirectly by affecting the structure of the UmuD' monomer. When expressed from a multicopy plasmid, the L40R umuD' mutant gene exhibited a dominant negative effect on a chromosomal umuD+ gene with respect to UV mutagenesis, suggesting that the mutation has an effect on UmuD' function that goes beyond its impairment of homodimer formation. The G129D mutation affects a highly conserved residue that lies at the end of the long C-terminal beta-strand and results in a mutant UmuD' protein that exhibits a strongly dominant negative effect on UV mutagenesis in a umuD+ strain. The A30V and E35K mutations alter residues in the N-terminal arms of the UmuD'2 homodimer, which are mobile in solution.     

Synchrotron X-ray studies suggest that the core of the transthyretin amyloid fibril is a continuous beta-sheet helix

BACKGROUND: Amyloid diseases, which include Alzheimer's disease and the transmissible spongiform encephalopathies, are characterized by the extracellular deposition of abnormal protein fibrils derived from soluble precursor proteins. Although different precursors seem to generate similar fibrils, no adequate molecular structure of amyloid fibrils has been produced using modern techniques. Knowledge of the fibril structure is essential to understanding the molecular mechanism of amyloid formation and could lead to the development of agents to inhibit or reverse the process. RESULTS: The structure of amyloid fibrils from patients with familial amyloidotic polyneuropathy (FAP), which are derived from transthyretin (TTR) variants, has been investigated by fibre diffraction methods using synchrotron radiation. For the first time a significant high-angle diffraction pattern has been observed showing meridional reflections out to 2 A resolution. This pattern was fully consistent with the previously reported cross-beta structure for the fibril, but also reveals a new large scale fibre repeat of 115 A. We interpret this pattern as that of a repeating unit of 24 beta strands, which form a complete helical turn of beta sheet about an axis parallel to the fibre axis. This structure has not been observed previously. We have built a model of the protofilament of the FAP amyloid fibril based on this interpretation, composed of four beta sheets related by a single helix axis coincident with the fibre axis, and shown that it is consistent with the observed X-ray data. CONCLUSIONS: This work suggests that amyloid fibrils have a novel molecular structure consisting of beta sheets extended in regular helical twists along the length of the fibre. This implies that the polypeptide chains in the fibres are hydrogen-bonded together along the entire length of the fibres, thereby accounting for their great stability. The proposed structure of the FAP fibril requires a TTR building block that is structurally different from the native tetramer. This is likely to be either a monomer or dimer with reorganized or truncated beta sheets, suggesting that amyloid formation may require significant structural change in precursor proteins.     

Surface stresses in plastically deformed steels by X-ray diffraction

Surface stresses in samples of wheel steel with different degree of plastic deformation are studied. Both surface stresses and the microstructure of the steel are changed in the dependence on the plastic deformation magnitude. The X-ray technique is shown can be used to study a change in the surface stresses due to development a plastic deformation under action of operating loads.     

滑石粉的X射线衍射快速定性筛选方法

考察了国内不同产地纯滑石粉的X射线衍射图谱,并与JADE谱图分析软件中所提供的滑石的JCPDS卡中标准数据比对,结果表明,国内不同产地滑石粉具有相同的XRD指纹特征。考察了滑石粉快速定性筛选的可靠性和精密度,以滑石三强线的面间距d值作为定性指标,以衍射峰相对强度(I/I₁)作为半定量指标,建立了滑石相含量大于70%的滑石粉的X射线衍射快速定性筛选判定方法,为出口滑石粉的检验监管提供快速准确的技术支持。     

A synchrotron X-ray diffraction study of the local residual strains around a single inclusion in an AI/W metal-matrix composite

An X-ray technique for local measurements of the internal residual stress near inclusions in metalmatrix composites (MMCs) is presented. The technique utilizes medium- to high-energy monochromatic X-rays from a synchrotron source and a combination of slits on the entry and exit sides of the sample in order to determine the strains from small volumes deep within the composite sample. The strains of the individual matrix grains are sampled and averaged, allowing for a much improved spatial resolution. An analyzer is used in order to avoid well-known systematic errors related to geometry and stability of the beam. First results are obtained on a model system consisting of a 500 μm continuous W fiber imbedded in an A1 matrix. Two specimens were investigated with typical Al grain sizes of 1 mm and 30 μm. With a gage volume of 10×10×170 μm³, we obtained count rates on the order of 5000 cps and an accuracy in the strain measurements of Δε≤5×10⁻⁵. For both specimens, we found no variations of the radial and transverse strain components with the distance to the fiber, indicating either a complete debonding of the system, a very weak interface, or interface deterioration due to chemical reaction. Intragranular strain fluctuations on the order of ε=±10⁻⁴ were found to build up close to the grain boundaries. For the specimen with the smaller Al grain size, sampling data from approximately 15 grains at the same distance to the fiber was sufficient for averaging out the intergranular fluctuations. Finally, we observe effects from the conventional metallurgic sample preparation up to 400 μm from the surface, emphasizing the relevance of bulk techniques.