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ABSTRACT: The response of 316 stainless steel has been examined under uniaxial tensile loading during a range of tests carried out between
20 °C and 650 °C. In-situ neutron diffraction was used to measure internal elastic strain in subsets of differently oriented crystallites within the
polycrystal aggregate. This allowed the determination of diffraction elastic constants. Further, results have been compared
with predictions from a slip-based elasto-plastic self-consistent model. Good agreement is obtained during both conventional
slip and when dynamic strain aging (DSA) is evidenced. The quality of agreement was reduced in the higher temperature regime,
where it is expected that other mechanisms become active.
Metallurgical and Materials Transactions A 04/2012; 37(6):1863-1873. · 1.54 Impact Factor
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ABSTRACT: Residual strain and texture variations were measured in two titanium matrix composites reinforced with silicon carbide fibers
(Ti/SiC) of similar composition but fabricated by different processing routes. Each composite comprised a Ti-6242 α/β matrix alloy containing vol 35 pct continuous SiC fibers. In one, the matrix was produced by a plasma sprayed (PS) route,
and in the other by a wiredrawn (WD) process. The PS and WD composites were reinforced with SCS-6 (SiC) and Trimarc (SiC)
fibers, respectively. The texture in the titanium matrices differed significantly. The titanium matrix for the PS material
exhibited random texture pre and post fabrication of the composite. For the WD material, the starting texture of the monolithic
titanium matrix was ≈17 times random, but after consolidation into composite form, it was ≈6 times random. No significant
differences were noted in the fiber-induced matrix residual strains between the composites prepared by the two procedures.
However, the Trimarc (WD) fibers recorded higher (≈1.3 times) compressive strains than the SCS-6 (PS) fibers. Stresses and
stress balance results are reported. Plane-specific elastic moduli, measured in load tests on the unreinforced matrices, showed
little difference.
Metallurgical and Materials Transactions A 04/2012; 31(3):889-898. · 1.54 Impact Factor
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ABSTRACT: A series of neutron diffraction measurements have been carried out to determine the elastic residual strains deep within a
large, 40-cm-diameter, forged and water-quenched IN718 aeroengine compressor disc. Neutron path lengths of up to 6 cm were
necessary to probe the thickest parts of the forging, and three-dimensional strain and stress components have been derived
for the first time in such a large superalloy specimen. Measurements have been compared with the results from a coupled thermal-mechanical
finite-element model of the quenching process, based upon appropriate temperature-dependent material properties, with some
success. The general residual stress state in the disc is one of near-surface compression, balanced by tension within the
disc interior. The steepest stress and strain gradients occur in the transition region from compression to tension, about
1 cm below the surface all around the disc. The largest stress component is in the disc tangential direction and reaches a
magnitude of 400 to 500 MPa near the disc surface and at its core. This exceeds the effective yield stress because of the
presence of significant hydrostatic stress.
Metallurgical and Materials Transactions A 04/2012; 37(2):459-467. · 1.54 Impact Factor
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ABSTRACT: Neutron diffraction has been used to measure the elastic strains in a silicon carbide particle-reinforced aluminum alloy during
cyclic plasticity. Strains were recorded in both phases of the material, in sufficient directions to allow for calculation
of the internal stresses. The shape misfit stress in the composite was calculated from the macroscopic stress data using an
Eshelby-based model. Changes in the misfit stress caused by plastic deformation can be clearly observed. Local plastic anisotropy
of the matrix material is also seen and was monitored by comparing results from the two diffraction planes, {111} and {200},
that were measured. The results have been compared to those obtained using an elasto-plastic self-consistent modeling approach,
which shows the evolution of load sharing between the matrix and reinforcement, as well as the origin of the plastic anisotropy
strains in the aluminum.
Metallurgical and Materials Transactions A 04/2012; 37(6):1977-1986. · 1.54 Impact Factor
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ABSTRACT: Microdamage and failure mechanisms have been well characterized in bovine trabecular bone. However, little is known about how elastic strains develop in the apatite crystals of the trabecular struts and their relationship with different deformation mechanisms. In this study, wide-angle high-energy synchrotron X-ray diffraction has been used to determine bulk elastic strains under in situ compression. Dehydrated bone is compared to hydrated bone in terms of their response to load. During compression, load is initially borne by trabeculae aligned parallel to loading direction with non-parallel trabeculae deforming by bending. Ineffective load partitioning is noted in dehydrated bone whereas hydrated bone behaves like a plastically yielding foam.
Acta biomaterialia 10/2010; 7(2):716-23. · 3.98 Impact Factor
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ABSTRACT: The microstructure and associated mechanical properties of antler trabecular bone have been studied using a variety of techniques. The local trabeculae properties, as well as the three-dimensional architecture were characterized using nanoindentation and X-ray microtomography, respectively. An elastic modulus of 10.9+/-1.1 GPa is reported for dry bone, compared with 5.4+/-0.9 GPa for fully hydrated bone. Trabeculae thickness and separation were found to be comparable to those of bovine trabecular bone. Uniaxial compression conducted in situ during X-ray microtomography showed that antler can undergo significant architectural rearrangement, dominated by trabeculae bending and buckling, due to its low mineral content. High-energy synchrotron X-ray diffraction was used to measure elastic strains in the apatite crystals of the trabeculae, also under in situ uniaxial compression. During elastic loading, strain was found to be accommodated largely by trabeculae aligned parallel to the loading direction. Prior to the macroscopic yield point, internal strains increased as trabeculae deformed by bending, and load was also found to be redistributed to trabeculae aligned non-parallel to the loading direction. Significant bending of trabecular walls resulted in tensile strains developing in trabeculae aligned along the loading direction.
Acta biomaterialia 06/2008; 4(6):1677-87. · 3.98 Impact Factor
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ABSTRACT: Time-of-flight neutron diffraction profiles were recorded in situ during uniaxial tensile loading and subsequent annealing of an Fe–Mn–Si shape memory alloy. Results are presented on the evolution of phase textures and internal strains. Stress-induced phase transformation occurred preferentially in austenite grains having a 110 direction aligned axially, and resulted in the formation of ϵ martensite with a {101} fibre texture. These results and the evolution of internal strains are rationalised by consideration of the Schmid factor for the motion of Shockley partial dislocations.
Journal of Neutron Research 09/2007; 15(3–4):179-184.
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ABSTRACT: The evolution of lattice strains in Zircaloy-2 was investigated in situ by time-of-flight neutron diffraction during uni-axial compression in three principal directions, normal, transverse and rolling. The material is a warm-worked Zircaloy-2 slab with basal plane normals mostly concentrated in ND. Lattice strains relative to the undeformed sample were measured by neutron diffraction during cyclic compression and unloading for three test directions and three scattering vector directions. Intensity variations of the reflections were monitored to assess the grain rotations. Substantial tensile twinning was inferred from the evolution of {0002} lattice strains and the intensity of prismatic and basal poles. The spread of lattice strains in the test direction (amongst different crystal orientations) is much greater in compression than in tension. The sources leading to this asymmetry were ascribed, through preliminary simulations using an elasto-plastic self-consistent model, to the compressive load, the occurrence of tensile twinning and thermal residual strains.
Journal of Neutron Research 06/2007; 15(2):121-130.
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ABSTRACT: In situ synchrotron X-ray diffraction measurements are used to create two-dimensional maps of elastic strain and texture, averaged over a compact-tension specimen thickness, near a crack tip in a martensitic NiTi alloy. After fatigue crack propagation, the material ahead of the crack and in its wake exhibits a strong texture, which is eliminated by subsequent shape-memory heat treatment, indicating that this texture is due to detwinning, the main deformation mechanism of NiTi. Upon subsequent application of a static tensile stresses, the highly textured zone reappears and grows around the crack tip as the applied stress is increased. At the highest applied stress intensity of 35 MPa m 1/2 , large tensile strains are measured ahead of the crack tip and considerable elastic anisotropy is observed. This detwinning zone is similar to the plastic zone produced by dislocation slip present around cracks in other metals. The texture in this zone is not significantly altered after mechanical unloading, despite the development of substantial triaxial compressive residual strains in this zone.
04/2007;
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ABSTRACT: An ultrahigh-carbon steel was heat-treated to form an in situ composite consisting of a fine-grained ferritic matrix with 34 vol.% sub-micron spheroidized cementite particles. Volume-averaged lattice elastic strains for various crystallographic planes of the a-Fe and Fe 3 C phases were measured by synchrotron X-ray diffraction for a range of uniaxial tensile stresses up to 1 GPa. In the elastic range of steel deformation, no load transfer occurs between matrix and particles because both phases have nearly equivalent elastic properties. In the steel plastic range after Lü ders band propagation, marked load transfer takes place from the ductile a-Fe matrix to the elastic Fe 3 C par-ticles. Reasonable agreement is achieved between phase lattice strains as experimentally measured and as computed using finite-element modeling.
02/2007;
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ABSTRACT: Shape memory alloys (SMAs) exhibit unique thermomechanical properties due to a reversible martensitic phase transformation. Their current high cost and the insufficient predictability of the thermomechanical responses of these materials hinders further growth in their application. Neutron diffraction offers a unique tool to probe the phase transformation of bulk materials in situ during thermal and mechanical loading, allowing the simultaneous monitoring of phase fraction, texture evolution, interphase and intergranular strains. This paper describes the initial results of thermomechanical loading tests of the shape memory material NiTi as it martensitically transforms in complex thermomechanical load cycles. When fully analyzed this experimental data will provide crucial information on the interaction between the transformed and untransformed material, and on the dependence of transformation on grain level orientation.
IEEE Transactions on Nuclear Science 03/2005; · 1.45 Impact Factor
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ABSTRACT: The elastoplastic properties of a stainless steel with an austenitic matrix and martensitic inclusions induced during cyclic tensile–compressive fatigue loading were studied using neutron diffraction. Specimens of annealed and quenched AISI type 321 steel were subjected to low cycle fatigue (strain amplitude of 1% at 0.5 Hz). Subsequent in situ loading tests provided the elastoplastic responses of both austenitic and martensitic phases via Rietveld refinement of neutron diffraction spectra. A clear trend of increasing elastic modulus with increasing fatigue level was noted in the austenite matrix. The results of modified refinements accounting for the elastic anisotropy in polycrystalline materials under load are presented. The residual strains in the austenitic matrix and the deviatoric components of both phases' residual microstresses were determined as a function of fatigue cycling. In the Appendix it is demonstrated that the presence of residual stresses can have an effect on the observed elastic moduli in a two phase system, but that for typical magnitudes the effect is negligible.
Materials Science and Technology 12/2004; 21(1):35-45. · 0.77 Impact Factor
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ABSTRACT: Neutron diffraction spectra have been recorded in situ during tensile testing of polycrystalline Fe–30.5 at % Pd at a range of temperatures, in order to investigate stress-induced martensitic transformation and variant changes. The selective transformation of preferentially oriented austenite grains and martensite variants is identified and related to elasticity-based theory. Rietveld refinement is applied to determine the variation of elastic stiffness with temperature, revealing a significant increase in stiffness upon transformation to the martensite phase, in contrast to macroscopic measurements.
Acta Materialia. 01/2003; 51:6453-6464.
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ABSTRACT: For many research areas it is important to know how long one must measure on a given instrument in order to obtain a required level of precision in the measurement of a diffraction peak. This count time is dependent on instrument characteristics, which in turn determine peak signal, shape and background. It is often useful to determine the expected level of precision before the experiment; to plan experiments, to compare instrument capabilities or to check the performance and reliability of peak fitting. We present a method for predicting the precision of peak location, width and intensity in terms of the diffracted signal, peak width and signal to noise ratio. The derivation is for Gaussian peaks, but is broadly applicable to other peak shapes. Our primary interest is the location of peak position for strain measurement, but the methods have a wider range of applicability.
Applied Physics A 11/2002; 74:s112-s114. · 1.63 Impact Factor
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ABSTRACT: Neutron diffraction has been used to follow in situ the stress-induced martensitic transformation during uniaxial tensile testing of polycrystalline Fe-25 wt% Ni-0.4 wt% C. This method provides data on the extent of transformation, the development of crystallographic texture and the evolution of lattice strain. Two types of specimen were examined: unswaged, and swaged at 200 C. The swaged material was further aged with greater yield strength, enhancing the transformation. Moreover, it exhibited a stronger texture than the unswaged material. The extent of the transformation is shown to be dependent on grain orientation, with austenite grains having 𘜄¢ parallel to the tensile axis transforming preferentially. The internal stress state shows that, during plastic flow, the load is transferred from the original austenite to the evolving martensite phase.
Applied Physics A 11/2002; 74:s1143-s1145. · 1.63 Impact Factor
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ABSTRACT: The fatigue behaviour of austenitic stainless steel, in which a martensitic phase is formed due to plastic deformation, is of some interest for practical reasons. Earlier, we reported the results of the first stage of the in situ stress-rig experiment on the ENGIN instrument at the ISIS facility, with samples from steel X6CrNiTi1810 subjected to different tensile-compressive loading cycles at a frequency of 5 Hz. This paper describes the results of the second stage of the experiment in which a series of samples subjected to 0.5 Hz was studied. Information about the mechanical properties of the austenitic matrix and the martensitic precipitates is obtained from the experimental mechanical and neutron-diffraction data.
Applied Physics A 11/2002; 74:s1391-s1393. · 1.63 Impact Factor
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ABSTRACT: A study of the elastic and plastic properties of the austenitic base matrix and martensitic precipitates induced in stainless steel during high cycling tensile-compressive loading was made in the in situ stress-rig experiment on the ENGIN instrument at the ISIS facility. It is observed that the elastic constants of the austenitic and martensitic phases for the axial and transverse directions determined by the Rietveld refinement are almost independent of the level of fatigue, while the bulk value of the Young modulus measured by a gauge extensometer decreases with increasing fatigue. The austenitic elastic response is linear throughout the measured stress range (0-500 MPa), while the martensitic response is linear only up to 300-320 MPa, almost the same stress level at which bulk plasticity is observed. Herewith, the ratio of the martensitic elastic constants in the axial and transverse directions is almost twice that expected based purely on the value of the Poisson ratio. Results of the modified Rietveld refinement accounting for the elastic anisotropy in polycrystalline materials under load are presented.
Applied Physics A 11/2002; 74:s1385-s1387. · 1.63 Impact Factor
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ABSTRACT: The new generation of dedicated Engineering Strain Scanners at neutron facilities such as ENGIN-X at ISIS and SMARTS at LANSCE offer considerable increases in both the throughput of samples and the density of measurements which are feasible within each sample. This trend is set to increase further with new neutron sources such as the SNS. In order to make full use of these advances the routine processes associated with setting up measurements, and analysing data need to be made as efficient as possible. This issue has been addressed on ENGIN-X by writing a new piece of software which provides support for many of these operations. The approach is based on a virtual lab consisting of three dimensional models of the sample and lab equipment such as collimators and positioner. A typical session using the package would be; 1) Generate the sample model using primitives or from surface points measured with a coordinate measurement machine, 2) Specify fiducial and measurement points on screen, 3) Locate the sample model within the virtual and real laboratories, 4) Execute the measurement sequence using automatically generated machine control scripts, 5). Analyse the data, 6). Display data using a variety of options including superimposed on the sample model. The inclusion of an accurate sample model within the virtual lab allows many other useful properties such as neutron path lengths and measurement gauge volumes to be determined; it is also a relatively simple matter to check for possible collisions between sample and lab equipment such as collimators thereby avoiding potentially costly mistakes. The software which is shortly to be installed at ENGIN-X has been designed with visiting industrial and academic researchers in mind; users who need to be able to control the instrument after only a short period of training.
11/2002;
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"Materials Science and Engineering A". 01/2002; "324":"225 -- 234".
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Journal of Neutron Research 12/2001; 9(2-4):289-294.
