Andrew Venter - Academia.edu (original) (raw)

Papers by Andrew Venter

AIP Advances, 2021

Measurements of the temperature dependence of electrical resistance (R) and neutron powder diffra... more Measurements of the temperature dependence of electrical resistance (R) and neutron powder diffraction (NPD) of a Cr84.7Re15.3 alloy is reported. The magnetism originates from the formation of a spin-density-wave (SDW) as a result of nesting between the electron and hole sheets at the Fermi surface (FS). NPD revealed the magnetic character of the SDW to be commensurate (C) with the lattice with a Neel temperature (TN) of 560 ± 5K that correlates to TN = 585 ± 5K determined from the saddle point in the R(T) results. A power law fit Iα(1−TTN)2β of the neutron data gave β = 0.36(1), indicating that the AF ordering follows the 3D Heisenberg model. An additional antiferromagnetic magnetic contribution, associated with Cr2O3, was observed to co-exist in the alloy. Quantification analyses revealed that the Cr2O3 content was less than 2 wt.%. A β value of 0.30(1) was determined for the magnetic phase transition of Cr2O3 in correspondence with the 3D Ising model.

Acta Physica Polonica A, 2018

The magnetic phase diagram of bulk Cr100¡xAlx shows interesting behaviour close to the triple poi... more The magnetic phase diagram of bulk Cr100¡xAlx shows interesting behaviour close to the triple point concentration of x 2. Since the magnetic properties of Cr are influenced by dimensionality, stress and strain, this study focussed on the investigation of Cr99Al1 thin films prepared on fused silica substrates with thicknesses t varying from 29 to 452 nm using sputtering techniques. Resistance measurements covering the temperature range 2 to 400 K did not show any clear anomalies that could be indicative of changes in the magnetic ordering. X-ray diffraction (XRD) and atomic force microscopy (AFM) studies indicate the films are polycrystalline textured and that the 80 nm sample has the smallest grain size. In-plane stresses in these thin films were studied using the specialised XRD sin 2 ψ-method. The results show that the stress varies with film thickness. The 29 nm sample has stress in the order of 833 MPa and with increasing film thickness the stress reaches 1471 MPa for the 80 nm layer, where after it systematically reduces for the thicker coatings to 925 MPa for the 452 nm film. The highest stress for the Cr99Al1 thickness sample series is seen in the film with the smallest grain size.

Journal of Magnetism and Magnetic Materials, 1986

Abstract The temperature dependence of the thermal expansion and elastic constants was measured f... more Abstract The temperature dependence of the thermal expansion and elastic constants was measured for dilute Cr-Mo alloys. Antiferromagnetism was found to disappear at (22±1) at% Mo. Good agreement is found between existing itinerant electron theories and experiment for the concentration dependence of the magnetovolume, Δω(0), at O K when one assumes the same proportionality factor for all the alloys in the expression Δω( T ) α 〈 m 2 ( T )〉. This confirms the conclusion previously reached from electrical resistivity measurements that the addition of Mo to Cr changes the amplitude of the SDW without altering its basic nature. The concentration dependence of the magnetic contribution to the bulk modulus at 0 K is qualitatively well accounted for by existing itinerant electron theories.

Residual Stresses 2016, Dec 22, 2016

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC.

ICAF 2011 Structural Integrity: Influence of Efficiency and Green Imperatives, 2011

... Also, the authors would like to express immense gratitude to Dr. Andrea Lausi and Dr. Jasper ... more ... Also, the authors would like to express immense gratitude to Dr. Andrea Lausi and Dr. Jasper Plaisier from Elettra Trieste Synchrotron ... 34, 327–337 (2000) [3] Heckenberger, U., Hombergsmeier, E., Holzinger, V., von Bestenbostel, W.: Advances in Laser Shock Peening theory ...

International Journal of Refractory Metals and Hard Materials, 2009

The interaction of uniaxial compressive load and thermal residual stress was measured in a WC-10 ... more The interaction of uniaxial compressive load and thermal residual stress was measured in a WC-10 wt.% (16 vol.%) Ni cemented carbide composite using neutron diffraction. Loading was from 0 to À2500 MPa in increments of 250 MPa, and measurements were made in situ during load-unload cycles 1, 2, 3, 10, 25, 50 and 100. Plasticity is observed in the Ni from the lowest levels of applied load, leading to continuous curvature of the WC-Ni stress-strain curves, and is believed to be a significant contribution to the composite's toughness. It is due to interaction between local extremes of the thermal residual microstress with the applied macrostress and leads to anisotropic relaxation of the thermal residual stress. Strain distribution and plasticity were observed through peak breadths. Although the initially strong hysteresis is reduced as the cycles increase, there are still changes taking place after 100 cycles.

ACS Omega, 2021

This study examines the role of defects in structure−property relationships in spinel LiMn 1.5 Ni... more This study examines the role of defects in structure−property relationships in spinel LiMn 1.5 Ni 0.5 O 4 (LMNO) cathode materials, especially in terms of Mn 3+ content, degree of disorder, and impurity phase, without the use of the traditional high-temperature annealing (≥700°C used for making disordered LMNO). Two different phases of LMNO (i.e., highly P4 3 32-ordered and highly Fd3̅ m-disordered) have been prepared from two different β-MnO 2−δ precursors obtained from an argon-rich atmosphere (β-MnO 2−δ (Ar)) and a hydrogen-rich atmosphere [β-MnO 2−δ (H 2)]. The LMNO samples and their corresponding β-MnO 2−δ precursors are thoroughly characterized using different techniques including high-resolution transmission electron microscopy, field-emission scanning electron microscopy, Raman spectroscopy, powder neutron diffraction, X-ray photoelectron spectroscopy, synchrotron X-ray diffraction, X-ray absorption near-edge spectroscopy, and electrochemistry. LMNO from β-MnO 2−δ (H 2) exhibits higher defects (oxygen vacancy content) than the one from the β-MnO 2−δ (Ar). For the first time, defective β-MnO 2−δ has been adopted as precursors for LMNO cathode materials with controlled oxygen vacancy, disordered phase, Mn 3+ content, and impurity contents without the need for conventional methods of doping with metal ions, high synthetic temperature, use of organic compounds, postannealing, microwave, or modification of the temperature-cooling profiles. The results show that the oxygen vacancy changes concurrently with the degree of disorder and Mn 3+ content, and the best electrochemical performance is only obtained at 850°C for LMNO-(Ar). The findings in this work present unique opportunities that allow the use of β-MnO 2−δ as viable precursors for manipulating the structure−property relationships in LMNO spinel materials for potential development of highperformance high-voltage lithium-ion batteries.

Physica B: Condensed Matter, 2018

Abstract Previous electrical resistivity ( ρ ) and specific heat ( C P ) studies on the ternary (... more Abstract Previous electrical resistivity ( ρ ) and specific heat ( C P ) studies on the ternary (Cr100-xAlx)95Mo5 alloy system suggested that it harbours two quantum critical points (QCPs). This study reports comprehensive investigations of this alloy system through Seebeck coefficient ( S ) , thermal conductivity ( κ ) , magnetic susceptibility ( χ ) and neutron diffraction ( N D ) measurements in the concentration range 0 ≤ x ≤ 8.6. The results of S and χ show that spin-density-wave (SDW) antiferromagnetism is suppressed to temperatures below 2 K for concentrations in the range 1.4 ≤ x ≤ 4.4. Plots of d S / d T in the limit as T → 2 K depict two minima, i.e. just above x = 1.4 and 4.4. This parameter has been used as a key indicator of quantum critical behaviour (QCB) in Cr alloys. Analyses against the Nordheim-Gorter relationship demonstrates a positive slope for the incommensurate (I) SDW alloys and a negative slope for the commensurate (C) SDW alloys. Extrapolations of these two slopes intercept at a concentration of 3.2 at.% Al indicating the occurrence of band structure modifications when Al is added into the Cr95Mo5 base alloy. The Lorenz number ( L ) for the alloys with x = 0 and 0.5 shows interesting anomalous behaviour associated with band structure effects and SDW magnetic ordering. N D measurements as a function of temperature confirm that alloys with x 4.4 show commensurate (C) SDW order. Power law fits of the form T N ∝ 1.40 − x 0.35 ± 0.05 for the ISDW to P phase transition and T N ∝ x − 4.40 0.63 ± 0.03 for the P to CSDW phase transition rendered the critical exponents 0.35 ± 0.05 and 0.63 ± 0.03 respectively. Overall the results of S , κ , χ and N D corroborate the existence of two QCPs at x ≈ 1.4 and 4.4.

The investigation of welding residual stress is very important for industry since they can affect... more The investigation of welding residual stress is very important for industry since they can affect considerably on the structural integrity of various components and products and their lifetime. The residual stress in specimens welded by electron and laser beam was analyzed using high resolution neutron diffraction in order to evaluate feasibility of various welding methods. The experiments have shown that the level of residual stress in welds can be quite high (up to 200 MPa) and this fact should be considered when choosing a welding technique.

Journal of Magnetism and Magnetic Materials, 2020

Abstract NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having uniq... more Abstract NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having unique structural and magnetic properties. This paper is a continuation of previous work [Mohanty et al., J. Magn. Magn. Mater. 451(2018)20] and is focused on the modification of ferrimagnetic properties in NiCr2O4 by substituting Co ions at Ni sites. In order to do so (Ni1−xCox)Cr2O4 (x = 0, 0.50, 0.75, 1.00) powder samples were synthesized using chemical co-precipitation techniques. Interest is primarily on (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4 as characterization results on the end members have been reported. It was determined with in-situ temperature dependent X-ray diffraction measurements that these compositions become fully crystallized cubic spinels with space group Fd 3 - m above 700 °C. All samples were subsequently calcined at 900 °C to ensure phase purity and uniform crystallinity. Transmission electron microscopy showed non-uniform distribution of particle sizes with the majority of the particles having bi-pyramidal structures with flat tops for both samples. A comparative account of different ferrimagnetic Curie temperature, θ f or TC, estimations are presented as determined from measured temperature dependences of the following techniques: Fits of the magnetic susceptibility data indicated a minimum value for the (Ni0.5Co0.5)Cr2O4 sample at 56.1 ± 0.9 K, followed by an increase to 76.8 ± 0.4 K for the (Ni0.25Co0.75)Cr2O4 sample; Neutron diffraction measurements gave values of 80 ± 1 K and 87 ± 1 K; Their paramagnetic Curie points, θ p , were determined to be 73.3 and 84.9 K, respectively. The magnetic frustration index f = ΘCW/TC was found to decrease continuously over the series from 11.9 for NiCr2O4 to 7.1 and 6.5 for x = 0.5 and 0.75 respectively with (Ni0.5Co0.5)Cr2O4 being more frustrated than (Ni0.25Co0.75)Cr2O4. Magnetization as a function of applied magnetic field measured at 3 K demonstrates unusually high coercivity for (Ni0.5Co0.5)Cr2O4, supporting the notion for the higher degree of magnetic frustration in this sample. No exchange bias was observed for M (μ0H) in the (Ni0.25Co0.75)Cr2O4 sample measured at 3 K, but did however show a significant anomaly in its hysteresis loop at this temperature. The anomaly in hysteresis behaviour disappeared when measured at 1.7 K and also at higher temperatures. Frequency dependent ac-susceptibility measurements revealed no shift in the peak position with frequency refuting the existence of spin-glass like behavior in both these samples.

Journal of Physics: Conference Series, 2017

Materials Characterization, 2019

Abstract Chemical phase quantification from neutron diffraction measurements relies on accurate B... more Abstract Chemical phase quantification from neutron diffraction measurements relies on accurate Bragg peak intensities at all diffraction angles. Amongst others these intensities are influenced by the total distance that the neutron beam has traversed through the sample in reaching the contributing volume segment and the distance from the contributing volume segment to the outer edge of the sample in the direction of the neutron detector. Since the total path length is dependent on the diffraction angle, gauge volume position inside the sample, as well as the sample dimensions and shape, the measured diffraction pattern should be corrected to account for this non-constant attenuation effect. Results show that in highly attenuation materials this can influence refined parameters during chemical phase quantification. A module was designed and integrated into the data reduction system ScanManipulator used at the neutron diffraction facility at the SAFARI-1 research reactor. The correction technique can further be used to accurately determine diffraction patterns obtained from position-resolved neutron diffraction experiments.

Solid State Sciences, 2017

Abstract The thermal production of 4PbO·PbSO4 was comprehensively studied and characterized for t... more Abstract The thermal production of 4PbO·PbSO4 was comprehensively studied and characterized for two syntheses routes, i.e. either heating 3PbO·PbSO4·H2O, or a mixture of 4PbO:PbSO4, in air to about 700 °C. In the 3PbO·PbSO4·H2O approach, the formation of an intermediate amorphous phase occurred at around 210 °C with the loss of H2O from the hydrated structure. Formation of 4PbO·PbSO4 initiated at around 270 °C with predominantly 4PbO·PbSO4 and 13% residual PbO·PbSO4 existing at 700 °C. With the synthesis route of mixing a stoichiometric ratio of 4PbO with PbSO4, an intermediate phase of PbO·PbSO4 formed at around 300 °C, before the 4PbO·PbSO4 phase started to form at around 500 °C. Upon further heating, 4PbO·PbSO4 was the predominant phase with 8% of PbO·PbSO4 remaining. Both samples decomposed upon further heating to 850 °C. Powder neutron diffraction studies of the final 4PbO·PbSO4 products from the two different synthesis routes showed similar crystallographic unit cell lattice parameters with slight differences in the PbO:PbSO4 contents. This could possibly be linked to differences observed in the microscopic crystallite shapes from the two synthesis routes.

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 2008

Laser-induced forming and microstructural modification of materials and components is a technique... more Laser-induced forming and microstructural modification of materials and components is a technique that offers interesting possibilities for improved control over the final state of the workpiece, and the possibility of generating carefully tailored residual stress states and spatially varying microstructures best suited to specific industrial applications. A key challenge in the development of this technique is the characterization of the relationship between process parameters and the resulting state of the workpiece; and the optimization of processing conditions in order to deliver a desired result. The results of an experimental study of microstructure and residual elastic strains (RES) in multi-scan laser-bent steel plates by the combination of microscopy and strain mapping using energy-dispersive synchrotron X-ray diffraction have been recently reported. This paper is devoted to the presentation of simplified eigenstrain analysis of the RES fields arising due to laser processin...

The Journal of Strain Analysis for Engineering Design, 2007

Residual elastic strain and microstructural transformations resulting from high thermal gradients... more Residual elastic strain and microstructural transformations resulting from high thermal gradients introduced by industrial-strength laser forming of mild steel are reported in this article. An 8 mm thick steel plate was bent to a total bending angle of 1.2° by running a laser three successive times across the length of the plate. Thin through-thickness slices of the plate were prepared for synchrotron energy-dispersive X-ray diffraction experiments from which lattice strains were calculated by measuring peak shifts in the diffraction patterns. The diffraction patterns were analysed by means of Rietveld refinement using the general structure analysis system allowing the spatially resolved mapping of relevant strain components needed for full-field eigenstrain determination in the plate. Comparison of the measured residual elastic strain fields and metallographic analyses of the steel plate suggests that a correlation exists between the residual elastic strains and metallurgical proce...

Journal of Physics: Conference Series, 2012

International Journal of Refractory Metals and Hard Materials, 2014

ABSTRACT This investigation had been conducted to determine the influence of residual stresses on... more ABSTRACT This investigation had been conducted to determine the influence of residual stresses on the abrasive wear resistance of HVOF thermal spray WC-17 wt% Co coatings, as well as to derive stress relaxation after cutting by wire electric discharge machining (EDM). The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The residual stress was measured by means of X-ray diffraction techniques, on the coated samples before and after the abrasive wear tests. Compressive residual stresses were observed in the surface layer of the large coated samples. However, stress relaxation results after cutting into small sizes were distinctly different. There was strong correlation between residual stresses in the surface layer and abrasive wear resistance, as well as yield strength of a material.

AIP Advances, 2021

Measurements of the temperature dependence of electrical resistance (R) and neutron powder diffra... more Measurements of the temperature dependence of electrical resistance (R) and neutron powder diffraction (NPD) of a Cr84.7Re15.3 alloy is reported. The magnetism originates from the formation of a spin-density-wave (SDW) as a result of nesting between the electron and hole sheets at the Fermi surface (FS). NPD revealed the magnetic character of the SDW to be commensurate (C) with the lattice with a Neel temperature (TN) of 560 ± 5K that correlates to TN = 585 ± 5K determined from the saddle point in the R(T) results. A power law fit Iα(1−TTN)2β of the neutron data gave β = 0.36(1), indicating that the AF ordering follows the 3D Heisenberg model. An additional antiferromagnetic magnetic contribution, associated with Cr2O3, was observed to co-exist in the alloy. Quantification analyses revealed that the Cr2O3 content was less than 2 wt.%. A β value of 0.30(1) was determined for the magnetic phase transition of Cr2O3 in correspondence with the 3D Ising model.

Acta Physica Polonica A, 2018

The magnetic phase diagram of bulk Cr100¡xAlx shows interesting behaviour close to the triple poi... more The magnetic phase diagram of bulk Cr100¡xAlx shows interesting behaviour close to the triple point concentration of x 2. Since the magnetic properties of Cr are influenced by dimensionality, stress and strain, this study focussed on the investigation of Cr99Al1 thin films prepared on fused silica substrates with thicknesses t varying from 29 to 452 nm using sputtering techniques. Resistance measurements covering the temperature range 2 to 400 K did not show any clear anomalies that could be indicative of changes in the magnetic ordering. X-ray diffraction (XRD) and atomic force microscopy (AFM) studies indicate the films are polycrystalline textured and that the 80 nm sample has the smallest grain size. In-plane stresses in these thin films were studied using the specialised XRD sin 2 ψ-method. The results show that the stress varies with film thickness. The 29 nm sample has stress in the order of 833 MPa and with increasing film thickness the stress reaches 1471 MPa for the 80 nm layer, where after it systematically reduces for the thicker coatings to 925 MPa for the 452 nm film. The highest stress for the Cr99Al1 thickness sample series is seen in the film with the smallest grain size.

Journal of Magnetism and Magnetic Materials, 1986

Abstract The temperature dependence of the thermal expansion and elastic constants was measured f... more Abstract The temperature dependence of the thermal expansion and elastic constants was measured for dilute Cr-Mo alloys. Antiferromagnetism was found to disappear at (22±1) at% Mo. Good agreement is found between existing itinerant electron theories and experiment for the concentration dependence of the magnetovolume, Δω(0), at O K when one assumes the same proportionality factor for all the alloys in the expression Δω( T ) α 〈 m 2 ( T )〉. This confirms the conclusion previously reached from electrical resistivity measurements that the addition of Mo to Cr changes the amplitude of the SDW without altering its basic nature. The concentration dependence of the magnetic contribution to the bulk modulus at 0 K is qualitatively well accounted for by existing itinerant electron theories.

Residual Stresses 2016, Dec 22, 2016

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licens... more Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC.

ICAF 2011 Structural Integrity: Influence of Efficiency and Green Imperatives, 2011

... Also, the authors would like to express immense gratitude to Dr. Andrea Lausi and Dr. Jasper ... more ... Also, the authors would like to express immense gratitude to Dr. Andrea Lausi and Dr. Jasper Plaisier from Elettra Trieste Synchrotron ... 34, 327–337 (2000) [3] Heckenberger, U., Hombergsmeier, E., Holzinger, V., von Bestenbostel, W.: Advances in Laser Shock Peening theory ...

International Journal of Refractory Metals and Hard Materials, 2009

The interaction of uniaxial compressive load and thermal residual stress was measured in a WC-10 ... more The interaction of uniaxial compressive load and thermal residual stress was measured in a WC-10 wt.% (16 vol.%) Ni cemented carbide composite using neutron diffraction. Loading was from 0 to À2500 MPa in increments of 250 MPa, and measurements were made in situ during load-unload cycles 1, 2, 3, 10, 25, 50 and 100. Plasticity is observed in the Ni from the lowest levels of applied load, leading to continuous curvature of the WC-Ni stress-strain curves, and is believed to be a significant contribution to the composite's toughness. It is due to interaction between local extremes of the thermal residual microstress with the applied macrostress and leads to anisotropic relaxation of the thermal residual stress. Strain distribution and plasticity were observed through peak breadths. Although the initially strong hysteresis is reduced as the cycles increase, there are still changes taking place after 100 cycles.

ACS Omega, 2021

This study examines the role of defects in structure−property relationships in spinel LiMn 1.5 Ni... more This study examines the role of defects in structure−property relationships in spinel LiMn 1.5 Ni 0.5 O 4 (LMNO) cathode materials, especially in terms of Mn 3+ content, degree of disorder, and impurity phase, without the use of the traditional high-temperature annealing (≥700°C used for making disordered LMNO). Two different phases of LMNO (i.e., highly P4 3 32-ordered and highly Fd3̅ m-disordered) have been prepared from two different β-MnO 2−δ precursors obtained from an argon-rich atmosphere (β-MnO 2−δ (Ar)) and a hydrogen-rich atmosphere [β-MnO 2−δ (H 2)]. The LMNO samples and their corresponding β-MnO 2−δ precursors are thoroughly characterized using different techniques including high-resolution transmission electron microscopy, field-emission scanning electron microscopy, Raman spectroscopy, powder neutron diffraction, X-ray photoelectron spectroscopy, synchrotron X-ray diffraction, X-ray absorption near-edge spectroscopy, and electrochemistry. LMNO from β-MnO 2−δ (H 2) exhibits higher defects (oxygen vacancy content) than the one from the β-MnO 2−δ (Ar). For the first time, defective β-MnO 2−δ has been adopted as precursors for LMNO cathode materials with controlled oxygen vacancy, disordered phase, Mn 3+ content, and impurity contents without the need for conventional methods of doping with metal ions, high synthetic temperature, use of organic compounds, postannealing, microwave, or modification of the temperature-cooling profiles. The results show that the oxygen vacancy changes concurrently with the degree of disorder and Mn 3+ content, and the best electrochemical performance is only obtained at 850°C for LMNO-(Ar). The findings in this work present unique opportunities that allow the use of β-MnO 2−δ as viable precursors for manipulating the structure−property relationships in LMNO spinel materials for potential development of highperformance high-voltage lithium-ion batteries.

Physica B: Condensed Matter, 2018

Abstract Previous electrical resistivity ( ρ ) and specific heat ( C P ) studies on the ternary (... more Abstract Previous electrical resistivity ( ρ ) and specific heat ( C P ) studies on the ternary (Cr100-xAlx)95Mo5 alloy system suggested that it harbours two quantum critical points (QCPs). This study reports comprehensive investigations of this alloy system through Seebeck coefficient ( S ) , thermal conductivity ( κ ) , magnetic susceptibility ( χ ) and neutron diffraction ( N D ) measurements in the concentration range 0 ≤ x ≤ 8.6. The results of S and χ show that spin-density-wave (SDW) antiferromagnetism is suppressed to temperatures below 2 K for concentrations in the range 1.4 ≤ x ≤ 4.4. Plots of d S / d T in the limit as T → 2 K depict two minima, i.e. just above x = 1.4 and 4.4. This parameter has been used as a key indicator of quantum critical behaviour (QCB) in Cr alloys. Analyses against the Nordheim-Gorter relationship demonstrates a positive slope for the incommensurate (I) SDW alloys and a negative slope for the commensurate (C) SDW alloys. Extrapolations of these two slopes intercept at a concentration of 3.2 at.% Al indicating the occurrence of band structure modifications when Al is added into the Cr95Mo5 base alloy. The Lorenz number ( L ) for the alloys with x = 0 and 0.5 shows interesting anomalous behaviour associated with band structure effects and SDW magnetic ordering. N D measurements as a function of temperature confirm that alloys with x 4.4 show commensurate (C) SDW order. Power law fits of the form T N ∝ 1.40 − x 0.35 ± 0.05 for the ISDW to P phase transition and T N ∝ x − 4.40 0.63 ± 0.03 for the P to CSDW phase transition rendered the critical exponents 0.35 ± 0.05 and 0.63 ± 0.03 respectively. Overall the results of S , κ , χ and N D corroborate the existence of two QCPs at x ≈ 1.4 and 4.4.

The investigation of welding residual stress is very important for industry since they can affect... more The investigation of welding residual stress is very important for industry since they can affect considerably on the structural integrity of various components and products and their lifetime. The residual stress in specimens welded by electron and laser beam was analyzed using high resolution neutron diffraction in order to evaluate feasibility of various welding methods. The experiments have shown that the level of residual stress in welds can be quite high (up to 200 MPa) and this fact should be considered when choosing a welding technique.

Journal of Magnetism and Magnetic Materials, 2020

Abstract NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having uniq... more Abstract NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having unique structural and magnetic properties. This paper is a continuation of previous work [Mohanty et al., J. Magn. Magn. Mater. 451(2018)20] and is focused on the modification of ferrimagnetic properties in NiCr2O4 by substituting Co ions at Ni sites. In order to do so (Ni1−xCox)Cr2O4 (x = 0, 0.50, 0.75, 1.00) powder samples were synthesized using chemical co-precipitation techniques. Interest is primarily on (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4 as characterization results on the end members have been reported. It was determined with in-situ temperature dependent X-ray diffraction measurements that these compositions become fully crystallized cubic spinels with space group Fd 3 - m above 700 °C. All samples were subsequently calcined at 900 °C to ensure phase purity and uniform crystallinity. Transmission electron microscopy showed non-uniform distribution of particle sizes with the majority of the particles having bi-pyramidal structures with flat tops for both samples. A comparative account of different ferrimagnetic Curie temperature, θ f or TC, estimations are presented as determined from measured temperature dependences of the following techniques: Fits of the magnetic susceptibility data indicated a minimum value for the (Ni0.5Co0.5)Cr2O4 sample at 56.1 ± 0.9 K, followed by an increase to 76.8 ± 0.4 K for the (Ni0.25Co0.75)Cr2O4 sample; Neutron diffraction measurements gave values of 80 ± 1 K and 87 ± 1 K; Their paramagnetic Curie points, θ p , were determined to be 73.3 and 84.9 K, respectively. The magnetic frustration index f = ΘCW/TC was found to decrease continuously over the series from 11.9 for NiCr2O4 to 7.1 and 6.5 for x = 0.5 and 0.75 respectively with (Ni0.5Co0.5)Cr2O4 being more frustrated than (Ni0.25Co0.75)Cr2O4. Magnetization as a function of applied magnetic field measured at 3 K demonstrates unusually high coercivity for (Ni0.5Co0.5)Cr2O4, supporting the notion for the higher degree of magnetic frustration in this sample. No exchange bias was observed for M (μ0H) in the (Ni0.25Co0.75)Cr2O4 sample measured at 3 K, but did however show a significant anomaly in its hysteresis loop at this temperature. The anomaly in hysteresis behaviour disappeared when measured at 1.7 K and also at higher temperatures. Frequency dependent ac-susceptibility measurements revealed no shift in the peak position with frequency refuting the existence of spin-glass like behavior in both these samples.

Journal of Physics: Conference Series, 2017

Materials Characterization, 2019

Abstract Chemical phase quantification from neutron diffraction measurements relies on accurate B... more Abstract Chemical phase quantification from neutron diffraction measurements relies on accurate Bragg peak intensities at all diffraction angles. Amongst others these intensities are influenced by the total distance that the neutron beam has traversed through the sample in reaching the contributing volume segment and the distance from the contributing volume segment to the outer edge of the sample in the direction of the neutron detector. Since the total path length is dependent on the diffraction angle, gauge volume position inside the sample, as well as the sample dimensions and shape, the measured diffraction pattern should be corrected to account for this non-constant attenuation effect. Results show that in highly attenuation materials this can influence refined parameters during chemical phase quantification. A module was designed and integrated into the data reduction system ScanManipulator used at the neutron diffraction facility at the SAFARI-1 research reactor. The correction technique can further be used to accurately determine diffraction patterns obtained from position-resolved neutron diffraction experiments.

Solid State Sciences, 2017

Abstract The thermal production of 4PbO·PbSO4 was comprehensively studied and characterized for t... more Abstract The thermal production of 4PbO·PbSO4 was comprehensively studied and characterized for two syntheses routes, i.e. either heating 3PbO·PbSO4·H2O, or a mixture of 4PbO:PbSO4, in air to about 700 °C. In the 3PbO·PbSO4·H2O approach, the formation of an intermediate amorphous phase occurred at around 210 °C with the loss of H2O from the hydrated structure. Formation of 4PbO·PbSO4 initiated at around 270 °C with predominantly 4PbO·PbSO4 and 13% residual PbO·PbSO4 existing at 700 °C. With the synthesis route of mixing a stoichiometric ratio of 4PbO with PbSO4, an intermediate phase of PbO·PbSO4 formed at around 300 °C, before the 4PbO·PbSO4 phase started to form at around 500 °C. Upon further heating, 4PbO·PbSO4 was the predominant phase with 8% of PbO·PbSO4 remaining. Both samples decomposed upon further heating to 850 °C. Powder neutron diffraction studies of the final 4PbO·PbSO4 products from the two different synthesis routes showed similar crystallographic unit cell lattice parameters with slight differences in the PbO:PbSO4 contents. This could possibly be linked to differences observed in the microscopic crystallite shapes from the two synthesis routes.

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 2008

Laser-induced forming and microstructural modification of materials and components is a technique... more Laser-induced forming and microstructural modification of materials and components is a technique that offers interesting possibilities for improved control over the final state of the workpiece, and the possibility of generating carefully tailored residual stress states and spatially varying microstructures best suited to specific industrial applications. A key challenge in the development of this technique is the characterization of the relationship between process parameters and the resulting state of the workpiece; and the optimization of processing conditions in order to deliver a desired result. The results of an experimental study of microstructure and residual elastic strains (RES) in multi-scan laser-bent steel plates by the combination of microscopy and strain mapping using energy-dispersive synchrotron X-ray diffraction have been recently reported. This paper is devoted to the presentation of simplified eigenstrain analysis of the RES fields arising due to laser processin...

The Journal of Strain Analysis for Engineering Design, 2007

Residual elastic strain and microstructural transformations resulting from high thermal gradients... more Residual elastic strain and microstructural transformations resulting from high thermal gradients introduced by industrial-strength laser forming of mild steel are reported in this article. An 8 mm thick steel plate was bent to a total bending angle of 1.2° by running a laser three successive times across the length of the plate. Thin through-thickness slices of the plate were prepared for synchrotron energy-dispersive X-ray diffraction experiments from which lattice strains were calculated by measuring peak shifts in the diffraction patterns. The diffraction patterns were analysed by means of Rietveld refinement using the general structure analysis system allowing the spatially resolved mapping of relevant strain components needed for full-field eigenstrain determination in the plate. Comparison of the measured residual elastic strain fields and metallographic analyses of the steel plate suggests that a correlation exists between the residual elastic strains and metallurgical proce...

Journal of Physics: Conference Series, 2012

International Journal of Refractory Metals and Hard Materials, 2014

ABSTRACT This investigation had been conducted to determine the influence of residual stresses on... more ABSTRACT This investigation had been conducted to determine the influence of residual stresses on the abrasive wear resistance of HVOF thermal spray WC-17 wt% Co coatings, as well as to derive stress relaxation after cutting by wire electric discharge machining (EDM). The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The residual stress was measured by means of X-ray diffraction techniques, on the coated samples before and after the abrasive wear tests. Compressive residual stresses were observed in the surface layer of the large coated samples. However, stress relaxation results after cutting into small sizes were distinctly different. There was strong correlation between residual stresses in the surface layer and abrasive wear resistance, as well as yield strength of a material.