Mukul Kumar - Academia.edu (original) (raw)

Papers by Mukul Kumar

Acta Materialia, 2007

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of small cr... more High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency cyclic loading, has been identified as one of the leading causes of turbine engine failures in aircraft. In this work, we consider the feasibility of using grain-boundary engineering to improve the HCF properties of a polycrystalline nickel-base superalloy, René 104 (also known as ME3), through systematic modification of the grain-boundary distribution. In particular, we investigate the growth of microstructurally small fatigue cracks at ambient temperature in microstructures with varying proportions of ''special'' vs. ''random'' boundaries, as defined by coincident-site lattice theory. Specifically, we examine the interaction of propagating small ($10-900 lm) surface cracks with grain boundaries of known character, with respect both to any deflection in crack trajectory that occurs at or near the boundary, and more importantly to any local changes in crack-growth rates. In addition, finite-element calculations are performed to evaluate the effective driving force and plastic-zone profile for such small-crack propagation, incorporating information from both the local microstructure (from electron backscattering diffraction scans) and the surface crack-path profile.

Journal of the American Chemical Society, Jan 21, 2015

CO electroreduction activity on oxide-derived Cu (OD-Cu) was found to correlate with metastable s... more CO electroreduction activity on oxide-derived Cu (OD-Cu) was found to correlate with metastable surface features that bind CO strongly. OD-Cu electrodes prepared by H2 reduction of Cu2O precursors reduce CO to acetate and ethanol with nearly 50 % Faradaic efficiency at moderate overpotential. Temperature programmed desorption of CO on OD-Cu revealed the presence of surface sites with strong CO binding that are distinct from the terraces and stepped sites found on polycrystalline Cu foil. After annealing at 350 °C, the specific current density for CO reduction is 44-fold lower and the Faradaic efficiency for ethanol and acetate is less than 5%. This loss of activity is accompanied by a reduction in the proportion of strong CO binding sites. We propose that the active sites for CO reduction on OD-Cu surfaces are strong CO binding sites that are supported by grain bounda-ries. Uncovering these sites is a first step towards understanding the surface chemistry necessary for efficient CO ...

Electron backscatter diffraction is a very powerful and relatively new materials characterization... more Electron backscatter diffraction is a very powerful and relatively new materials characterization technique aimed at the determination of crystallographic texture, grain boundary character distributions, lattice strain, phase identification, and much more. The purpose of this book is to provide the fundamental basis for electron backscatter diffraction in materials science, the current state of both hardware and software, and illustrative examples of the applications of electron backscatter diffraction to a wide-range of materials including ...

Acta Materialia, 2012

We present experimental validation of a method for estimating three-dimensional (3-D) relative nu... more We present experimental validation of a method for estimating three-dimensional (3-D) relative numerical populations of grain boundaries from measurements of individual two-dimensional (2-D) cross-sections. Such numerical populations are relevant to network topology and the modeling of intergranular failure modes in grain boundary engineered materials, and are distinct from geometrical population measures such as area per volume. We examine 3-D reconstructions of stainless steel and copper, with varying populations of twin-related boundaries, generated by serial-section electron backscatter diffraction and high-energy X-ray diffraction microscopy. We show that 2-D length fractions, 2-D number fractions and 3-D number fractions are all distinct quantities when grain boundary type is correlated with grain boundary size. We also demonstrate that the last quantity may be reliably inferred from the first two, provided the experimental spatial resolution is much finer than the grain size, eliminating the need to use 3-D experimental methods to access at least some information about 3-D network properties. Many of the R3 boundaries are extremely complex, with highly re-entrant shapes that can intersect a sample plane many times, giving a false impression of multiple separate boundaries.

Acta Materialia, 2009

The feasibility of using ''grain-boundary engineering" techniques to reduce the susceptibility of... more The feasibility of using ''grain-boundary engineering" techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of ''special" grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J c fracture toughness values were some 20-30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

The phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and... more The phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and 760°C for 2000 h (2.7 mo) and 16,000 h (1.8 yr). The tensile properties and the Charpy impact toughness of these samples were measured in the mill annealed condition as well as after aging. The microstructures of samples aged 16,000 hours were examined using scanning and transmission electron microscopy (SEM and TEM). Preliminary TEM results suggest that µ phase forms at all temperatures investigated. Discrete carbide particles in addition to a film with very uniform thickness which appears to be µ phase formed on grain boundaries in the sample aged at 593°C. The ordered Ni 2 (Cr, Mo) phase was also seen in this sample. At the higher aging temperatures, mainly µ phase forms covering all the grain boundaries and also distributed throughout the bulk. Although strength increased somewhat with aging, the ductility decreased due to the formation of these grain boundary precipitates and brittle intermetallics.

Http Dx Doi Org 10 1080 095008397178823, Nov 14, 2010

ABSTRACT

Acta Mater, 1997

Deformation twinning in a two phase Ni-25Mo-8Cr (in wt%) alloy containing Ni*(Mo, Cr) domains/pre... more Deformation twinning in a two phase Ni-25Mo-8Cr (in wt%) alloy containing Ni*(Mo, Cr) domains/precipitates in a face centred cubic (f.c.c.) matrix has been studied by detailed crystallographic analysis, coupled with transmission electron microscopy and experimental and computer simulated electron diffraction. Correlation of crystallography with diffraction evidence indicates that all the twinning modes that lead to the formation of true twins (combined, types I and II) in the NilMo superlattice are in operation. Long range order within the domains/precipitates of the ordered phase is preserved during deformation by the reorientation of the sheared regions into anti-parallel or perpendicular twin variants, and the rationale for this, from crystallographic considerations, is provided. Defect analysis, through weak-beam dark field imaging, revealed that the twins in the NizMo domains form as a consequence of shearing by a( 112) Shockley partial dislocations with the same Burgers vector on adjacent { I1 1) planes and eventual overlapping of intrinsic stacking faults. Twin nuclei are composed of three-layer or nine-layer stacking faults, depending on the geometry of the twinning plane within the NizMo domains. A new dislocation model, based on the dissociation of a single set of i(Ol I) unit dislocations-in slips bands at the precipitate/matrix interface-to release the twinning Shockley partial dislocations, is able to satisfactorily account for twin formation within the NizMo precipitates. 0 1997 Acta Metallurgica Inc.

Recovery based observations of high pressure material behavior generated under high explosively d... more Recovery based observations of high pressure material behavior generated under high explosively driven flyer based loading conditions are reported. Two shock pressures, 25, and 55 GPa and four orientations (100), (110), (111), (123) were considered. Recovered material was characterized using electron backscatter diffraction along with a limited amount of transmission electron microscopy to assess the occurrence of twinning under each

Philosophical Magazine A, 2000

... and VIJAY K. VASUDEVAN~ Department of Materials Science and Engineering, University of Cincin... more ... and VIJAY K. VASUDEVAN~ Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012, USA ... Mag. A, 73, p. 575. WANG, P., KUMAR, M.,VEERARAGHAVAN, D., and VASUDEVAN, VK, 1998, Acta mater., 46, ...

MRS Online Proceeding Library

ABSTRACT

We have investigated the microstructural changes in ductile porous metals during high pressure-hi... more We have investigated the microstructural changes in ductile porous metals during high pressure-high strain rate loading employing atomistic simulations and explored their relation to recent experiments on polycrystalline copper samples. Molecular-dynamics ...

Acta Materialia, 2015

Grain boundary networks should play a dominant role in determining the mechanical properties of n... more Grain boundary networks should play a dominant role in determining the mechanical properties of nanocrystalline metals. However, these networks are difficult to characterize and their response to deformation is incompletely understood. In this work, we study the grain boundary network of nanocrystalline Ni and explore whether it can be modified by plastic deformation.

International Journal of Solids and Structures, 2015

Acta Materialia, 2007

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of small cr... more High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency cyclic loading, has been identified as one of the leading causes of turbine engine failures in aircraft. In this work, we consider the feasibility of using grain-boundary engineering to improve the HCF properties of a polycrystalline nickel-base superalloy, René 104 (also known as ME3), through systematic modification of the grain-boundary distribution. In particular, we investigate the growth of microstructurally small fatigue cracks at ambient temperature in microstructures with varying proportions of ''special'' vs. ''random'' boundaries, as defined by coincident-site lattice theory. Specifically, we examine the interaction of propagating small ($10-900 lm) surface cracks with grain boundaries of known character, with respect both to any deflection in crack trajectory that occurs at or near the boundary, and more importantly to any local changes in crack-growth rates. In addition, finite-element calculations are performed to evaluate the effective driving force and plastic-zone profile for such small-crack propagation, incorporating information from both the local microstructure (from electron backscattering diffraction scans) and the surface crack-path profile.

Journal of the American Chemical Society, Jan 21, 2015

CO electroreduction activity on oxide-derived Cu (OD-Cu) was found to correlate with metastable s... more CO electroreduction activity on oxide-derived Cu (OD-Cu) was found to correlate with metastable surface features that bind CO strongly. OD-Cu electrodes prepared by H2 reduction of Cu2O precursors reduce CO to acetate and ethanol with nearly 50 % Faradaic efficiency at moderate overpotential. Temperature programmed desorption of CO on OD-Cu revealed the presence of surface sites with strong CO binding that are distinct from the terraces and stepped sites found on polycrystalline Cu foil. After annealing at 350 °C, the specific current density for CO reduction is 44-fold lower and the Faradaic efficiency for ethanol and acetate is less than 5%. This loss of activity is accompanied by a reduction in the proportion of strong CO binding sites. We propose that the active sites for CO reduction on OD-Cu surfaces are strong CO binding sites that are supported by grain bounda-ries. Uncovering these sites is a first step towards understanding the surface chemistry necessary for efficient CO ...

Electron backscatter diffraction is a very powerful and relatively new materials characterization... more Electron backscatter diffraction is a very powerful and relatively new materials characterization technique aimed at the determination of crystallographic texture, grain boundary character distributions, lattice strain, phase identification, and much more. The purpose of this book is to provide the fundamental basis for electron backscatter diffraction in materials science, the current state of both hardware and software, and illustrative examples of the applications of electron backscatter diffraction to a wide-range of materials including ...

Acta Materialia, 2012

We present experimental validation of a method for estimating three-dimensional (3-D) relative nu... more We present experimental validation of a method for estimating three-dimensional (3-D) relative numerical populations of grain boundaries from measurements of individual two-dimensional (2-D) cross-sections. Such numerical populations are relevant to network topology and the modeling of intergranular failure modes in grain boundary engineered materials, and are distinct from geometrical population measures such as area per volume. We examine 3-D reconstructions of stainless steel and copper, with varying populations of twin-related boundaries, generated by serial-section electron backscatter diffraction and high-energy X-ray diffraction microscopy. We show that 2-D length fractions, 2-D number fractions and 3-D number fractions are all distinct quantities when grain boundary type is correlated with grain boundary size. We also demonstrate that the last quantity may be reliably inferred from the first two, provided the experimental spatial resolution is much finer than the grain size, eliminating the need to use 3-D experimental methods to access at least some information about 3-D network properties. Many of the R3 boundaries are extremely complex, with highly re-entrant shapes that can intersect a sample plane many times, giving a false impression of multiple separate boundaries.

Acta Materialia, 2009

The feasibility of using ''grain-boundary engineering" techniques to reduce the susceptibility of... more The feasibility of using ''grain-boundary engineering" techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of ''special" grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J c fracture toughness values were some 20-30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

The phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and... more The phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and 760°C for 2000 h (2.7 mo) and 16,000 h (1.8 yr). The tensile properties and the Charpy impact toughness of these samples were measured in the mill annealed condition as well as after aging. The microstructures of samples aged 16,000 hours were examined using scanning and transmission electron microscopy (SEM and TEM). Preliminary TEM results suggest that µ phase forms at all temperatures investigated. Discrete carbide particles in addition to a film with very uniform thickness which appears to be µ phase formed on grain boundaries in the sample aged at 593°C. The ordered Ni 2 (Cr, Mo) phase was also seen in this sample. At the higher aging temperatures, mainly µ phase forms covering all the grain boundaries and also distributed throughout the bulk. Although strength increased somewhat with aging, the ductility decreased due to the formation of these grain boundary precipitates and brittle intermetallics.

Http Dx Doi Org 10 1080 095008397178823, Nov 14, 2010

ABSTRACT

Acta Mater, 1997

Deformation twinning in a two phase Ni-25Mo-8Cr (in wt%) alloy containing Ni*(Mo, Cr) domains/pre... more Deformation twinning in a two phase Ni-25Mo-8Cr (in wt%) alloy containing Ni*(Mo, Cr) domains/precipitates in a face centred cubic (f.c.c.) matrix has been studied by detailed crystallographic analysis, coupled with transmission electron microscopy and experimental and computer simulated electron diffraction. Correlation of crystallography with diffraction evidence indicates that all the twinning modes that lead to the formation of true twins (combined, types I and II) in the NilMo superlattice are in operation. Long range order within the domains/precipitates of the ordered phase is preserved during deformation by the reorientation of the sheared regions into anti-parallel or perpendicular twin variants, and the rationale for this, from crystallographic considerations, is provided. Defect analysis, through weak-beam dark field imaging, revealed that the twins in the NizMo domains form as a consequence of shearing by a( 112) Shockley partial dislocations with the same Burgers vector on adjacent { I1 1) planes and eventual overlapping of intrinsic stacking faults. Twin nuclei are composed of three-layer or nine-layer stacking faults, depending on the geometry of the twinning plane within the NizMo domains. A new dislocation model, based on the dissociation of a single set of i(Ol I) unit dislocations-in slips bands at the precipitate/matrix interface-to release the twinning Shockley partial dislocations, is able to satisfactorily account for twin formation within the NizMo precipitates. 0 1997 Acta Metallurgica Inc.

Recovery based observations of high pressure material behavior generated under high explosively d... more Recovery based observations of high pressure material behavior generated under high explosively driven flyer based loading conditions are reported. Two shock pressures, 25, and 55 GPa and four orientations (100), (110), (111), (123) were considered. Recovered material was characterized using electron backscatter diffraction along with a limited amount of transmission electron microscopy to assess the occurrence of twinning under each

Philosophical Magazine A, 2000

... and VIJAY K. VASUDEVAN~ Department of Materials Science and Engineering, University of Cincin... more ... and VIJAY K. VASUDEVAN~ Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012, USA ... Mag. A, 73, p. 575. WANG, P., KUMAR, M.,VEERARAGHAVAN, D., and VASUDEVAN, VK, 1998, Acta mater., 46, ...

MRS Online Proceeding Library

ABSTRACT

We have investigated the microstructural changes in ductile porous metals during high pressure-hi... more We have investigated the microstructural changes in ductile porous metals during high pressure-high strain rate loading employing atomistic simulations and explored their relation to recent experiments on polycrystalline copper samples. Molecular-dynamics ...

Acta Materialia, 2015

Grain boundary networks should play a dominant role in determining the mechanical properties of n... more Grain boundary networks should play a dominant role in determining the mechanical properties of nanocrystalline metals. However, these networks are difficult to characterize and their response to deformation is incompletely understood. In this work, we study the grain boundary network of nanocrystalline Ni and explore whether it can be modified by plastic deformation.

International Journal of Solids and Structures, 2015