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Articles 1 - 30 of 36
Full-Text Articles in Engineering
Primary Dendrite Trunk Diameter In Al-7wt% Si Alloy Directionally Solidified Aboard The International Space Station, S. R. Upadhyay, Surendra N. Tewari, M. Ghodes, R. N. Grugel, D. R. Poirier, M. Lauer
Primary Dendrite Trunk Diameter In Al-7wt% Si Alloy Directionally Solidified Aboard The International Space Station, S. R. Upadhyay, Surendra N. Tewari, M. Ghodes, R. N. Grugel, D. R. Poirier, M. Lauer
Chemical & Biomedical Engineering Faculty Publications
Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified at growth speeds varying from 10 to 50 µm s-1 aboard the International Space Station to determine the effect of mitigating convection on the primary dendrite array. The observed primary dendrite trunk diameters during steady-state growth of MICAST samples show a good agreement with predictions from a coarsening based model developed by the authors. The trunk diameters in …
Effect Of Cross-Section-Change Induced Advective Flow On The Primary Dendrite Array Morphology Of Hypoeutectic Pb-Sb Alloys During Directional Solidification, K. Pandit, S. R. Upadhyay, Surendra N. Tewari
Effect Of Cross-Section-Change Induced Advective Flow On The Primary Dendrite Array Morphology Of Hypoeutectic Pb-Sb Alloys During Directional Solidification, K. Pandit, S. R. Upadhyay, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
The morphology and distribution of primary dendrites have been examined in Pb-2.2, 5.8 and 10.8 wt. pct. Sb alloy samples directionally solidified (DSed) in ampoules shaped like an hour-glass to examine the influence of cross-section change induced advective flow on the cellular/dendritic interface. This sample design increases the advective flow of the melt towards the array tips, as the liquid-solid interface enters the neck of the ampoule, and then decreases it as the interface exits the neck. The warm solute-rich melt flowing towards the growth front suppresses the extent of side-branching, decreases the primary dendrite spacing, and increases the primary …
Ebc Development For Hot-Pressed Y2O3/Al2O3 Doped Silicon Nitride Ceramics, Sivakumar Ramasamy, Surendra N. Tewari, Kang N. Lee, Ramakrishna T. Bhatt, Dennis S. Fox
Ebc Development For Hot-Pressed Y2O3/Al2O3 Doped Silicon Nitride Ceramics, Sivakumar Ramasamy, Surendra N. Tewari, Kang N. Lee, Ramakrishna T. Bhatt, Dennis S. Fox
Chemical & Biomedical Engineering Faculty Publications
Multilayer EBCs of alcohol and sol-based slurries containing (a) 45% SiO2-34% Y2O3-%Al2O3, (b) mullite/Gd2SiO5 (88/12wt.%) with and without B2O3 addition, (c) mullite/rare-earth silicates (94/6wt.%, (Gd2SiO5, Lu2SiO5, Er2SiO5, and HfSiO4)), and (d) only mullite, were applied on hot pressed Y2O3/Al2O3 doped Si3N4 substrates. Of the four major EBC systems studied, only mullite/Gd2SiO5 (88/12wt.%) EBCs sintered in air at …
Effect Of Synthesis Time And Treatment On Porosity Of Mesoporous Silica Materials, Asli Ertan, Pradeep Kodumuri, Orhan Talu, Surendra N. Tewari
Effect Of Synthesis Time And Treatment On Porosity Of Mesoporous Silica Materials, Asli Ertan, Pradeep Kodumuri, Orhan Talu, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Nitrogen adsorption at 77 K on mesoporous silica materials (MPS) with varying synthesis time and treatment conditions was investigated. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were also used to characterize the mesoporous materials. This study was performed at 6, 24 and 72-h synthesis times. It is shown that 6-h is not enough for complete formation of the MPS material and at least 24-h is necessary. The pore structure starts decaying for the 72-h synthesis time. The three-after-synthesis treatment conditions used were 1) washed, 2) washed and calcined and 3) directly calcined after synthesis. Ethanol/HCl …
Electrodeposition Of Nickel Nanowires And Nanotubes Using Various Templates, Asli Ertan, Surendra N. Tewari, Orhan Talu
Electrodeposition Of Nickel Nanowires And Nanotubes Using Various Templates, Asli Ertan, Surendra N. Tewari, Orhan Talu
Chemical & Biomedical Engineering Faculty Publications
Nickel nanotubes and nanowires are grown by galvanostatic electrodeposition in the pores of 1000, 100, and 15 nm polycarbonate as well as in anodised alumina membranes at a current density of 10 mA cm-2. The effects of pore size, porosity, electrodeposition time, effective current density, and pore aspect ratio are investigated. Nickel nanotube structures are obtained with 1000 nm pore size polycarbonate membrane without any prior treatment method. At the early stages of electrodeposition hollow nickel nanotubes are produced and nanotubes turn into nanowires at longer depositon times. As effective current density accounting for the membrane porosity decreases, the axial …
Mushy-Zone Rayleigh Number To Describe Macrosegregation And Channel Segregate Formation During Directional Solidification Of Metallic Alloys, Surendra N. Tewari, R. Tiwari, G. Magadi
Mushy-Zone Rayleigh Number To Describe Macrosegregation And Channel Segregate Formation During Directional Solidification Of Metallic Alloys, Surendra N. Tewari, R. Tiwari, G. Magadi
Chemical & Biomedical Engineering Faculty Publications
A recently defined mushy-zone Rayleigh number (R-aM) that includes side-branching contributions to the mushy-zone permeability has been examined for its correlation with the longitudinal macrosegregation and channel segregate formation. The Rayleigh number shows (1) a strong correlation between the extent of longitudinal macrosegregation and increase in the mushy-zone convection and (2) a good ability to predict the formation of channel segregates during directional solidification.
A Comparison Between Growth Morphology Of "Eutectic" Cells/Dendrites And Single-Phase Cells/Dendrites, Surendra N. Tewari, S. V. Raj, L. E. Locci
A Comparison Between Growth Morphology Of "Eutectic" Cells/Dendrites And Single-Phase Cells/Dendrites, Surendra N. Tewari, S. V. Raj, L. E. Locci
Chemical & Biomedical Engineering Faculty Publications
Directionally solidified (DS) intermetallic and ceramicbased eutectic alloys with an in-situ composite microstructure microstructure containing finely distributed, long aspect ratio, fiber, or plate reinforcements are being seriously examined for several advanced aero-propulsion applications. In designing these alloys, additional solutes need to be added to the base eutectic composition in order to improve their high temperature strength, and provide for adequate toughness and resistance to environmental degradation. Solute addition, however, promotes instability at the planar liquid-solid interface resulting in the formation of two-phase eutectic “colonies.”[1–4] Because morphology of eutectic colonies is very similar to the single-phase cells and dendrites, the stability …
Effect Of Crucible Diameter Reduction On The Convection, Macrosegregation, And Dendritic Morphology During Directional Solidification Of Pb-2.2 Wt Pct Sb Alloy, Jun Chen, Surendra N. Tewari, G. Magadi, H. C. Degroh Iii
Effect Of Crucible Diameter Reduction On The Convection, Macrosegregation, And Dendritic Morphology During Directional Solidification Of Pb-2.2 Wt Pct Sb Alloy, Jun Chen, Surendra N. Tewari, G. Magadi, H. C. Degroh Iii
Chemical & Biomedical Engineering Faculty Publications
The Pb-2.2 wt pct Sb alloy has been directionally solidified in 1, 2-, 3-, and 7-mm-diameter crucibles with planar and dendritic liquid-solid interface morphology. For plane front solidification, the experimentally observed macrosegregation along the solidified length follows the relationship proposed by Favier.([17,18]) Application of a 0.4 T transverse magnetic field has no effect on the extent of convection. Reducing the ampoule diameter appears to decrease the extent of convection. However, extensive convection is still present even in the 1-mm-diameter crucible. An extrapolation of the observed behavior indicates that nearly diffusive transport conditions require ampoules that are about 40 mum in …
A Mushy-Zone Rayleigh Number To Describe Interdendritic Convection During Directional Solidification Of Hypoeutectic Pb-Sb And Pb-Sn Alloys, Surendra N. Tewari, R. Tiwari
A Mushy-Zone Rayleigh Number To Describe Interdendritic Convection During Directional Solidification Of Hypoeutectic Pb-Sb And Pb-Sn Alloys, Surendra N. Tewari, R. Tiwari
Chemical & Biomedical Engineering Faculty Publications
Based on measurements of the specific dendrite surface area (S-nu), fraction of interdendritic liquid (phi), and primary dendrite spacing (lambda(1)) on transverse sections in a range of directionally solidified hypoeutectic Pb-Sb and Pb-Sn alloys that were grown at thermal gradients varying from 10 to 197 K cm(-1) and growth speeds ranging from 2 to 157 mum s(-1), it is observed that S-nu = lambda(1)(-1) S*(-0.33) (3.38 - 3.29 phi + 8.85 phi(2)), where S* = D-l G(eff)/V m(1) C-o (k - 1)/k, with D-l being the solutal diffusivity in the melt, G(eff) being the effective thermal gradient, V being the …
The Effect Of Convection On Disorder In Primary Cellular And Dendritic Arrays, R. Trivedi, P. Mazumder, Surendra N. Tewari
The Effect Of Convection On Disorder In Primary Cellular And Dendritic Arrays, R. Trivedi, P. Mazumder, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Directional solidification studies have been carried out to characterize the spatial disorder in the arrays of cells and dendrites. Different factors that cause array disorder are investigated experimentally and analyzed numerically. In addition to the disorder resulting from the fundamental selection of a range of primary spacings under given experimental conditions, a significant variation in primary spacings is shown to occur in bulk samples due to convection effects, especially at low growth velocities. The effect of convection on array disorder is examined through directional solidification studies in two different alloy systems, Pb-Sn and Al-Cu. A detailed analysis of the spacing …
Primary Dendrite Distribution And Disorder During Directional Solidification Of Pb-Sb Alloys, Jun Hui, R. Tiwari, X. Wu, Surendra N. Tewari, R. Trivedi
Primary Dendrite Distribution And Disorder During Directional Solidification Of Pb-Sb Alloys, Jun Hui, R. Tiwari, X. Wu, Surendra N. Tewari, R. Trivedi
Chemical & Biomedical Engineering Faculty Publications
Pb-2.2 wt pct Sb and Pb-5.8 wt pet Sb alloys have been directionally solidified from a single-crystal seed with its [100] orientation parallel to the growth direction, to examine the primary dendrite distribution and disorder of the dendrite arrays. The dendrite distribution and ordering have been investigated using analysis techniques such as the Gauss-amplitude fit to the frequency distribution of nearest and higher-order spacings, minimum spanning tree (MST), Voronoi polygon, and Fourier transform (FT) of the dendrite centers. Since the arrangement of dendrites is driven by the requirement to accommodate side-branch growth along the (100) directions, the FT images of …
Compressive Properties Of Zone-Directionally Solidified Β-Nial And Its Off-Eutectic Alloys With Chromium And Tungsten, R. Asthana, R. Tiwari, Surendra N. Tewari
Compressive Properties Of Zone-Directionally Solidified Β-Nial And Its Off-Eutectic Alloys With Chromium And Tungsten, R. Asthana, R. Tiwari, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
The ordered intermetallic compound β-NiAl and its pseudo-binary off-eutectic alloys with 1 at.% tungsten and 9.7 at.% chromium were directionally solidified (DS) in the ‘floating-zone’ mode, and tested for compressive strength and fracture behavior in the temperature range 300–800 K. The dual-phase structures created by the DS of ternary NiAl alloys led to improvements in both the compressive strength and the ductility. The room-temperature (300 K) 0.2% compressive yield strength (CYS) of DS NiAl(W) (623 MPa) is larger than the CYS of DS NiAl(Cr) (565 MPa) and DS NiAl (435 MPa). The CYS of the three alloys dropped …
Cellular Array Morphology During Directional Solidification, Surendra N. Tewari, Y-Husuan Weng, G. L. Ding, R. Trivedi
Cellular Array Morphology During Directional Solidification, Surendra N. Tewari, Y-Husuan Weng, G. L. Ding, R. Trivedi
Chemical & Biomedical Engineering Faculty Publications
Cellular array morphology has been examined in the shallow cell, deep cell, and cell-to-dendrite transition regime in Pb-2.2 wt pct Sb and Al-4.1 wt pct Cu alloy single-crystal samples that were directionally solidified along [100]. Statistical analysis of the cellular spacing distribution on transverse sections has been carried out using minimum spanning tree (MST), Voronoi polygons, radial distribution factor, and fast Fourier transform (FFT) techniques. The frequency distribution of the number of nearest neighbors and the MST parameters suggest that the arrangement of cells may be visualized as a hexagonal tessellation with superimposed 50 pct random noise. However, the power …
Heat Transfer In A Thin Liquid Film In The Presence Of Electric Field For Non-Isothermal Interfacial Condition, Rama S.R. Gorla, Jorge E. Gatica, Bahman Ghorashi, Pijarn In-Eure, Larry W. Byrd
Heat Transfer In A Thin Liquid Film In The Presence Of Electric Field For Non-Isothermal Interfacial Condition, Rama S.R. Gorla, Jorge E. Gatica, Bahman Ghorashi, Pijarn In-Eure, Larry W. Byrd
Chemical & Biomedical Engineering Faculty Publications
Heat transfer enhancement in an evaporating thin liquid film using the electric field under non-isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, the capillary pressure and the electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of an electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that the electric field can enhance heat transfer in the thin liquid film significantly. In addition, …
Mushy Zone Morphology During Directional Solidification Of Pb-5.8 Wt Pct Sb Alloy, L. Yu, G. L. Ding, J. Reye, S. N. Ojha, Surendra N. Tewari
Mushy Zone Morphology During Directional Solidification Of Pb-5.8 Wt Pct Sb Alloy, L. Yu, G. L. Ding, J. Reye, S. N. Ojha, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
The Pb-5.8 wt pet Sb alloy was directionally solidified with a positive thermal gradient of 140 K cm(-1) at a growth speed ranging from 0.8 to 30 mu m s(-1), and then it was quenched to retain the mushy zone morphology. The morphology of the mushy zone along its entire length has been characterized by using a serial sectioning and three-dimensional image reconstruction technique. Variation in the cellular/dendritic shape factor, hydraulic radius of the interdendritic region, and fraction solid along the mushy zone length has been studied. A comparison with predictions from theoretical models indicates that convection remarkably reduces the …
Reactive Infiltration Of Silicon Melt Through Microporous Amorphous Carbon Preforms, P. Sangsuwan, Surendra N. Tewari, Jorge E. Gatica, M. Singh, R. Dickerson
Reactive Infiltration Of Silicon Melt Through Microporous Amorphous Carbon Preforms, P. Sangsuwan, Surendra N. Tewari, Jorge E. Gatica, M. Singh, R. Dickerson
Chemical & Biomedical Engineering Faculty Publications
The kinetics of unidirectional capillary infiltration of silicon melt into microporous carbon preforms has been investigated as a function of the pore morphology and melt temperature. The infiltrated specimens showed alternating bands of dark and bright regions, which corresponded to the unreacted free carbon and free silicon regions, respectively. The decrease in the infiltration front velocity for increasing infiltration distances is in qualitative agreement with the closed-form solution of capillarity-driven fluid flow through constant-cross-section cylindrical pores. However, drastic changes in the thermal response and infiltration front morphologies were observed for minute differences in the preform's microstructure. This suggests the need …
Cellular/Dendritic Array Tip Morphology During Directional Solidification Of Pb-5.8 Wt Pct Sb Alloy, L. Yu, G. L. Ding, J. Reye, S. N. Ojha, Surendra N. Tewari
Cellular/Dendritic Array Tip Morphology During Directional Solidification Of Pb-5.8 Wt Pct Sb Alloy, L. Yu, G. L. Ding, J. Reye, S. N. Ojha, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Cellular/dendritic array tip morphology has been examined in directionally solidified and quenched Pb-5.8 wt pet Sb alloy by a serial sectioning and three-dimensional image reconstruction technique. There is a large scatter in the tip radius, the nearest neighbor spacing, and the mushy zone length, even among the immediately neighboring cells and dendrites. This scatter may be caused by the natural convection (in the mushy zone and in the bulk melt at the array tip), which also produces macrosegregation along the length of the directionally solidified samples. Even in the presence of convection, however, the tip radii are observed to be …
Macrosegregation Caused By Thermosolutal Convection During Directional Solidification Of Pb-Sb Alloys, S. N. Ojha, G. L. Ding, Y. Lu, J. Reye, Surendra N. Tewari
Macrosegregation Caused By Thermosolutal Convection During Directional Solidification Of Pb-Sb Alloys, S. N. Ojha, G. L. Ding, Y. Lu, J. Reye, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Pb-2.2 and 5.8 wt pet Sb alloys were directionally solidified with a positive thermal gradient of 140 K cm(-1) at growth speeds ranging from 0.8 to 30 mu m s(-1), and then quenched to retain the mushy-zone morphology. Chemical analysis along the length of the directionally solidified portion and in the quenched melt ahead of the dendritic array showed extensive longitudinal macrosegregation, Cellular morphologies growing at smaller growth speeds are associated with larger amounts of macrosegregation as compared with the dendrites growing at higher growth speeds. Convection is caused, mainly, by the density inversion in the overlying melt ahead of …
Cell/Dendrite Distribution In Directionally Solidified Hypoeutectic Pb-Sb Alloys, S. P. O'Dell, G. L. Ding, Surendra N. Tewari
Cell/Dendrite Distribution In Directionally Solidified Hypoeutectic Pb-Sb Alloys, S. P. O'Dell, G. L. Ding, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
The alloys Pb-2.2 wt pet Sb and Pb-5.8 wt pet Sb were directionally solidified with three different thermal gradients of 40, 86, and 140 K cm(-1) at growth rates ranging from 0.6 to 30 mu m s(-1). A Gaussian peak amplitude analysis of the cell/dendrite spacing distribution shows a distinct peak corresponding to the nearest neighbors. The peaks corresponding to the second and the third nearest neighbor spacings are also brought out by this technique. The ratios of the second to the first nearest neighbor spacings and that of the third to the first neighbor spacings are 1.85 +/-: 0.11 …
Copper Alloy-Impregnated Carbon-Carbon Hybrid Composites For Electronic Packaging Applications, S. K. Datta, Surendra N. Tewari, Jorge E. Gatica, W. Shih, L. Bentsen
Copper Alloy-Impregnated Carbon-Carbon Hybrid Composites For Electronic Packaging Applications, S. K. Datta, Surendra N. Tewari, Jorge E. Gatica, W. Shih, L. Bentsen
Chemical & Biomedical Engineering Faculty Publications
Porous carbon-carbon preforms, based on three-dimensional networks of PAN (Polyacrylonitrile)-based carbon fibers and various volume fractions of chemical vapor-deposited (CVD) carbon, were impregnated by oxygen-free, high-conductivity (OFHC) Cu, Cu-6Si-0.9Cr, and Cu-0.3Si-0.3Cr (wt pct) alloys by pressure infiltration casting. The obtained composites were characterized for their coefficient of thermal expansion (CTE) and thermal conductivity (K) along the through-thickness and two in-plane directions. One composite, with a 28 vol pct Cu-0.3Si-0.3Cr alloy, showed outstanding potential for thermal management applications in electronic applications. This composite exhibited approximately isotropic thermal expansion properties (CTE = 4 to 6.5 ppm/K) and thermal conductivities (k greater than …
Strength Degradation Of Sapphire Fibers During Pressure Casting Of A Sapphire-Reinforced Ni-Base Superalloy, R. Asthana, Surendra N. Tewari, S. L. Draper
Strength Degradation Of Sapphire Fibers During Pressure Casting Of A Sapphire-Reinforced Ni-Base Superalloy, R. Asthana, Surendra N. Tewari, S. L. Draper
Chemical & Biomedical Engineering Faculty Publications
Transient-liquid-phase (TLP) bonding was used to fabricate a Haynes 230 Ni-base superalloy/sapphire fiber composite for high-temperature applications. Boron was used as a melting-point depressant for the Ni, to aid superalloy infiltration of the fibers. Preliminary study of the composite indicated an incomplete TLP bonding cycle. Therefore, microstructural and microchemical analyses were carried out to determine the TLP bonding mechanism. It was found that the TLP process did not occur under local thermodynamic equilibrium conditions at the solid/liquid interfaces, contrary to the primary assumption of conventional models, so a modified model for TLP bonding is proposed. The main differences between the …
Dynamic Analysis Of Unidirectional Pressure Infiltration Of Porous Preforms By Pure Metals, Dhiman K. Biswas, Jorge E. Gatica, Surendra N. Tewari
Dynamic Analysis Of Unidirectional Pressure Infiltration Of Porous Preforms By Pure Metals, Dhiman K. Biswas, Jorge E. Gatica, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Unidirectional pressure infiltration of porous preforms by molten metals is investigated numerically. A phenomenological model to describe fluid flow and transport phenomena during infiltration of fibrous preforms by a metal is formulated. The model describes the dynamics of the infiltration process, the temperature distribution, and solid fraction distribution. The numerical results are compared against classical asymptotic analyses and experimental results. This comparison shows that end effects may become important and render asymptotic results unreliable for realistic samples. Fiber volume fraction and initial temperature appear as the factors most strongly influencing infiltration. Metal superheating affects not only the length of the …
Wear And Friction Behavior Of Metal Impregnated Microporous Carbon Composites, Gultekin Goller, D. P. Koty, Surendra N. Tewari, M. Singh, A. Tekin
Wear And Friction Behavior Of Metal Impregnated Microporous Carbon Composites, Gultekin Goller, D. P. Koty, Surendra N. Tewari, M. Singh, A. Tekin
Chemical & Biomedical Engineering Faculty Publications
Metal-matrix composites have been prepared by pressure-infiltration casting of copper-base alloy melts into microporous carbon preforms. The carbon preforms contained varying proportions of amorphous carbon and graphite. Load dependence of the wear and friction behavior of the composite pins has been examined under ambient conditions against cast-iron plates, using a pin-on-plate reciprocating wear tester. The wear resistance of the composite is significantly improved, as compared with the base alloy. Contrary to the normally expected behavior, the addition of graphite to the amorphous carbon does not reduce the friction coefficient, especially at high loads. The wear and friction behavior of the …
Permeability Of Microporous Carbon Preforms, S. K. Datta, N. Simhai, Surendra N. Tewari, Jorge E. Gatica, M. Singh
Permeability Of Microporous Carbon Preforms, S. K. Datta, N. Simhai, Surendra N. Tewari, Jorge E. Gatica, M. Singh
Chemical & Biomedical Engineering Faculty Publications
The permeability of microporous amorphous carbon preforms with varying pore size and pore distributions has been experimentally examined. The porous structures have been characterized by mercury porosimetry and by quantitative metallography of pressure-infiltration-cast metal matrix composites based on the carbon preforms. The permeability shows a linear correlation with the fraction porosity and the square of the pore diameter.
Macrosegregation During Dendritic Arrayed Growth Of Hypoeutectic Pb-Sn Alloys: Influence Of Primary Arm Spacing And Mushy Zone Length, Surendra N. Tewari, Rajesh Shah
Macrosegregation During Dendritic Arrayed Growth Of Hypoeutectic Pb-Sn Alloys: Influence Of Primary Arm Spacing And Mushy Zone Length, Surendra N. Tewari, Rajesh Shah
Chemical & Biomedical Engineering Faculty Publications
Thermosolutal convection in the dendritic mushy zone occurs during directional solidification of hypoeutectic lead tin alloys in a positive thermal gradient, with the melt on the top and the solid below. This results in macrosegregation along the length of the solidified samples. The extent of macrosegregation increases with increasing primary dendrite spacings for constant mushy zone length. For constant primary spacings, the macrosegregation increases with decreasing mushy zone length. Presence of convection reduces the primary dendrite spacings. However, convection in the interdendritic melt has significantly more influence on the spacings as compared with that in the overlying melt, which is …
Convection During Thermally Unstable Solidification Of Pb-Sn In A Magnetic Field, Hui Song, Surendra N. Tewari, H. C. Degroh Iii
Convection During Thermally Unstable Solidification Of Pb-Sn In A Magnetic Field, Hui Song, Surendra N. Tewari, H. C. Degroh Iii
Chemical & Biomedical Engineering Faculty Publications
Convection and macrosegregation in directionally solidified hypoeutectic Pb-38 wt pct Sn and hypereutectic Pb-64.5 wt pct Sn have been examined during upward and downward growth. Temperature fluctuations are observed along the length of the melt column during downward growth. With increasing Rayleigh number, these fluctuations change from none, to cyclic, to time periodic having multiple harmonics, and finally to random. At the higher convective driving force of 350 K temperature inversion, the transverse magnetic field decreased convective levels, strong random temperature fluctuations (flows) becoming smaller and periodic. The maximum field of 0.45 T was unable to completely eliminate convection. For …
Time Dependence Of Tip Morphology During Cellular Dendritic Arrayed Growth, Hui Song, Surendra N. Tewari
Time Dependence Of Tip Morphology During Cellular Dendritic Arrayed Growth, Hui Song, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Succinonitrile-1.9 wt pct acetone has been directionally solidified in 0.7 X 0.7-cm-square cross section pyrex ampoules in order to observe the cell/dendrite tip morphologies, not influenced by the ''wall effects,'' which are present during growth in the generally used thin (about 200 mu m) crucibles. The tips do not maintain a steady-state shape, as is generally assumed. Instead, they fluctuate within a shape envelope. The extent of fluctuation increases with decreasing growth speed, as the micro structure changes from the dendritic to cellular. The influence of natural convection has been examined by comparing these morphologies with those grown, without convection, …
Influence Of Cr And W Alloying On The Fiber-Matrix Interfacial Shear-Strength In Cast And Directionally Solidified Sapphire Nial Composites, R. Asthana, R. Tiwari, Surendra N. Tewari
Influence Of Cr And W Alloying On The Fiber-Matrix Interfacial Shear-Strength In Cast And Directionally Solidified Sapphire Nial Composites, R. Asthana, R. Tiwari, Surendra N. Tewari
Chemical & Biomedical Engineering Faculty Publications
Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in …
Development Of Nial-Based Intermetallic Alloys: Effect Of Chromium Addition, R. Tiwari, Surendra N. Tewari, R. Asthana, A. Garg
Development Of Nial-Based Intermetallic Alloys: Effect Of Chromium Addition, R. Tiwari, Surendra N. Tewari, R. Asthana, A. Garg
Chemical & Biomedical Engineering Faculty Publications
The mechanical behavior of dual-phase NiAl(Cr) microstructures, consisting of elongated primary NiAl grains aligned with an intergranular NiAl-Cr eutectic phase, produced by extrusion of a cast NiAl(Cr) alloy, has been examined. Chromium addition to create a dual phase NiAl-based aligned microstructure leads to large increases in the yield strength but no significant toughness improvement. This is achieved primarily by solid solution hardening and precipitation hardening. The constitutional hardening rate resulting from deviations from stoichiometry in the nickel-rich NiAl was estimated to be about 66 MPa per atomic per cent of nickel.
Influence Of Interfacial Reactions On The Fiber-Matrix Interfacial Shear-Strength In Sapphire Fiber-Reinforced Nial(Yb) Composites, Surendra N. Tewari, R. Asthana, R. Tiwari, R. R. Bowman, J. Smith
Influence Of Interfacial Reactions On The Fiber-Matrix Interfacial Shear-Strength In Sapphire Fiber-Reinforced Nial(Yb) Composites, Surendra N. Tewari, R. Asthana, R. Tiwari, R. R. Bowman, J. Smith
Chemical & Biomedical Engineering Faculty Publications
The influence of microstructure of the fiber-matrix interface on the interfacial shear strength, measured using a fiber-pushout technique, has been examined in a sapphire-fiber-reinforced NiAl(Yb) matrix composite under the following conditions: (1) as-fabricated powder metallurgy (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM composites after directional solidification (DS). The fiber-pushout stress-displacement behavior consisted of an initial ''pseudoelastic'' region, wherein the stress increased linearly with displacement, followed by an ''inelastic'' region, where the slope of the stress-displacement plot decreased until a maximum stress was reached, and the subsequent gradual stress decreased to a ''frictional'' stress. Energy-dispersive …