Jainagesh Sekhar - Academia.edu (original) (raw)

Papers by Jainagesh Sekhar

PICMET '09 - 2009 Portland International Conference on Management of Engineering & Technology, 2009

... ARI author contributions to radical innovation include Bers [6-10, 59], Dismukes [25-32], Mil... more ... ARI author contributions to radical innovation include Bers [6-10, 59], Dismukes [25-32], Miller [63], and Sekhar [86 ... The four prominent 20th Century radical innovations selected and illustrated in Table 3 below are: Atomic Bomb, GMO in Food Products, Powered Flight, and Wind ...

International Journal of Innovation and Technology Management, 2012

In-depth historical assessment of innovation over the past several hundred years shows that the i... more In-depth historical assessment of innovation over the past several hundred years shows that the incremental innovation and cost reduction emphasized in the late 20th century will prove inadequate and even counterproductive in the 21st century for achieving sustained, global competitive advantage requiring radical innovation. Accordingly, a comprehensive, holistic innovation life cycle model is needed. Ongoing research since the PICMET'07 Conference has confirmed this need for a viable model, and developed an Accelerated Radical Innovation (ARI) methodology as part of the life cycle framework for a viable model. A significant contribution of the ARI methodology has been to develop improved techniques for measuring and guiding innovation progress, based on focused assessment of 10 innovation attributes at each of the 10 ARI steps in the first two phases of a six-phase life cycle envisioned to describe any radical innovation life cycle. This paper first summarizes systematically th...

Si' implant activation efficiencies above 90%, even at doses of 5~1 0 '~ cm-2, have been achieved... more Si' implant activation efficiencies above 90%, even at doses of 5~1 0 '~ cm-2, have been achieved in GaN by RTP at 1400-1500 "C for 10 secs. The annealing system utilizes with MoSi2 heating elements capable of operation up to 1900 "C , producing high heating and cooling rates (up to 100 "C s-I). Unencapsulated GaN show severe surface pitting at 1300 "C , and complete loss of the film by evaporation at 1400 "C. Dissociation of nitrogen fiom the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for A1N and 3.4 eV for InN). Encapsulation with either rf-magnetron reactively sputtered or MOMBE-grown AlN thin films provide protection against GaN surface degradation up to 1400 "C, where peak electron concentrations of-5 x 1 O2' cm-3 can be achieved in Si-implanted GaN. SIMS profiling showed little measurable redistribution of Si, suggesting Dsi 5 cm2 s-' at 1400 "C. The implant activation efficiency decreases at higher temperatures, which may result fiom SiGa to SiN site switching and resultant self-compensation. 5 MASTER DISCLAIMER This report was prepared as an account of work sponsored by an agency of the

GaN implanted with donor(Si, S, Se, Te) or acceptor (Be, Mg, C) species was annealed at 900-1500 ... more GaN implanted with donor(Si, S, Se, Te) or acceptor (Be, Mg, C) species was annealed at 900-1500 °C using AlN encapsulation. No redistribution was measured by SIMS for any of the dopants and effective diffusion coefficients are ≤2×10-13 cm 2 ⋅ s-1 at 1400 °C, except Be, which displays damage-enhanced diffusion at 900 °C and is immobile once the point defect concentration is removed. Activation efficiency of ~90% is obtained for Si at 1400 °C. TEM of the implanted material shows a strong reduction in lattice disorder at 1400-1500 °C compared to previous results at 1100 °C. There is minimal interaction of the sputtered AlN with GaN under our conditions, and it is readily removed selectively with KOH.

Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of... more Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of 3-5~1 0 '~ cmV2 and annealed at temperatures up to 1450 "C. No redistribution of any of the elements is detectable by Secondary Ion Mass Spectrometry, except for Be, which displays an apparent damage-assisted diffusion at 900 "C. At higher temperatures there is no further movement of the Be, suggesting that the point defect flux that assists motion at lower temperatures has been annealed. Effective difiivities are , < 2~l O ' '~ cm2.sec-' at 1450 "C for each ofthe dopants in GaN.

... convection in aluminium and superalloys GS REDDY*, JA SEKHAR Defence Metallurgical Research L... more ... convection in aluminium and superalloys GS REDDY*, JA SEKHAR Defence Metallurgical Research Laboratory, PO Kanchanbagh, Hyderabad 500258, India ... Mr GS Reddy is also a post graduate student registered at the Banaras Hindu University, Varanasi, India. ...

A high temperature nickel-base superalloy (Rene 108) was directionally solidified by imposing var... more A high temperature nickel-base superalloy (Rene 108) was directionally solidified by imposing various growth rates and thermal gradients using a modified Bridgeman apparatus. The scaling of the solidification structure was recorded as a function of the imposed growth variables. A special Gleeble testing procedure, developed previously where the solidified samples were quickly raised to a predetermined temperature in the semi-solid zone and fractured, was used for the measurement of fracture conditions in the semi-solid region. The effect of the solidification process variables, namely, the temperature gradient and velocity, on the fracture stress in the transverse direction was to increase the fracture stress at a given temperature. The upper hot-tearing temperature was noted to be a function of the solidification variables. The amount of strain accommodation and the hot tearing resistance was found to be influenced by the solidification microstructure. Fracture maps, which include the temperature, transverse fracture stress and temperature gradient during solidification (T-cyv-G), for the directionally solidified microstructures are presented. Castability maps are created from the microstructure and the fracture data and display the porosity and semi-solid strength as a function of the casting variables.

A new process for the forming of metal alloys into net or near-net shapes by forgingtype operatio... more A new process for the forming of metal alloys into net or near-net shapes by forgingtype operations was investigated. The process, denoted as thixoforging, uses a partially solidified, vigorously agitated rheocast metal as initial charge material. It was shown that intermittent operation of the slurry producer coupled with direct transfer of the semi-solid charge to the lower die of the forging press permits production of sound heat-treatable components that are free of entrapped oxides and which have excellent static and dynamic properties. Commercial 2024 aluminum-base alloy was used in the investigation. The solution heat treatment response of rheocast ingots and the structure and properties of thixoforged parts of the alloy were compared with conventionally solidified ingots and with forgings made from a completely liquid initial charge respectively. It was found that the initial spheroidal rheocast structure contains less nonequilibrium second phase and solutionizes more rapidly than the plate-like dendritic structure. The disk-shaped parts 0.12 m in diameter and 0.025-0.04 m high produced from both rheocast and liquid charge materials respond to heat treatment and possess properties comparable with those of the wrought alloy. For example, the tensile properties of the thixoforged parts in the T-6 condition were as follows: yield strength, approximately 35 kgf mm-2 ; ultimate tensile strength, 46.4 kgf mm-2 ; elongation, 11.2%. These values are slightly lower than those measured on squeeze-cast parts made in this study. The fatigue properties of the thixoforged parts, however, were slightly superior.

This article reports on the influence of the processing and material parameters, namely initial t... more This article reports on the influence of the processing and material parameters, namely initial temperature, compaction pressure, particle size, diluent, and chemical composition on the dimensional changes of the product during micropyretic synthesis. The relationship between the porosity change and elongation is established. From a knowledge of this relationship it is possible to tailor the dimensions of a synthesized product for proper design in an engineering application. Conversely, from the noted dimensional changes it is possible to obtain the final porosity values and thus the mechanical properties.

NiAl has been produced by micropyretic synthesis in a tougher state by adding Cr, V, and Fe. The ... more NiAl has been produced by micropyretic synthesis in a tougher state by adding Cr, V, and Fe. The mechanical properties of the alloys have been evaluated. Fracture toughness values as high as 9.5 M Pa√m have been obtained. The effect of different process variables like particle size, diluents, and extent of preheat on the process of synthesis and the microstructure

The effect of alloying additions of Ti and B on the process of micropyretic synthesis on NiAl and... more The effect of alloying additions of Ti and B on the process of micropyretic synthesis on NiAl and on the microstructure of the synthesized alloy was examined. It was observed that the combustibility of the quaternary alloy is good despite the presence of the alloying elements because of an additional combustion reaction between Ti and B. The microstructure of the quaternary alloy was found to consist primarily of the NiAl and Ti boride phases. The effect of preheating of the specimen prior to synthesis on the process of synthesis was also examined. It was observed that preheating not only can change the morphology of the phases but also influence the nature of the phases present in the alloy. The mechanism of the formation of the two phase microstructure during the synthesis from the elemental powders was established by stopping the combustion front and by carrying out a detailed microstructural characterization of regions around the stopped combustion front.

The heat flow model previously developed for a pure metal is extended to the solidification of an... more The heat flow model previously developed for a pure metal is extended to the solidification of an alloy over a range of temperatures. The equations are then applied to rapid surface melting and solidification of an alloy substrate. The substrate is subjected to a pulse of stationary high intensity heat flux over a circular region on its bounding surface. The finite difference form of the heat transfer equation is written in terms of dimensionless nodal temperature and enthalpy in an oblate spheroidal coordinate system. A numerical solution technique is developed for an alloy which precipitates a eutectic at the end of solidification. Generalized solutions are presented for an A1-4.5 wt pct Cu alloy subjected to a uniform heat flux distribution over the circular region. Dimensionless temperature distributions, size and location of the "mushy" zone, and average cooling rate during solidification are calculated as a function of the product of absorbed heat flux, q, the radius of the circular region, a, and time. General trends established show that for a given product of qa all isotherms are located at the same dimensionless distance for identical Fourier numbers. The results show that loss of superheat and shallower temperature gradients during solidification result in significantly larger "mushy" zone sizes than during melting. Furthermore, for a given set of process parameters, the average cooling rate increases with distance solidified from the bottom to the top of the melt pool.

During combustion/micropyretic synthesis, conditions that give rise to rapid solidification and r... more During combustion/micropyretic synthesis, conditions that give rise to rapid solidification and rapidly solidified microstructures may be encountered. In this article, many such conditions are identified for the first time in a Ni–Al system. In addition, the banded structures and aligned dendrites that are encountered in this system are also examined. The various techniques of rapid solidification that may be initiated with combustion synthesis are examined and discussed.

A combined theoretical and experimental program was carried out to study the effect of applied pr... more A combined theoretical and experimental program was carried out to study the effect of applied pressure and die coatings on the heat transfer coefficient at the metal-die interface during solidification of Al-Si eutectic alloy against an H-13 die. The castings were unidirectionally solidified in a commercial hydraulic press modified to accept a controlled temperature die set. The die thermal behavior was experimentally determined from thermocouples located at different distances from the metal-die interface. A onedimensional computer heat flow program was used to correlate measured and simulated die temperatures and hence to deduce the heat transfer coefficients. It was found that application of apressure of 19.6 X 107 Pa increases the heat transfer coefficient from about 3.4 X 103 to 5.25 X 104 Wm-2 K-1. The effectsof different die coatings on the heat transfer coefficient were also determined.

PICMET '09 - 2009 Portland International Conference on Management of Engineering & Technology, 2009

... ARI author contributions to radical innovation include Bers [6-10, 59], Dismukes [25-32], Mil... more ... ARI author contributions to radical innovation include Bers [6-10, 59], Dismukes [25-32], Miller [63], and Sekhar [86 ... The four prominent 20th Century radical innovations selected and illustrated in Table 3 below are: Atomic Bomb, GMO in Food Products, Powered Flight, and Wind ...

International Journal of Innovation and Technology Management, 2012

In-depth historical assessment of innovation over the past several hundred years shows that the i... more In-depth historical assessment of innovation over the past several hundred years shows that the incremental innovation and cost reduction emphasized in the late 20th century will prove inadequate and even counterproductive in the 21st century for achieving sustained, global competitive advantage requiring radical innovation. Accordingly, a comprehensive, holistic innovation life cycle model is needed. Ongoing research since the PICMET'07 Conference has confirmed this need for a viable model, and developed an Accelerated Radical Innovation (ARI) methodology as part of the life cycle framework for a viable model. A significant contribution of the ARI methodology has been to develop improved techniques for measuring and guiding innovation progress, based on focused assessment of 10 innovation attributes at each of the 10 ARI steps in the first two phases of a six-phase life cycle envisioned to describe any radical innovation life cycle. This paper first summarizes systematically th...

Si' implant activation efficiencies above 90%, even at doses of 5~1 0 '~ cm-2, have been achieved... more Si' implant activation efficiencies above 90%, even at doses of 5~1 0 '~ cm-2, have been achieved in GaN by RTP at 1400-1500 "C for 10 secs. The annealing system utilizes with MoSi2 heating elements capable of operation up to 1900 "C , producing high heating and cooling rates (up to 100 "C s-I). Unencapsulated GaN show severe surface pitting at 1300 "C , and complete loss of the film by evaporation at 1400 "C. Dissociation of nitrogen fiom the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for A1N and 3.4 eV for InN). Encapsulation with either rf-magnetron reactively sputtered or MOMBE-grown AlN thin films provide protection against GaN surface degradation up to 1400 "C, where peak electron concentrations of-5 x 1 O2' cm-3 can be achieved in Si-implanted GaN. SIMS profiling showed little measurable redistribution of Si, suggesting Dsi 5 cm2 s-' at 1400 "C. The implant activation efficiency decreases at higher temperatures, which may result fiom SiGa to SiN site switching and resultant self-compensation. 5 MASTER DISCLAIMER This report was prepared as an account of work sponsored by an agency of the

GaN implanted with donor(Si, S, Se, Te) or acceptor (Be, Mg, C) species was annealed at 900-1500 ... more GaN implanted with donor(Si, S, Se, Te) or acceptor (Be, Mg, C) species was annealed at 900-1500 °C using AlN encapsulation. No redistribution was measured by SIMS for any of the dopants and effective diffusion coefficients are ≤2×10-13 cm 2 ⋅ s-1 at 1400 °C, except Be, which displays damage-enhanced diffusion at 900 °C and is immobile once the point defect concentration is removed. Activation efficiency of ~90% is obtained for Si at 1400 °C. TEM of the implanted material shows a strong reduction in lattice disorder at 1400-1500 °C compared to previous results at 1100 °C. There is minimal interaction of the sputtered AlN with GaN under our conditions, and it is readily removed selectively with KOH.

Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of... more Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of 3-5~1 0 '~ cmV2 and annealed at temperatures up to 1450 "C. No redistribution of any of the elements is detectable by Secondary Ion Mass Spectrometry, except for Be, which displays an apparent damage-assisted diffusion at 900 "C. At higher temperatures there is no further movement of the Be, suggesting that the point defect flux that assists motion at lower temperatures has been annealed. Effective difiivities are , < 2~l O ' '~ cm2.sec-' at 1450 "C for each ofthe dopants in GaN.

... convection in aluminium and superalloys GS REDDY*, JA SEKHAR Defence Metallurgical Research L... more ... convection in aluminium and superalloys GS REDDY*, JA SEKHAR Defence Metallurgical Research Laboratory, PO Kanchanbagh, Hyderabad 500258, India ... Mr GS Reddy is also a post graduate student registered at the Banaras Hindu University, Varanasi, India. ...

A high temperature nickel-base superalloy (Rene 108) was directionally solidified by imposing var... more A high temperature nickel-base superalloy (Rene 108) was directionally solidified by imposing various growth rates and thermal gradients using a modified Bridgeman apparatus. The scaling of the solidification structure was recorded as a function of the imposed growth variables. A special Gleeble testing procedure, developed previously where the solidified samples were quickly raised to a predetermined temperature in the semi-solid zone and fractured, was used for the measurement of fracture conditions in the semi-solid region. The effect of the solidification process variables, namely, the temperature gradient and velocity, on the fracture stress in the transverse direction was to increase the fracture stress at a given temperature. The upper hot-tearing temperature was noted to be a function of the solidification variables. The amount of strain accommodation and the hot tearing resistance was found to be influenced by the solidification microstructure. Fracture maps, which include the temperature, transverse fracture stress and temperature gradient during solidification (T-cyv-G), for the directionally solidified microstructures are presented. Castability maps are created from the microstructure and the fracture data and display the porosity and semi-solid strength as a function of the casting variables.

A new process for the forming of metal alloys into net or near-net shapes by forgingtype operatio... more A new process for the forming of metal alloys into net or near-net shapes by forgingtype operations was investigated. The process, denoted as thixoforging, uses a partially solidified, vigorously agitated rheocast metal as initial charge material. It was shown that intermittent operation of the slurry producer coupled with direct transfer of the semi-solid charge to the lower die of the forging press permits production of sound heat-treatable components that are free of entrapped oxides and which have excellent static and dynamic properties. Commercial 2024 aluminum-base alloy was used in the investigation. The solution heat treatment response of rheocast ingots and the structure and properties of thixoforged parts of the alloy were compared with conventionally solidified ingots and with forgings made from a completely liquid initial charge respectively. It was found that the initial spheroidal rheocast structure contains less nonequilibrium second phase and solutionizes more rapidly than the plate-like dendritic structure. The disk-shaped parts 0.12 m in diameter and 0.025-0.04 m high produced from both rheocast and liquid charge materials respond to heat treatment and possess properties comparable with those of the wrought alloy. For example, the tensile properties of the thixoforged parts in the T-6 condition were as follows: yield strength, approximately 35 kgf mm-2 ; ultimate tensile strength, 46.4 kgf mm-2 ; elongation, 11.2%. These values are slightly lower than those measured on squeeze-cast parts made in this study. The fatigue properties of the thixoforged parts, however, were slightly superior.

This article reports on the influence of the processing and material parameters, namely initial t... more This article reports on the influence of the processing and material parameters, namely initial temperature, compaction pressure, particle size, diluent, and chemical composition on the dimensional changes of the product during micropyretic synthesis. The relationship between the porosity change and elongation is established. From a knowledge of this relationship it is possible to tailor the dimensions of a synthesized product for proper design in an engineering application. Conversely, from the noted dimensional changes it is possible to obtain the final porosity values and thus the mechanical properties.

NiAl has been produced by micropyretic synthesis in a tougher state by adding Cr, V, and Fe. The ... more NiAl has been produced by micropyretic synthesis in a tougher state by adding Cr, V, and Fe. The mechanical properties of the alloys have been evaluated. Fracture toughness values as high as 9.5 M Pa√m have been obtained. The effect of different process variables like particle size, diluents, and extent of preheat on the process of synthesis and the microstructure

The effect of alloying additions of Ti and B on the process of micropyretic synthesis on NiAl and... more The effect of alloying additions of Ti and B on the process of micropyretic synthesis on NiAl and on the microstructure of the synthesized alloy was examined. It was observed that the combustibility of the quaternary alloy is good despite the presence of the alloying elements because of an additional combustion reaction between Ti and B. The microstructure of the quaternary alloy was found to consist primarily of the NiAl and Ti boride phases. The effect of preheating of the specimen prior to synthesis on the process of synthesis was also examined. It was observed that preheating not only can change the morphology of the phases but also influence the nature of the phases present in the alloy. The mechanism of the formation of the two phase microstructure during the synthesis from the elemental powders was established by stopping the combustion front and by carrying out a detailed microstructural characterization of regions around the stopped combustion front.

The heat flow model previously developed for a pure metal is extended to the solidification of an... more The heat flow model previously developed for a pure metal is extended to the solidification of an alloy over a range of temperatures. The equations are then applied to rapid surface melting and solidification of an alloy substrate. The substrate is subjected to a pulse of stationary high intensity heat flux over a circular region on its bounding surface. The finite difference form of the heat transfer equation is written in terms of dimensionless nodal temperature and enthalpy in an oblate spheroidal coordinate system. A numerical solution technique is developed for an alloy which precipitates a eutectic at the end of solidification. Generalized solutions are presented for an A1-4.5 wt pct Cu alloy subjected to a uniform heat flux distribution over the circular region. Dimensionless temperature distributions, size and location of the "mushy" zone, and average cooling rate during solidification are calculated as a function of the product of absorbed heat flux, q, the radius of the circular region, a, and time. General trends established show that for a given product of qa all isotherms are located at the same dimensionless distance for identical Fourier numbers. The results show that loss of superheat and shallower temperature gradients during solidification result in significantly larger "mushy" zone sizes than during melting. Furthermore, for a given set of process parameters, the average cooling rate increases with distance solidified from the bottom to the top of the melt pool.

During combustion/micropyretic synthesis, conditions that give rise to rapid solidification and r... more During combustion/micropyretic synthesis, conditions that give rise to rapid solidification and rapidly solidified microstructures may be encountered. In this article, many such conditions are identified for the first time in a Ni–Al system. In addition, the banded structures and aligned dendrites that are encountered in this system are also examined. The various techniques of rapid solidification that may be initiated with combustion synthesis are examined and discussed.

A combined theoretical and experimental program was carried out to study the effect of applied pr... more A combined theoretical and experimental program was carried out to study the effect of applied pressure and die coatings on the heat transfer coefficient at the metal-die interface during solidification of Al-Si eutectic alloy against an H-13 die. The castings were unidirectionally solidified in a commercial hydraulic press modified to accept a controlled temperature die set. The die thermal behavior was experimentally determined from thermocouples located at different distances from the metal-die interface. A onedimensional computer heat flow program was used to correlate measured and simulated die temperatures and hence to deduce the heat transfer coefficients. It was found that application of apressure of 19.6 X 107 Pa increases the heat transfer coefficient from about 3.4 X 103 to 5.25 X 104 Wm-2 K-1. The effectsof different die coatings on the heat transfer coefficient were also determined.