The demand for high performance lithium-ion batteries (LIBs) is increasing due to widespread use ... more The demand for high performance lithium-ion batteries (LIBs) is increasing due to widespread use of portable devices and electric vehicles. Silicon (Si) is one of the most attractive candidate anode materials for next generation LIBs. However, the high-volume change (>300%) during lithium ion alloying/de-alloying leads to poor cycle life. When Si is used as the anode, conductive carbon is needed to provide the necessary conductivity. However, the traditional carbon coating method could not overcome the challenges of pulverization and unstable Solid Electrolyte Interphase (SEI) layer during long-term cycling. Since 2010, Si/Graphene composites have been vigorously studied in hopes of providing a material with better cycling performance. This paper reviews current progress of Si/Graphene nanocomposites in LIBs. Different fabrication methods have been studied to synthesize Si/Graphene nanocomposites with promising electrochemical performances. Graphene plays a key enabling role in Si/Graphene anodes. However, the desired properties of graphene for this application have not been systematically studied and understood. Further systematic investigation of the desired graphene properties is suggested to better control the Si/Graphene anode performance.
Each year millions of dollars are lost as a result of distortion, cracking and mechanical propert... more Each year millions of dollars are lost as a result of distortion, cracking and mechanical property variations due to unexpected problems in the quenching process. A thorough understanding of the variations in quenching fluid's performance as a function of the medium's physical properties, system variables and part geometry is not currently available.
The concept of the new category materials high entropy ceramics (HECs) has been proposed several ... more The concept of the new category materials high entropy ceramics (HECs) has been proposed several years ago, which is directly borrowed from high entropy alloys (HEAs). It quickly attracts a lot of interests and displays promising properties. However, there is no clear definition of HECs differentiating it from HEAs, as it is still in its early research stage. In the current work, we are trying to use the classic perovskite LaMnO 3±δ (LMO) to demonstrate the fundamental differences between HECs and HEAs. We have adopted the integrated defect chemistry and CALPHAD approach to investigate the mixing behavior and how it is affected by the control parameters, i.e. PO 2 , T, and composition. We have developed a new way to visualize the mixing behavior of the species including the cations, anions, and defects (vacancies), which linked the mixing behavior to the thermo-chemical properties including enthalpy, entropy, and Gibbs energy. It was found that entropy plays the most important role on the mixing behavior in LMO. The present work paves the way for the HECs investigation and the design of new HECs for the various applications.
A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quench... more A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quenching system in distilled water while varying bath temperature and the agitation level. Time versus temperature data were collected during the quench using a thermocouple embedded inside the probe. The surface heat transfer coefficients as a function of temperature were calculated using the Newtonian cooling approximation. The maximum heat transfer coefficient values ranged from 1054 W/m2⋅K for 100°C water with no agitation to 3822 W/m2⋅K for 5°C water with the agitation of 1850 r/min. The data also showed that at higher levels of agitation and lower bath temperatures, the maximum heat transfer coefficient increased.
Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science, Aug 1, 2007
for providing the interesting discussions, for their creative ideas, and for helping me in conduc... more for providing the interesting discussions, for their creative ideas, and for helping me in conducting some of the experiments at the WPI advanced casting v laboratory. My sincere thanks go to Rita Shilansky for her patience, consideration, and assistance during all the years when I was at WPI. All the faculties, my friends, and my colleagues at the Materials Science & Engineering program and the Metal Processing Institute are acknowledged for providing me a friendly atmosphere and encouraging me throughout this research. Finally my sincere gratitude is given to my husband, Torbjorn S. Bergstrom, my parents, Fengtong Ma and Hongzhen Sun, my brother, Qingquan Ma, and my sisters, Shuhua Ma and Shufang Ma, for their understanding, continuous encouragement, care, and infinite love.
This article provides an overview of surface contaminants that may affect the heat treatment proc... more This article provides an overview of surface contaminants that may affect the heat treatment processes and end-product quality. It presents information on the chemicals used to clean different surface contaminants of steels. The article discusses three types of cleaning methods, namely, mechanical, chemical, and electrochemical and their effectiveness and applicability. The mechanical cleaning methods include grinding, brushing, steam or flame jet cleaning, abrasive blasting, and tumbling. Solvent cleaning, emulsion cleaning, alkaline cleaning, acid cleaning, pickling, and descaling are chemical cleaning methods. The electrochemical cleaning methods include electropolishing, electrolytic alkaline cleaning, and electrolytic pickling. The article provides information on cleanliness measurement methods such as qualitative tests and quantitative tests to ensure product quality. Health hazards that may be associated with each cleaning method and the general control measures to be used for each hazard are tabulated.
Quenching heat treatment in a liquid medium is a very complex heat transfer process. Heat extract... more Quenching heat treatment in a liquid medium is a very complex heat transfer process. Heat extraction from the part surface occurs through several different heat transfer mechanisms in distinct temperature ranges, namely, film boiling, partial film boiling (i.e. transition), nucleate boiling and convection. The maximum heat transfer occurs during the nucleate boiling stage. Experimental study shows that, the effective surface heat transfer coefficient varies more than two orders of magnitude with the temperature during the quenching. For quenching process simulation, accurate prediction of the time-temperature history and microstructure evolution within the part largely depends on the accuracy of the boundary condition supplied. The heat transfer coefficient is the most important boundary condition for process simulation. This study focuses on creating a database of heat transfer coefficients for various liquid quenchant-metallic alloy combinations through experimentation using three different quench probes. This database is a web-based tool for use in quench process simulation. It provides at-a-glance information for quick and easy analysis and sets the stage for a Decision Support System (DSS) and Data Mining for heat-treating process.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Apr 1, 2011
ABSTRACT The effect of porosity on the kinetics of the austenite to ferrite isothermal transforma... more ABSTRACT The effect of porosity on the kinetics of the austenite to ferrite isothermal transformation in powder metallurgy steels was characterized using high-speed quench dilatometery. The measurements reveal that the presence of porosity in these steels reduces the stability of austenite and hence shortens the incubation time of the transformation. An Avrami-type equation was fitted to the measured data in order to quantify the effect of porosity on the Avrami constants. In addition, samples with varying levels of porosity were interruptedly quenched after holding them at 650 °C for 900 s. Quantitative microscopic measurements performed on these samples showed an increase in the number and a decrease in the average diameter of the ferrite grains with increasing porosity. It is hypothesized that pores in powder metallurgy steels increase the rate of nucleation of ferrite from austenite by providing high diffusivity paths for carbon atoms that help accelerate their partitioning during the transformation.
Journal of Materials Engineering and Performance, Feb 1, 1994
Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilize... more Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilized zirconia (PSZ, 8 wt. %Y203) and a diffusion aluminide bond coat, were characterized as a function of time after oxidative isothermal heat treatment at 1373 K in air. The experimental characterizations was conducted by X-ray diffraction analysis and scanning electron microscopy (SEM) with energy-dispersive spectroscopy. During cooling to room temperature, spallation of the PSZ ceramic coatings occurred after 200 and 350 h of isothermal heat treatment. This failure was always sudden and violent, with the TBC popping from the substrate. The monoclinic phase of zirconia was first observed on the bottom surface of the PVD PSZ after 200 h of isothermal heat treatment. The failure of TBCs occurred either in the bond coat oxidation products of (zAI203 and rutile TiO2 or at the interface between the oxidation products and the diffusion aluminide bond coat or the PSZ coating.
Thermal barrier coated high pressure turbine blades were characterized before and after the servi... more Thermal barrier coated high pressure turbine blades were characterized before and after the service by microstructural analysis and Cr3+ photostimulated luminescence piezo-spectroscopy. Thermal barrier coatings, in this study, consisted of electron beam physical vapor deposited yttria partially stabilized zirconia (YSZ; ZrO2–8 wt.% Y2O3), vapor-deposited aluminide bond coat and Ni-base superalloy. Compressive residual stress in thermally grown oxide, measured by Cr3+ photostimulated luminescence piezo-spectroscopy, was observed to be in the order of 2.5∼3.0 GPa and varied slightly as a function of substrate geometry. X-Ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy were utilized to investigate the microstructural development of thermal barrier coatings. The as-deposited non-equilibrium tetragonal (t′) phase in the YSZ coatings was observed to decompose after the service, but the monoclinic (m) phase was only found in the YSZ coatings with concave substrate curvature on the pressure side of the HPT blade. Also, a significant sintering of ZrO2–8 wt.% Y2O3 coating after the service was observed in the microstructure. Localized spallation of YSZ occurred within the thermally grown oxide (mostly α-Al2O3) and within the ZrO2–8 wt.% Y2O3 coating for pressure and suction sides of the serviced high pressure turbine blade near the tip, respectively.
Initiated by the need of industry for gas carburizing process control and optimization, this pape... more Initiated by the need of industry for gas carburizing process control and optimization, this paper focuses on understanding the effect of the time, temperature and carbon potential on the mass transfer coefficient and carbon diffusivity in Austenite. A method for direct flux integration has previously been proposed to calculate these kinetic parameters from the experimental carbon concentration profiles. AISI 8620 steel discs were gas carburized at different levels of atmosphere carburizing potential for selected austenizing temperatures. Analyses of the carburized parts included experimental measurement of weight gain, surface carbon concentration and carbon concentration profiles. The time-dependent weight gain and surface carbon content measurements allowed calculation of the time average mass transfer coefficient, while carbon concentration profiles were used to calculate the concentration dependent carbon diffusivity for selected process parameters. Excellent agreement was found between the calculated mass transfer coefficient and carbon diffusivity values and those reported in the literature. The calculated values served as input in the previously developed carburizing model validating the predicted results by comparison with the experimental concentration profiles.
International Heat Treatment & Surface Engineering, Sep 1, 2011
The Lehrer diagram of pure iron is widely used to select the nitriding process of alloy steels. I... more The Lehrer diagram of pure iron is widely used to select the nitriding process of alloy steels. In the present work, the Lehrer diagram of AISI 4140 was determined for the first time by computational thermodynamics. It is demonstrated that the Lehrer diagram of AISI 4140 differs from that of pure iron significantly. Nitriding experiments for AISI 4140 were also carried out to verify the predictions from computational thermodynamics. The scanning electron microscopy results show the existence of two phases in the compound layer and the transmission electron microscopy results at the interface between the compound layer and the diffusion zone verified the coexistence of γ′-Fe4N and ϵ-Fe2–3(C,N) phases. These experimental results agree well to the customised Lehrer diagram of AISI 4140 constructed from computational thermodynamics.
The effect of porosity on the kinetics of the austenite to bainite isothermal transformation in p... more The effect of porosity on the kinetics of the austenite to bainite isothermal transformation in powder metallurgy steels was characterized using a high-speed quenching dilatometer. The measurements revealed that the presence of porosity shortens the incubation time as well as the overall isothermal transformation time. An Avrami-type equation was fitted to the measured data, and the effect of porosity on the nucleation rate of bainite was quantified. In addition, the activation energy for diffusion of carbon atoms during nucleation of bainite was calculated and was found to decrease with increasing porosity.
In mining graphical data the default Euclidean distance is often used as a notion of similarity. ... more In mining graphical data the default Euclidean distance is often used as a notion of similarity. However this does not adequately capture semantics in our targeted domains, having graphical representations depicting results of scientific experiments. It is seldom known a-priori what other distance metric best preserves semantics. This motivates the need to learn such a metric. A technique called LearnMet is proposed here to learn a domainspecific distance metric for graphical representations. Input to LearnMet is a training set of correct clusters of such graphs. LearnMet iteratively compares these correct clusters with those obtained from an arbitrary but fixed clustering algorithm. In the first iteration a guessed metric is used for clustering. This metric is then refined using the error between the obtained and correct clusters until the error is below a given threshold. LearnMet is evaluated rigorously in the Heat Treating domain which motivated this research. Clusters obtained using the learned metric and clusters obtained using Euclidean distance are both compared against the correct clusters over a separate test set. Our results show that the learned metric provides better clusters.
Journal of Phase Equilibria and Diffusion, Dec 1, 2006
In the early 1970s, Professor Dayananda developed a technique for the direct integration of fluxe... more In the early 1970s, Professor Dayananda developed a technique for the direct integration of fluxes from the concentration profiles in vapor-solid diffusion couples to determine diffusion coefficients and atomic mobilities. As part of a project to control and optimize the industrial carburization process in mild-and low-alloyed steels, a modified integration analysis was applied to determine the mass transfer coefficient in the gas boundary layer and carbon diffusivity in austenite. Because carbon flux and surface carbon content vary with time during single-stage carburizing even with a fixed carbon potential in the atmosphere, a mass balance at the gas-solid interface must serve as a boundary condition. This article discusses the numerical modeling of gas carburizing, and focuses on calculating the mass transfer and carbon diffusivity parameters using the simulated concentration profiles. This approach validates the proposed method by comparing the calculated parameters with those used in simulation. The results were compared with previous determinations and predictions reported in the literature.
A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quench... more A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quenching system in distilled water while varying bath temperature and the agitation level. Time versus temperature data were collected during the quench using a thermocouple embedded inside the probe. The surface heat transfer coefficients as a function of temperature were calculated using the Newtonian cooling approximation. The maximum heat transfer coefficient values ranged from 1054 W/m2⋅K for 100°C water with no agitation to 3822 W/m2⋅K for 5°C water with the agitation of 1850 r/min. The data also showed that at higher levels of agitation and lower bath temperatures, the maximum heat transfer coefficient increased.
Various data mining techniques developed by CHTE to enhance the position of the heat treating ind... more Various data mining techniques developed by CHTE to enhance the position of the heat treating industry, by applying fundamental research, to solve problems and to advance heat treatment technology are discussed. A technique, called QuenchMiner™ system, provides decision support in heat treating to optimize the processes involved in heat treatment. CHTE has proposed a technique called AutoDomainMine, which is based on data mining guided by the basic knowledge of the domain. AutoDomainMine helps in estimating a set of input conditions that would produce the desired graph for a heat transfer curve in a quenching experiment. The AutoDomainMine estimation can be used to select process parameters for quenching in the industry. AutoDomainMine is also useful for intelligent tutoring systems in heat treating
The demand for high performance lithium-ion batteries (LIBs) is increasing due to widespread use ... more The demand for high performance lithium-ion batteries (LIBs) is increasing due to widespread use of portable devices and electric vehicles. Silicon (Si) is one of the most attractive candidate anode materials for next generation LIBs. However, the high-volume change (>300%) during lithium ion alloying/de-alloying leads to poor cycle life. When Si is used as the anode, conductive carbon is needed to provide the necessary conductivity. However, the traditional carbon coating method could not overcome the challenges of pulverization and unstable Solid Electrolyte Interphase (SEI) layer during long-term cycling. Since 2010, Si/Graphene composites have been vigorously studied in hopes of providing a material with better cycling performance. This paper reviews current progress of Si/Graphene nanocomposites in LIBs. Different fabrication methods have been studied to synthesize Si/Graphene nanocomposites with promising electrochemical performances. Graphene plays a key enabling role in Si/Graphene anodes. However, the desired properties of graphene for this application have not been systematically studied and understood. Further systematic investigation of the desired graphene properties is suggested to better control the Si/Graphene anode performance.
Each year millions of dollars are lost as a result of distortion, cracking and mechanical propert... more Each year millions of dollars are lost as a result of distortion, cracking and mechanical property variations due to unexpected problems in the quenching process. A thorough understanding of the variations in quenching fluid's performance as a function of the medium's physical properties, system variables and part geometry is not currently available.
The concept of the new category materials high entropy ceramics (HECs) has been proposed several ... more The concept of the new category materials high entropy ceramics (HECs) has been proposed several years ago, which is directly borrowed from high entropy alloys (HEAs). It quickly attracts a lot of interests and displays promising properties. However, there is no clear definition of HECs differentiating it from HEAs, as it is still in its early research stage. In the current work, we are trying to use the classic perovskite LaMnO 3±δ (LMO) to demonstrate the fundamental differences between HECs and HEAs. We have adopted the integrated defect chemistry and CALPHAD approach to investigate the mixing behavior and how it is affected by the control parameters, i.e. PO 2 , T, and composition. We have developed a new way to visualize the mixing behavior of the species including the cations, anions, and defects (vacancies), which linked the mixing behavior to the thermo-chemical properties including enthalpy, entropy, and Gibbs energy. It was found that entropy plays the most important role on the mixing behavior in LMO. The present work paves the way for the HECs investigation and the design of new HECs for the various applications.
A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quench... more A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quenching system in distilled water while varying bath temperature and the agitation level. Time versus temperature data were collected during the quench using a thermocouple embedded inside the probe. The surface heat transfer coefficients as a function of temperature were calculated using the Newtonian cooling approximation. The maximum heat transfer coefficient values ranged from 1054 W/m2⋅K for 100°C water with no agitation to 3822 W/m2⋅K for 5°C water with the agitation of 1850 r/min. The data also showed that at higher levels of agitation and lower bath temperatures, the maximum heat transfer coefficient increased.
Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science, Aug 1, 2007
for providing the interesting discussions, for their creative ideas, and for helping me in conduc... more for providing the interesting discussions, for their creative ideas, and for helping me in conducting some of the experiments at the WPI advanced casting v laboratory. My sincere thanks go to Rita Shilansky for her patience, consideration, and assistance during all the years when I was at WPI. All the faculties, my friends, and my colleagues at the Materials Science & Engineering program and the Metal Processing Institute are acknowledged for providing me a friendly atmosphere and encouraging me throughout this research. Finally my sincere gratitude is given to my husband, Torbjorn S. Bergstrom, my parents, Fengtong Ma and Hongzhen Sun, my brother, Qingquan Ma, and my sisters, Shuhua Ma and Shufang Ma, for their understanding, continuous encouragement, care, and infinite love.
This article provides an overview of surface contaminants that may affect the heat treatment proc... more This article provides an overview of surface contaminants that may affect the heat treatment processes and end-product quality. It presents information on the chemicals used to clean different surface contaminants of steels. The article discusses three types of cleaning methods, namely, mechanical, chemical, and electrochemical and their effectiveness and applicability. The mechanical cleaning methods include grinding, brushing, steam or flame jet cleaning, abrasive blasting, and tumbling. Solvent cleaning, emulsion cleaning, alkaline cleaning, acid cleaning, pickling, and descaling are chemical cleaning methods. The electrochemical cleaning methods include electropolishing, electrolytic alkaline cleaning, and electrolytic pickling. The article provides information on cleanliness measurement methods such as qualitative tests and quantitative tests to ensure product quality. Health hazards that may be associated with each cleaning method and the general control measures to be used for each hazard are tabulated.
Quenching heat treatment in a liquid medium is a very complex heat transfer process. Heat extract... more Quenching heat treatment in a liquid medium is a very complex heat transfer process. Heat extraction from the part surface occurs through several different heat transfer mechanisms in distinct temperature ranges, namely, film boiling, partial film boiling (i.e. transition), nucleate boiling and convection. The maximum heat transfer occurs during the nucleate boiling stage. Experimental study shows that, the effective surface heat transfer coefficient varies more than two orders of magnitude with the temperature during the quenching. For quenching process simulation, accurate prediction of the time-temperature history and microstructure evolution within the part largely depends on the accuracy of the boundary condition supplied. The heat transfer coefficient is the most important boundary condition for process simulation. This study focuses on creating a database of heat transfer coefficients for various liquid quenchant-metallic alloy combinations through experimentation using three different quench probes. This database is a web-based tool for use in quench process simulation. It provides at-a-glance information for quick and easy analysis and sets the stage for a Decision Support System (DSS) and Data Mining for heat-treating process.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, Apr 1, 2011
ABSTRACT The effect of porosity on the kinetics of the austenite to ferrite isothermal transforma... more ABSTRACT The effect of porosity on the kinetics of the austenite to ferrite isothermal transformation in powder metallurgy steels was characterized using high-speed quench dilatometery. The measurements reveal that the presence of porosity in these steels reduces the stability of austenite and hence shortens the incubation time of the transformation. An Avrami-type equation was fitted to the measured data in order to quantify the effect of porosity on the Avrami constants. In addition, samples with varying levels of porosity were interruptedly quenched after holding them at 650 °C for 900 s. Quantitative microscopic measurements performed on these samples showed an increase in the number and a decrease in the average diameter of the ferrite grains with increasing porosity. It is hypothesized that pores in powder metallurgy steels increase the rate of nucleation of ferrite from austenite by providing high diffusivity paths for carbon atoms that help accelerate their partitioning during the transformation.
Journal of Materials Engineering and Performance, Feb 1, 1994
Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilize... more Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilized zirconia (PSZ, 8 wt. %Y203) and a diffusion aluminide bond coat, were characterized as a function of time after oxidative isothermal heat treatment at 1373 K in air. The experimental characterizations was conducted by X-ray diffraction analysis and scanning electron microscopy (SEM) with energy-dispersive spectroscopy. During cooling to room temperature, spallation of the PSZ ceramic coatings occurred after 200 and 350 h of isothermal heat treatment. This failure was always sudden and violent, with the TBC popping from the substrate. The monoclinic phase of zirconia was first observed on the bottom surface of the PVD PSZ after 200 h of isothermal heat treatment. The failure of TBCs occurred either in the bond coat oxidation products of (zAI203 and rutile TiO2 or at the interface between the oxidation products and the diffusion aluminide bond coat or the PSZ coating.
Thermal barrier coated high pressure turbine blades were characterized before and after the servi... more Thermal barrier coated high pressure turbine blades were characterized before and after the service by microstructural analysis and Cr3+ photostimulated luminescence piezo-spectroscopy. Thermal barrier coatings, in this study, consisted of electron beam physical vapor deposited yttria partially stabilized zirconia (YSZ; ZrO2–8 wt.% Y2O3), vapor-deposited aluminide bond coat and Ni-base superalloy. Compressive residual stress in thermally grown oxide, measured by Cr3+ photostimulated luminescence piezo-spectroscopy, was observed to be in the order of 2.5∼3.0 GPa and varied slightly as a function of substrate geometry. X-Ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy were utilized to investigate the microstructural development of thermal barrier coatings. The as-deposited non-equilibrium tetragonal (t′) phase in the YSZ coatings was observed to decompose after the service, but the monoclinic (m) phase was only found in the YSZ coatings with concave substrate curvature on the pressure side of the HPT blade. Also, a significant sintering of ZrO2–8 wt.% Y2O3 coating after the service was observed in the microstructure. Localized spallation of YSZ occurred within the thermally grown oxide (mostly α-Al2O3) and within the ZrO2–8 wt.% Y2O3 coating for pressure and suction sides of the serviced high pressure turbine blade near the tip, respectively.
Initiated by the need of industry for gas carburizing process control and optimization, this pape... more Initiated by the need of industry for gas carburizing process control and optimization, this paper focuses on understanding the effect of the time, temperature and carbon potential on the mass transfer coefficient and carbon diffusivity in Austenite. A method for direct flux integration has previously been proposed to calculate these kinetic parameters from the experimental carbon concentration profiles. AISI 8620 steel discs were gas carburized at different levels of atmosphere carburizing potential for selected austenizing temperatures. Analyses of the carburized parts included experimental measurement of weight gain, surface carbon concentration and carbon concentration profiles. The time-dependent weight gain and surface carbon content measurements allowed calculation of the time average mass transfer coefficient, while carbon concentration profiles were used to calculate the concentration dependent carbon diffusivity for selected process parameters. Excellent agreement was found between the calculated mass transfer coefficient and carbon diffusivity values and those reported in the literature. The calculated values served as input in the previously developed carburizing model validating the predicted results by comparison with the experimental concentration profiles.
International Heat Treatment & Surface Engineering, Sep 1, 2011
The Lehrer diagram of pure iron is widely used to select the nitriding process of alloy steels. I... more The Lehrer diagram of pure iron is widely used to select the nitriding process of alloy steels. In the present work, the Lehrer diagram of AISI 4140 was determined for the first time by computational thermodynamics. It is demonstrated that the Lehrer diagram of AISI 4140 differs from that of pure iron significantly. Nitriding experiments for AISI 4140 were also carried out to verify the predictions from computational thermodynamics. The scanning electron microscopy results show the existence of two phases in the compound layer and the transmission electron microscopy results at the interface between the compound layer and the diffusion zone verified the coexistence of γ′-Fe4N and ϵ-Fe2–3(C,N) phases. These experimental results agree well to the customised Lehrer diagram of AISI 4140 constructed from computational thermodynamics.
The effect of porosity on the kinetics of the austenite to bainite isothermal transformation in p... more The effect of porosity on the kinetics of the austenite to bainite isothermal transformation in powder metallurgy steels was characterized using a high-speed quenching dilatometer. The measurements revealed that the presence of porosity shortens the incubation time as well as the overall isothermal transformation time. An Avrami-type equation was fitted to the measured data, and the effect of porosity on the nucleation rate of bainite was quantified. In addition, the activation energy for diffusion of carbon atoms during nucleation of bainite was calculated and was found to decrease with increasing porosity.
In mining graphical data the default Euclidean distance is often used as a notion of similarity. ... more In mining graphical data the default Euclidean distance is often used as a notion of similarity. However this does not adequately capture semantics in our targeted domains, having graphical representations depicting results of scientific experiments. It is seldom known a-priori what other distance metric best preserves semantics. This motivates the need to learn such a metric. A technique called LearnMet is proposed here to learn a domainspecific distance metric for graphical representations. Input to LearnMet is a training set of correct clusters of such graphs. LearnMet iteratively compares these correct clusters with those obtained from an arbitrary but fixed clustering algorithm. In the first iteration a guessed metric is used for clustering. This metric is then refined using the error between the obtained and correct clusters until the error is below a given threshold. LearnMet is evaluated rigorously in the Heat Treating domain which motivated this research. Clusters obtained using the learned metric and clusters obtained using Euclidean distance are both compared against the correct clusters over a separate test set. Our results show that the learned metric provides better clusters.
Journal of Phase Equilibria and Diffusion, Dec 1, 2006
In the early 1970s, Professor Dayananda developed a technique for the direct integration of fluxe... more In the early 1970s, Professor Dayananda developed a technique for the direct integration of fluxes from the concentration profiles in vapor-solid diffusion couples to determine diffusion coefficients and atomic mobilities. As part of a project to control and optimize the industrial carburization process in mild-and low-alloyed steels, a modified integration analysis was applied to determine the mass transfer coefficient in the gas boundary layer and carbon diffusivity in austenite. Because carbon flux and surface carbon content vary with time during single-stage carburizing even with a fixed carbon potential in the atmosphere, a mass balance at the gas-solid interface must serve as a boundary condition. This article discusses the numerical modeling of gas carburizing, and focuses on calculating the mass transfer and carbon diffusivity parameters using the simulated concentration profiles. This approach validates the proposed method by comparing the calculated parameters with those used in simulation. The results were compared with previous determinations and predictions reported in the literature.
A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quench... more A 6061 aluminum alloy probe was quenched using a Center for Heat Treating Excellence probe-quenching system in distilled water while varying bath temperature and the agitation level. Time versus temperature data were collected during the quench using a thermocouple embedded inside the probe. The surface heat transfer coefficients as a function of temperature were calculated using the Newtonian cooling approximation. The maximum heat transfer coefficient values ranged from 1054 W/m2⋅K for 100°C water with no agitation to 3822 W/m2⋅K for 5°C water with the agitation of 1850 r/min. The data also showed that at higher levels of agitation and lower bath temperatures, the maximum heat transfer coefficient increased.
Various data mining techniques developed by CHTE to enhance the position of the heat treating ind... more Various data mining techniques developed by CHTE to enhance the position of the heat treating industry, by applying fundamental research, to solve problems and to advance heat treatment technology are discussed. A technique, called QuenchMiner™ system, provides decision support in heat treating to optimize the processes involved in heat treatment. CHTE has proposed a technique called AutoDomainMine, which is based on data mining guided by the basic knowledge of the domain. AutoDomainMine helps in estimating a set of input conditions that would produce the desired graph for a heat transfer curve in a quenching experiment. The AutoDomainMine estimation can be used to select process parameters for quenching in the industry. AutoDomainMine is also useful for intelligent tutoring systems in heat treating
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Papers by Richard Sisson