Hastinapur is one of the prominent painted grey ware archaeological sites in the Ganga Valley. In... more Hastinapur is one of the prominent painted grey ware archaeological sites in the Ganga Valley. In the present communication, we report the results of the metallurgical analysis of the excavated slag samples from the site. The existence of a large number of iron objects for this site can be dated between c. 6th century BCE and c. 16th century CE, with early iron from the 8th century BCE. The morphological, microstructural and elemental studies of the excavated slag indicate a high amount of carbon in iron, indicating crucible carburization activities at this site. In addition, the presence of niobium, cobalt, etc. in the slag indicates the usage of non-traditional iron ores in the primary ironmaking stage.
The famed Wootz steels are hypothesized to contain carbon nanotubes. However, the primary context... more The famed Wootz steels are hypothesized to contain carbon nanotubes. However, the primary context of such nanostructures in Wootz steels is not known. In this letter, we present the microarchitectured and nanoscale hierarchical carbon structures in ancient archaeological carburization slags from 400 BCE to 16th century CE. We show that carbon microrods at the micro-scale are composed of finer carbon microrods, which are arranged in a co-axial manner. Carbon microrods are composed of twisted carbon nanotubes at the nanoscale. Such carbon nanostructure formation in the crucible process provides the possible source of carbon nanostructures in crucible steels of antiquity.
Multicomponent materials containing a comparatively large number of different elemental component... more Multicomponent materials containing a comparatively large number of different elemental components, yet exhibiting simple crystal structures have opened up a new era of materials design with the possibility of tuning properties of materials with greater degrees of freedom. This poses a formidable challenge in terms of design as the number of parameters involved in simulating such systems increase significantly with the increasing number of components. This work reports a sampling methodology based on hybrid genetic algorithm-molecular dynamics for sampling positional-disordered materials such as high-entropy systems. This investigation also demonstrates the influence of individual cationic species on the evolution of distortion in single-phase solid solution with the rock-salt structure, when oxides such as CoO, CuO, MgO, NiO and ZnO are mixed together. Additionally, the relationship between the number of atomic species and its effect on the lattice distortion has been presented. Th...
The deviation of positions of atoms from their ideal lattice sites in crystalline solid state sys... more The deviation of positions of atoms from their ideal lattice sites in crystalline solid state systems causes distortion and can lead to variation in structural [1] and functional properties [2]. Distortion in molecular systems has been traditionally understood in the term of Jahn-Teller distortion [3], while for the one-dimensional chain of metals, the Peierls distortion mechanism has been proposed [4]. In real three dimensional metallic systems, a fundamental description of the distortion is missing, which we need to design alloys with attractive structural properties. The present investigation presents the evolution of distortion in metallic systems in terms of magnetovolume effects arising due to magnetic ground-state of the system. Particularly the significant distortion in the case of Cr due to presence of other transition metals is seen. Cr with transition metals as a candidate system to study the correlation between charge disproportion, spin-flip, magnetovolume effects and Fermi surface nesting. This study provides a novel explanation of strengthening effect of Cr observed in alloys due to its unique magnetic properties. Distortion in metallic systems is a topic of debate in the context of multi-component concentrated alloys or so called "high-entropy alloys (HEA)" [5, 6, 7, 8, 9, 10]. Such distortions are known to influence the structural properties of these alloys [11, 12]. Local lattice distortion is instrumental 1
The deviation of positions of atoms from their ideal lattice sites in crystalline solid state sys... more The deviation of positions of atoms from their ideal lattice sites in crystalline solid state systems causes distortion and it causes variation in structural [1] and functional properties [2]. Severe lattice distortion has been proposed to be one of the core-effect in high-entropy alloys. But the fundamental mechanism of distortion at atomic scale is missing for real three-dimensional metallic systems. The present investigation aims to develop mechanistic understanding of atomic scale distortion in metallic systems in terms of the magneto-volume effects. The correlation between charge-disproportion, spin fluctuations, magnetovolume effects and Fermi surface nesting has been highlighted.
During the last couple of decades, treatment of microstructure in materials science has been shif... more During the last couple of decades, treatment of microstructure in materials science has been shifted from the diagnostic to design paradigm. Design of microstructure is inherently complex problems due to non linear spatial and temporal interaction of composition and parameters leading to the target properties. In most of the cases, different properties are reciprocally correlated i.e., improvement of one lead to the degradation of other. Also, the design of microstructure is a multiscale problem, as the knowledge of phenomena at range of scales from electronic to mesoscale is required for precise compositionmicrostructure-property determination. In the view of above, present chapter provides the introduction to computationally driven microstructure engineering in the framework of constitutive length scale in microstructure design. The important issues pertaining to design such as phase stability and interfaces has been explained. Additionally, the bird-eye view of various computational techniques in order of length scale has been introduced, with an aim to present the picture of combination of various techniques for solving microstructural design problems under various scenarios.
This study examines the comparative phase stability of transition and late transition element bas... more This study examines the comparative phase stability of transition and late transition element based high entropy alloys in body-centred cubic (BCC) or face-centred cubic (FCC) forms using a combined classical molecular dynamics and statistical mechanics based approach. Multi-configurational sampling was carried out using a hybrid genetic algorithm-molecular dynamics (GA-MD) based method. The calculations demonstrate that comparative BCC or FCC phase stability is influenced by configurational entropy. The present study also provides a theoretical explanation of the recently reported occurrence of BCC phase in CoCrFeNi HEA, where the high temperature structure may be retained.
ABSTRACT The present study addresses the evolution of texture and microstructure during annealing... more ABSTRACT The present study addresses the evolution of texture and microstructure during annealing in a cryo-rolled copper. Transition from copper to brass texture during the cryo-rolling has been illustrated. Twinning and interaction between twins and shear bands have been found to play the important role in grain refinement and strengthening. The low temperature vacancy clustering and its effect on the recrystallization have been experimentally demonstrated. Fine scale twinning, and grain refinement have been attributed to the higher yield strength found in the case of samples subjected to cryo-rolling.
Steel design is traditionally governed by the imprecise knowledge acquired from the experimental ... more Steel design is traditionally governed by the imprecise knowledge acquired from the experimental efforts occasionally invoking the empirical, physical and thermodynamic models supported by numerous assumptions and boundary conditions. The available frameworks are independently inadequate for complete and deterministic composition-process-microstructure-property (CPMP) correlation. The situation thus calls for judicious integration of the potential techniques with the capability of a priori prediction/optimization of the complete system. In view of the above the present article aims to provide a bird’s eye view of the deterministic approach invoked so far in the traditional framework of design and development of formable steels. An attempt has also been made to propose the scheme for hybridization of the potential techniques for describing the complete CPMP correlation in the production schedule of hot strip coil.
Journal of The Institution of Engineers (India): Series D, 2012
The deformation behavior of interstitial free/ferritic steel has been investigated at cryogenic a... more The deformation behavior of interstitial free/ferritic steel has been investigated at cryogenic and room temperature. The study concerns the occurrence of microstructural damages during low temperature deformation conditions for such high stacking fault energy (SFE) body centered cubic material and their role on the evolution of microstructure and mechanical properties.
In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti... more In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800 °C to 950 °C) in steps of 298 K (25 °C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise a-Fe + v, v, FeTi + k, FeTi + b-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni 3 Ti, NiTi, and NiTi 2 are formed at the nickel alloytitanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875 °C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe 5 Cr 35 Ni 40 Ti 15 have been observed within the Ni 3 Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~477 MPa and shear strength of ~356.9 MPa along with ~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900 °C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875 °C), the bond tensile and shear strengths increase to ~523.6 and ~389.6 MPa, respectively, along with 6.2 pct elongation.
In the traditional physical metallurgy approach, the microstructure evolves as the product of the... more In the traditional physical metallurgy approach, the microstructure evolves as the product of the interactions between composition and process parameters, controlled by thermodynamic and kinetic conditions. Thus, the approach is more evolutionary than constructive. An architecturally designed microstructure implies planning, design and construction of microstructure considering nature, size, morphology and distribution of the constituent phases on a suitably conceived topological framework. This investigation puts emphasis on performance driven construction of the microstructure with precise calculation of response of microstructural constituents under the given loading conditions. With the aforesaid motivation, the present study elucidates the principle involving microstructural generation by Voronoi tessellation and subsequent elastoplastic finite element simulation. In addition, the present study also explores the correlation between architecturing and initiation of instability in the microstructure through genetic programming.
This study examines the comparative phase stability of transition and late transition element bas... more This study examines the comparative phase stability of transition and late transition element based high entropy alloys in body-centred cubic (BCC) or face-centred cubic (FCC) forms using a combined classical molecular dynamics and statistical mechanics based approach. Multi-configurational sampling was carried out using a hybrid genetic algorithm-molecular dynamics (GA-MD) based method. The calculations demonstrate that comparative BCC or FCC phase stability is influenced by configurational entropy. The present study also provides a theoretical explanation of the recently reported occurrence of BCC phase in CoCrFeNi HEA, where the high temperature structure may be retained. High entropy alloys (HEAs) are a class of multicomponent metallic alloys [1], which exhibit simple crystal structures such as body-centred cubic (BCC), face-centred cubic (FCC), hexagonal cubic phase (HCP) or orthorhombic [2–4] crystal structures. The stabilisation of a solid solution phase over intermetallic and amorphous alloy formation in such multicomponent systems was initially attributed to the high configura-tional entropy of mixing [5]. The prediction of the comparative phase stability of BCC and FCC phases for HEAs remains a major debate in the literature. For example, CoCrFeNi shows the FCC structure [6], but addition of Al causes its transformation to the BCC crystal structure [6]. The phase transformation and existence of HEAs in two main types of crystal structures was initially rationalised in terms of the conventional metallurgical concept of FCC and BCC stabilising elements [7]. Traditionally, phase stability in alloys has been understood in terms of the number of electrons per atom (e/a) [8], which has been extensively used for Hume-Rothery electron phase stabilisation. In the case of HEAs, which are mostly composed of transition metals, the accurate determination of the e/a ratio is problematic. Hence, the valence electron concentration (VEC) has been employed to rationalise FCC and BCC formation, where higher VEC (≥ 8.0) leads to FCC phases, while lower VEC (≤ 6.87) causes BCC formation, leaving a mixture of BCC and FCC phase for 6.87 ≤ VEC ≤ 8.0 [9]. This criterion, however, does not apply to HEAs containing Mn [10], and in certain cases the effect of cooling conditions dominates the phase formation, i.e., alloys with the same composition but subjected to different processing conditions produce different phase occurrence characteristics [11]. Therefore phase selection remains an open debate. Computationally expensive ab-initio approaches, such as coherent potential approximation [12] and special quasi-random structures with density functional theory (DFT) [13] have been used to study HEAs. Given the potential significance of (con-figurational) entropy in the phase stability in HEAs, there is a need for an extensive sampling of configurations, which cannot be achieved easily with ab-initio methods that are limited to small cells. The present study attempts to address this issue by a using atomistic simulation with a genetic algorithm (GA) as the sampling strategy followed by a statistical mechanics technique for the calculation of the configurational entropy. The DL_POLY code was employed to carry out Molecular Dynamics (MD) simulations [14]. Position, velocity and acceleration of the atoms were updated via the velocity-verlet integration algorithm [15]. The GULP code [16] was used to generate 7 × 7 × 7 (FCC) and 9 × 9 × 9 (BCC) cubic supercells for starting configurations. The disordered structures of the alloys were generated by randomising the elements on the lattice sites. The simulations were performed with a 1 fs time-step. The long-range interaction cutoff was set to be 6 Å. The MD simulations were carried out for 10 ps, where the system was equilibrated for first 5000 time steps. The configurational energy was shown to converge over this time (see Fig. A1 in supplementary information). A Nóse-Hoo-ver NPT ensemble with thermostat and barostat relaxation times of 0.01 ps and 0.1 ps, respectively, was used to keep temperature (300 K) and pressure (1 atm) constant. The initial lattice parameters for the BCC and the FCC variant of AlCoCrFeNi, CoCrFeNi and CoCrFeNiTi were taken from first-principle calculations, while for Al 0·5 CoCrFeNi, the equilibrium lattice parameter was calculated from finding the minimum of the energy-volume curve in NVT calculation. The initial lattice Scripta Materialia 124 (2016) 90–94
Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near... more Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near-cryogenic (−150°C to −100°C) rolling followed by annealing have been investigated. Twining has been found to be the dominant mode of deformation during rolling. Both the rolled and annealed sample exhibited limited work hardening due to high matrix resistance. In the rolled sample cleavage, nuclei acted as potential fracture initiation site. Annealing has resulted in effective blunting of cleavage nuclei giving rise to large total elongation due to prolonged damage growth process.
The present study addresses the evolution of texture and microstructure during annealing in a cry... more The present study addresses the evolution of texture and microstructure during annealing in a cryo- rolled copper. Transition from copper to brass texture during the cryo-rolling has been illustrated. Twinning and interaction between twins and shear bands have been found to play the important role in grain refinement and strengthening. The low temperature vacancy clustering and its effect on the recrystallization have been experimentally demonstrated. Fine scale twinning, and grain refinement have been attributed to the higher yield strength found in the case of samples subjected to cryo-rolling.
In the traditional physical metallurgy approach, the microstructure evolves as the product of the... more In the traditional physical metallurgy approach, the microstructure evolves as the product of the interactions between composition and process parameters, controlled by thermodynamic and kinetic conditions. Thus, the approach is more evolutionary than constructive. An architecturally designed microstructure implies planning, design and construction of microstructure considering nature, size, morphology and distribution of the constituent phases on a suitably conceived topological framework. This investigation puts emphasis on performance driven construction of the microstructure with precise calculation of response of microstructural constituents under the given loading conditions. With the aforesaid motivation, the present study elucidates the principle involving microstructural generation by Voronoi tessellation and subsequent elastoplastic finite element simulation. In addition, the present study also explores the correlation between architecturing and initiation of instability in the microstructure through genetic programming.
The deformation behaviour of interstitial free/ ferritic steel has been investigated at cryogenic... more The deformation behaviour of interstitial free/ ferritic steel has been investigated at cryogenic and room temperature. The study concerns the occurrence of microstructural damages during low temperature deformation conditions for such high stacking fault energy body centred cubic material and their role on the evolution of microstructure and mechanical properties.
In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti... more In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800°C to 950°C) in steps of 298 K (25°C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise a-Fe + v, v, FeTi + k, FeTi + b-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni 3 Ti, NiTi, and NiTi 2 are formed at the nickel alloytitanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875°C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe 5 Cr 35 Ni 40 Ti 15 have been observed within the Ni 3 Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of~477 MPa and shear strength of~356.9 MPa along with~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900°C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875°C), the bond tensile and shear strengths increase to~523.6 and~389.6 MPa, respectively, along with 6.2 pct elongation.
Steel design is tradit ionally governed by the imprecise knowledge acquired fro m the experimenta... more Steel design is tradit ionally governed by the imprecise knowledge acquired fro m the experimental efforts occasionally invoking the empirical, physical and thermodynamic models supported by numerous assumptions and boundary conditions. The available frameworks are independently inadequate for co mplete and deterministic composition-process-microstructure-property (CPMP) correlation. The situation thus calls for judicious integration of the potential techniques with the capability of a priori prediction/optimization of the comp lete system. In v iew of the above the present article aims to provide a bird's eye view of the determin istic approach invoked so far in the tradit ional framework of design and development of formable steels. An attempt has also been made to propose the scheme for hybridization of the potential techniques for describing the complete CPM P correlation in the production schedule of hot strip coil.
Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near... more Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near-cryogenic (−150°C to −100°C) rolling followed by annealing have been investigated. Twining has been found to be the dominant mode of deformation during rolling. Both the rolled and annealed sample exhibited limited work hardening due to high matrix resistance. In the rolled sample cleavage, nuclei acted as potential fracture initiation site. Annealing has resulted in effective blunting of cleavage nuclei giving rise to large total elongation due to prolonged damage growth process.
Hastinapur is one of the prominent painted grey ware archaeological sites in the Ganga Valley. In... more Hastinapur is one of the prominent painted grey ware archaeological sites in the Ganga Valley. In the present communication, we report the results of the metallurgical analysis of the excavated slag samples from the site. The existence of a large number of iron objects for this site can be dated between c. 6th century BCE and c. 16th century CE, with early iron from the 8th century BCE. The morphological, microstructural and elemental studies of the excavated slag indicate a high amount of carbon in iron, indicating crucible carburization activities at this site. In addition, the presence of niobium, cobalt, etc. in the slag indicates the usage of non-traditional iron ores in the primary ironmaking stage.
The famed Wootz steels are hypothesized to contain carbon nanotubes. However, the primary context... more The famed Wootz steels are hypothesized to contain carbon nanotubes. However, the primary context of such nanostructures in Wootz steels is not known. In this letter, we present the microarchitectured and nanoscale hierarchical carbon structures in ancient archaeological carburization slags from 400 BCE to 16th century CE. We show that carbon microrods at the micro-scale are composed of finer carbon microrods, which are arranged in a co-axial manner. Carbon microrods are composed of twisted carbon nanotubes at the nanoscale. Such carbon nanostructure formation in the crucible process provides the possible source of carbon nanostructures in crucible steels of antiquity.
Multicomponent materials containing a comparatively large number of different elemental component... more Multicomponent materials containing a comparatively large number of different elemental components, yet exhibiting simple crystal structures have opened up a new era of materials design with the possibility of tuning properties of materials with greater degrees of freedom. This poses a formidable challenge in terms of design as the number of parameters involved in simulating such systems increase significantly with the increasing number of components. This work reports a sampling methodology based on hybrid genetic algorithm-molecular dynamics for sampling positional-disordered materials such as high-entropy systems. This investigation also demonstrates the influence of individual cationic species on the evolution of distortion in single-phase solid solution with the rock-salt structure, when oxides such as CoO, CuO, MgO, NiO and ZnO are mixed together. Additionally, the relationship between the number of atomic species and its effect on the lattice distortion has been presented. Th...
The deviation of positions of atoms from their ideal lattice sites in crystalline solid state sys... more The deviation of positions of atoms from their ideal lattice sites in crystalline solid state systems causes distortion and can lead to variation in structural [1] and functional properties [2]. Distortion in molecular systems has been traditionally understood in the term of Jahn-Teller distortion [3], while for the one-dimensional chain of metals, the Peierls distortion mechanism has been proposed [4]. In real three dimensional metallic systems, a fundamental description of the distortion is missing, which we need to design alloys with attractive structural properties. The present investigation presents the evolution of distortion in metallic systems in terms of magnetovolume effects arising due to magnetic ground-state of the system. Particularly the significant distortion in the case of Cr due to presence of other transition metals is seen. Cr with transition metals as a candidate system to study the correlation between charge disproportion, spin-flip, magnetovolume effects and Fermi surface nesting. This study provides a novel explanation of strengthening effect of Cr observed in alloys due to its unique magnetic properties. Distortion in metallic systems is a topic of debate in the context of multi-component concentrated alloys or so called "high-entropy alloys (HEA)" [5, 6, 7, 8, 9, 10]. Such distortions are known to influence the structural properties of these alloys [11, 12]. Local lattice distortion is instrumental 1
The deviation of positions of atoms from their ideal lattice sites in crystalline solid state sys... more The deviation of positions of atoms from their ideal lattice sites in crystalline solid state systems causes distortion and it causes variation in structural [1] and functional properties [2]. Severe lattice distortion has been proposed to be one of the core-effect in high-entropy alloys. But the fundamental mechanism of distortion at atomic scale is missing for real three-dimensional metallic systems. The present investigation aims to develop mechanistic understanding of atomic scale distortion in metallic systems in terms of the magneto-volume effects. The correlation between charge-disproportion, spin fluctuations, magnetovolume effects and Fermi surface nesting has been highlighted.
During the last couple of decades, treatment of microstructure in materials science has been shif... more During the last couple of decades, treatment of microstructure in materials science has been shifted from the diagnostic to design paradigm. Design of microstructure is inherently complex problems due to non linear spatial and temporal interaction of composition and parameters leading to the target properties. In most of the cases, different properties are reciprocally correlated i.e., improvement of one lead to the degradation of other. Also, the design of microstructure is a multiscale problem, as the knowledge of phenomena at range of scales from electronic to mesoscale is required for precise compositionmicrostructure-property determination. In the view of above, present chapter provides the introduction to computationally driven microstructure engineering in the framework of constitutive length scale in microstructure design. The important issues pertaining to design such as phase stability and interfaces has been explained. Additionally, the bird-eye view of various computational techniques in order of length scale has been introduced, with an aim to present the picture of combination of various techniques for solving microstructural design problems under various scenarios.
This study examines the comparative phase stability of transition and late transition element bas... more This study examines the comparative phase stability of transition and late transition element based high entropy alloys in body-centred cubic (BCC) or face-centred cubic (FCC) forms using a combined classical molecular dynamics and statistical mechanics based approach. Multi-configurational sampling was carried out using a hybrid genetic algorithm-molecular dynamics (GA-MD) based method. The calculations demonstrate that comparative BCC or FCC phase stability is influenced by configurational entropy. The present study also provides a theoretical explanation of the recently reported occurrence of BCC phase in CoCrFeNi HEA, where the high temperature structure may be retained.
ABSTRACT The present study addresses the evolution of texture and microstructure during annealing... more ABSTRACT The present study addresses the evolution of texture and microstructure during annealing in a cryo-rolled copper. Transition from copper to brass texture during the cryo-rolling has been illustrated. Twinning and interaction between twins and shear bands have been found to play the important role in grain refinement and strengthening. The low temperature vacancy clustering and its effect on the recrystallization have been experimentally demonstrated. Fine scale twinning, and grain refinement have been attributed to the higher yield strength found in the case of samples subjected to cryo-rolling.
Steel design is traditionally governed by the imprecise knowledge acquired from the experimental ... more Steel design is traditionally governed by the imprecise knowledge acquired from the experimental efforts occasionally invoking the empirical, physical and thermodynamic models supported by numerous assumptions and boundary conditions. The available frameworks are independently inadequate for complete and deterministic composition-process-microstructure-property (CPMP) correlation. The situation thus calls for judicious integration of the potential techniques with the capability of a priori prediction/optimization of the complete system. In view of the above the present article aims to provide a bird’s eye view of the deterministic approach invoked so far in the traditional framework of design and development of formable steels. An attempt has also been made to propose the scheme for hybridization of the potential techniques for describing the complete CPMP correlation in the production schedule of hot strip coil.
Journal of The Institution of Engineers (India): Series D, 2012
The deformation behavior of interstitial free/ferritic steel has been investigated at cryogenic a... more The deformation behavior of interstitial free/ferritic steel has been investigated at cryogenic and room temperature. The study concerns the occurrence of microstructural damages during low temperature deformation conditions for such high stacking fault energy (SFE) body centered cubic material and their role on the evolution of microstructure and mechanical properties.
In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti... more In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800 °C to 950 °C) in steps of 298 K (25 °C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise a-Fe + v, v, FeTi + k, FeTi + b-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni 3 Ti, NiTi, and NiTi 2 are formed at the nickel alloytitanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875 °C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe 5 Cr 35 Ni 40 Ti 15 have been observed within the Ni 3 Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~477 MPa and shear strength of ~356.9 MPa along with ~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900 °C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875 °C), the bond tensile and shear strengths increase to ~523.6 and ~389.6 MPa, respectively, along with 6.2 pct elongation.
In the traditional physical metallurgy approach, the microstructure evolves as the product of the... more In the traditional physical metallurgy approach, the microstructure evolves as the product of the interactions between composition and process parameters, controlled by thermodynamic and kinetic conditions. Thus, the approach is more evolutionary than constructive. An architecturally designed microstructure implies planning, design and construction of microstructure considering nature, size, morphology and distribution of the constituent phases on a suitably conceived topological framework. This investigation puts emphasis on performance driven construction of the microstructure with precise calculation of response of microstructural constituents under the given loading conditions. With the aforesaid motivation, the present study elucidates the principle involving microstructural generation by Voronoi tessellation and subsequent elastoplastic finite element simulation. In addition, the present study also explores the correlation between architecturing and initiation of instability in the microstructure through genetic programming.
This study examines the comparative phase stability of transition and late transition element bas... more This study examines the comparative phase stability of transition and late transition element based high entropy alloys in body-centred cubic (BCC) or face-centred cubic (FCC) forms using a combined classical molecular dynamics and statistical mechanics based approach. Multi-configurational sampling was carried out using a hybrid genetic algorithm-molecular dynamics (GA-MD) based method. The calculations demonstrate that comparative BCC or FCC phase stability is influenced by configurational entropy. The present study also provides a theoretical explanation of the recently reported occurrence of BCC phase in CoCrFeNi HEA, where the high temperature structure may be retained. High entropy alloys (HEAs) are a class of multicomponent metallic alloys [1], which exhibit simple crystal structures such as body-centred cubic (BCC), face-centred cubic (FCC), hexagonal cubic phase (HCP) or orthorhombic [2–4] crystal structures. The stabilisation of a solid solution phase over intermetallic and amorphous alloy formation in such multicomponent systems was initially attributed to the high configura-tional entropy of mixing [5]. The prediction of the comparative phase stability of BCC and FCC phases for HEAs remains a major debate in the literature. For example, CoCrFeNi shows the FCC structure [6], but addition of Al causes its transformation to the BCC crystal structure [6]. The phase transformation and existence of HEAs in two main types of crystal structures was initially rationalised in terms of the conventional metallurgical concept of FCC and BCC stabilising elements [7]. Traditionally, phase stability in alloys has been understood in terms of the number of electrons per atom (e/a) [8], which has been extensively used for Hume-Rothery electron phase stabilisation. In the case of HEAs, which are mostly composed of transition metals, the accurate determination of the e/a ratio is problematic. Hence, the valence electron concentration (VEC) has been employed to rationalise FCC and BCC formation, where higher VEC (≥ 8.0) leads to FCC phases, while lower VEC (≤ 6.87) causes BCC formation, leaving a mixture of BCC and FCC phase for 6.87 ≤ VEC ≤ 8.0 [9]. This criterion, however, does not apply to HEAs containing Mn [10], and in certain cases the effect of cooling conditions dominates the phase formation, i.e., alloys with the same composition but subjected to different processing conditions produce different phase occurrence characteristics [11]. Therefore phase selection remains an open debate. Computationally expensive ab-initio approaches, such as coherent potential approximation [12] and special quasi-random structures with density functional theory (DFT) [13] have been used to study HEAs. Given the potential significance of (con-figurational) entropy in the phase stability in HEAs, there is a need for an extensive sampling of configurations, which cannot be achieved easily with ab-initio methods that are limited to small cells. The present study attempts to address this issue by a using atomistic simulation with a genetic algorithm (GA) as the sampling strategy followed by a statistical mechanics technique for the calculation of the configurational entropy. The DL_POLY code was employed to carry out Molecular Dynamics (MD) simulations [14]. Position, velocity and acceleration of the atoms were updated via the velocity-verlet integration algorithm [15]. The GULP code [16] was used to generate 7 × 7 × 7 (FCC) and 9 × 9 × 9 (BCC) cubic supercells for starting configurations. The disordered structures of the alloys were generated by randomising the elements on the lattice sites. The simulations were performed with a 1 fs time-step. The long-range interaction cutoff was set to be 6 Å. The MD simulations were carried out for 10 ps, where the system was equilibrated for first 5000 time steps. The configurational energy was shown to converge over this time (see Fig. A1 in supplementary information). A Nóse-Hoo-ver NPT ensemble with thermostat and barostat relaxation times of 0.01 ps and 0.1 ps, respectively, was used to keep temperature (300 K) and pressure (1 atm) constant. The initial lattice parameters for the BCC and the FCC variant of AlCoCrFeNi, CoCrFeNi and CoCrFeNiTi were taken from first-principle calculations, while for Al 0·5 CoCrFeNi, the equilibrium lattice parameter was calculated from finding the minimum of the energy-volume curve in NVT calculation. The initial lattice Scripta Materialia 124 (2016) 90–94
Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near... more Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near-cryogenic (−150°C to −100°C) rolling followed by annealing have been investigated. Twining has been found to be the dominant mode of deformation during rolling. Both the rolled and annealed sample exhibited limited work hardening due to high matrix resistance. In the rolled sample cleavage, nuclei acted as potential fracture initiation site. Annealing has resulted in effective blunting of cleavage nuclei giving rise to large total elongation due to prolonged damage growth process.
The present study addresses the evolution of texture and microstructure during annealing in a cry... more The present study addresses the evolution of texture and microstructure during annealing in a cryo- rolled copper. Transition from copper to brass texture during the cryo-rolling has been illustrated. Twinning and interaction between twins and shear bands have been found to play the important role in grain refinement and strengthening. The low temperature vacancy clustering and its effect on the recrystallization have been experimentally demonstrated. Fine scale twinning, and grain refinement have been attributed to the higher yield strength found in the case of samples subjected to cryo-rolling.
In the traditional physical metallurgy approach, the microstructure evolves as the product of the... more In the traditional physical metallurgy approach, the microstructure evolves as the product of the interactions between composition and process parameters, controlled by thermodynamic and kinetic conditions. Thus, the approach is more evolutionary than constructive. An architecturally designed microstructure implies planning, design and construction of microstructure considering nature, size, morphology and distribution of the constituent phases on a suitably conceived topological framework. This investigation puts emphasis on performance driven construction of the microstructure with precise calculation of response of microstructural constituents under the given loading conditions. With the aforesaid motivation, the present study elucidates the principle involving microstructural generation by Voronoi tessellation and subsequent elastoplastic finite element simulation. In addition, the present study also explores the correlation between architecturing and initiation of instability in the microstructure through genetic programming.
The deformation behaviour of interstitial free/ ferritic steel has been investigated at cryogenic... more The deformation behaviour of interstitial free/ ferritic steel has been investigated at cryogenic and room temperature. The study concerns the occurrence of microstructural damages during low temperature deformation conditions for such high stacking fault energy body centred cubic material and their role on the evolution of microstructure and mechanical properties.
In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti... more In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800°C to 950°C) in steps of 298 K (25°C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise a-Fe + v, v, FeTi + k, FeTi + b-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni 3 Ti, NiTi, and NiTi 2 are formed at the nickel alloytitanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875°C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe 5 Cr 35 Ni 40 Ti 15 have been observed within the Ni 3 Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of~477 MPa and shear strength of~356.9 MPa along with~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900°C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875°C), the bond tensile and shear strengths increase to~523.6 and~389.6 MPa, respectively, along with 6.2 pct elongation.
Steel design is tradit ionally governed by the imprecise knowledge acquired fro m the experimenta... more Steel design is tradit ionally governed by the imprecise knowledge acquired fro m the experimental efforts occasionally invoking the empirical, physical and thermodynamic models supported by numerous assumptions and boundary conditions. The available frameworks are independently inadequate for co mplete and deterministic composition-process-microstructure-property (CPMP) correlation. The situation thus calls for judicious integration of the potential techniques with the capability of a priori prediction/optimization of the comp lete system. In v iew of the above the present article aims to provide a bird's eye view of the determin istic approach invoked so far in the tradit ional framework of design and development of formable steels. An attempt has also been made to propose the scheme for hybridization of the potential techniques for describing the complete CPM P correlation in the production schedule of hot strip coil.
Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near... more Microstructure and tensile deformation behavior of interstitial-free (IF) steel subjected to near-cryogenic (−150°C to −100°C) rolling followed by annealing have been investigated. Twining has been found to be the dominant mode of deformation during rolling. Both the rolled and annealed sample exhibited limited work hardening due to high matrix resistance. In the rolled sample cleavage, nuclei acted as potential fracture initiation site. Annealing has resulted in effective blunting of cleavage nuclei giving rise to large total elongation due to prolonged damage growth process.
During the last couple of decades, treatment of microstructure in materials science has been shif... more During the last couple of decades, treatment of microstructure in materials science has been shifted from the diagnostic to design paradigm. Design of microstructure is inherently complex problems due to non linear spatial and temporal interaction of composition and parameters leading to the target properties. In most of the cases, different properties are reciprocally correlated i.e., improvement of one lead to the degradation of other. Also, the design of microstructure is a multiscale problem, as the knowledge of phenomena at range of scales from electronic to mesoscale is required for precise composition-microstructure-property determination. In the view of above, present chapter provides the introduction to computationally driven microstructure engineering in the framework of constitutive length scale in microstructure design. The important issues pertaining to design such as phase stability and interfaces has been explained. Additionally, the bird-eye view of various computational techniques in order of length scale has been introduced, with an aim to present the picture of combination of various techniques for solving microstructural design problems under various scenarios.
Uploads
Papers by Gautam Anand