Sanjay Vajpai | Ritsumeikan University (original) (raw)

Papers by Sanjay Vajpai

Advances in Materials and Processing Technologies, 2015

Journal of Alloys and Compounds, 2016

In the present work, Tie40 mass%Nb alloys were successfully fabricated by a powder metallurgy rou... more In the present work, Tie40 mass%Nb alloys were successfully fabricated by a powder metallurgy route consisting of mechanical alloying (MA) of TiH 2 eNb powder mixture and spark plasma sintering (SPS). The use of brittle TiH 2 powder, instead of ductile elemental powder, led to significant increment in the yield of mechanically alloyed (MAed) powder. The MAed powder consisted of homogeneously distributed nano-sized Ti/Nb hydride particles together with micron-sized pure Nb particles. The MA also led to the lowering of dehydrogenation temperature of hydride particles. Sintering of MAed powders under low temperature conditions (1223 K, & 1373 K) resulted in the fine-grained heterogeneous microstructure consisting of a, b, and unreacted pure Nb phase. On the other hand, sintering at higher temperatures (1523 K) resulted in a relatively coarse-grained chemically homogeneous microstructure with almost complete b phase. Coarse-grained homogeneous b TieNb alloy exhibited higher average hardness as compared to that of heterogeneous fine grained microstructures. An attempt has been made to illustrate the correlation between the microstructural characteristics and mechanical properties of the sintered Ti e40Nb compacts.

Materials science & engineering. C, Materials for biological applications, 2016

In the present work, Co-Cr-Mo alloy compacts with a unique bimodal microstructural design, harmon... more In the present work, Co-Cr-Mo alloy compacts with a unique bimodal microstructural design, harmonic structure design, were successfully prepared via a powder metallurgy route consisting of controlled mechanical milling of pre-alloyed powders followed by spark plasma sintering. The harmonic structured Co-Cr-Mo alloy with bimodal grain size distribution exhibited relatively higher strength together with higher ductility as compared to the coarse-grained specimens. The harmonic Co-Cr-Mo alloy exhibited a very complex deformation behavior wherein it was found that the higher strength and the high retained ductility are derived from fine-grained shell and coarse-grained core regions, respectively. Finally, it was observed that the peculiar spatial/topological arrangement of stronger fine-grained and ductile coarse-grained regions in the harmonic structure promotes uniformity of strain distribution, leading to improved mechanical properties by suppressing the localized plastic deformation...

MATERIALS TRANSACTIONS, 2015

Through many years, conventional material developments have emphasized on microstructural refinem... more Through many years, conventional material developments have emphasized on microstructural refinement and homogeneity. However, "Nano- and Homogeneous" microstructures do not, usually, satisfy the need to be both strong and ductile, due to the plastic instability in the early stage of the deformation. As opposed to such a “nano- and homo-“microstructure design, we have proposed “Harmonic Structure” design. The harmonic structure has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of fine and coarse grains. In other words, the harmonic structure is heterogeneous on micro- but homogeneous on macro-scales. In the present work, the harmonic structure design has been applied to pure-Ti and Ti-6Al-4V alloy via a novel powder metallurgy route consisting of controlled severe plastic deformation of the fine-sized powder particles (pre-alloyed in the case of Ti-6Al-4V) via jet milling and subsequent cons...

A new tailored heterogeneous "Harmonic Structure" design is proposed for titanium PM materials wi... more A new tailored heterogeneous "Harmonic Structure" design is proposed for titanium PM materials with outstanding mechanical properties. The harmonic structure consists of a three-dimensional network structure with continuously connected ultra-fine grain regions and homogeneously dispersed coarse grain colonies. In the present study, pure titanium and Ti-6Al-4V alloy compacts with the harmonic structure are fabricated by a combination of Mechanical Milling (MM) and Powder Metallurgy (PM) processes. The MM process is one of severe plastic deformation (SPD) processes, and it enables to fabricate a bimodal microstructure in the powders. That is, in MM powder, the surface and the inner regions are composed of nano scale grains and micron-scale grains, respectively. The sintered compacts with the harmonic structure demonstrate outstanding mechanical properties compared to the conventional materials.

Duplex stainless steels are attractive structural materials for applications in hostile environme... more Duplex stainless steels are attractive structural materials for applications in hostile environments due to their excellent mechanical properties together with superior corrosion resistance. Although the micro-duplex structure exhibits high strength, its poor ductility limits their toughness. In the present study, a high-strength duplex stainless steel was produced by combining a fine micro duplex structure with a coarse structure, thus retaining satisfactory work hardening ability. This is accomplished by applying the new concept of harmonic structure design, which involves the mechanical milling followed by a spark-plasma sintering of a two phase steel (Fe-25%Cr-5%Ni-2%Mo) powder, leading to a microstructure with a gradual and continuous transition between coarse and ultrafine-grained regions at the microscale. The UFG regions on the surface of powder particles – the “shell” – forms 3-D nano duplex (Mid-Shell) and micro duplex (Outer-Shell) network within the sintered compacts. Th...

Metallurgical and Materials Transactions A, 2014

ABSTRACT The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique mi... more ABSTRACT The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique microstructure with bimodal grain size distribution, termed as “harmonic structure.” The Ti-6Al-4V compacts with harmonic structure design were successfully prepared via a powder metallurgy approach consisting of controlled mechanical milling and spark plasma sintering of the pre-alloyed Ti-6Al-4V powders. The microstructural evolution at each stage of processing has been investigated to establish a correlation between the processing conditions and the microstructural evolution. The Ti-6Al-4V compacts with heterogeneous harmonic structure exhibited better mechanical properties as compared to their homogeneous fine/coarse-grained counterparts. An attempt has also been made to explain the deformation mechanism of the harmonic-structured Ti-6Al-4V specimens with the help of the experimental evidences. The superior mechanical properties of the harmonic structure Ti-6Al-4V were found to be related to the peculiar topological distribution of strong fine-grained and ductile coarse-grained regions, which promotes uniform distribution of strain during plastic deformation and results in improved mechanical properties by avoiding the localized plastic deformation in the early stages of deformation.

IOP Conference Series: Materials Science and Engineering, 2014

In the present work the harmonic structure design has been successfully applied for achieving a c... more In the present work the harmonic structure design has been successfully applied for achieving a combination of high strength and high ductility, simultaneously, in a two-phase steel. The compacts of two-phase stainless steels with harmonic structure were prepared by controlling mechanical milling (MM) of pre-alloyed stainless steel powders followed by spark plasma sintering. The controlled MM leads to the formation of severely deformed "shell" region, wherein the subsurface region in the immediate vicinity of the powder surface consists of a nanocrystalline structure followed by the inner region consisting of dislocation cell structure. These severely deformed regions form fine-grained network during subsequent sintering, resulting in Harmonic structure. This networked structure displayed high strength, high ductility, and better uniform plastic deformation as compared to the homogeneous fine/coarse grained structure. Such a unique combination of properties in the two-phase stainless steel powder compacts was found to be associated with the ability of the harmonic structure to evenly distribute the strain during plastic deformation.

Advanced Engineering Materials, 2014

ABSTRACT Creating bimodal grain size distribution in crystalline materials is an effective strate... more ABSTRACT Creating bimodal grain size distribution in crystalline materials is an effective strategy to improve structural efficiency. Present investigation demonstrates that further control over topology through the formation of bimodal ‘harmonic’ structure simultaneously increases average values of strength and ductility, reduces the variation of corresponding properties, and thwarts the localization of plastic flow.

MATERIALS TRANSACTIONS, 2015

MATERIALS TRANSACTIONS, 2014

IOP Conf. Ser.: Mater. Sci. Eng., 2014

Ti-6Al-4V alloy is an advanced structural material having applications in a wide range of areas s... more Ti-6Al-4V alloy is an advanced structural material having applications in a wide range of areas spanning from biomedical to aerospace sectors due to the excellent combination of mechanical and chemical properties. In the present work, a new tailored heterogeneous microstructural design with a specific topological distribution of fine and coarse grained areas, called "harmonic structure", has been proposed for the strengthening of Ti-6Al-4V alloy to achieve improved performance of the components in service. It has been demonstrated that Ti-6Al-4V alloy with harmonic structure can be successfully prepared via a powder metallurgy route consisting of controlled severe plastic deformation of pre-alloyed powders via mechanical milling followed by their consolidation. The Ti-6Al-4V compacts with harmonic structure design exhibited significantly better strength and ductility, under quasi-static as well as rapid loading conditions, as compared to their homogeneous fine and coarse grained counterparts. It was found that the harmonic structure design has the ability to promote the uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding localized plastic instability.

Current Applied Physics, 2012

a b s t r a c t (Bi 0.5 Na 0.5 )TiO 3 was doped in situ with 5, 8, and 11 mol% BaTiO 3 (BNT-BT x ... more a b s t r a c t (Bi 0.5 Na 0.5 )TiO 3 was doped in situ with 5, 8, and 11 mol% BaTiO 3 (BNT-BT x ; x ¼ 0.05, 0.08 and 0.11) using a sol-gel technique. The resulting powders from gel precursors showed microstructures consisting of nano-sized grains and crystalline perovskite structure. Spark plasma sintering (SPS) technique was used to prepare high densified (98e99%r theor ) BNT-BT x ceramics from these nanopowders. The results confirm the spark plasma sintering method applied to nano-scale powders, obtained by sol-gel, as a viable route in producing nanostructured ceramics. The evolution of the structure and electrical properties of the ceramics with BaTiO 3 concentration (x) was investigated. The permittivity of BNT-BT 0.08 ceramic is higher (ε r ¼ 2090, at 100 kHz) than that for x ¼ 0.05 (ε r ¼ 1350) and x ¼ 0.11 (ε r ¼ 1800). BNT-BT 0.08 ceramic shows maximum values for the frequency constants (N p , N t ), piezoelectric charge coefficient (d 31 ) and piezoelectric voltage coefficient (g 31 ), and minimum values for the electromechanical coupling factor (k p ) and piezoelectric charge coefficient (d 33 ). The electrical properties of these ceramics are influenced by grains size, oxygen deficiency and non-uniform internal stresses due to these oxygen deficiencies. BNT-BT x ceramics sintered by SPS seem to be good ceramic resonators with high mechanical quality factor (Q m ).

Acta Materialia, 2011

Copper-based shape memory alloys High temperature shape memory alloys Powder metallurgy processin... more Copper-based shape memory alloys High temperature shape memory alloys Powder metallurgy processing Cu-Al-Ni SMA a b s t r a c t Cu-Al-Ni high temperature shape memory alloy (HTSMA) strips were successfully prepared from rapid solidified water atomized Cu-Al-Ni pre-alloyed powders via hot densification rolling of unsheathed sintered powder preforms. Finished heat-treated Cu-Al-Ni alloy strips had fine-grained structure, average grain size approximately 16 mm, and exhibited a combination of high strength and high ductility. It has been demonstrated that the redistribution of nano-sized alumina particles, present on the surface as well as inside the starting water atomized Cu-Al-Ni pre-alloyed powder particles, due to plastic deformation of starting powder particles during hot densification rolling resulted in the fine grained microstructure in the finished SMA strips. The finished SMA strips were almost fully martensitic in nature, consisting of a mixture of b 0 1 and g 0 1 martensite. The average fracture strength and fracture strain of the finished SMA strips were 810 MPa and 12%, respectively, and the fractured specimens exhibited primarily micro-void coalescence type ductile nature of fracture. Finished Cu-Al-Ni SMA strips exhibited high characteristic transformation temperatures and an almost 100% one-way shape recovery was obtained in the specimens up to 4% applied deformation pre-strain. The retained two-way shape memory recovery increased with increasing applied training pre-strain, achieving a maximum value of 16.25% at 5% applied training pre-strain.

Materials Science and Engineering: A, 2014

Journal of Powder Metallurgy and Mining, Jun 2014

Conventional material developments have emphasized ultrafine grain refinement and homogenization;... more Conventional material developments have emphasized ultrafine grain refinement and homogenization; however, “nano and homo” materials do not usually satisfy the need to be both strong and ductile, which are of course rather contradictory characteristics. On the other hand, "Harmonic Structure Materials Design" creates a "nano- and heterogeneous microstructure", and the harmonic structure materials can overcome that antinomy through its unique
microstructure. The procedure to create the harmonic structure is accomplished by using one of the non-equilibrium powder metallurgy (PM) processes called the Severe Plastic Deformation (SPD) PM process. The harmonic structure materials consist of Ultra-Fine Grain (UFG) and coarse grain structures known as “shell” and “core”, respectively. They have a network structure of continuously connected shells, and simultaneously demonstrate both high strength and elongation, especially a large uniform elongation. In the present study, pure Ni powder is processed so as to produce the harmonic structure materials. The deformation mechanism is analyzed based on the strain hardening behavior.

Advanced Materials Research, 2014

Co-­Cr-­Mo alloy powders were subjected to controlled mechanical milling at room temperature unde... more Co-­Cr-­Mo alloy powders were subjected to controlled mechanical milling at room temperature under Ar atmosphere to fabricate bimodal microstructure in the MM powders, having nano-­sized grains in the surface region and micron-­sized coarse grains in the center of the milled powders. Subsequently, the MM powder was compacted by spark-­plasma sintering (SPS) process. The sintered compacts indicated two structure areas: (i) ultra-­fine grained (UFG) regions, called "shell", and (ii) the coarse grained region called "core". The shell and the core correspond to the surface and center of the MM powders, respectively. The shell regions established a continuous three dimensional network of high strength ultra-­fine grained regions, which surrounded the discrete coarse grained ductile regions. Such a microstructure is referred as "harmonic structure". The sintered Co-­Cr-­Mo alloy compacts exhibited outstanding mechanical properties. The yield strength increased from 605 to 635 MPa, and ultimate tensile strength increased from 1201 to 1283 MPa. Moreover, the elongation was maintained more or less same as that of coarse grained compacts. Therefore, the harmonic structure design leads to the new generation microstructure of Co-­Cr-­Mo alloy, which demonstrates outstanding mechanical properties, i.e. superior strength and
excellent ductility as compared to conventional materials.

Materials Science and Engineering A, 2014

The present work deals with achieving improvement in the mechanical properties of SUS304L stainle... more The present work deals with achieving improvement in the mechanical properties of SUS304L stainless steel through the application of a unique microstructure design termed as ‘Harmonic structure’, and establishing a co-relationship between various microstructural characteristics and mechanical properties. Harmonic structure essentially means a bimodal grain size distribution with a specific periodic arrangement of coarse-and ultrafine-grain fractions. SUS304L stainless steel samples having such microstructure were fabricated by a powder metallurgy route involving the mechanical milling of pre-alloyed steel powder followed by spark plasma sintering. Due to these peculiar microstructural characteristics, theharmonic-structured SUS304L stainless steels demonstrated a winning combination of high strength, large uniform elongation, and large total elongation to failure, simultaneously. It was also found that the fraction of a shell area (a three-dimensional continuously connected network of
ultrafine-grained structure) is an important parameter controlling the balance of the mechanical properties oftheharmonic-structured SUS304L steel compacts.

Advances in Materials and Processing Technologies, 2015

Journal of Alloys and Compounds, 2016

In the present work, Tie40 mass%Nb alloys were successfully fabricated by a powder metallurgy rou... more In the present work, Tie40 mass%Nb alloys were successfully fabricated by a powder metallurgy route consisting of mechanical alloying (MA) of TiH 2 eNb powder mixture and spark plasma sintering (SPS). The use of brittle TiH 2 powder, instead of ductile elemental powder, led to significant increment in the yield of mechanically alloyed (MAed) powder. The MAed powder consisted of homogeneously distributed nano-sized Ti/Nb hydride particles together with micron-sized pure Nb particles. The MA also led to the lowering of dehydrogenation temperature of hydride particles. Sintering of MAed powders under low temperature conditions (1223 K, & 1373 K) resulted in the fine-grained heterogeneous microstructure consisting of a, b, and unreacted pure Nb phase. On the other hand, sintering at higher temperatures (1523 K) resulted in a relatively coarse-grained chemically homogeneous microstructure with almost complete b phase. Coarse-grained homogeneous b TieNb alloy exhibited higher average hardness as compared to that of heterogeneous fine grained microstructures. An attempt has been made to illustrate the correlation between the microstructural characteristics and mechanical properties of the sintered Ti e40Nb compacts.

Materials science & engineering. C, Materials for biological applications, 2016

In the present work, Co-Cr-Mo alloy compacts with a unique bimodal microstructural design, harmon... more In the present work, Co-Cr-Mo alloy compacts with a unique bimodal microstructural design, harmonic structure design, were successfully prepared via a powder metallurgy route consisting of controlled mechanical milling of pre-alloyed powders followed by spark plasma sintering. The harmonic structured Co-Cr-Mo alloy with bimodal grain size distribution exhibited relatively higher strength together with higher ductility as compared to the coarse-grained specimens. The harmonic Co-Cr-Mo alloy exhibited a very complex deformation behavior wherein it was found that the higher strength and the high retained ductility are derived from fine-grained shell and coarse-grained core regions, respectively. Finally, it was observed that the peculiar spatial/topological arrangement of stronger fine-grained and ductile coarse-grained regions in the harmonic structure promotes uniformity of strain distribution, leading to improved mechanical properties by suppressing the localized plastic deformation...

MATERIALS TRANSACTIONS, 2015

Through many years, conventional material developments have emphasized on microstructural refinem... more Through many years, conventional material developments have emphasized on microstructural refinement and homogeneity. However, "Nano- and Homogeneous" microstructures do not, usually, satisfy the need to be both strong and ductile, due to the plastic instability in the early stage of the deformation. As opposed to such a “nano- and homo-“microstructure design, we have proposed “Harmonic Structure” design. The harmonic structure has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of fine and coarse grains. In other words, the harmonic structure is heterogeneous on micro- but homogeneous on macro-scales. In the present work, the harmonic structure design has been applied to pure-Ti and Ti-6Al-4V alloy via a novel powder metallurgy route consisting of controlled severe plastic deformation of the fine-sized powder particles (pre-alloyed in the case of Ti-6Al-4V) via jet milling and subsequent cons...

A new tailored heterogeneous "Harmonic Structure" design is proposed for titanium PM materials wi... more A new tailored heterogeneous "Harmonic Structure" design is proposed for titanium PM materials with outstanding mechanical properties. The harmonic structure consists of a three-dimensional network structure with continuously connected ultra-fine grain regions and homogeneously dispersed coarse grain colonies. In the present study, pure titanium and Ti-6Al-4V alloy compacts with the harmonic structure are fabricated by a combination of Mechanical Milling (MM) and Powder Metallurgy (PM) processes. The MM process is one of severe plastic deformation (SPD) processes, and it enables to fabricate a bimodal microstructure in the powders. That is, in MM powder, the surface and the inner regions are composed of nano scale grains and micron-scale grains, respectively. The sintered compacts with the harmonic structure demonstrate outstanding mechanical properties compared to the conventional materials.

Duplex stainless steels are attractive structural materials for applications in hostile environme... more Duplex stainless steels are attractive structural materials for applications in hostile environments due to their excellent mechanical properties together with superior corrosion resistance. Although the micro-duplex structure exhibits high strength, its poor ductility limits their toughness. In the present study, a high-strength duplex stainless steel was produced by combining a fine micro duplex structure with a coarse structure, thus retaining satisfactory work hardening ability. This is accomplished by applying the new concept of harmonic structure design, which involves the mechanical milling followed by a spark-plasma sintering of a two phase steel (Fe-25%Cr-5%Ni-2%Mo) powder, leading to a microstructure with a gradual and continuous transition between coarse and ultrafine-grained regions at the microscale. The UFG regions on the surface of powder particles – the “shell” – forms 3-D nano duplex (Mid-Shell) and micro duplex (Outer-Shell) network within the sintered compacts. Th...

Metallurgical and Materials Transactions A, 2014

ABSTRACT The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique mi... more ABSTRACT The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique microstructure with bimodal grain size distribution, termed as “harmonic structure.” The Ti-6Al-4V compacts with harmonic structure design were successfully prepared via a powder metallurgy approach consisting of controlled mechanical milling and spark plasma sintering of the pre-alloyed Ti-6Al-4V powders. The microstructural evolution at each stage of processing has been investigated to establish a correlation between the processing conditions and the microstructural evolution. The Ti-6Al-4V compacts with heterogeneous harmonic structure exhibited better mechanical properties as compared to their homogeneous fine/coarse-grained counterparts. An attempt has also been made to explain the deformation mechanism of the harmonic-structured Ti-6Al-4V specimens with the help of the experimental evidences. The superior mechanical properties of the harmonic structure Ti-6Al-4V were found to be related to the peculiar topological distribution of strong fine-grained and ductile coarse-grained regions, which promotes uniform distribution of strain during plastic deformation and results in improved mechanical properties by avoiding the localized plastic deformation in the early stages of deformation.

IOP Conference Series: Materials Science and Engineering, 2014

In the present work the harmonic structure design has been successfully applied for achieving a c... more In the present work the harmonic structure design has been successfully applied for achieving a combination of high strength and high ductility, simultaneously, in a two-phase steel. The compacts of two-phase stainless steels with harmonic structure were prepared by controlling mechanical milling (MM) of pre-alloyed stainless steel powders followed by spark plasma sintering. The controlled MM leads to the formation of severely deformed "shell" region, wherein the subsurface region in the immediate vicinity of the powder surface consists of a nanocrystalline structure followed by the inner region consisting of dislocation cell structure. These severely deformed regions form fine-grained network during subsequent sintering, resulting in Harmonic structure. This networked structure displayed high strength, high ductility, and better uniform plastic deformation as compared to the homogeneous fine/coarse grained structure. Such a unique combination of properties in the two-phase stainless steel powder compacts was found to be associated with the ability of the harmonic structure to evenly distribute the strain during plastic deformation.

Advanced Engineering Materials, 2014

ABSTRACT Creating bimodal grain size distribution in crystalline materials is an effective strate... more ABSTRACT Creating bimodal grain size distribution in crystalline materials is an effective strategy to improve structural efficiency. Present investigation demonstrates that further control over topology through the formation of bimodal ‘harmonic’ structure simultaneously increases average values of strength and ductility, reduces the variation of corresponding properties, and thwarts the localization of plastic flow.

MATERIALS TRANSACTIONS, 2015

MATERIALS TRANSACTIONS, 2014

IOP Conf. Ser.: Mater. Sci. Eng., 2014

Ti-6Al-4V alloy is an advanced structural material having applications in a wide range of areas s... more Ti-6Al-4V alloy is an advanced structural material having applications in a wide range of areas spanning from biomedical to aerospace sectors due to the excellent combination of mechanical and chemical properties. In the present work, a new tailored heterogeneous microstructural design with a specific topological distribution of fine and coarse grained areas, called "harmonic structure", has been proposed for the strengthening of Ti-6Al-4V alloy to achieve improved performance of the components in service. It has been demonstrated that Ti-6Al-4V alloy with harmonic structure can be successfully prepared via a powder metallurgy route consisting of controlled severe plastic deformation of pre-alloyed powders via mechanical milling followed by their consolidation. The Ti-6Al-4V compacts with harmonic structure design exhibited significantly better strength and ductility, under quasi-static as well as rapid loading conditions, as compared to their homogeneous fine and coarse grained counterparts. It was found that the harmonic structure design has the ability to promote the uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding localized plastic instability.

Current Applied Physics, 2012

a b s t r a c t (Bi 0.5 Na 0.5 )TiO 3 was doped in situ with 5, 8, and 11 mol% BaTiO 3 (BNT-BT x ... more a b s t r a c t (Bi 0.5 Na 0.5 )TiO 3 was doped in situ with 5, 8, and 11 mol% BaTiO 3 (BNT-BT x ; x ¼ 0.05, 0.08 and 0.11) using a sol-gel technique. The resulting powders from gel precursors showed microstructures consisting of nano-sized grains and crystalline perovskite structure. Spark plasma sintering (SPS) technique was used to prepare high densified (98e99%r theor ) BNT-BT x ceramics from these nanopowders. The results confirm the spark plasma sintering method applied to nano-scale powders, obtained by sol-gel, as a viable route in producing nanostructured ceramics. The evolution of the structure and electrical properties of the ceramics with BaTiO 3 concentration (x) was investigated. The permittivity of BNT-BT 0.08 ceramic is higher (ε r ¼ 2090, at 100 kHz) than that for x ¼ 0.05 (ε r ¼ 1350) and x ¼ 0.11 (ε r ¼ 1800). BNT-BT 0.08 ceramic shows maximum values for the frequency constants (N p , N t ), piezoelectric charge coefficient (d 31 ) and piezoelectric voltage coefficient (g 31 ), and minimum values for the electromechanical coupling factor (k p ) and piezoelectric charge coefficient (d 33 ). The electrical properties of these ceramics are influenced by grains size, oxygen deficiency and non-uniform internal stresses due to these oxygen deficiencies. BNT-BT x ceramics sintered by SPS seem to be good ceramic resonators with high mechanical quality factor (Q m ).

Acta Materialia, 2011

Copper-based shape memory alloys High temperature shape memory alloys Powder metallurgy processin... more Copper-based shape memory alloys High temperature shape memory alloys Powder metallurgy processing Cu-Al-Ni SMA a b s t r a c t Cu-Al-Ni high temperature shape memory alloy (HTSMA) strips were successfully prepared from rapid solidified water atomized Cu-Al-Ni pre-alloyed powders via hot densification rolling of unsheathed sintered powder preforms. Finished heat-treated Cu-Al-Ni alloy strips had fine-grained structure, average grain size approximately 16 mm, and exhibited a combination of high strength and high ductility. It has been demonstrated that the redistribution of nano-sized alumina particles, present on the surface as well as inside the starting water atomized Cu-Al-Ni pre-alloyed powder particles, due to plastic deformation of starting powder particles during hot densification rolling resulted in the fine grained microstructure in the finished SMA strips. The finished SMA strips were almost fully martensitic in nature, consisting of a mixture of b 0 1 and g 0 1 martensite. The average fracture strength and fracture strain of the finished SMA strips were 810 MPa and 12%, respectively, and the fractured specimens exhibited primarily micro-void coalescence type ductile nature of fracture. Finished Cu-Al-Ni SMA strips exhibited high characteristic transformation temperatures and an almost 100% one-way shape recovery was obtained in the specimens up to 4% applied deformation pre-strain. The retained two-way shape memory recovery increased with increasing applied training pre-strain, achieving a maximum value of 16.25% at 5% applied training pre-strain.

Materials Science and Engineering: A, 2014

Journal of Powder Metallurgy and Mining, Jun 2014

Conventional material developments have emphasized ultrafine grain refinement and homogenization;... more Conventional material developments have emphasized ultrafine grain refinement and homogenization; however, “nano and homo” materials do not usually satisfy the need to be both strong and ductile, which are of course rather contradictory characteristics. On the other hand, "Harmonic Structure Materials Design" creates a "nano- and heterogeneous microstructure", and the harmonic structure materials can overcome that antinomy through its unique
microstructure. The procedure to create the harmonic structure is accomplished by using one of the non-equilibrium powder metallurgy (PM) processes called the Severe Plastic Deformation (SPD) PM process. The harmonic structure materials consist of Ultra-Fine Grain (UFG) and coarse grain structures known as “shell” and “core”, respectively. They have a network structure of continuously connected shells, and simultaneously demonstrate both high strength and elongation, especially a large uniform elongation. In the present study, pure Ni powder is processed so as to produce the harmonic structure materials. The deformation mechanism is analyzed based on the strain hardening behavior.

Advanced Materials Research, 2014

Co-­Cr-­Mo alloy powders were subjected to controlled mechanical milling at room temperature unde... more Co-­Cr-­Mo alloy powders were subjected to controlled mechanical milling at room temperature under Ar atmosphere to fabricate bimodal microstructure in the MM powders, having nano-­sized grains in the surface region and micron-­sized coarse grains in the center of the milled powders. Subsequently, the MM powder was compacted by spark-­plasma sintering (SPS) process. The sintered compacts indicated two structure areas: (i) ultra-­fine grained (UFG) regions, called "shell", and (ii) the coarse grained region called "core". The shell and the core correspond to the surface and center of the MM powders, respectively. The shell regions established a continuous three dimensional network of high strength ultra-­fine grained regions, which surrounded the discrete coarse grained ductile regions. Such a microstructure is referred as "harmonic structure". The sintered Co-­Cr-­Mo alloy compacts exhibited outstanding mechanical properties. The yield strength increased from 605 to 635 MPa, and ultimate tensile strength increased from 1201 to 1283 MPa. Moreover, the elongation was maintained more or less same as that of coarse grained compacts. Therefore, the harmonic structure design leads to the new generation microstructure of Co-­Cr-­Mo alloy, which demonstrates outstanding mechanical properties, i.e. superior strength and
excellent ductility as compared to conventional materials.

Materials Science and Engineering A, 2014

The present work deals with achieving improvement in the mechanical properties of SUS304L stainle... more The present work deals with achieving improvement in the mechanical properties of SUS304L stainless steel through the application of a unique microstructure design termed as ‘Harmonic structure’, and establishing a co-relationship between various microstructural characteristics and mechanical properties. Harmonic structure essentially means a bimodal grain size distribution with a specific periodic arrangement of coarse-and ultrafine-grain fractions. SUS304L stainless steel samples having such microstructure were fabricated by a powder metallurgy route involving the mechanical milling of pre-alloyed steel powder followed by spark plasma sintering. Due to these peculiar microstructural characteristics, theharmonic-structured SUS304L stainless steels demonstrated a winning combination of high strength, large uniform elongation, and large total elongation to failure, simultaneously. It was also found that the fraction of a shell area (a three-dimensional continuously connected network of
ultrafine-grained structure) is an important parameter controlling the balance of the mechanical properties oftheharmonic-structured SUS304L steel compacts.