Oleg Senkov - Academia.edu (original) (raw)

Papers by Oleg Senkov

The Physics of Metals and Metallography, 1987

A novel refractory alloy produced in the Institute for Problems of Materials Science, Kiev, Ukrai... more A novel refractory alloy produced in the Institute for Problems of Materials Science, Kiev, Ukraine, has been characterized. The composition of the alloy is Ti-6Si-5Al-7Zr (wt.%) with a specific density of 4.6 g/cm3. The powder particles produced from this alloy by the plasma rotating electrode process (PREP) are spherical in shape with a very small variation in size (the average particle size was about 350 m). The particles have a dendritic microstructure and consist of three phases, disordered alpha-Ti, ordered TiAl and ordered Ti5Si3. The Ti5Si3 phase is stable on heating up to 1100C while disordering of the Ti3Al phase occurs near 1000C. Tensile properties of the cast alloy were studied at temperatures in the range of 20C to 800C. The yield strength and tensile strength of the cast alloy are higher than those of a gamma-TiAl-based alloy. The fracture toughness of the alloy is about 17-20 MPam within the temperature range of 20C to 600C. The oxidation resistance of the alloy was ...

Materials Science Forum, 2013

An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fin... more An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fine equiaxed structure with the average grain/particle size of ~1.5 µm. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C. For example, during deformation at a strain rate of 10-3 s-1, tensile ductility increased from 400% to 860% when the temperature increased from 800°C to 1000°C. An increase in strain rate from 10-4 to 10-2 s-1 at T = 1000°C did not affect ductility: elongation to failure was about 800%. The strain rate sensitivity of the flow stress was rather high, m = 0.6, which is typical to the superplastic behavior. The equiaxed morphology of grains and particles retained after the superplastic deformation, although some grain/particle growth was observed.

Physical Review B, 2013

The atomic structure of the supercooled liquid has often been discussed as a key source of glass ... more The atomic structure of the supercooled liquid has often been discussed as a key source of glass formation in metals. The presence of icosahedrally-coordinated clusters and their tendency to form networks have been identified as one possible structural trait leading to glass forming ability in the Cu-Zr binary system. In this work, we show that this theory is insufficient to explain glass formation at all compositions in that binary system. Instead, we propose that the formation of ideally-packed clusters at the expense of atomic arrangements with excess or deficient free volume can explain glass-forming by a similar mechanism. We show that this behavior is reflected in the structural relaxation of a metallic glass during constant pressure cooling and the time evolution of structure at a constant volume. We then demonstrate that this theory is sufficient to explain slowed diffusivity in compositions across the range of Cu-Zr metallic glasses.

Metallurgical and Materials Transactions A, 1996

Metallurgical and Materials Transactions A, 2005

The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the... more The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the direct chill (DC) cast ingots of developmental Al-Zn-Mg-Cu alloys has been evaluated in this work. The properties in both the longitudinal and transverse directions were determined in as-cast and cast-plusheat-treated conditions, at room temperature (RT) and cryogenic temperature (CT). Extensive microstructural evaluation was carried out using optical microscopy and scanning electron microscopy, including orientation image microscopy (OIM) by the electron backscatter diffraction (EBSD) technique. The Sc-containing developmental cast alloys showed the tensile properties, which are much better than the properties of commercial cast Al alloys and are similar or even superior to the properties of 7075-T6 alloy forgings. The microstructural evolution, the strengthening mechanisms, the optimum content of the dispersoid-forming elements, and the processing-structure-property correlations are discussed.

Materials Science and Engineering: A, 2013

Using Government drawings, specifications, or other data included in this document for any purpos... more Using Government drawings, specifications, or other data included in this document for any purpose other than Government procurement does not in any way obligate the U.S. Government. The fact that the Government formulated or supplied the drawings, specifications, or other data does not license the holder or any other person or corporation; or convey any rights or permission to manufacture, use, or sell any patented invention that may relate to them. Qualified requestors may obtain copies of this report from the Defense Technical Information Center (DTIC) (http://www.dtic.mil).

JOM, 2002

Thermohydrogen processing, a technique in which hydrogen is used as a temporary alloying element ... more Thermohydrogen processing, a technique in which hydrogen is used as a temporary alloying element in titanium, allows enhanced microstructural control and improved mechanical properties. This paper provides an improved definition of the phase transformations taking place in the Ti-6Al-4V-hydrogen system, which should lead to increased application of the technique.

Journal of Materials Science, 1992

ABSTRACT The TiAl intermetallic compound was used to illustrate an approach which enables the cre... more ABSTRACT The TiAl intermetallic compound was used to illustrate an approach which enables the creation of a submicrocrystalline structure (d0.1 m) in massive semifinished products made of hard-to-deform materials by means of their deformation at elevated temperatures. Tensile mechanical properties of the TiAl intermetallic compound with a mean grain size of 0.4 m were tested. In this state, the lower temperature limit of superplasticity in TiAl was found to be 800C. At this temperature and at an initial strain rate of 8.310–4s–1, the relative elongation to rupture attains 225%.

Journal of Materials Science, 2012

Using Government drawings, specifications, or other data included in this document for any purpos... more Using Government drawings, specifications, or other data included in this document for any purpose other than Government procurement does not in any way obligate the U.S. Government. The fact that the Government formulated or supplied the drawings, specifications, or other data does not license the holder or any other person or corporation; or convey any rights or permission to manufacture, use, or sell any patented invention that may relate to them. This report was cleared for public release by the USAF 88 th Air Base Wing (88 ABW) Public Affairs Office and is available to the general public, including foreign nationals. Copies may be obtained from the Defense Technical Information Center (DTIC) (http://www.dtic.mil).

Journal of Materials Processing Technology, 2001

A Ti–6Al–4V alloy sheet with a submicrocrystalline structure (grain size of 0.3μm) was produced b... more A Ti–6Al–4V alloy sheet with a submicrocrystalline structure (grain size of 0.3μm) was produced by multistep isothermal forging followed by flat rolling at temperatures below 650°C. The sheet showed a very strong basal texture and isotropic properties in the sheet plane. Room temperature yield strength, ultimate tensile strength, and fatigue limit of the sheet were 1.2–1.3 times higher than that

Journal of Electronic Materials, 2001

Applied Surface Science, 2011

A fully dense nanocrystalline compact of the Ti-47Al-3Cr (at. %) alloy was produced by mechanical... more A fully dense nanocrystalline compact of the Ti-47Al-3Cr (at. %) alloy was produced by mechanical alloying and hot isostatic pressing at 725 ± C. Microstructure characteristics and grain growth behavior of this compact were studied after annealing for up to 800 h in the temperature range of 725 to 1200 ± C, using analytical transmission electron microscopy techniques. The temperature and time dependencies of the grain sizes and the grain size distributions were determined. The grain growth occurred, with a timeand temperature-invariant single-peak grain size distribution (when normalized by the mean grain size), which was consistent with normal grain growth. The experimentally measured grain growth exponent decreased from 10 to 4.6 when the temperature was increased. The grain growth kinetics was described by a single thermally activated rate process limited by a permanent pinning force on the grain boundaries. The microhardness decreased on annealing and followed the Hall-Petch relationship with the parameters H yo 5.8 GPa and K H 1.6 MPa ? m 0.5 .

Applied Physics Letters, 2012

Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where ... more Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order.

ABSTRACT Two fully dense nanocrystalline TiAl-based compacts, Ti-47.5Al-3Cr and Ti-48Al-2Nb-2Cr, ... more ABSTRACT Two fully dense nanocrystalline TiAl-based compacts, Ti-47.5Al-3Cr and Ti-48Al-2Nb-2Cr, were synthesized by hot isostatic pressing (HIP'ing) from mechanically alloyed (MA'd) powders. Microstructure evolution and phase transformations after MA'ing, HIP'ing, and following annealing were studied. Formation of amorphous phases was detected in both powders after 15 hour MA'ing. During HIP'ing, crystallization of the amorphous phases occurred and very fine equiaxed gamma-TiAl grains were formed. The grain size decreased when the temperature of HIP'ing was decreased, and it was 42 nm after HIP'ing at 725 C. Grain growth occurred during annealing of the HIP'd alloys, which could be described by a single thermally activated process limited by a permanent pinning force. The activation energy of the grain growth was close to the activation energy for lattice diffusion of Ti in TiAl. Microhardness decreased with the annealing time and followed the Hall-Perch dependence on the grain size.

ABSTRACT The production cost of titanium and its alloys can be reduced considerably by synthesizi... more ABSTRACT The production cost of titanium and its alloys can be reduced considerably by synthesizing them from precursor compounds, by direct chemical reduction at ambient temperatures. In the present work, mechanochemical processing has been employed to synthesize Ti, Ti-Al, Ti-V, and Ti-Al-V in powder form by chemically reducing combinations of the respective metal chlorides with Mg or CaH2. Use of CaH2 for the reduction reactions resulted in the formation of hydrides of titanium and its alloys. The hydrided powders are passive against oxidation during leaching. In an attempt to reduce the cost of titanium production even further, studies of the direct reduction of TiO2 have been carried out. In all these processes complete removal of by-product salts from metal powder agglomerates has still to be achieved along with a minimization of interstitial pick-up during milling.

Metallurgical and Materials Transactions A, 2012

A coupled thermal/material flow model of friction stir welding was developed and applied to the j... more A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanis...

The Physics of Metals and Metallography, 1987

A novel refractory alloy produced in the Institute for Problems of Materials Science, Kiev, Ukrai... more A novel refractory alloy produced in the Institute for Problems of Materials Science, Kiev, Ukraine, has been characterized. The composition of the alloy is Ti-6Si-5Al-7Zr (wt.%) with a specific density of 4.6 g/cm3. The powder particles produced from this alloy by the plasma rotating electrode process (PREP) are spherical in shape with a very small variation in size (the average particle size was about 350 m). The particles have a dendritic microstructure and consist of three phases, disordered alpha-Ti, ordered TiAl and ordered Ti5Si3. The Ti5Si3 phase is stable on heating up to 1100C while disordering of the Ti3Al phase occurs near 1000C. Tensile properties of the cast alloy were studied at temperatures in the range of 20C to 800C. The yield strength and tensile strength of the cast alloy are higher than those of a gamma-TiAl-based alloy. The fracture toughness of the alloy is about 17-20 MPam within the temperature range of 20C to 600C. The oxidation resistance of the alloy was ...

Materials Science Forum, 2013

An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fin... more An AlCoCrCuFeNi high entropy alloy was multiaxially isothermally forged at 950°C to produce a fine equiaxed structure with the average grain/particle size of ~1.5 µm. The forged alloy exhibited superplastic behavior in the temperature range of 800-1000°C. For example, during deformation at a strain rate of 10-3 s-1, tensile ductility increased from 400% to 860% when the temperature increased from 800°C to 1000°C. An increase in strain rate from 10-4 to 10-2 s-1 at T = 1000°C did not affect ductility: elongation to failure was about 800%. The strain rate sensitivity of the flow stress was rather high, m = 0.6, which is typical to the superplastic behavior. The equiaxed morphology of grains and particles retained after the superplastic deformation, although some grain/particle growth was observed.

Physical Review B, 2013

The atomic structure of the supercooled liquid has often been discussed as a key source of glass ... more The atomic structure of the supercooled liquid has often been discussed as a key source of glass formation in metals. The presence of icosahedrally-coordinated clusters and their tendency to form networks have been identified as one possible structural trait leading to glass forming ability in the Cu-Zr binary system. In this work, we show that this theory is insufficient to explain glass formation at all compositions in that binary system. Instead, we propose that the formation of ideally-packed clusters at the expense of atomic arrangements with excess or deficient free volume can explain glass-forming by a similar mechanism. We show that this behavior is reflected in the structural relaxation of a metallic glass during constant pressure cooling and the time evolution of structure at a constant volume. We then demonstrate that this theory is sufficient to explain slowed diffusivity in compositions across the range of Cu-Zr metallic glasses.

Metallurgical and Materials Transactions A, 1996

Metallurgical and Materials Transactions A, 2005

The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the... more The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the direct chill (DC) cast ingots of developmental Al-Zn-Mg-Cu alloys has been evaluated in this work. The properties in both the longitudinal and transverse directions were determined in as-cast and cast-plusheat-treated conditions, at room temperature (RT) and cryogenic temperature (CT). Extensive microstructural evaluation was carried out using optical microscopy and scanning electron microscopy, including orientation image microscopy (OIM) by the electron backscatter diffraction (EBSD) technique. The Sc-containing developmental cast alloys showed the tensile properties, which are much better than the properties of commercial cast Al alloys and are similar or even superior to the properties of 7075-T6 alloy forgings. The microstructural evolution, the strengthening mechanisms, the optimum content of the dispersoid-forming elements, and the processing-structure-property correlations are discussed.

Materials Science and Engineering: A, 2013

Using Government drawings, specifications, or other data included in this document for any purpos... more Using Government drawings, specifications, or other data included in this document for any purpose other than Government procurement does not in any way obligate the U.S. Government. The fact that the Government formulated or supplied the drawings, specifications, or other data does not license the holder or any other person or corporation; or convey any rights or permission to manufacture, use, or sell any patented invention that may relate to them. Qualified requestors may obtain copies of this report from the Defense Technical Information Center (DTIC) (http://www.dtic.mil).

JOM, 2002

Thermohydrogen processing, a technique in which hydrogen is used as a temporary alloying element ... more Thermohydrogen processing, a technique in which hydrogen is used as a temporary alloying element in titanium, allows enhanced microstructural control and improved mechanical properties. This paper provides an improved definition of the phase transformations taking place in the Ti-6Al-4V-hydrogen system, which should lead to increased application of the technique.

Journal of Materials Science, 1992

ABSTRACT The TiAl intermetallic compound was used to illustrate an approach which enables the cre... more ABSTRACT The TiAl intermetallic compound was used to illustrate an approach which enables the creation of a submicrocrystalline structure (d0.1 m) in massive semifinished products made of hard-to-deform materials by means of their deformation at elevated temperatures. Tensile mechanical properties of the TiAl intermetallic compound with a mean grain size of 0.4 m were tested. In this state, the lower temperature limit of superplasticity in TiAl was found to be 800C. At this temperature and at an initial strain rate of 8.310–4s–1, the relative elongation to rupture attains 225%.

Journal of Materials Science, 2012

Using Government drawings, specifications, or other data included in this document for any purpos... more Using Government drawings, specifications, or other data included in this document for any purpose other than Government procurement does not in any way obligate the U.S. Government. The fact that the Government formulated or supplied the drawings, specifications, or other data does not license the holder or any other person or corporation; or convey any rights or permission to manufacture, use, or sell any patented invention that may relate to them. This report was cleared for public release by the USAF 88 th Air Base Wing (88 ABW) Public Affairs Office and is available to the general public, including foreign nationals. Copies may be obtained from the Defense Technical Information Center (DTIC) (http://www.dtic.mil).

Journal of Materials Processing Technology, 2001

A Ti–6Al–4V alloy sheet with a submicrocrystalline structure (grain size of 0.3μm) was produced b... more A Ti–6Al–4V alloy sheet with a submicrocrystalline structure (grain size of 0.3μm) was produced by multistep isothermal forging followed by flat rolling at temperatures below 650°C. The sheet showed a very strong basal texture and isotropic properties in the sheet plane. Room temperature yield strength, ultimate tensile strength, and fatigue limit of the sheet were 1.2–1.3 times higher than that

Journal of Electronic Materials, 2001

Applied Surface Science, 2011

A fully dense nanocrystalline compact of the Ti-47Al-3Cr (at. %) alloy was produced by mechanical... more A fully dense nanocrystalline compact of the Ti-47Al-3Cr (at. %) alloy was produced by mechanical alloying and hot isostatic pressing at 725 ± C. Microstructure characteristics and grain growth behavior of this compact were studied after annealing for up to 800 h in the temperature range of 725 to 1200 ± C, using analytical transmission electron microscopy techniques. The temperature and time dependencies of the grain sizes and the grain size distributions were determined. The grain growth occurred, with a timeand temperature-invariant single-peak grain size distribution (when normalized by the mean grain size), which was consistent with normal grain growth. The experimentally measured grain growth exponent decreased from 10 to 4.6 when the temperature was increased. The grain growth kinetics was described by a single thermally activated rate process limited by a permanent pinning force on the grain boundaries. The microhardness decreased on annealing and followed the Hall-Petch relationship with the parameters H yo 5.8 GPa and K H 1.6 MPa ? m 0.5 .

Applied Physics Letters, 2012

Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where ... more Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order.

ABSTRACT Two fully dense nanocrystalline TiAl-based compacts, Ti-47.5Al-3Cr and Ti-48Al-2Nb-2Cr, ... more ABSTRACT Two fully dense nanocrystalline TiAl-based compacts, Ti-47.5Al-3Cr and Ti-48Al-2Nb-2Cr, were synthesized by hot isostatic pressing (HIP'ing) from mechanically alloyed (MA'd) powders. Microstructure evolution and phase transformations after MA'ing, HIP'ing, and following annealing were studied. Formation of amorphous phases was detected in both powders after 15 hour MA'ing. During HIP'ing, crystallization of the amorphous phases occurred and very fine equiaxed gamma-TiAl grains were formed. The grain size decreased when the temperature of HIP'ing was decreased, and it was 42 nm after HIP'ing at 725 C. Grain growth occurred during annealing of the HIP'd alloys, which could be described by a single thermally activated process limited by a permanent pinning force. The activation energy of the grain growth was close to the activation energy for lattice diffusion of Ti in TiAl. Microhardness decreased with the annealing time and followed the Hall-Perch dependence on the grain size.

ABSTRACT The production cost of titanium and its alloys can be reduced considerably by synthesizi... more ABSTRACT The production cost of titanium and its alloys can be reduced considerably by synthesizing them from precursor compounds, by direct chemical reduction at ambient temperatures. In the present work, mechanochemical processing has been employed to synthesize Ti, Ti-Al, Ti-V, and Ti-Al-V in powder form by chemically reducing combinations of the respective metal chlorides with Mg or CaH2. Use of CaH2 for the reduction reactions resulted in the formation of hydrides of titanium and its alloys. The hydrided powders are passive against oxidation during leaching. In an attempt to reduce the cost of titanium production even further, studies of the direct reduction of TiO2 have been carried out. In all these processes complete removal of by-product salts from metal powder agglomerates has still to be achieved along with a minimization of interstitial pick-up during milling.

Metallurgical and Materials Transactions A, 2012

A coupled thermal/material flow model of friction stir welding was developed and applied to the j... more A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanis...