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

Papers by Oleg Senkov

Scientific Reports, 2021

Two classical criteria, by Pugh and Pettifor, have been widely used by metallurgists to predict w... more Two classical criteria, by Pugh and Pettifor, have been widely used by metallurgists to predict whether a material will be brittle or ductile. A phenomenological correlation by Pugh between metal brittleness and its shear modulus to bulk modulus ratio was established more than 60 years ago. Nearly four decades later Pettifor conducted a quantum mechanical analysis of bond hybridization in a series of intermetallics and derived a separate ductility criterion based on the difference between two single-crystal elastic constants, C12–C44. In this paper, we discover the link between these two criteria and show that they are identical for materials with cubic crystal structures.

Journal of Non-Crystalline Solids, 2009

The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

Intermetallics, 2011

The structures of Ca 60 Mg 15 Cu 25 , Ca 60 Mg 20 Cu 20 and Ca 60 Mg 25 Cu 15 metallic glasses ha... more The structures of Ca 60 Mg 15 Cu 25 , Ca 60 Mg 20 Cu 20 and Ca 60 Mg 25 Cu 15 metallic glasses have been investigated by neutron and X-ray diffraction. The correlation functions show a peak manifold in the region 2.2-4.5 Å, arising from the various atom pairs for these glasses. The results show clearly that there are contacts between the solute atoms (Cu and Mg) which agree with a simple estimate of the maximum solute atom fraction beyond which solute-solute contact becomes topologically necessary. The Cu-Mg and Cu-Ca distances are consistent with the sum of covalent radii, whereas all other interatomic distances are consistent with the sum of metallic radii. The neutron and X-ray diffraction correlation functions were simultaneously fitted with a series of symmetric peaks to obtain coordination numbers and interatomic distances, but only the results from the first two peaks, Cu-Cu and Cu-Mg, are of reasonable reliability. The Percus-Yevick approximation for binary hard sphere systems has been used to simulate the results. This shows that the results of fits to the Cu-Cu and Cu-Mg correlations give a reasonable description for atoms which are in close contact, but do not include an additional contribution at longer distance due to atoms which are almost in contact. The fits to the Cu-Ca peak are strongly affected by overlaps with both the longer distance peaks used in the fit, and the broad trailing edge in the distributions of interatomic distances that is not taken into account by fitting with symmetric peaks. Final results show that copper has a total of 6.5 neighbours which are in close contact, but has a total coordination number of about 12-13, when atoms which are almost in contact are included.

Glass-forming ability and thermal stability of Nd70−Fe20Al10Y alloys

Acta Materialia, 2002

The glass-forming ability (GFA) and thermal stability of Nd70−xFe20Al10Yx (0≤x≤15) alloys produce... more The glass-forming ability (GFA) and thermal stability of Nd70−xFe20Al10Yx (0≤x≤15) alloys produced by melt spinning and copper-mold casting have been investigated. Ribbon samples in the composition range show a fully amorphous structure. Except Y=5 at.%, bulk amorphous alloys at least 2 mm in diameter were obtained. Both the amorphous ribbon and the bulk cylinder for the Nd55Fe20Al10Y15 alloy show a

Metals, 2022

Metallic materials with extreme and often unusual combinations of properties are always in high d... more Metallic materials with extreme and often unusual combinations of properties are always in high demand in the competitive world market [...]

Entropy, 2018

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and f... more We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, cont...

Entropy, 2016

Microstructure, phase composition and mechanical properties of a refractory high entropy superall... more Microstructure, phase composition and mechanical properties of a refractory high entropy superalloy, AlMo 0.5 NbTa 0.5 TiZr, are reported in this work. The alloy consists of a nano-scale mixture of two phases produced by the decomposition from a high temperature body-centered cubic (BCC) phase. The first phase is present in the form of cuboidal-shaped nano-precipitates aligned in rows along <100>-type directions, has a disordered BCC crystal structure with the lattice parameter a 1 = 326.9 ˘0.5 pm and is rich in Mo, Nb and Ta. The second phase is present in the form of channels between the cuboidal nano-precipitates, has an ordered B2 crystal structure with the lattice parameter a 2 = 330.4 ˘0.5 pm and is rich in Al, Ti and Zr. Both phases are coherent and have the same crystallographic orientation within the former grains. The formation of this modulated nano-phase structure is discussed in the framework of nucleation-and-growth and spinodal decomposition mechanisms. The yield strength of this refractory high entropy superalloy is superior to the yield strength of Ni-based superalloys in the temperature range of 20 ˝C to 1200 ˝C.

Journal of Materials Engineering and Performance, 2007

The effect of Nd addition on the microstructure and mechanical properties of a die-cast AZ91 allo... more The effect of Nd addition on the microstructure and mechanical properties of a die-cast AZ91 alloy was investigated in the present work. The results show that the die-cast AZ91 alloy is composed of a-Mg matrix and c-Mg 17 Al 12 phase. Nd addition into the AZ91 alloy leads to the formation of rare earth containing intermetallic phase. Al 4 Nd phase forms when Nd content is less than or equal to 1.0 wt.%. Al 2 Nd phase appears simultaneously when Nd content reaches to 3.0 wt.%. The size and volume fraction of c-Mg 17 Al 12 phase decrease, because of the newly formed Al-Nd phase. And the c-Mg 17 Al 12 phase distributes from reticular to dispersive. Nd addition has a little effect on the room temperature properties of the die-cast AZ91 alloy, but greatly improves the elevated temperature properties. The tensile strength of AZ91-0.5Nd and AZ91-1.0Nd alloy tested at 150 °C is even close to the room temperature strength. The AZ91-1.0Nd alloy has the optimal properties. Keywords die-cast magnesium alloys, microstructure, elevated temperature properties, Nd

Acta Materialia, 2019

Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex ... more Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, T m ) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000°C and 1200°C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 T m , so that the alloys with higher T m retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000°C is probably due to dissolution of secondary phases.

Journal of Materials Research, 2018

Open literature publications, in the period from 2010 to the end of January 2018, on refractory h... more Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

Entropy, 2018

This study examines one of the limitations of CALPHAD databases when applied to high entropy allo... more This study examines one of the limitations of CALPHAD databases when applied to high entropy alloys and complex concentrated alloys. We estimate the level of the thermodynamic description, which is still sufficient to correctly predict thermodynamic properties of quaternary alloy systems, by comparing the results of CALPHAD calculations where quaternary phase space is extrapolated from binary descriptions to those resulting from complete binary and ternary interaction descriptions. Our analysis has shown that the thermodynamic properties of a quaternary alloy can be correctly predicted by direct extrapolation from the respective fully assessed binary systems (i.e., without ternary descriptions) only when (i) the binary miscibility gaps are not present, (ii) binary intermetallic phases are not present or present in a few quantities (i.e., when the system has low density of phase boundaries), and (iii) ternary intermetallic phases are not present. Because the locations of the phase bo...

Journal of Alloys and Compounds, 2016

A simple thermodynamic criterion is proposed to predict the presence or absence of equilibrium in... more A simple thermodynamic criterion is proposed to predict the presence or absence of equilibrium intermetallic phases in a high entropy alloy at a given temperature T. The criterion was verified using 45 currently available HEAs, for which equilibrium phases and respective annealing temperature are reported. The present model shows good correlation with experiment and gives an improved ability to predict formation of solid solution and intermetallic phases compared to earlier models.

Nature communications, 2015

Recent multi-principal element, high entropy alloy (HEA) development strategies vastly expand the... more Recent multi-principal element, high entropy alloy (HEA) development strategies vastly expand the number of candidate alloy systems, but also pose a new challenge-how to rapidly screen thousands of candidate alloy systems for targeted properties. Here we develop a new approach to rapidly assess structural metals by combining calculated phase diagrams with simple rules based on the phases present, their transformation temperatures and useful microstructures. We evaluate over 130,000 alloy systems, identifying promising compositions for more time-intensive experimental studies. We find the surprising result that solid solution alloys become less likely as the number of alloy elements increases. This contradicts the major premise of HEAs-that increased configurational entropy increases the stability of disordered solid solution phases. As the number of elements increases, the configurational entropy rises slowly while the probability of at least one pair of elements favouring formation...

Physical Review B, 2010

The amorphous structure of four Ca 60 Mg X Zn 40-X (X=10, 15, 20, and 25 at. %) ternary metallic ... more The amorphous structure of four Ca 60 Mg X Zn 40-X (X=10, 15, 20, and 25 at. %) ternary metallic glasses (MGs) has been investigated by neutron and x-ray diffraction, using Reverse Monte Carlo modeling to simulate the results. A critical analysis of the resultant models, corroborated by ab initio molecular-dynamics simulations, indicate that the glass structure for this system can be described as a mixture of Mg-and Zn-centered clusters, with Ca dominating in the first coordination shell of these clusters. A coordination number (CN) of 10 [with about 7 Ca and 3(Mg+Zn) atoms] is most common for the Zn-centered clusters. CN=11 and 12 [with about 7-8 Ca and 4 (Mg+Zn) atoms] are most common for Mg-centered clusters. Fivefold bond configurations (pentagonal pyramids) dominate (~60%) in the first coordination shell of the clusters, suggesting dense atomic packing. Bond-angle distributions suggest near-equilateral triangles and pentagonal bipyramids to be the most common nearest atom configurations. This is the systematic characterization of the structure of Ca-Mg-Zn MGs, a category of bulk MGs with interesting properties and intriguing applications. It is also the experimental verification of the principle of efficient packing of solute-centered clusters in ternary MGs. 15. SUBJECT TERMS bulk metallic glasses (BMGs), molecular-dynamics simulations, phase diagrams 16. SECURITY CLASSIFICATION OF: 17. LIMITATION

Journal of Materials Engineering and Performance, 2015

Microstructure, tensile properties, and fracture behavior of the inertia friction weld joints of ... more Microstructure, tensile properties, and fracture behavior of the inertia friction weld joints of dissimilar superalloys, cast Mar-M247 and wrought LSHR, were studied to assess the weld quality. Tensile tests were conducted at 23 and 704 °C on the samples containing different areas of the weld interface of the same welded material. The stress-strain curves were registered at different axial distances from the weld interface. In all tested samples, plastic deformation was localized on Mar-M247 side, outside the heat-affected zone (HAZ), and the resistance to plastic deformation of Mar-M247 increased with a decrease in the distance from the weld interface inside HAZ. Only elastic deformation occurred on the LSHR side. Fracture occurred on the Mar-M247 side, outside HAZ, or at the weld interface. In the latter case, welding defects in the form of clusters of nanometer-sized oxide and carbide particles were observed at the fracture surfaces. These results revealed that the IFW process is capable of producing the weld joints between Mar-M247 and LSHR with the fracture strength higher than that of Mar-M247. However, optimization of the IFW processing parameters is required to minimize clustering of oxide/carbide particles at the weld interface in this alloy pair.

Scripta Materialia, 2002

The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. -YY) 2. REPORT TYPE 3. DATES COVERED (From -To) February 2002 Journal Article 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER CRYSTALLIZATION KINETICS OF AN AMORPHOUS TiAl SHEET F33615-01-C-5214 PRODUCED BY PVD 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 61102F 6. AUTHOR(S) 5d. PROJECT NUMBER 0. N. Senkov (UES, Inc.) 2306 M. D. Uchic (AFRL/MLLMD) 5e. TASK NUMBER S. Menon (UES, Inc.) A3 D. B. Miracle (AFRL/MLLMD) 5f. WORK UNIT NUMBER 02 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Metals Branch, Metals Development Section (AFRL/MLLMD) REPORT NUMBER Metals, Ceramics & NDE Division Materials and Manufacturing Directorate

Physical Review B, 2006

A thermodynamic analysis of the onset driving force for crystallization of super-cooled liquid ha... more A thermodynamic analysis of the onset driving force for crystallization of super-cooled liquid has been conducted to explain strong composition dependency of the glass forming ability in the Ca-Mg-Zn ternary alloy system observed experimentally. In addition to the onset driving force, other energetic and kinetic factors are discussed to explain the observed glass stabilities. The universality of this method is discussed, thus establishing an analytic approach for determining the most stable glass in a given alloy system.

Metallurgical and Materials Transactions A, 2012

The atomic and electronic structures of several Ca-Mg-TM amorphous alloys (TM is Cu or Zn) have b... more The atomic and electronic structures of several Ca-Mg-TM amorphous alloys (TM is Cu or Zn) have been analyzed using ab initio molecular dynamics simulation and neutron diffraction. Partial pair distribution functions have been produced and the pair bond distances and partial coordination numbers have been reported for these alloys. Similarities and differences in the amorphous structures of the Ca-Mg-Cu and Ca-Mg-Zn alloys have been discussed. Strong interactions between Ca-Cu, Mg-Cu and Ca-Zn atom pairs rooted from the orbital hybridization of the s-p-d electrons have been recognized to result in noticeable shortening of respective atom pair bond distances and pronounced chemical short range ordering near the TM atoms. Voronoi tessellation analysis has shown that the polytetrahedral-type clusters and fivecoordinated atom pairs dominate in the amorphous structures, which indicates that tetrahedra and pentagonal bi-pyramids are the main building blocks in these amorphous alloys.

Metallurgical and Materials Transactions A, 2009

SAR 18. NUMBER OF PAGES 9 19a. NAME OF RESPONSIBLE PERSON (Monitor) a. REPORT

Metallurgical and Materials Transactions A, 2007

Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of... more Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of BMGs. Many of them have excellent GFA, which can be related to their efficient atomic packing, low onset driving force for crystallization, and high viscosity (high relaxation time) of the supercooled liquid. The Ca-based glasses have the lowest density and elastic moduli among all BMGs discovered to date. Unfortunately, as many other glasses, Ca-based BMGs are brittle below the glass transition temperature, and they also have marginal oxidation and corrosion resistance. The latter can be improved by proper selection of alloying elements. In this article, we review recent work on the development of low-density Ca-based BMGs and discuss the effect of alloy composition on the thermal, physical, and chemical properties of these glasses.

Scientific Reports, 2021

Two classical criteria, by Pugh and Pettifor, have been widely used by metallurgists to predict w... more Two classical criteria, by Pugh and Pettifor, have been widely used by metallurgists to predict whether a material will be brittle or ductile. A phenomenological correlation by Pugh between metal brittleness and its shear modulus to bulk modulus ratio was established more than 60 years ago. Nearly four decades later Pettifor conducted a quantum mechanical analysis of bond hybridization in a series of intermetallics and derived a separate ductility criterion based on the difference between two single-crystal elastic constants, C12–C44. In this paper, we discover the link between these two criteria and show that they are identical for materials with cubic crystal structures.

Journal of Non-Crystalline Solids, 2009

The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

Intermetallics, 2011

The structures of Ca 60 Mg 15 Cu 25 , Ca 60 Mg 20 Cu 20 and Ca 60 Mg 25 Cu 15 metallic glasses ha... more The structures of Ca 60 Mg 15 Cu 25 , Ca 60 Mg 20 Cu 20 and Ca 60 Mg 25 Cu 15 metallic glasses have been investigated by neutron and X-ray diffraction. The correlation functions show a peak manifold in the region 2.2-4.5 Å, arising from the various atom pairs for these glasses. The results show clearly that there are contacts between the solute atoms (Cu and Mg) which agree with a simple estimate of the maximum solute atom fraction beyond which solute-solute contact becomes topologically necessary. The Cu-Mg and Cu-Ca distances are consistent with the sum of covalent radii, whereas all other interatomic distances are consistent with the sum of metallic radii. The neutron and X-ray diffraction correlation functions were simultaneously fitted with a series of symmetric peaks to obtain coordination numbers and interatomic distances, but only the results from the first two peaks, Cu-Cu and Cu-Mg, are of reasonable reliability. The Percus-Yevick approximation for binary hard sphere systems has been used to simulate the results. This shows that the results of fits to the Cu-Cu and Cu-Mg correlations give a reasonable description for atoms which are in close contact, but do not include an additional contribution at longer distance due to atoms which are almost in contact. The fits to the Cu-Ca peak are strongly affected by overlaps with both the longer distance peaks used in the fit, and the broad trailing edge in the distributions of interatomic distances that is not taken into account by fitting with symmetric peaks. Final results show that copper has a total of 6.5 neighbours which are in close contact, but has a total coordination number of about 12-13, when atoms which are almost in contact are included.

Glass-forming ability and thermal stability of Nd70−Fe20Al10Y alloys

Acta Materialia, 2002

The glass-forming ability (GFA) and thermal stability of Nd70−xFe20Al10Yx (0≤x≤15) alloys produce... more The glass-forming ability (GFA) and thermal stability of Nd70−xFe20Al10Yx (0≤x≤15) alloys produced by melt spinning and copper-mold casting have been investigated. Ribbon samples in the composition range show a fully amorphous structure. Except Y=5 at.%, bulk amorphous alloys at least 2 mm in diameter were obtained. Both the amorphous ribbon and the bulk cylinder for the Nd55Fe20Al10Y15 alloy show a

Metals, 2022

Metallic materials with extreme and often unusual combinations of properties are always in high d... more Metallic materials with extreme and often unusual combinations of properties are always in high demand in the competitive world market [...]

Entropy, 2018

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and f... more We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, cont...

Entropy, 2016

Microstructure, phase composition and mechanical properties of a refractory high entropy superall... more Microstructure, phase composition and mechanical properties of a refractory high entropy superalloy, AlMo 0.5 NbTa 0.5 TiZr, are reported in this work. The alloy consists of a nano-scale mixture of two phases produced by the decomposition from a high temperature body-centered cubic (BCC) phase. The first phase is present in the form of cuboidal-shaped nano-precipitates aligned in rows along <100>-type directions, has a disordered BCC crystal structure with the lattice parameter a 1 = 326.9 ˘0.5 pm and is rich in Mo, Nb and Ta. The second phase is present in the form of channels between the cuboidal nano-precipitates, has an ordered B2 crystal structure with the lattice parameter a 2 = 330.4 ˘0.5 pm and is rich in Al, Ti and Zr. Both phases are coherent and have the same crystallographic orientation within the former grains. The formation of this modulated nano-phase structure is discussed in the framework of nucleation-and-growth and spinodal decomposition mechanisms. The yield strength of this refractory high entropy superalloy is superior to the yield strength of Ni-based superalloys in the temperature range of 20 ˝C to 1200 ˝C.

Journal of Materials Engineering and Performance, 2007

The effect of Nd addition on the microstructure and mechanical properties of a die-cast AZ91 allo... more The effect of Nd addition on the microstructure and mechanical properties of a die-cast AZ91 alloy was investigated in the present work. The results show that the die-cast AZ91 alloy is composed of a-Mg matrix and c-Mg 17 Al 12 phase. Nd addition into the AZ91 alloy leads to the formation of rare earth containing intermetallic phase. Al 4 Nd phase forms when Nd content is less than or equal to 1.0 wt.%. Al 2 Nd phase appears simultaneously when Nd content reaches to 3.0 wt.%. The size and volume fraction of c-Mg 17 Al 12 phase decrease, because of the newly formed Al-Nd phase. And the c-Mg 17 Al 12 phase distributes from reticular to dispersive. Nd addition has a little effect on the room temperature properties of the die-cast AZ91 alloy, but greatly improves the elevated temperature properties. The tensile strength of AZ91-0.5Nd and AZ91-1.0Nd alloy tested at 150 °C is even close to the room temperature strength. The AZ91-1.0Nd alloy has the optimal properties. Keywords die-cast magnesium alloys, microstructure, elevated temperature properties, Nd

Acta Materialia, 2019

Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex ... more Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, T m ) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000°C and 1200°C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 T m , so that the alloys with higher T m retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000°C is probably due to dissolution of secondary phases.

Journal of Materials Research, 2018

Open literature publications, in the period from 2010 to the end of January 2018, on refractory h... more Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

Entropy, 2018

This study examines one of the limitations of CALPHAD databases when applied to high entropy allo... more This study examines one of the limitations of CALPHAD databases when applied to high entropy alloys and complex concentrated alloys. We estimate the level of the thermodynamic description, which is still sufficient to correctly predict thermodynamic properties of quaternary alloy systems, by comparing the results of CALPHAD calculations where quaternary phase space is extrapolated from binary descriptions to those resulting from complete binary and ternary interaction descriptions. Our analysis has shown that the thermodynamic properties of a quaternary alloy can be correctly predicted by direct extrapolation from the respective fully assessed binary systems (i.e., without ternary descriptions) only when (i) the binary miscibility gaps are not present, (ii) binary intermetallic phases are not present or present in a few quantities (i.e., when the system has low density of phase boundaries), and (iii) ternary intermetallic phases are not present. Because the locations of the phase bo...

Journal of Alloys and Compounds, 2016

A simple thermodynamic criterion is proposed to predict the presence or absence of equilibrium in... more A simple thermodynamic criterion is proposed to predict the presence or absence of equilibrium intermetallic phases in a high entropy alloy at a given temperature T. The criterion was verified using 45 currently available HEAs, for which equilibrium phases and respective annealing temperature are reported. The present model shows good correlation with experiment and gives an improved ability to predict formation of solid solution and intermetallic phases compared to earlier models.

Nature communications, 2015

Recent multi-principal element, high entropy alloy (HEA) development strategies vastly expand the... more Recent multi-principal element, high entropy alloy (HEA) development strategies vastly expand the number of candidate alloy systems, but also pose a new challenge-how to rapidly screen thousands of candidate alloy systems for targeted properties. Here we develop a new approach to rapidly assess structural metals by combining calculated phase diagrams with simple rules based on the phases present, their transformation temperatures and useful microstructures. We evaluate over 130,000 alloy systems, identifying promising compositions for more time-intensive experimental studies. We find the surprising result that solid solution alloys become less likely as the number of alloy elements increases. This contradicts the major premise of HEAs-that increased configurational entropy increases the stability of disordered solid solution phases. As the number of elements increases, the configurational entropy rises slowly while the probability of at least one pair of elements favouring formation...

Physical Review B, 2010

The amorphous structure of four Ca 60 Mg X Zn 40-X (X=10, 15, 20, and 25 at. %) ternary metallic ... more The amorphous structure of four Ca 60 Mg X Zn 40-X (X=10, 15, 20, and 25 at. %) ternary metallic glasses (MGs) has been investigated by neutron and x-ray diffraction, using Reverse Monte Carlo modeling to simulate the results. A critical analysis of the resultant models, corroborated by ab initio molecular-dynamics simulations, indicate that the glass structure for this system can be described as a mixture of Mg-and Zn-centered clusters, with Ca dominating in the first coordination shell of these clusters. A coordination number (CN) of 10 [with about 7 Ca and 3(Mg+Zn) atoms] is most common for the Zn-centered clusters. CN=11 and 12 [with about 7-8 Ca and 4 (Mg+Zn) atoms] are most common for Mg-centered clusters. Fivefold bond configurations (pentagonal pyramids) dominate (~60%) in the first coordination shell of the clusters, suggesting dense atomic packing. Bond-angle distributions suggest near-equilateral triangles and pentagonal bipyramids to be the most common nearest atom configurations. This is the systematic characterization of the structure of Ca-Mg-Zn MGs, a category of bulk MGs with interesting properties and intriguing applications. It is also the experimental verification of the principle of efficient packing of solute-centered clusters in ternary MGs. 15. SUBJECT TERMS bulk metallic glasses (BMGs), molecular-dynamics simulations, phase diagrams 16. SECURITY CLASSIFICATION OF: 17. LIMITATION

Journal of Materials Engineering and Performance, 2015

Microstructure, tensile properties, and fracture behavior of the inertia friction weld joints of ... more Microstructure, tensile properties, and fracture behavior of the inertia friction weld joints of dissimilar superalloys, cast Mar-M247 and wrought LSHR, were studied to assess the weld quality. Tensile tests were conducted at 23 and 704 °C on the samples containing different areas of the weld interface of the same welded material. The stress-strain curves were registered at different axial distances from the weld interface. In all tested samples, plastic deformation was localized on Mar-M247 side, outside the heat-affected zone (HAZ), and the resistance to plastic deformation of Mar-M247 increased with a decrease in the distance from the weld interface inside HAZ. Only elastic deformation occurred on the LSHR side. Fracture occurred on the Mar-M247 side, outside HAZ, or at the weld interface. In the latter case, welding defects in the form of clusters of nanometer-sized oxide and carbide particles were observed at the fracture surfaces. These results revealed that the IFW process is capable of producing the weld joints between Mar-M247 and LSHR with the fracture strength higher than that of Mar-M247. However, optimization of the IFW processing parameters is required to minimize clustering of oxide/carbide particles at the weld interface in this alloy pair.

Scripta Materialia, 2002

The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. -YY) 2. REPORT TYPE 3. DATES COVERED (From -To) February 2002 Journal Article 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER CRYSTALLIZATION KINETICS OF AN AMORPHOUS TiAl SHEET F33615-01-C-5214 PRODUCED BY PVD 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 61102F 6. AUTHOR(S) 5d. PROJECT NUMBER 0. N. Senkov (UES, Inc.) 2306 M. D. Uchic (AFRL/MLLMD) 5e. TASK NUMBER S. Menon (UES, Inc.) A3 D. B. Miracle (AFRL/MLLMD) 5f. WORK UNIT NUMBER 02 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Metals Branch, Metals Development Section (AFRL/MLLMD) REPORT NUMBER Metals, Ceramics & NDE Division Materials and Manufacturing Directorate

Physical Review B, 2006

A thermodynamic analysis of the onset driving force for crystallization of super-cooled liquid ha... more A thermodynamic analysis of the onset driving force for crystallization of super-cooled liquid has been conducted to explain strong composition dependency of the glass forming ability in the Ca-Mg-Zn ternary alloy system observed experimentally. In addition to the onset driving force, other energetic and kinetic factors are discussed to explain the observed glass stabilities. The universality of this method is discussed, thus establishing an analytic approach for determining the most stable glass in a given alloy system.

Metallurgical and Materials Transactions A, 2012

The atomic and electronic structures of several Ca-Mg-TM amorphous alloys (TM is Cu or Zn) have b... more The atomic and electronic structures of several Ca-Mg-TM amorphous alloys (TM is Cu or Zn) have been analyzed using ab initio molecular dynamics simulation and neutron diffraction. Partial pair distribution functions have been produced and the pair bond distances and partial coordination numbers have been reported for these alloys. Similarities and differences in the amorphous structures of the Ca-Mg-Cu and Ca-Mg-Zn alloys have been discussed. Strong interactions between Ca-Cu, Mg-Cu and Ca-Zn atom pairs rooted from the orbital hybridization of the s-p-d electrons have been recognized to result in noticeable shortening of respective atom pair bond distances and pronounced chemical short range ordering near the TM atoms. Voronoi tessellation analysis has shown that the polytetrahedral-type clusters and fivecoordinated atom pairs dominate in the amorphous structures, which indicates that tetrahedra and pentagonal bi-pyramids are the main building blocks in these amorphous alloys.

Metallurgical and Materials Transactions A, 2009

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Metallurgical and Materials Transactions A, 2007

Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of... more Ca-based bulk metallic glasses (BMGs) have unique properties and represent a new seventh group of BMGs. Many of them have excellent GFA, which can be related to their efficient atomic packing, low onset driving force for crystallization, and high viscosity (high relaxation time) of the supercooled liquid. The Ca-based glasses have the lowest density and elastic moduli among all BMGs discovered to date. Unfortunately, as many other glasses, Ca-based BMGs are brittle below the glass transition temperature, and they also have marginal oxidation and corrosion resistance. The latter can be improved by proper selection of alloying elements. In this article, we review recent work on the development of low-density Ca-based BMGs and discuss the effect of alloy composition on the thermal, physical, and chemical properties of these glasses.