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Dean L. Jacobson - One of the best experts on this subject based on the ideXlab platform.
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Sub-surface Iridium depletion in dilute solution tungsten-Iridium Alloys due to high temperature work function testing
International Journal of Refractory Metals & Hard Materials, 1992Co-Authors: L.a.d. Cruz, Dean L. JacobsonAbstract:Abstract Dilute solution tungsten-Iridium Alloys with less than 2 wt % Ir have been found to exhibit the highest work functions among candidate Alloys for thermionic emitters. Recent studies have investigated the effect of Iridium on the work function of tungsten-Iridium Alloys fabricated either by arc-melting or sintering. In the present paper, an attempt has been made to understand the interrelationship between work function, composition and microstructure in these Alloys. The ultimate aim was to ascertain the role of Iridium in the modification of the work function. The effective work function was determined using a vacuum emission vehicle (VEV) employing the thermionic method. Electron probe microanalysis was used to characterize the sub-surface Iridium depletion caused by elevated temperature testing. Theoretical estimates of the equilibrium segregation of Iridium in tungsten could not adequately account for the magnitude of Iridium depletion found at the electrode surfaces. An alternative explanation for the sub-surface Iridium depletion has been proposed and involves the creation of Iridium-rich surface layers during the pre-test heat treatment of the Alloys.
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ultrahigh temperature tensile properties of arc melted tungsten and tungsten Iridium Alloys
Scripta Metallurgica Et Materialia, 1991Co-Authors: Anhua Luo, Kwang S. Shin, Dean L. JacobsonAbstract:Tungsten is one of the most important metals for high temperature applications. The major deterrents to the use of tungsten are poor fabricability at room temperature and rapid decrease in strength at temperatures above 1600 K. Previous investigations have shown that the addition of rhenium to tungsten improves both room- temperature fabricability and high-temperature strength. Based on its similarity to rhenium's electron structure, Iridium as another potential alloying element in tungsten. It was recently confirmed that tungsten-Iridium Alloys with less than 1.0w/o Iridium exhibit better fabricability than tungsten-rhenium Alloys at room temperature. The strength properties of tungsten-Iridium Alloys at ultrahigh temperatures have not yet been researched. The present paper reports the tensile properties of dilute tungsten-Iridium Alloys in the temperature range 1600 to 2600 K. The focus of the present paper is to examine the effects of Iridium concentration and test temperature on the strength and fracture behavior of tungsten-Iridium Alloys at ultrahigh temperatures.
E P George - One of the best experts on this subject based on the ideXlab platform.
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effects of alloying elements on dendritic segregation in Iridium Alloys
Journal of Alloys and Compounds, 2008Co-Authors: Lee Heatherly, E P George, C T Liu, Yong LiuAbstract:Abstract The effects of alloying elements on dendritic segregation in Ir Alloys were studied by Auger Electron Spectroscopy (AES). The addition of 10 at.% Nb induces significant segregation of carbon and thorium to dendritic interfaces. The addition of 5 at.% Zr to the Ir alloy leads to the formation of an Ir 3 Zr intermetallic phase, which results in less dendritic segregation of carbon and thorium. This dendritic segregation appears to be linked to the severe cracking observed in the Ir–Nb alloy, but not in the Ir–Zr alloy, after casting and heat treatment. The mechanism for the dendritic segregation was discussed by consideration of the solidification process and the change of carbon solubility in Ir by Zr addition.
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impurity effects on high temperature tensile ductility of Iridium Alloys at high strain rate
Scripta Materialia, 1999Co-Authors: C G Mckamey, Evan Keith Ohriner, Lee Heatherly, E H Lee, E P George, J W CohronAbstract:The current study was undertaken to determine what effects, if any, larger amounts of certain impurities (Al,Cr,Fe,Ni, and Si) might have on the physical metallurgy and mechanical properties of the DOP-26 Iridium alloy. This report summarizes the effects of these impurities on grain growth behavior and high-temperature high-strain-rate tensile ductility. Comparisons are made to the grain growth behavior and high-strain-rate tensile properties of the DOP-26 alloy without intentional impurity additions.
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segregation of lutetium and yttrium to grain boundaries in Iridium Alloys
Acta Materialia, 1998Co-Authors: A N Gubbi, Evan Keith Ohriner, E P George, Ralph H. ZeeAbstract:The equilibrium segregation of lutetium and yttrium to grain boundaries in Iridium Alloys containing 0.3 wt% tungsten was studied. The segregation level of Lu decreased with increasing annealing temperature whereas that of Y initially increased with increasing temperature but then decreased with further increases in temperature. At all temperatures, Y segregated more strongly than lutetium. The observed segregation behavior of the two dopants was explained on the basis of their free energies of segregation and solid solubilities, and the solvus lines for Lu and Y were obtained from the results. Energies of segregation and energies of solution for Lu and Y in the Iridium alloy were also determined.
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Influence of cerium additions on high-temperature-impact ductility and fracture behavior of Iridium Alloys
Metallurgical and Materials Transactions A, 1997Co-Authors: A N Gubbi, Evan Keith Ohriner, E P George, Ralph H. ZeeAbstract:High-temperature tensile impact testing was carried out on Ir + 0.3 wt pct W Alloys doped with cerium and thorium individually, and with cerium and thorium together. Impact ductility was evaluated as a function of grain size and test temperature. Cerium by itself was not as effective as thorium in improving the grain boundary cohesion, even though it segregated more strongly than thorium to the grain boundaries. This lower grain boundary cohesion was responsible for lower impact ductility and higher brittle-to-ductile transition temperature of cerium-doped Alloys compared to those of the thorium- or thorium plus cerium-doped Alloys. Reduction in thorium content by a factor of 5 (from 50 to 10 appm) in the bulk did not result in any significant reduction in hightemperature impact ductility or an increase in the brittle-to-ductile transition temperature as long as sufficient cerium was added to provide grain refinement. Grain boundary strengths of thorium- and thorium plus cerium-doped Alloys were almost identical.
Evan Keith Ohriner - One of the best experts on this subject based on the ideXlab platform.
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Processing of Iridium and Iridium Alloys
Platinum Metals Review, 2008Co-Authors: Evan Keith OhrinerAbstract:Iridium and its Alloys have been considered to be difficult to fabricate due to their high melting temperatures, limited ductility, sensitivity to impurity content, and chemical properties. The variety of processing methods used for Iridium and its Alloys are reviewed, including purification, melting, forming, joining, and powder metallurgy techniques. Also included are coating and forming by the methods of electroplating, chemical and physical vapor deposition, and melt particle deposition.
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welding and weldability of thorium doped Iridium Alloys
International Symposium on Iridium Nashville TN March 12-15 2000, 2000Co-Authors: S.a. David, Evan Keith Ohriner, J.f. KingAbstract:Ir-0.3%W Alloys doped with thorium are currently used as post-impact containment material for radioactive fuel in thermoelectric generators that provide stable electrical power for a variety of outer planetary space exploration missions. Welding and weldability of a series of Alloys was investigated using arc and laser welding processes. Some of these Alloys are prone to severe hot-cracking during welding. Weldability of these Alloys was characterized using Sigmajig weldability test. Hot-cracking is influenced to a great extent by the fusion zone microstructure and composition. Thorium content and welding atmosphere were found to be very critical. The weld cracking behavior in these Alloys can be controlled by modifying the fusion zone microstructure. Fusion zone microstructure was found to be controlled by welding process, process parameters, and the weld pool shape.
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impurity effects on high temperature tensile ductility of Iridium Alloys at high strain rate
Scripta Materialia, 1999Co-Authors: C G Mckamey, Evan Keith Ohriner, Lee Heatherly, E H Lee, E P George, J W CohronAbstract:The current study was undertaken to determine what effects, if any, larger amounts of certain impurities (Al,Cr,Fe,Ni, and Si) might have on the physical metallurgy and mechanical properties of the DOP-26 Iridium alloy. This report summarizes the effects of these impurities on grain growth behavior and high-temperature high-strain-rate tensile ductility. Comparisons are made to the grain growth behavior and high-strain-rate tensile properties of the DOP-26 alloy without intentional impurity additions.
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segregation of lutetium and yttrium to grain boundaries in Iridium Alloys
Acta Materialia, 1998Co-Authors: A N Gubbi, Evan Keith Ohriner, E P George, Ralph H. ZeeAbstract:The equilibrium segregation of lutetium and yttrium to grain boundaries in Iridium Alloys containing 0.3 wt% tungsten was studied. The segregation level of Lu decreased with increasing annealing temperature whereas that of Y initially increased with increasing temperature but then decreased with further increases in temperature. At all temperatures, Y segregated more strongly than lutetium. The observed segregation behavior of the two dopants was explained on the basis of their free energies of segregation and solid solubilities, and the solvus lines for Lu and Y were obtained from the results. Energies of segregation and energies of solution for Lu and Y in the Iridium alloy were also determined.
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Influence of cerium additions on high-temperature-impact ductility and fracture behavior of Iridium Alloys
Metallurgical and Materials Transactions A, 1997Co-Authors: A N Gubbi, Evan Keith Ohriner, E P George, Ralph H. ZeeAbstract:High-temperature tensile impact testing was carried out on Ir + 0.3 wt pct W Alloys doped with cerium and thorium individually, and with cerium and thorium together. Impact ductility was evaluated as a function of grain size and test temperature. Cerium by itself was not as effective as thorium in improving the grain boundary cohesion, even though it segregated more strongly than thorium to the grain boundaries. This lower grain boundary cohesion was responsible for lower impact ductility and higher brittle-to-ductile transition temperature of cerium-doped Alloys compared to those of the thorium- or thorium plus cerium-doped Alloys. Reduction in thorium content by a factor of 5 (from 50 to 10 appm) in the bulk did not result in any significant reduction in hightemperature impact ductility or an increase in the brittle-to-ductile transition temperature as long as sufficient cerium was added to provide grain refinement. Grain boundary strengths of thorium- and thorium plus cerium-doped Alloys were almost identical.
L.a.d. Cruz - One of the best experts on this subject based on the ideXlab platform.
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Sub-surface Iridium depletion in dilute solution tungsten-Iridium Alloys due to high temperature work function testing
International Journal of Refractory Metals & Hard Materials, 1992Co-Authors: L.a.d. Cruz, Dean L. JacobsonAbstract:Abstract Dilute solution tungsten-Iridium Alloys with less than 2 wt % Ir have been found to exhibit the highest work functions among candidate Alloys for thermionic emitters. Recent studies have investigated the effect of Iridium on the work function of tungsten-Iridium Alloys fabricated either by arc-melting or sintering. In the present paper, an attempt has been made to understand the interrelationship between work function, composition and microstructure in these Alloys. The ultimate aim was to ascertain the role of Iridium in the modification of the work function. The effective work function was determined using a vacuum emission vehicle (VEV) employing the thermionic method. Electron probe microanalysis was used to characterize the sub-surface Iridium depletion caused by elevated temperature testing. Theoretical estimates of the equilibrium segregation of Iridium in tungsten could not adequately account for the magnitude of Iridium depletion found at the electrode surfaces. An alternative explanation for the sub-surface Iridium depletion has been proposed and involves the creation of Iridium-rich surface layers during the pre-test heat treatment of the Alloys.
Ralph H. Zee - One of the best experts on this subject based on the ideXlab platform.
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segregation of lutetium and yttrium to grain boundaries in Iridium Alloys
Acta Materialia, 1998Co-Authors: A N Gubbi, Evan Keith Ohriner, E P George, Ralph H. ZeeAbstract:The equilibrium segregation of lutetium and yttrium to grain boundaries in Iridium Alloys containing 0.3 wt% tungsten was studied. The segregation level of Lu decreased with increasing annealing temperature whereas that of Y initially increased with increasing temperature but then decreased with further increases in temperature. At all temperatures, Y segregated more strongly than lutetium. The observed segregation behavior of the two dopants was explained on the basis of their free energies of segregation and solid solubilities, and the solvus lines for Lu and Y were obtained from the results. Energies of segregation and energies of solution for Lu and Y in the Iridium alloy were also determined.
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Influence of cerium additions on high-temperature-impact ductility and fracture behavior of Iridium Alloys
Metallurgical and Materials Transactions A, 1997Co-Authors: A N Gubbi, Evan Keith Ohriner, E P George, Ralph H. ZeeAbstract:High-temperature tensile impact testing was carried out on Ir + 0.3 wt pct W Alloys doped with cerium and thorium individually, and with cerium and thorium together. Impact ductility was evaluated as a function of grain size and test temperature. Cerium by itself was not as effective as thorium in improving the grain boundary cohesion, even though it segregated more strongly than thorium to the grain boundaries. This lower grain boundary cohesion was responsible for lower impact ductility and higher brittle-to-ductile transition temperature of cerium-doped Alloys compared to those of the thorium- or thorium plus cerium-doped Alloys. Reduction in thorium content by a factor of 5 (from 50 to 10 appm) in the bulk did not result in any significant reduction in hightemperature impact ductility or an increase in the brittle-to-ductile transition temperature as long as sufficient cerium was added to provide grain refinement. Grain boundary strengths of thorium- and thorium plus cerium-doped Alloys were almost identical.
