Dean L. Jacobson

Solution softening mechanism of iridium and rhenium in tungsten at room temperature

Abstract With the highest melting temperature among metals, tungsten possesses a combination of properties desired for space nuclear power applications. One of the major deterrents to the application of tungsten lies in its low-temperature brittleness. The poor ductility of tungsten at room temperatures results in great difficulty in fabrication. Various approaches, including purification and alloying, have been employed to ductilize tungsten at room temperature. The fact that rhenium additions increase the ductility of tungsten was first reported by Geach and Hughes 1 in 1956, and following study confirmed that rhenium added to tungsten not only improves the room temperature fabricability, but also increases the high-temperature strength. The alloy system studied most in the past was tungsten-rhenium binary system, and the mechanical test conducted was limited to the bending test which was far from enough to examine the solution softening mechanism of rhenium in tungsten. The effect of rhenium on the room temperature mechanical properties of tungsten-rhenium-thoria ternary system has not been researched. Besides, based on its similarity to rhenium in electron structure, iridium (Ir) appears to be another potential softening element in tungsten. But so far, there are no mechanical data available for tungsten-iridium system, either at room temperature or at high temperatures. In the present study, a comprehensive investigation has been performed on the room-temperature hardness, tensile properties, microstructure and fracture behavior of various tungsten-iridium and tungsten-rhenium-thoria alloys. The objective of this study was to examine the solid-solution softening mechanism of iridium in unalloyed tungsten and that of rhenium in thoriated tungsten at room temperature. The effect of alloying elements on the fracture behavior of tungsten-iridium and tungsten-rhenium-thoria alloys was also examined.

High temperature tensile properties of WReThO2 alloys

The tensile properties of WRe1wt.%ThO2 alloys with rhenium concentrations of 0–26wt.% were examined in a temperature range 1600–2600 K. The effects of rhenium concentrations, testing temperature and strain rate on the strength properties and deformation behavior of these alloys were investigated. It was found that rhenium had a moderate strengthening effect of the thoriated tungsten at temperatures up to about 2000 K. In the temperature range 2000–2600 K, rhenium concentrations above 11 wt.% in the thoriated tungsten resulted in a strength degradation instead of strengthening owing to rhenium-introduced structural instability. The addition of rhenium increased both the temperature sensitivity and the strain rate sensitivity of the thoriated tungsten at high temperatures. The post-test specimens were characterized by scanning electron microscopy and transmission electron microscopy. The dominant deformation mechanism was the cross-slip of screw dislocations when deformation occurred in the temperature range 1600–2000 K and was grain boundary sliding when deformation occurred at temperatures above 2000 K. An examination on the fracture surfaces of the tensile-fractured specimens showed that the fracture mode of the WRe1wt.%ThO2 alloys experienced a transition from transgranular to intergranular with either increasing temperature or increasing rhenium concentrations.

High-temperature tensile properties of molybdenum and a molybdenum-0.5%hafnium carbide alloy

Molybdenum (Mo) has melting temperature of 2,890 K and good fabricability at room temperature. It also possesses a relatively low density (10.22 g/cm[sup 3]) as compared with other common refractory metals. The major deterrent to the implementation of Mo as a high-temperature structural material lies in its weak strength at temperatures above 0.5 T[sub m] (melting temperature of Mo in Kelvin). It has been found that dispersion strengthening is a very effective method of increasing the strength of refractory metals subjected to high temperatures. Hafnium carbide (HfC), a compound with a NaCl structure and the highest melting temperature (4,163 K) among all carbides, is an ideal strengthener to Mo. Previous study has shown that dispersed HfC particles could increase the recrystallization temperature of Mo by up to 600 K. Therefore, it is advantageous to develop HfC strengthened Mo for high-temperature applications.

Hafnium carbide strengthening in a tungsten-rhenium matrix at ultrahigh temperatures

Tungsten--rhenium--hafnium carbide (W---Re---HfC) alloy is the strongest metallic material at temperatures greater than 2000 K. In the this paper, the mechanical properties of tungsten and a W--3.6Re--0.26HfC alloy were determined from 1700 to 2980 K in a vacuum below 10[sup [minus]5] Pa. hfC particles had an exceptional strengthening effect n the tungsten-rhenium matrix at temperatures up to 2700 K. The strengthening was attributed to the high thermodynamic stability of HfC particles at ultrahigh temperatures. The growth behavior of HfC particles in the tungsten-rhenium matrix is examined. Carbon is found to be the rate-limiting element in the growth process of HfC particles. The strengthening mechanisms in a W--3.6Re--0.26HfC were discussed. It was concluded that the strength of a dispersion-strengthened material was proportional to the square root of the volume fraction of the particles. The calculation of a W--3.6Re--0.26HfC alloys yield strength, calculated based on the dislocation pinning and the particle statistical distribution, was in good agreement with the experimental data over the entire temperature range.

Creep behavior of molybdenum and a molybdenum-hafnium carbide alloy from 1600 to 2100 K

Abstract The creep behavior of arc-melted molybdenum-0.5% hafnium carbide and commercial purity molybdenum was evaluated at temperatures from 1600 to 2100 K while subjected to stresses of 10–60 MPa in a vacuum below 1.3 × 10 −6 Pa (1.0 × 10 −8 Torr). The effects of temperature and stress on the steady-state creep rate of these materials were examined. The stress exponent and activation energy for creep deformation were determined. The stress exponents for molybdenum-0.5% hafnium carbide and molybdenum were 4.1 and 2.27. The activation energies for molybdenum-0.5% hafnium carbide and molybdenum were determined to be 104 and 66 kcal mol −1 , respectively. The creep strength of molybdenum-0.5% hafnium carbon at a creep rate of 10 −6 was determined as a function of temperature and compared with that of molybdenum. Hafnium carbide particles were found to be effective in strengthening molybdenum at high temperatures. The steady-state creep rate of molybdenum-0.5% hafnium carbide was approximately two orders of magnitude lower than molybdenum and the creep strength of molybdenum-0.5% hafnium carbide was about two times greater than that of molybdenum over the entire temperature range. The microstructures of post-test molybdenum-0.5% hafnium carbide specimens were examined with a transmission electron microscope. The creep strength of molybdenum-0.5% hafnium carbide was correlated with its microstructures that developed during high temperature deformation. The results illustrate that the great creep resistance of this alloy was associated with the presence of HfC particles which retarded the movements of dislocations, resulting in a dispersion strengthening.

Ultrahigh temperature tensile properties of arc-melted tungsten and tungsten-iridium alloys

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 rheniums 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.

Thermionic work function of chemically vapor deposited rhenium on tungsten

The vacuum work function of chemically vapor deposited (CVD) rhenium was investigated. The thermionic emission technique was used in order to obtain high temperature data. Data were collected as a function of temperature and time, and the total pressure was recorded. X-ray analysis showed that the rhenium was preferentially deposited with the 0001 planes approximately parallel to the emitting surface. The effective work function data were compared with the results of Campbell et al. [1]. It was concluded that the residual gas pressure was responsible for conflicting work function data.

Molybdenum work function determined by electron emission microscopy

A polycrystalline molybdenum sample was recrystallized and thermally stabilized. Quantitative measurements of the emission from each individual grain were obtained with an electron emission microscope. The effective work function for each grain was then calculated. The crystallographic orientation of each grain was determined by Laue back-reflection techniques. A polar plot of effective work function vs crystallographic orientation for the sample was constructed to provide a correlation between effective work function and crystallographic orientation.

Effects of thoria particles on the high-temperature tensile properties of a W-26wt.%Re alloy

Abstract The tensile properties of powder-metallurgy-processed W-26wt.%Re and W-26wt.%Re-1wt.%ThO 2 were examined over the temperature range 1600–2600 K in a strain rate range 10 −3 –10 −2 s −1 . It was found that the addition of ThO 2 particles significantly reduced grain boundary migration and hence stabilized the microstructure of W-26Re at elevated temperatures. ThO 2 particle strengtheninge in W-26Re was effective up to 2200 K. The strengthening mechanisms were attributed to microstructure stabilization, misfit dilatation, and inhomogeneous deformation in the vicinity of ThO 2 particles. The addition of ThO 2 increased both temperature sensitivity and strain rate sensitivity of W-26Re at high temperatures. Post-test specimens were characterized by scanning electron microscopy and transmission electron microscopy. Both W-26Re-1ThO 2 and W-26Re exhibited an intergranular fracture mode when fractured at high temperatures.

Ultrahigh-temperature tensile properties of a powder metallurgy processed tungsten-3.6w/o rhenium-1.0w/o thoria alloy

Metals that have been dispersion hardened with stable oxides provide ideal systems to study the strengthening effect of second-phase particles at high temperatures. Thoria (ThO{sub 2}), a compound having the highest melting temperature among all the metallic oxides, is one of the most promising second-phase particles in strengthening tungsten at ultrahigh temperatures. On the other hand, tungsten has been severely limited due to the difficulties experienced in fabrication. An effective way to lower the ductile-brittle transition temperature of tungsten is to alloy with rhenium at a composition of about 4.0 weight percent rhenium. Therefore, it is necessary to add both thoria and rhenium to tungsten in order to improve the high-temperature strength and room-temperature fabricability. The present paper reports the tensile properties of a thoria particle strengthened W-Re alloy at ultrahigh temperatures. The present study was focused on the strengthening mechanism of dispersed thoria particles in the W-Re matrix and the deformation behavior of a power metallurgy processed W-Re-ThO{sub 2} alloy in a temperatures range of 1400 to 2600 K.