Tungsten-rhenium alloys

The rhenium ductilizing effect is more pronounced at higher Re additions (Fig. 6.1) and reaches a maximum at the solid solubility limit (approximately 27 wt% Re). Beyond that, embrittlement occurs due to the formation of a hard and brittle a-phase. The hardness of W-26 Re alloy is 20% larger than that of unalloyed tungsten and the hot tensile strength 

Compositions. The most important alloy compositions are W-(3-5)Re, W-lORe, and W- 25-26)Re. 

Thermocouples based on W-Re alloys can be used to measure temperatures up to 2300 °C and, for short times, up to 2700 °C (W-3Re/W-25Re W-5Re/W-26Re). 

Production. The classical method is to mix Re and W powders in the desired ratio prior to compaction and sintering. In order to achieve a more even W-Re distribution, restricting the danger of local a-phase formation, mixtures of tungstic acid or tungsten trioxide or tungsten blue oxide with ammonium perrhenate can be used as raw materials. These mixtures are co-reduced by hydrogen to metal powder. 

Tungsten-rhenium alloys are hot-swaged and hot-rolled in air and are well suited for shaping, including tube fabrication. 

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Rhenium hexafluoride is a cosdy (ca 3000/kg) material and is often used as a small percentage composite with tungsten or molybdenum. The addition of rhenium to tungsten metal improves the ductility and high temperature properties of metal films or parts (11). Tungsten—rhenium alloys produced by CVD processes exhibit higher superconducting transition temperatures than those alloys produced by arc-melt processes (12). 

Tungsten with the addition of as much as 5% thoria is used for thermionic emission cathode wires and as filaments for vibration-resistant incandescent lamps. Tungsten—rhenium alloys are employed as heating elements and thermocouples. Tantalum and niobium form continuous soHd solutions with tungsten. Iron and nickel are used as ahoy agents for specialized appHcations. 

The resistance wires, made of tungsten or a tungsten-rhenium alloy, are heated with a DC source to a temperature above the block temperature... 

The TCD is a differential detector that measures the thermal conductivity of the analyte in carrier gas, compared to the thermal conductivity of pure carrier gas. In a conventional detector at least two cell cavities are required, although a cell with four cavities is more common. The cavities are drilled into a metal block (usually stainless steel) and each contains a resistance wire or filament (so-called hot wires). The filaments are either mounted on holders, as shown in Figure 7.11, or are held concentrically in the cylindrical cavity, a design that permits the cell volume to be minimized. They are made of tungsten or a tungsten-rhenium alloy (so-called WX filaments) of high resistance. 

W.R. Holman, FJ. Huegel, CVD tungsten and tungsten-rhenium alloys for structural applications part I - process development, in Proceedings of the Conference on Chemical Vapor Deposition of Refractory Metals, Alloys and Compounds, American Nuclear Society, 1967, p. 127. 

Molybdenum hexafluoride is used in the manufacture of thin films (qv) for large-scale integrated circuits (qv) commonly known as LSIC systems (3,4), in the manufacture of metallised ceramics (see MetaL-MATRIX COMPOSITES) (5), and chemical vapor deposition of molybdenum and molybdenum—tungsten alloys (see Molybdenumand molybdenum alloys) (6,7). The latter process involves the reduction of gaseous metal fluorides by hydrogen at elevated temperatures to produce metals or their alloys such as molybdenum—tungsten, molybdenum—tungsten—rhenium, or molybdenum—rhenium alloys. 

Two alloys containing tungsten are commercially available. The first, containing about 3 wt % rhenium, is used for heating filaments. The rhenium contributes improved resistance to thermal and mechanical shock. The second alloy contains about 25 wt % rhenium. This latter alloy is sold as sheet, rod, and heavy wire and may be fabricated for various uses. An important use of these rhenium alloys is in the constmction of thermocouples. Various combinations, 3 wt % Re—97 wt % W, or 25 wt % Re—75 wt % W, are usehil for measurement of temperatures to 2500°C (see Temperaturemeasurement). 

Two different kinds of metals are found in chondrites. Small nuggets composed of highly refractory siderophile elements (iridium, osmium, ruthenium, molybdenum, tungsten, rhenium) occur within CAIs. These refractory alloys are predicted to condense at temperatures above 1600 from a gas of solar composition. Except for tungsten, they are also the expected residues of CAI oxidation. 

When rhenium is added to other refractory metals, such as molybdenum and tungsten, ductility and tensile strength are improved. These improvements persist even after heating above the rccrystallization temperature. An excellent example is the. complete, ductility shown by a molybdenum-rhenium fusion weld. Rhenium and rhenium alloys have gained some acceptance in semiconductor, thermocouple, and nuclear reactor applications. The alloys also axe used in gyroscopes, miniature rockets, electrical contacts, electronic-tube components, and thermionic converters. 

Tantalum-hafnium and tantalum-zirconium alloys are less suitable for aggressive acid environments. Compared with tantalum-tungsten alloys, tantalum rhenium alloys are superior in corrosion resistance and to hydrogen embrittlement, but the major disadvantage is the high cost of rhenium. 

Some of the materials that have been examined as catalysts include Pure Platinum, Platinum-Iridium Alloys, Various Compositions of Platinum-Rhodium Alloys, Platinum-Palladium Alloys, Platinum-Ruthenium Alloys, Platinum-Rhenium Alloys, Platinum-Tungsten Alloys, FejOj-M CVI Oj (Braun Oxide), CoO-Bi20j, CoO with AI2O3, Thorium, Cerium, Zinc and Cadmium. 

Phillips and Timms [599] described a less general method. They converted germanium and silicon in alloys into hydrides and further into chlorides by contact with gold trichloride. They performed GC on a column packed with 13% of silicone 702 on Celite with the use of a gas-density balance for detection. Juvet and Fischer [600] developed a special reactor coupled directly to the chromatographic column, in which they fluorinated metals in alloys, carbides, oxides, sulphides and salts. In these samples, they determined quantitatively uranium, sulphur, selenium, technetium, tungsten, molybdenum, rhenium, silicon, boron, osmium, vanadium, iridium and platinum as fluorides. They performed the analysis on a PTFE column packed with 15% of Kel-F oil No. 10 on Chromosorb T. Prior to analysis the column was conditioned with fluorine and chlorine trifluoride in order to remove moisture and reactive organic compounds. The thermal conductivity detector was equipped with nickel-coated filaments resistant to corrosion with metal fluorides. Fig. 5.34 illustrates the analysis of tungsten, rhenium and osmium fluorides by this method. 

Molybdenum, niobium, and tantalum (bcc crystal structure) form a continuous series of solid solutions with tungsten, but only Nb and Ta additions lead to a strong straining effect. Higher additions of Nb and Ta raise the recrystallization temperature but also increase the DBTT and thus decrease the workability. Therefore, none of these binary or ternary tungsten base alloys developed in the past [6.2] has attained commercial importance. The only important tungsten-base solid-solution alloy today is tungsten-rhenium. 

One key factor was not investigated here. Several researchers have reported in presentations that there may be an interaction between the tungsten-rhenium tool and some titanium alloys during FSW that makes welding difficult. At present, no quantitative explanation has been offered for this interaction. Nonetheless, the choice of tool material in the current study may have had unintended consequences on the results. 

R. I. Jaffee, C.T. Sims, J.J. Harwood The effect of rhenium on the fabricabilily and ductility of molybdenum and tungsten, Proc. 3rd Plansee Seminar (Plansee AG, Reutte 1958) p. 380 J.G. Booth, R.I. Jaffee, E. I. Salkovitz The mechanisms of the rhenium-alloying effect in group Vl-A metals, Proc. 5th Plansee Seminar (Plansee AG, Reutte 1964) p. 547... 

One problem encountered in all experiments on fluid metals is the difficulty of confining samples at high temperatures without contamination from reactions with the cell material. Only a few materials are suitable. For studies of mercury and the alkali metals near their criticeil points, acceptable materials include pure tungsten, molybdenum, and the alloys tungsten-rhenium and tungsten-molybdenum. Sintered aluminum oxide or single crystal sapphire may be used to contain alkali metals up to about 1500 °C and to somewhat higher temperatures for mercury. 

Annealed rhenium is very ductile, and can be bent, coiled, or rolled. Rhenium is used as an additive to tungsten and molybdenum -based alloys to impart useful properties. 

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