Evan Keith Ohriner

Growth of intermetallic layers in the iridium-molybdenum system

Abstract Growth of intermetallic layers between molybdenum and an iridium alloy (Ir-0.3 wt.%W-0.006 wt.%Th-0.005 wt.%Al) has been evaluated over the temperature range 1473–1748 K, in argon and in vacuum. Two intermetallic layers, with compositions corresponding to IrMo and Ir 3 Mo, are formed over the entire temperature range. The two layers have approximately equal thicknesses, and the total layer thickness formed during a 4 h exposure can be expressed as t =1200 exp(- Q / RT ), where t is the total thickness of the intermetallic layers in micrometers, Q =64 kJ mol −1 , R is the gas constant, and T is the absolute temperature. Vapor-phase transport of molybdenum (as MoO 3 ) is suggested as a possible mechanism for the growth of these intermetallic layers in (oxygen-contaminated) argon. There was no indication of molybdenum transport through the iridium alloy grain boundaries in advance of the intermetallic layer interface. The resistance of these intermetallies to attack by several acids and acid mixtures is similar to that of iridium.

High-Temperature Galling Characteristics of TI-6AL-4V with and without Surface Treatments

Galling is a severe form of surface damage in metals and alloys that typically arises under relatively high normal force and low sliding speed and in the absence of effective lubrication. It can lead to macroscopic surface roughening and seizure. The occurrence of galling can be especially problematic in high-temperature applications like diesel engine exhaust gas recirculation system components and adjustable turbocharger vanes, because suitable lubricants may not be available, moisture desorption promotes increased adhesion, and the yield strength of metals decreases with temperature. Oxidation can counteract these effects to some extent by forming lubricative oxide films. Two methods to improve the galling resistance of titanium alloy Ti-6Al-4V were investigated: (a) applying an oxygen diffusion treatment and (b) creating a metal–matrix composite with TiB2 using a high-intensity infrared heating source. A new oscillating three-pin-on-flat, high-temperature test method was developed and used to characterize galling behavior relative to a cobalt-based alloy (Stellite 6B, HP Alloys, Windfall, IN). The magnitude of the oscillating torque, the surface roughness, and observations of surface damage were used as measures of galling resistance. Due to the formation of lubricative oxide films, the galling resistance of the Ti alloy at 485°C, even nontreated, was considerably better than it was at room temperature. The infrared (IR)-formed composite displayed reduced surface damage and lower torque than the substrate titanium alloy.

High-Heat-Flux Testing of Irradiated Tungsten-Based Materials for Fusion Applications Using Infrared Plasma Arc Lamps

Abstract Testing of advanced materials and component mock-ups under prototypical fusion high-heat-flux conditions, while historically a mainstay of fusion research, has proved challenging, especially for irradiated materials. A new high-heat-flux–testing (HHFT) facility based on water-wall plasma arc lamps (PALs) is now introduced for materials and small-component testing. Two PAL systems, utilizing a 12 000°C plasma arc contained in a quartz tube cooled by a spiral water flow over the inside tube surface, provide maximum incident heat fluxes of 4.2 and 27 MW/m2 over areas of 9×12 and 1×10 cm2, respectively. This paper will present the overall design and implementation of a PAL-based irradiated material target station (IMTS). The IMTS is primarily designed for testing the effects of heat flux or thermal cycling on material coupons of interest, such as those for plasma-facing components. Temperature results are shown for thermal cycling under HHFT of tungsten coupon specimens that were neutron irradiated in HFIR. Radiological surveys indicated minimal contamination of the 36-× 36-× 18-cm test section, demonstrating the capability of the new facility to handle irradiated specimens at high temperature.

Weldability of DOP‐26 iridium alloy: Effects of welding gas and alloy composition

The DOP‐26 iridium alloy, containing 2000‐ to 4000‐ppm W, 30‐ to 90‐ppm Th, and 20‐ to 80‐ppm Al by weight, is used as a cladding material. The closure weld on the fueled clad is performed by gas‐tungsten‐arc (GTA) welding, which under certain conditions results in hot cracking of the fusion zone. The effects of variations in the chemical composition of the alloy and in the welding gas atmosphere on the welding behavior are evaluated using the Sigmajig test. In this test, sheet materials are welded in an argon atmosphere with measured levels of applied stress to determine a threshold stress for hot cracking. The threshold stress for cracking of DOP‐26 alloy increases by a factor of two as the thorium content is decreased from 94 to 37 ppm. There is no effect on threshold cracking stress for variations in oxygen content of the argon welding atmosphere from 10 to 2000 ppm or for variations in water vapor content from 10 to 1000 ppm.

Improvements in manufacture of iridium alloy materials

Iridium alloys are used as fuel‐cladding material in radioisotope thermoelectric generators (RTGs). Hardware produced at the Oak Ridge National Laboratory (ORNL) has been used in Voyager 1 and 2, Galileo, and Ulysses spacecrafts. This hardware was fabricated from small, 500‐g drop‐cast ingots. Porosity in these ingots and the resulting defects in the rolled sheets caused rejection of about 30% of the product. An improved manufacturing process was developed with the goal of substantially reducing the level of defects in the rolled sheets. The ingot size is increased to 10 kg and is produced by vacuum arc remelting. In addition, the ingot is hot extruded prior to rolling. Since implementation of the process in 1989, the average rate of rejection of the product has been reduced to below 10%.

Facility for high-heat flux testing of irradiated fusion materials and components using infrared plasma arc lamps

A new high-heat flux testing (HHFT) facility using water-wall stabilized high-power high-pressure argon plasma arc lamps (PALs) has been developed for fusion applications. It can accommodate irradiated plasma facing component materials and sub-size mock-up divertor components. Two PALs currently available at Oak Ridge National Laboratory can provide maximum incident heat fluxes of 4.2 and 27 MW m−2, which are prototypic of fusion steady state heat flux conditions, over a heated area of 9 × 12 and 1 × 10 cm2, respectively. The use of PAL permits the heat source to be environmentally separated from the components of the test chamber, simplifying the design to accommodate safe testing of low-level irradiated articles and materials under high-heat flux. Issues related to the operation and temperature measurements during testing of tungsten samples are presented and discussed. The relative advantages and disadvantages of this photon-based HHFT facility are compared to existing e-beam and particle beam facilities used for similar purposes.