Weld Metal Grain Structure and Mechanical Properties of a Th-Doped Ir-0.3 Pct W Alloy (DOP-26) C. T. LIU and S. A. DAVID Weld metal grain structure and mechanical properties of the Ir-0.3 pct W alloy (DOP-26) doped with 60 ppm Th and 50 ppm A1 have been investigated by use of a gas tungsten arc (GTA) welding process. The fusion zone grain structure is strongly influenced by heat input and puddle shape and therefore by the bead width. With increasing bead width from 2.5 to 3.7 mm, the grains in the fusion zone show a sharp change in growth direction near the centerline region and develop a fine columnar structure with grains growing parallel to the welding direction. Mechanical properties of the welds and base metal were characterized by tensile and impact tests from 650 to 1150 ~ The ductility and fracture behavior of DOP-26 welds are sensitive to weld bead width, postweld heat treatment, and weld-test orientation. The ductility of the welded specimens increases with increasing test temperature and decreasing weld bead width. The transverse weld specimen with a wide-bead width (3.7 mm) has the lowest impact ductility, and the longitudinal weld with a narrow-bead width (2.5 mm) has the highest elongation at all the test temperatures. The impact ductility of the transverse weld specimen with the narrow-bead width falls between the limits. All the results are discussed in terms of the fusion zone grain structure and fracture path of the welds. I. INTRODUCTION IRIDIUM is an fcc metal having very unusual mechanical properties. Iridium metal has an exceptionally high elastic modulus, ~ critical resolved shear stress, 2'3'4 and work hard- ening rate. 2'3'4It exhibits a cleavage-type fracture on tensile tests, which is not observed in other fcc metals. 5 The cleav- age fracture has been observed in iridium at temperatures as high as 1000 ~ Polycrystalline iridium shows grain boundary fracture with limited ductility in tensile tests be- low 800 ~ at conventional speed (10 -4 m per second) or in impact tests to at least 1500 ~ at high velocities (> 10 m per second). 6'8 Recent studies of intergranularly fractured sur- faces with Auger electron spectroscopy showed no evidence of impurity segregation that might be responsible for the grain boundary failure of iridium and its alloys. 7'9 Ir-0.3 pct W alloys doped with 30 to 60 ppm Th are currently used as postimpact containment material for radio- active fuel in thermoelectric generators that provide stable electrical power for a variety of outer planetary mis- sions.l~ alloys were chosen for this application because of their high melting point (2450 ~ good high- temperature strength, oxidation resistance, and com- patibility with oxide fuel forms and insulation materials. Tungsten at a level of 0.3 pct has been alloyed to improve the fabricability of unalloyed iridium. Thorium is added as a grain boundary strengthener, ~~ segregating strongly to the grain boundaries and inhibiting intergranular fracture during high-temperature, high-velocity reentry impact from space. Thorium additions also react with iridium to form Thlr5 precipitates, which pin the grain boundaries and retard the grain growth during long-term service at 1200 to 1400 ~ the operating temperatures. Weldability studies have shown that iridium alloys con- taining less than 100 ppm Th can be successfully welded by a gas tungsten arc (GTA) process without hot cracking.~2 However, the mechanical properties of these iridium alloys C. T. LIU and S. A. DAVID are both with the Research Staff, Oak Ridge National Laboratory, Oak Ridge, TN 37830. Manuscript submitted September 28, 1981. METALLURGICAL TRANSACTIONSA depend strongly on grain structure, 8'11 and development of coarse unfavorable microstructures in GTA welds may severely reduce the ductility and impact strength of a welded joint. To overcome this problem we judiciously selected weld process parameters to control heat input and solidi- fication rate and thereby solidification structure. 12A3'14An Ir-0.3 pct W alloy doped with 60 ppm Th and 50 ppm A1, designated as DOP-26 alloy, was welded by a GTA process under controlled conditions, and the mechanical properties of the welds were characterized by tensile and impact tests from 650 to 1150 ~ The results thus obtained are also compared with the base-metal properties. Emphasis is on correlation of the mechanical properties with fusion zone grain structure, heat treatment, and fracture path. II. EXPERIMENTAL PROCEDURES The DOP-26 alloy used in this study was arc melted and drop cast in a chilled copper mold. The ingots were clad in molybdenum and hot rolled to 0.89 mm at 1000 to 1200 ~ Following hot rolling, the sheets were ground to a final thickness of 0.64 mm (0.025 inch) and cleaned in a potas- sium cyanide solution to remove impurities or oxide scales from the surface. Table I lists the chemical composition of the alloy as determined by spark source mass spectroscopy (SSMS), fusion, and carbon analyses. Autogeneous bead-on-plate and butt welds were made on 0.64-mm-thick samples in a glove box containing 75 pct He-25 pct Ar. To provide a fully recrystallized grain struc- ture, all samples were vacuum-annealed for one hour at 1500 ~ The fusion zone grain structure was controlled by transverse arc oscillation, heat input, and postweld heat treatment. On the basis of early studies involving a range of frequency and welding speed, 12'13'~4we selected and used a transverse arc oscillation frequency of 375 cycles per minute and a welding speed of 76 cm per minute. The surface bead width of the welds was governed by the power input and chilling conditions. Welds with two surface bead widths, ISSN 0360-2133/82/0611-1043500.75/0 9 1982 AMERICAN SOCIETY FOR METALS AND VOLUME 13A, JUNE 1982-- 1043 THE METALLURGICAL SOCIETY OF AIME