Grain growth behaviour and high strain rate tensile properties of gas tungsten are welds in iridium alloy DOP-26

The high strain rate tensile ductilities of gas tungsten arc welds in an Ir -0.3 wt-%W alloy containing 60 wt-ppm Th (designated DOP-26) have been dete rmined at test temperatures of 900-1200 degreesC. Within this temperature r ange, the welded specimens of DOP-26 exhibited tensile ductilities of 9-15% , independent of the test temperature. These values are comparable to those of unwelded DOP-26 tensile specimens tested at temperatures below 1000 deg reesC, but significantly lower than (approximately half) those of unwelded DOP-26 tested above 1000 degreesC. Elongation measurements at points along the gauge length of tensile tested specimens indicated that ductility was f airly uniform across the base metal and weld regions. At a tensile test tem perature of 900 degreesC, fracture occurred in the base metal with a mixed intergranular-transgranular failure mode. At 980 degreesC and above, fractu re occurred along the grain boundaries in the centreline of the weld. Scann ing electron microscopy of fracture surfaces revealed the presence of numer ous secondary phase particles along grain boundaries in the weld region. Th ese particles were rich in thorium and were identified as an Ir. Th cutecti c phase (melting point similar to 2080 degreesC) that formed as the weld po ol cooled. These particles, and the larger grain size of the fusion zone co mpared with the base metal, contributed to the lower tensile ductilities of the welded specimens compared with unwelded specimens. Because high strain rate tensile ductility in this alloy is strongly dependent on grain size, th grain growth behaviour of welded specimens of the alloy was also studied . In as welded specimens, the average grain diameters (measured through the thickness of the specimens in a plan perpendicular to the welding directio n) in the base metal, weld centreline, and fusion zone were similar to 21, 41, and 72 mum respectively. For annealing times up to 1065 h at 1400 degre esC and up to 100 h at 1500 degreesC, grain sizes in the weld centreline an d in the fusion zone did not change significantly. For these same anneals t he base metal grain size increased gradually to 45 and 58 mum for 1400 and 1500 degreesC annealing respectively. The base metal grain sizes were compa rable to previous data from unwelded specimens of this alloy. However, exce ssive grain growth for an annealing time of 250 h at 1500 degreesC was obse rved and as yet is unexplained.