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

Abstract

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 determined at test temperatures of 900–1200°C. Within this temperature range, 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°C, but significantly lower than (approximately half) those of unwelded DOP–26 tested above 1000°C. Elongation measurements at points along the gauge length of tensile tested specimens indicated that ductility was fairly uniform across the base metal and weld regions. At a tensile test temperature of 900°C, fracture occurred in the base metal with a mixed intergranular–transgranular failure mode. At 980°C and above, fracture occurred along the grain boundaries in the centreline of the weld. Scanning electron microscopy of fracture surfaces revealed the presence of numerous secondary phase particles along grain boundaries in the weld region. These particles were rich in thorium and were identified as an Ir–Th eutectic phase (melting point ∼2080°C) that formed as the weld pool cooled. These particles, and the larger grain size of the fusion zone compared 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, the 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 plane perpendicular to the welding direction) in the base metal, weld centreline, and fusion zone were ∼21, 41, and 72 µm respectively. For annealing times up to 1065 h at 1400°C and up to 100 h at 1500°C, grain sizes in the weld centreline and in the fusion zone did not change significantly. For these same anneals the base metal grain size increased gradually to 45 and 58 µm for 1400 and 1500°C annealing respectively. The base metal grain sizes were comparable to previous data from unwelded specimens of this alloy. However, excessive grain growth for an annealing time of 250 h at 1500°C was observed and as yet is unexplained.