The influence of microstructure on fracture of drawn tungsten wire

The microstructures and longitudinal fracture resistances of 0.635 mm diam lamp-doped and undoped tungsten wire were examined in the as-drawn condition and after anealing at temperatures between 600 and 1500°C. A variety of experimental techniques were employed, including Auger Electron Spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and a newly developed mechanical testing technique. The longitudinal fracture mode was intergranular for all wires and a second phase was observed on the grain boundaries of all doped wires. High concentrations of the dopant element potassium were present on the fracture surfaces of doped wires and experimental evidence was obtained which suggests they may be due to postfracture surface diffusion. Doped wires demonstrated increasing amounts of structure coarsening up to 1500°C whereas large equiaxed grains were formed in undoped wires annealed at 1300 and 1500°C. The longitudinal fracture resistance of undoped wire was unaltered by annealing at 1050°C and below, but decreased dramatically after annealing at 1300 and 1500°C. In contrast the fracture resistance of doped wire decreased after annealing at 1050 and 1300°C, but increased after annealing at 1500°C. Fracture resistance is discussed in terms of microstructure and fracture surface chemistry. A. W. FUNKENBUSCH, formerly Research Metallurgist with General Electric Refractory Metals Product Department