Internal Distribution of Lattice Defects and Splitting in Lamp Grade Tungsten Wires Deformed by Torsion

Abstract

Commercial non-sag tungsten wires can be susceptible to splitting, depending on the drawing temperature and on the specific properties of the wire, and the different splitting properties are easily distinguished from each other by using torsion tests. Three different types of wires, called A, B and C, are now investigated, of which wire A is most deformable by torsion, but wire C is already split at wire drawing, and wire B is easily split at torsion together with the formation of Luders bands. In twisted wires annealing diagrams separate the different types of lattice defects, which are very different from those found after wire drawing (or coiling), especially due to the anisotropy of grains after torsion. After stress-relief annealing all types of wires were deformed by torsion to surface strains γ 0.75, when splitting appeared on their surfaces. On decreasing their diameters step by step to about half, splitting could still be seen on wires B and C, but not on wire A. After each thinning step excess electrical resistivity was measured, revealing that the concentration of lattice defects in wire A decreased about proportionally to the radius, while in wires B and C it even increased inwards, probably due to the sliding of the material along the splits during torsion.