Splitting and Lüders Band Formation During Torsion in Lamp Grade Tungsten

Abstract

Torsion testing permits the separation of stress relieved commercial non-sag tungsten wires into three classes of split propensity. The most ductile sorts of wire deform split-free in free-end torsion in a nearly homogeneous way up to quite large strains and undergo considerable elongation. Wires rejected by the split test form Luders bands at low twist angles. In these wires the formation and propagation of Luders bands are connected with splitting and considerable shortening. There is also an intermediate sort of wires. These pass the split test but form Luders bands at low twist angles. These Luders bands propagate with a very small change in sample length. The strain distribution along the wire was measured by means of a marker line, and the splits were revealed by scanning electron microscopy. The resistivity contribution of the splits and the lattice defects were separated by measuring the effective resistivity and the resistance ratio at 77 and 300 K of the samples. The origin of the three classes of split propensity is discussed with respect to the nature of the Luders bands and to the possible effect of the drawing temperature.