EFFECTS OF ANNEAL UPON TRANSMISSION LOSS AND MODE GROUP-VELOCITY IN POLYCRYSTALLINE METAL WIRES

Relative transmission coefficients of pulses of ultrasound, over the f requency range 1.0-5.0 MHz, propagating in cold-drawn and heat-treated wires of aluminum, stainless steel, brass, and copper are compared. V arious heating cycles were used to alter the structure of the grains i n the wires and particularly the grain size itself. It was found almos t without exception that starting from an initially cold-worked sample of wire, ultrasound transmission first of all decreased with heat tre atment; then, as the grains started recrystallizing, ultrasound transm ission increased, reaching a maximum at complete recrystallization, an d thereafter decreased once again with the process of grain growth. Th e change in ultrasound transmission with heat treatment was most notic eable at higher frequencies, with a dramatic increase in transmission loss for wires subjected to extensive amounts of heat treatment. Durin g the investigation it was discovered that, contrary to expectation, g roup velocities of modes propagating in stainless-steel wires were dep endent upon the amount of heat treatment that the wires had been subje cted to. Further study revealed that group velocities of modes propaga ting in brass wires were similarly affected by heat treatment. In part icular, the lowest longitudinal mode which is predicted by Pochhammer' s theory to be equal to the bar velocity at the lower end of its frequ ency range, increased with amount of heat treatment and approached the dilatational velocity. No such marked changes were observed in the pu re metal wires studied.