Phase selection and transformation kinetics of copper-zinc powder mixture subjected to ultrasonic powder consolidation

Abstract

Ultrasonic powder consolidation (UPC) is a new variant of ultrasonic welding (USW) in which a monolithic or blended powder is subjected to ultrasonic vibration under simultaneous normal pressure to produce metallurgically consolidated bulk materials at temperatures much below the melting point in a few seconds. Previous studies have revealed that the high strain rate plastic deformation experienced by materials subjected to ultrasonic vibration causes the vacancy concentration to increase far above the thermal equilibrium value, affecting the structural changes in the material through enhanced diffusion, alteration of relative thermodynamic stability and increased dislocation mobility. The present study was conducted to investigate mainly the structural changes in Cu-48 wt% Zn powder mixtures subjected to UPC under systematically varied conditions, with main focus on the effects of strain-induced excess vacancies on the phase transformations during UPC. The primary discovery in this research is that the application of ultrasonic vibration caused rapid formation of γ-brass in samples consolidated at or above 200 °C, with more than 75% of the sample volume transforming to γ in only 4 s at 300 °C. The rapid γ growth was found parabolic with vibration time, indicative of diffusion-controlled growth kinetics. Simple pressing under otherwise identical conditions produced no new phase in the same powder compact. The γ phase formation in the UPC samples was contrasted by the direct formation of β-brass observed over hours of static annealing of the same powder mixture. Kinetic analysis based on diffusion-controlled γ growth yielded diffusivity values 4-5 orders of magnitude above the normal values. Both the rapid growth and phase selection in UPC was attributed to enhanced diffusion during UPC. The rapid densification and phase transformation in UPC Cu-Zn samples are discussed in terms of effects of excess vacancies on diffusion, phase stability and dynamic recovery in materials deforming at high strain rate.