A universal law for metallurgical effects on acoustoplasticity

Acoustoplasticity, namely, the softening of metals induced by ultrasonic vibration, is routinely utilized in a large variety of industrial applications. Yet, a quantitative description of how metallurgical factors govern acoustoplastic softening is still not available to-date. In this paper, indentation experiments were carried out to study the ultrasound softening behavior of materials spanning a wide range of stacking fault energies (SFE). Analysis of these results as well as a large database from the literature unveils a universal law in which the acoustoplastic softening is proportional to the vibrational stress amplitude and the square of the yield stress normalized by the shear modulus which, according to Taylor's hardening law, scales with the dislocation density. For different materials, the proportionality constants in the above relation increase with the SFE. This law is consistent with the conjecture that vibrations enhance dislocation dipole annihilation which is easier in higher SFE materials, and lead to lower Taylor hardening. This law describes in a quantitative manner how metallurgical factors control acoustoplasticity, and provides the scientific basis for making the best use of acoustoplasticity in real applications, including choosing the optimized conditions to enhance ultrasound forming, and selecting vibration proof materials for structural applications in extreme vibratory conditions.