Observation of local internal friction and plasticity onset in nanocrystalline nickel by atomic force acoustic microscopy

Atomic force acoustic microscopy (AFAM) is a near-field microscopy technique exploiting the vibrational behavior of the atomic force microscope cantilever. With its tip in contact with a surface, it is sensitive to its elastic and anelastic properties. We show how AFAM can be used to investigate the onset of plasticity in nanocrystalline nickel. To this end cantilever resonance curves are recorded with varying tip-loading force P. From the resonance frequencies and the width of the resonance curves, one obtains the contact stiffness k* and the contact damping Q?1. Plotting these quantities vs. P, one observes damping peaks, as well as a reduction of the contact stiffness at specific P (?1 ?N) due to the nucleation of partial dislocation loops at grain boundaries. The local Q?1 value is most likely caused both by the nucleation and by the interaction of the loop with the phonon and the electron baths. There is a background damping which is related to the global ultrasonic absorption.