Anelastic contributions to the behavior of freestanding Al thin films

The anelastic contribution to the mechanical behavior of Al thin films was evaluated using a micro-tensile test system. The samples consisted of 2 mu m thick freestanding micromachined microbeams of nominally 99.999% pure Al surrounded by a Si frame. Elastic moduli of the Al beams were measured using uniaxial tensile tests performed at strain rates from 7.6x10(-6) to 7.6x10(-2) s(-1). Large changes in modulus were observed with changes in strain rate up to 7.6x10(-4). Stress relaxation tests and anelastic reloading tests were performed on similar samples to confirm the presence of significant anelastic processes, and relaxation times were extracted. We suggest that the observed modulus behavior may be attributed to two anelastic mechanisms: a fast mechanism such as grain boundary sliding, and a slower mechanism possibly associated with dislocation motion. A discrete-spectrum model for these two mechanisms is used to explain the measured relaxed elastic moduli as a function of strain rate.