Effective Modulus and Stress Relaxation of Freestanding Aluminum Microtensile Beams

Abstract

Freestanding Al thin film microtensile beams 600 μm long, 100 μm wide, and with thicknesses of 0.25 μm, 0.50 μm, 0.75 μm, and 1.00 μm were tested under uniaxial tension with a custom-built microtensile system. Displacement controlled experiments employing five strain rates between 8.3 × 10⁻⁴ s⁻¹ and 8.3 × 10⁻¹ s⁻¹ were performed. Three crosshead displacements of 1.00 μm (0.17% strain), 2.60 μm (0.43%), and 3.60 μm (0.60% strain) were exercised in order to investigate behavior in the elastic, anelastic, and the inelastic sections of the stress/strain curve.

At a 0.17% strain amplitude, the 0.50 μm thick microtensile beams exhibit an effective modulus that increases with increasing strain rate. The 1.00 μm, 0.75 μm, and 0.25 μm microtensile beams do not show a similar trend. The 0.25 μm and the 0.75 μm thick microtensile beams have similar grain sizes and similar moduli; the 0.50 μm and the 1.00 μm microtensile beams have similar but smaller grain sizes and higher effective moduli. This suggests that grain size is more significant than film thickness in determining the effective modulus of freestanding Al thin films. Furthermore, no stress relaxation behavior was identified for any of the films during the hold at the 0.17% strain amplitude.

At a strain amplitude of 0.43%, the effective moduli are very similar to the values measured at a 0.17% strain amplitude; however, the 0.50 μm and 1.00 μm thick microtensile beams show measurable stress relaxation while the 0.25 μm and 0.75 μm microtensile beams do not. This result combined with a lower observed modulus suggests that the particular anelastic mechanisms operating in these films at these time scales activate sooner and provide a faster stress relaxation process in the larger grained films.