Anelasticity in freestanding aluminum thin films.

摘要

Anelastic deformation is present in all metals, but it is generally negligible in bulk form and at low homologous temperatures. In thin films, anelastic deformation contributes a great deal to the overall mechanical properties. Anelastic deformation is any portion of the total deformation of a body that occurs as a function of time when load is applied and which disappears completely after a period of time when the load is removed. Two major anelastic mechanisms proposed for pure, metal thin films are reversible dislocation glide and grain boundary sliding. Dislocation glide is expected at low homologous temperatures while grain boundary sliding is expected at high homologous temperatures. With increasing grain boundary density (i.e. smaller grain sizes), grain boundary sliding is expected to dominate, and with larger grain sizes dislocation glide is expected to dominate.; To investigate this phenomenon in thin metal films, a custom-built microtensile system was designed and fabricated. Uniaxial tensile tests were performed on four different thicknesses (1.00 mum, 0.75 mum, 0.50 mum, and 0.25 mum) of freestanding Al thin films. Strain rate sensitivity and stress relaxation experiments were performed to investigate anelasticity. Film quality turned out to be a major factor in all of the mechanical measurements made, including the ability to observe anelasticity.; Uniaxial tensile experiments performed on Al and Cu micro-wires lead to the supposition that a dislocation gliding mechanism is responsible for the observed strain rate sensitivity. Stress relaxation experiments were performed at four different temperatures (38°C, 50°C, 60°C, and 70°C) on freestanding Al thin films. The results indicate that the freestanding films can be modeled by a linear anelastic solid. In addition, a time constant for fast relaxation (0.5 seconds) was determined at each temperature. From these values, an activation energy was calculated. The small value of the activation energy (approximately 14 kJ/ol) suggests that only one mechanism is operating over the time scale of 0.5 seconds: dislocation glide.