Stress relaxation in free-standing aluminum beams

The stress-relaxation behavior of 2-μm-thick free-standing thin-film aluminum beams was evaluated by using a piezoelectric-actuated mechanical tester, which subjected them to quasi-static microtensile stress relaxation tests. The microstructure of the Al beams was modified in two ways: by annealing at different temperatures or by alloying with 1.5 at.% Ti. The features of the relaxation curves, such as stress recovery after unloading, suggest an anelastic relaxation behavior. Based on the anelastic relaxation model, the relaxation parameters were estimated from these relaxation curves and samples of various microstructures were compared. The results show that the relaxation strength and relaxation time change markedly with a decreasing grain size. It was also found in Ti-alloyed samples that the presence of the precipitates along the grain boundaries appears to suppress the relaxation significantly. In addition, the measured relaxation time was found to be comparable with the value predicted by the grain boundary sliding model. As such, we conclude that the grain boundary sliding is the dominant relaxation mechanism in free-standing thin films.