Bauschinger effect and anomalous thermomechanical deformation induced by oxygen in passivated thin Cu films on substrates

Annealed thin metal films on Si substrates often show early yielding behavior, similar to the well-known Bauschinger effect in bulk metals, during thermomechanical cycling. Small amounts of oxygen added to a Cu film dramatically enhance early yielding, and lead to extensive plastic deformation at zero stress and increasing stress with temperature during heating. These anomalous plastic deformation effects were explored using tests in which the stresses in the films were measured during thermal cycles with varying temperature endpoints. Anomalous behavior is attributed to a mechanism whereby reversible plastic strain is enhanced by recovery of misfit dislocation line length, which is apparently prevented by strain hardening in films showing normal thermoelastic behavior. The mechanism can account for a variety of thermomechanical behaviors including stress asymmetry and memory effects in thin films.