Drop In Flow Stress Research Articles

The origin of superplasticity in the sn-5%bi alloy

A study was made of the plastic deformation processes in an extruded Sn-5 % Bi alloy by means of mechanical measurements and microstructural observations. The flow stress, and strain rate sensitivity of the flow stress, d log σ/ d log ϵ, are strong functions of strain rate and Sn grain size. The log σ-log ϵ curve is S-shaped with a region of maximum rate sensitivity at an intermediate strain rate, which increases with decreasing grain size. In this region, superplastic elongations of 1000 % are observed. Concurrent study of the development of microstructure on a polished surface during deformation confirms that distinct deformation mechanisms are dominant at high and low strain rates. At high rates the dominant mechanism is slip, at low rates, grain boundary sliding. High rate sensitivity is associated with the rapid drop in flow stress occurring at strain rates at which grain boundary sliding becomes an important mode of deformation. This interpretation is consistent with a body of evidence in the literature of high temperature creep.