A New Criterion for Construction of Instability Maps in Hot Deformation

A new instability criterion has been developed to identify safe and unsafe regions in hot deformation of metals. The instability function takes into account both the material variable (m) and process variable ((epsilon)) over dot as well as the efficiency of power dissipation (eta). The proposed instability function is based on Narayana Murty's efficiency of power dissipation expression and can be used for a wide range of materials, including materials in which the stress-strain curve does not obey the power law. Different criteria as well as the new one were used for construction of the instability maps for the Mg-4Sn alloy, single-phase brass, and aluminum alloy AFNOR 7020. Concerning the Mg-4Sn alloy, compression tests were conducted at the temperature range of 300 degrees C-500 degrees C and strain rates of 0.001-1 s(-1). It has been shown that the prediction of Prasad and Narayana Murty instability criteria is more conservative than the new presented criterion.