A simple instability condition is derived following Ziegler's continuum principles for identifying the regions of unstable metal flow during hot deformation in the processing maps. This criterion is examined by considering the flow stress data on 2124 Al-SiCp metal matrix composites and comparing with the microstructural observations of the deformed compression specimens in the as-vacuum hot pressed condition and vacuum hot pressed and extruded condition. The optimum hot working conditions are proposed from the identified stable regions (having comparatively high values of workability parameters, namely, strain rate sensitivity parameter and efficiency of power dissipation) in the developed processing maps.