A model for rate independent hysteresis, based on the Duhem-Madelung framework, is examined in the context of temperature induced phase transitions such as those which occur in shape memory metals. Connection to experiment follows from its dependence upon the hysteresis envelope for complete transitions. Various requirements upon partial transitions, notably that such transitions proceed at a pace that is proportional to the phase fraction of the parent phase, complete the model. Restrictions on the hysteresis envelope are obtained that ensure monotonicity, containment and orientation requirements upon partial transition paths.