Additivity of strengthening mechanisms in ultrafine grained Al-Mg-Sc alloy

In the light of unique and anomalous properties of ultrafine grained (UFG) alloys, an effort was made to develop a predictive capability of the yield strength (YS) of UFG Al-Mg-Sc alloy. UFG microstructure was introduced using friction stir processing. Microstructural characterization of grain size, dislocations, and nano-sized Al3(Sc,Zr) particles was carried out using electron backscatter diffraction and transmission electron microscope. The contribution from Peierls-Nabarro stress, solid solution strengthening, precipitation strengthening, grain boundary strengthening, and dislocation strengthening were assessed using existing strengthening models. Additivity law to predict the YS of the alloy was chosen based on the microstructural state of the alloy. The microstructural state of coarse grained and UFG alloy favored the use of linear additivity rule over Pythagorean. The use of a mixed (linear and Pythagorean) additivity rule was also carried out to assess its YS prediction capability. A difference in the range of 33-55% was observed between predicted and experimentally obtained YS for UFG alloy. The reason was related to the overprediction from grain boundary strengthening model. A smaller HP slope than the normal slope value was able to predict the YS of the UFG alloy more closely.