Plastic Strain and Temperature Dependences of Hall-Petch Effect

Abstract

Material strengthening can be achieved by means of effectively intercepting the dislocation motion by some obstacles. The representative one is grain boundary (GB) strengthening, called Hall-Petch relationship (H-P effect). Since such an interaction between dislocation and GB always happens at any plastic deformation stage, H-P effect could have plastic strain dependence. Also the dislocation motion is extremely sensitive to temperature, thus temperature dependence should be involved as well. In the present paper, both dependences of H-P effect were investigated using pure aluminum specimens with six different averaged grain sizes at four different temperatures. The former dependence leads to a relationship with work hardening behavior which can be fitted to n-power law, and the latter might successfully induce the activation barrier for the plastic deformation to be evolved across GB. H-P coefficient, which is a coefficient of the term with power -1/2 of averaged grain size on the H-P relationship, was summarized as a function with power m of plastic strain, where m-value is newly defined as plastic strain grain boundary strengthening sensitivity exponent. Considering two simple models for description of fundamental plastic deformation across GB in comparison with experimental results, H-P coefficient is found to be proportional to power n of plastic strain (that is, m-value is equal to n-value of work hardening exponent). This corresponding model suggests that the internal stress due to the piled-up dislocations in front of GB activates the dislocation sources in adjoining grain. However, this relation doesn't hold at elevated temperature. From Arrhenius plots concerning to the relationship between plastic strain and H-P coefficient, activation energy was obtained as 67kJ/mol, which is almost half of that of creep behavior (140kJ/mol).