Formation of Cells and Subgrains and Its Influence on Properties

Abstract

During plastic deformation, cells and subgrains are created in most alloys. This is collectively referred as the formation of a substructure. There is extensive qualitative information about substructures in the literature, but quantitative modeling has only appeared recently. In this paper, basic models for the formation of substructure during creep and deformation at constant strain rate are presented. It is demonstrated that the models can give at least an approximate description of available experimental data. The presence of substructure can have a dramatic impact on properties. It is well-known that prior cold work can significantly increase the creep strength. Cold work of copper can raise the creep rupture time by up to six orders of magnitude. During plastic deformation dislocations with opposite Burgers vectors move in different directions creating polarized or unbalanced dislocations. Since the unbalanced dislocations are not exposed to static recovery, they form a stable dislocation structure. Taking the role of the unbalanced dislocations into account, the full increase of the creep strength after cold work can quantitatively be explained (without the use of adjustable parameters). Additionally, the shape of the creep curves that varies with the amount of cold work can be modeled. The substructure is also of importance for the modeling of creep curves for material without cold work. In power-law breakdown, the stress exponent can be 50 or more. This should imply that there would be a huge increase in the creep rate with increasing strain, but that is not observed. The reason is that the unbalanced dislocations form a back stress that acts against the increase in the true stress. Taking the back stress into account, it has been possible to model creep curves for copper at near ambient temperatures. This effect must be taken into account in stress analysis to avoid overestimating the creep rate by many orders of magnitude.