| Current research is aimed towards making contribution in the areas of (i) understanding the details of precipitation-dislocation interactions, (ii) determining the significance of these interactions on the flow stress response, and (iii) on developing a multiscale model which is based on the interaction of dislocations and precipitates. In this research project the precipitation sequence of a precipitation hardened alloy, AA6022, was characterized by means of transmission electron microscopy, differential scanning calorimetry, and hardness measurements. The metastable phases during the isothermal and dynamic decomposition of supersaturated alloy were identified and a new precipitation sequence was proposed. To understand the effect of dislocations on the precipitates, the precipitation sequence in predeformed specimens were analyzed and compared with those in un-deformed conditions. It was found that the Q' phase is more favorable to form in the presence of dislocations. The effect of precipitate types and morphologies on dislocation structures and orientation evolution were characterized by performing electron backscatter diffraction measurements on deformed specimens. The results showed that the density of geometrically necessary dislocations and their patterning are governed by the morphology of precipitates. Also, it was found that the presence of peak-aged precipitates resulted in significant orientation spreading within the individual grains while this effect was less observed for over-aged precipitates. To understand the significance of interaction between precipitates and dislocations on the flow stress, the precipitate and dislocation structures parameters were characterized and the most important microstructural parameters were fitted into a phenomenological yield stress model. Experimental and statistical analysis showed a linear relationship between yield stress and average GND density. Based on the interaction of dislocations and precipitates, a rigorous micromechanical model for elastic-plastic deformation of crystals containing elastic inclusions was developed. The model is based on a novel method for calculation of elastic strain energy associated with incompatible deformation of matrix and inclusion, by representing it with the interaction energy of geometrically necessary dislocations. The resulting model is remarkably simple. It falls within the framework of classical crystal plasticity with a specific elastic-plastic constitutive law that accounts for shape of inclusions. |
