MOLECULAR DYNAMICS SIMULATION OF DEFORMATION BEHAVIOUR IN NANOCRYSTALLINE FCC METALS

Abstract

Mechanical behaviour of nanocrystalline metals have been a topic of discussion for decades. A lot of work has been done in this area which are usually summarized as deformation mechanism maps. Still there is scope for study. Generally the samples used for experiment has been found to be to porous. Also, the samples for test are generally prepared by compaction. This means the samples when tested are deformed for the second time. In the present work, we studied the effect of porosity on mechanical behavior of nanocrystalline FCC metals via Molecular dynamics (MD) simulations using EAM potential. Cu and Pd were taken as a representative of low stacking fault energy (SFE) and high SFE material. Structures with grain size of 5nm and 10nm were studied. It was found that porosity resulted in decrease in Young's modulus and flow stress for both Cu and Pd of 5nm and 10nm grain size. Some grain boundary relaxation was observed at low compressive load. The potential energy curves for 5nm grain structures and 10nm grain structures were found to behave completely differently after onset of plasticity, implying that different deformation mechanisms are dominant for the two grain size. The effect of reloading was also studied (for compression). During first reloading cycle (phase I), the stress was removed after 3.5% strain (phase IV) or after 10% strain and relaxed for 140ps. They were then compressed again to get a final strain of 10%. The 5nm and 10nm structures for both Cu and Pd showed different behaviour. The 5nm grain size structures showed increase in flow stress for both phase IV and phase III. The 10nm grain size structures showed increase in flow stress for phase IV while a decrease in flow stress and Young's modulus for phase III. The difference in the behaviour of 5nm and 10nm grain size structures is due to dominance of different deformation mechanism in the plastic regime. The relaxation process was studied. It was found that during relaxation grain boundary thinning occured. Depending upon the dislocation density, stacking faults, grain size and stacking fault energy dislocations ending within grains can either progress and cross the grain forming a twin or retrieve into grain boundary. In the present study, some difference was found in the potential energy curve of Cu and Pd. But the overall behaviour of the two FCC metals was same. Lastly, a brief creep test was performed. Some interesting results were found. But the data is not sufficient enough to make generalized conclusions