Footprints of deformation mechanisms during in-situ x-ray diffraction : nanocrystalline and ultrafine grainded materials

Nanocrystalline metals have been an area of great interest in recent years due to their enhanced characteristics. One of the most striking is implied by the Hall-Petch relation: with decreasing grain size the material becomes stronger. This promise is fulfilled in nanocrystalline metals which display strengths up to 10 times higher than their coarse grain counter parts. With such high strengths they show potential significance for engineering and structural applications. However, for these materials to be of use in structural applications it is paramount to improve their low ductility. This in turn requires a thorough understanding of the fundamental principles governing plastic deformation at the nanograin length scales. In coarse grain metals, with grain sizes in the micrometer range, plastic deformation is governed by dislocations that are generated by sources within the grains, propagate and interact with pre-existing structures and also with each other. Upon unloading at a given level of deformation all the dislocation segments that have not yet annihilated make up the final microstructure of the deformed state. As the operation of a traditional dislocation source is grain size dependent there would be a critical length scale below which such a source can no longer operate. For FCC metals such as Ni this grain size is between 20-40 nm and thus the mechanisms governing classical plasticity in coarse grain materials would appear to break down in the nanocrystalline regime. Whether plasticity in nc materials with average grain sizes below 100 nm and an intrinsic grain size distribution is still governed by dislocation mediated processes is still an open question. Post-mortem TEM observations have not found dislocation debris and in-situ TEM methods have noted some dislocation activity during deformation. Molecular dynamics studies have, however suggested that in this nanograin regime and with the absence of internal dislocation sources, GBs of nc FCC metals can act not only as a source but also as a sink for lattice dislocations. These suggestions motivated us to investigate x-ray diffraction (XRD) signature through an in-situ experiment designed specifically for providing insight into plastic deformation behaviour of bulk nanocrystalline materials during deformation. Thanks to the high intensity of the radiation at the Swiss Light Source and the development of the Microstrip detector allowing time-resolved measurements and covering an angle of 60° we have built-up an experiment which allows us to perform tensile deformation in-situ. The uniqueness of the experiment is that it allows both a continuous and simultaneous monitoring of peak position and peak broadening during tensile testing. The Microtensile Machine mounted at the SLS had been specifically designed for this experiment and allows a variety of different sample shapes and sizes to be mounted and observed in the beam. Measurements can be conducted using a range of different strain rat