Fatigue in Nanocrystalline, Bimodal and Ultrafine-grained Nickel in Respect of Thermal and Mechanical StabilityThursday (29.09.2016) 15:00 - 15:15 Part of:
Motivated by the high strengthening effect of grain refinement, bulk nanocrystalline materials become more and more important in current studies and applications in material science. One research focus lies on the stability of the microstructure regarding the undesired and distinct propensity of grain growth at slightly elevated temperatures or mechanical stresses. The problem thereby is the loss of the enhanced mechanical properties. The reason for grain growth is found in the reduction of the surface energy which is especially high in nanocrystalline materials due to the big amount of grain boundaries. In the case of nanocrystalline, electrodeposited nickel, which is stabilized by the organic additive saccharin, grain growth occurs even at 200°C, whereby some grains grow preferred (abnormal grain growth) and form a bimodal microstructure. Furthermore mechanical induced grain growth occurs in the stress field of a crack. The influence of this behavior in respect to fatigue, crack initiation and crack growth was investigated by measuring crack propagation and SN curves. Astonishingly, the nanocrystalline samples showed better crack propagation behavior then the bimodal ones. This might be due to the underlying mechanism of the grain refinement and stabilization by additives like saccharin, known as solute-drag-effect, which is based on the drag of impurity atoms on grain boundaries. A big disadvantage of this effect is the embrittlement of grain boundaries by segregation after a heat treatment. Hence an alternative method is presented for production of a nanocomposite with higher stability against grain growth and minor grain boundary embrittlement.