INVESTIGATIONS OF LOADED CRACK TIPS IN NIAL BY ATOMIC-FORCE MICROSCOPY

In this paper it was demonstrated, that the AFM can be used as a new tool for examining local processes at crack tips. Examples are given, which show that dislocation distributions can be measured as a function of applied load in suitably oriented single crystals. From the slip step height as a function of distance from the crack tip the size of the plastic zone and number of dislocation within a slip plane can be estimated and correlated with the measured crack tip opening. With a small four point bending device constructed for in situ loading in the AFM, stable quasi-brittle crack growth was observed in NiAl single crystals. It was found that the crack propagated in micro-pop-ins. The critical COD for such a pop-in could be obtained directly from the AFM images aud compared with the fracture toughness KIC. The calculated value of 1.5 Mpa√m compares favourably with results obtained from standard fracture tests. © 1995.