[PO-L1] Poster Session 1

[P1-68] Collective motion of dislocation associated with local plasticity initiation and subsequent behavior in bcc metals

^○Takahito Ohmura (National Institute for Materials Science, Japan)

Nanoindentation-induced mechanical behavior was investigated for bcc metals. Pop-in event that corresponds presumably to local plasticity initiation was detected on load-displacement curves with major parameters of critical load P_c and corresponding excursion depth deltah. In a plot of P_c vs deltah, the P_c increases monotonically with the deltah, and the slope depends on alloy systems. On the other hand, brittle-to-ductile transition temperature (BDTT) also depends on the alloy systems, and the alloy with higher BDTT shows higher slope in the P_c vs deltah plot. Dislocation structures underneath the indenter were observed through TEM before and after a pop-in event. No dislocations were observed before initiation while considerable dislocations were generated right after the event. These results suggest that dislocation nucleation and multiplication occur drastically upon plasticity initiation based on collective dislocation motion. Since the BDTT is closely consistent with pop-in event, crack propagation might be related with plasticity initiation. Statistical analysis of the events suggests physical models of the phenomena. In the case of the first pop-in event, the probability of the event to the P_c is Gaussian distribution, which indicates that a thermally activated process dominates the event. For the second or later cases, the probability to the magnitude of the event shows power law function like Gutenberg-Richter model, which suggests a dislocation avalanche.