In-situ Tem Investigation Of The Plastic Behaviors On The Crack Tip Of Small-sized Single Crystal Niobium

Micon/nano-scaled body-centered cubic(BCC)metals have been broadly applied into structural materials due to their outstanding mechanical properties.However,the plasticity of BCC meatals,especially the plasticity during the process of crack propagation,are not systematically investigated,which makes our understanding the deformation mechanism of micon/nano-scaled BCC metals are heavily rely on the theoretical prediction.Thus,more incisive studies are highly expected to enrich relevant theories.In-situ transmission electron microscopy(TEM)enables efficient and incisive observations of microstructure evolutions and thereby has been broadly applied to revealing the time-resolved plastic behaviors of metals.This dissertation is focus on the plastic deformation mechanisms of BCC Nb during crack propagation through in-situ TEM tension tests,which take the advantage of our self-developed in-situ bimetal tension actuators.These studies will provide a new insight into the plastic deformation mechanisms of BCC metals.The mainly covers innovative concerns as followed:(1)Nb sample for in-situ TEM tension tests were prepared by focused-ion beam technique and transferred onto the tension actuators for in-situ tension tests.The multiplication of dislocation loops and activities of arc-like dislocations in front of the crack during tensile tests in BCC Nb were revealed by in-situ TEM tests.Reasonable models were proposed to rationalize the interesting dislocation phenomena and provide experimental evidences for previous theories;(2)The plastic deformation near the crack tip of BCC Nb was revealed by in-situ high-resolution TEM(HRTEM)at the atomic scale.It turns out that dislocation activities,phase transition and polycrystallization can cooperate to accommodate the plastic deformation of BCC Nb in front of crack;(3)The in-situ atomic-scale investigation showed the stress-induced phase transition and phase transition-induced lattice reorientation in BCC Nb.Temperature,residual stress and sample thickness can promise the stability of FCC Nb.We discovered that the lattice reorientation through BCC ? HCP(hexagonal closed-packed)?BCC phase transition process can enhance the fracture toughness.