Study On Micromechanisms Of Fracture In Engineering Materials And Optimum Microstructure Design In Ductile Iron

The micromechanisms of fracture in engineering materials have been studied in this thesis, with particular emphasis on the dislocations behavior in the front of crack tip and the processes of crack nucleation and propagation on nanometer scale. The relation between the microprocesses of fracture and the macromechanical behaviors were investigated, which will be helpful to the strengthening and toughening of engineering materials. As an example, optimum microstructures of ductile iron were designed according to the observations on the microprocesses of fracture in the last part of this thesis. The results shown that the ductile irons with optimized microstructure exhibit excellent mechanical properties.The microprocesses of fracture in commercial purity metals (Fe, Al, Ti and Mg) were observed by in-suit tension in TEM. It was found that many dislocations were emitted from the crack tip during the early stage of crack propagation and were driven out of the crack tip area, and them piled up inversely. A closed dislocation free zone(DFZ) was formed between the crack tip and the pile-up dislocations. Keeping displacement constant, the dislocations were still emitted from the crack tip for a while. The less the lattice friction is, the longer time the dislocated emission keeps.The size of DFZ and the morphology of pile-up dislocations would change during the crack propagation. The length of DFZ increased as applied stress increased. Keeping displacement constant, the length of DFZ decreased .The less the lattice resistance is, the longer the DFZ is.During the crack propagation, the pile-up dislocations may move forward in the previous direction or in another direction which caused shape of DFZ changed .The dislocations emitted from the crack tip might pile up in the DFZ and can be served as dislocation source in the successive loading.Nanocrack might nucleate at a crack tip or in the DFZ. In the thicker area, a ribbon thin area or a zigzag thin area might be formed by slip. In the ribbon thin area, the crack propagated by straight manner or by zigzag manner. While in the Zigzag thin area, the crack can only propagate by zigzag manner.In the fracture of titanium, deformation twins might forme in the front of crack. TheVAbstractcrack propagated through the deformation twins by step manner.The grainboundary repealed the dislocations emitted from the crack tip. Many dislocations were emitted from the grainboundary in the front of crack and were driven out of grain boundary area, leaving dislocation free Zone in the front of grainboundary. A concept of grainboundary dislocation free zone is proposed.The deformation in the front of crack tip in magnesium was single slip, which resulted in the thinning in the area of crack tip. And then a laminar structure was formed in the front of the crack tip. A microcrack might nucleate at the crack tip and propagate ahead perpendicular to the laminar structure. Local cleavage fracture might happen in the front of crack tip.The microprocesses of deformation and fracture in 18-8 Stainless steel were observed by in -situ tension in TEM. It was found that the dislocations were emitted from the grainboundary and dislocation source inside a grain and slip along their slip system respectively. Partial dislocation could also be emitted from the grainboundary, and moved into the grain, and that leaving behind a stacking fault. The width of stacking fault increased as partial dislocations move forward.In the fracture of 18-8 stainless steel, many dislocations were emitted from the crack tip during the early stage of crack propagation and moved forward, and then a DFZ was formed between the crack tip and the pile-up dislocations. The DFZ is an elastic Zone. The DFZ might fragment and rotate as crack propagation.The grainboundary might absorb dislocations emitted from the crack tip, and emitted dislocations into neighbor grain, and them leaving behind a dislocation free Zone in the front of a grainboundary, thi...