Studies On Microstructure Mechanisms Of Plastic Deformation And Fracture Of Coarse And Fine-Grained Magnesium Alloys

Magnesium(Mg)alloys have poor plastic forming ability,which often causes rupture failure during deformation processes.The primary reason is that the relevant microstructure mechanisms are unclear.Applying a local strain nucleus model and a crystal rotation defect model based on the complex potential method in plane elasticity and conducting the characterization of Election Backscatter Diffraction(EBSD)and optical microscopy,this study theoretically analyzes and experimentally verifies the microstructure mechanisms of plastic deformation and fracture for coarse and fine-grained Mg alloys.In coarse-grained Mg alloys,fracture is mainly originated from the stress concentration resulting from the dislocation pile-ups at the site of twinning junctions within grains.This paper models and calculates the local stress field and the stress intensity factors arising from the deformation twinning based on the dislocation-based strain nucleus model and the Green's function method.The results show that: the twin junction is a main origin of brittle cracks(in accordance with the EBSD results),the critical tension of crack nucleation increases with decreasing grain size(d),and the dependence of the critical tension on d is similar to the classical Hall-Petch relationship;with the increase of the ratio between the thickness and length of barrier twin(q),the critical tension and critical size respectively increase and decrease;cracks may propagate along the twin boundaries and deflect at the high density twinning region and the site of twin junctions(in accordance with the corresponding metallographic characterization).In fine-grained Mg alloys,fracture is mainly based on the cracking of precipitates and the void nucleation at triple junctions.This study models the toughening effect of the nanscale twinning induced by the stress concentration at the particle/matrix interfacial crack tip on fine-grained Mg alloys by using the rotational disclination model.It is found that: the fracture toughness of the interface strongly depends on the particle radius and the distribution under the toughening effect of the nanoscale twinning;the optimum precipitate radius and distribution can significantly strengthen the interface,which is consistent with previous experiments;the study on nanotwinning toughening effect can essentially mend the underestimation of classical fracture toughness model.Furthermore,this study also discusses the competition and interaction between coordination mechanisms during superplastic deformation.The effects of grain sliding cooperating with migration during plastic deformation on the precipitate cracking of fine-grained Mg alloys were theoretically modeled and analyzed.The results show that: the precipitate cracking and grainboundary migration are two typical coordination mechanisms,which can coordinate the stress concentration resulting from grain boundary sliding at the sites of precipitate interfaces and triple junctions and continue the plastic deformation;the strain accumulation around a precipitate during superplastic deformation is more inclined to cause particle fracture for a larger particle radius;the fracture toughness of the particle is significantly improved as the shear modulus ratio of the particle to Mg matrix increases;in addition to the hardening and refining of the precipitates,the recrystallization based on grain boundary migration can also weaken the coordination mechanism on the basis of the particle cracking and improve the fracture toughness.In this thesis,the study of micromechanics can determine the parameters of microstructures and the deformation process that enhance the fracture toughness,and aid to optimize the plastic deformation process and to improve the finished product rate of Mg alloys.