Study On Toughening Mechanism Of Amorphous AlN And Its Application In Dual-phase AlN And Nanoglass AlN

Ceramics are widely used in many fields due to their excellent properties.However,the brittleness of ceramics strongly restricts their applications.It has been reported recently that amorphous state can efficiently improve the toughness of ceramics,but the underlying mechanism remains unclear.In this dissertation,taking AlN as the research object,we investigate improvement of the mechanical properties induced by amorphous AlN?a-AlN?and the corresponding deformation mechanisms using molecular dynamics simulations.We further study the mechanical properties of dual-phase AlN and nanoglass AlN based on properties of amorphous structure.The main progresses achieved are listed as follows:?1?It is found that the elongation ratio of a-AlN is 33.3%larger than that of crystalline w-AlN,which can be attributed to self-repairing mechanism in deformed a-AlN.Different from the brittle fracture along the cleavage plane in w-AlN,in a-AlN,shear bands form by alternately inducing mechanism of the shear transition zone?STZ?and vortex,then voids nucleate,grow and coalesce in the shear bands,leading to the final failure of a-AlN.a-AlN has three short-range ordered structures?SRO?and different kinds of medium-range ordered structures?MRO?,which are randomly made up of SRO structures.The tensile and compressive responses of a-AlN are asymmetric.The asymmetric yield surface can be well described by Drucker-Prayer yield criterion.The crystallization in a-AlN under compression is more obvious than that under tension,which leads to the asymmetry before yield point.The difficulty in the formation of shear bands and the intersection of PSBs during compression result in the asymmetry after yield point.?2?The strength of a-AlN is less sensitive to the notch compared with that of w-AlN,and the fracture toughness of a-AlN is better than that of w-AlN,which can be attributed to the short-range ordered atomic arrangement,which is induced by reconstruction of broken bonds and the rotation of bonds during deformation.The ductility and failure process of intact a-AlN and notched a-AlN are different.The deformation in intact a-AlN is dominated by the formation of shear bands,followed by the nucleation,growth and coalescence of voids.While in notched a-AlN,STZs begin to form near to the notch,followed by the nucleation,growth and coalescence of voids.?3?The study of the effects of volume fraction,specific surface area and distribution pattern of crystalline nanoparticles on the mechanical properties of dual-phase AlN nanocomposites?nc-composites,with a-AlN matrix reinforced with crystalline w-AlN nanoparticles?indicates that the nc-composite containing volume fraction of f_v?40.9%and triangular distribution of particles would possess both higher strength and ductility.Stress concentration exists in the interface zone,and the particles and matrix are subjected to tensile and compressive stress,respectively.Strain hardening occurs in the cases of f_v?40.9%,attributed to the intersection between shear bands.Transformation from wurtzite structure?B4?to graphene-like structure?GL?occurs layer by layer in the cases of f_v=40.9%,as a result of high hydrostatic stress.However,the GL phase does not further transform into rock-salt phases?B1?,which is different from the crystalline structure under compression.Plastic performance of nc-AlN is less dependent on the specific surface area,because the extension of the shear bands is restricted by the crystalline particles of high shear modulus.The distribution pattern of the crystalline nanoparticles plays an important role in the mechanical performance of nc-AlN.Triangular distribution of crystalline nanoparticles?TD?may be reasonable compared with the square and random distributions?SD and RD?.Although the immature shear bands can form in nc-AlN with either TD or SD,their movement would be restricted in the cases of TD.After yield point,the deformation of nc-AlN with RD would be similar to that of a-AlN.?4?The effects of nano-glassy particle size,temperature,and strain rate on the mechanical properties of nanoglass AlN?ng-AlN?are investigated with MD simulations.When the grain size is reduced to 1 nm,the ng-AlN exhibits super ductility,and the deformation is uniform without void formation,which can be attributed to the formation of a large number of uniformly distributed STZs during deformation.The elastic modulus of ng-AlN_d=1nm is not sensitive to temperature,but ultimate strength is and follows the scaling law of T^2/3.At high temperature,uniform diffusive flow accompanied by diffusive rearrangement may occur under low stress conditions,leading to the decrease of the strength.The ultimate strength is positively related to the strain rate by a power function.At higher strain rate,the time for atomic diffusion and free volume rearrangement is shorter,which leads to the decrease of the available free volume,which hinders the activation of STZ.The results obtained in this dissertation are of great significance for understanding the enhancement mechanisms of ductility of amorphous ceramics and can provide support for design of high performance ceramic materials.