Preparation of Bulk Amorphous Al2O3-ZrO2-Y2O3 Ceramics and Investigation on the Moderate Temperature Elastic-plastic Deformation

Author:Xu Xi Qing

Supervisor:liu jia chen

Database:Doctor

Degree Year:2017

Download:19

Pages:139

Size:13103K

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Based on the structural nature of brittleness in ceramics,we introduced the amorphous structure into ceramics;through destroying the crystalline structures,weakening the interatomic bonding,we aimed to get new deformation mechanism and achieve macroscopic deformation in ceramics.In this work,bulk amorphous Al2O3-ZrO2-Y2O3 ceramics were prepared and their elastic-plastic deformation at moderate temperatures was studied;furthermore,the deformation mechanisms were analyzed.Two-step hot pressing solved the competition between densification and crystallization,and bulk amorphous ceramics with uniform amorphous phase were achieved,with relative density of up to 97.7%.Compared to polycrystal counterparts,amorphous samples had disordering microstructures,low density,and exhibited low hardness,low strength,low elastic modulus and high deformation strain at room temperature.Under uniaxial compression,the amorphous Al2O3-ZrO2-Y2O3 ceramics suffered brittle fracture at 500°C,but exhibited significant plastic deformation of 15.4%and21.5%at 600°C and 700°C,respectively.The yield stress in plastic deformation was independent of the strain rate;the deformed sample maintained amorphous and exhibited multiple shear bands.The plastic strain was resulted from shear banding along with structural densification,and shear banding was confirmed as the dominating mechanism.Free volume provided open space for atomic migration,and shear bands were formed due to directional migration of mass atoms under shear stress,contributing the structural basis for plastic deformation of amorphous ceramics.The elastic deformation of amorphous ceramics at moderate temperature was achieved by round trip migration of free volume in amorphous matrix;the large interatomic space and disordering microstructure in amorphous phases provided large free path for free volume to make round trip migration;therefore,amorphous ceramics exhibited large elastic strain and high rebound ratio.The residual deformation was mainly resulted from structural densification and formation of microcracks or shear bands.With higher metastability degree,the amorphous phase owned larger free volume and more disordering microstructure,beneficial to the migration of free volume and atoms;therefore,the amorphous ceramics exhibited better plasticity and higher rebound rate.With increasing crystallinity degree,amorphous ceramics exhibited an unusual behavior of brittle-plastic-brittle transition.Tiny nanocrystalline particles resulted in larger fluctuation of free volume distribution,and led to increased free volume in the vicinity of nanocrystalline/amorphous matrix interfaces,promoting the shear banding and plastic deformation.As the crystallinity degree was too high,the total amount of free volume was too low,which prevented the nucleation and propagation of shear bands.Nanocrystalline particles also promoted the formation of microcracks,which was harmful to the rebound rate of amorphous ceramics.Phase separation had similar influence on moderate temperature deformation of amorphous ceramis,leading to enhanced plastic deformation and decreased rebound rate.Under uniaxial compression at 500°C,brittle fracture occurred in amorphous A20Z and A40Z;however,plastic deformation of 9.6%and 13%were exhibited in amorphous A60Z and A80Z,respectively,and tiny t-ZrO2 crystallite were detected after deformation.Ductile transformation temperature(Td)should be noticed between brittle fracture and ductile deformation;Td was an intuitive criterion for plasticity,and lower Td represented better plasticity.The crystallization activation energy(ΔG)of amorphous ceramics exhibited similar variation with Td,as plastic deformation and crystallization were both related to atomic migration ability;therefore,ΔG could be regarded as an intrinsic factor dominating the plastic deformation ability in amorphous ceramics.