A Phase Field Study On The Phase Transformation And Fracture Behavior Of Tetragonal Zirconia

Zirconia ceramics are widely used in aerospace,biomaterials and other fields because of their excellent properties such as high strength and high fracture toughness.The mechanical behavior of zirconia ceramics is closely correlated to the phase transformation from tetragonal to monoclinic(t-m).The high fracture toughness of zirconia ceramics mainly originates from the transformation toughening effect of the stress induced t-m transformation on the propagating crack.The reversibility of t-m transformation makes zirconia ceramics a potential high-temperature shape memory ceramic.However,spontaneous transformation in zirconia ceramics may also be accompanied by the nucleation of intergranular microcracks and the degradation of the mechanical properties.Meanwhile,the mechanical behavior of zirconia ceramics is also related to the ambient temperature and crystal orientations,etc.The current researches on zirconia ceramics are mostly based on experiments and transformation criterions or macroscopic constitutive models,while the mesoscopic numerical models to study the mechanical behavior of zirconia ceramics from microstructure evolution are still rare.In this thesis,the phase field(PF)models for simulating the evolution of t-m transformation and crack propagation in tetragonal zirconia were established,and the effects of grain size and temperature on t-m phase transformation were taken into account.For the first time the mechanical mechanisms of the fracture behavior and transformation toughening effect in tetragonal zirconia,and transformation induced intergranular microcracking were revealed from mesoscopic scale.And the effects of grain size and ambient temperature on the phase transformation and macroscopic mechanical response of tetragonal zirconia were discussed.The main contents of the thesis are as follows:(1)The coupled phase field model for simulating t-m transformation and crack propagation in single crystalline tetragonal zirconia was established,and the effect of t-m transformation on crack propagation behavior in single crystals with crystal orientations of0° and 90° was studied.The simulation results showed that the t-m transformation nucleated at the crack tip and then propagates around.The t-m transformation could significantly reduce the stress level at the crack tip and restrain the crack propagation,bringing about the toughening effect.For the crack propagating along the vertical direction,the toughening effect of the single crystal with crystal orientation of 90° was stronger than that of 0°.(2)In order to simulate the transformation induced microcrack nucleation at the grain boundary in tetragonal zirconia polycrystal(TZP),the phase field model for simulating the fracture process was improved with tension-compression decomposition of the elastic energy,so that compression induced unreasonable fracture was avoided.On this basis,the coupled PF model for simulating the t-m transformation and grain boundary microcrack nucleation in TZP was constructed.With the developed crack phase field model,the stress field and the microcrack nucleation induced by a single variant with pure shear eigenstrain was studied.The validity of the phase field model was verified by comparing the simulation results with the theoretical results.Then,with the coupled PF model,the t-m transformation induced intergranular microcrack nucleation at the grain boundary in TZP was studied.By analyzing the microstructure of the martensitic variants and the distribution of microcracks,it was found that the width of the variant and the angle between the variant and the grain boundary had important influences on the transformation induced microcrack nucleation.The mechanical mechanism of transformation induced intergranular microcrack nucleation was revealed in this work.(3)The phase field model characterizing the temperature effect and grain size effect of t-m transformation in TZP was established.In this model,the chemical free energy functional characterizing the t-m transformation was modified to consider the effect of the temperature on the energy barrier of t-m transformation.Meanwhile,additional grain boundary energy was introduced into the PF model to describe the inhibition of the grain boundary on phase transformation,and the interface energy between martensite twins was differentiated from that of austenite-martensite interface.With the established PF model,the mechanical response of TZP under uniaxial tension and compression was studied.The effects of grain size and ambient temperature on the shape memory effect and pseudoelasticity of TZP were discussed,and the physical mechanism of the asymmetry of the mechanical response of TZP under tension and compression was analyzed.(4)Based on the crack PF model with single well potential energy functional,a coupled PF model was constructed to simulate the crack propagation and nucleation process and the t-m transformation process in TZP.With the constructed PF model,the stress-induced transformation at the static crack tip was studied,and the effects of the ambient temperature,load and crystal orientations on the size and shape of the transformation zone at the crack tip were discussed.The toughening effect of phase transformation on static crack propagation was revealed by stress analysis.Then,with the developed coupled PF model,the crack propagation behavior in TZP was studied.The simulation results showed that crack propagation in TZP was in complex patterns such as crack deflection,branching and convergence.On this basis,the effects of the toughness of grain boundary,crystal orientations and external load on the crack propagation path and morphology were discussed.When the crack propagated along a fixed path,the toughening effect of t-m transformation on crack propagation was quantitatively characterized by calculating the energy release rate of crack propagation.