- 學位論文
以微觀力學模型模擬超高溫複合陶瓷之潛變破壞
Micromechanics Modeling of Creep Fracture of the Ultra-High Temperature Ceramic Composites
摘要
超高溫陶瓷是一可廣泛應用於高溫環境的耐火材料，其中ZrB2-SiC的複合陶瓷因其低密度與優異的高溫耐火性質，目前受到相當大的重視，然其高溫潛變破壞會限制其於高溫中之應用，是亟待解決的問題。許多實驗結果都指出ZrB2-SiC的複合陶瓷潛變抗性會隨著工作溫度的增加而減少，並發現當溫度高於1500 oC之後，其潛變機制由晶界擴散轉換為晶界滑移的破壞機制。最近的實驗觀察更指出在溫度高於1800 oC時，於ZrB2-SiC的ZrB2晶粒中觀察到差排的滑動，而SiC晶粒的變形行為則比較像剛體運動。 本論文的目的即為發展微觀力學理論模型，並透過有限元素計算模擬探討ZrB2-SiC複合陶瓷之潛變破壞與其微觀機制。由於潛變破壞牽涉到許多不同尺度的機制，其主要的微觀機制可以分為三種：晶界的擴散效應、晶粒本身的潛變與晶界間的相對滑移。因此本研究發展了一考慮晶界開裂，晶界相對滑動與晶粒潛變或差排滑動之微觀力學理論模型，以釐清超高溫陶瓷在不同溫度與不同材料組成之破壞機制與潛變行為。透過有限元素法，將其微觀力學模型實作在知名的商業軟體ABAQUS中。利用ABAQUS提供的使用者自訂材料(UMAT)分別實作描述等向性晶粒的潛變微觀力學模型，以及考慮差排滑動的晶體塑性力學去模擬晶粒的行為。另一方面我們利用ABAQUS中的使用者自訂元素(UEL)實作時變率相關之內聚力模型(CZM)，以模擬晶界的滑動以及空孔生成與擴張行為。 我們首先考慮晶界的非均質性對潛變破壞的影響。在不考慮非等向性與非均質的ZrB2材料系統中，其材料的強度會受到應變率以及晶界的性質所影響，晶界的非均質性會導致裂痕生長，進而而造成潛變破壞。利用等向性的潛變模型(UMAT)模擬晶粒的行為，配合內聚力模型(UEL)來模擬晶界的特殊行為，如晶界的孔隙生成係數以及擴散係數，以及控制所外加的應變率，以了解晶界的非均質性對潛變破壞的影響。模擬結果顯示晶界的非均質性會降低材料的潛變抵抗能力。 我們接著考慮在沒有差排滑動的情況下，利用等向性的潛變模型(UMAT)模擬ZrB2與SiC晶粒的行為，配合內聚力模型(UEL)模擬ZrB2之間與ZrB2以及SiC之間的介面，以探討ZrB2與SiC複合陶瓷的潛變機制，當升溫超過1500 oC的情況下，ZrB2- SiC複合陶瓷的破壞行為將由晶界相對滑動所控制，模擬的結果指出，ZrB2- SiC之間的介面必須有空孔生成的能力，其晶粒滾動的現象將會大大的降低ZrB2- SiC複合陶瓷抵抗潛變的能力。 在溫度超過1800 oC的環境下，本研究使用能模擬材料塑性滑動的非等向性晶體塑性力學以模擬ZrB2與SiC晶粒的塑性行為，配合內聚力模型(UEL)模擬之ZrB2- SiC複合陶瓷間的晶界，以探討ZrB2與SiC複合陶瓷的潛變機制，本研究利用此一晶體塑性力學力學模擬，驗證幾何必要差排成形成為材料晶粒軟化的一種機制，模擬結果與實驗結果相當吻合，晶界的非均質性提供了一個較容易產生孔隙的位址，其亦為晶界滑動所必須的互補機制。模擬結果亦顯示出差排的產生是因為ZrB2與SiC之晶粒相對滑移產生的不諧和變形所造成的。
並列摘要
Ultra-high temperature ceramics has great applications as refectory materials at high temperature. ZrB2-SiC has been considered as an excellent candidate of the ultra-high temperature ceramics due to its relatively low density and excellent refractory ability. However, the creep fracture of ZrB2-SiC limits its potential utilities. To mediate the creep fracture is thus imperative. It has been concluded from several experiments that the creep resistance of ZrB2-SiC reduces when an elevated temperature increases. There also exists a transition region of creep resistance for temperature above 1500oC for ZrB2-SiC composites. Moreover, recent experiments have shown that above 1800oC, dislocations were observed within the ZrB2 grains but the SiC grains behaved like a rigid body. The thesis is aiming to develop a micromechanics model by means of finite element analysis to simulate the creep fracture and related deformation mechanisms. Since several length scales are involved in creep fracture, there are three major micromechanisms: grain creep, grain boundary diffusion and sliding. A micromechanics model considering grain boundary separation, grain boundary sliding and the creep or dislocation slip of grain was hence developed to simulate intergraunlar creep fracture of the ZrB2-SiC at elevated temperature in the present study. By finite element method, this micromechanics model was implemented in a commercial software ABAQUS. An isotropic creep deformation behavior and a crystal plastic model considering crystalline slip were implemented via ABAUQS UMAT, a user-defined material, for grains. For grain boundaries, a rate–dependent cohesive zone model was implemented using the user-defined element (UEL) in ABAQUS. The influence of creep resistance by the effect of grain boundary heterogeneity was studied first. The creep resistance of an isotropic, homogeneous ZrB2 polycrystalline material is affected by the applied strain rate and the grain boundary properties. Grain boundary heterogeneity would initiate the microcrack and thus lead to fracture. A polycrystalline model composed by grain interiors constituted by the isotropic creep (UMAT) and grain boundaries modeled by a rate-dependent cohesive zone model (UEL) was built to study the heterogeneity effect on grain boundary, e.g. grain boundary nucleation and diffusivity, and rate dependent effect on different applied strain rates was studied. Simulation results indicate that the grain boundary heterogeneity reduces the creep resistance for ZrB2 polycrystalline materials. Our focus then moved to the inhomogeneous grain aggregates without slip deformation. An isotropic grain interior modeled by UMAT along with the grain boundary simulated by a rate-dependent cohesive zone modeling (UEL) was constructed to study the creep fracture of ZrB2-20%SiC composites. The micromechanism of ZrB2-20%SiC composites when temperature is raised above 1500 oC is predominated by grain boundary sliding. Numerical results indicate that cavity nucleation at ZrB2-SiC interface is a necessary accommodation for grain boundary sliding. The phenomena of grain rotation jeopardize the creep resistance of ZrB2-20%SiC at high temperature region. An advanced micromechanics model which composed by crystal plasticity for grain interior and cohesive zone model for grain boundary was used to study the creep mechanism for ZrB2-SiC composites when temperature is above 1800oC. The slip deformation of ZrB2 grain is considered as a complementary mechanism for grain boundary sliding. The simulation results showed a good agreement with experimental EBSD data, such that the grain boundary heterogeneity provided a preferred site for cavity nucleation which is necessary to grain boundary sliding. The simulation results also revealed the dislocation was induced by an incompatible deformation nearby the ZrB2-SiC interface.