The Infiltration Mechanism Study Of Hf-based Alloy Into The Carbon Materials At Low Temperature

Carbon fiber reinforced ultra-high-temperature ceramic matrix composites(C_f/UHTCs) synthesize the advantages of both excellent high-temperature mechanical properties of C/C composites and outstanding ultra-high-temperature ceramic anti-oxidative ablation performance of ultra-high-temperature ceramic, and are expected to become advanced aircraft's ultra-high-temperature structural materials under extreme conditions. Reactive infiltration method has the characteristics of simple process, short preparation cycle, near net shape, low cost, no pollution and so on, becomes the optimized preparation method of carbon fiber reinforced ultra-high-temperature ceramic matrix composites. Hafnium carbide(Hf C) has high melting point, of which oxide has higher melting point and low evaporation rate. In the case, Hf C posesses outstanding ultra-high-temperature anti-oxidative ablation performance. This thesis describes how to prepare C_f/( Hf C+MC) composites using reactive infiltration. In addition, The microstructure and composition of the composites configurated by different alloys were studied by XRD and SEM. The mechanism of formation of transient liquid when high hafnium alloy infiltrated carbon/carbon conposites, alloy elements' effects on the formation of alloy liquid, carbonation reaction and infiltration depth, the factors which may affect infiltration competence and structure and the characteristic of the anti-oxidative ablation performance and mechanical properties of C_f/(Hf C+MC)composites have been analysed.In the process of infiltration, the bcc-Hf phase plays a dominant role in the formation of the instantaneous liquid phase. Compared with the Hf's silicic, it is easier for the bcc-Hf phase to react with C and release more heat, making alloy temperature increase nearly 480 K regionally. As a result, the instantaneous liquid phase forms. All kinds of Zr's silicides can react with carbon under alloy infiltration melting point and a small quantity of Si phase forms in the reaction product when the content of Zr is 60%. This fact is beneficial to the formation of but makes little contribution to the instantaneous liquid phase' formation. Additionally, Ta element can boost catbonization reaction of Hf base alloy, and Ta can expand the areas of bcc-Hf phase, which increases bcc-Hf phase content in the alloy at high temperature. Furthermore, the liquid phase' formation is accelerated.Through thermodynamic theoretical model, the effects of Ta and Si on surface tension of binary alloy of hafnium-based are calculated. With increasing Ta' content, the surface tension of the Hf-Ta binary alloy increases; on the contrary, with increasing Si' content, the surface tension of the Hf-Si binary alloy decreases.By perforating on the graphite material, the author simulates C/C composites' porosity and studies the process of infiltration structure of Hf-Si-Ta ternary alloy and factors which influence infiltrated capabilities during the process of infiltrated stucture' formation.The process of infiltration organization' formation of Hf-Si-Ta ternary alloy can be described like follows: the infiltrated liquid goes into the C/C precast body pore?the first eutectic phase(bcc-Hf(Ta) phase) separates out on the wall of hole?the liquid freezes and the eutectic phase separates out in the middle of the hole?the carbon of composites react with the first eutectic phase(bcc-Hf(Ta) phase) and the Hf C, Ta C form?the carbon of composites diffuses into the middle of the hole, reacts with the bcc-Hf(Ta) of the eutectic phase and the Hf C, Ta C form.When the composition of the alloy is the same, the size of aperture has little effect on infiltration depth when the size of aperture is smaller than 4mm. Ta can affect the infiltration ability, specifically, when the content of Ta is low, the content of the eutectic phase in liquid is small during the solidification, so the dispersed eutectic phase forms, adheres to the first eutectic phase and generates the thick bcc-Hf phase. Subsequently, the thick bcc-Hf phase is carbonized to Hf C which will jam the hole and decrease the infiltration depth. When the content of Ta is high,the content of eutectic phase in liquid is high, and the classical eutectic organization forms. In that case, the distribution of phases in the infiltration composites is homogeneous and there are no thick Hf C phase. With the increase of content of Ta, the surface tension of Hf-Ta binary alloy increases, which raises the infiltration depth. When the content of Ta is 10at%, the infiltration depth of liquid will increase sharply, reaching 40 mm.The author applies designed four hafnium base alloy and reactive infiltration to prepare C_f/( Hf C+Zr C+Ta C) composites, whose densities are 2.1~2.4g/cm3 and porosities are 6~8%. Using oxyacetylene torch ablation instrument, the author tests composites and uses three point method to examine bending strength. The examination result shows that with the increase of Ta's content, the densities of prepared materials increase; porosities decrease; the mass ablation rate decreases; the mass ablation rate of composite made by 50Hf-30Zr-10Si-10 Ta alloy is 0.053g/s, the materials forms an oxide layer. But the bending strength of prepared composites will decrease with the increase of Ta's content and the bending strength of composite made by 50Hf-25Zr-10Si-15 Ta alloy is only 100 MPa. Their load-displacement curve changes from typical brittle fracture mode to "pseudo plastic" failure mode.