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粉末冶金是一种低成本制备碳化钛及钛零部件的工艺技术。然而，在粉末冶金制备碳化钛及钛材料过程中，均存在烧结温度高，致密化难度大等问题。引入合适的烧结助剂，可以有效降低材料烧结温度，促进其致密化过程。铁作为一种低成本粉末冶金的理想的烧结助剂，被广泛应用于碳化钛及钛的烧结致密化过程中。常规的铁烧结助剂添加方法为机械混合法，但由于铁与粉体材料存在密度差，混合过程中难以均匀分散，严重影响到烧结产品微观组织的均匀性，制约了材料的应用。为解决这一问题，本文分别以碳化钛及钛两种粉体为基体，通过流化床化学气相沉积技术制备了具有核壳结构的铁包覆碳化钛复合粉体及铁包覆钛复合粉体，有效改善了复合材料的微观均匀性，提高了材料的烧结性能。论文取得了如下研究成果：合适的气相前驱体是流化床化学气相沉积制备铁包覆复合粉体的关键和基础。本文系统研究了FeCl3的气化及分解热力学过程。FeCl3蒸气压随温度变化较为显著，气化温度应控制在327 °C以内。在FeCl3-H2体系中，为抑制杂质FeCl2的生成，提高反应转化率，可以将摩尔配H2比提高至FeCl3:H2=1:50以上，并控制反应温度在560 °C以上。FeCl3作为一种性能优良的铁前驱体，可以应用于制备TiC-Fe复合粉体，但在Ti的Fe包覆过程中会腐蚀Ti粉。采用二茂铁代替FeCl3作为铁前驱体，可以有效避免沉积过程对Ti的腐蚀。以FeCl3为铁前驱体制备TiC-Fe复合粉体，对Fe沉积过程动力学规律及失流机理展开深入的讨论。流化过程中的失流现象是由于微米级自形核Fe颗粒与TiC表面定向生长的Fe颗粒发生烧结团聚，形成巨大的TiC-Fe团聚体导致的。优化后的气化温度为275 °C，反应温度为600 °C。制备的TiC-Fe复合粉体包覆均匀致密，界面结合优良。TiC-14Fe包覆粉体热压烧结后材料致密度比混粉法制备的材料提高了2.36%。以二茂铁为前驱体，制备得到了Fe含量可控的核壳结构Ti-Fe复合粉体。沉积过程较优的气化温度为160 °C，反应温度为500 °C。在烧结过程中，包覆型Ti-Fe复合粉体可形成各向同性较好的α-Ti等轴晶，可在各个方向上均匀的承受及传递外加载荷。沉积过程产生的少量C，在烧结过程中可生成TiC，极大提高了材料的抗压强度及屈服强度，但会导致材料塑性有一定程度降低。 ;Powder metallurgy is a low-cost process for preparing titanium carbide and titanium parts. During the preparation of titanium carbide and titanium materials by powder metallurgy process, some problems including high sintering temperature and hard to densification are not ignorable. Introduction of suitable sintering aids can effectively reduce the sintering temperature of the material and promote its densification process. Iron is an ideal sintering aid for low cost powder metallurgy. It has been widely used in sintering and densification of titanium carbide and titanium. The conventional way for adding iron sintering aid is powder mixing method. However, due to the density difference between iron and the base material, some problems such as uneven dispersion are easy to occur during the mixing process. It will seriously affect the uniformity of the microstructure of the sintered product, which restricts the application of materials. In order to solve this problem, titanium carbide and titanium powders are used as substrates in this paper. The Fe-coated TiC composite powder with core-shell structure and the Fe-coated Ti composite powder were prepared by the process of fluidized bed chemical vapor deposition. It improves the microscopic uniformity of the composite material effectively and improves the sintering performance of the material. The following results were obtained:Proper gas-phase precursor is the key and foundation for preparing iron-coated composite powder by fluidized bed chemical vapor deposition. The thermodynamic process of gasification and decomposition of FeCl3 was systematically studied. The vapor pressure of FeCl3 changed significantly with temperature, and the gasification temperature should be controlled within 327 °C. In the FeCl3-H2 system, in order to control the formation of impurity FeCl2 and improve the reaction conversion rate, The mole ratio of H2 should be increased to above FeCl3:H2=1:50, and the decomposition temperature should be controlled above 560 °C. As an excellent iron precursor, FeCl3 could be used to prepare TiC-Fe composite powder. But in the process of Ti-Fe coating, FeCl3 would corrode the Ti powder. Using ferrocene as the iron precursor avoided the corrosion of the Ti powder effectively during the deposition process.FeCl3 was used as Fe precursor. The deposition process dynamics rule of Fe and defluidization mechanism were deeply studied. Due to the sintering and agglomeration of micron-scale self-nucleating Fe particles and the directional growth of Fe particles on TiC surface, the defluidization phenomenon during the fluidization process occured easily after the formation of giant TiC-Fe agglomerates. The optimized gasification temperature of the deposition process was 275 °C, and the reaction temperature was 600 °C. The coating layer was uniform and dense and a good interfacial bonding condition was observed. After hot-pressed sintering, the density of the TiC-14Fe material was about 2.36% higher than the material prepared by mixing powder.The coated Ti-Fe composite powder with controllable Fe content was prepared by using ferrocene as the precursor. The preferred gasification temperature during deposition was 160 °C and the reaction temperature was 500 °C. During the sintering process, the coated Ti-Fe composite powder could form equiaxed crystals of α-Ti with better isotropy. It could withstand and transfer external loads evenly in all directions. The C deposited during the coating process would react with Ti and TiC was generated. It could greatly improve the compressive strength and yield strength of the composites. But the plasticity of titanium matrix composites reduced.
|刘城. 铁包覆复合粉体的可控制备及烧结行为研究[D]. 中国科学院大学,2020.|
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