亚快速定向凝固Fe-Al-Ta三元合金的组织形成机理

发布时间：2018-06-18 23:28

本文选题：定向凝固 + Fe-Al金属间化合物　；参考：《西安建筑科技大学》2017年硕士论文

【摘要】：金属间化合物具有低密度、高熔点、高强度以及优异的抗氧化腐蚀等特点,是一类极具潜力的新型中高温结构材料。金属间化合物既优于高温合金的耐温性,又优于陶瓷材料的韧性与可热加工性,受到材料研究者的广泛关注。在备受关注的金属间化合物种类中,Fe-Al系金属间化合物以其较小的密度、低廉的价格以及优越的性能,使其一度成为中高温结构材料研究的热点。本文采用亚快速定向凝固技术,通过向Fe-Al金属间化合物中添加微量的Ta元素,制备出Fe-Al-Ta共晶自生复合材料。借助先进的分析设备和测试仪器,在较大范围的凝固速率内深入分析亚快速定向凝固的工艺参数对相成分、固液两相界面形态以及凝固组织生长特征等的影响。同时也探讨了Fe-Al-Ta共晶自生复合材料凝固组织与力学性能的关系,分析了共晶合金的表面显微硬度、室温拉伸断口、三点弯曲断口以及强化机制。通过EDS和XRD分析,真空感应熔炼态(铸态)和定向凝固不同凝固速率下的Fe-Al-Ta共晶自生复合材料均由基体相Fe(Ta,Al)和增强相Fe2Ta(Al)两相组成。在不同凝固速率下Fe-Al-Ta共晶自生复合材料的稳态生长区横截面宏观组织形貌逐渐细化,出现均匀到非均匀的转变。结合稳态生长区的微观纵截面和横截面得出,合金凝固组织形态呈现出由棒状→球状→乱序短棒状的演化规律。且在凝固速率R为90μm/s到600μm/s的范围内,合金的增强相体积分数出现先减后增的变化,当凝固速率为R=150μm/s,合金增强相体积分数最小为36.11%。合金组织的片层(棒状)间距逐渐减小,其关系符合λ=75.99R-0.79,基本满足J-H理论模型。Fe-Al-Ta共晶自生复合材料的固液界面受凝固速率影响较小,主要是胞枝状界面。但随着凝固速率的增大,胞枝状组织分枝出现变化,由规则的一次分枝到多分枝再到不规则的多分枝演变。在不凝固速率下,合金的表面硬度差距较大,且有逐渐增大的趋势。在室温拉伸试验时,真空感应熔炼态Fe-Al-Ta共晶合金和亚快速定向凝固下Fe-Al-Ta共晶合金均表现为脆性断裂。定向凝固Fe-Al-Ta合金中大的Laves强化相体积分数逐渐减小的层片(棒状)间距以及较高的过冷度均有利于共晶合金室温抗拉强度的提高,使得其最大抗拉强度远远高于铸态Fe-Al-Ta共晶合金。由三点弯曲试验曲线以及宏观断口形貌分析,Fe-Al-Ta共晶合金属于解理断裂。但在高倍显微镜下对试样分析发现,定向凝固条件下Fe-Al-Ta共晶合金的断口有少部分韧窝出现,为Fe-Al-Ta共晶复合材料室温断裂韧性提高做出了贡献。
[Abstract]:Intermetallic compounds have the characteristics of low density, high melting point, high strength and excellent oxidation resistance. Intermetallics are better than superalloys in temperature resistance, toughness and heat processability of ceramic materials, and have been paid more attention by material researchers. Among the most concerned intermetallic compounds, Fe-Al intermetallics have become a hot topic for their low density, low price and superior properties. In this paper, Fe-Al-Ta eutectic in-situ composites were prepared by adding a small amount of Ta to Fe-Al intermetallic compounds by subrapid directional solidification technique. With the help of advanced analysis equipment and testing instruments, the effects of technological parameters of sub-rapid directional solidification on phase composition, solid-liquid two-phase interface morphology and solidification microstructure growth characteristics were analyzed in a wide range of solidification rates. The relationship between solidification microstructure and mechanical properties of Fe-Al-Ta eutectic in-situ composites was also discussed. The surface microhardness, tensile fracture at room temperature, three-point bending fracture and strengthening mechanism of eutectic alloy were analyzed. By means of EDS and XRD analysis, the Fe-Al-Ta eutectic in-situ composites in vacuum induction melting (as-cast) and directionally solidified Fe-Al-Ta eutectic composites at different solidification rates are composed of two phases, the matrix phase (Fe _ (10) Ta-Al) and the reinforced phase (Fe _ (2) Tao _ (Al). At different solidification rates, the microstructure of the stable growth zone of Fe-Al-Ta eutectic in-situ composites was gradually refined and changed from uniform to non-uniform. Combined with the microcosmic longitudinal section and cross section of the steady growth zone, it is found that the solidification microstructure of the alloy shows the evolution law of the short rod shape from the rod-shaped orbicular order. In the range of solidification rate R from 90 渭 m / s to 600 渭 m / s, the volume fraction of the reinforcing phase of the alloy decreases first and then increases. When the solidification rate is 150 渭 m / s, the minimum volume fraction of the reinforcing phase is 36.11. The lamellar (rod) spacing of the alloy structure decreases gradually, and the relationship accords with 位 _ (75.99R) -0.79, which basically meets the J-H theoretical model. The solid-liquid interface of Fe-Al-Ta eutectic in-situ composites is less affected by solidification rate, mainly the dendritic interface. However, with the increase of solidification rate, the branching of cellular dendritic tissue changes, from regular primary branching to multi-branching to irregular multi-branching. At the unsolidification rate, the surface hardness of the alloy has a large gap and tends to increase gradually. In the tensile test at room temperature, both the vacuum induction melting Fe-Al-Ta eutectic alloy and the sub-rapid directionally solidified Fe-Al-Ta eutectic alloy exhibit brittle fracture. The large volume fraction of Laves strengthened phase in directionally solidified Fe-Al-Ta alloy decreases the lamellar (rod) spacing and the higher undercooling degree is beneficial to the increase of the room temperature tensile strength of the eutectic alloy. The maximum tensile strength of Fe-Al-Ta eutectic alloy is much higher than that of as-cast Fe-Al-Ta eutectic alloy. The fracture of Fe-Al-Ta eutectic alloy belongs to cleavage fracture by means of three-point bending test curve and macroscopic fracture morphology. However, the fracture surface of Fe-Al-Ta eutectic alloy under unidirectional solidification condition has a few dimples, which contributes to the improvement of fracture toughness of Fe-Al-Ta eutectic composites at room temperature.
【学位授予单位】：西安建筑科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG21

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