摘要
近年来,由于其相对钛合金更为优异的综合性能,钛基复合材料引起人们广泛关注。目前,钛基复合材料最重要、最有潜力的应用领域之一是在航空航天结构材料以及航空航天发动机材料。为提高高温钛合金的性能及使用温度,钛基复合材料应该具有高比强度、高比模量,更为重要的是,应在高温条件下有高的强度、优异的抗蠕变性能、可靠的热稳定性、抗氧化性以及高的疲劳强度。针对这一目标,本论文开展了一系列基础性研究。研究了耐热钛基复合材料的微观结构,室温、高温力学性能,高温蠕变性能,其中,重点研究了复合材料的高温强化机理,取得了以下主要研究结果:
运用真空自耗电弧炉熔炼以及热加工、热处理技术,简洁、低成本地原位合成了以近α合金为基体,TiB、TiC和La_2O_3多元强化的耐热钛基复合材料。经热加工和热处理,复合材料和基体合金以层片组织为主。复合材料中的增强体为TiB短纤维、TiC颗粒以及La_2O_3颗粒,TiB短纤维沿加工方向形成了定向排布,增强体分布均匀,其中La_2O_3主要以纳米级的细小颗粒弥散地分布在基体中。
耐热钛基复合材料的室温和高温拉伸性能相对基体合金有大幅度的提高。增强体体积分数10%的三元增强耐热钛基复合材料(TiB+TiC+La_2O_3)/Ti相对基体合金的强化效果最高。复合材料的塑性是晶粒细化与增强体含量综合作用的结果,增强体含量为2.4%的复合材料实现了强度和塑性的同时提高。室温失效机制主要是TiB短纤维的承载断裂,强化效果取决于增强体含量。耐热钛基复合材料在高温条件下以韧性断裂为主。在选定的增强体含量范围,600℃时增强体含量5%的三元增强钛基复合材料具有最高的抗拉强度。而在650-700℃时,增强体含量2.4%复合材料具有最高的抗拉强度。耐热钛基复合材料的高温断裂机理与室温时明显不同,TiB短纤维长径比较低的复合材料以短纤维端部与基体脱粘为主,增强体含量2.4%的复合材料优异的高温性能主要来源于其高的TiB短纤维长径比。
耐热钛基复合材料的高温拉伸性能对应变速率非常敏感,应变速率降低时,高温抗拉强度降低,断裂延伸率提高。在10-3/s应变速率的条件下,基体的等强温度在600℃附近,而10-5/s应变速率的条件下基体的等强温度远低于600℃。随温度上升、应变速率降低,TiB短纤维的临界长径比提高,使得界面脱粘更容易发生,从而严重降低复合材料的高温抗拉强度。温度越高,应变速率越低,复合材料的高温强度对TiB短纤维长径比分布的依赖性越强,并超过了对增强体体积分数的依赖。
耐热钛基复合材料的稳态蠕变速率比基体合金以及IMI834合金低了1~2个数量级,增强体的加入显著提高了复合材料的蠕变抗力。门槛蠕变应力理论很好地解释了复合材料较高的表观应力指数,经门槛应力补偿后,复合材料的真应力指数与基体合金相同。复合材料减速蠕变阶段耗时远大于基体合金,且减速蠕变阶段与稳态蠕变阶段之间存在较长的过渡区。复合材料和基体合金的持久强度和断裂延伸率均有明显的下降,这主要是由于晶界和界面结合强度进一步降低,以及空气中长时间高温持久试验造成的表面氧化。
随试验应力的不同,基体合金和复合材料均出现低应力区和高应力区,基体合金在低应力区和高应力区的应力指数分别为2和4.5。复合材料蠕变抗力的强化主要来自于门槛应力和应力传递效应。复合材料中高的表观应力指数主要是来源于门槛应力,而应力传递效应对高应力区的表观应力因子有一定的影响,但其作用远低于门槛应力。La_2O_3颗粒对门槛应力的提高作用主要体现在高应力区,且门槛应力值主要取决于增强体在复合材料中的弥散程度,而与增强体体积分数关系不大。应力传递效应主要依赖于TiB短纤维的长径比,随增强体总体积分数单调递减,且低应力区的传递因子显著高于高应力区。复合材料蠕变抗力的强化效果对增强体的形态特征依赖更强。
600℃和650℃热暴露均使得耐热钛基复合材料的室温塑性有所降低,而700℃热暴露使室温塑性有所回升。引起室温塑性降低的主要因素为α层片晶界处析出的Ti3Al有序相,这种有序相在基体中的固溶温度在650℃~ 700℃之间。增强体在热暴露过程中稳定性良好,与基体没有界面反应发生。镧元素在熔炼过程中可吸收熔体中的氧元素,形成纳米La_2O_3陶瓷颗粒,弥散强化基体合金,进一步提高复合材料的热稳定性。
Recently, titanium matrix composites have drawn great attention due to the superior properties over titanium alloys. At present, the most important and promising application of titanium matrix composites lies in the field of aero-space, where they serve as structural materials and materials for aero-space engines. As the potential materials in place of high temperature titanium alloys, titanium matrix composites should be of high specific strength and specific modulus. Moreover, the performances at high temperatures call for high strength, excellent creep resistance, solid thermal stability and oxidation resistance, and high fatigue strength. Aiming at the above goal, fundamental research was done in the thesis. Via observing microstructures of the matrix alloy and TMCs, testing tensile properties at room and high temperatures, high temperature creep behaviors, and thermal stability of the matrix alloy and TMCs were tested, the enhancement mechanisms of TMCs at high temperatures were investigated in particular.
Near-alpha based heat resistant titanium matrix composites (TMCs) reinforced with TiB, TiC and La_2O_3 were synthesized by common casting and hot-forging technology. After hot working and heat treatment, the basic microstructures of the matrix alloy and TMCs were fully lamellar. The reinforcements in the composites were TiB whiskers, TiC particles and La_2O_3 particles. TiB whiskers showed good alignment along the hot working direction. The reinforcements were uniformly distributed in the TMCs, and sizes of La_2O_3 particles were in the nano-scale.
Tensile strengths of TMCs at room and high temperatures were significantly enhanced by the reinforcements. The tri-reinforced composite (TiB+TiC+La_2O_3)/Ti with 10% volume fraction of reinforcements showed the highest tensle strength. Ductilities of TMCs were deterimined by the competing effect between the refinement of matrix microstructure and the volume fraction of reinforcements. Both strength and ductility of the composite with 2.4% voume fraction of reinforcements were enhanced. Failure mechanisms at room temperature were load bearing and fracture of TiB short fibers, and the enhancement of strength was mainly determined the volume fraction of reinforcements. Fracture mechanisms of TMCs at high temperature were mainly ductile. The composite with 5% volume fraction of reinforcements showed the highest strength at 600℃. At 650℃and 700℃, the composite with 2.4% volume fraction of reinforcements showed the highest tensile strength. The fractreu mechanism of TMCs with low aspect ratios of TiB whisker was mainly the interfacial debongding between the matrix and ends of TiB whiskers. The excellent high temperature tensile properties of the composite with 2.4% volume fraction of reinforcements were attributed to the high aspect ratios of TiB whiskers.
High temperature tensile properties of TMCs were quite sensitive to the strain rates. Tensile strength decreased and the fracture strains increased when the strain rate was lower. The equicohesive temperature of the matrix was around 873K at the strain rate 10?3s?1, and well below 873K at 10?5s?1. The critical aspect ratio of TiB whiskers increased when the temperature was higher or the strain rate was lower, leading to more drastic interfacial debonding and worse tensile properties of TMCs. At higher temperatures or lower strain rates, aspect ratios distribution of TiB short fibers had the dominant influence on the strength enhancement rather than the total volume fraction in TMCs.
Steady state creep rates of the TMCs were 1 ~ 2 orders of magnitude lower than those of the matrix alloy and IMI 834. Creep resisitance were effectively enhanced by the reinforcements. High apparent stress exponents of TMCs were successfully explained by threshold stress theory. And after compensated with threshold stresses, the stress exponents of the TMCs and matrix alloys were the same. The decelerating creep time of the composites was much lower than the matrix alloy, and there was a long transitional period between the primary decelerating creep and the second steady state creep for the composites. The matrix alloy and composites were embrittled by further weakening of interfaces and surface oxidation under creep rupture conditions.
Depending on stress levels, creep behavior of both the matrix alloy and TMCs showed two stress regions. Stress exponents of the matrix alloy were 2.0 and 4.5 in the low and high stress regions respectively. Enhancement of creep resistance in TMCs was mainly attributed to threshold stresses and stress transfer effects. The relatively high apparent stress exponents of TMCs were mainly attributed to the threshold stresses. Stress transfer effects could influence the apparent stress exponents in the high stress region, but the attribution was much lower than threshold stresses. La_2O_3 paticles were only effective to increase the threshold stresses in the high stress region. Threshold stresses were mainly dependent on the dispersion of reinforcements, while the volume fractions of reinforcements were not influential. Stress transfer effects showed a monotone decreasing relationship with the volume fractions of reinforcements, and the stress transfer effects in the low stress region were higher than those in the high stress region. The enhancement of creep resistance was mainly dependent on the morphologies of reinforcements.
Thermal exposure at 600℃and 650℃led to loss of room temperature ductility of TMCs. However, thermal exposure at 700℃could bring back some ductility. The loss of room temperature ductility was mainly attributed to the ordered intermetallic Ti3Al, whose solution temperature was estimated to be between 650℃and 700℃. The reinforcements were stable during the thermal exposure, and showed no interfacial reaction with the matrix. Lathanum can absorb oxygen in the matrix to form La_2O_3 particles, which provided further dispersion strengthening and enhance the thermal stability of the composites.
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