【作者】 刘延斌；

【导师】 刘咏； 汤慧萍；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 钛合金及钛基复合材料由于具有优异的室温、高温力学性能,特别是高比强度、高比模量在航空航天上有着广泛的应用。由于原料价格较高,加工困难等原因,限制了钛材在民用领域的应用。粉末冶金技术可以实现中小零部件生产短流程化,而所需原料钛粉的生产工艺不断革新,生产成本不断下降,因而有望拓宽钛材在民用领域的应用。本文以开发汽车发动机用钛连杆和气门为应用目标展开研究。采用金属碳化物、6A1-4V合金粉、YH2粉、钛粉等为原料粉,通过混料,冷等静压,真空烧结等工艺获得烧结坯,采用热轧、热锻和热挤压等热变形手段实现材料的全致密化和获得必要的尺寸制得粉末冶金钛材。对所制备的材料进行了热变形行为、室温和高温力学性能、抗氧化性能以及耐磨性能等进行了系统研究,并对材料制备过程中的微观组织演化,拉伸断口组织,氧化试验后和摩擦磨损试验后样品表面的微观组织进行了详细分析。探讨了材料制备过程中的组织演化机理,材料的强化机制以及抗氧化性能的影响因素。最终采用优化后的钛材成分和加工工艺,制备出了钛基复合材料连杆和气门,并进行台架试验。通过以上研究得到了如下结论：1.金属碳化物的种类和烧结后的热变形对TiC/Ti基复合材料中TiC颗粒的分布影响较大,金属碳化物和6A1-4V合金粉的添加量对TiC颗粒分布影响很小。金属碳化物与钛的反应驱动力大小影响两相区扩张和TiC颗粒形核与长大对C元素的获取能力,从而影响最终TiC颗粒的分布。反应驱动力大,TiC颗粒形核与长大获取C能力强,因而在较窄的区域内形成大量的TiC颗粒,使得其分布不均；反应驱动力小,两相区扩张获取C能力强,因而在较为宽的区域内形成均匀分布的TiC颗粒。热变形能够促进TiC颗粒的均匀分布。2.金属碳化物、6A1-4V合金粉添加量以及金属碳化物的种类对TiC/Ti基复合材料的室温和高温力学性能及不同强化机制的强化效果有显著影响。主要强化机制有：Al, Mo, V等合金元素产生的固溶强化,TiC颗粒在热变形过程中促进基体晶粒细化产生的细晶强化以及TiC颗粒在拉伸变形过程中基于载荷转移机制产生的颗粒强化。在室温时,颗粒强化效果随不均匀分布的TiC颗粒体积分数增加而减小,随均匀分布的TiC颗粒体积分数增加而增加；钛基体强度对颗粒强化的影响甚小。在高温时,增加TiC颗粒的体积分数以及在TiC颗粒体积分数一定时通过增加合金元素来提高钛基体的强度有利于延缓钛基复合材料的强度衰退。3.基体合金的种类对基体p-Ti相和TiC颗粒在氧化过程中的演化有重要影响。富Mo的p-Ti相在氧的作用下转变成a-Ti相,并且Mo也固溶于相应a-Ti相中,有利于提高材料的抗氧化性能。富V的p-Ti相氧化产物中的V205具有很强的挥发性,使得氧化层产生大量微裂纹加速氧的扩散,同时为TiC颗粒的氧化提供有利条件,降低材料的抗氧化性能。4.热加工图表明烧结态Y2O3/Ti基复合材料具有十分优异的热变形行为,理想的变形区间为：700-850℃和应变速率为0.001-0.01s-1的范围内以及温度为900-1000℃和应变速率为0.001-0.1s-1的范围内。前者的变形机制为片层球化或α相的动态再结晶；后者的变形机制为p相的动态再结晶。Y203强化相颗粒在烧结后的热机处理过程中形成,并且能够显著提高材料的强度和塑性。5.尽管有残余孔隙的存在,细小的基体组织使得粉末冶金钛基复合材料烧结坯体具有十分优异的热变形性能,可进行进一步的热锻造、热轧制和热挤压等。采用烧结,热挤压,电镦粗和模锻工艺路线制备出了综合性能优异的TiC/Ti基基复合材料气门。通过烧结,预成型和模锻工艺路线制备出力学性能优异的Y2O3/Ti基复合材料连杆。更多还原

【Abstract】 Titanium alloys and titanium matrix composites have widely application in aerospace due to their excellent mechanical properties at ambient and elevated temperature, especially high specific strength and modulus. The high cost from both alloy materials and metalworking limits titanium parts for civil use. Recently, the cost of the Ti powder becomes effectiveness due innovation of manufacturing process. As a near net forming process, the powder metallurgy may expand the use of titanium in civil application. The present work aims to fabrication automobile parts such as valve and connect rod with good application properties and low cost. The metal carbids,6A1-4V powder, YH2 powder and titanium powder were blended,cold isostatically pressed, sintered, and finally hot deformed such as hot rolled or hot pressed to achieve fully densified titanium matrix composites. The deformation behavior, mechanical properties at ambient and elevated temperature, oxidation and wear behavior of the composites were systematic investigated. The microstructure evolution during fabrication, fracture surface, surface microstructure after oxidation and wear test were obseverd. The mechanism of the microstructure evolution, strengthen and oxidation were discussed. The optimized titanium matrix composites and their deformation process were used to fabrication connect rod and valve. These automobile parts are bench tested. The results show:1. The distribution of TiC particles is greatly influenced by the type of metal carbides and hot deformation after sintering and is less dependent on the addition amount of metal carbides and 6A1-4V powder. The carbon element is completely obtained by the expanding of the two-phase reaction zone and precipitation of TiC during reaction sintering. The ability is determined by the driving force for reaction between Ti and metal carbide in the case of Mo2C and VC. The high driving force is benefit to precipitation of TiC, then the particles is formed in a narrow area and inhomogenously distributed. While the low driving force is benefit to the expanding of the two-phase reaction zone, and the homogenous distribution of TiC particles is achieved.2.The mechanical properties and the effectiveness of the strengthening mechanism at ambient and elevate temperature are influenced by the addition amount and type of metal carbides and amount of 6Al-4V powder. The main strengthening mechanisms are:solid solution strengthening of Al,Mo and V, grain refinement strengthening and particulate strengthening due to load transformation. At ambient temperature, the strengthening increment is decreased with increasing of inhomogenous distribution of TiC particles and increased with increasing of homogenous distribution of TiC particles. At elevate temperature, increasing the volume fraction of TiC particles and the titanium matrix strength by addition alloy element with constant volume fraction of TiC particles can delay the degration of mechanical properties.3.The matrix alloy elements greatly influence the evolution ofβ-Ti and TiC particles during oxidation. The Mo richβ-Ti is transferred to a-Ti promoted by the oxygen and the Mo element is soluted in the correspondeda-Ti, then increasing the oxidation resistance of the composites. The micro-cracks are formed in the oxidation layer due to the volatilization of V2O5, hence the V richβ-Ti is fast passageway of oxidation and oxygen diffusion. On the other hand, the oxidation of TiC is promoted by the micro-cracks. Hence, the V element is detrimental to the oxidation resistance of the composites.4.The processing map shows that the as sintered Y2O3/Ti matrix composite has excellent hot deformation properties. Efficiency peaks in the temperature-strain rate regions of 700-900℃and 10-3-10-2 s-1 and of 950-1000℃and 10-3-10-2 s-1 are identified to dynamic recrystallization and grain coarsening, respectively. Y-rich particles with a low content of oxygen form after sintering, instead of oxides. The Y2O3 particles are formed during thermal mechanical treatment and can greatly improve the mechanical properties of the composites.5. Despite existing of residual pores, the powder metallurgical titanium matrix composites has good hot deformation behavior due to the fine matrix microstructure and can be further hot forged, rolled and pressed. TiC/Ti matrix composite valve with good application properties is fabricated by sintering, hot pressing, upsetting and mold forging. Y2O3/Ti matrix composite connected rod with good mechanical properties is fabricated by sintering, pre-forming and mold forging.更多还原