【作者】 刘鑫旺；

【导师】 郭景杰； 苏彦庆；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2011， 博士

【摘要】 γ-TiAl基合金具有优良的高温强度、抗蠕变、抗氧化和阻燃性能,而且密度低、弹性模量高,有望作为结构材料应用于航空航天及汽车等领域,但TiAl合金的热成形仍然存在一些问题,如铸造成形后组织粗大,锻造成形时流变应力高。热氢处理技术可以细化组织,改善热变形性能,其原理是将氢作为一种临时性合金元素,利用氢的作用细化合金的组织,改善合金的高温变形性能。但由于氢在固态合金中的扩散速率相对较慢,热氢处理技术主要应用于小型薄壁工件。为克服传统热氢处理技术置氢周期长,效率低的不足,本课题组发明了一种新的置氢技术,定义为液态置氢,该技术的实施过程是在氢气/氩气中熔炼合金,氢气分解并扩散进入合金熔体进而达到置氢的目的。液态置氢技术具有较高的置氢效率,可与合金熔炼同时进行,适用于对各种尺寸的工件进行置氢。本文将液态置氢技术应用于TiAl合金,研究液态置氢对TiAl合金凝固组织和力学性能的影响,以及液态置氢对TiAl合金熔体的净化作用。本文首先对液态置氢的置氢量和置氢时间的控制这一基础问题进行了研究,包括氢在TiAl合金熔体中的溶解度及扩散系数。结合热力学理论和实验数据,得到了氢在TiAl二元合金熔体中的溶解度模型,建立了氢在TiAl二元合金熔体中的溶解度与合金成分、氢分压和熔体温度的关系。经实验验证,氢在TiAl合金熔体中溶解度的实验值与计算值吻合较好。在本实验条件下,将氢在TiAl合金熔体中的扩散近似为一维扩散,利用菲克第二定律,并采用误差函数法估算得到了氢在TiAl合金熔体中的等效扩散系数约为5.5×10-3 cm2/s。氢在TiAl合金熔体中溶解度和扩散系数的获得为实施液态置氢技术奠定了基础。研究了液态置氢去除Ti-47Al合金熔体中的杂质元素O、C和N的作用。发现液态置氢对O和C具有比较显著的脱除作用,而对N的脱除作用极其微弱。在液态置氢过程中,随着氢分压和熔炼时间的增大,Ti-47Al合金中的O和C含量逐渐降低。利用吉布斯自由能的变化对液态置氢去除TiAl合金中的O和C元素的机理进行分析,O和C元素含量的降低是由于氢与这些元素在合金熔体表面发生化学反应所造成的。对Ti-44Al,Ti-47Al及Ti-49Al二元合金进行了液态置氢并研究了液态置氢对TiAl合金的凝固组织的影响。液态置氢后,这三种不同成分的TiAl合金的凝固组织均得到了显著的细化,TiAl合金由粗大的柱状晶变成比较细小的等轴晶。液态置氢后合金中除了四重对称的树枝晶之外,还出现了六重对称的树枝晶。液态置氢细化TiAl合金的机理是由于氢降低了临界形核功,同时促进了固液界面前沿溶质Al原子的富集,增强了固液界面前沿的成分过冷,提高了成分过冷度。液态置氢还能够细化Ti-47Al合金的固态相变组织,减小Ti-47Al合金的片层厚度。对经液态置氢处理的Ti-47Al合金进行真空退火除氢后发现,细小的晶粒得以保留。利用Gleeble热模拟机研究了液态置氢后的Ti-47Al合金高温塑性变形行为,发现在所研究的变形条件下,经液态置氢处理的合金的流变应力和峰值应力均有所降低,降低的程度随着置氢量的提高而增强。如在1150℃,1×10^-2 s-1变形时,1 at.%的置氢量可将峰值应力降低约33 %。TEM观察发现,置氢后的Ti-47Al合金中动态再结晶的程度显著增强,还发现氢的存在减少了合金中的位错缠结,同时还观察到置氢后孪生变形的堆垛层错数量有所增加。更多还原

【Abstract】 γ-TiAl based alloys have excellent high-temperature strength, creep resistance, oxidation resistance, flame retardant, low density and high elastic modulus, which is promising to be applied to aerospace and automotive industries. However, there are some disadvantages for forming of TiAl alloy, such as the coarse structure after the casting and high flow stress during the forging. The thermohydrogen treatment （THT） is a technique in which hydrogen is used as a temporary alloying element to be introduced into the alloys. Grain refinement and improvement of hot deformation performance can be obtained by the positive effect of hydrogen.Because the diffusion of hydrogen in solid alloy is relatively slow, the conventional THT is usually applied to small-scale or thin-wall samples. In order to overcome the disadvantage of low efficiency of the conventional THT, a novel hydrogenation technique was put forward by our research group, which is melt hydrogenation technique （MH）. In the MH processing, the alloys are melted in argon/hydrogen mixture. The hydrogen decomposes and diffuses into alloy melt and retains in the alloys during the cooling process. The MH is applied to TiAl alloy in this study, and its effect on solidification structure, mechanical property and removal of impurities are studied.The control of hydrogen content and duration during MH process was studied first, including solubility and diffusion coefficient of hydrogen in TiAl alloy melt. A formula of hydrogen solubility in TiAl alloy melt was obtained based on thermodynamic theory and experimental data. The relationship of hydrogen solubility in TiAl binary alloy melt between composition, partial pressure of hydrogen and melt temperature was established. The experiment value of hydrogen solubility is consistent with the calculated values. The equivalent diffusion coefficient of hydrogen in TiAl alloy melt was estimated by assuming one-dimention conduction, together with Fick’s second law and error function method, which is 5.5×10^-3 cm2/s.The removal effect of MH on the impurities O, C and N in TiAl alloy melt was investigated. The method showed good deoxidation, decarburization and weak denitrification effect on TiAl alloy melt, as evidence by the decrease of impurities when the partial pressure of hydrogen and the melting duration were increased. The deoxidation, decarburization and denitrification effect of hydrogen was analyzed via change of Gibbs free energy together with the hydrogen action in alloy melt, which is due to the reactions of hydrogen with the impurities near certain regions of the melt surface.The MH was performed on Ti-44Al, Ti-47Al, Ti-49Al alloys and its effect on solidification structures was studied. After MH, the coarse columnar grains of TiAl alloys become much refiner columnar plus equiaxed grains, and some grains with the six-fold symmetry appear except the four-fold symmetrical grains. The hydrogen-induced refinement is due to hydrogen-decreased critical nucleation energy and hydrogen-enhanced constitutional supercooling ahead of the solid-liquid interface. In addition, the lamellar spacing of Ti-47Al alloy was decreased by MH.The effect of hydrogen on hot deformation of Ti-47Al alloy was studied using a Gleeble hot simulator. At the appropriate temperature and strain rate, the flow stress and peak stress of Ti-47Al alloy are reduced with increasing hydrogen addition. The peak stress can be reduced by about 33 % with 1 at.% hydrogen addition at 1150°C, 1×10-2 s-1. There are less dislocation tangles and more twinning and stacking faults in hydrogenated alloys. The decrease of flow stress and peak stress by melt hydrogenation is attributed to hydrogen-induced dislocation mobility and hydrogen-promoted dynamic recrystallization and twinning.更多还原