【作者】 高强；

【导师】 张俊善； 郭建亭；

【作者基本信息】 大连理工大学 ， 材料学， 2006， 博士

【摘要】 长程有序B2结构金属间化合物NiAl由于具有密度低、导热性好、良好的抗高温氧化能力和较好的高温稳定性等优点,成为当前令人瞩目的一类材料,并有希望取代现有的Ni基、Fe基高温合金应用于更高的温度和更恶劣的环境中。但NiAl的室温脆性和高温强度低,阻碍了NiAl及其合金的实际应用。在脆性的NiAl基体中加入难熔的金属相形成共晶合金是改善NiAl合金性能的一种有效的方法。其中,等轴晶或柱状晶NiAl-28Cr-（6-x）Mo-xHf（1≥x≥0.15,原子百分比）共晶合金具有优异的高温强度和蠕变性能,而比重仅比NiAl增加6%,抗氧化性能也与NiAl相当。但存在于NiAl/Cr（Mo）相界的Ni₂AlHf相使相界呈台阶状,这使得合金的室温压缩塑性和韧性降低。本文研究了B、Dy的微合金化和喷铸、吸铸、粉末冶金及喷射沉积等先进的制备工艺对NiAl-Cr（Mo）共晶合金的性能的影响以及其机制。 在真空非自耗炉中分别制备不同B和Dy含量合金的钮扣锭来进行微合金化研究,所有合金锭均在高温箱式电阻炉中进行匀质化退火处理,制度为:1523K/24h,炉冷。快速凝固、粉末冶金和喷射沉积所用的母合金在真空感应炉中熔炼。快速凝固工艺采用喷铸和吸铸两种方法,粉末冶金采用热压预合金粉的方法,喷射沉积采用氩气为雾化气体,对沉积坯进行了1573K/150MPa/3h热等静压处理。 微量B和Dy的加入细化了铸造NiAl-28Cr-5.8Mo-0.2Hf合金的显微组织并偏聚到共晶胞界起到强化晶界的作用。B除了偏聚到共晶胞界外还分布在Cr（Mo）相中,过量B的加入会导致枝晶组织的出现,当B含量达到0.1wt%时,合金的微观组织主要为枝晶组织。Dy则完全偏聚到共晶胞的胞界,在所加入的范围,Dy没有引起枝晶组织的出现。微量B和Dy的加入强化了晶界,使铸造NiAl-28Cr-5.8Mo-0.2Hf合金的室温断裂方式由胞间断裂变为胞内断裂。微量B加入使铸造NiAl-28Cr-5.8Mo-0.2Hf合金的室温压缩塑性提高,高温强度不变。Dy加入量为0.1wt%时,合金的综合压缩性能最佳。合金性能的改善可归结为共晶层片细化及晶界强化。通过对真空感应熔炼NiAl-28Cr-6Mo（含有0.1Dy+0.015B,wt%）和NiAl-28Cr-5.7Mo-0.3Hf普通铸造合金的室温和高温压缩性能的对比表明微量B和Dy在NiAl-Cr（Mo）合金中的作用优于少量Hf的作用。 利用喷铸法和吸铸法两种快速凝固工艺分别制备了NiAl-28Cr-6Mo（含有0.1Dv+0.015B,wt%）和NiAl-28Cr-5.7Mo-0.3Hf两种共晶合金。由于共晶NiAl-Cr（Mo）合金的固液区间狭窄,其喷铸的成形性较差,吸铸能促进液态金属的流动从而改善更多还原

【Abstract】 NiAl has received much attention as a promising candidate in the search for a low-density, high-strength structural alloy for high-performance turbine engine applications owing to its attractive advantages over the currently used nickel-based superalloys, including low density, high melting point, excellent thermal conductivity and exceptional oxidation resistance. Unfortunately, the room temperature ductility and toughness as well as the high temperature strength of NiAl based alloys have not been successfully overcome which hold up the practical implementation of them. Research on NiAl has indicated that a number of eutectic alloys consisting of NiAl possess both improved toughness and creep strength. Additionally, incorporation of a refractory metallic phase within the brittle NiAl matrix is one useful technique which has been investigated as a method to obtain better property of NiAl alloy. In particular, the equiaxed and columar NiAl-Cr（Mo）-Hf alloys developed by Guo and his group exhibits much better elevated temperature strength and creep resistance with only 6% weight increase and inappreciable loss of oxidation resistance. But the Ni₂AlHf Heusler phase, existing at the NiAl/Cr（Mo） interface, makes the NiAl/Cr（Mo） interface serrated and leads to the lower ductility and toughness. In this paper, the microalloying and various fabrication methods are tried to improve the mechanical properties of the NiAl-Cr（Mo） eutectic alloys by microstructure refinement.The button billets containing different amount of Dy or B used for microalloying study are arc-melted under an argon atmosphere using a non-consumable tungsten electrode. The button billets are homogenized at 1523K for 24h in air followed by furnace cooling to RT. The maser alloys for rapid solidification, powder metallurgy and spray and deposition investigations are prepared in vacuum induction furnace. Both injection casting and suction casting are conduction for rapid solidification investigation. The hot pressing is used to consolidate atomized powders in powder metallurgy route. The molten master alloy is atomized by argon, and then at 1573K the deposited billet is hot isostatic pressed under 150MPa for 3 hours.The minor Dy and B additions lead to microstructural refinement and eutectic cell boundary strengthening through segregation at the cellular boundary in cast NiAl-28Cr-5.8Mo-0.2Hf alloy. The B is partitioned both into Cr（Mo） phase and at the phase interface in 0.1wt% B containing alloy, the extra B results in the dendritic NiAl and the dendriticmicrostructure dominates in 0.1 wt% B containing alloy. The Dy is distributed at the cellular boundary and no dendrite occurs in Dy containing alloys in the investigated Dy range of 0-0.2 wt%. The RT compressive ductility is proportional to the B content in the B containing alloys without improvement in high temperature compressive strength and the balance of properties is attained at the amount of 0.1wt% Dy for Dy containing alloy. The improvement in compressive is attributed to the boundary strengthening and microstructural refinement. The effecct of trace B and Dy is superior to that of small amount of Hf in the conventionally cast NiAl-Cr（Mo） eutectic alloy in terms of compressive behavior.The lamellar refinement and composition change in constituent phase takes place during the rapid solidification. The rapid solidification castability is weak for the NiAl-Cr（Mo） eutectic alloy owing to the narrow liquid/solid zone and the suction casting can improve the castability by accelerating the flow of the molt. The primary NiAl dendrites are introduced by the rapid solidification because the pseudo-eutectic region is moved to the high-molting-point phase in pseudo binary phase diagram. The improvement of high temperature compressive strength has been achieved for the injection cast NiAl-28Cr-6Mo and suction cast NiAl-28Cr-5.7Mo-0.3Hf alloys and dissolution of Hf in NiAl phase is beneficial to theThe atomization process produces the powders which are uniformly refined size. The powders are contaminated in the atomization process and the porosity was the main defect in the powder metallurgy alloy and the microstructure of the hot pressed prealloyed powders is also refined and uniform. In creep, the microstructure of the hot pressed NiAl-28Cr-6Mo is invarious and the cracks originate fron the porosity. The hot pressed NiAl-28Cr-6Mo alloy exhibited a high-level combination of fracture toughness and tensile strength, whereas the tensile creep resistance of the PM alloy was degraded. The addition of 0.5wt% Zr further enhances the mechanical properties of the PM alloy.Many irregular pores are unevenly distributed through thickness of the deposited NiAl-28Cr-6Mo alloy and the hot-isostatic-pressing can decrease the porosity greatly. The microstructure of the alloy produce by spray deposition technique is refined and the alloying element concentration of the constituent phases is extended. No new phase is introduced in the spray deposition process. The pores of the sprayed billet are decreased and the microstructure is hardly changed by HIPing. The compressive properties are improved by atomization and spray technique compared to convention! casting and the compressive creep mechanism is attributed to the dislocation climbing by self-diffusion.At the strain rate of 2×10^-3s^-1, the suction cast alloy containing Hf has the highest high temperature compressive strength, the spray deposited alloy exhibits the best RT compressive ductility and the hot pressed NiAl-28Cr-6Mo alloy containing Zr presents the optimal combination of RT and high temperature compressive properties in the all studed alloys.更多还原