| Ho 合金化、快凝工艺及强磁场处理对NiAl 基共晶合金组织和性能的影响 | |
| Alternative Title | Effects of Ho alloying addition, rapid solidification and strong magnetice field treatment on the microstructures and mechanical properties of NiAl-based eutectic alloys |
| 盛立远 | |
| Subtype | 博士 |
| Thesis Advisor | 叶恒强 |
| 2009-05-21 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料物理与化学 |
| Keyword | Nial-cr(Mo)共晶合金 微合金化 喷铸工艺 热等静压 强磁场 室温塑性 高温强度 |
| Abstract | 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，成为潜在的新一代高温结构材料，但是其严重的室温脆性和相对低的高温强度阻碍了NiAl及其合金的实际应用。定向凝固工艺制备出的NiAl-28Cr -6Mo共晶合金具有较高的室温断裂韧性和高温性能，使此合金成为有望实用化的合金之一，但其高温强度仍低于镍基高温合金。适量Hf的加入虽然可以进一步提高该合金的高温强度，然而存在于NiAl/Cr(Mo)相界和共晶胞界的Heusler相却明显地降低了该合金的室温断裂韧性和压缩塑性。从期望取得平衡的高温强度和室温力学性能的角度出发，本文开展了三方面的研究工作，首先研究了微量Ho对NiAl-28Cr-6Mo-0.15Hf共晶合金微观组织和力学性能的影响；其次采用快速凝固喷铸工艺制备了NiAl-28Cr-6Mo-0.15Hf-0.15Ho共晶合金，研究了快速凝固工艺对合金微观组织和力学性能的影响，以及热等静压处理对快速凝固合金微观组织和力学行为的影响；最后，研究了强磁场热处理工艺对NiAl-28Cr-6Mo-0.2Hf共晶合金组织和力学性能的影响及其机理。 少量Ho的添加细化了NiAl-28Cr-6Mo-0.15Hf共晶合金的层片间距和共晶胞尺寸；当Ho的添加量超过0.2 at.%时，合金胞界组织开始粗化；当Ho量达到1 at.%时，合金开始失去胞状共晶组织的特征。微量Ho的添加导致了Ni2Al3Ho相的析出，该相具有Hexagonal晶体结构，而且在[124]晶向上发现相内存在着显微孪晶缺陷。除了Ni2Al3Ho相外，在合金中还发现了Ni3.5Al1.5Ho相和Ho2O3氧化物颗粒，而且发现Ni3.5Al1.5Ho相和NiAl基体有着一定的晶体取向关系： 。适量Ho的添加能够进一步提高NiAl-Cr(Mo)-Hf合金的室温压缩性能和高温强度，但是Ho的添加并未有效的改善合金的断裂韧性。高温热处理和热等静压处理粗化了NiAl和Cr(Mo)析出颗粒，高温热处理促使了Ho2Hf2O7氧化颗粒的析出，而热等静压处理则细化了共晶胞界的强化相并促使Hf和Ho元素向共晶胞内扩散，并析出细小的Ni2AlHf和Ni2Al3Ho颗粒。 对喷铸工艺制备的NiAl-Cr(Mo)-Hf-Ho共晶合金的微观组织和力学性能的研究发现：相对于普通铸态合金，喷铸工艺制备的合金具有细化的共晶层片间距、共晶胞尺寸和共晶胞界区域，而且层片的细化比共晶胞的细化更显著；快速凝固的高过冷度扩展了NiAl-Cr(Mo)-Hf-Ho合金的伪共晶区，减少了初生NiAl 相，由于共晶胞界的细化，NiAl-Cr(Mo)-Hf-Ho共晶胞体积分数明显增加；快速凝固能使NiAl-Cr(Mo)-Hf-Ho合金各组成相发生明显的合金元素固溶度扩展。此外，共晶胞界的Ni2AlHf和Ni2Al3Ho相更加细小，分布也更加均匀。喷铸工艺使得合金的共晶胞界附近形成了Hf和Ho的固溶体相，其分别具有Cubic和Hexagonal晶体结构；此外在共晶胞间形成一种Cr7Ni3相，具有Tetragonal晶体结构，而且相内具有层错和显微孪晶缺陷。细化的微观组织、扩展的固溶度和增加的共晶体积分数是喷铸态NiAl-Cr(Mo)-Hf合金室温力学性能提高的主要原因。此外，Ni2AlHf和Ni2Al3Ho相的细化和均布以及共晶胞内的Hf、Ho固溶体相都对合金室温性能的改善有一定的帮助。尽管组织的细化对合金的高温强度不利，但是共晶胞界弥散分布的Ni2AlHf和Ni2Al3Ho强化相完全可以弥补这一不足。热等静压处理使得喷铸合金部分共晶组织发生了粗化，同时造成部分Ni2AlHf和Ni2Al3Ho相转变为Hf和Ho的固溶体相。此外，合金中NiAl和Cr (Mo)相中沉淀析出更多的颗粒，NiAl基体中也形成了大量的可动位错，这些微观组织的优化进一步改善了合金的室温塑性和高温强度。 强磁场处理加速了NiAl-Cr(Mo)-Hf共晶合金内部的元素扩散，细化了共晶胞界的Ni2AlHf相，并使得部分Ni2AlHf相转变为Hf固溶体相。此外，强磁场处理使得Cr(Mo)相和NiAl基体之间形成了一富Cr的NiAl过渡层，并促使一些细小Cr(Mo)层片开始发生颈缩及球化变化。较高温度的强磁场处理改变了共晶合金内部的组织状态，首先是部分共晶胞被粗大的Cr (Mo)相所代替，即使存留的共晶胞，其内部的Cr(Mo)层片也发生了颈缩及球化等变化，且粗化的Cr(Mo)层片呈现出沿磁场排列的趋势，部分Ni2AlHf相存在于NiAl晶内。强磁场处理显著提高了合金的室温压缩性能，特别是室温压缩应变。 |
| Other Abstract | NiAl has been paid more attentions as a potential high temperature structural material because of its high melting point, good thermal conductivity, moderate density and excellent oxidation resistance at all elevated temperatures, but its poor high-temperature strength and serious lack in fracture toughness and ductility at room temperature limit the application of NiAl alloys. Fortunately, the NiAl-28Cr-6Mo fabricated by directionally solidification not only has relative high fracture toughness but also shows improved creep strength. But, the balanced mechanical properties of NiAl-Cr(Mo) eutectic alloy between room temperature fracture toughness and high temperatures creep strength need to improve further in order to put it in practical application. Recent research found that hafnium (Hf) is very effective in improving the high temperature strength of NiAl-Cr(Mo) eutectic alloy. Unfortunately, the Hf addition weakens the fracture toughness and compression ductility at room temperature severely due to the formed Ni2AlHf near the cell boundaries. In this paper, the effect of minor Ho addition, injection casting technique and strong magnetic field treatment on the microstructure and mechanical properties of NiAl-Cr(Mo)-Hf eutectic alloy and their mechanism were investigated, respectively. The investigations on the effect of Ho addition reveal that the Ho addition changs the microstructure of NiAl-Cr(Mo)-Hf eutectic alloy by influence the undercooling in front of solid/liquid interface. The minor addition of Ho refines the NiAl/Cr(Mo) lamellar and eutectic cell size of the NiAl-28Cr-6Mo-0.15Hf eutectic alloy, and the 0.2 at.% Ho addition coarsens the intercellular microstructure. When the Ho addition increase to 1 at.%, the alloy becomes to lose the original microstructure characteristics. The trace addition of Ho leads to the formation of Ni2Al3Ho phase, which has Hexagonal crystal structure, and along the [124] crystal orientation the micro-twin crystal defects are observed. In addition, some Ni3.5Al1.5Ho phases and Ho2O3 particles are observed in the alloy as well. It is found that the Ni3.5Al1.5Ho phase has the orientation relationship with the NiAl matrix of . The appropriate addition of Ho can improve the compressive properties of the NiAl-Cr(Mo)-Hf eutectic alloy further. But the fracture toughness of the NiAl-28Cr-6Mo-0.15Hf-0.15Ho eutectic alloy improves just a little. The high temperature treatment and hot isostatic pressing (HIP) treatment result in the coarsening of NiAl and Cr(Mo) precipitating particles. Additionally, the high temperature treatment causes the Hf2Ho2O7 precipitate; while the HIP treatment accelerates the diffusion of Hf and Ho elements into the eutectic cell. The studies on the NiAl-28Cr-6Mo-0.15Hf-0.15Ho eutectic alloy prepared by injection casting reveal that the injection-cast alloy presents a fine microstructure, i.e. the refined eutectic cell size, interlamellar spacing, intercellular zone, Ni2AlHf and Ni2Al3Ho phases, compared with the conventional cast alloy. In addition, the injection casting extends the solid solubility of elements in NiAl and Cr(Mo) phases and decreases the amount of primary NiAl phases. Moreove, Hf solid solution phase and Ho solid solution phase are observed near the eutectic cell boundary. The Cr7Ni3 stick like phase is observed in the intercellular zone, which has stack fault and micro-twin crystal defects inside. The injection-cast alloy owns much better mechanical properties than the conventional cast alloy, which should be attributed to the refined microstructure, extension of solid solubility and homogeneous distributed fine Ni2AlHf, Ni2Al3Ho phases. The HIP treatment results in the coarsening of parts of microstructure of injection-cast alloy, and the transformations of Ni2AlHf and Ni2Al3Ho phases into Hf solid solution and Ho solid solution phases respectively. More NiAl and Cr(Mo) particles precipitate in the alloy and some moved dislocations form in the NiAl matrix. The microstructure optimization caused by the HIP treatment improves the mechanical properties further, especially the elevated temperature properties. The results of strong magnetic field treated NiAl-28Cr-6Mo-0.2Hf alloy show that the strong magnetic field treatment can accelerate the elements diffusion, refine the Ni2AlHf phase and transform some Ni2AlHf phases into Hf solid solution phases at relative low temperature. Additionally, the strong magnetic field treatment still leads to the formation of a Cr containing NiAl diffusion zone between NiAl and Cr(Mo) phases, and promotes the necking and spheroidizing of fine Cr(Mo) plates. With the increase of treatment temperature, the microstructure of the NiAl-Cr(Mo)-Hf alloy changed greatly. Some eutectic cells are substituted by coarse NiAl and Cr(Mo) phases. The Cr(Mo) plates inside eutectic cell become necking and spheroidizing and aligning along the strong magnetic field direction. Furthermore some fine Ni2AlHf particles form in the NiAl phase. The strong magnetic field treatment improves the room temperature compressive ductility significantly. |
| Pages | 121 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17144 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 盛立远. Ho 合金化、快凝工艺及强磁场处理对NiAl 基共晶合金组织和性能的影响[D]. 金属研究所. 中国科学院金属研究所,2009. |
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