Effect of theθ-α-Al2O3 transformation on the oxidation behavior ofβ-NiAl + Zr

Isothermal oxidation of NiAl + Zr has been performed over the temperature range of 800–1200°C and studied by TGA, XRD, and SEM. A discontinuous decrease in growth rate of two orders of magnitude was observed at 1000° C due to the formation of -Al₂O₃ from -Al₂O₃. This transformation also resulted in a dramatic change in the surface morphology of the scales, as a whisker topography was changed into a weblike network of oxide ridges and radial transformation cracks. It is believed that the ridges are evidence for a shortcircuit outward aluminum diffusion growth mechanism that has been documented in a number of¹⁸O tracer studies.     

NiAl表面能的分子动力学计算

利用嵌入原子类型的势函数，通过分子动力学方法计算了Ｂ２结构标准化计量比ＮｉＡｌ合金的四个低指数晶面在６种温度时的表面能。计算结果表明，表面能随温度的升高呈增加趋势；最密排面的表面能最低；一些表面在高温下发生了重构或成分偏析等表面反应，用类似方法计算了ｆｃｃ结构Ｎｉ的表面能，计算结果与已知实验研究基本一致。     

Effect of NiAl Microcrystalline Coating on the High-Temperature Oxidation Behavior of NiAl–28Cr–5Mo–1Hf

The effect of an NiAl microcrystalline coating prepared by magnetron sputtering on the high-temperature oxidation behavior of NiAl–28Cr–5Mo–1Hf was investigated in static air at 1000–1150°C. The additions of Cr, Mo, and Hf changed the single -phase structure into a multiphase structure [-NiAl, -Cr(Mo), and Heusler phase]. The NiAl–28Cr–5Mo–1Hf alloy formed a nonprotective mixed scale of Al₂O₃+Cr₂O₃+HfO₂ and exhibited relatively large weight gains. The large weight gains were attributed to extensive internal oxidation. The sputtered NiAl microcrystalline coating remarkably improved the oxidation resistance of NiAl–28Cr–5Mo–1Hf due to the formation of a compact and adherent Al₂O₃ scale at all test temperatures. It was found that the --Al₂O₃ transformation caused the anomalous behavior of the oxidation–kinetics curves of the NiAl microcrystalline coating in the temperature range 1000–1150°C. A change in the morphology of scales occurred with the transformation.     

Tungsten wires and porous NiAl prepared through directional solidification and selective dissolution

The combination of directional solidification and selective dissolution was applied to fabricate tungsten (W) wires and porous NiAl matrix. A NiAl–W pseudobinary eutectic alloy with 1.5?at.% tungsten was directionally solidified in a Bridgman-type oven at 1700°C. Results confirmed that the relationships of the growth rate with the interfibrous spacing and diameter of W fibrous phases in the directionally solidified samples are in accordance with the Jackson and Hunt (J?H) model. Afterward, the NiAl matrix was selectively dissolved in an HCl:H₂O₂ solution to reveal W wires, which present various three-dimensional (3D) morphologies at different growth rates. The W fibrous phases in the NiAl–W alloy samples were then selectively removed with a mixed etchant of ammonium acetate to form a porous NiAl matrix at a constant potential. Dynamic corrosion curves revealed that etching W from the NiAl matrix was inhibited after 2–3?h. The porous structures of NiAl after removing W phases are linked to the 3D morphologies of W fibrous phases embedded in the NiAl matrix. The aspect ratio of W wires and the structures of porous NiAl can be adjusted by selecting the process parameters of this combined technology.     

Using Selective Electron Beam Melting to Enhance the High-Temperature Strength and Creep Resistance of NiAl–28Cr–6Mo In Situ Composites

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material.     

快速凝固NiAl-Cr（Mo）-Zr合金的显微组织与力学性能

研究了快速凝固NiAl-Cr（Mo）-Zr合金及其均匀化热处理后的显微组织和力学性能。结果表明：快速凝固NiAl-Cr（Mo）-Zr合金的组织由NiAl和Cr（Mo）两相组成,并有少量初生NiAl相,在共晶胞界处存在少量不连续分布的Heusler（Ni2AlZr）相。随着Zr含量的增加,Heusler相的数量增加,Cr（Mo）相的片层间距增大。均匀化热处理后,共晶胞界上不连续分布的Heusler相或大量消失、或全部消失,NiAl和Cr（Mo）层片状组织形貌基本没有变化。NiAl-Cr（Mo）-0.5Zr合金在应变速率为2.78×10-3s-1时,随着测试温度的升高,合金的压缩屈服强度降低。     

Chemical Reaction of In-situ Processing of NiAl/Al2O3 Composite by Using Thermite Reaction

1Introduction Alumina,asanexcellentconstructionceramics,pos sessestheattractivepropertiesofgoodhardness,chemical stability,refractorycharacterandlowdensity.However,it isbrittleatlowtemperatureforthelackoffundamental dislocationmobilityandofinsufficientsli…     

定向凝固NiAl合金的拉伸行为研究

研究了定向凝固NiAl-Mo(Hf)和NiAl-Fe(Nb)合金的拉伸行为和显微组织变化.结果表明,两种合金在一定的拉伸条件下均具有反常的屈服行为和中温脆性.反常屈服和中温脆性行为与合金中含有的Ni3Al相有关.两种合金在高温时还表现出高应变速率的超塑性变形行为.超塑性变形的主要机理是位错滑移和攀移产生的应变硬化与动态回复和动态再结晶的应变软化作用相平衡.超塑性变形试样的断口呈韧性特征,在断裂区有孔洞产生.     

多相NiAl-Cr合金的微观组织和韧脆转变行为的研究

研究了NiAl-25Cr合金微观组织和韧脆转变行为.NiAl-25Cr合金的铸态组织为β-Ni(Al,Cr),γ'-Ni3(Al,Cr)和α-Cr三相组成,挤压后沿着挤压方向共晶区被显著拉长、变直,共晶区之间并列平行排列.经挤压变形后材料为完全再结晶的细小等轴晶,晶粒大小约3～5μm.韧脆转变温度明显依赖于应变速率,应变速率提高2个数量级,韧脆转变温度(BDTT)相应提高80K.在BDTT以下,断口形貌以β/γ'解理为主,而在BDTT以上,断口形貌出现大量的裂纹,表明合金的韧脆转变后的塑性变形由相界强度所控制.     

NiAl金属间化合物的研究进展

对NiAl金属间化合物的国内外研究现状进行了系统综述,包括为改善NiAl合金力学性能和高温抗氧化性能等所采用的合金化、制备多相合金、制备复合材料、定向凝固、机械合金化、热压及热等静压、燃烧合成、微晶涂层等工艺,以及NiAl合金的超塑性行为,着重介绍及论述了合金化及定向凝固等工艺.简单介绍了NiAl合金的固溶强化磁行为.