La对富Ni的NiAl系合金组织与性能的影响

通过压缩和三点弯曲试验对加Ｌａ后富Ｎｉ的ＮｉＡｌ合金力学性能进行了考察。结果发现,Ｌａ的加入可以明显地细化合金的晶粒及改善ＮｉＡｌ和Ｎｉ３Ａｌ的两相分布。适量Ｌａ可提高合金的室温屈服强度和塑性,但超过一定的含量,合金的塑性反而有所下降。在合金中,引入Ｂ韧化的Ｎｉ３Ａｌ对合金的塑性有利。     

热压反应烧结NiAl金属间化合物的初步研究

高熔点（１６３８℃）既是ＮｉＡｌ的优点也是其研制的障碍之一。本文报导用热压反应烧结法制备单相ＮｉＡｌ和（ＮｉＡｌ＋Ｎｉ３Ａｌ）两相金属间化合物及其室温微观组织结构和压缩力学性能。     

NiAl基金属间化合物研究现状与前景

介绍了ＮｉＡｌ基金间化合物研究的现状与前景。ＮｉＡｌ由于具有高熔点、低密度和良好的抗氧化性等性能而被认为是下一代的高温结构材料，然而ＮｉＡｌ在室温时塑性低和高温时强度低限制了它作为工程材料的应用。对ＮｉＡｌ的晶体结构和缺陷、力学性能进行研究，采用合金化、控制显微结构和改进加工技术等方法使ＮｉＡｌ的室温塑性和高温强度都得到了提高。     

NiAl—TiC原位复合材料的室温韧化机制研究

通过热压放热反应合成工艺制备的ＮｉＡｌ－２００ｖｏｌ％ＴｉＣ原位复合材料，它的平均室温断裂韧性比单相ＮｉＡｌ提高了５０％，断口形貌及裂纹扩展路径观查表明，裂纹偏转机制是复合材料的主要韧化机制。     

富Ni的NiAl退火组织和室温力学性能

快速凝固Ｎｉ－３４．６ａ．％Ａｌ薄带经１５２３Ｋ退火２ｈ并以较快速度冷却扣形成以ＮｉＡｌ马氏体为基体，γ－Ｎｉ３Ａｌ沿晶界网状分布和少量残β－ＮｉＡｌ的组织，退火，室温弯曲延性良好。     

NiAl金属间化合物的快速凝固组织

用单辊快速凝固法制备了Ｎｉ原子分数为０．５３－０．６０的Ｎｉ－Ａｌ金属间化合物薄带，研究了成分与微观组织的关系，发现试样均为Ｂ２型单相ＮｉＡｌ，Ｎｉ０．５３Ａｌ０．４７的快速凝固组织为１０μｍ左右的等轴晶，Ｎｉ０．５６Ａｌ０．４４－Ｎｉ０．６Ａｌ０．４的组织分别为５μｍ左右的不规则和规则柱状晶，发现近理想配比的ＮｉＡｌ在快速凝固时形成针状晶亚结构。     

原位形成Al3Ti颗粒对SiCp／2024Al复合材料性能及微观结构的影响

本文采用粉末冶金法制制备了含Ｔｉ的ＳｉＣｐ／２０２４Ａｌ复合材料。研究表明，在复合材料制备工艺条件下，部分Ｔｉ与Ａｌ反应形成了Ａｌ３Ｔｉ，粗大的Ａｌ３Ｔｉ／Ｔｉ复合颗粒的存在降低了复合材料的室温拉伸强度和塑性，但可以提高屈服强度和弹性模量。     

一种第二相粒子强化的再结晶Ni3Al基合金的拉伸性能

在不同温度下对一种ＴｉＣ和Ｃｒ２３Ｃ６第二相粒子沉淀强化的变形再结晶Ｎｉ３Ａｌ基合金进行了拉伸试验。结果发现，该合金的拉伸屈服强度的峰伍温度升高到８５０℃左右。与相同处理条件下的Ｎｉ３Ａｌ基合金ＩＣ２１８相比较，该合金的高温屈服强度和拉伸强度都有显著的提高，而高温韧性损失不大。在７５０℃左右该合金的动态脆化程度最大，在该温度下的延伸率大于１０．０％。     

α—SiAlON／SiCnp复合材料在室温和高温下的显微结构与力学性能

本文采用机械混合Ｓｉ３Ｎ４，ＡｌＮ，Ａｌ２Ｏ３，Ｄｙ２Ｏ３和纳米β－ＳｉＣ粉料，通过热压烧结，制备了１０ｗｔ％纳米ＳｉＣ颗粒增强，α－ＳｉＡｌＯＮ复合材料，力学性能测试表明，在室温时复合材料的维氏硬度，压痕断裂韧性和三点弯曲强度比单相α－ＳｉＡｌＯＮ略高，但复合材料的三点弯曲强度可以保持到１０００℃，其值为时单相α－ＳｉＡｌＯＮ的两倍，断口形貌表明复合材料的晶粒尺寸比单相α－ＳｉＡｌＯＮ的小，这两     

α-SiAlON/SiCnp复合材料在室温和高温下的显微结构与力学性能

本文采用机械混合Ｓｉ３Ｎ４，ＡｌＮ，Ａｌ２Ｏ３，Ｄｙ２Ｏ３和纳米β－ＳｉＣ粉料，通过热压烧结，制备了１０ｗｔ％纳米ＳｉＣ颗粒增强α－ＳｉＡｌＯＮ复合材料。力学性能测试表明，在室温时复合材料的维氏硬度，压痕断裂韧性和三点弯曲强度比单相α－ＳｉＡｌＯＮ略高。但复合材料的三点弯曲强度可以保持到１０００℃，其值为这时单相α－ＳｉＡｌＯＮ的两倍。断口形貌表明复合材料的晶粒尺寸比单相α－ＳｉＡｌＯＮ的小，这两种材料的室温断裂方式均以穿晶断裂为主。研究表明，低粘度的玻璃相是造成单相α－ＳｉＡｌＯＮ高温性能下降的主要原因，而纳米ＳｉＣ的加入可以促使晶界相结晶，从而使复合材料的高温性能维持到较高的温度。     

Microstructures and room temperature mechanical properties of NiAl prepared by high pressure reaction sintering

The microstructures and room temperature compressive mechanical properties of a nearstoichiometric NiAl manufactured by a new high pressure reaction sintering (HPRS) process are investigated. Applying a very high uniaxial pressure (2 GPa) leads to considerable lowering of the sintering temperature and reducing the hold time gives NiAl with a good sintering density. The HPRS NiAl consists of -NiAl with a B2 structure containing a high density of dislocations, 3.5×10⁹ cm^–2, and very fine Al₂O₃ particles. The NiAl exhibits quite high true compressive strain, 14.5%, and a reasonably high yield strength, 526 MPa. The effects of employing the high pressure in the HPRS process on the reaction sintering, microstructures and mechanical properties of the NiAl are studied.     

Mechanical properties of extruded dual-phase NiAl Alloys

The mechanical behaviour of dual-phase microstructures, consisting of elongated primary NiAl grains aligned with an intergranular NiAl/X eutectic phase, produced by extrusion of cast NiAl-X (whereX=Cr and W) alloys, has been examined. Chromium, added to create a dual-phase NiAl-based aligned microstructure, resulted in large increases in the yield strength, but only marginal improvement in the toughness. In contrast, tungsten alloying leads to ductility or toughness increases with no significant strengthening. The constitutional hardening rate due to deviations from stoichiometry in Ni rich NiAl was estimated to be about 66 MPa (Ni^–1 at%).     

Tensile properties and microstructures of NiAl-20TiB2 and NiAl-20TiC in situ composites

A hot-pressing aided exothermic synthesis (HPES) technique was developed to fabricate NiAl matrix composites reinforced with TiB₂ and TiC particles which were in situ reaction synthesized from elemental powders. These particles were uniformly dispersed in the matrix. The resulting products were hot isostatically pressed to nearly complete densification. It was found that the tensile yield strengths of the composites at 900°C were about two times stronger than that of unreinforced NiAl and were approximately three times stronger at 980°C. The interfaces between NiAl and TiC or TiB₂ were atomically flat, sharp and free from any interfacial phases in most cases, however, a thin interfacial amorphous layer or overlapped interfacial layer was observed at the interfaces in some cases. This type of interfacial structure may be beneficial to the strength of the composites.     

Simultaneous improvement of strength and ductility in NiAl-Cr(Mo)-Hf near eutectic alloy by small amount of Ti alloying addition

Effects of Ti alloying addition on the microstructure and room temperature compression deformation behavior of a NiAl-Cr(Mo)-Hf near eutectic alloy were investigated by SEM, TEM, EDX and compression test. The results showed that compared with base alloy, the compressive fracture strain and 0.2% yield strength of the Ti-containing alloy were enhanced simultaneously. Disordered (Ti,Hf) solid solution phase together with the blocky Heusler phase Ni₂Al(Ti,Hf) precipitation presented at eutectic cell boundaries is responsible for the enhancement in ductility due to the better deformation ability of (Ti,Hf) solid solution phase. The improvement in strength depends mainly on the solid solution strengthening in NiAl matrix by large amount of Ti due to its larger solid solubility relative to Hf in NiAl.     

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.     

The microstructure and interface behaviour of Ni/NiAl composites produced by the explosive compaction of powders

Explosive compaction of Ni and NiAl powders was utilized for the processing of Ni/NiAl metal-matrix composites containing up to 57 vol% NiAl particulate. The microstructure, the Vickers microhardness and the Ni/NiAl interfacial bonding strength were studied. The resulting microstructure had a very low volume fraction of porosity ( 1 vol%) except for the melting zone formed in the upper portion of a cylindrical specimen. NiAl particles underwent welding during explosive compaction; this was particularly pronounced at the highest volume fractions of NiAl. The lowest microhardness of the Ni matrix was observed in the central portion of a cylindrical specimen. Other parts of the matrix were heavily cold-worked, indicating that a recrystallization had occurred in the centre. NiAl particles were also highly cold-worked regardless of their volume fraction in the composite. The Ni/NiAl interfacial bond strength, measured by the indentation debonding technique, was highest in a composite containing 57 vol% of NiAl particulate.     

Effect of Ag alloying on microstructure,mechanical and electrical properties of NiAl intermetallic compound

In this paper, the effects of Ag on microstructure, mechanical and electrical properties of NiAl intermetallic compound were investigated. The present results show that the NiAl–Ag alloys consisted of two phases: β-NiAl and Ag-rich solid solution. The amount of the Ag-rich phase increased with increasing Ag content. Ag has very low solubility in NiAl and they form pseudo-binary monotectic systems. The addition of 0.5–1at.% Ag to NiAl increased its strength while the addition of more than 1at.% Ag decreased its strength. The strengthening and weakening effects come from solid solution hardening and second ductile phase softening. In addition, Ag alloying can improve NiAl's room temperature compressive ductility. The NiAl–Ag alloy has high hardness and electrical conductivity that make it an attractive candidate for electrical contact material.     

Development of oxidation resistant high temperature intermetallics

Abstract

The phase NiAl with its wide homogeneity range, and especially when of high Al content, is a reliable alumina scale former and exhibits remarkable oxidation resistance at high temperatures. By contrast, the line phase NbAl₃ oxidises with formation of mixed layers of Al₂O₃ and fast growing oxides. However, the mechanical strength of NiAl is not sufficient for application at high temperatures, but the strength is enhanced in multiphase alloys in the systems Ni–Al–Cr and Nb–Ni–Al. Increasing the Cr content of two phase NiAl–Cr alloys reduces the oxidation resistance, but for three phase NbAl₃–NbNiAl–NiAl alloys the oxidation resistance is comparable to that of NiAl, since the Al content of the phases NiAl and NbNiAl is high in the phase equilibrium.

MST/1570     

Structural Applications of NiAl

Alloys based on NiAl offer significant payoffs as structural materials in gas turbine applications due to a unique range of physical and mechanical properties These properties include high melting temperature. low density. high thermal conductivity. and excellent environmental resistance Very significant improvements in the strength and ductility of NiAl single crystals have been achieved through alloying. Tensile strength and stress rupture properties which compete with current Ni-base Superalloys have been achieved through precipitation of an ordered L21 Heusler phase in NiAl single crystals Room temperature tensile ductility as high as 6% has been produced in NiAl single crystals containing less than 0.5% (atomic) of Fe. Ga or Mo. However. a single alloy with both room temperature ductility and sufficient high temperature strength has not yet been developed. While activity to develop on alloy with both high temperature strength and room temperature ductility continues. the Current approach also emphasizes design and test methodologies which can accept a material with limited ductility and damage tolerance More work is required on measuring and understanding strain rate sensitivity and impact behaviour While several significant challenges still remain. excellent progress has been made in many areas, and the prognosis for using NiAl alloys as high temperature structural materials is promising     

Yield or martensitic phase transformation conditions and dissipation functions for isotropic, pressure-insensitive alloys exhibiting SD effect

Summary A class of nonsingular yield conditions depending on three parameters is analyzed for isotropic materials exhibiting strength differential effect and pressure insensitivity. The yield condition can then be expressed in terms of the second and third stress deviator invariants. The convexity requirement is considered and the constraints imposed on the material parameters are discussed in detail. The dual dissipation function is derived in the analytical form. The condition can be applied in the analysis of high strength alloys (such as Inconnel 718) or of shape memory alloys (such as NiTi, NiAl, CuZnGa, or CuAlNi) in order to specify the onset of yield, or of martensitic or austenitic transformation. The conditions can easily be generalized to account for mixed hardening and back stress anisotropy. Some experimental data are provided to verify the proposed conditions. Dedicated to Professor Franz Ziegler on the occasion of his 70th birthday