Concentration Heterogeneity in Structure of Intermetallic Synthesized under Compression

By X-ray analysis, TEM and SEM methods the phase composition, microstructure, and deviation from stoichiometric composition were investigated for the Ni3Al intermetallic compound obtained by cast and self-propagation high-temperature synthesis under compression with followed homogenization anneal, with the Al concentration being 24.0 and 25.0 at. pct with and without B (0.5 at. pct). It was discovered that in SHS intermetallic alloys an average local deviation from stoichiometric composition is significantly more than in the cast alloy. Dependence of the local deviation from the stoichiometric composition on the distance has a zig zag nature at the level of the grain groups. Deviation from the stoichiometric composition at the boundaries of the general type is less than at the boundaries of the special type. When the intensity of boundary migrations of the general type decreases, the degree of deviation from the stoichiometric composition increases. There is correlation between the solubility of B in the crystalline lattice of the Ni3Al phase and the value of the deviation from the stoichiometric composition in the grains. When the B concentration in a solid solution increases, the deviation from stoichiometric composition decreases.

Structure of the Intermetallic Compound Ni_3Al Synthesized under Compression of the Powder Mixture of Pure Elements Part Ⅰ: Phase Composition and Microstructure of Main Phase

It was shown by TEM and X-ray analysis that there are four types of grains of the main Ni3Al phase in the structure of the intermetallic obtained by the self-propagation high temperature method (SHS). Every type of grains has its own domain and dislocation structure. There are mono- and polydomains with and without dislocations. The grains of the main phase of monoand polydomains without dislocations and polydomains with dislocations were formed by diffusion in the solid phase. In these conditions NiAl3 phase is located on the grain boundary of the main phase. The Ni2Al3 phase is located at the triple joints of the main phase.

Structure of the Intermetallic Compound Ni_3Al Synthesized under Compression of the Powder Mixture of Pure Elements Part Ⅱ: Influence of Alloying by Boron on the Phase Composition and the Microstructure of Grains of the Main Phase

The Ni_3B phase was formed when boron (0.5 at. pct B) was added to the intermetallic of sto- ichiometric and off-stoichiometric (Ni-24 at. pct Al) compounds. In the alloy of stoichiometric composition the particles of Ni_3B phase has the size around 0.1μm and is located on the grain boundary of the main phase. The decreasing of concentrations of Al in the ofF-stoichiometric alloy leads to increase in the degree of the long-range order parameter, increasing the concen- trations of boron in the solid solution and decreasing its localization on the grain boundary. Microalloying of boron leads to increasing in the fraction of grain monodomains with disloca- tions up to 0.7 in the alloy of the off-stoichiometric composition and up to 1 in the alloy of the stoichiometric composition. It was established the correlation between the degree of the concentration inhomogeneity, average density of the dislocations and the average long range-order parameter.

INTERACTION OF H_2O AND O_2 WITH INTERMETALLIC ALLOYS

The interaction of water vapor and oxygen with Ni 3(Al, Ti) and Ni 3Fe were investigated using Auger electron spectroscopy, X ray photo emission and thermal desorption. These studies showed that water dissociates on Ni 3(Al, Ti) (001) and polycrystalline Ni 3Fe to produce atomic hydrogen above 200 K, while remaining intact when exposed to Ni 3(Al, Ti) (111). In addition, pre adsorbed or co adsorbed oxygen on these surfaces strongly suppresses hydrogen production due to water dissociation. It was shown that adsorbed oxygen reacts with water vapor to produce surface hydroxyls, thus quenching the production of atomic hydrogen. Recent studies revealed an unexpected embrittling effect of boron in Ni 3Al in dry hydrogen. Experiments were proposed to elucidate the different possible roles of boron.

Effect of cooling rate on the 3D morphology of the proeutectic Al3Ni intermetallic compound formed at the Al/Ni interface after solidification

Effect of cooling rate on the 3 D morphology and the growth mechanism of the proeutectic Al3Ni intermetallic compound(IMC) that forms at the Al/Ni interface after solidification was investigated by synchrotron X-ray microtomography in combination with EBSD analysis. The proeutectic Al3Ni phase that forms under an average cooling rate of 0.1 Ks^(-1) shows a characteristic faceted growth behavior and presents a typical 3 D morphology as partially hollow quadrangular prisms. On the contrary, that forms under an average cooling rate of 10 Ks^(-1) shows complicated dendritic morphology with asymmetrically distributed arms and faceted V-shape groove at the distal end, indicating a gradual transition of the growth behavior from non-faceted to faceted during the solidification process. These results reveal that the morphology of the proeutectic Al3Ni is highly sensitive to the solidification condition so that fine control of the desired morphology may be achieved by carefully manipulating the cooling profile.

Yield anisotropy and tension/compression asymmetry of a Ni_3 Al based intermetallic alloy

In order to investigate the yielding behavior of the newly developed Ni 3 Al-based intermetallic alloy IC10, yield stresses have been measured in tension and compression with different orientations. The specimens were cut from a sheet with different angles inclined from the solidification direction. The inclined angles were taken to be 0 , 22.5 , 45 , 67.5 and 90 . All experiments were conducted at room temperature except for orientation 0 , whose deformation temperatures ranged from 298 to 1273 K. Experimental results show that the yield strength of alloy IC10 has the anomalous behavior which has been observed for other Ll 2 -long-range ordered intermetallic alloys, but it is less pronounced. The abnormalities show the following characteristics： （i） the yield strength increases as the temperature is raised below the peak temperature, （ii） yield strength anisotropy, （iii） tension/compression asymmetry. Compared to Ni 3 Al single crystals, the polycrystalline exhibits some different yielding behaviors which may be due to the high volume fraction of c phase.

WC-Ni_3Al-B composites prepared through Ni+Al elemental powder route

In order to develop the liquid phase sintering process of WC-Ni3Al-B composites,the preparation process of WC＋Ni3Al prealloyed powder by reaction synthesis of carbonyl Ni,analytical purity Al and coarse WC powders was investigated.DSC and XRD were adopted to study the procedure of phase transformation for the 3Ni＋Al and 70%WC＋（3Ni＋Al） mixed powders in temperature ranges of 550-1200 °C and 25-1400 °C,respectively.The results demonstrate that the formation mechanism of Ni3Al depends on the reaction temperature.Besides WC phase,there exist Ni2Al3,NiAl and Ni3Al intermetallics in the powder mixture after heat treatment at 200-660 °C,while only NiAl and Ni3Al exist at 660-1100 °C.Homogeneous WC＋Ni3Al powder mixture can be obtained in the temperature range of 1100-1200 °C.The WC-30%（Ni3Al-B） composites prepared from the mixed powders by conventional powder metallurgy technology show nearly full density and the shape of WC is round.WC-30%（Ni3Al-B） composites exhibit higher hardness of 9.7 GPa,inferior bending strength of 1800 MPa and similar fracture toughness of 18 MPa-m1/2 compared with commercial cemented carbides YGR45（WC-30%（Co-Ni-Cr））.

Effect of extrusion process on microstructure and mechanical properties of Ni_3Al-B-Cr alloy during self-propagation high-temperature synthesis

The well-densified Ni3Al-0.5B-5Cr alloy was fabricated by self-propagation high-temperature synthesis and extrusion technique. Microstructure examination shows that the synthesized alloy has fine microstructure and contains Ni3Al, Al2O3, Ni3B and Cr3Ni2 phases. Moreover, the self-propagation high-temperature synthesis and extrusion lead to great deformation and recrystallization in the alloy, which helps to refine the microstructure and weaken the misorientation. In addition, the subsequent extrusion procedure redistributes the Al2O3 particles and eliminates the γ-Ni phase. Compared with the alloy synthesized without extrusion, the Ni3Al-0.5B-5Cr alloy fabricated by self-propagation high-temperature synthesis and extrusion has better room temperature mechanical properties, which should be ascribed to the microstructure evolution.

TiC／Ni_3Al表面复合涂层及其耐磨性

采用铸造工艺结合SHS技术制备TiC／Ni3Al表面复合涂层,研究了涂层的显微组织及其耐磨性。结果表明:表面复合涂层中直径为1μm～2μm的TiC颗粒呈球形镶嵌在Ni3Al基体上,涂层致密。与钢基体为良好的冶金结合,界面随TiC含量的变化而呈现不同的形貌,在TiC含量<45％时,涂层为一整体;在TiC含量≥45％时,涂层出现了分层。表面复合涂层的HV值最高达8970 MPa,沿界面呈梯度变化;涂层具有高的耐磨性,在室温下约是钢基体的3～5倍;在400℃下是钢基体的20倍左右。

NiAl金属间化合物的热爆合成过程研究

以Al粉和Ni粉为主要原料,采用自蔓延高温合成技术,制备Ni-Al基金属间化合物。绘制了热爆反应曲线、描述了热爆反应的过程,分析了热爆反应机理及产物的显微组织特征,研究了热爆反应的边界条件以及各种反应参数对热爆反应的影响。研究结果表明,升温速率和颗粒尺寸对热爆反应均有不同程度的影响。提高升温速率、减小颗粒尺寸均可缩短热爆反应的起始时间,降低热爆反应的起始温度。初步确定了反应起始温度低于Ni-Al系的最低共晶温度(640℃),Ni-Al系的热爆反应是由固-固扩散反应引发的,Ni-32Al的反应产物为单相、均一的NiAl相。

反应合成Ni-Al金属间化合物多孔材料的研究

采用自蔓燃高温合成技术(SHS)制备Ni-Al金属间化合物多孔材料。采用XRD和SEM分析了SHS反应生成的多孔材料的相组成和微观形貌。结果表明:利用自蔓燃高温合成法制备的Ni-Al金属间化合物多孔材料,产物主要相组成为Ni3Al和NiAl,还有少量的Ni固溶体;反应形成的多孔材料的孔洞形状不规则、结构复杂,孔道曲折,孔壁粗糙,大大增加了多孔材料的比表面积;Ni/Al质量比为4∶1的反应产物比质量比为3∶1的反应产物致密,且孔洞直径较小,颗粒也较细小、均匀。

Fabrication of Ni_3Al by hot pressing from element powders

Ni3Al intermetaUic was synthesized by hot pressing from element powders of nickel, aluminum, and boron. The influences of parameters on the properties of Ni3Al were investigated. The parameters include the particle size of nickel powder, adding or without boron powder, hot pressing temperature, etc. The properties include the density of hot-pressed samples, resultant redo of Ni3A1 phase, and bending strength. The microstructures of hot-pressed samples were investigated by X-ray diffraction and scan electronic microscopy, and the properties, such as density and bending strength, were also measured. The results show that a higher bending strength was obtained under the same hot pressing conditions by the fine nickel powder than the coarse one, and there is little difference about density. Boron powder added in this process accelerates the formation of Ni3Al and markedly increases the hot pressed density. In the temperature range of this study, the density increases along with the hot pressing temperature. Full dense Ni3Al samples were obtained under the condition of 860℃, 10 min, 45 MPa from Ni-22.89A1-0.5B powder.

Co-Ni-Al-Ti系L2_1/L1_2型双相金属间化合物合金热处理显微组织研究

对由 Ｌ２１型（Ｃｏ，Ｎｉ）２ＡｌＴｉ和Ｌ１２型（Ｃｏ，Ｎｉ）３（Ａｌ，Ｔｉ）两种有序相组成的Ｃｏ－Ｎｉ－Ａｌ－Ｔｉ系新型金属间化合物高温结构实验用合金进行了不同制度的热处理，并对相应的显微组织特征进行了分析研究，找到了控制显微组织以优化性能的热处理参数。

Co-Ni-Al-Ti系L2_1/L1_2型双相金属间化合物合金的显微组织和力学行为研究

对于由 L2 1 型 ( Co,Ni) 2 Al Ti和 L12 型 ( Co,Ni) 3( Al,Ti)两种有序相组成的 Co- Ni- Al- Ti系新型金属间化合物高温结构用合金 ,经热处理后获得不同显微组织的高温压缩和室温断裂等力学行为进行了分析研究 ,确立了合金成分、显微组织和机械性能之间的关系 ,找到了进一步提高高温强度和室温塑性的途径。

自蔓延燃烧合成法制备分层及梯度结构的Ni-Al/金刚石复合材料研究

采用自蔓延燃烧合成法制备了分层及梯度结构的Ni-Al/金刚石复合材料。研究了多层及金刚石粒度梯度结构对Ni-Al/金刚石复合材料其自蔓延反应过程及微观相貌的影响。结果表明:随着分层数的增加,自蔓延反应的燃烧波速度下降;自蔓延反应在金刚石粒度梯度结构的燃烧波速率比其在金刚石分层结构的速率快。微观形貌分析表明,自蔓延反应使得钎料合金Ni-Cr与金刚石生成了强碳化合物Cr_3C_2和Cr_7C_3,增强了金刚石和合金粉末基体的结合力。

MECHANICAL PROPERTIES OF DIRECTIONALLY-SOLIDIFIED Ni_3Al CONTAINING BORON AND CARBON

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THE ALLOYING BEHAVIOR OF BORON AND CARBON INNi_3Al

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Coarsening Kinetics of γ' Precipitates in Dendritic Regions of a Ni_3 Al Base Alloy

Coarsening behavior of γ＇ precipitates in the dendritic regions of a Ni 3 Al base alloy containing chromium,molybdenum,zirconium and boron was investigated.Annealing treatment was performed up to 50 h at 900,1000 and 1100℃.The alloy was produced by vacuum-arc remelting technique.Results show that coarsening of the γ＇ precipitates in this complex alloy containing high volume fractions of γ＇ phase follows Lifshitz-Slyozov-Wagner（LSW） theory.Coarsening activation energy of the γ＇ precipitates was evaluated to be about 253.5 kJ.mol-1 which shows that the growth phenomenon is controlled by volume diffusion of aluminum.With an innovative approach,diffusion coefficient of the solute element（s） and the interfacial energy between γ＇ precipitates and γ＇（matrix） were estimated at 900,1000 and 1100℃.Accordingly,the interfacial energies at 900,1000 and 1100℃ are 4.49±1.48,2.08±0.69 and 0.98±0.32 mJ.m-2,respectively.Also the diffusivities of solute element（s） at these temperatures are 3.41±1.08,30±9.5 and 145.15±45.85（10-15 m-2.s-1）,respectively.

The electronic structure and grain boundary segregation by boron addition——A binding energy shifting criterion for the brittle-ductile fracture transition in Ni_3Al

The electronic structure and grain boundary segregation caused by boron addition to Ni3Al have been studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy, respectively. The obtained results show that the Ni2p3/2 electron binding energy rises gradually in the sequence of pure Ni< Ni76Al24< Ni74Al26 < Ni25Al25, while it reduces monotonously with an increase in boron addition to Ni3Al. Besides, it is found that the gram boundary segregation of boron occurring in Ni3Al is a combined equilibrium and non-equilibrium type in nature. Based on the concept of the bonding environmental inhomogeneity, measured by the shift in Ni2p3/2 electron binding energy from the nickel atoms in the simple substance nickel to those in the intermetallic compound Ni3Al (ΔEB), being responsible for the brittle behavior of the alloy, a binding energy shifting criterion for the brittle-ductile fracture transition in Ni3Al is presented: when ΔEB>0, the brittle failure occurs in Ni3Al; when ΔEB<0, the ductile one appears. Combined with the above experimental rules, the criterion predicts that pure Ni3Al is brittle, and there exist the stoichiometric effect and concentration effect in the ductilization process for Ni3Al by boron addition. Hence the criterion can be taken as a theoretical guide to alloy design in developing ductile intermetallics.

ELECTRONIC STRUCTUKE OF γ’ PHASE CONTAINING Mg IN NICKEL-BASED SUPERALLOY

The electronic structure of γ′ phase containing Mg in nickel-based superalloy is inves-tigated with SCF-X_n-SW method. Based on the model cluster presented for the strengtheningmechanism of Mg in γ′ phase, the energy levels, the density of states, the partial, wave localdensity of states and one-electron properties such as charge distribution, have been obtained.After substituting Mg for Al, the bonding states in low energy region increase, unoccupiedstates move towards more shallow potential well and energy gap becomes larger. All these canpredict that the proper Mg makes γ′ phase more stable. The charge transfer of Mg in γ′phase benefits Mg to be dissolved in γ′ phase.