Study on behavior of NiAl coating with different Ni/Al ratios

β-NiAl coatings with different Ni/Al ratios were deposited on K403 superalloy substrates via magnetron sputtering. The phase transformation and diffusion phenomenon of the NiAl/Ni-based superalloy system after vacuum annealing at 900 and 1000 °C were analyzed using X-Ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectrometry (EDS). The effect of coating concentrations on the outward diffusion behavior of substrate elements was discussed. The high Cr concentrations in the Al-rich NiAl coatings were caused by the intense interdiffusion between Al and Cr. The Ti, W and Mo partitioned to γ′-Ni₃Al in the coatings. Several possible reasons for the formation of γ′-Ni₃Al at the surface of Ni-rich NiAl coating were identified, including: diffusion behavior of W and Mo in β-NiAl, destabilizing effect of substrate elements on β-NiAl, and diffusion rates of Ni and Al in β-NiAl. The volume change in β ⇛ γ′ transformation process shows Ni uphill diffused to the γ′-Ni₃Al islands at the surface of Ni-rich NiAl coatings. The IDZ (interdiffusion zone) thickness and precipitates in IDZ were related to the Al initial concentrations in the coatings.     

Effect of Heat Treatment on the Microstructure of Multiphase NiAl-based Alloy

The alloy Ni-Al26.6-Cr13.4-Co8.1-Ti4.3-W1.3-Mo0.9 （at. pct） was fabricated from superalloy K44 and Al element using vacuum induction and casting technique. Investigations to this alloy reveal that a new phase Cr3Ni2 possessing low melting point and poor ductility is formed, which is distributed as a network along NiAI matrix grain boundaries. Subsequent different solution and aging treatments are carried out and lead to microstructural changes to various extents. Rapid cooling after solution at 1250℃ for 20 h gives rise to macrocracks in the specimen while slow cooling after the same treatment results in the formation of spheric α-Cr solid solution and needle-like Ni3Al phase, which are embedded in NiAl matrix. It is comfirmed that aging treatments initiate lath-shaped Ni3Al phase and pearl-shaped α-Cr phase to precipitate from the NiAl matrix, which own orientation relationships with these precipitates.     

Positron Lifetime Study of the Single-Phase B2 Ni100-xAlx and Ni50-xCoxAl50 Intermetallics

To whom correspondence should be addressedE-mail: jtguo@imr.ac.cn1. IntroductionIt is now well established that the mechanicalproperties of NIAI are strong functions of composition and often of thermal history. These dependencieshave been correlated in part to the presence of pointdefects. The observation of off-stoichiometric hardening in NiAI, for instance, has been attributed tothe constitutional point defects[1'2]. Previous investigators have attempted to relate this hardening tothe co…     

B2－NiAl力学性质Cr合金化效应的第一原理计算

采用第一原理赝势平面渡方法和基于虚拟晶体势函数近似（VCA）,计算了Cr合金化（浓度x〈1．0at％）时完整与缺陷B2-NiAl晶体的弹性性质,并采用弹性常数G44、Cauchy压力参数（C12-C44）、杨氏模量E、剪切模量G及其与体模量B0的比值G／B0等,表征和评判了Cr合金化浓度X对NiAl金属间化合物延性与硬度的影响。结果表明：Cr合金化浓度在0～1at％范围内均对NiAl晶体的硬度有明显影响。无论点缺陷存在与否,Cr合金化均可使B2-NiAl晶体的硬度大幅提高。Cr合金化浓度在0at％～0．5at％区间,有利于无缺陷NiAl晶体材料延性的提高,且以0．05at％的合金化浓度为最好。Ni空位或Ni反位降低B2-NiAl晶体的本征延性。Cr合金化浓度在x〈1at％时,Ni空位的NiAl晶体增塑效应并不明显,而对于Ni反位的缺陷NiAl晶体,Cr合金化浓度在0．9at％附近存在比较明显的增塑效应。     

Synthesis of nanocrystalline NiAl over a wide composition range by mechanical alloying

The paper reports the synthesis of nanocrystalline NiAl by mechanical alloying of pure metal mixture and a mixture of prealloyed powder with Ni/Al. A large number of compositions have been studied to establish the phase field of NiAl in the milled state. The phase field of NiAl in the ball milled condition was found to be much wider (10–68 at.% Ni) than its equilibrium phase field (45–59 at.% Ni). The metastable equilibrium achieved by mechanical alloying was identical for a given composition irrespective of the starting ingredients. The crystallite size of NiAl reached a minimum (5 nm) at the phase boundary of NiAl/Ni₃Al.     

Effect of Ru on interdiffusion dynamics of β-NiAl/DD6 system: A combined experimental and first-principles studies

As a diffusion barrier between thermal barrier coating (TBC) and advanced single crystal (SC) superalloy DD6, RuNiAl coating has attracted increasing attention recently. In this work, different diffusion couples including NiAl/DD6, RuNiAl/DD6 and RuAl/DD6, were prepared and their interdiffusion behavior was investigated at 1100 °C, to understand the diffusion barrier mechanism of the RuNiAl coating. The addition of Ru to NiAl effectively reduced the interdiffusion coefficient of Al, thereby delaying the phase transformation from β to γ′ and suppressing the formation of topologically closed-packed (TCP) phases and primary interdiffusion zone (IDZ). It is verified by first-principles calculations that Ru restrained the formation of Ni and Al vacancies and Ni and Al antisite atoms. According to the calculations, the addition of Ru increases the defect formation energies, thus inhibiting the diffusion of Al in β-NiAl. The calculation results are consistent with the experimental data.     

Formation of B2 intermetallic NiAl and FeAl by mechanical alloying

Nanostructud B2 intermetallic compounds NiAl and FeAl have been prepared by mechanical alloying (MA) the elemental powder mixtures and subsequent heating. The structural evolution during MA was monitored by in situ thermal analysis and X-ray diffraction (XRD). The final products were characterized by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the nanocrystalline intermetallic compound NiAl, which is difficult to disorder by milling, was synthesized directly after an exothermic explosive reaction; whereas FeAl compound was formed after a thermal process of asmilled Fe(Al) solid solution obtained through interdiffusion during MA. The large heat of formation of NiAl compound is the main driving force for the exothermic explosive reaction, and the difference in diffusivity between NiAl system and FeAl system is suggested to be the main cause of the different behaviors of formation between NiAl and FeAl compounds by MA.     

Structure of the Intermetallic Compound Ni3Al 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 mono- and 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.     

Mechanical alloying synthesis and structural characterization of ternary Ni-Al-Fe alloys

Ternary Ni-Al-Fe alloys with different Fe additions have been synthesized by mechanical alloying of elemental powder mixtures. The effects of Fe-substitution for Al in an equi-atomic NiAl alloy on the mechanical alloying process and on the final Ni-Al-Fe alloys were investigated experimentally. Lower Fe additions have been found to prolong the milling time prior to explosive formation of the NiAl(Fe) compound, while ≥ 15 at % Fe addition has been found not only to eliminate the explosive reaction during milling but also to produce a Ni-based supersaturated solid solution (the 15 at % Fe addition results in the formation of an amorphous-like phase). The addition of Fe improves the plastic deformation ability of the alloys and hinders the tendency of fracture during milling, resulting in the formation of larger alloy particles. Upon heating, the as-milled samples with lower Fe addition remain as NiAl(Fe) compound, whereas the Ni-based supersaturated solid solutions decompose into a mixture of compounds of β-(Ni, Fe) (Al, Fe) + γ′-(Ni, Fe)₃Al. It is suggested that the ternary addition into the binary intermetallic compound might be a possible route to improve ductility by forming the dual phase of β + γ′. This revised version was published online in November 2006 with corrections to the Cover Date.     

合金元素在MCrAIY涂层中的行为

采用电弧离子镀在新型γ′-Ni     

Formation Mechanism of NiAl/TiB_2 Nanocomposite by Mechanical Alloying Elemental Powders

A NiAl/TiB2 nanocomposite is synthesized by mechanical alloying elemental powders. Upon milling for a certain time, an abrupt exothermic reaction occurs and a large amount of NiAl and TiB2 compounds form simultaneously. It is suggested that two separate chemical reactions,i.e. Ni+Al →NiAl and Ti+2B→TiB2, are involved during the exothermic reaction. Additionof Ti and B to Ni-Al system impedes the structural evolution of Ni and Al powders and delays the abrupt reaction. The final products are equilibrium phases without any metastable phases formed. This type of reaction is suggested to be suitable for alloy systems with two large heatrelease reactions.     

Density functional theory study of alloy element interstitials in Al

Density Functional Theory calculations were used to study Mg, Si, Cr, Mn, Fe, Co, Ni, and Cu interstitial configurations in Al. Energies of these elements in (100) dumbbell and octahedral configurations were determined. Results show that it is energetically favourable for metal alloying element atoms to replace Al selfinterstitials if the alloying atoms are smaller than the Al atoms, as expected. The system energy can thus be decreased by up to 2 eV. The difference between the energies of (100) dumbbell and octahedral configurations is only a few tenth eV for the alloys with metallic alloying elements. For Si, the difference can be up to 0.9 eV. This exceptional behavior of Si is most likely due to its angularly dependent bonding characteristics. Short ab-initio Molecular Dynamics simulations were performed on Mg and Si interstitials to allow these systems to evolve into different interstitial configurations rather than just the (100) dumbbell and octahedral configurations. For Si an alternative configuration with tetrahedral-like coordination was found. Consequences of the calculation results for radiation-induced segregation are discussed.     

Microstructure evolution of Al-Mg alloy during solidification under high pressure

The microstructure of Al-21.6Mg alloy solidified under high pressure was investigated. The results show that the amount of β-Al₃Mg₂ phase decreases with increasing pressure and a supersaturated Al(Mg) solid solution is formed under 2 GPa. The distribution of Mg in the solid solution is inhomogeneous, causing peak asymmetry of the XRD patterns under high pressures. The Mg concentration in the interdendritic region extends up to about 30 at.%, which is higher than that in the dendrite. Besides solution treatment, mechanical alloying, mechanical deformation of pre-alloy ingots and rapid solidification, solidification under high pressure is proved to be a new way to prepare supersaturated Al(Mg) solid solution.     

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.     

Obtaining NiAl intermetallic compound using different milling devices

NiAl intermetallic compound was synthesized by mechanical alloying technique in planetary and attritor mills. The starting powders consisted of elemental mixtures of Ni and Al at Ni₅₀Al₅₀ (at%) composition. In the planetary mill, compound formation occurred gradually during mechanical alloying, while the occurrence of a mechanically induced self-propagating reaction (MSR) can be suggested in the attritor mill. The NiAl obtained in both mill types was partially disordered with long-range order parameter not inferior to 0.66. Quantitative phase analysis using the Rietveld method was performed in as-milled samples, and this method was also employed to estimate changes in crystallite size and lattice strain of the NiAl produced during mechanical alloying.     

Synthesis and structural characterization of nanocrystalline (Ni,Fe)3Al intermetallic compound prepared by mechanical alloying

Synthesis of (Ni, Fe)₃Al intermetallic compound by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures with composition Ni₅₀Fe₂₅Al₂₅ was successfully investigated. The effects of Fe-substitution in Ni₃Al alloy on mechanical alloying process and on the final products were investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry, scanning electron microscopy and microhardness measurements. At the early stages, mechanical alloying resulted in a Ni (Al, Fe) solid solution with a layered nanocrystalline structure consisting of cold welded Ni, Al and Fe layers. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound which increased the degree of L1₂ ordering upon heating. In comparison to Ni–Al system, Ni (Al, Fe) solid solution formed at longer milling times. Meanwhile, the substitution of Fe in Ni₃Al alloy delayed the formation of Ni (Al, Fe) solid solution and (Ni, Fe)₃Al intermetallic compound. The microhardness for (Ni, Fe)₃Al phase produced after 80 h milling was measured to be about 1170HV which is due to formation of nanocrystalline (Ni, Fe)₃Al intermetallic compound.     

In-situ synchrotron radiation photoemission spectroscopy study of the initial atomic layer deposition of Al2O3 film on Si(001) substrate

In-situ synchrotron radiation photoemission spectroscopy and X-ray photoemission spectroscopy have been used to investigate the initial stages of Al2O3 growth on a Si(001) substrate by atomic layer deposition (ALD). The core level spectra of Si 2p, O 1s, and Al 2p as well as the valence band spectra were measured at every half reaction in the trimethylaluminum (TMA)-H2O ALD process. The line shape changes and binding energy shifts of the core level spectra reveal that Al2O3 is predominantly formed with a small amount of Si oxide in the initial stages without the formation of Al silicate. All core level spectra were alternately shifted toward higher and lower binding energies sides at every half ALD reaction. This can be explained by the band bending effect induced by different chemical species on the surface during the TMA-H2O ALD reaction. The valence band spectra showed that four cycles of ALD reactions were necessary to complete the electronic structure of the Al2O3 film with a valence band offset of 3.73 eV.     

The effect of long range order on the activation energy for atomic migration in NiAl alloys; resistivity study

The influence of the degree of long range order (S) on the migration energy in NiAl was investigated by electrical resistivity and X-ray diffraction. Iron (12at.%) was over-alloyed to the stoichiometric NiAl in order to decrease the order-disorder transition temperature.(NiAl12Fe) The degree of LRO in NiAl12Fe decreased gradually as temperature increased. The activation energies for migration of iron in NiAl12Fe (Q _A ^Fe* that were measured from the temperature dependence of jumping frequency (1/_Fe*) were 1.42 ± 0.19 eV, 1.35 ± 0.18 eV, 1.16 ± 0.06 eV for the alloys quenched from 1073 K, 1273 K, and 1473 K respectively. The activation energies for migration of excess vacancy (Q _A ^v* in NiAl single crystal, were 1.55 ± 0.02 eV for the specimen quenched from 1473 K and 1.40 ± 0.11 eV for one quenched from 1673 K. The degree of LRO (S) in single crystalline NiAl calculated from the result of Hughes Lautensclager, Cohen and Brittain, J. Appl. Phys. 42 (1971) 3705 seems to be consistent with this decreasing tendency in the activation energy. The reduction of the degree of LRO (S) can explain these variations in the migration energy as the Girifalco's model.     

Synthesis of nanocrystalline alloys and intermetallics by mechanical alloying

Nanocrystalline Al₃Ni, NiAl and Ni₃Al phases in Ni-Al system and theα, β, γ, ɛ and deformation induced martensite in Cu-Zn system have been synthesized by mechanical alloying (MA) of elemental blends in a planetary mill. Al₃Ni and NiAl were always ordered, while Ni₃Al was disordered in the milled condition. MA results in large extension of the NiAl and Ni₃Al phase fields, particularly towards Al-rich compositions. Al₃Ni, a line compound under equilibrium conditions, could be synthesized at nonstoichiometric compositions as well by MA. The phases obtained after prolonged milling (30 h) appear to be insensitive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (∼ 6 nm) when NiAl and Ni₃Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn blends containing 15 to 85 at.% Zn, the Zn-rich phases were first to form, and the final crystallite sizes were coarser (15–80 nm). Two different modes of alloying have been identified. In case of NiAl and Al₃Ni, where the ball milled product is ordered, as well as, the heat of formation (ΔH _f) is large (> 120 kJ/mol), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases, irrespective of the magnitude of ΔH _f, a gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.     

Formation of Nanoscale Intermetallic Phases in Ni Surface Layer at High Intensity Implantation of Al Ions

The results of experimental study of nanoscale intermetallic formation in surface layer of a metal target at ion implantation are presented. To increase the thickness of the ion implanted surface layer the high intensive ion implantation is used. Compared with the ordinary ion implantation, the high intensive ion implantation allows a much thicker modified surface layer. Pure polycrystalline nickel was chosen as a target. Nickel samples were irradiated with Al ions on the vacuum-arc ion beam and plasma flow source "Raduga-5". It was shown that at the high intensity ion implantation the fine dispersed particles of Ni3AI, NiAl intermetallic compounds and solid solution Al in Ni are formed in the nickel surface layer of 200 nm and thicker. The formation of phases takes place in complete correspondence with the Ni-AI phase diagram.