Ni3Al Grain Research Articles

Особенности тонкой структуры Ni-Al покрытий, полученных методом HV-APS

Introduction. Development of Ni-Al intermetallic compounds is one of the priority directions of modern machine building. Due to such characteristics as high heat resistance, high temperature strength, and low density, nickel aluminides are used as functional coatings in the aerospace industry. The main methods of Ni-Al coating surfacing are High-Velocity Oxygen-Fuel and High-Velocity Air-Fuel spraying (HVOF and HVAF), atmospheric plasma spraying (APS) and its modification such as High-Velocity Atmospheric Plasma spraying (HV-APS) which provides non-equilibrium cooling conditions. Since there are eight different intermetallic compounds, as well as martensite transformation, Ni-Al coatings is quite interesting to study. The work purpose is to study the features of the martensitic structure in HV-APS coatings, and also to establish the effect of heating temperature on its decomposition. Materials and methods. Ni-Al coatings were surfaced onto a low-carbon steel substrate using the HV-APS method. Studies of the fine structure of the coatings were carried out using transmission electron microscopy (TEM). In addition, the influence heating temperature on structural transformations of the coatings was analyzed. Results and discussion. Two types of particles are formed in HV-APS coatings: with a dendritic and granular structure. The most part of HV-APS coatings consists of particles with a two-phase grain structure (NiхAl1-х and γ'-Ni3Al grains). Only NiхAl1-x grains undergo martensitic transformation at cooling. Martensite in large grains (sizes greater than 500 nm) has a lamellar structure, while small grains are completely transformed into one martensite plate. In addition, the coatings contain grains in which martensite plates (NiхAl1-х) and β-phases alternated. It is shown the behavior of martensitic plates at colliding with each other, as well as with the γ′-Ni3Al grain. Heating up to 400 °C contribute the begins of martensite decomposition in individual grains with the release of a secondary phase; after heating up to 600 °C all martensite dissolves.

Theoretical study of synergistic effect of P and Mg on the cohesive properties of Ni3Al grain boundaries

The addition of specific alloying elements can effectively control the microstructure of alloys, so as to improve the cohesive properties of the GBs. In this work, the first-principles plane-wave pseudopotential method is used to investigate the segregation behavior of P and Mg doping at the Ni3Al GBs and reveal the physical mechanism. Different 100%Ni Σ5 (210) [010] GB systems with and without doping elements are relaxed and larger volume expansions are obtained. The calculated segregation energies show that P or Mg atoms tend to segregate to the GBs relative to the bulk. P atoms tend to stay at the interstitial sites surrounded by 8 Ni atoms in the pure Ni holes of the GBs while Mg atoms tend to substitute Ni atoms. The segregation of P or Mg to the GBs leads to an increase of the Griffith work at the GBs, indicating that the segregation can improve the bonding properties of the GBs. Especially, the synergistic segregation of Mg can improve the detrimental effect of P in the mixed hole on the cohesive properties of the GBs. It is also found that the site occupation of Mg can be reversed from the Ni site to the Al site by the synergistic alloying of P at the 50%Ni Σ5 GBs. P and Mg tend to bond with the host atom Ni regardless of GB structure. The investigation of electronic structure shows that GB strengthening of alloying elements is attributed to the increasing P/Mg-Ni interactions across the GBs.

Synergistic effect of Mn and B on the cohesive properties of grain boundaries in Ni3Al: A first-principles study

Ni-based superalloys are known as potential structural materials under extreme conditions due to their good high temperature properties. However, some alloys such as Ni3Al exhibit intrinsic brittleness which severely restricts their practical applications as structural materials. It is well known that the moderate addition of B can significantly improve the ductility of Ni3Al grain boundaries (GB) but restricted to the Ni-rich side. Adding Mn together with B can improve the ductility of Ni3Al in a wider composition range. To provide some fundamental understandings on such a cooperative effect, the segregation of Mn or/and B to the Ni3Al GBs and their effects on the cohesive properties of the GBs are systematically investigated by using a first-principles method. It is shown that B tends to segregate to the Ni-rich holes of the GBs, which means that B prefers to form stronger covalent bonds with the Ni atoms. The calculation of Griffith’s work shows its improved effect on the cohesion of the GBs. Different from site occupation tendency in bulk Ni3Al, Mn tends to segregate to the GBs and substitute the Ni sites at the GBs. It is interesting to notice that B can segregate to the Ni-deficient hole and improve the cohesive properties of the GBs in the presence of Mn substituting Ni atoms at the GBs. Whether at the Ni-rich holes or Ni-deficient holes of B segregation, Mn substituting the Al sites won’t take place at the GBs. The improvement of the ductility of composition-dependent Ni3Al extends from the Ni-rich side to the stoichiometric side with the co-doping of Mn and B. The electronic structure mechanism behind is discussed based on the calculated electronic density of states. It is suggested that the beneficial cooperative effect of Mn and B on the cohesive properties of the GBs is attributable to the increasing B-Al interaction.

The coupling effects of temperature and stress on the growth kinetics of Ni3Al in Ni//NiAl diffusion couples

The growth kinetics of Ni3Al were investigated in Ni//NiAl macroscopic diffusion couples annealed at 800–1000 °C under applied elastic compressive stress. By utilizing electron backscatter diffraction (EBSD) technology in conjunction with appropriate statistical methods, the mechanism underlying the impact of elastic stress on Ni3Al growth kinetics is elucidated. Based on the findings, the application of compressive elastic stress can enhance Ni3Al growth kinetics by increasing both atomic vibration frequency (ν) and activation entropy (Δs). The newly formed Ni3Al phase at the Ni/NiAl interface exhibits a specific crystallographic relationship with adjacent NiAl phase, with most Ni3Al grains maintaining a Kurdjumov-Sachs (KS) orientation relationship. Remarkably, the application of elastic compressive stress in conjunction with an increase in temperature results in a reduction of Ni3Al grains grown according to the KS orientation relationship.

Impact of crystallography at Ni/NiAl interfaces on the nucleation of Ni3Al

Early stages of phase formation at (Ni)/NiAl interfaces with different crystallographic configurations were studied. As opposed to earlier work, macroscopic diffusion couples and a low annealing temperature of 650°C were applied. The morphology of the newly forming Ni3Al phase, its crystallographic relationships to the different (Ni)/NiAl interfaces and the growth kinetics of Ni3Al were evaluated with appropriate statistics combining EBSD with STEM EDX. The Ni3Al phase forms in two layers and mainly grows into the NiAl phase, establishing a specific crystallographic relationship with respect to the adjoining phases. The majority of Ni3Al grains growing into the NiAl phase maintain Kurdjumov-Sachs orientation relationships. The grains in the second Ni3Al layer mostly grow in a near cube-on-cube orientation relationship (misorientation < 15°) towards the (Ni) phase. The formation of large Al enriched (Ni) grains at the (Ni)/Ni3Al interface is observed and interpreted as diffusion induced recrystallization.

Intermetallic/Ceramic Composites Synthesized from Al–Ni–Ti Combustion with B4C Addition

The fabrication of intermetallic/ceramic composites by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) was investigated in the Al–Ni–Ti system with the addition of B4C. Two reaction systems were employed: one was used to produce the composites of xNiAl–2TiB2–TiC with x = 2–7, and the other was used to synthesize yNi3Al–2TiB2–TiC with y = 2–7. The reaction mechanism of the Al–Ni–Ti system was strongly influenced by the presence of B4C. The reaction of B4C with Ti was highly exothermic, so the reaction temperature and combustion velocity decreased due to increasing levels of Ni and Al in the reactant mixture. The activation energies of Ea = 110.6 and 172.1 kJ/mol were obtained for the fabrication of NiAl- and Ni3Al-based composites, respectively, by the SHS reaction. The XRD (X-ray diffraction) analysis showed an in situ formation of intermetallic (NiAl and Ni3Al) and ceramic phases (TiB2 and TiC) and confirmed no reactions taking place between Ti and Al or Ni. The microstructure of the product revealed large NiAl and Ni3Al grains and small TiB2 and TiC particles. With the addition of TiB2 and TiC, the hardness of NiAl and Ni3Al was considerably increased and the toughness was also improved.

Characterization of Ni3Al Alloy Fabricated by Thermal Explosion and Hot Extrusion Process

In the present paper, the trace B doped Ni3Al alloy was prepared by the thermal explosion and hot extrusion (TH/HE) technique. Its micro-structure characterization and mechanical properties were carried out by OM, XRD, TEM, compression and micro-hardness tests. Micro-structure examinations exhibit that the TH/HE technology has transformed the powders into Ni3Al alloy with less porosity. In the synthesized part, coarse and ultrafine Ni3Al grains comprise the dual-scale grain structure with segregated Al2O3 particles along grain boundary. TEM observations reveal that γ-Ni, Al2O3 and Ni3Al phases coexist in the synthesized part. Moreover, the Al2O3 particles in the synthesized part have α-Al2O3 and γ-Al2O3 two crystal structures. In addition, there is a transition region along the interface of Al2O3 and Ni3Al grain. The subsequent hot extrusion optimize the micro-structure of the extruded part by homogenizing the Ni3Al grain, uniformly distributing Al2O3 and generating massive substructures. The great deformation results in the pile-up of dislocations, intersected dislocations and substructures. Moreover, the movement of dislocation promotes the formation of micro-twinning in the Ni3Al grain, which is accompanied with the stacking faults and nano-structure. The mechanical properties exhibit that the micro-structure optimization caused by the TE/HE technology improves the compressive ductility and strength of the Ni3Al alloy obviously. DOI: http://dx.doi.org/10.5755/j01.ms.24.4.19460

Effect of Vacuum Carburizing on the Structure and the Properties of Aluminide Coatings on Nickel Superalloys

The processes that occur during vacuum carburizing of turbine blades made of carbon-free nickel superalloys are considered. Disperse alloying-element carbide particles are found to form in the surface layer of the VZhM5 alloy. These particles increase the fatigue life of the alloy during high-temperature oxidation. The disperse carbide particles are shown to hinder the formation of coarse γ′-Ni3Al grains and topologically close-packed phases under a coating; hence, they prevent premature fracture of the alloy.

Phase field modeling the growth of Ni 3Al layer in the β/γ diffusion couple of Ni–Al binary system

The growth behavior of Ni 3Al phase layer in the β/γ diffusion couple of Ni–Al binary system, including the shape evolution and growth kinetics, have been simulated by using the KKS multiphase field model. Simulation results indicate that, when Ni 3Al layer growth is controlled completely by volume diffusion, it could be regarded as parabolic growth. However, if the fast grain boundary diffusion is taken into account, the growth rate of Ni 3Al phase is accelerated, and the growth kinetics deviates from parabolic growth, which is consistent with experiments and other simulation results. Simulation results also demonstrate some details of shape evolution of grains, such as the uneven Ni 3Al/γ and Ni 3Al/β interfaces and the grain boundary migration of Ni 3Al grains caused by fast grain boundary diffusion.

The dual-species model study of co-enrichment of trace elements and Ni at Ni3Al grain boundary

The dual-species model is based on the Monte Carlo simulation conjoined with the embedded atom method (EAM) potentials. The model suggests that during relaxation of Ni3Al grain boundary，the trace element atoms can be seen not only as segregating species but also as inducing species. As segregating species the trace element atoms segregate (or enrich) to the grain boundary, but as inducing species they induce Ni atoms to segregate (or enrich) to the grain boundary. Evidently, the model can explain why the trace element atoms and Ni atoms co-segregate (or co-enrich) to the grain boundary. According to combination of positive (as inducing species) and negative (as segregating species) effects, the model explains the most obvious Ni-enrichment phenomenon at Ni3Al grain boundaries.

Synthesis and Characterization of Nanocrystalline Ni&lt;sub&gt;3&lt;/sub&gt;Al Intermetallic via Mechanical Alloying and Reaction Synthesis

Stoichiometric nanocrystalline Ni3Al was prepared by mechanical alloying of elemental Ni and Al powders under argon gas atmosphere for different time (4-48h). The nanostructured Ni3Al powders were consolidated into bulk compacts and sintered in a small DTA furnace under flowing Argon to observe the exothermic reaction between the stoichiometric Ni and Al. The estimated crystallite size showed that the mechanically alloyed Ni3Al grain size decrease from 127 nm to 9.36nm with increasing mechanical alloying time from 4h to 48h. Agglomerations of the powder particles prevalently occurred as observed from the SEM micrographs. Saturation magnetization, Ms value of the mechanically alloyed powders decreases as milling time increases due to smaller amount of elemental nickel responding to the applied fields. Following reaction synthesis of the compacted powders, thermal profile analysis revealed the presence of exothermic peaks in the DTA curves at about 400oC. Relative densities of the sintered compact were measured and found to be from 77- 88% with the exception for the 48h mechanically alloyed sintered compact from milling balls contaminations. XRD results of the sintered compacts mechanically alloyed for 18h and above revealed the formation of pure nanocrystalline Ni3Al. Crystallites size estimations showed the occurrence of grain growth during sintering.

Diamond nucleation on Ni 3Al substrate using bias enhanced nucleation method

Diamond deposition with positive and negative bias enhanced nucleation (BEN) pretreatments on mirror-polished polycrystalline Ni 3Al substrates has been investigated, respectively. It was found that diamond deposition on the substrates under both biasing exhibited significant variations among grains of different orientations. The substrate surface was found to be rough in the case of negative biasing, whereas it was smooth in the case of positive biasing. Thus, the correlation of the crystallographic orientation of grains on the samples with the diamond nucleation behavior was systematically characterized for the case of positive biasing by electron backscattered diffraction method with scanning electron microscopy. Diamond deposition on Ni 3Al grains near (111) orientation results in higher nucleation densities, while the densities are low on (110) and (100) oriented grains. Also, the interfacial microstructure between diamond deposited and Ni 3Al was characterized by cross-sectional transmission electron microscopy.

A double species model for study of relaxation of impure Ni 3Al grain boundaries

Based on the Monte Carlo simulation conjoined with the embedded atom method (EAM) potentials, the double species model is established to study relaxation of impure Ni 3Al grain boundaries. The present double species model suggests that the impure atoms are not only segregating species but also inducing species. The present model also suggests that study of combination of the positive (the impure atoms induce Ni atoms to substitute into Al sites) and the negative (the impure atoms substitute into Ni sites) bulk effects of impure atoms is available, in order to understand dependence of the cohesion of the impure Ni 3Al grain boundary (or the Ni enrichment at the impure Ni 3Al grain boundary) on the bulk concentration of impure atoms. The double species model is clarified in comparison between the Ni 3AlB and the Ni 3AlMg systems.

Calculation of the Griffith Cohesive Energy of theNi3AlBx Symmetrical Grain Boundary

A Monte Carlo simulation, with the energetics described by the embeddedatom method, has been employed to study the physical behaviour of boron atomsduring relaxation of the Ni3AlBx grain boundary. It has also been usedto calculate not only the peak concentrations of Ni and B and the valleyconcentration of Al at the grain boundary, but also the dependence of thegrain boundary cohesion on the B bulk concentration. During relaxationof impure Ni3Al grain boundaries, we suggest that, as thesegregating species, the B atoms either insert into interstices in thegrain boundary or substitute Ni atoms. Meanwhile, as the inducingspecies, they induce Ni atoms to substitute for Al atoms. Calculationsshow that in the equilibrium, when the B bulk concentration xincreases from 0.1 to 0.9, the peak concentration of B increases, thepeak concentration of Ni maximizes while the valley concentration of Alminimizes at x = 0.5. The calculations also show the best cohesion of thegrain boundary at x = 0.5.

A comparative study of two inward grown Pt modified Al diffusion coatings on a single crystal Ni base superalloy

Two inward grown Pt modified aluminide diffusion coatings (RT22 and SS82A) on the same Ni-based superalloy substrate (CMSX-4) have been studied. The specimens were oxidised in still laboratory air at 1050°C for times up to 2000 h and investigated by gravimetry. Specimens oxidised up to 500 h were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS). The gravimetric investigations revealed that the oxide on RT22 grew faster than the oxide on SS82A. Moreover, it was found that in the as-coated condition, both coatings contained precipitates of α-W, α-Cr, μ-phase and σ-phase. After short time heat treatment at 1050°C, only μ-phase precipitates were present. The volume fraction of the precipitates was measured and compared for different heat treatments. Compositional profiles across the coatings were acquired by TEM-EDS before and after heat treatment. They showed that considerable redistribution of elements takes place during oxidation and the initially steep Pt and Al profiles became smeared out. The RT22 coating formed γ′-Ni 3Al grains at the coating/oxide interface.

Synthesis and sintering of Ni 3Al intermetallic compound by combustion synthesis process

In order to develop the production process of the Ni 3Al intermetallic compound, combustion synthesis of the Ni 3Al intermetallic compound was investigated. Some kinds of Ni and Al powders with differential powder size were used as initial substances. These powders were mixed, and the Ni/Al mixed compacts (Ni:Al = 3:1 in atomic ratio) were prepared. The compacts were heated in a vacuum furnace, and the Ni 3Al compound was produced by the thermal explosion combustion synthesis. For the compounds obtained, density measurement, microstructure observation, phase analysis, and strength test were performed. Furthermore, the combustion synthesis of the boron added Ni 3Al compound was investigated. The Ni, Al and B powders were mixed, and the boron added compacts were prepared. The combustion synthesis experiment was attempted as well. A nearly dense intermetallic compound could be obtained by controlling the raw powder size and the density of the green compact. The products consisted of the Ni 3Al grains of about 50 μm. From compressive test, it was found that the products obtained were equal to the compound produced by the fusion process. To improve brittle nature of the Ni 3Al compound, the synthesis of the boron added Ni 3Al was attempted. Although, the boron added compact fused during the combustion synthesis, the boron added compound could be synthesized by controlling the charge ratio of the Ni, Al and B powders. In the compressive test, plastic deformation was measured, and it was confirmed that the boron addition was effective to improve the brittle nature. The production process by the combustion synthesis route could be determined in this paper.

Monte Carlo simulation of Mg segregation to Ni 3Al grain boundary

Monte Carlo computer simulation based on the embedded atom method (EAM) has been employed to study the segregation of Mg to Ni 3Al grain boundary. The results show that the segregation is localized within a few atomic layers nearby the grain boundary plane and the solute concentration profile is approximately symmetrical with respect to the boundary plane. The results also show that nearly 94% Mg atoms substituted into Ni or Al sites at the grain boundaries and Mg atoms substituted more Al atoms than Ni atoms. The simulated results agree with the experimental measurements.

The effects of boron and hydrogen on the embrittlement of polycrystalline Ni 3Al

The discrete-variational method within the framework of density functional theory is used to study the effects of both boron and hydrogen on the embrittlement of polycrystalline Ni 3Al. The calculated results show that there are strong repulsive interaction between the boron and the hydrogen atoms, if they occupy the nearest interstitial sites, respectively, in the Ni 3Al grain boundaries. It indicates that the boron atoms inhibit the diffusion of hydrogen atoms along the grain boundary. It may be the main reason why boron can suppress the moisture induced hydrogen embrittlement. Our results also show that the attractive interactions between boron and some substrate atoms are weakened, but the attractive interactions between boron and other substrate atoms are enhanced, when hydrogen atoms are forced into the grain boundary and occupy the nearest interstitial sites to boron atoms. As a result, the bonding states are polarized in the local region of the grain boundary. It may suppress the movement of slips across the grain boundary. Furthermore, the weakening effects of hydrogen to the grain boundary are hardly affected by the boron atoms, even though they are very near to each other. It can be concluded that hydrogen embrittlement takes place when the boron-doped polycrystalline Ni 3Al are charged with hydrogen.

Effects of composition on microstructures of rapidly solidified Ni–Al alloys

Microstructures of melt-spun Ni–Al alloys with compositions from 61–85 at% Ni were studied by means of transmission electron microscopy, X-ray diffraction analysis and optical microscopy. The microstructures of as-quenched ribbons exposed to cooling rates of the order of 106 K s-1 reflect the transition from primary β-NiAl to γ-Ni solidification with increasing nickel content. In 70 at% Ni alloy ribbons, martensitic NiAl grains were detected near the wheel-side surface contrasting with anomalous and lamellar eutectic microstructure in the top part. Directly ordered Ni3Al grains with single (or large) antiphase domains (APDs) and a minor eutectic fraction were observed in 75 at% Ni alloy ribbons. Samples containing 80 at% Ni exhibit mainly single-phase Ni3Al grains with 10–20 nm sized APDs indicating sequential ordering. Weak L12 ordering was even detected in 85 at% Ni ribbons which displayed ordered antiphase zones of 1 nm size. Disordered γ-(Ni) films on grain boundaries can be discounted for 80 at% Ni ribbons, but occurred near the top of 85 at% Ni samples. The results are explained in terms of the reassessed Ni–Al phase diagram employing recent corrections near to the Ni3Al composition and new results on phase formation in undercooled Ni–Al melts. © 1998 Kluwer Academic Publishers

Structure of Ni20Cr13Al alloy with and without carbon addition annealed at 800°C

The phase analysis of Ni20Cr13Al showed the presence of the (hypostoichiometric) γ′ (Ni 3Al) phase in chill-cast alloy. The spheroid particles of γ′ change their shape to cuboidal at the very beginning of the annealing at 800°C. This change is accompanied by a decrease of volume fraction of γ′, and at the same time by an increase of hardness. The annealing also leads to the precipitation of Cr-rich α-phase. The precipitation starts at grain boundaries. After longer times at the annealing temperature, the particles appear in the grain interior as well. In the vicinity of grain interior particles, electron microdiffraction patterns contain the spots corresponding to Ni 2Cr long-range order. At grain boundaries, the precipitation of Cr-rich phase induces the growth of Ni 3Al particles and grain boundary migration. Carbides (Cr 23C 6) are very rare in an alloy with only traces (impurities) of carbon while in carbon enriched (0.02%) alloys they appear after 500 h of annealing mainly in association with the Cr-rich particles.