Ni Al Cr System Research Articles

Mechanical properties of NiAl–Cr alloys in relation to microstructure and atomic defects

AbstractThe quasi-binary eutectic NiAl–Cr of the ternary Ni–Al–Cr system is composed of B2 type NiAl(Cr) solid solutions and the b.c.c. transition metal chromium as second phase. This constitution enables a systematic alloy design in dependence on the Cr content for improving elastic stiffness, high temperature strength, creep resistance, and fracture toughness of intermetallic NiAl. Elasticity and mechanical properties of NiAl(Cr) reinforced by nano-sized Cr particles or quasi-continuous Cr fibres of several microns in diameter are presented and discussed in respect to single phase NiAl and literature data. The microstructural characterization and discussion of mechanical properties of NiAl–Cr alloys are supplemented by atom probe field ion microscopy analysis (APFIM). APFIM was used to determine the atomic defect structures, the Cr site preference and reveals Cr segregations at antiphase boundaries.

Diffusion behavior of fcc and L12 Ni–Al–Cr alloys

In this paper, 12 groups of single-phase (γ and γ′) Ni–Al–Cr diffusion couples were carefully fabricated, and their concentration profiles at 1423, 1473, and 1523 K were measured by means of electron probe microanalysis technique. Atomic mobilities of γ phase in ternary Ni–Al–Cr system were then established directly from the experimental concentration profiles by using the HitDIC (High-throughput Determination of Interdiffusion Coefficients) software. The reliability of the atomic mobilities was confirmed by the coincidence between the calculated and experimental properties including the composition profiles and diffusion paths. Moreover, based on the atomic mobilities together with the available thermodynamic descriptions, the three-dimensional interdiffusivities planes at 1423–1523 K were constructed, from which the activation energies in γ Ni–Al–Cr alloys were extracted. Lower diffusion activation energy corresponds to higher interdiffusion coefficient. After that, the composition-dependent interdiffusion coefficients in γ′ Ni–Al–Cr alloys were determined and the magnitude relation of D˜AlAlNi and D˜CrCrNi in γ’ Ni–Al–Cr alloys was verified.

High-Throughput Determination of Interdiffusivities for Ni-Al-Cr Alloys at 1173 K through a Combination of Diffusion Multiple and Numerical Inverse Method

In this report, a combination of the diffusion multiple technique and the recently developed numerical inverse method was employed for a high–throughput determination of interdiffusivity matrices in Ni–Al–Cr alloys. A face–centered cubic (fcc) quinary Ni–Al–Cr diffusion multiple at 1173 K was carefully prepared by means of the hot–pressing technique. Based on the composition profiles measured by the field emission electron probe micro analysis (FE–EPMA), the composition–dependent interdiffusivity matrices in ternary Ni–rich Ni–Al–Cr system at 1173 K were then efficiently determined using the numerical inverse method.

P27] Determination of phase boundaries of Ni-rich corner of Ni–Al–Cr system by in situ experimental methods

P27] Determination of phase boundaries of Ni-rich corner of Ni–Al–Cr system by in situ experimental methods

High‐Throughput Investigation of the Oxidation and Phase Constitution of Thin‐Film Ni–Al–Cr Materials Libraries

Thin‐film materials libraries of the intermetallic model system Ni–Al–Cr were fabricated and their oxidation behavior was studied by compositional, optical, electrical, and structural high‐throughput characterization methods. The study reveals the compositional regions of the binary and ternary compositions which withstand longest to annealing in air (up to 700 °C), and are, therefore, resistant to oxidation and delamination under these conditions. A complete ternary thin‐film phase diagram for the Ni–Al–Cr system in its state after 9 h annealing in air at 500 °C was determined. Optical high‐throughput characterization is shown to be valid for rapid identification of oxidizing phases. Generally, the initially metallic phases show different oxidation behavior in air. We find that the ternary compositions are more resistant to oxidation than the binary phases. Compositions around Ni25Al12.5Cr62.5 were found to show very good oxidation resistance. These results were supported by additional information from corresponding electrical and optical property investigations. The presented high‐throughput approach is generic for the efficient study of multinary thin‐film materials in harsh environments.

Nanocrystalline intermetallic compounds in the Ni–Al–Cr system synthesized by mechanical alloying and their thermodynamic analysis

Mechanical alloying (MA) of the ternary Ni–Al–Cr mixtures having the Ni50Al50, Ni25Cr25Al50 and Ni50Cr25Al25 compositions was investigated. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that for all three compositions the gradual interdiffusion of elements during MA leads to the development of different phases. The final structures for the Ni50Al50 and Ni25Cr25Al50 compositions are nanocrystalline NiAl and (Ni, Cr)Al intermetallic compounds, respectively. However MA of the Ni50Cr25Al25 composition led to the formation of a Ni3Al intermetallic compound as well as Cr which transformed to the (Ni, Cr)3Al intermetallic compound after subsequent heat treatment. The structural transition upon annealing was investigated. Furthermore, thermodynamic analysis using the Miedema model for all three systems is carried out and discussed in detail.

New possibilities of recently constructed high-temperature solution calorimeter

The calorimeter presented in this paper is constructed under supervision of authors. The most substantial innovation of construction of the device was introduction of intermediate container with ten sockets. As a result the calorimeter is used for determination of enthalpy of formation for alloys in a wide range of temperature (up to 1,400 K). As an example results from alloys from the Ni–Al–Cr system are presented, they are aimed to determine directly γ′ / γ′ + γ / γ phase boundaries with use of this “in situ” calorimetric method. Precise indication of phase boundaries is crucial for determination of phase diagrams.

Thermal conductivity mapping of the Ni–Al system and the beta-NiAl phase in the Ni–Al–Cr system

We use two examples to highlight the power of micron-scale-resolution thermal conductivity mapping on graded compositions created in a diffusion-multiple sample in rapid establishment of composition–phase–property relationships. The Ni–NiAl diffusion couple example clearly shows the effects of solid solutioning, order–disorder transition, and compositional point defects on thermal conductivity. Thermal conductivity mapping of the beta-NiAl phase region of an Ni–Cr–NiAl diffusion triple clearly shows that Cr substitutes for both Ni and Al sites in beta-NiAl without a strong preference, which is also found not to be strongly composition dependent.

Synthesis and characterization of lamellar and fibre-reinforced NiAl-Mo and NiAl-Cr

Directionally solidified (DS) alloys of the eutectic systems NiAl-10Mo and NiAl-34Cr (at.%) are potential candidates for high-temperature structural applications. Here, these alloys were first arc-melted and drop-cast. Thereafter, they were directionally solidified (DS) at growth rates of 20 and 80 mm/h while rotating at a fixed rotation speed of 60 revolutions per minute. Specimens of the DS alloys were tested in three-point-bending and uniaxial compression to obtain mechanical properties, including the ductile to brittle transition temperature (DBTT). For the NiAl-Cr system DBTT was found to be around 300 °C. Microstructural observations revealed that in the section perpendicular to the growth direction a uniform distribution of fibres was observed. The expected decrease of the fibre diameter with increasing growth rate was not observed. Instead, the fibre diameter slightly increased with increasing crystal growth rates. First compression tests were performed to get insights into the creep behaviour of these fibre-reinforced microstructures.

Reply to Comments on “Interdiffusion in the β phase region of the Ni–Al–Cr system”

A recent comment on a previously published paper addressed the invalid explanation of the off-diagonal interdiffusion coefficients of the β-Ni(Al,Cr) phase in the Ni–Cr–Al system according to the symmetric property of the thermodynamic matrix. In this paper, the experimental data presented by Hou et al. was reanalyzed and the interdiffusion coefficient matrix was estimated again. The results of the analysis have been discussed in terms of a brief reply to the comments of Liu and Liang.

Assessment of the diffusion mobilites in the γ′ and B2 phases in the Ni–Al–Cr system

This work presents the assessment of the diffusion mobilities in both the γ′ (Ni 3Al-L1 2) and B2 (NiAl) phases in the Ni–Al–Cr system utilizing the phenomenological model developed by Helander and Ågren. Available experimental tracer diffusivity, interdiffusion coefficients and activation energies were evaluated and then used to optimize the composition- and temperature-dependent diffusion mobilities. For both the B2 and γ′ phases, the assessed diffusion mobility descriptions reproduce the Arrhenius temperature dependence for the Ni, Al and Cr tracer diffusivities and interdiffusion coefficients. The assessment reproduces the strong composition dependence of the diffusivities in the B2 phase observed experimentally. The measured composition dependences of the diffusivities in the γ′ phase are also replicated by the present mobility descriptions. The assessed mobility descriptions are validated by comparing calculated and measured composition profiles for a variety of Ni–Al and Ni–Al–Cr diffusion couples, including B2/B2, γ (fcc)/ γ′ and γ/B2 couples.

Interdiffusion in the β phase region of the Ni–Al–Cr system

This paper describes an investigation of the interdiffusion behavior in the beta single-phase region of the Ni-Al-Cr system by the modified Boltzmann-Matano method. The interdiffusion coefficient matrix is estimated and the cross-interdiffusion coefficients found to be smaller than the main interdiffusion coefficients at the examined composition ranges. Combining the thermodynamic calculation of the isochemical potential lines reveals that the thermodynamic interaction between Al and Cr in Al-rich beta-Nl(AI,Cr) is possibly negligible. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Application of the cluster/site approximation to fcc phases in Ni–Al–Cr system

The cluster/site approximation (CSA) has been used to model the face-centered cubic (fcc) phases (disordered γ with A1 structure and ordered γ′ with L1 2 structure) in the Ni–Al–Cr ternary system. The CSA takes into account short-range order (SRO), which is essential to satisfactorily describe the thermodynamics of order/disorder transitions such as occur between the fcc phases in the Ni–Al–Cr system. It possesses computational advantages over the cluster variation method (CVM) while offering comparable accuracy in the calculation of multi-component phase diagrams. This makes the CSA a practical method for carrying out calculations on real alloy systems. These points are illustrated in the application of the CSA to fcc phases in the Ni–Al–Cr system. The CSA-calculated phase diagrams, which use fewer model parameters than used in previous descriptions, based on the point approximation, are in good accord with the experimental data. In addition, the fcc metastable phase diagrams calculated by using the CSA show reasonable ordering/disordering behavior and phase relationships.

Mechanical properties of NiAl–Cr alloys in relation to microstructure and atomic defects

Abstract The quasi-binary eutectic NiAl–Cr of the ternary Ni–Al–Cr system is composed of B2 type NiAl(Cr) solid solutions and the b.c.c. transition metal chromium as second phase. This constitution enables a systematic alloy design in dependence on the Cr content for improving elastic stiffness, high temperature strength, creep resistance, and fracture toughness of intermetallic NiAl. Elasticity and mechanical properties of NiAl(Cr) reinforced by nano-sized Cr particles or quasi-continuous Cr fibres of several microns in diameter are presented and discussed in respect to single phase NiAl and literature data. The microstructural characterization and discussion of mechanical properties of NiAl–Cr alloys are supplemented by atom probe field ion microscopy analysis (APFIM). APFIM was used to determine the atomic defect structures, the Cr site preference and reveals Cr segregations at antiphase boundaries.

Theoretical and experimental study of the influence of Cr on the γ+γ′ phase field boundary in the Ni–Al–Cr system

The Ni–Al–Cr system, as a relatively simple yet sufficiently representative model system for commercial Ni-based superalloys, has been studied in the γ+γ′ two-phase field. The phase boundaries and chemical composition of γ and γ′ phases were calculated by means of the software Thermo-Calc applying both the general compound energy model (J. Alloys Comp. 247 (1997) 20) and the compound energy model with constraints to bond energy for the γ′ phase (Intermetallics 6 (1998) 487). Values obtained are compared with experimental values for four alloys with Cr contents in the range 0–15 at.% after long-term annealing at 900°C. The experimental determination of chemical composition of existing phases was performed by means of energy-dispersive X-ray analysis in a scanning transmission microscope. The comparison shows acceptable agreement between calculated and experimental data.

Thermodynamic properties of the Ni–Al–Cr system

Phase equilibrium and thermodynamic data of the Ni–Al–Cr ternary system were reviewed and analyzed by thermodynamic modeling. The liquid, fcc(Al, Ni) and bcc(Cr) phases were modeled as substitutional solution phases. Two-sublattice models were used for the ordered B2 and L1 2 phases. The other intermetallic compounds were treated as binary phases without ternary solubilities. No ternary phases were included. A self-consistent thermodynamic description was obtained by optimization. In comparison with a previous Ni–Al–Cr thermodynamic description, the present one gives a better account of experimental data and, at the same time, has fewer model parameters with more reasonable values.