Effect of ternary alloying elements addition on atomic ordering characteristics of Fe–Al intermetallics

doi:10.1016/S1359-6454(99)00087-7

Acta Materialia

Volume 47, Issue 7, 28 May 1999, Pages 2067-2075

https://doi.org/10.1016/S1359-6454(99)00087-7 Get rights and content

Abstract

The energetic and structural characteristics of atomic ordering processes in Fe_0.5(Al_1−nX_n)_0.5 intermetallics have been qualitatively analyzed based on the statistico-thermodynamical theory of ordering by means of a quasi-chemical method combined with electronic theory in the pseudopotential approximation. The effects of ternary impurities on order–disorder phase transformation temperature and the characteristics of atomic short-range order in Fe–Al type intermetallics have been calculated. Impurity elements in Fe_0.5(Al_1−nX_n)_0.5 where X=Ni, Co, Mn, Cr, Ti, Si, Zr, Hf, Nb, Ta, Re, Mo or W, are considered up to 1 at.% concentration. The results of the calculation indicate that the impurity elements, X, with regard to their lattice site occupancy characteristics (SRO) can be divided into two groups; X_I=Ni, Co, Mn or Cr element atoms substitute mainly for Al sublattice sites, whereas X_II=Ti, Si, Zr, Hf, Nb, Ta, Re, Mo or W element atoms substitute preferentially for Fe sublattice sites in Fe_0.5(Al_1−nX_n)_0.5 intermetallics. It has been found that the absolute values of partial ordering energies of the W^Al–X(R₁) and W^Fe–X(R₁) have a profound effect on the order–disorder transition temperature of Fe_0.5(Al_1−nX_n)_0.5 alloys that would either increase or remain unchanged depending on the type and content of the ternary substitutional alloying elements. The impurities X=Zr, Hf, Nb, Ta, Re, Mo or W which are preferentially distributed Fe sublattice sites are more effective in increasing order–disorder transition temperature in Fe–Al(B2) intermetallics. The results of the present calculation are in good qualitative agreement with experimental observation for most of the third component impurity elements X in Fe_0.5(Al_1−nX_n)_0.5 intermetallics.

Introduction

The unique physical and mechanical properties of iron aluminides based on Fe₃Al(DO₃) and FeAl(B2) intermetallics are attributed to the long-range ordered (LRO) superlattices1, 2, 3, 4. Fe–Al alloy containing more than about 37 at.% of Al forms a single-phase ordered B2-type structure based on a body-centered cubic (b.c.c.) lattice, Fig. 13, 5. The B2-type ordered Fe–Al intermetallics are currently being investigated for potential technological and industrial applications because of their attractive low density, low material cost, excellent oxidation and corrosion resistance and conservation of strategic elements6, 7, 8, 9, 10, 11, 12, 13, 14, 15. However, exploitation of these intermetallics as structural components has been limited in the past by their brittle fracture and low ductility at ambient temperatures. The mechanical properties of Fe–Al intermetallics strongly depend on deviation from alloy stoichiometry and ternary alloy additions. It was shown by Titran et al.[16] that the effects of 1–5 at.% ternary alloying addition into Fe–Al alloys with B2 structure can be classified into three categories:

1.
Class I elements, X_I≡Ni, Co, Ti, Si, Mn or Cr, form single B2 phase Fe–Al–X alloys after homogenization. These elements did not show significant improvements in high temperature (1300 K) compressive flow strength in comparison with binary B2 Fe–Al alloys.
2.
Class II elements, X_II≡Zr, Hf, Nb, Ta or Re, show incomplete solubility in Fe–Al even after a long homogenization treatment for 175 h at high temperature, 1525 K. It has been reported that addition of class II elements increases the flow stress by a factor of three that of the binary Fe–Al alloys.
3.
Class III elements, X_III≡Mo or W, appear not to show any significant solubility in Fe–Al alloys and increase the flow stress by more than a factor of six compared to binary Fe–Al alloy at high temperature, 1300 K.

It appears that the mechanical properties of Fe–Al–X alloys strongly depend on the type of constituent elements. Nevertheless, from an atomistic point of view it seems most likely that the improvement of the observed mechanical properties would be mainly attributed to the arrangement of ternary alloying element atoms in the submicro volume of B2-type ordered crystal lattice, i.e. energetic and structural characteristics of short-range ordering (SRO) processes in Fe–Al intermetallics.

Recently, the first-principles statistical mechanics of intermetallic compounds[17] and the Korringa–Kohn–Rostoker (KKR) coherent potential approximation (CPA) combined with the local density approximation (LDA), called the LDA–KKR–CPA method[18], have been developed for quantitative calculations of atomic ordering processes for high temperature ordered intermetallics and disordered alloys, respectively. However, utilization of such methods for ab initio calculations of the physical and mechanical properties of complex multicomponent alloy systems has no widespread applications in materials science and solid state physics due to the solution of both quantum and statistical mechanical problems which requires very powerful high-speed computers and computer codes. Therefore, in this study, attempts have been made by using the statistico-thermodynamical method which has been around a long time, for qualitative analysis of the characteristics of multicomponent alloys.

The purpose of this work is to investigate the effect of type and content of the ternary alloying element additions on the SRO characteristics of Fe_0.5(Al_1−nX_n)_0.5 intermetallics with B2-type ordered structure by combining the statistico-thermodynamical theory of ordering with the electronic theory of alloys in pseudopotential approximation. The utilization of such a combination of methods for the qualitative and/or semi-quantitative study to elucidate the effect of alloying elements additions has already been established and satisfactory results have been obtained for L1₂-type and DO₃-type ordered intermetallics of Ni₃Fe19, 20, 21, 22, Ni₃Al[23] and Fe₃Al24, 25, 26, 27, respectively.

Section snippets

Effect of ternary alloying additions on atomic ordering characteristics of B2-type ordered intermetallics

The statistical theory of atomic ordering in ternary substitutional alloys was developed by Krivoglaz and co-workers28, 29 by means of the quasi-chemical method in which atomic ordering processes in ternary A–B–C substitutional alloys with two types of sites have been considered. The configurational part of the free energy of the A–B–C alloy, in which the number of sites is equal to each other and each site is surrounded only by sites of the other type in the nearest neighbor interaction

Calculation of ordering energy using the electronic theory of alloys in pseudopotential approximation

The partial ordering energies W_αα′(R) on the basis of electronic theory of multicomponent alloys in the pseudopotential approximation can be calculated using the following equations19, 21, 22, 23, 24, 25, 26, 27, 31, 32: $W_{αα′} (R_{l})= Ω ̄_{0} π^{2} ∫ 0 ∞ F_{αα′} (q) sin qR_{l} qR_{l} q^{2} d q$ where $F_{αα′} (q)=− Ω ̄_{0} 8π |ω^{0}_{α} (q)−ω^{0}_{α′} (q)|^{2} q^{2} ε(q)−1 ε^{∗} (q) + 2π Ω ̄_{0} q^{2} |Z∗_{α} −Z∗_{α′} |^{2} exp − q^{2} 4ξ .$ In , $Ω ̄_{0}$ is the average atomic volume of the ternary alloy; ε(q) is the dielectric constant in the Hartree approximation; $ε∗(q)$ is the modified dielectric

Results and discussion

In the present study, the partial ordering energies, W_αα′(R), order–disorder phase transformation temperatures, T_oc, and pairwise SRO parameters, α_αα′(R), have been calculated for Fe_0.5Al and Fe_0.5(Al_1−nX_n)_0.5 alloys (X≡Ni, Co, Mn, Cr, Ti, Si, Zr, Hf, Nb, Ta Re, Mo or W). The non-local bare pseudopotential form factors, proposed in Refs33, 47, 48, 49, are used in quasi-on the Fermi sphere approximations and have been screened using a modified dielectric constant50, 51. Moreover, the function

Conclusions

The present study has shown that the combination of statistico-thermodynamical theory of ordering by means of the quasi-chemical method combined with the electronic theory of multicomponent alloys can be successfully applied to the qualitative and/or semi-quantitative analysis of substitutional impurity elements effect on order–disorder transformation temperature and atomic short-range ordering characteristics in Fe(Al,X) ternary alloys. The partial ordering energies, W_αα′(R_l), order–disorder

Acknowledgements

This work is supported through DPT project No. AFP-03-08-DPT.98K122560, which the authors gratefully acknowledge. The authors are also grateful to Tarik Ogurtani for helpful discussions.

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Acta Materialia

Effect of ternary alloying elements addition on atomic ordering characteristics of Fe–Al intermetallics

Abstract

Introduction

Section snippets

Effect of ternary alloying additions on atomic ordering characteristics of B2-type ordered intermetallics

Calculation of ordering energy using the electronic theory of alloys in pseudopotential approximation

Results and discussion

Conclusions

Acknowledgements

Acta metall.

Acta metall. mater.

Acta mater.

J. Alloys Compounds

Acta mater.

Scripta metall. mater.

J. Alloys Compounds

J. Phys. Chem. Solids

Scripta metall. mater.

Scripta metall. mater.

Rev. High-Temp. Mater.

ISIJ Int.

Mater. Sci. Engng

Physica status solidi (B)

Fizika Metall.

Fizika Metall.

Turkish J. Engng Environ. Sci.