Research Progress on Brittleness of Iridium

Rare Metal Materials and Engineering Volume 44, Issue 10, October 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(10): 2363-2367.

ARTICLE

Research Progress on Brittleness of Iridium Wang Peng,

Yu Jie,

Zhou Xiaolong

Chen Jingchao

Key Laboratory of Advance Material of Rare Precious and Nonferrous Metals, Education Ministry of China; Key Laboratory of Advanced Materials of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, China

Abstract: Due to the high melting point, excellent high temperature strength and anticorrosive property, iridium is the unique material which can be used under extremely hostile environments. However, iridium exhibits an anomalous brittle fracture behavior, a mixed brittle intergranular fracture (BIF) and brittle transgranular fracture (BTF), even though it is of face-centred cubic (fcc) crystal structure. A great deal of efforts have been made to explore the embrittlement mechanisms since the anomalous fracture behavior was recognized in 1960 s, up to now, there has not been a reasonable conclusion yet. This paper emphatically reviewed the possible embrittlement mechanism of iridium, including impurity-induced brittleness, intrinsic brittleness and special defect structure induced embrittlement, discussed the research status quo about the deformation and failure mechanisms of iridium. Finally, the research direction and research method of the embrittlement mechanism of iridium were forecasted. Key words: iridium; BTF; BIF; intrinsic brittleness

Iridium (Ir) is one of the platinum group metals (PGMS). It is not only the most anticorrosive among all metals, but also has a high melting point (2443 oC) and is the only metal to maintain good mechanical properties in air at temperatures above 1600 oC[1-3]. Due to its high melting point, excellent high temperature strength and anticorrosive properties, iridium is the unique material used under extremely hostile environments. For example, it is used for crystal growth crucibles, nuclear fuel containers in thermoelectric generators, coatings of advanced rocket thrusters, automotive spark plugs, etc. In recent years, the application requirements of iridium in industry have progressively increased, and the price for iridium also has increased rapidly (Table 1). Unfortunately, iridium exhibits poor workability (even at elevated temperatures), and is very difficult to be fabricated [1,5]. The brittleness substantially restricts its industrial applications. Iridium belongs to face-centred cubic (fcc) metals. Generally, metals with fcc lattice are considered to be high plasticity, and the fracture mode has also been widely accepted as ductile fracture. However, the fcc-metal iridium

is somewhat strange, as its fractures in a brittle manner (Fig.1): at room temperature, high pure iridium single crystal is a highly plastic material, but it fails by brittle transgranular cleavage (with the features of river patterns) after considerable plastic deformation (the elongations up to 80% under tension [5] ), and the necking is absent. Under compression, the anisotropy of single iridium is vanishing[6,7]. Obviously, the brittle fracture of ductile materials rules out empirical theories. Iridium in polycrystalline state displays a brittle-ductile transition as a function of temperature [8] , which is the typical character of body-centred cubic (bcc) metals and its alloys, and exhibits both brittle intergranular fracture (BIF) and brittle transgranular fracture (BTF) at temperatures up to 1000 oC even in inert environments and at moderate strain rates [9,10]. But fine-grain iridium, prepared from a massive single crystal, never fails under compression [7] and can be forged like platinum [11] . The anomalous fracture behaviors had been recognized since 1960s, and grabbed the attention of researchers. Although a great deal of efforts has been made to explore

Received date: October 25, 2014 Foundation item: Yunnan Provincial Innovation Team (2009CI003); National Natural Science Foundation of China (51361016) Corresponding author: Yu Jie, Associate Professor, Kunming University of Science and Technology, Kunming 650093, P. R. China, Tel: 0086-871-5189490, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Table 1

Variation in price and consumption of iridium (t) over industry branches in 2006~2011

Consumption 2006 Chemical industry 1.0 Electronic, electrical 0.9 engineering Electrochemistry 1.1 Others 1.1 Average price (USD/troy 350 ounce)

[4]

2007 2008 2009 2010 2011 0.7 0.7 0.7 0.4 0.4 0.8

0.5

0.2

2.3

2.3

0.7 1.0

0.8 1.3

1.0 0.9

2.3 1.2

2.3 1.4

447

450

425

606

950

a

b

Fig.1

Fracture surface of iridium

[6]

: (a) single crystal and

(b) polycrystalline

the mechanism of unusual fracture, up to now there is still a big controversy on the factors which control fundamental mechanisms in the materials science community. The deformation and failure mechanisms of iridium are not clear. And the nature of both BTF and inclination to GB brittleness continues to be unsolved. At the first international symposium on iridium, Professor P. E. Panfilov (Urals State University, Russia) commented that “It is unbelievable that there is a face-centred-cubic metal whose properties continue to be puzzling at the end of the twentieth century” [12]. Although some physical properties of iridium are generally in agreement with empirical cleavage criteria, its fundamental mechanisms are not clarified, and controversial with each other in the literatures. At present， the main viewpoints of embrittlement mechanism of iridium are as following.

1 Impurity-Induced Brittleness Grain boundaries (GBs) are important components of polycrystalline materials and the segregation of harmful impurities usually deteriorates the performance of materials. It is well known that the BIF in fcc metals is mainly attributed to the influence of the segregation of dangerous

elements to GBs. Iridium is one of refractory metals (refractory metals are difficult to be refined and their mechanical properties are very sensitive to the influence of non-metallic impurities [13] ). In addition, high pure iridium single crystal exhibits high plasticity. Hence, the brittleness of polycrystalline iridium might be an impurity-induced brittleness. Early studies[5,14] suggested that a small quantity of harmful impurities (such as C, P, Si) have a great effect upon critical resolved shear stress, and might also affect the Peierls-Nabarro (P-N) force resisting the movement of dislocations. Thus, the brittleness of iridium is due to the effect of impurities, not only on GBs but also in the interior of the grains. In contrary, Hecker [15] et al. showed that impurity segregation to GBs was not necessary for grain boundary fracture by analysis of freshly fractured grain boundaries using Auger electron spectroscopy. Hence, they concluded that brittle transgranular cleavage and BIF of iridium are intrinsic and not caused by impurities. However, Handley[11] suggested that the sensitivity of iridium to dangerous impurities is so high that standard procedures did not allow detecting the critical level of impurities on fracture surfaces of iridium samples. For example, the presence of very small amounts of impurities (10 μg/g is the critical level for carbon) will result zero plasticity. Therefore, this makes it extremely difficult to be verified by direct experiment [16]. Indeed, the segregation of dangerous impurities or the diffusion of harmful impurity atoms on GBs in recrystallization annealing can induce GBs brittleness, as GBs are the most probable sites for crack growth. Although that is the most logical explanation for the BIF of polycrystalline iridium in common sense, many experimental studies [16-19] indicated that the fracture mode of high purity polycrystalline iridium and its alloys is a mixture of BIF and BTF, and the fracture surface of fine grained polycrystalline iridium without non-metallic impurities can consist of approximately 100% BTF, despite the fact that the plasticity of iridium with (or without) non-metallic impurities will become worse after recrystallization annealing in vacuum. So impurities (C, Si, P, etc.) can exacerbate GBs brittleness, but its influence is only a secondary factor for BIF. Hence, BTF may be the inherent fracture mode of polycrystalline iridium, while BIF in iridium is considered as an impurity-induced fracture mode, as no environmental factors have so far been implicated in the brittle fracture of iridium. Now, it is clear that impurities and most non-metals (metal impurities (such as Fe, Ni, Al, Cr) do not induce brittleness [7,20] , and some metal elements are beneficial for ductility, such as Liu [21] discovered that doping with “10-6 levels” of Th can suppress grain-boundary fracture and increase the ductility at high-strain rates), can considerably

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reinforce the inclination to BIF. However, whether the BIF of polycrystalline is an intrinsic fracture or impurityinduced fracture mode both does not gain an agreement on this issue in the academia. As single and polycrystalline iridium should have the same inherent fracture, recent experimental studies [3,22] suggested that the intrinsic fracture mode of iridium free of non-metallic impurities is BTF, which does not depend on the presence of grain boundaries in the matrix.

2

Intrinsic Brittleness

Iridium exhibits high melting point, small and negative Cauchy pressure, the highest shear modulus of any metal [23]. In addition, it belongs to 5d transition metal (partially filled valence d-electron band). All those indicate that iridium might have some inherent properties which are different from other fcc metals, such as the peculiarities of interatomic interactions. Hecker [15] and collaborators were the first to speculate the intrinsic brittleness of iridium, which is a result of strong interatomic bonds. Later, a series of first-principles calculations [23-25] suggested that the “pseudocovalent” effects, which occur as a result of the extremely directional charge density redistribution of d states and strong bonds, are responsible for its brittleness. Hence, Liang [26] et al. considered that the mechanical properties of iridium would be more similar to covalent or ionic materials. But Kamran [27] et al. found aluminum (Al) also has directional and moderately covalent bonds by ab initio calculation, and concluded that the angular characteristics of the chemical bonding is not the unique factor controlling the shear distortion scope of fcc crystals. Consequently, the directional bonds might not be the main cause for the brittleness of iridium, which might also be associated with other factors. So the strong directional bonding couldn’t interpret the unambiguously macroscopic brittleness. While octahedral slip in fcc crystals causes single iridium to have high plasticity, iridium may have intrinsic grain boundary brittleness due to special interatomic bond which result in weak cohesive strength of boundaries. However, Chen [28] found the GBs structures of iridium are similar to classical polycrystalline fcc metals by simulation using local volume potentials, but the GBs of polycrystalline iridium have high grain boundary energies and low cohesive energies, which results in the weak cohesive strength of the boundaries and are responsible for BIF in polycrystalline iridium. In fact, the high plasticity of iridium single crystals and mixed BTF and BIF of polycrystalline iridium do not support the idea on inherent brittleness of this metal.

3

Special Defect Structure Induced Brittleness At room temperature, the major deformation mechanisms

of crystal materials are dislocation sliding and mechanical twinning in grains during the deformed processing. As a high stacking fault energy (about 420 mJ/m [29], which is much higher than that of fcc metals), the deformation mechanisms should be in the form of dislocation sliding. This is confirmed by experiments [7,30] and ab initio calculations [24]: octahedral slip of perfect dislocations having <110>/2 Burgers vector is the sole deformation mechanism of single crystal iridium at room temperature, and no other deformation mechanisms (such as mechanical twinning or non-octahedral slip) are found. As dislocation property of sliding is controlled by dislocation core structure and distribution, special dislocation core structure and abnormal dislocation density may be the cause of brittle fracture of iridium. Cawkwell et al. published the paper on Science[31] and Acta Materialia [32], shown that screw dislocations in iridium have specific core structures, including planar and nonplanar core configurations that enable cross-slip to occur more readily than in other fcc metals, leading to extremely intense work hardening. Therefore, they suggested that abnormally high dislocation density makes further dislocation glide extremely difficult such that stress concentrators cannot be relaxed via dislocation mediated plasticity, thereby resulting in atomically brittle cleavage or intergranular fracture. However, Ref.[7] pointed out that the dislocation structures of iridium crystals are normal and similar to that of other fcc metals, but the dislocation density of iridium was so high that it could be compared with the density of dislocations in irradiated metals. On the other hand, literature [33] indicated that the characteristics of strain hardening of iridium is similar to other fcc metals, dislocation density is not abnormally high after deformation, and the stress-strain curve don’t support the mechanism of unusual cross-slip by experimental observations. While octahedral slip is the dominant mechanism for deformation, the low mobility of dislocations and their hindered path on GBs may be responsible for brittle fracture of iridium. Panfilov[19] considered that as the mobility of <110>/2 dislocations is considerably lower than that in other fcc metals, dislocation nets are the sole permitted configuration of iridium at room temperatures. So they cannot transform into small angle boundary. Meanwhile, considerable width of diffusion zone of point defects around grain boundaries will hinder the movement of <110>/2 dislocations through the GBs, so cracks are formed on the GBs. Beside, only refractory iridium displays GBs brittleness. Perhaps it may be related with the specific structure of GB in this fcc metal. However, experimental data by TEM studies [9] do not support this hypothesis, and show that there are no differences between the GB structure of

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iridium and that of other fcc metals. Latterly, Lynch [34] held that the anomalous fracture behavior is probably associated with unusual crack-tip surface structure and bonding characteristics rather than with some unusual bulk property using metallographic and fractographic observations. However, the information on structure and bonding at non-atomically sharp crack tip is little. In short, the nature of the brittleness of iridium continues to be a puzzle, even though the discussion about its deformation and failure mechanisms began fifty years ago. Nevertheless, the excellent high temperature mechanical properties and chemical resistance make it have irreplaceable advantages in the field of high-temperature -resistant materials. For example, Ir-Rh thermocouple is the only temperature-measurement material up to 2100 oC in air, iridium-based superalloy is the optimal candidate material for aero-engine and space-engine, which has better high-temperature mechanical properties than those of traditional Ni-based superalloys. Besides, iridium, the unique case of fcc metals, violates the traditional cognition of perfect toughness and plasticity of fcc metals. Hence, making clear the microscopic mechanism of brittleness of iridium has intensely realistic significance for production and processing of iridium system materials, and profoundly theoretical value for thoroughly understanding of mechanisms of plastic deformation. At present, there are a lot of scientific literatures about abnormal mechanism behaviors of iridium, but the results are always not in accordance with, eventually in contradict to, others. Obviously, the brittleness of iridium has become engineering and theoretical challenge. Now it is clear that electronic structure characterized by the partially filled d-band plays a crucial role in the macroscopic mechanism properties. It has long been recognized that there is the connection between mechanical behavior of materials and atomistic properties (or electronic structure), but it is different to be clarified, because the relationship is complex, and the macroscopic properties of materials do not result unambiguously from atomistic (electronic) properties. In fact, macroscopic properties are heavily influenced by defects, since the defects destroy the integrity of crystal structure. Unfortunately, the current in formations on the defects of iridium are very limited. Therefore, the structure and the properties of the defects of iridium should be a research emphasis such as: dislocation nucleation and motion, micro crack nucleation and propagation, grain boundary properties and segregation. With the help of reliable first-principles calculations, defect properties, especially the electronic structure effect of defects, will be predicted in theory. Together with experiments and advanced tests methods (such as HRTEM observation, situ TEM observation), a better understanding

of the unusual mechanical behavior of iridium will be achieved in the time to come.

4

Summary

At present, the possible embrittlement mechanisms of iridium include impurity-induced brittleness, intrinsic brittleness and special defect structure. Although there is still no a reasonable conclusion after a great deal of efforts explored the embrittlement mechanisms since the anomalous fracture behaviors had been recognized in 1960s, it is clear that electronic structure characterized by the partially filled d-band may play a crucial role in the anomalous fracture behaviors of iridium. Hence, the researches on electronic and atomic scale, especially electronic structure effects of defects, are the key links for understanding the nature of the brittleness of iridium. Meanwhile, further investigations should be studied in the future.

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