Hydrogen Embrittlement

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1 Hydrogen Embrittlement 1. Introduction Hydrogen, plasticity interaction is one of the most controversial of all fracture related phenomena. The condition results because nearly all technologically important corroding materials in water evolve hydrogen and, as far as we know plasticity precedes fracture in all environmentally influenced cracking phenomena. Despite extensive study, the mechanism(s) of hydrogen embrittlement have remained unclear. Several candidate mechanisms have evolved, each of which is supported by sets of experimental observations. One reasonable certain aspect of this controversy is that there are several viable mechanisms of hydrogen – related failure and that the search for a single mechanism to explain all the observations is doomed to failure. While both recent in-situ electron microscopy experiments [1-8] and atomistic simulations of hydrogen in discrete lattices [9-15] are not necessarily incompatible, the majority of studies have tended toward separate path regarding fracture mechanisms. In the middle ground are experimentalists studying bulk failure behaviour [16-20] some of them favour hydrogen enhanced localized plasticity (HELP)[16-18] and some of them favour hydrogen enhanced decohision (HEDE) [19,20]. 2. Overview of mechanisms Of the many suggestions for hydrogen embrittlement mechanism three mechanisms appear to be viable. Three basic mechanism associated with hydrogen embrittlement are indicated in table 1. The first mechanism is concerned mostly with hydrides although hydrogen-induced phases in metastable stainless steels have been cited often [21]. HEDE has been investigated for over forty years [22] while HELP is more recent [1] even though Beachem [16] suggested the phenomena thirty years ago. No. Mechanism 1 Hydride-induced embrittlement (Second-phase mechanism) 2 Hydrogen-enhanced decohesion mechanism, HEDE (brittle fracture) 3 Hydrogen-enhanced localized plasticity mechanism HELP (ductile fracture) Table 1. Hydrogen embrittlement mechanisms. 2.1. Hydride-induced embrittlement The stress-induced hydride formation and cleavage mechanism is one of the well- established hydrogen embrittlement mechanisms with extensive experimental and theoretical support [23-25]. The nucleation and growth of an extensive hydride field ahead of a crack has been observed dynamically by Robertson et al. [26] in -Ti charged from the gas phase in situ in a controlled environment transmission electron microscope [27]. In their observations the hydrides first nucleated in the stress-field of the crack and grew to large sizes not by the growth of individual hydrides but by the nucleation and growth of new hydrides in the stress

Transcript of Hydrogen Embrittlement

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Hydrogen Embrittlement

1. Introduction

Hydrogen, plasticity interaction is one of the most controversial of all fracture related phenomena. The condition results because nearly all technologically important corroding materials in water evolve hydrogen and, as far as we know plasticity precedes fracture in all environmentally influenced cracking phenomena. Despite extensive study, the mechanism(s) of hydrogen embrittlement have remained unclear. Several candidate mechanisms have evolved, each of which is supported by sets of experimental observations. One reasonable certain aspect of this controversy is that there are several viable mechanisms of hydrogen –related failure and that the search for a single mechanism to explain all the observations is doomed to failure. While both recent in-situ electron microscopy experiments [1-8] and atomistic simulations of hydrogen in discrete lattices [9-15] are not necessarily incompatible, the majority of studies have tended toward separate path regarding fracture mechanisms. In the middle ground are experimentalists studying bulk failure behaviour [16-20] some of them favour hydrogen enhanced localized plasticity (HELP)[16-18] and some of them favour hydrogen enhanced decohision (HEDE) [19,20].

2. Overview of mechanisms

Of the many suggestions for hydrogen embrittlement mechanism three mechanisms appear to be viable.

Three basic mechanism associated with hydrogen embrittlement are indicated in table 1. The first mechanism is concerned mostly with hydrides although hydrogen-induced phases in metastable stainless steels have been cited often [21]. HEDE has been investigated for over forty years [22] while HELP is more recent [1] even though Beachem [16] suggested the phenomena thirty years ago.

No. Mechanism

1 Hydride-induced embrittlement (Second-phase mechanism)

2 Hydrogen-enhanced decohesion mechanism, HEDE (brittle fracture)

3 Hydrogen-enhanced localized plasticity mechanism HELP (ductile fracture)

Table 1. Hydrogen embrittlement mechanisms.

2.1. Hydride-induced embrittlement

The stress-induced hydride formation and cleavage mechanism is one of the well-

established hydrogen embrittlement mechanisms with extensive experimental and theoretical support [23-25]. The nucleation and growth of an extensive hydride field ahead of a crack has been observed dynamically by Robertson et al. [26] in �-Ti charged from the gas phase in situ

in a controlled environment transmission electron microscope [27]. In their observations the hydrides first nucleated in the stress-field of the crack and grew to large sizes not by the growth of individual hydrides but by the nucleation and growth of new hydrides in the stress

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field of the others. They showed that these small hydrides grew together to form the larger hydrides.

This auto-catalytic process of hydride nucleation and growth together with brittle nature of them seems to be the main cause of embrittlement of typical hydride former element, i.e. the element of the group Vb; e.g., V, Nb, Ti and Zr.

2.2. Hydrogen-enhanced decohesion

The decohesion model is one of the oldest models used to represent the change of properties as a result of atomar hydrogen. It was described first in 1941 by Zapffe and Sims [28]. It is based on the increased solubility of hydrogen in a tensile strength field, for instance on the tip of a crack or in areas with internal tensile strength or in the tension field of edge dislocations (fig 1.). The increased solubility of hydrogen in this tension field results in a decrease in the atom binding forces of the metal lattice. The influence of stress results in a premature brittle-material fracture along the grain boundaries (intergranular cleavage) or network levels (transgranular cleavage) owing to the decrease of the binding forces.

Figure 1. Schematic decohesion model

In it’s basic form, the mechanism is interpreted in terms of the energy of two half solids as they are separated. This energy can include the influence of elastic distortion of the two half solids, but it does not include inelastic effects. This mechanism is supported primarily by the observation that in some non-hydride forming systems, hydrogen embrittlement appears to occur in the absence of significant local deformation, by theoretical calculation of the effect of hydrogen on the atomic potentials [29] and by thermodynamic arguments [30,31]. Direct evidence for this mechanism has not been obtained and measurement which have been made on the effects of hydrogen on small strain aspect of lattice potential suggest no softening of the lattice potential [32].

In summary , consideration of binding forces decrease and energy changes during segregation of hydrogen to interfaces and surfaces leads one to expect only a very small hydrogen induced decrease in the separation energy during transgranular fracture, but a substantially larger decrease during intergranular fracture in those cases where hydrogen segregation to the grain boundaries has taken place. Direct experimental characterization of this effect currently lacking. Such decohesion is competitive with hydrogen enhanced plasticity, and fractography and microstructure studies indicated that the latter process frequently intervenes, even when the fracture appears to follow grain boundaries. The decohesion mechanism has been unambiguously observed, however. In systems where H and other embirittling solutes co-segregate to boundaries.

There is a compelling need for better understanding of the influence of hydrogen on cohesion. This goal should be pursued experimentally in at least three areas. First more comprehensive work on hydrogen modified elastic constant and phonon-dispersion relations would illuminate the small-strain regime. Second, careful measurements on surface energy under equilibrium and non-equilibrium conditions are needed for variety of systems. Finally, it’s highly desirable to develop new techniques that probe the presently

Hydrogen atom

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inaccessible large-displacement regions of the lattice potential. One possible approach would utilize atomic force microscopy.

2.3. Hydrogen-enhanced localized plasticity

The most recent process model by far is the so-called HELP (Hydrogen Enhanced Local plasticity) process [1]. A prerequisite for the HELP process is, as is the case with the decohesion model, the accumulation of hydrogen in the field of stress, for instance, in the vicinity of the tips of cracks or in the stress areas of dislocations (carriers of plastic deformation in a metal grid). During the initiation of a dislocation movement by introducing external stresses, the existing active hydrogen considerably eases the dislocation movement through shielding the fields of stress of the dislocations against each other as well as against other grid defects (see Fig. 2). Therefore, a local dislocation movement will already occur at low levels of shearing stress, which is caused by a local drop of yield stress due to hydrogen. A sliding localization occurs, leading to a micro crack caused by the formation of micro pores and shearing action. As soon as the crack leaves the area of reduced yield stress, it will not propagate any further.

Figure 2. Schematic HELP model

This counter-intuitive process says that the macroscopic ductility is limited by the

onset of extensive localized plasticity [33]. Sliding localization and formation of micro pores up to micro cracks can be found in nearly all hydrogen induced brittle fracture surfaces in form of short micro deformation lines, running mostly in the vicinity of micro pores, i.e. on inter-crystalline separation surfaces. In the past, they were often referred to as "crow’s feet” and they are probably the most striking sign for a hydrogen induced brittle fracture.

Experiments on which the HELP mechanism is based are founded on the premise that detailed understanding of fracture mechanisms required observations at sufficiently high resolution to allow the mechanistic details to be revealed. High-resolution fractography of hydrogen embrittled metals, such as Ni and Fe, show extensive plastic deformation localized along the fracture surfaces [33]. Particularly revealing results have been obtained with the technique of in situ TEM environmental cell deformation and fracture. Although these methods allow observation of the fracture process in real time, at high spatial resolution, and in vacuum or in hydrogen atmospheres, correlating the information with effects in bulk samples must be made cautiously. TEM samples are typically 200 nm thick, which raises the issue of the influence, of nearby surfaces on reaction pathways and products. In addition, the act of observing the sample can cause change by, for example,

Hydrogen atom

Dislocation

Elastic obstacle

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the electron beam increasing the sample temperature or displacing atoms from their lattice sites [27]. Also it should be noted that the fugacity at the sample surface is considerably higher because atomic hydrogen is created by the electron beam dissociating the hydrogen molecules. Bond et al. [34] determined that the hydrogen fugacity in the range of 34-750 MPa for a gas pressure in the environmental cell of 10-16 kPa.

3. Difficulties of investigation of hydrogen embrittlement

Hydrogen enhanced fracture introduces a range of complexities beyond the normal fracture process. This is because hydrogen/deformation interactions have many facets with broad implications. Numerous systems and circumstances in terms of chemical potential or even purely mechanical aspects are only one side of the issue. The additional aspect concerns the wide range of extrinsic and/or intrinsic variables which resulted in an enormous number of sometimes controversial findings and/or interpretations. Figure 3, shows a global description of hydrogen embrittlement interaction aspects. Even with no hydrogen the role of plasticity is critical [35]. It means that, the assessment of micro-plasticity findings which leading to the conclusion that brittle processes are actually “ductile” has provided some uncertainty. Considering the interwoven nature of plasticity enhancing brittle fracture in semi-brittle materials, this highlights the coupled nature of the fracture process even in the absence of the hydrogen. This is exacerbate with the addition of the hydrogen. Furthermore the complexity is heightened by substructural change or local damage introduced by different hydrogenation methods. For example, some trap binding sites such as grain boundaries versus precipitate interfaces may compete differently for hydrogen depending on the relative coherency of each boundary. Superimpose on this the relative strength of those interfaces in the presence and absence of hydrogen, the prediction of initiation sites becomes problematic. Finally, since initiation may favour grain boundary failure in one instance and precipitate shearing in another, either boundary decohesion versus shear decohesion as enhanced by plasticity may result. That either result may be favoured by changing the external state of stress, yield strength level or thermal history makes the general separation between stress criteria and strain criteria for hydrogen-enhanced fracture an unresolved issue. Local softening as claimed in stainless steel does not necessarily result in a stress-free material as might be

achieved with plastic strain relief. In fact, hydrogenation of 5 µm thickness strips has produced internal stress fields during gas release sufficiently large to produce surface micro-cracking. After out-gassing, the fracture micro-mechanism reverts from brittle to ductile[35].

It is seen then that depending on the “critical experiment” one might conclude that a given material is failing by enhanced cleavage, enhanced grain boundary fracture, enhanced localized microvoid formation or all of the above. We must admit that much of the hydrogen embrittlement research has been conducted on commercial microstructures or model materials that were nearly impossible to fully characterize. As such, “critical experiment” have most often addressed a innumerable of competing phenomena without addressing the basic building blocks. Know time is ripe on two fronts for these basic building blocks to be examined in more fundamental ways. First, there are in situ spectroscopies and microscopies that can probe small volumes, i.e. the building blocks themselves. That is not to say that such efforts are totally absent as there has been, for example, an effort involving electron microscopy using environmental stages. Second, there are computational materials sciences modelling efforts that can either be calibrated against or incorporated those building blocks in embrittlement predictions.

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Figure 3. Global description of HE interaction aspects.

4. Nano-probing hydrogen interactions

To introduce the use of SPM for probing hydrogen interactions, A brief outline of the different SPM techniques first will be given. Then SPM technique for mechanical characterisation and fracture investigation of materials will be addressed.

4.1. Scaning probe microscopy

Scanning probe microscopes (SPMs) are a family of instruments used to measure properties of surfaces. In their first applications, SPMs were used solely for measuring surface topography and, although they can now be used to measure many other surface properties, that is still their primary application. SPMs are the most powerful tools for surface metrology of our time, measuring surface features whose dimensions are in the range from interatomic spacing to a tenth of a millimeter.

The main feature that all SPMs have in common is that the measurements are performed with a sharp probe scanning over the surface while maintaining a very close spacing to the surface. With most SPM technologies, this produces an atomically short depth of focus such that only the top layer of rigidly bound (chemisorbed) atoms is seen. Excellent spatial resolution can be obtained by using a very sharp probe (on the order of a few nanometers radius of curvature at the end, and with a very steep sidewall angle) and keeping it's spacing from the surface very small (usually within a nanometer). These instruments were the first to produce real space images of atomic arrangements on flat surfaces. SPMs are most commonly used to perform very precise, three dimensional measurements on the nanometer-to-micron scale. Table 2, gives a comparison over different common microscopy techniques for surface imaging.

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Table 2. Comparison of Common Techniques for Imaging and Measuring Surface

Morphology

Optical

Microscope SEM SPM

Operating environment ambient, liquid, vacuum

vacuum ambient, liquid, vacuum

Depth of field small large medium

Depth of focus medium small small

Resolution: x,y 1.0 µm 5 nm 0.1 - 3.0 nm

Resolution: z N/A N/A 0.01 nm

Magnification range 1X - 2 x 103X 10X – 106X 5 x 102X - 108X

Sample preparation required

little freeze drying, coating none

Characteristics required of sample

must not be completely transparent to light wave

no charge build-up on surface

must not have excessive variations in surface height

The basic scanning probe microscope consists of the Scanning System, Probe, Probe

Motion Sensor, Electronics, Vibration Isolation and Computer as schematically shown in the figure 4.

Figure 4. Basic SPM Components

There is various Techniques for Scanning Probe Microsocopy as outlined below;

1. Scanning Tunnelling Microscopy (STM):

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Measures topography of surface electronic states using the tunnelling current which is dependent on the separation between the probe tip and a highly conductive sample surface.

2. Contact Mode AFM: Measures topography by sliding the probe tip across sample surface.

3. Lateral Force Microscopy (LFM): Measures frictional forces between the probe tip and the sample surface.

4. Force Modulation: Measures relative stiffness of surface features.

5. Tapping Mode AFM: Measures topography by tapping the surface with an oscillating probe tip; eliminates shear forces which can damage soft samples and reduce image resolution.

6. Phase Imaging: Measures variations in surface properties (stiffness, adhesion, etc.) as the phase lag of the cantilever oscillation relative to the piezo drive.

7. Non-contact Mode AFM: Measures topography by sensing Van der Waals attractive forces between surface and probe tip held above surface; provides lower resolution than either contact mode or Tapping Mode AFM.

8. LiftMode: A combined two-pass technique that separately measures topography using Tapping Mode and another selected property (magnetic force, electric force, etc.) using topographical information to track the probe tip at a constant distance above the surface; provides the best resolution and eliminates cross-contamination of images.

9. Magnetic Force Microscopy (MFM): Measures magnetic force gradient and distribution above the sample surface using amplitude, phase or frequency shifts; best performed using LiftMode to track topography.

10. Electric Force Microscopy (EFM) Measures electric field gradient and distribution above the sample surface, best performed using Lift Mode to track topography.

11. Scanning Thermal Microscopy (SThM): Measures temperature distributions on the sample surface.

12. Scanning Capacitance Microscopy (SCM): Measures carrier (dopant) concentration profiles on semiconductor surfaces.

13. Nanoindentation: For indenting and scratching thin films and other surfaces.

14. Lithography: Use of probe tip to write patterns in either STM or AFM contact mode. Out of 14 different above mentioned SPM techniques two techniques seems to be really promising for nano probeing of hydrogen interaction with metals which will be discussed in more detail below.

4.1.1. Atomic force microscopy

AFM uses a very sharp tip to probe and map the morphology of a surface. The key element of the AFM is its microscopic force sensor, or cantilever. The cantilever is usually formed by one or more beams of silicon or silicon nitride with a dimension of 100 ~ 500 microns long and 0.5 ~ 5 microns wide. Mounted on the end of the cantilever is a sharp tip used to sense the force between the tip and the sample surface.

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For normal topographic imaging, the probe tip is brought into continuous or intermittent contact with the sample and scanned over the surface by a piezoelectric scanner that generate the precision motion needed for topographic images and force measurements.

4.1.2. Nanoindentation

Using a diamond tip mounted to a metal-foil cantilever, you can indent a surface and immediately image the indentation. This in situ imaging ability eliminates the need to move the sample, switch tips, relocate the area for scanning, or use an entirely different instrument to image the indentation. Although indentation cantilevers have higher spring constants than typical imaging cantilevers, it is still possible to image soft samples with relatively low forces. This is possible using Tapping Mode which requires less force to image a sample than contact mode operation. The diamond tips are sufficiently sharp to provide good image resolution. The nanoindentation capability also includes the ability to perform scratch and wear tests using the same cantilevers.

Figure. 5. A typical Nano-indentor

4.2. Nanoprobe measures of deformation and fracture

AFM and nanoindentation techniques has been widely used for characterisation of fracture mechanics, crack growth investigations and mechanical characterization of materials. Vehoff et al. used AFM equipped with in situ straining module for studying of crack tip deformation in intermetallic materials [36-38]. They also used Nanoindentation technique for characterization of thin coatings and metals [39,40].

Even though this nanoprobe technique within the last 15 years has became a widespread technique for probing materials on the micrometer and nanometer scale the works done in the field of mechanical properties, deformation and fracture influenced by hydrogen with the means of this techniques are few in number. Bahr et al. used nanoindentation and AFM coupled with OIM (Orientation Imaging Microscopy) to investigate hydrogen effect on mechanical properties of Ti-30wt.% Mo as a BCC structure alloy and commercial 304 stainless steel as a FCC alloy [41, 42].

5. Intermetallic alloys and Hydrogen embrittlement

Intermetallic compounds have been the subject of study for the past thirty five years, both as intellectually interesting phases from an alloy theory standpoint and as material that have unusual physical and mechanical properties. More recently, there has been recognition that intermetallics have promise as high temperature structural materials because they exhibit excellent strength retention at high homologous temperature[45]. But

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for intermetallics, the most challenging problem is still the brittleness at room temperature.

For the past decade, extensive investigations have shown that the occurrence of moisture induced hydrogen embrittlement is a major cause of the room temperature brittleness of many intermetallics at ambient temperatures. It has been thus recognised that the study of environmental effects and associated hydrogen embrittlement is critical for the development of intermetallic alloys as engineering materials for structural use.

5.1. Iron aluminides intermetallic alloys

Ordered intermetallics which form long range ordered crystal structures below their critical ordering temperatures constitute an emerging important class of metallic materials. Between different intermetallic alloys the iron aluminides Fe3Al and FeAl have been among the most widely studied intermetallics because of their low cost, low density, good wear resistance, ease of fabrication, resistance to corrosion, oxidation and sulfidation and conservation of strategic materials such as chromium. In addition, Fe3Al is one of the few structural intermetallics that passes trough two ordered structures (D03 and B2) before becoming disordered, as shown in figure 6. These structures (D03 and B2) are both ordered BCC structures and are presented in figure 7. It must be noted that a perfect ordered crystallographic structure exists only at the exact stoichiometry corresponding to its stoichiometric formula. However, ordered iron aluminides exist in relatively narrow compositional ranges around simple stoichiometric ratios (fig. 6) but the degree of order decreases as the deviation from stoichiometry increases.

Figure 6 – Fe-Al equilibrium phase diagram showing the disordered iron solid solution

(�) and the ordered intermetallic phases (Fe3Al and FeAl) that are present from

approximately 11-35wt%Al.

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Figure 7 – The D03 and B2 ordered crystal structures of Fe-Al

Table 3 presents some typical room temperatures properties and critical ordering

temperature for different allotropic modifications of iron aluminides [46].

Alloys Crystal

structure

Critical ordering

temperature (˚C)

Melting point (˚C)

Density (gr/cm3)

Room temperature

yield strength (Mpa)

Room temperature elongation

(%)

Fe3Al D03 540 1540 6.72 300 3-7

Fe3Al B2 760 1540 6.72 380 4-1

FeAl B2 1250 1250 5.56 360 2-2

Table 3 –Typical room temperatures properties of Fe-Al

Figure 8 – Dark field TEM image showing APB’s in Fe3Al (small rounded boundaries).

Due to the formation of FeAl prior to the formation of Fe3Al during cooling, FeAl APB’s

can be seen as well (arrows).

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Ordered phases in iron aluminides can be identified using the TEM due to the presence of anti-phase boundaries (APB) in ordered structures. APB’s are boundaries within an ordered structure where the atomic periodicity or order is interrupted. Ordered phases cannot be identified using light optical or scanning electron microscopy, but APB’s can be seen using dark field imaging techniques on the TEM (Figure 8) [47].

5.2. Hydrogen embrittlement of iron aluminides

Iron aluminides exhibit poor ductility at ambient temperature and environment. That hydrogen is the main damaging agent causing poor ductilities can be inferred from tensile tests conducted in different environments, the results of which are summarized in table 4 [48].

Material Environment Yield

strength (MPa)

Ultimate tensile

strength (MPa)

Ductility (%)

Heat treated for 1 hour at 900˚C and 2 hours at 700˚C [B2 structure]

Vacuum (~1 × 10-4 Pa) 387 851 12.8

Oxygen (6.7 × 104 Pa) 392 867 12.0

Ar + 4% H2 (6.7 × 104 Pa) 385 371 8.4

Air 387 559 4.1

H2O vapour (1.3 × 103 Pa) 387 475 2.1

Heat treated for 1 hour at 850˚C and 120 hours at 500˚C [D03 structure]

Vacuum (~1 × 10-4 Pa) 316 813 12.4

Oxygen (6.7 × 104 Pa) 298 888 11.7

Air 279 514 3.7

Fe 3

Al

H2O vapour (Saturated Air) 322 439 2.1

Heat treated for 1 hour at 900˚C and 2 hours at 700˚C [B2 structure]

Vacuum (<1 × 10-4 Pa) 352 501 5.4

Oxygen (6.7 × 104 Pa) 360 805 17.6

Ar + 4% H2 (6.7 × 104 Pa) 379 579 6.2

Air 360 412 2.2

FeA

l

H2O vapour (67 Pa) 368 430 2.4

Testing in water vapour environment show the least ductility and vacuum/oxygen

environments provide high ductility. Despite of all investigations which has been done, there is still no unique mechanism

to explain the hydrogen embrittlement in iron aluminides. This is mainly caused by the complexities in investigation of hydrogen embrittlement that mentioned before in part 3. Subject of the big portion of investigations on hydrogen embrittlement in iron aluminides based on the application shortages of this material and improvement of mechanical properties for application as a functional material and there is a lack in fundamental investigations to understand this phenomena.

In situ AFM and nanoindentation techniques combined with controlled atmosphere chamber could be used to investigate the mechanism of hydrogen embrittlement in iron aluminides due to the unique characteristics of this material and special abilities of these techniques to probe the materials in nanoscale. This investigations may reveal the facts

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about hydrogen embrittlement as a physical phenomena as well as unknown mechanism of hydrogen embrittlement in iron aluminides.