Environmental Embrittlement Research Articles

Moisture induced environmental embrittlement of intermetallics

This paper reviews recent investigations of the kinetic aspects of moisture induced environmental embrittlement of intermetallic alloys. The main kinetic steps of the embrittlement include surface adsorption, oxidation to form atomic hydrogen, diffusion of dissolved hydrogen [H] from surface to bulk, and hydrogen induced embrittlement. The first part of the paper reviews the chemical aspects of surface adsorption and oxidation reactions using thermodynamic approaches and kinetic equations. The second part of the paper reviews the aspects of hydrogen diffusion in fcc and bcc related ordered structures and the hydrogen concentration profile at room temperature. Then, three possible cases (diffusion rate limited, adsorption reaction rate limited, and mixed rate limited) and their related kinetic equations are discussed. The influence of certain important kinetic factors on the hydrogen concentration profile is also considered. The beneficial effect of B additions on environmental embrittlement is illustrated by using the kinetic approach. The third part of the paper reviews experimental results of the environmental embrittlement in aluminides, silicides, and other intermetallics. The temperature and strain rate dependence of embrittlement, the susceptibility of various intermetallics to environmental embrittlement, as well as the effects of alloy composition and test environment, are discussed. The last part of the paper describes alloy design principles used for alleviating moisture induced environmental embrittlement at ambient temperatures. As an example, the steps taken to design FeAl alloys resistant to environmental embrittlement are briefly mentioned.

On the effect of the B2 thermomechanical treatment in improving the room temperature ductility of Fe 3Al-based alloys

The effect of the B2 thermomechanical treatment on the room temperature ductility of Fe 3Al-based alloys has been systematically investigated by studying the order structure, the microstructure, the surface state and the texture evolution during rolling and annealing. A comparison between a single crystal Fe 3Al-alloy and the polycrystalline Fe 3Al alloy is presented in order to discuss the effect of the pancake shaped grains and the transverse grain boundaries on the room temperature ductility. The experimental results demonstrate that the order structure, microstructure and texture are mainly responsible for the improved intrinsic ductility, while an appropriate surface state can suppress the extrinsic environmental embrittlement by delaying the formation of microcracks. The pancake shaped grains hardly influence the hydrogen embrittlement, but could be responsible for the improved ductility at room temperature by increasing the resistance to crack propagation.

Moisture and hydrogen-induced embrittlement of Fe3Al alloys

Abstract Environmental embrittlement types of Fe3Al and Fe3Al–5Cr alloys have been evaluated using hydrogen microprint technique and tensile test in this study. The hot rolled iron aluminides offer better tensile strength and ductility in comparison to the as-cast iron aluminides in all test environments. Due to the fact that hydrogen transport in Fe3Al and Fe3Al–5Cr alloys is mainly lattice diffusion, the cleavage strength is decreased significantly by hydrogen.

Hydrogen Assisted Intergranular Crack Propagation during Environmental Embrittlement in an Al-Zn-Mg-Cu Alloy

Hydrogen absorbed from the test environment was detected experimentally from an Al-4.9%Zn-2.2%Mg-1.3%Cu alloy deformed in laboratory air, which is a direct evidence of hydrogen embrittlement. The alloy exhibited grain boundary embrittlement when deformed at a strain rate of 10 -7 s -1 in laboratory air and hydrogen behavior was studied using deuterium as a tracer of hydrogen. The direct detection of hydrogen supports the idea that gradual propagation of intergranular crack at sufficiently low strain rate may enable continuous absorption and accumulation of hydrogen in the stress field near the crack tip and eventually embrittle the material.

Effects of Grain Size and Temperature on Environmental Embrittlement of Ni<SUB><B>3</B></SUB>(Si, Ti) Alloy

The effects of grain size and temperature on environmental embrittlement of an Ll 2 -type Ni 3 (Si,Ti) ordered alloy were investigated as a function of strain rate by tensile tests and scanning electron microscope fractography. Irrespective of grain size and temperature, ductile-brittle transition (DBT) occurred when the strain rate decreased. Corresponding to the DBT, fracture surfaces changed from transgranular to intergranular fracture. DBT of a coarse-grained material occurred in a higher strain rate region than that of a fine-grained material. DBT of materials deformed at 423 K occurred in a lower strain rate region than that of materials deformed at room temperature. In a very low strain rate region, an anomalous increase of tensile elongation was observed for a fine-grained material deformed at 423 K.

Fracture Analysis of Hydrogen-Charged Nickel-Titanium Superelastic Alloy

Superelastic Ni–Ti alloys are widely used as engineering and medical materials. However, the alloy is susceptible to environmental embrittlement in a corrosive atmosphere. Accelerated tests of hydrogen embrittlement of the Ni–Ti alloy were carried out. The effect of electrolytic charging hydrogen on the properties of the alloy was measured. The rupture process in tension of the alloy with charged hydrogen was discussed on the basis of the tensile stress-strain curve, hardness in a cross-sectional area and fractography. The results of the measurements suggested that hydrogen concentration on the surface of the alloy and immersion time in an electrolytic bath had different effects on embrittlement. This will be related with the distribution of the hydrogen concentration, which was determined from the diffusion equation and boundary conditions.

Environmental embrittlement of intermetallics

In this paper the effect of alloying elements on the environmental embrittlement of Ll2 type intermetallics is summarized. The results show that the ductilizing effect of boron doping in Ni3Al is mainly to suppress the moisture-induced environmental embrittlement. The mechanism of this suppression effect is proved to be related to its severely reducing the hydrogen diffusivity along the grain boundaries. However, the boron doping in Co3Ti alloys does not have the same effect of suppressing the environmental embrittlement. The different behavior of boron doping in Ni3Al and Co3Ti may be attributed to its different segregation behavior on the grain boundaries. Boron in Co3Ti does not segregate on the grain boundaries and can not effectively reduce the hydrogen diffusivity along the grain boundaries. The moisture-induced environmental embrittlement of Co3Ti alloy can be completely suppressed by the addition of Fe. It is proved by Auger that this suppression effect is due to its obvious reduction of the kinetics of the surface reaction with water vapor.

Environmental embrittlement and preferred orientation of cold-rolled and annealed Fe3(Al,Cr,Zr) alloy at ambient temperature

The room temperature tensile properties of cold-rolled and annealed Fe3(Al,Cr,Zr) alloy are similar to those of warm-rolled Fe3Al alloys. The cold-rolled Fe3(Al,Cr,Zr) alloy is also susceptible to test environments. It has been shown that the ductility in various environments decreases in sequence of oxygen-oil-air-distilled water. The results of X-ray diffraction analysis show that (211) preferred orientation of B2 phase appears in cold-rolled Fe3(Al,Cr,Zr) alloy after recrystallization annealing.

Mechanical properties of aluminide matrix composites fabricated by reactive hot-pressing in several environments

The mechanical properties of Ni and Fe alminide matrix composites with low volume fraction of ceramic particles and fibers, fabricated by reactive hot-pressing, were evaluated. These composites reveal particular mechanical behaviors depending on characteristics of these matrix alloys. FeAl and Ni3Al matrix composites with ceramic particles exhibit significant loading rate dependence of toughness due to the moisture induced environmental embrittlement at ambient temperatures. The ultimate strength of the composites with ceramic continuous fibers, which exhibit the multiple fracture of fibers prior to the matrix cracking, also depends on the environmental embrittlement. Although the ductility and toughness of these composites at ambient temperatures is improved by the B doping, those of Ni3Al composites drastically decrease at intermediate temperatures due to dynamic oxygen embrittlement, deterioration of grain boundary cohesion and unique behavior of dislocations. On the other hand, NiAl composites are insensitive to the chemical effect of environmental factors because the matrix is inherently brittle. The alloy design for the matrix needs to be adequately applied to develop high performance intermetallic matrix composites.

Nu-Phase in Fe-Al-B Alloys

Abstract FeAl alloys with 40 to 50 at % Al have an impressive combination of oxidation and sulfidation resistance, low density, and low cost and are thus considered as possible substitutes for stainless steels. However, before these materials can be used in structural applications, the fracture characteristics of the alloy's grain boundaries need to be improved. Small additions of carbon and boron have been considered and the resulting properties have been reported. Boron is found to prevent intergranular fracture in iron-rich alloys and to mitigate environmental embrittlement although the improvement is not as dramatic as is seen with Ni3Al alloyed with boron. Boron segregates to the grain boundaries in Fe-Al alloys and boride precipitates at the boundaries and within the grains have been observed. In this paper, we report the composition and atomic arrangement in a recently discovered phase (designated v phase) found in Fe-Al alloys containing small boron additions using a combination of Z-contrast imaging and electron energy-loss spectroscopy.

Environmental embrittlement of SiC f/SiC composites

The effects of surface condition, moisture and temperature on the mechanical behavior of SiC f/SiC composites produced by Dupont Lanxide are described. Elastic moduli and tensile strengths have been established at temperatures between 25 and 1200°C. A degradation in properties is noted by thermal exposure alone, but the effect is altered by the presence of moisture in the test atmosphere. The influence of a surface coating on the observed behavior is described. Changes in microstructure accompanying environmental embrittlement are described. Implications for structural applications of these composites are discussed.

Recent developments in high temperature intermetallics research in China

A comprehensive survey was made of various advances of intermetallics research in China. The investigation focussed on the fundamental research and materials development. Charge density distribution and site occupancy of alloying elements, environmental embrittlement and chemical reaction, interface structures and phase transformation at the atomic scale, nanocrystalline intermetallics and its thermal stability, superplastic behavior and anomalous yield strength peak are reviewed. Several Ti–Al and Ni–Al based alloys have been manufactured, and show good mechanical properties. Diverse components have been fabricated successfully.

Effect of Fe on the environmental embrittlement of Co 3Ti alloy

The mechanical properties of Co 3Ti-based alloys were investigated. The tensile results show that the environment embrittlement of the alloy was completely suppressed when third element Fe was added to L1 2-type Co 3Ti alloy. The AES result shows that the surface reaction of Co 3Ti–Fe alloy with water vapor saturates at exposure of 2×10 −3 Pa s, but it does not saturate at 0.1 Pa s exposure for Co 3Ti alloy, and that the extent of the surface reaction of Co 3Ti–Fe with water vapor is much smaller than that of Co 3Ti at the same exposure. It illustrates that the beneficial effect of Fe in Co 3Ti on the environmental embrittlement is attributed to its obvious reduction of the kinetics of the surface reaction with water vapor.

High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy

A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB2 particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed, above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature. The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature.

Environmental embrittlement of Ti-48Al alloys

The tensile behaviour of two TiAl 48 alloys, namely Ti 47 Al 48 Cr 2 Nb 2 B 1 and Ti 52 Al 48 , was studied with respect to the gaseous environment. The samples were annealed to give them a duplex microstructure, consisting of equiaxed γ-TiAl grains and lamellar γ/α 2 grains. They were tensile tested at room temperature in pure argon gas or in water vapour saturated argon gas. Elongation was found rather low in the dry gas. It was reduced still further in moisture with respect to dry argon even in the case of Ti 47 Al 48 Cr 2 Nb 2 B 1 where additions, such as Cr, were thought to lessen the environmental embrittlement.

Hydrogen transport and environmental embrittlement effects in iron aluminides

Environmental embrittlement types of six iron aluminides have been systematically evaluated using electrochemical hydrogen permeation measurement, hydrogen microprint technique and tensile test in this study. The results of hydrogen permeation and microprint technique show that three α - disordered solid solution structure of binary and ternary iron aluminides (Fe-10Al, Fe-18Al, and Fe-18Al-5Cr) have higher effective diffusivity and permeation rate than B2 ordered structure of binary and ternary iron aluminides (Fe-28Al, Fe-28Al-5Cr, and Fe-40Al). The tensile test of six iron aluminides in air, vacuum treatment and hydrogen precharged were analyzed and concluded. Three α - disordered solid solution structure of iron aluminides suffer in hydrogen environment which are quite different embrittlement mechanisms from those B2 ordered structure of iron aluminides with serious tensile loss in air by moisture induced embrittlement.

Environmental effects on the tensile properties of two Ni 3Si-Based alloys

There is considerable potential for structural intermetallic alloys based on the nickel-silicon system. This is due to their good corrosion resistance, reasonable room-temperature ductility, and excellent strength to temperatures of 600 C or more. Several previous studies have been performed on Ni{sub 3}Si as well as Ni{sub 3}(Si,Ti) based alloys, yielding promising results. However, these alloys have been reported to suffer from environmental embrittlement at room as well as elevated temperatures. The present study was performed to develop a further understanding of such effects. Toward this end, the tensile properties of two Ni{sub 3}Si based alloys were evaluated over a range of temperatures in environments including air, vacuum, and dry oxygen.

Environmental and internal hydrogen embrittlement of a directionally solidified Ni-rich Ni3Al intermetallics

The influence of tensile orientation, test environments and internal hydrogen contents on the room temperature tensile properties of a directionally solidified Ni3Al alloy was investigated. The specimens parallel to the growth direction exhibited a good ductility and little susceptibility to test environment. The values of elongation in vacuum, air and H2 are 39.7%, 39.2% and 29.7%, respectively. Also, a transgranular fracture mode was observed in the specimens. However, the specimens perpendicular to the growth direction exhibited lower ductility, much more sensitivity to test environment and intergranular fracture mode. The elongation values in vacuum, air and H2 are 13.7%, 10.3% and 3.3%, respectively. The results indicate that the cohesive strength of grain boundaries in the alloy is low and they are more susceptible to test environment than are grain interiors. In addition, only a slight embrittlement of the internal hydrogen was found in the specimens parallel to the growth direction.

The effect of Nb addition on environmental embrittlement of a Ni 3(Si,Ti) alloy

The effect of Nb addition on the moisture-induced embrittlement of a Ni 3(Si,Ti) alloy was investigated at room temperature by tensile test and SEM fractography. Embrittlement/ductility was assessed as functions of strain rate and environment. The Nb-containing second-phase dispersion was found to be effective in reducing the moisture-induced embrittlement of the Ni 3(Si,Ti) alloy, while Nb as a solute in the Ll 2 matrix was shown to enhance the moisture-induced embrittlement of the Ni 3(Si,Ti) alloy. Possible mechanisms accounting for the beneficial effect of the Nb-containing second-phase dispersion on the moisture induced embrittlement of the Ni 3(Si,Ti) alloy was discussed in terms of microstructural modification by the second-phase, hydrogen transportation kinetics and deformation properties in the constituent phases or L1 2 matrix/second-phase interface.

Environmental embrittlement caused by hydrogen for intermetallic compounds: Preliminary model of ductility reduction

The reduction of fracture elongation caused by environmental embrittlement has been dealt with for intermetallic alloys as follows. The fresh surface that is formed by plastic deformation increases with an increase in tensile deformation, and hydrogen permeating through the fresh surface into the specimen increases and embrittles its surface layer. Thus, a microcrack nucleates at the surface layer. The crack propagates unstably through the specimen because of the low fracture toughness of intermetallic alloys, resulting in the fracture of the specimen. Hydrogen permeating into the specimen increases with the increasing fresh surface area and increasing period of exposure to the environment. The dependence of fracture elongation on strain rate and hydrogen pressure has been described by a phenomenological and preliminary model, which has been proposed according to the consideration mentioned previously. The experimental data which were reported for intermetallic alloys have been compared to the phenomenological model.