Effect of atomic ordering on the hydrogen-induced environmental embrittlement of Ni4Mo intermetallics

Hydrogen induced environmental embrittlement of a Ni₄Mo alloy in different degree of ordered conditions was investigated by tensile tests in various atmosphere. The results show that the disordered Ni₄Mo alloy is not susceptible to embrittlement in hydrogen gas, but very susceptible to embrittlement in hydrogen charging. However, for the ordered Ni₄Mo alloy, there is similar deterioration in ductility when the environment changes from oxygen to hydrogen gas and simultaneous hydrogen charging. It indicates that the atomic ordering does not influence the dynamic hydrogen charging-induced environmental embrittlement, but has a considerable effect on the gaseous hydrogen-induced environmental embrittlement. In addition, hydrogen absorption and desorption of the Ni₄Mo alloy with disordered and ordered structures were also investigated using gas chromatographic analysis. The results show that the atomic ordering can promote gaseous hydrogen absorption at room temperature. This suggests that the atomic ordering accelerates the kinetics of the catalytic reaction for the dissociation of molecular hydrogen into atomic hydrogen due to the change of the outer layer electron structure and therefore exacerbates the hydrogen gas-induced environmental embrittlement.     

H2-induced environmental embrittlement in ordered and disordered Ni3Fe: An electronic structure approach

The different behaviors in H₂-induced environmental embrittlement in ordered and disordered Ni₃Fe are associated with differences in their electronic structures. The experimental study on electronic structures of ordered and disordered Ni₃Fe has been carried out by electron energy-loss spectroscopy (EELS). The onset energy of Ni L_2,3 edges from ordered phase is 0.3 eV lower than that from disordered phase, while the 3d occupancy of Ni atoms in ordered phase is 0.07 electrons/atom less than that in disordered phase. Severe H₂-induced environmental embrittlement in ordered phase is attributed to rather negative dissociative adsorption energy of hydrogen at surfaces, which arises from upward shifting of the valence band center of Ni.     

The effect of atomic ordering on the hydrogen absorption and desorption of Ni4Mo alloy

Hydrogen absorption and desorption of disordered and ordered Ni₄Mo alloys were investigated. The results show that the atomic ordering can promote gaseous hydrogen absorption at room temperature and therefore exacerbates the hydrogen gas-induced environmental embrittlement.     

有序和无序态Ni3Fe的环境氢脆研究

研究了有序和无序Ｎｉ３Ｆｅ在室温下不同气氛中的力学性能。结果表明：有序态或无序态Ｎｉ３Ｆｅ不存在由空气中水汽诱发的环境氢脆;无序态Ｎｉ３Ｆｅ也不存在由氢气诱发的环境氢脆,但有序态Ｎｉ３Ｆｅ在Ｈ２中却有强烈的脆化作用,且随形变速率降低而加剧;有序Ｎｉ３Ｆｅ的拉伸曲线出现了明显的屈服平台,而无序态没有。     

Effect of atomic ordering on environmental embrittlement of （Co,Fe）3V alloy in gaseous hydrogen

The diffusible hydrogen contents in precharged(Co,Fe)3V alloy were measured.It is found that atomic ordering can not promote hydrogen peretration in the(Co,Fe)3V alloy.The ultimate tensile strength(UTS)and ductilities in various condition were also investigated.The results show that the UTS and elongation of disordered alloy are higher than that of ordered one with fixed diffusible hydrogen content and(Co,Fe)3V alloy with ordered structure is highly sus ceprible to the embrittlement in hydrogen gas.The factor which may affect the susceptibility to the embrittlement of (Co,Fe)3V alloy in hydrogen gas in mainly due to that the atomic ordering may accelerate the kinetics of the catalytic reaction for the dissociation of molecular hydrogen into atomic hydrogen.However,it can not be roled out that atomic ordering intensifies planar slip and restricts cross-slip at the grain boundaries and enhances the suscptibility of the alloy to hydrogen embrittlement.     

Interaction of H2O and O2 with Ni3Fe and their effects on ductility

Possible reasons for the high ductility of Ni₃Fe and its insensitivity toward the testing environment have been investigated. Thermal desorption experiments have shown that water dissociates on clean Ni₃Fe surfaces to produce atomic hydrogen. Ductility measurements of cast and cold-rolled polycrystalline Ni₃Fe demonstrated that the reduced ductility was obtained only when the testing was performed with oxygen carefully removed. X-ray photoemission studies indicate that oxygen interacts with water to form hydroxyls, thereby suppressing the production of atomic hydrogen. Hydrogen desorption from the Ni₃Fe surface requires a lower activation energy, resulting in a smaller surface hydrogen concentration at a given temperature. Hence it is possible that there is insufficient hydrogen to cause the nucleation and growth of brittle cracks for severe embrittlement.     

Sol-gel preparation and characterization of NiCo and Ni3Fe nanoalloys

Sol-gel process is effective in preparation of metallic oxides. Here we report that sol-gel process based on chelating of citric acid is also effective in fabricating NiCo and Ni₃Fe nanoalloys under N₂ or H₂ atmosphere during heating treatment. With the introducing of surfactant, the average grain size of the nanoalloys is less than 10 nm and the grain size distribution is narrow. The formation of nanoalloys with equilibrium ordered phase rather than metastable disordered phase is confirmed by the occurrence of superlattice diffraction rings obtained by selected area electron diffraction. The Ni₃Fe nanoalloy shows typical ferromagnetic behavior at room temperature.     

Hydrogen diffusivity in B-doped and B-free ordered Ni3Fe alloys

The hydrogen diffusion coefficient of the ordered Ni₃Fe–B alloys with and without boron additions was measured by a method of the cathodical precharging with hydrogen. The apparent hydrogen diffusion coefficient decreases with increasing the boron concentration doped in the ordered Ni₃Fe alloy. Comparing with the B-free ordered Ni₃Fe alloy, the activation energy of hydrogen diffusion for the ordered B-doped Ni₃Fe alloy increases by as high as 42% when the boron content is sufficient. The doping boron in the Ni₃Fe alloy is effective in reducing the hydrogen diffusion at the grain boundary.     

Hydrogen embrittlement of a Ti-strengthened 250 grade maraging steel

Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 °C (800 °F) underaged specimens are very sensitive to strain rate; the 482 °C (900 °F) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 °C (1100 °F) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni₃Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.     

Ni2Cr合金室温环境氢脆的研究

对Ni2Cr合金在不同环境气氛及动态渗氢中的脆化进行了系统研究,并从能量学方面分析了影响合金脆化的因素。结果表明,Ni2Cr合金在室温空气及氢气中不存在明显的环境氢脆,动态渗氢拉伸时Ni2Cr合金存在严重的氢脆敏感性,无序和高度有序合金脆化严重,部分有序合金氢脆敏感性较小。     

氢对有序态Ni_3Fe合金脆性的影响(英文)

研究了有序态Ni3Fe合金在不同氢气压力中和在不同电流密度电解渗氢时合金的拉伸性能。结果表明：随着氢气压力或电流密度的增加,合金的延伸率先快速下降,随后逐渐趋于恒定;合金的氢脆因子与氢气压力或电流密度之间呈相同的依赖关系。有序态Ni3Fe合金在氢气中的脆化机制是催化反应生成的氢原子进入合金所致,合金的脆化程度与进入合金的氢原子数量有关。     

The effect of Nb addition on environmental embrittlement of a Ni3(Si,Ti) alloy

The effect of Nb addition on the moisture-induced embrittlement of a Ni₃(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 Ni₃(Si,Ti) alloy, while Nb as a solute in the Ll₂ matrix was shown to enhance the moisture-induced embrittlement of the Ni₃(Si,Ti) alloy. Possible mechanisms accounting for the beneficial effect of the Nb-containing second-phase dispersion on the moisture induced embrittlement of the Ni₃(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 L1₂ matrix/second-phase interface.     

Welding of unique and advanced ductile intermetallic alloys for high-temperature applications

Intermetallic alloys represent a unique class of materials with atomic arrangements that are different from those of conventional disordered alloys. Among them are alloys based on Ni₃Al, Fe₃Al, and TiAl. Intermetallic alloys have unique properties, such as high melting point, low density, high-temperature strength, and high-temperature corrosion and oxidation resistance. Their only disadvantage is the lack of ductility at room temperature and at elevated temperatures. However, they can be ductilised by micro- and macroalloying. Application of intermetallic alloys for structural use at elevated temperature depends on their ability to be welded using conventional welding procedures. This paper focuses on the development of these alloys, their behaviour when subjected to weld thermal cycles, and their weldability. Most intermetallic alloys are susceptible to cracking during or after welding, but some can be modified to have good weldability. The paper discusses welding and weldability of Ni₃Al-, Fe₃Al-, and TiAl-based intermetallic alloys. In addition, the weldability of other long-range ordered alloys, of the type (Fe, Ni)₃V and (Fe, Co)₃V, are briefly discussed.     

硼含量对有序态(Fe,Co)3V合金环境氢脆的影响

通过在真空和氢气环境下的拉伸实验,研究了硼含量对有序态(Fe,Co)_3V合金力学性能和环境氢脆的影响。结果表明,当在有序态(Fe,Co)_3V合金中添加0.02%B(质量分数)时,相对于无硼合金,合金的晶粒尺寸减小了27.5%,合金在真空和氢气中的抗拉强度和延伸率均达到最大值;而当硼含量继续增加时,合金的晶粒尺寸、合金在真空和氢气中的力学性能均保持不变。无硼有序态(Fe,Co)_3V合金在氢气中呈现严重的环境氢脆,当在合金中添加0.02%B后,合金氢脆因子降低了34.4%,合金的断口形貌由完全沿晶断口转变为穿晶和沿晶的混合断口;当合金中的硼含量继续增加时,合金的氢脆因子不再降低,恒定在50%左右,即硼原子只能部分抑制有序态(Fe,Co)_3V合金在氢气中的环境氢脆。     

First-principles study of the influence of lattice misfit on the segregation behaviors of hydrogen and boron in the Ni–Ni3Al system

A first-principles method is employed to investigate the segregation behaviors of hydrogen and boron in Ni-based and Ni₃Al-based alloys using two models. Chemical binding energy analysis shows that both boron and hydrogen are able to segregate to the interstices in the Ni phase, Ni₃Al phase and Ni/Ni₃Al interface. Boron, however, is bound to its neighbor atoms more tightly than hydrogen in both models and its stable state exists over a broader lattice misfit range compared with hydrogen. The bond order analysis we have proposed reveals the origin of the boron-induced ductility and hydrogen-induced embrittlment at the Ni/Ni₃Al interface with different lattice misfit. The calculations indicate that hydrogen causes more severe embrittlement at the Ni/Ni₃Al interface in Ni₃Al-based than in Ni-based alloys. Furthermore, it is found that the boron-induced ductility and hydrogen-induced embrittlement are changed, and thus controllable, by the lattice misfit. Our results provide a quantitative explanation of many experimental phenomena caused by the addition of boron and hydrogen to Ni-based and Ni₃Al-based alloys.     

Effect of long-range order on the cyclic deformation behaviour of Ni3Fe single crystals

Effect of ordering on cyclic deformation in disordered and ordered Ni₃Fe single crystals was investigated focusing on stress–strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni₃Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening.     

Comparison of mechanical properties of Ni3Al thin films in disordered FCC and ordered L12 phases

We report the results of several experiments isolating the effect of long-range order on mechanical properties of intermetallic compounds. Kinetically disordered FCC Ni₃Al (Ni 76%) thin films were produced by rapid solidification following pulsed laser melting. For comparison, compositionally and microstructurally identical films with ordered L1₂ structure were produced by subsequent annealing at 550°C for 2 h. These FCC and L1₂ Ni₃Al thin films were tested by nanoindentation for hardness and Young's modulus, and the critical strain to fracture was measured by straining the substrate under four-point bending. Ni₃Al thin films in the disordered phase were found to have nearly twice the critical strain to fracture, more than three times the fracture toughness, and about 20% lower hardness than in the ordered counterpart. Blunter crack tips and crack bridging observed in the disordered phase also illustrate increased ductility. The increased plasticity of Ni₃Al due to chemical disorder is manifested both within the grains and at the grain boundaries. Young's moduli of the ordered and disordered materials were found to be indistinguishable.     

Concentration dependence of Cr for alleviating environmental embrittlement in Fe30Ni20Mn35Al15

Previously, it has been shown that Fe₃₀Ni₂₀Mn₃₅Al_15, which consists of alternating submicron B2 and f.c.c. lamellae, exhibits a room temperature yield strength of 770 MPa and an elongation of ∼10% at a strain rate of 3 × 10⁻³ s⁻¹ in air, but at the slower strain rate of 3 × 10⁻⁶ s⁻¹ the alloy exhibits an elongation <1% [1]. An addition of 6 at% Cr has been proven to not only solve this environmental problem, but also to increase the elongation to 15–18% irrespective of strain rate [2]. Since we do not know whether Cr additions less than 6 at% can suppress this environmental embrittlement, in this paper we examined the room temperature mechanical properties of several alloys based on Fe₃₀Ni₂₀Mn₃₅Al₁₅ with Cr additions ≤6 at%. We show that additions as low as 0.5 at% Cr alleviate the environmental embrittlement and that additions of ≥2 at% completely suppress the embrittlement with little change in microstructure. X-ray photoelectron spectroscopy examination suggested that the suppression is mostly due to the formation of protective oxide scales on the surface that provide rapid passivation. The lower yield strength when Cr is present may also contribute to the improved ductility, possibly by easing dislocation cross-slip in the deforming f.c.c. phase where most of the Cr resides.     

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.     

The effects of boron and hydrogen on the embrittlement of polycrystalline Ni3Al

The discrete-variational method within the framework of density functional theory is used to study the effects of both boron and hydrogen on the embrittlement of polycrystalline Ni₃Al. The calculated results show that there are strong repulsive interaction between the boron and the hydrogen atoms, if they occupy the nearest interstitial sites, respectively, in the Ni₃Al grain boundaries. It indicates that the boron atoms inhibit the diffusion of hydrogen atoms along the grain boundary. It may be the main reason why boron can suppress the moisture induced hydrogen embrittlement. Our results also show that the attractive interactions between boron and some substrate atoms are weakened, but the attractive interactions between boron and other substrate atoms are enhanced, when hydrogen atoms are forced into the grain boundary and occupy the nearest interstitial sites to boron atoms. As a result, the bonding states are polarized in the local region of the grain boundary. It may suppress the movement of slips across the grain boundary. Furthermore, the weakening effects of hydrogen to the grain boundary are hardly affected by the boron atoms, even though they are very near to each other. It can be concluded that hydrogen embrittlement takes place when the boron-doped polycrystalline Ni₃Al are charged with hydrogen.