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.     

The influence of boron-doping on the effectiveness of grain boundary hardening in Ni3Al

The influence of boron-doping on the effectiveness of grain boundary hardening in Ni₃Al has been investigated by measuring microhardness profiles across grain boundaries of binary and boron-doped Ni₃Al bicrystals. It was found that although boron gives rise to significant solution strengthening in Ni₃Al, the effectiveness of grain boundary hardening in Ni₃Al is lessened by the addition of boron. Furthermore, the contribution of grain boundary hardening to the overall strength decreases as the segregation extent of boron at the grain boundary increases. A theoretical model of grain boundary hardening considering the various effects of boron-doping has been developed. Application of the model can deconvolute the individual effects of boron-doping on solution hardening, distribution of microcavities along grain boundaries and the interaction of dislocations on different slip systems. Analyzing the experimental results with the model suggests that boron-doping can (i) improve the transfer efficiency of shear stress across a grain boundary by reducing the amount of microcavities along the grain boundary; (ii) suppress the hardening effect from the interaction of dislocations moving on different slip systems and (iii) cause a significant solution hardening effect.     

Atomic structural environment of grain boundary segregated Y and Zr in creep resistant alumina from EXAFS

Dopants Y and Zr at 100 p.p.m. level (Y or Zr to Al atomic ratio) in ultra-high purity polycrystalline alumina have been found to be mainly segregated to the alumina grain boundaries. The atomic structural environments around the Y and Zr segregants have been investigated by Extended X-ray Absorption Fine Structure (EXAFS). On average, Y ions in α-Al₂O₃ grain boundaries are coordinated by four oxygens, at a distance of 2.30 Å, which corresponds nearly to the Y–O bond length in cubic Y₂O₃, and Zr ions are coordinated by five oxygens at a distance of 2.14 Å, which is approximately the same as the average Zr–O bond length in monoclinic ZrO₂. However, in the EXAFS radial distribution function, the Y-cation and Zr-cation next nearest neighbor shell cannot be clearly identified. These results suggest that Y and Zr at 100 p.p.m. concentrations in α-Al₂O₃ occupy grain boundary sites with well defined nearest neighbor cation–oxygen bond lengths similar to those in their parent oxides but with the next nearest neighbor cation–cation distances varying considerably from site to site. From EXAFS, the Y grain boundary saturation concentration is estimated to be 6.0 atoms/nm², which is consistent with the estimate from STEM of 4.4 atoms/nm². The differences in the Zr–O and Y–O nearest neighbor distances and coordination numbers in Al₂O₃ grain boundaries are related to the Y–Al and Zr–Al size mismatches.     

Structural features of Y-saturated and supersaturated grain boundaries in alumina

Grain boundary segregation of Y in α-Al₂O₃ and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α-Al₂O₃ grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y₃Al₅O₁₂ (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α-Al₂O₃, and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y₂O₃. This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.     

SURFACE REACTION，HYDROGEN DIFFUSIVITY AND ENVIRONMENTAL EMBRITTLEMENT OF INTERMETALLIC COMPOUNDS Ni_3Al AND Fe_3Al

ＳＵＲＦＡＣＥＲＥＡＣＴＩＯＮ，ＨＹＤＲＯＧＥＮＤＩＦＦＵＳＩＶＩＴＹＡＮＤＥＮＶＩＲＯＮＭＥＮＴＡＬＥＭＢＲＩＴＴＬＥＭＥＮＴＯＦＩＮＴＥＲＭＥＴＡＬＬＩＣＣＯＭＰＯＵＮＤＳＮｉ_３ＡｌＡＮＤＦｅ_３ＡｌＷＡＮＸｉａｏｊｉｎｇ；ＺＨＵＪｉａｈｏｎｇ；...     

Influence of grain boundary composition on the moisture-induced embrittlement of Ni3(Si,Ti) alloys

Ni₃(Si,Ti) alloys containing different levels of boron were tensile tested in air at room temperature at two different strain rates. The grain boundary compositions and fracture modes of these alloys were determined by Auger spectroscopy and scanning electron microscopy, respectively. Tensile elongation and fracture mode depend upon the fabrication procedure, heat treatment, and strain rate. Widely different boron concentrations were observed at the grain boundaries, depending on the fabrication procedure and heat treatment. In addition, silicon and titanium were depleted while nickel was enriched at the grain boundaries in all specimens examined. Tensile elongations correlated well with the grain boundary concentration of boron and also with an embrittlement parameter defined as (Si+Ti−B)/Ti. A sharp brittle-ductile transition was found to occur with increasing grain boundary concentration of boron and with decreasing values of the embrittlement parameter (Si+Ti−B)/Ti. The critical grain boundary concentrations corresponding to this transition were found to be sensitive to the strain rate. All the results can be explained in terms of the effect of grain-boundary composition on moisture-induced environmental embrittlement.     

First-principles investigations of interatomic interactions in Ni3Al alloyed by interstitial and substitutional impurities

First-principles calculations of the total energy of interstitial and substitutional solid solutions in intermetallic compound Ni₃Al were performed based on methods using Vienna ab-initio simulation package (VASP). The results of the calculations for interstitial solutions of carbon in Ni₃Al confirmed the priority role of chemical interactions over deformational ones for the nearest neighbors. We attempted to use first-principles methods of calculation of the deformation interaction and continuum approaching in the theory of solutions to calculate coefficients of the concentration changes of the lattice spacing. Comparison of the calculation results with experimental data of substitutional impurities in Ni₃Al has shown that the proposed method can aid in the study of the distribution of impurity atoms on the sublattices of the ordered phases, intermetallic compounds. We have proposed a method of calculating the partial molar volume of impurity in interstitial solid solutions.     

Influence of recrystallization and environment on tensile behavior of cold-rolled Ni3Al（Zr） alloys

1 Introduction Intermetallic compounds have long been the subjects of considerable interest for high temperature applications. In particular, Ni3Al is one of the more promising intermetallic compounds due to its anomalous temperature dependence of the yie…     

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.     

Fe3Al氢脆机理的研究

用余氏理论计算了氢对Ｆｅ３Ａｌ金属间化合物键结构、键能和电子分布的影响，分析发现，Ｆｅ３Ａｌ的氢脆是由于氢原子溶入扣引起Ｆｅ，Ａｌ原子状态变化。使晶体内晶格电子数大大减少，导致晶体局域金属性减弱，并构成具有严重各向异性的键络的结果。同时得出，氢在Ｆｅ３Ａｌ中占据间隙位置。     

The influence of atomic order on H2-induced environmental embrittlement of Ni3Fe intermetallics

The different sensitivity to H₂-induced environmental embrittlement for the ordered and disordered Ni₃Fe alloys has been investigated. The results show no environmental embrittlement in disordered Ni₃Fe in gaseous H₂ when tested at room temperature. However, the H₂-induced environmental embrittlement for the ordered Ni₃Fe becomes severer as the degree of order increases. The results of testing on simultaneous hydrogen charging show that disordered Ni₃Fe embritted as hydrogen atoms are forced into the material, implying that the embrittlement of ordered Ni₃Fe in gaseous H₂ is due to the acceleration of the kinetics of catalytic reaction to produce more atomic hydrogen. Further more, the hydrogen adsorption test of Ni₃Fe powder shows that the amount of chemically adsorbed hydrogen in the ordered state at room temperature is significantly larger than that adsorbed by the disordered materials, indicating that the more sensitive to H₂-induced embrittlement in the ordered Ni₃Fe is essentially due to accelerated catalytic reaction to produce more atomic hydrogen.     

Co3Ti金属间化合物的环境氢脆及氢扩散

研究了硼对Ｃｏ３Ｔｉ环境脆性的影响,证明硼对Ｃｏ３Ｔｉ合金无抑制环境氢脆的作用,用一种新方法测定了Ｃｏ３Ｔｉ中氢的扩散系数,证明加硼或不加硼Ｃｏ３Ｔｉ合金中氢的扩散系数相同。表明硼下降低氢的沿晶扩散。加硼Ｃｏ３Ｔｉ合金仍然对水汽诱发的环境氢脆敏感是由于硼不偏聚在Ｃｏ３Ｔｉ晶界,不能有效抑制氢的沿晶扩散所致。     

Environmental effect on mechanical properties of aluminide matrix composites

This paper investigates the validity of the toughness measurement with a variation of the loading rate for distinguishing the fracture mechanism of aluminide intermetallics and their composites. The ductility and fracture toughness of Ni₃Al alloys and their composites are governed by inherent grain boundary brittleness and moisture-induced embrittlement at ambient temperatures. Although B doping is effective in suppressing both factors, remarkable improvement of toughness mainly depends on grain boundary strengthening. The toughness of the alloys is influenced by the dislocation locking mechanism and the extrinsic embrittlement promoted by diffusion of oxygen at intermediate temperatures. Extrinsic embrittlement is the predominant mechanism in determining the toughness at 673 K. Restriction of the dislocation motion is the predominant factor in determining toughness at 873 and 1073 K. The composites reinforced with TiC particles exhibit exceptionally constant toughness at 300 to 900 K.     

The control of brittleness and development of desirable mechanical properties in polycrystalline systems by grain boundary engineering

Grain boundaries can be effectively controlled to produce or enhance their beneficial effects and also to diminish or reduce their detrimental effects on bulk properties in polycrystalline materials. Particular attention has been paid to the control of intergranular brittleness which remains a serious problem of material processing and development. Recent studies are presented and discussed, which have been successfully performed to control intergranular brittleness of “intrinsically brittle” materials such as the refractory metal molybdenum and the ordered intermetallic alloy Ni₃Al and to produce superplasticity in an Al–Li alloy, by grain boundary engineering through controlling a new microstructural factor termed the grain boundary character distribution (GBCD). The optimization of GBCD and the grain boundary connectivity has been found to be a key to produce desirable bulk mechanical properties in both structural and functional polycrystalline materials.     

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 ALLOYING BEHAVIOR OF BORON AND CARBON INNi_3Al

ＴＨＥＡＬＬＯＹＩＮＧＢＥＨＡＶＩＯＲＯＦＢＯＲＯＮＡＮＤＣＡＲＢＯＮＩＮＮｉ３Ａｌ①ＺｈａｎｇＹｕｎ，ＨｕａｎｇＪｉｎ，ＬｉｎＤｏｎｇｌｉａｎｇＳｈａｎｇｈａｉＪｉａｏｔｏｎｇＵｎｉｖｅｒｓｉｔｙ，Ｓｈａｎｇｈａｉ２０００３０ＡＢＳＴＲＡＣＴＴｏｆ...     

Calculation of electronic properties of boundaries in Ni3Al

The effects of boron doping on both the Ni-d DOS and the cohesive properties of Ni₃Al have been studied through the use of supercell models having the local composition of Ni₃Al with and without B at grain boundaries. The calculations of the density of states are compared with experimental results obtained from electron energy loss near edge structures. The results suggest an increase in the intensity of the Ni-L_2,3 white line when there is Ni enrichment; they also suggest that this increase disappears when B is added to the Ni-enriched model boundary, in agreement with experiment. The relationship between these results and cohesive enhancement as observed in B-doped Ni₃Al is discussed.     

Ni_3Al合金中硼原子的占位

采用特征晶体模型计算了Ｎｉ３Ａｌ合金中的Ｎｉ和Ａｌ原子的半径。依据特征晶体模型理论和从硬球模型所得到的不同间隙的间隙半径方程,计算了不同间隙的间隙半径、间隙能和固溶度。计算结果表明：硼原子趋向于占据６个Ｎｉ原子所围成的间隙。     

Phase field modeling the growth of Ni3Al layer in the β/γ diffusion couple of Ni–Al binary system

The growth behavior of Ni₃Al phase layer in the β/γ diffusion couple of Ni–Al binary system, including the shape evolution and growth kinetics, have been simulated by using the KKS multiphase field model. Simulation results indicate that, when Ni₃Al layer growth is controlled completely by volume diffusion, it could be regarded as parabolic growth. However, if the fast grain boundary diffusion is taken into account, the growth rate of Ni₃Al phase is accelerated, and the growth kinetics deviates from parabolic growth, which is consistent with experiments and other simulation results. Simulation results also demonstrate some details of shape evolution of grains, such as the uneven Ni₃Al/γ and Ni₃Al/β interfaces and the grain boundary migration of Ni₃Al grains caused by fast grain boundary diffusion.     

Synergistic effect of co-alloying elements on site preferences and elastic properties of Ni3Al: A first-principles study

The site preferences of co-alloying elements (Mo–Ta, Mo–Re, Mo–Cr) in Ni₃Al are studied using first-principles calculations, and the effects of these alloying elements on the elastic properties of Ni₃Al are evaluated by elastic property calculations. The results show that the Mo–Ta, Mo–Re and Mo–Cr atom pairs all prefer Al–Al sites and the spatial neighbor relation of substitution sites almost has no influence on the site preference results. Furthermore, the Young's modulus of Ni₃Al increases much higher by substituting Al–Al sites with co-alloying atoms, among which Mo–Re has the best strengthening effect. The enhanced chemical bondings between alloying atoms and their neighbor host atoms are considered to be the main strengthening mechanism of the alloying elements in Ni₃Al.