Interfacial segregation and grain boundary embrittlement: An overview and critical assessment of experimental data and calculated results

One of the most dangerous technical failures of materials is intergranular brittle fracture (temper embrittlement) as it proceeds very quickly and its appearance is often hardly predictable. It is known that this phenomenon is closely related to the chemistry of grain boundaries and to the difference of the segregation energies of the grain boundaries and the free surfaces (Rice–Wang model). To elucidate the effect of individual solutes on embrittlement of various materials such as steels and nickel-base superalloys, grain boundary and surface segregation was extensively studied in many laboratories. As a result, numerous data on surface and grain boundary segregation have been gathered in literature. They were obtained in two main ways, by computer simulations and from experiments. Consequently, these results are frequently applied to quantify the embrittling potency of individual solutes. Unfortunately, the values of the segregation energy of a solute at grain boundaries as well as at the surfaces obtained by various authors sometimes differ by more than one order of magnitude: such a difference is unacceptable as it cannot provide us with representative view on the problem of material temper embrittlement. In some cases it seems that these values do not properly reflect physical reality or are incorrectly interpreted. Due to the above mentioned large scatter of the segregation and embrittlement data a critical assessment of the literature results is highly needed which would enable the reader to avoid both the well known and less well known pitfalls in this field. Here we summarize the available data on interfacial segregation and embrittlement of various solutes in nickel and bcc iron and critically discuss their reliability, assessing also limitations of individual approaches employed to determine the values of segregation and strengthening/embrittling energies, such as density functional theory, Monte Carlo method, molecular statics and dynamics and tight binding on the theoretical side, and Auger electron spectroscopy, 3D tomographic atom probe, and electron microscopy techniques on the experimental side. We show that experimental methods have serious limitations which can be overcome by accepting reasonable assumptions and models. On the other hand, the theoretical approaches are limited by the size of the computational repeat cell used for the calculations of the segregation energy. In both cases, a careful critical analysis of the available segregation energy and/or enthalpy reflecting physical reality allows to assess the reliability of these values and their applicability in analysis of intergranular brittle fracture in steels and nickel-base alloys.     

Interfacial dislocations dominated lateral growth of long-period stacking ordered phase in Mg alloys

Understanding the interface between strengthening precipitates and matrix in alloys, especially at the atomic level, is a critical issue for tailoring the precipitate strengthening to achieve desired mechanical properties. Using high-resolution scanning transmission electron microscopy, we here clarify the semicoherent interfaces between the matrix and long-period stacking ordered(LPSO) phases, including 18 R and 14 H, in Mg–Zn–Y alloys. The LPSO/Mg interface features the unique configuration of the Shockley partial dislocations, which produces a near zero macroscopic strain because the net Burgers vectors equal zero. The 18 R/Mg interface characterizes a dissociated structure that can be described as a narrow slab of 54 R. There are two dislocation arrays accompanied to the 18 R/54 R and 54 R/Mg interface, resulting a slight deviation(about 2.3°). The 14 R/Mg interface exhibits the dislocation pairs associated with solute atoms. We further evaluate the stability and morphology of the corresponding interfaces based on elastic interaction, via calculating the mutual strong interactions between dislocation arrays, as well as that between the dislocations and solute atoms. The synchronized migration of interfacial dislocations and solute atoms, like move-drag behavior, dominates the lateral growth of LPSO phases in Mg alloys.     

The effect of segregated sp-impurities on grain-boundary and surface structure, magnetism and embrittlement in nickel

We present a detailed theoretical study of segregation and strengthening/embrittling energy of sp-elements from the 3rd, 4th and 5th period (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Σ5(2 1 0) grain boundary (GB) in fcc ferromagnetic nickel. For comparison, we investigate also the segregation of these impurities at the (2 1 0) free surface (FS). On the basis of ab initio electronic structure calculations, full relaxation of the geometric configuration of the GB and FS without and with impurities is performed and the effect of impurities on the distribution of magnetic moments is analysed. Whereas there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk nickel (3-7% and 24%, respectively), the studied impurities entirely kill or strongly reduce ferromagnetism at the GB and in its immediate neighbourhood so that magnetically dead layers are formed. This effect, which is due to the hybridization of the impurity sp-states and nickel d-states, is even more pronounced at the impurity-decorated (2 1 0) FS. We determine the preferred segregation sites at the Σ5(2 1 0) GB for the sp-impurities studied, their segregation enthalpies and strengthening/embrittling energies with their decomposition into the chemical and mechanical components. We find interstitially segregated Si as a GB cohesion enhancer, substitutionally segregated Al and interstitially segregated P with none or minimum strengthening effect and interstitially segregated S, Ge, As, Se and substitutionally segregated Ga, In, Sn, Sb and Te as GB embrittlers in nickel. As there is very little experimental information on GB segregation in nickel most of the present results are theoretical predictions which may motivate future experimental work.     

Atomic-scale analysis of light alloys using atom probe tomography

The present paper reviews recent progress in atomic-scale characterisation of composition and nanostructure of light alloy materials using the technique of atom probe tomography. In particular, the present review will highlight atom-by-atom analysis of solid solution architecture, including solute clustering and short-range order, with reference to current limitations of spatial resolution and detector efficiency of atom probe tomography and methods to address these limitations. This leads to discussion of prediction of mechanical properties by simulation and modelling of the strengthening effect exerted by solute clusters and the role of experimental atom probe data to assist in this process. The unique contribution of atom probe tomography to the study of corrosion and hydrogen embrittlement of light alloys will also be discussed as well as a brief insight into its potential application for the investigation of solute strengthening of twinning in Mg alloys.     

Time-dependent interfacial failure in metallic alloys

Time-dependent intergranular brittle fracture has now been studied experimentally in a number of alloy systems, and the generic features are becoming clear. Mobile surface-adsorbed elements are caused to diffuse inward along grain boundaries under the influence of a tensile stress, and this can lead to sub-critical crack growth by decohesion. Oxygen is found to play this role in nickel-base superalloys and intermetallics, as well as in a precipitation-strengthened Cu–Be alloy. Crack-growth rates lie in the range 10⁻⁷–10⁻⁴ m sec⁻¹. The same kind of cracking is found in steels treated so that free sulfur is able to segregate to the surface, as well as in Cu-Sn alloys, in which the embrittling element is surface-segregated Sn. The latter has been studied in bicrystals, and the importance of the variation in diffusivity with grain boundary structure has been documented. Hydrogen-induced cracking is a special case of an extremely mobile embrittling element and is responsible for much of the brittleness found in intermetallics. The effect of boron in retarding brittle behavior in Ni₃Al has been shown to result partly from its interaction with hydrogen. This is a prime example of how segregated solutes can be used to ameliorate the tendency for diffusion-controlled brittle fracture.     

Ab initio identified design principles of solid-solution strengthening in Al

Abstract

Solid-solution strengthening in six Al–X binary systems is investigated using first-principle methods. The volumetric mismatch parameter and the solubility enthalpy per solute were calculated. We derive three rules for designing solid-solution strengthened alloys: (i) the solubility enthalpy per solute is related to the volumetric mismatch by a power law; (ii) for each annealing temperature, there exists an optimal solute–volume mismatch to achieve maximum strength; and (iii) the strengthening potential of high volumetric mismatch solutes is severely limited by their low solubility. Our results thus show that the thermodynamic properties of the system (here Al–X alloys) set clear upper bounds to the achievable strengthening effects owing to the reduced solubility with increasing volume mismatch.     

Nucleation and growth mechanisms of phase with single-unit-cell height in Mg-RE-Zn(Ag) series alloys:a first-principles study

The highly thermal stability precipitates strengthen creep resistance for alloys,which require precipitates to resist coarsen at elevated temperature.In the Mg-RE-Zn (Ag) series alloys,the key strengthening phase-γ'phase,remains its single-unit-cell height throughout aging process.The origin of such extremely thermal stability of γ'phase is still unclear.By using the first-principles calculations,it is found that the formation ofγ'phase introduces compressive strain to its surrounding α-Mg lattice and simultaneously alter charge distribution of α-Mg lattice near the α-Mg/γ'hetero-interface.These two variations over α-Mg lattice lead to lower vacancy formation energies and migration energy barriers for solute atoms near the α-Mg/γ'hetero-interface in comparison with that of bulk condition.Consequently,the variations facilitate rapid diffusion of solute atoms near the γ'phase and promote nucleation rate of other γ'plates around it.On the basis of ledge-thickening model,the origin of nucleation/growth of γ'phase with single-unit-cell height was explained.Thermodynamically and kinetically,the solute atoms are not apt to migrate into the nearest basal plane adjacent to the γ'phase.Moreover,the lower aging temperature(～200 ℃) and almost completely coherent α-Mg/γ'hetero-interface lead to the ledges are extremely hard to nucleate on the hetero-interface.Therefore,γ'plates maintain single-unit-cell height throughout aging process.     

Theoretical and experimental studies of the effect of pressure on solute retention in liquid chromatography

Pressure is often assumed to have a negligible influence on solute retention in liquid chromatography because of the small compressibility of the mobile and stationary phases. The range of pressures commonly encountered in reversed-phase separations is considerable, however, and may give rise to significant changes in solute capacity factor. In this study, the retention of model solutes is measured directly along the chromatographic column as a function of the local pressure. The model solutes, a homologous series of derivatized fatty acids, exhibit a significant increase in capacity factor ranging from +9.3% for n-C(10) to +24.4% for n-C(20) for inlet pressures from 1500 to 5000 psi. These experimental results are compared with a thermodynamic model derived from regular solution theory. This model suggests that state effects alone are not sufficient to describe the measured change in solute retention and that variations in interaction energy with density must also be considered. By using the simple relationship of van der Waals for the interaction energy (E ∝ 1/V), the change in capacity factor with density is slightly underestimated. However, by using an extended relationship that better describes polar fluids (E ∝ 1/V(2)), good agreement is observed. Finally, the correlation of experimental results with this thermodynamic model reveals that all components in the chromatographic system, including the solute, mobile phase, and stationary phase, must be considered compressible. The results of this study have clear implications for the determination of fundamental physicochemical parameters, as well as for the everyday practice of liquid chromatography.     

On the background damping in the vicinity of the grain-boundary damping peak in zinc

The background damping in the vicinity of the grain-boundary damping decreases with increasing grain size. An analysis of the strain amplitude dependence of this damping shows that as the grain size increases the distance between solute pinning atoms on dislocation decreases. This can be explained in terms of a movement of solute away from grain boundaries and to dislocations. Thus as the grain size increases the total number of solute atoms at grain boundaries decreases and is rejected both into the lattice and to dislocations. A mathematical model is used to explain this result. As a consequence an activation energy of 0.05 eV is obtained for the binding energy of the solute to the dislocation.     

Participation of Cluster Species in the Solvation Mechanism of a Weak Polar Solute in a Methanol/Water Mixture over a 0.2-0.7 Water Fraction Range: High-Performance Liquid Chromatography Study

It is known that a methanol/water mixture has a quaternary organization consisting of free water W, free methanol M, and two types of methanol/water clusters. A 1:1 methanol/water cluster has been observed at a high methanol concentration, and a 5:1 cluster has been observed at a low methanol concentration. In the water fraction range used in this paper, 0.2-0.7, the number of MW(5) clusters was always inferior to the number of MW clusters. When a weak polar solute is introduced into such a mixture, it is preferentially solvated by free methanol and the MW water clusters. Using this mixture as the mobile phase, a novel mathematical theory is presented to describe, in this water fraction range, the variations of the retention factor k' of alkyl benzoate esters and benzodiazepines in reversed-phase liquid chromatography. Excellent predictions of k' versus free methanol and methanol/water fractions were obtained. For the first time, using this model, enthalpy, entropy, and the Gibbs free energy of the two solute solvation processes were evaluated. Enthalpy-entropy compensation revealed that the main parameters determining retention in the range of the water fraction (0.2-0.7) increased as follows: free methanol ? solute solvation > methanol/water cluster ? solute solvation > RP18 stationary phase ? solute interaction. These results agree well with values obtained when ACN was used instead of methanol.     

Annealing‐Induced Hardening in Ultrafine‐Grained Ni–Mo Alloys

The influence of Mo alloying on annealing‐induced hardening in ultrafine‐grained (UFG) Ni is studied. The hardening observed after low temperature annealing is explained by the annihilation of mobile dislocations and a concomitant clustering of the remaining dislocations into low energy configurations. This study reveals that, with increasing Mo concentration, the hardening effect decreases as the Mo solute atoms hinder the annihilation and rearrangement of dislocations. This trend is the opposite to that observed in electrodeposited Ni–Mo alloys where the larger alloying element concentration yields a higher annealing‐induced strengthening effect. The difference is attributed to the different deformation mechanisms in UFG and nanocrystalline Ni–Mo alloys.

     

Effects of manganese dispersoid on the mechanical properties in Al-Zn-Mg alloys

In order to investigate the strengthening effect of manganese dispersoid in manganeseadded Al-Zn-Mg alloys, specially designed alloys with various manganese content were prepared and evaluated. The manganese dispersoid in the alloy is found to increase the strength significantly without losing much elongation. The strengthening effect originates from the fact that the manganese dispersoid behaves as the non-shearable particle which is composed of Al, Mn and Zn. It can be concluded that the improvement of mechanical properties by the manganese dispersoid without losing much elongation is due to the strengthening effect produced by the pinning action of the dispersoid on dislocation glide and the enhancement effect in elongation caused by a homogenization of slip. Meanwhile, the manganese dispersoid and soluble manganese element in the peak-aged manganeseadded Al-Zn-Mg alloys are observed not to influence the width of the precipitate-free zones (PFZ) and the kinetics of precipitates such as and GP zone (solute rich cluster).     

Effect of silicon on recrystallisation of V–Ti microalloyed steel

Abstract

The dynamic recrystallisation (DRX) and static recrystallisation (SRX) behaviours of three V–Ti microalloyed steels were studied by the analysis of the true stress–strain curves and the stress relaxation curves under different deformation conditions. The results of DRX showed that deformation activation energy Q_def, peak stress and peak strain increased, as a result of the solute strengthening and dragging effect due to Si. The results of SRX showed that Si increased the SRX activation energy Q_SRX. The solute retardation parameter for static recrystallisation of Si was calculated. Based on the SRX results, to quantify the drag effect of Si and V, a new model was proposed to describe the time for 50% recrystallisation (t_0·5), which was tested and verified by previously published data on similar steels. Precipitation during recrystallisation could lead to a lower value of the Avrami exponent.     

Congeneric behavior in estimations of octanol-water partition coefficients by micellar electrokinetic chromatography

Linear Solvation Energy Relationships (LSERs) are used to explain the congeneric behavior observed when using Micellar Electrokinetic Chromatography (MEKC) to estimate the octanol-water partition coefficient scale of solute hydrophobicity. Such studies provide useful insights about the nature of solute interactions that are responsible for the sources of congeneric relationships between MEKC retention and log Po/w. It was determined that solute dipolarity/polarizability and hydrogen-bonding character play the most important roles in the congeneric behavior observed for many surfactant systems. The individual dipolarity/polarizability and hydrogen-bonding contributions to the free energy of transfer were also investigated.     

Calculation of solute-vacancy binding energy in dilute fcc and bcc alloys by diffusion

Due to excess charge of the solute with respect to solvent, the free energy of vacancy formation and migration in the neighbourhood of the solute will change. This results in a change in the solvent diffusivity. A relation for the solute vacancy binding energy for fcc and bcc lattices using enhancement factor has been derived considering the solute vacancy interactions to be limited to first neighbour and neglecting the changes in the solvent correlation factor.     

A predictive mechanism for dynamic strain ageing in aluminium-magnesium alloys

Dynamic strain ageing (DSA) is the phenomenon in which solute atoms diffuse around dislocations and retard dislocation motion, leading to negative strain-rate sensitivity (nSRS) and thus to material instabilities during processing, an important issue in commercial metal alloys. Here, we show the mechanism of DSA and nSRS on experimental strain-rate, temperature and stress scales for Al-Mg to be single-atomic-hop motion of solutes from the compression to the tension side of a dislocation core. We derive an analytic expression for the strengthening versus strain rate and temperature that justifies widely used phenomenological forms, provides specific dependences of the parameters on material properties and is supported by atomistic kinetic Monte Carlo simulations. Using literature material properties, the predicted strengthening quantitatively agrees with the experimentally derived behaviour of Al-2.5% Mg at 300 K, and qualitatively agrees with the strain rate and temperature ranges of DSA and nSRS in Al-Mg alloys. The analyses herein show a clear path for multiscale design, from quantum to continuum mechanics, of solute strengthening in face-centred-cubic metal alloys.     

Diffusive-convective transport into a porous membrane. A comparison of theory and experiment using scanning electrochemical microscopy operated in reverse imaging mode

Molecule transfer at the interface between a single ion-selective micropore and aqueous solutions is quantitatively investigated using scanning electrochemical microscopy operated in reverse imaging mode (SECM-RIM). Accumulation of two electroactive solute molecules, acetaminophen and ferrocenylmethyltrimethylammonium, at the pore/solution interface is observed when an electrical current is passed through the pore. Slow interfacial transfer of solute relative to the solvent as the solution is driven across the membrane by electroosmosis is responsible for solute accumulation. A theoretical expression for the concentration distribution of solute molecules above an individual pore opening is obtained by analytical solution of the convective-diffusive flux equation. The fluid velocity through the pore at constant electroosmotic force is determined by fitting the theoretical expression to SECM-RIM concentration profiles and is found, as anticipated, to be independent of the solute species and the bulk solute concentration. The results provide a theoretical basis for the SECM-RIM imaging of biological membranes as well as a general method for characterizing interfacial molecule/ion transfer kinetics.     

多级结构的金属材料强韧化机理研究进展

强度和塑韧性是金属结构材料主要的性能指标,然而通常会出现强度与塑韧性倒置的现象,即传统的固溶强化、纳米晶强化、弥散强化和加工硬化在追求强度的同时会不可避免地牺牲金属材料的塑韧性.根据多级多尺度仿生结构可协同提高强度和韧性的思路,系统介绍了两级Ti-TiBw/Ti复合材料、不锈钢复合板、多层复合钢、层/网耦合结构钢和超细纤维晶钢的构型设计,并揭示其强韧化机理和断裂机制,通过改变裂纹的扩展方式与裂纹的竞争机制,以及残余内应力的释放等途径,有效实现材料的强韧化,可为金属材料强韧化提供新的设计思路和技术支撑.     

Sb合金化对AE41镁合金耐热性能的影响

采用X射线、光学显微镜、电子探针、扫描电镜等手段研究了Sb合金化对AE41（Mg-4Al-1RE）镁合金组织和耐热性能的影响．结果表明,Sb取代Al优先与RE形成以RE2Sb相为主的高熔点弥散颗粒质点,而枝条状Al11RE3相数量和尺寸减小．Al11RE3相对基体的割裂作用的减弱,RE～Sb质点的弥散强化作用、以及Sb、RE等元素的固溶强化作用,使合金的常温和高温力学性能尤其是塑韧性显著提高,并且有效地改善了高温抗蠕变能力．过量的Sb反而降低了合金的力学性能和耐热性能．合金的断裂为具有塑性特征的准解理断裂．     

First-principles study of 4d solute diffusion in nickel

Diffusion of the 4d transition elements in Ni has been investigated within the five-frequency model framework using migration energy barriers calculated from the first principles. Agreement with counterintuitive experimental/calculated data is observed; atoms in the middle of 4d row have the smallest atomic radii while exhibiting the lowest diffusivity as compared to larger atoms at the beginning and the end of 4d row. We show that 4d solute diffusion is controlled mainly by the size misfit. The larger atoms have higher solute–vacancy binding energies and lower migration barriers. Both were shown to correlate with a displacement of the equilibrium solute position toward the adjacent vacancy. The difference in mechanisms controlling sp- and transition elements diffusion rates in Ni is discussed.