Anisotropy of P Grain Boundary Segregation in a Rapidly Solidified Fe-0.6wt%P Alloy

P grain boundary segregation in an Fe-0.6wt%P alloy quenched from the melt was quantified by X-ray Mapping (XRM) in a Scanning Transmission Electron Microscope (STEM). The misorientation across the boundaries was determined by using ACT (Automatic Crystallography for TEM (Transmission Electron Microscopy)) and CBED (Convergent Beam Electron Diffraction). A significant range of the degree of P segregation to individual grain boundaries was found. Combination of chemical and structural studies provides evidence that P segregation to low-angle grain boundaries is reduced.     

Threshold stress for superplasticity in solid solution magnesium alloys

The effect of alloying elements on the threshold stress for superplasticity was investigated using two binary solid solutions, namely, Mg–Al and Mg–Y alloys. Both alloys exhibited superplasticity, and in spite of the absence of fine particles showed threshold-stress-like behavior. Different origins were suggested for the threshold-stress-like behavior after considering grain growth during deformation. The threshold-stress-like behavior in Mg–Al alloys originates from the effects of microstructural instability (grain-growth hardening). On the other hand, analysis of grain-boundary segregation suggested that the threshold-stress-like behavior in Mg–Y alloy originates from the segregation of yttrium in grain boundaries and its interaction with grain-boundary dislocations.     

Grain boundary compositions in stoichiometric and off-stoichiometric Y1Ba2Cu3O7−x

The grain boundary segregation behavior of stoichiometric and off-stoichiometric YBa₂Cu₃O_7−x has been studied with analytical electron microscopy (AEM) to complement previous results obtained with Auger electron spectroscopy (AES). It was observed that the grain boundary segregation levels varied from boundary to boundary. In stoichiometry material, excess copper and deficient oxygen were observed at the grain boundaries. In materials containing excess barium, excess yttrium and deficient copper levels were observed at the grain boundaries. The grain boundary segregation levels in the materials containing excess barium can be related to the resistivity and critical current density results. Preferential segregation to specific sites in grain boundaries was observed by AEM. The effect of grain boundary segregation on the superconducting properties of these materials is discussed in terms of the weak link behavior and a possible percolative mechanism in order to maintain a continuous current path.     

Structure,energy and solute segregation behaviour of [110] symmetric tilt grain boundaries in yttria-stabilized cubic zirconia

Yttria-stabilized cubic zirconia bicrystals with [110] symmetric tilt grain boundaries are systematically fabricated by the diffusion bonding method. It is revealed that the grain-boundary atomistic structures, excess energies and solute segregation behaviours are strongly dependent on the macroscopic geometries of the boundaries. High-resolution transmission electron microscopy combined with lattice statics calculations suggests that the grain-boundary structures are characterized by the accumulation of coordination-deficient cation sites at their cores, whose densities have a clear correlation with excess energies and amounts of solute segregation. The orientation dependence of grain-boundary properties in cubic zirconia can thus be linked and understood via local grain-boundary atomistic structures with the characteristic miscoordinated cation sites.     

Solute segregation to grain boundaries and free surfaces in an Fe---Si multicomponent alloy

Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si_xN_y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmann's theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmann's theory.     

Connectivity and percolation in simulated grain-boundary networks

Random percolation theory is a common basis for modelling intergranular phenomena such as cracking, corrosion or diffusion. However, crystallographic constraints in real microstructures dictate that grain boundaries are not assembled at random. In this work a Monte Carlo method is used to construct physically realistic networks composed of high-angle grain boundaries that are susceptible to intergranular attack, as well as twin-variant boundaries that are damage resistant. When crystallographic constraints are enforced, the simulated networks exhibit triple-junction distributions that agree with experiment and reveal the non-random nature of grain-boundary connectivity. The percolation threshold has been determined for several constrained boundary networks and is substantially different from the classical result of percolation theory; compared with a randomly assembled network, about 50-75% more resistant boundaries are required to break up the network of susceptible boundaries. Triple-junction distributions are also shown to capture many details of the correlated percolation problem and to provide a simple means of ranking microstructures.     

Twin boundaries in antimony(V)-doped rutile titanium(IV) oxide

Titanium-antimony oxides formed by the calcination of precipitates at elevated temperatures contain low concentrations of antimony(V) within the rutile titanium(IV) oxide lattice. Electron microscopy has revealed the presence of planar defects which have been shown by electron diffraction to be twin boundaries. Energy dispersive X-ray analysis has not revealed any evidence for the segregation of antimony to these planar faults. No evidence has been found for the occurrence of crystallographic shear planes which are frequently found in rutile when doped with trivalent cations.     

The relationship between grain-boundary structure and segregation in a rapidly solidified Fe-P alloy

It is well known that the amount of solute segregation can vary from one grain boundary to another. Though it is accepted that this variation is due to differences in boundary structure and crystallography, direct correlation between the degree of segregation and specific boundary structural characteristics is not well documented. In the present paper, P grain-boundary segregation in rapidly solidified Fe was studied by X-ray mapping (XRM) in a scanning transmission electron microscope (STEM). The boundary structure was characterized by convergent beam electron diffraction (CBED). To explore the relationship between the degree of segregation and boundary structure, a parameter?β?is introduced, which describes how well the two crystal planes on either side of a grain boundary match each other in a manner similar to the long-established plane matching theory. The introduction of this parameter enables us to relate the degree of segregation to boundary structure in a consistent way, e.g., both small-angle and low?Σ?symmetric boundaries correspond to low angle of β, leads to a low degree of segregation.     

Temperature and orientation dependences of the grain-boundary diffusion coefficient of antimony in copper bicrystals

An investigation is made of the diffusion of antimony through the bulk and along grain boundaries in copper bicrystals containing a symmetric 〈100〉 misorientation boundary with misorientation angles from 20 to 37.2°. The bicrystals are grown by the method of horizontal zone recrystallization. The temperature range for these studies is 480–580 °C, where the solubility of Sb in Cu is about 6 atomic % and practically temperature-independent. The concentration profiles are obtained by x-ray spectral microanalysis, and the grain-boundary diffusion parameters are computed by the method of Whipple and Suzuoka. The orientation dependence of the triple product P=sδD _b (where s is the segregation coefficient, δ the width of the grain boundary, and D _b the grain-boundary diffusion coefficient) is nonmonotonic, with a maximum for the special ∑5 misorientation boundary (36.9°). The effective activation energy for grain-boundary diffusion ranges from ∼70 kJ/mol for ∑5 to140 kJ/mol for general boundaries. Fiz. Tverd. Tela (St. Petersburg) 39, 1153–1157 (July 1997)     

Hydrogen sorption in titanium alloys with a symmetric Σ5(310) tilt grain boundary and a (310) surface

The hydrogen sorption in intermetallic B2 TiM (M = Ni, Co, Pd) with a symmetric ??5(310) tilt grain boundary and a (310) surface is studied by density functional theory methods. The effect of hydrogen on the electronic characteristics of the alloys is analyzed as a function of a sorption position at the interfaces. The hydrogen sorption energy is shown to depend on the local environment of hydrogen; on the whole, hydrogen at the interfaces prefers titanium-rich positions. The hydrogen sorption energy in metal-rich positions decreases when the d shell of the second alloy component is filled with electrons. The grain-boundary energy, the surface energy, and the hydrogen segregation energies to the interfaces are calculated. Hydrogen sorption in titanium alloys is shown to decrease Griffith work and to favor brittle fracture along tilt grain boundaries.     

Effect of thermomechanical processing on the creep behaviour of Udimet alloy 188

Udimet alloy 188 was subjected to grain-boundary engineering involving thermomechanical processing in an attempt to improve the creep performance and determine the effects on creep deformation processes. The as-received sheet was cold-rolled to either 10, 25 or 35% reduction per pass followed by a solution treatment at 1191°C for 1 h plus air cooling. This sequence was repeated four times and the resultant microstructure and grain-boundary character distribution were described using electron backscatter diffraction. The fraction of general high-angle grain boundaries tended to increase with increased cold rolling. The 10 and 25% cold-rolled materials exhibited lower creep rates than the 35% cold-rolled material. The measured creep stress exponents and activation energies suggested that dislocation creep with lattice self-diffusion was dominant at 760°C for stresses ranging between 100 and 220 MPa. A transition in the creep exponent below the applied stresses of 100 MPa indicated that a different secondary creep mechanism was rate-controlling at low stresses. A significant amount of grain-boundary cracking was observed both on the surface and subsurface of deformed samples, but surface cracks were greater in number and size than those within the bulk. The cracking behaviour was similar in both vacuum and air environments, indicating that grain-boundary cracking was not caused by environment. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during elevated-temperature (T ≤ 760°C) tensile-creep deformation. Sequential secondary electron imaging and electron backscatter diffraction orientation mapping were performed in situ to allow the evolution of crack nucleation and linkage to be followed. Cracking occurred preferentially along general high-angle grain boundaries and less than 15% of the cracks were found on low-angle grain boundaries and coincident site lattice boundaries. A fracture initiation parameter analysis was performed to identify the role of slip system interactions at the boundaries and their impact on crack nucleation. The parameter was successful in separating the population of intact and cracked general high-angle boundaries at lower levels of strain, but not after crack coalescence dominated the fracture process. The findings of this work have significant implications regarding grain-boundary engineering of this alloy and potentially for other alloy systems.     

AES/STEM grain boundary analysis of stabilized zirconia ceramics

Semiquantitative Auger Electron Spectroscopy (AES) on pure monophasic (ZrO₂)_0.83(YO_1.5)_0.17 was used to determine the chemical composition of the grain boundaries. Grain boundary enrichment with Y was observed with an enrichment factor of about 1.5. The difference in activation energy of the ionic conductivity of the grain boundary compared with the bulk can be explained by the Y segregation.When Bi₂O₃ is introduced into this material and second phase appears along the grain boundaries of the cubic main phase. Energy dispersive X-ray analysis (EDS) on a scanning transmission electron microscope (STEM) shows an enrichment of bismuth at the grain boundaries of this second phase.     

Effect of rare earth on the microstructures and properties of a low expansion superalloy

A new Fe-Ni-Co-Nb-Ti-Si superalloy containing trace additions of selective rare earths and having good combination of very low thermal expansion coefficient, high-resistance to stress accelerated grain boundary oxygen embrittlement and fairly good notch-bar rupture strength has been successfully developed. The resistance to oxidation for long time exposure at high-temperatures and the stress rupture life has been improved significantly with trace yttrium addition. The microstructures of the alloys have been studied by means of analytical electron microscopy, chemical and X-ray analysis techniques. The results reveal that the trace yttrium segregates in the strengthening phase with platelet morphology, and helps in transforming A(3)B type epsilon phase into A(5)B type H. The morphology and crystal structures of the grain boundary phases also change with selective additions of rare earth elements. Compared with those in the conventional alloy, the platelet precipitates in the yttrium-containing alloy densely segregate within the grains and along the grain boundaries with smaller size. The segregation of the platelet precipitates within the grains is helpful in improving the strength of the alloy. In addition, its precipitation along the grain boundaries can improve the resistance to stress accelerated grain boundary oxidation and stress rupture property of the alloy and thereby contribute to its temperature stability.     

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni₃Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.Tomsk Engineering-Construction Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1992.     

Formation of low-temperature deformation-induced segregations of nickel in Fe–Ni-based austenitic alloys

ABSTRACT

The authors present the results of an investigation in Fe–Ni-Cr austenitic alloys of the low-temperature deformation-induced segregations of nickel that form in the micro regions being (i) located close to grain- and subgrain boundaries and (ii) characteristic of the concentration and magnetic inhomogeneities indicated by the appearance of a dark diffraction contrast at the electron diffraction patterns taken from these regions typical (at the same time) of an enhanced value of Curie temperature. The observed effects were connected with the micro distortions caused by the local change of lattice parameter because of an increase in nickel concentration, as well as in the result of a magnetostriction dilatation. Using methods of the X-ray energy dispersive spectroscopy (XEDS) and atomic-probe body-section radiography (tomography – APT) has made it possible to determine the borders of those regions of austenite that were characteristic of an enhanced concentration of nickel in the fields of the localisation of a deformation-induced segregation of nickel in the vicinity of grain (subgrain) boundaries of austenitic alloys of the types Fe–13Cr–30Ni and Fe–37Ni–3Ti.     

Grain boundary ridge formation during initial high temperature oxidation of Mn/Al TRIP steel

Confocal scanning laser microscopy (CSLM) was used in real-time observation of alloy element oxidation of a Mn/Al TRIP steel in an Ar–O₂ atmosphere. CSLM images reveal a marked role of grain boundaries in the overall initial oxidation kinetics of the alloy, and consequently in the morphology of the initial surface oxide. The oxidation on the alloy surface is dominated by the formation of Mn-rich oxide ridges along grain boundary traces on the surface. Oxide ridge formation kinetics was quantified by measurements on images extracted from real-time recordings of surface oxide evolution. Oxide ridge growth was found to take place at a constant rate. Scanning electron microscopy (SEM) images of the oxidized surfaces showed homogenous oxide ridges along straight grain boundary traces and heterogeneous oxide ridges along non-straight grain boundary traces. A transport mechanism of Mn to the surface is proposed, which relies on grain boundary segregation of Mn and on a relationship between grain boundary diffusivity and grain boundary character. It is suggested that when regarding alloys with significant grain boundary segregation of a solute, separate Wagner balances for internal vs. external oxidation is required for the grain lattices and the grain boundaries, respectively.     

Influence of nitrogen on the mechanism of austenitic stainless steel hardening

On the example of a C18N12M2 austenitic stainless steel, the influence of nitrogen (whose content varied from 0 to 0.45 wt.%) on the grain boundary hardening coefficient k _h entering into the Hall-Patch equation is analyzed. High values of k _h in steels with and without nitrogen are found. The data of the Auger analysis show that the hardening coefficient in the steel without nitrogen is determined by the grain-boundary segregation of carbon and oxygen. The grain-boundary hardening in the steel with nitrogen is not connected with the predominant segregation of nitrogen at grain boundaries. It is completely governed by intragranular processes—interaction of nitrogen atoms with dislocations. Omsk State Pedagogical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 47–52, July, 1999.     

Alloying Effect on the Creep Mechanism and Creep Resistance of Polycrystals in the Concepts of Physical Mesomechanics

The effect of different slightly soluble alloy additions on the creep and individual creep-component behavior of lead at T = 0.5 T _cr under controlled conditions (high-purity polycrystals, the same grain size) is investigated on the basis of the concepts of physical mesomechanics. The additions used are shown to reduce the creep rate under these conditions. The effect of the slightly soluble alloy additions to the polycrystal on the steady-state creep rate is produced through grain-boundary sliding and localized-deformation banding near grain boundaries.     

On the Relationship Between Entropy and Enthalpy of Grain Boundary Segregation

An extensive study of anisotropy of grain boundary segregation in -iron, performed in the last decade, confirmed the existence of a linear relationship between entropy and enthalpy of solute segregation at individual grain boundaries. A thermodynamic analysis of this relationship is performed in the present work. It is shown that this dependence defines a compensation temperature and an entropy-like parameter. The compensation temperature is not necessarily connected with a phase transformation, but it represents exclusively a mean temperature at which all boundaries approach to a mean value of the Gibbs free energy of segregation. The most important consequences of this dependence are outlined: a reversed anisotropy of grain boundary segregation at temperatures above the compensation temperature, and a new method for the prediction of the enthalpy and entropy of solute segregation at individual grain boundaries.