Pressureless sintering of SiAlON ceramics

Pressureless sintering of β-SiAlON ceramics has been carried out successfully without the use of additives. Grinding the raw material powders (AlN, Si₃N₄ and Al₂O₃) for long periods was shown to be an important factor in obtaining ceramics of high quality. The problem of volatilisation of sub-oxides such as “SiO” has been solved by carrying out the sintering in a special closed crucible. The sintering time is approximately 2 hours at temperatures between 1740 and 1780°C. In this way a material having a density corresponding to 99 % of the theoretical density was obtained. Bend strengths of 420 MN/m² have been measured on the best SiAlON obtained. The ease of sintering depends on the composition of the particular SiAlON being apparently more difficult as the mixture becomes richer in Si₃N₄.     

Vacancies in selected special tilt grain boundaries in Ni3Al

The structure of selected special (Σ=5) symmetrical tilt grain boundaries in Ni₃Al with tilt axis [1 0 0] was simulated by the Monte Carlo method. Vacancies were generated both in Al and Ni sites in their relaxed structure and the vacancy formation and migration energy was estimated.     

Structure and chemistry of grain boundaries in SiO2-doped TZP

The addition of glass phase can control the grain boundary structure and hence the mechanical properties of tetragonal zirconia polycrystals (TZP). To reveal the effect of the glass dopant on the high-temperature deformation behavior of TZP, SiO₂-doped TZP, (SiO₂–Al₂O₃)-doped TZP, (SiO₂–MgO)-doped TZP and undoped TZP were prepared and their grain boundary structure, chemical composition and chemical bonding state were investigated by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) using a field-emission-type transmission electron microscope (FE-TEM). It was found that no amorphous film was formed along the grain boundaries in any of the specimens examined, but amorphous pockets formed at multiple grain boundary junctions in three kinds of glass-doped specimens. In the glass-doped specimens, the segregation of yttrium, silicon and the added metal ions (Al³⁺ or Mg²⁺) was observed over a width of several nm across the grain boundaries. The addition of pure SiO₂ much enhanced the ductility in TZP, although further addition of a small amount of Al₂O₃ or MgO to SiO₂ phase resulted in a marked reduction in the tensile ductility of SiO₂-doped TZP. EELS measurements and molecular orbital (MO) calculations using a cluster model revealed that the ductility of TZP was related to the bond overlap population (BOP) at the grain boundaries, which was influenced by the kinds of segregated dopants. That is, the presence of Si⁴⁺ increases the BOP, strengthening the grain boundary bonding strength and thus preventing cavity formation, but Al³⁺ and Mg²⁺ decrease the BOP, enhancing the grain boundary cavitation and thus reducing the ductility. Furthermore, the dynamic behavior of SiO₂ in TZP was observed using a TEM in situ heating technique, and the results supported the fact that that Si segregates along the grain boundaries.     

Structure of grains and grain boundaries in cryo-mechanically processed Ti alloy

A Ti5Ta1.8Nb alloy with the major phase as α (hcp) Ti has been subjected to severe plastic deformation by means of cryo-rolling. Significant grain refinement (from ~5 μm to ~60 nm) has been observed. The mechanism of grain refinement was studied by analysis of lattice strain variations with increase in cold work using XRD technique. Various intermediate stages, such as hardening, alignment of dislocations, cell formation and criticality before new grain formation, were identified. Formation of cells with dislocations alignment at the boundaries and then finally forming an ultra-fine grain structure was confirmed by transmission electron microscopy studies. Detailed grain boundary characterisation has been carried out using high-resolution transmission electron microscopy studies and crystallographic texture analysis. The grain-refined structure was found to possess a large fraction of high angle boundaries identified also as special boundaries by evaluating the misorientation angle/axis sets for a pair of adjacent grain boundaries.     

Structure and chemistry of grain boundaries in SiO2-doped TZP

The addition of glass phase can control the grain boundary structure and hence the mechanical properties of tetragonal zirconia polycrystals (TZP). To reveal the effect of the glass dopant on the high-temperature deformation behavior of TZP, SiO₂-doped TZP, (SiO₂—Al₂O₃)-doped TZP, (SiO₂—MgO)-doped TZP and undoped TZP were prepared and their grain boundary structure, chemical composition and chemical bonding state were investigated by high resolution electron microscopy ,HREM), energy dispersive X-ray spectroscopy ,EDS) and electron energy loss spectroscopy (EELS) using a field-emission-type transmission electron microscope (FE-TEM). It was found that no amorphous film was formed along the grain boundaries in any of the specimens examined, but amorphous pockets formed at multiple grain boundary junctions in three kinds of glass-doped specimens. In the glass-doped specimens, the segregation of yttrium, silicon and the added metal ions (Al³¹ or Mg²¹) was observed over a width of several nm across the grain boundaries. The addition of pure SiO₂ much enhanced the ductility in TZP, although further addition of a small amount of Al₂O₃ or MgO to SiO₂ phase resulted in a marked reduction in the tensile ductility of SiO₂-doped TZP. EELS measurements and molecular orbital (MO) calculations using a cluster model revealed that the ductility of TZP was related to the bond overlap population (BOP) at the grain boundaries, which was influenced by the kinds of segregated dopants. That is, the presence of Si⁴¹ increases the BOP, strengthening the grain boundary bonding strength and thus preventing cavity formation, but Al³¹ and Mg²¹ decrease the BOP, enhancing the grain boundary cavitation and thus reducing the ductility. Furthermore, the dynamic behavior of SiO₂ in TZP was observed using a TEM in situ heating technique, and the results supported the fact that that Si segregates along the grain boundaries.     

热压BN掺杂SiAlON陶瓷的致密化过程研究

以Si3N4粉、AlN粉、Y2O3粉以及BN粉末为原料，采用热压烧结（HP）方法在N2气氛、1900℃条件下制备BN掺杂α-SiAlON陶瓷，研究BN掺杂SiAlON陶瓷的致密化过程。BN粉末经造粒形成粒径约为0．5mm的球形颗粒，其掺量分别为SiAlON原始粉末质量的5％和10％。研究结果表明：BN的掺量对BN／SiAlON陶瓷的收缩趋势和致密化过程的温度范围无影响，BN掺量不同的两条相对收缩量曲线吻合得非常好，收缩均始于1650℃左右，结束于1705℃；在烧结温度相同的情况下，较高的BN掺量有助于提高BN／SiAlON陶瓷烧结过程的收缩速率和样品的总收缩量，在促进致密化进程的同时能够提高陶瓷的致密度。     

Electromigration of grain boundaries in aluminum

The electromigration of grain boundaries has been investigated with aluminum wires (99.999% purity, 1 mm in diameter) in the temperature range 340°–540°C under a current stress of 6×10³ A cm^-2. Grain boundary migration in aluminum is found to be reduced or enhanced by the currents stress when the grain boundary migrates in or against the current direction, respectively. The effects of additions of copper, silicon and zirconium to aluminum on the electromigration of the grain boundaries have also been examined. The contribution of electromigration to normal grain boundary migration is found to be increased by the addition of solutes. The results are discussed on the basis of the impurity drag mechanism of grain boundary migration.     

Strengths of grain boundaries in pure metals

Abstract

The relative surface energies for brittle fracture along grain boundaries or along crystal planes, at low temperatures, are estimated and used in a criterion for the relative strengths of boundaries and cleavages. It is concluded that the boundary is weaker than the crystal in a wide range of metals; that it becomes weaker as the ratio of shear modulus to bulk modulus increases; and that brittle pure metals, such as iridium and molybdenum, fracture preferentially on grain boundaries. The critical modulus ratio is in all cases lower than that for the ductile–brittle cleavage transition.

MST/1154     

Layering transitions at grain boundaries

We review various simplified models that have been advanced to describe layering (complexion) transitions at grain boundaries in multicomponent solids. In particular, we first outline lattice-gas, off-lattice atomistic and thermodynamic models that have been employed to investigate phase-like behavior at segregated grain boundaries. The results of these investigations are summarized in the form of complexion diagrams in different thermodynamic planes, and we highlight important features of these diagrams, such as complexion transition lines and critical points. Finally, we describe current issues and provide a future outlook.     

Dislocation interactions with grain boundaries

Recent progress in understanding dislocation interactions with grain boundaries and interfaces in metallic systems via static and in situ dynamic experimental approaches is reviewed.     

Bismuth-induced embrittlement of copper grain boundaries

Catastrophic brittle fracture of crystalline materials is one of the best documented but most poorly understood fundamental phenomena in materials science. Embrittlement of copper by bismuth is a classic example of this phenomenon. Because brittle fracture in any structural material can involve human tragedy, a better understanding of the mechanisms behind it is of the highest interest. In this study, we use a combination of two state-of-the-art atomic characterization techniques and ab initio theoretical materials simulations to investigate the geometric and electronic structure of a copper grain boundary with and without bismuth. Only with this unique combination of methods are we able to observe the actual distribution of bismuth in the boundary and detect changes in the electronic structure caused by the bismuth impurity. We find that the copper atoms that surround the segregated bismuth in the grain boundary become embrittled by taking on a more zinc-like electronic structure.     

Computing the mobility of grain boundaries

As current experimental and simulation methods cannot determine the mobility of flat boundaries across the large misorientation phase space, we have developed a computational method for imposing an artificial driving force on boundaries. In a molecular dynamics simulation, this allows us to go beyond the inherent timescale restrictions of the technique and induce non-negligible motion in flat boundaries of arbitrary misorientation. For different series of symmetric boundaries, we find both expected and unexpected results. In general, mobility increases as the grain boundary plane deviates from (111), but high-coincidence and low-angle boundaries represent special cases. These results agree with and enrich experimental observations.     

Electrodiffusion damage in sodium chloride grain boundaries

The continuous passage of an electric current through pure sodium chloride bicrystals has been found to produce grain-boundary damage similar to that caused by electromigration in conducting thin films. The damage takes the form of an array of voids that seriously reduces the strength of the boundary. As in electromigration, the voids appear to be due to the condensation of vacancies following removal of ions by the current. However, the voids produced in tilt boundaries by current at 250 C have regularities of distribution and shape that suggest they are associated with separation of a second phase from segregated boundary impurity, the phase change disorder resulting from the precipitation reaction leading to rapid production of damage. The phase change process is discussed.     

Further refinements in the energy of grain boundaries

Earlier surface dislocation analysis of a grain boundary recognized the tendency of the grain-boundary surfaces to coalesce in order to reduce surface energy. The coalescence process is described by a distribution of surface dislocations on the grain-boundary surfaces. In the present paper, previous analysis is further refined. In particular, the sum of the Burgers vectors of the surface array of grain-boundary dislocations is not equal to the Burgers vector of the grain-boundary lattice dislocation. Instead, the Burgers vector of the surface array is determined as a function of the coalescence of the grain-boundary surfaces. The conservation of Burgers vectors of dislocations is used to predict the presence of a screening array of dislocations. The screening array of dislocations is determined by minimization of the total energy of the configuration. The distortion around the boundary is relaxed by the screening array. In general, the distribution of the screening array is two dimensional. This result has been proved by the presence of a minimum energy configuration for two sets of screening arrays of dislocations situated at different distances from the boundary.     

Properties of tilt grain boundaries in ordered alloys

Computer simulation of symmetric tilt grain boundaries (GB) Σ = 5 [100](012) in ordered alloys Ni₃Al and NiAl was carried out. The energy of GB ws calculated by a method construction of γ-surface construction using Morse's empirical centralforce potentials. Results show GB having several steady states: (i) one is stable; or (ii) metastable. These states differ by energy and atomic structure of GB. Transition of GB from one state to the other is investigated and Burgers vector of GB dislocations are determined. It is shown that a direction fo GB slip in both alloys is [100].