A computer calorimetry technique employing molecular dynamics simulation has been used to calculate formation free energies for a copper impurity atom in a perfect nickel crystal in which the atoms are interacting through embedded-atom potentials. The method has also been used to calculate segregation free energies for a copper impurity at different site of a Σ=5 (031)[100] symmetical tilt boundary in nickel. Comparison of energetic and entropic contributions to segregation free energies indicate that knowledge of the entropic contribution is essential for prediciton of impurity site selection at grain boundaries above room temperatures.

The elastic distortions of two adjacent crystals separated by a phase boundary are in general of great complexity. Three types of phase boundaries are considered, depending on their apparent ability to maintain continuity of 1, 2 or 3 plane orientations. The elastic fields caused by particular interfacial defects are discussed with special attention paid to recent high resolution electron microscope observations on pairs of dissimilar crystals, like Al/AI2Cu, Ni/Ni3Nb, Si/TiSi2.

The behaviour of a twin boundary under different strains at different temperatures Is observed. The evolution of the structure is studied and the non direct transmission of the deformation induced dislocations is considered in the light of the movement of the grain-boundary dislocations.

The quantitative TEM characterization of lattice dislocation pile-ups at grain boundaries in polycrystalline materials can provide detailed information about the crystallography and mechanisms of slip transmission across a particular interface. This is used as input data in anisotropic elastic calculations of the local stress field and forces on both sides of the interface, and gives pile-up obstacle stresses varying from 280 to 870 MPa for different boundaries in the same type 304 stainless steel sample. These calculations have also led to an improved criterion for predicting slip systems activated across an interface; a double-ended pile-up interacting with two different boundaries is used as an illustration. In addition, dynamic in situ straining experiments in the HVEM reveal mechanisms for dislocation generation not predicted from static TEM studies. Finite element modeling (FEM) is also being used to predict the complete stress and strain state of specially oriented bicrystals.

The role of slip/grain boundary interaction in intergranular fracture has been analyzed for a Σ = 9 tilt boundary in L12 ordered alloys by use of the anisotropic elasticity theory of dislocations and fracture. Screw superpartials cross slip easily at the boundary onto the (111) and the (001) planes for low and high temperatures, respectively. Transmission of primary slip dislocations onto the conjugate slip system occurs with some difficulty, which is eased by localized disordering. Unless a symmetric double pile-up occurs simultaneously, cleavage fracture is predicted to occur on the (111) plane, not intergranular fracture. Absorption (or emission) of superpartials occurs only when the boundary region is disordered. The inherent weakness of grain boundaries in Ni3AI and its improvement by boron segregation are discussed.

The effect of 0.2 at.% boron on grain boundary hardness in Ni3Al containing 24 to 26 at.% Al was studied. Normal grain boundary hardening was observed in all alloys with and without boron. However, the addition of boron decreases the magnitude of the grain boundary hardness, and the degree of “softening” depends on alloy stoichiometry. The occurrence of boron-induced softening is likely due to the increase in mobility of grain boundary dislocations.

At elevated temperatures, crystal dislocations impinging onto the grain boundary become trapped and are confined to move in the interface. They form pile-ups at places where their movements are impeded in the interface the strength of which is determined by the rate of slip impingement and that of recovery in the interface. During steady state creep, a ‘steady state’ pile-up of grain boundary dislocations (GBDs) will form in the interface, providing a condition conducive to the formation of creep cavities. Calculations have shown that cavity nucleation ahead of a GBD pile-up during steady state creep is feasible under usual creep conditions in single-phase metals and alloys. It has also shown that solutes and impurities can exert a profound influence on the cavitation behavior of materials. For a solute/impurity which is surface active, which retards boundary diffusion and reduces the creep resistance of the material, it could reduce the threshold stress for cavitation failure by about an order of magnitude, thereby rendering the material highly susceptible to cavitation damage.

Nimonic 80A and Alloy 800H have been deformed at room temperature in tension and in compression and subsequently heat treated stress-free and under stress. The cavity densities which were investigated by scanning electron microscopy exhibit a different dependence on the plastic strain. If the heat treatment was performed under stress new and bigger cavities developed on unexpected grain boundaries in Nimonic 80A. The mechanisms are discussed in terms of the model of Kikuchi, Shiozawa and Weertman.

Recent studies of long life fatigue have shown fractures to initiate primarily at grain boundaries, even in copper. This is considered surprising in light of early work where crack nucleation in persistent slip bands (PSB´s) seemed to be the rule. To investigate the causes of this behavior, studies have been carried out on polycrystalline copper with emphasis on the following factors: grain size, environment, method of starting the test, frequency and mode of test control.

The results show that the cyclic plasticity is the controlling factor in grain boundary initiation and propagation of fatigue cracks. These conducted in load or strain control, even at low amplitudes, homogenized the deformation and cause grain boundary failure. Tests started by ramp loading emphasize localized strain, PSB formation and thus, transgranular cracking. An aggressive environment stimulates intergranular failure but is not controlling. Likewise a small grain size tends to promote strain localization.

The computer simulation of the structure and fracture of interfaces on an atomic scale requires a computationally efficient prescription for the total energy that is reliable both for small deviations from the bulk as well as for the free surfaces produced during fracture. The recently developed Embedded Atom Method is such a method. It will be briefly described and compared to traditional pair interaction approaches. In particular, it will be shown that the many-body effects inherent in the Embedded Atom Method are essential to correctly describe the experimentally observed surface reconstructions of Au surfaces.

The necessary first step in simulating the fracture of an interface, such as a grain boundary, is the determination of the initial or equilibrium atomic configuration of the interface. Equilibrium Monte Carlo simulations using the Embedded Atom Method can determine this structure. This approach will be outlined and various results for grain boundary structure in fcc metals will be presented. The atomic structure of symmetric tilt boundaries is found to be significantly different from that deduced from energy minimization techniques. In addition, the Monte Carlo technique allows for the determination of thermal effects such as the vibrational amplitudes at the interface and the thermal expansion of the interface.

Atomistic simulations of [001] symmetric tilt grain boundaries in Ni, Al, and Ni3Al are presented. The atomistic structures of the simulated grain boundaries have been analyzed in terms of the structural unit model, which is found to be of limited utility for intermetallics. Simulation results show that boron segregates more strongly to grain boundaries than to free surfaces, and strengthens the grain boundary. Good cohesive properties of the grain boundaries occur when both boron and some segregated Ni are present. The Ni and B are found to co-segregate to the Ni3AI boundary with an energy advantage of ∼ 0.5eV.

We analyze the stressing of a crack along a grain interface on which there have segregated solute atoms which retain a residual misfit volume relative to atoms of the host solid. Results suggest that the critical stress intensity factor necessary to emit a dislocation from the crack tip is increased in proportion to the misfit volume, so that oversized solutes tend to pin crack tip dislocations. This is a complementary embrittling mechanism to the often discussed effects of segregants on reducing the reversible work of interfacial separation. It is consistent with empirical correlations of the embrittling potency of solutes with their atomic size differences relative to the host, e.g., as for Sb, Sn, and S in Fe.

The ductile versus brittle response of Cu bicrystal interfaces with different misorientations and Bi segregation levels has been investigated. The fracture behavior of the bicrystals and the morphology of the fracture surfaces depends on the type of the interface and the amount of impurity segregation. The different fracture behaviors of the bicrystals can to some extent be understood by considering the difference in grain boundary structure and comparing the theoretically predicted values of crack tip energy release rates for dislocation emission from the crack tip versus for cleavage decohesion of the grain boundary. However, the effects of Bi segregation in inducing embrittlement seem greater than can currently be rationalized by such a concept and, also, there is evidence that fracture surfaces may not always coincide, microscopically, with the interface.

The embedded atom model was utilized to study the atomic displacements and the energy of (111) interfaces in Ni-Au and Ni-Pt bicrystals. A sphere of 459 atoms was utilized with the Ni (111) surface as a great circle through the crystal. The Au or Pt atoms were initially placed in perfect registry with the Ni atoms and then the crystal was allowed to relax to equilibrium positions. Shockley partial dislocations formed at the interface. The elastic strains were not in agreement with classical elasticity calculations. The simulation shows that it is easier to pull atoms apart than to push them together; and thus the larger gold lattice was strained less than the smaller nickel lattice even though the elastic modulus of nickel is greater than that of gold.

The Ni and Pt crystals exhibited strong bonding at the interface; whereas, the Ni and Au crystals exhibited little evidence of interface bonding. This result is in agreement with alloy heats of solution and the phase diagrams of these alloys.

The grain boundary segregation of solutes and the effects of alloying on the phosphorus induced intergranular fracture were investigated in high purity iron-phosphorus alloys with carbon, molybdenum, chromium, nickel or silicon. The solutes were dissolved and the specimens were heat-treated in the ∝phase region, unless dislocations were intentionally introduced through the γ-∝ transformation. Thus the effects of each alloying elements were studied under simplified conditions.

Carbon is the most important element in affecting the intergranular fracture. The carbon at grain boundaries increases the boundary cohesion as its inherent effect. It also drives off phosphorus atoms from the grain boundary. With these two effects, carbon reduces the intergranular fracture very effectively. Molybdenum and chromium reduces the intergranular fracture, perhaps through the attractive interaction with phosphorus to neutralize its detrimental effect. They may increase the grain boundary cohesion as their inherent effect. Nickel reduces the intergranular fracture through the solution softening. Silicon has the same effect, of which mechanism is now being studied.

Creep cavity nucleation at second phase particles in iron and steel is examined in the light of recent theoretical and experimental results. Theoretical considerations show it to be extremely unlikely that thermal nucleation will occur at Fe3C, FeO or A1203 particles, or that athermal nucleation will take place at Fe3C particles. Consistent with this, it is experimentally found that carbides and oxides almost never nucleate cavities in iron and steel, as long as harmful impurities like sulfur are not present. In the presence of segregated impurities like sulfur, oxides do nucleate cavities, but there is insufficient data to determine whether this is because the impurities make thermal or athermal nucleation easier. Finally, sulfides are thought to be nonwetting in iron and steel. As a result, there should be no barrier to thermal nucleation at sulfides, and experiments show that sulfides do nucleate cavities with great ease in iron and steel.

To investigate the mechanisms of grain boundary embrittlement, sustained load tests were conducted on a medium carbon, high strength martensitic steel at slightly elevated temperatures. Slow crack growth was observed from 185°C to 500°C.

Fracture surfaces of the specimens tested were examined by scanning electron microscopy after sustained load tests. It was found that severe grain boundary embrittlement occurred in this material with the predominant fractographic feature being intergranular fracture.

Auger electron and x-ray photoelectron spectroscopy of the intergranular facets were utilized to analyze the embrittling species. Prior suggestions for this “strain-aging embrittlement” phenomena have centered around carbide precipitation. The present results suggest that it is carbon, driven by thermal energy and the stress field, that segregates on the grain boundaries and weakens them.

Since recent years it is well known that under cyclic as well as monotonic loading conditions, traces of water vapor play an important role for crack propagation in Al based alloys [1–5]. In many cases it is often difficult to understand the difference between corrosion induced crack propagation and true hydrogen embrittlement, because the two phenomena lead to the same result,i.e. enhanced crack propagation. In order to study the H-embrittlement in more detail, fatigue crack propagation tests have been undertaken on single crystal Al-Zn-Mg specimens as well as specimens containing a single grain boundary. In both cases it is expected that water vapor makes a contribution to the embrittling process, but the mechanism leading to hydrogen enhanced fatigue crack growth are different. The aim of this paper is to discuss mechanism controlling hydrogen induced fatigue crack propagation in the light of microstructural aspects such as grain boundary structures and precipitation morphology in single crystals.

The large elongations associated with superplasticity are observed only in materials with fine grain sizes. An Al-Li alloy processed thermomechanically to develop a fine grain size exhibits superplastic characteristics and large elongations to failure of up to ∼900%. Measurements at large elongations reveal that the grain boundary sliding contribution to superplastic deformation is ∼75%. Microstructural inspection of superplastically deformed specimens reveals also the occurrence of extensive cavitation.

An experiment is reported where the proportion of potentially “special”, coincident site lattice boundaries is increased by manipulation of the kinetics of anomalous grain growth. A relatively new technique, electron back scattering in a scanning electron microscope, is used to obtain the data.