Diana Farkas

Structure and Mechanical Behavior of Bulk Nanocrystalline Materials

The reduction of grain size to the nanometer range (˜2-100 nm) has led to many interesting materials properties, including those involving mechanical behavior. In the case of metals, the Hall-Petch equation, which relates the yield stress to the inverse square root of the grain size, predicts great increases in strength with grain refinement. On the other hand, theory indicates that the high volume fraction of interfacial regions leads to increased deformation by grain-boundary sliding in metals with grain size in the low end of the nanocrystalline range. Nanocrystalline ceramics also have desirable properties. Chief among these are lower sintering temperatures and enhanced strain to failure. These two properties acting in combination allow for some unique applications, such as low-temperature diffusion bonding (the direct joining of ceramics to each other using moderate temperatures and pressures). Mechanical properties sometimes are affected by the fact that ceramics in a fine-grained form are stable in a different (usually higher pressure) phase than that which is considered “normal” for the ceramic. To the extent that the mechanical properties of a ceramic are dependent on its crystal-lographic structure, these differences will become evident at the smaller size scales. It is uncertain how deformation takes place in very fine-grained nanocrystalline materials. It has been recognized for some time that the Hall-Petch relationship, which usually is explained on the basis of dislocation pileups at grain boundaries, must break down at grain sizes such that a grain cannot support a pileup. Even some of the basic assumptions of dislocation theory may no longer be appropriate in this size regime. Recently considerable progress has been made in simulating the behavior of extremely fine-grained metals under stress using molecular-dynamics techniques. Molecular-dynamics (MD) simulations of deformation in nanophase Ni and Cu were carried out in the temperature range of 300–500 K, at constant applied uniaxial tensile stresses between 0.05 GPa and 1.5 GPa, on samples with average grain sizes ranging from 3.4 nm to 12 nm.

A molecular dynamics study of polycrystalline fcc metals at the nanoscale: grain boundary structure and its influence on plastic deformation

Molecular dynamics computer simulation of nanocrystalline Ni and Cu show that grain boundaries in nanocrystalline metals have the short range structure of most grain boundaries found in conventional polycrystalline materials. The simulations also indicate the presence of a critical grain size below which all plastic deformation is accommodated in the grain boundary and no intra-grain deformation is observed.

Are nanoporous materials radiation resistant

The key to perfect radiation endurance is perfect recovery. Since surfaces are perfect sinks for defects, a porous material with a high surface to volume ratio has the potential to be extremely radiation tolerant, provided it is morphologically stable in a radiation environment. Experiments and computer simulations on nanoscale gold foams reported here show the existence of a window in the parameter space where foams are radiation tolerant. We analyze these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (i) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically ∼5 nm for primary knock on atoms (PKA) of ∼15 keV in Au), while (ii) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKAs energy and dose rate used in this study). In between these dimensions, (i.e., ∼100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose-rate. We conclude that foams can be tailored to become radiation tolerant.

Embedded atom potential for Fe-Cu interactions and simulations of precipitate-matrix interfaces

A new empirical interatomic potential of the embedded atom type is developed for the Fe-Cu system. The potential for the alloy system was constructed to reproduce known physical parameters of the alloy, such as the heat of solution of Cu in Fe and the binding energy of a vacancy and a Cu atom in the matrix. The potential also reproduces first-principle calculations of the properties of metastable phases in the system. This atomic interaction model was used in simulation studies of the interface of small coherent Cu precipitates in and of dislocation core structure. The phase stability of the body-centred cubic Cu precipitates was also analysed.

Atomistic simulation of point defects and diffusion in B2 NiAl: Part I. Point defect energetics

Abstract In part I of this work we studied point defect energetics in the ordered B2 compound NiAl by means of computer simulations using ‘molecular statics’ and the embedded atom method. In the present paper we apply the computation technique and results of part I to study atomic mechanisms of tracer self-diffusion in NiAl. We calculate the activation energy of Ni and Al self-diffusion in perfectly stoichiometric NiAl for three atomic mechanisms: the mechanism of next-nearest-neighbour (NNN) vacancy jumps, the 6-jump vacancy mechanism and the 4-ring mechanism. The results of our simulations indicate that self-diffusion in stoichiometric NiAl is dominated by the mechanism of next-nearest-neighbour vacancy jumps. Diffusion of Al by this mechanism is likely to occur more slowly and with a higher activation energy than diffusion of Ni. The mechanism of 6-jump cycles is less favourable but still highly competitive to the NNN vacancy mechanism. The 4-ring mechanism is the least effective for both Ni and Al dif...

Interatomic potentials for B2 NiAl and martensitic phases

Interatomic potentials of the embedded atom type were developed for the Ni-Al system by empirical fitting to the properties of B2 NiAl and Ni5Al3. Consideration was also given to the properties of L12 Ni3Al as well as the martensitic L10 phase. The B2 phase is predicted as the stable phase for the equi-atomic composition. The potentials also predict the stability of the 3R martensitic structure with respect to the B2 phase for 62.5% Ni alloys. The globally stable phase for this composition is the Ni5Al3 structure. The predicted lattice parameters and tetragonality ratios for NiSAl5 and 3R martensite are very close to experimental values. The structure and energy of various defects was calculated using the new potentials and the results compared with those given by other potentials in the literature.

Molecular statics simulation of fracture in -iron

The behaviour of mode I cracks in -Fe is investigated using molecular statics computer simulation methods with an EAM potential. A double-ended crack of finite size embedded in a cylindrical simulation cell and fixed boundary conditions are prescribed along the periphery of the cell, whereas periodic boundary conditions are imposed parallel to the crack front. The displacement field of the finite crack is represented by that of an equivalent pile-up of opening dislocations distributed in a manner consistent with the anisotropy of the crystal and traction-free conditions of the crack faces. The crack lies on the plane and the crack front is located along , or directions. The crack tip response is rationalized in terms of the surface energy of the cleavage plane and the unstable stacking energies of the slip planes emanating from the crack front.

Effect of grain size on the elastic properties of nanocrystalline α-iron

The effect of grain size on the elastic properties of nanocrystalline α-iron is reported using atomistic simulations. A softening of the elastic properties is observed for grain sizes ranging from 12 nm down to 6 nm. The decrease in the Young’s and shear moduli with decreasing grain size is in agreement with experimental data and matches an analytical model based on the rule of mixtures for composite materials.

Atomistic simulation of an f.c.c./b.c.c. interface in NiCr alloys

Abstract The embedded atom method is applied to study the atomic structure and energy of an f.c.c./b.c.c. interface in NiCr. The two phases are oriented in a Kurdjumov-Sachs orientation relationship, and the interface considered is the (1 2 1) f habit plane adopted by precipitate laths of the b.c.c. phase. The interfacial energy and coherent strain energy at 0 K are calculated for boundaries between an f.c.c. NiCr solid solution and b.c.c. Cr. The calculated interfacial energy varies from 216 mJ/m 2 when the f.c.c. phase is pure Ni to 200 mJ/m 2 when the f.c.c. phase is Ni50 at. % Cr. Atomic relaxations appear limited to atoms in contact with the interphase boundary. Most of the interfacial energy is attributed to the structural difference across the f.c.c./b.c.c. boundary, and the chemical contribution to the energy is estimated to be less than 20% of the total energy. The value of the calculated energies and the widespread occurrence of the (1 2 1) f habit plane in a variety of alloy systems indicate this boundary orientation has a relatively low interfacial energy.

New eutectic alloys and their heats of transformation

Eutectic compositions and congruently melting intermetallic compounds in binary and multi-component systems among common elements such as AI, Ca, Cu, Mg, P, Si, and Zn may be useful for high temperature heat storage. In this work, heats of fusion of new multicomponent eutectics and intermetallic phases are reported, some of which are competitive with molten salts in heat storage density at high temperatures. The method used to determine unknown eutectic compositions combined results of differential thermal analysis, metallography, and microprobe analysis. The method allows determination of eutectic compositions in no more than three steps. The heats of fusion of the alloys were measured using commercial calorimeters, a differential thermal analyzer, and a differential scanning calorimeter.