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Multilayered Cu-Ni has a peak yield strength four orders of magnitude higher than either Cu or Ni because the multitude of interfaces obstruct glissile dislocations. The barrier strengths of the interfaces may be traced to four mismatches across an interface: modulus, lattice parameter, chemical and slip geometry. This paper describes sample embedded atom method (EAM) simulations of dislocations crossing interfaces, designed to separate the effects of the four mismatches. The results confirm some classical calculations and emphasize the importance of three new effects (i) an interface-chemical effect in which dislocations are trapped by core spreading in the interface, (ii) a coherency-chemical effect caused by coherency strains changing effective stacking fault energies and (iii) a coherency-modulus effect in which coherency strains change elastic moduli (and hence the Koehler stress) significantly.
Calculations by Krivogtaz  dealing with quasiline formation in highly distorted lattices undergoing phase separation have been extended for randomly arranged particles. Qualitative experimental evidence from powder patterns, already in the literature for Cu-Be,Ni-Be,Cu-Ti and Nimonic alloys[2-5], have demonstrated the existence of quasilines. This extended calculation deals with ellipsoids of revolution and allows one to examine different shapes and transformation strains in an anisotropic medium. it is shown that the precipitate transformation strains play a very important role in shaping the Bragg-like profiles. This is most obvious in the intermediate stage which includes Bragg scattering from the lattice, regular static diffuse scattering and the quasiline. For precipitate sizes associated with maximum age hardening, all three normally become scrambled into a broad assymetrically shaped Bragg-like peak. However, a comparison of the theoretical calculations with experimental data from an age hardened Cu-Be alloy shows qualitative agreement, which we believe is due to the non-random nature of precipitation in this system.
Recent studies have suggested a particular relationship between the degree of covalent bonding in TiAl and the mobility of dislocation[1,2]. Ultimately such electronic effects In ordered compounds must dictate the dislocation core structures and at the same time the dislocation mobility within a given compound. However, direct modelling of line defects Is beyond the capability of todays electronic structure techniques. Alternatively, significant steps toward extending our understanding of the flow behaviour of structural intermetallics may come through general application of empirical interatomic potential methods for calculating the structure and mobility of defects. Toward this end, we have constructed semi-empirical interatomic potentials within the embedded atom formalism for L1O and B2 type structures. These potentials have been determined by fitting to known bulk structural and elastic properties of TIAl and NiAl, using least squares procedures. Simple expressions that relate the parameters of the potentials to the bulk properties are used in the fitting procedure. Calculations of dislocation core structures and planar fault energies using these potentials are considered. The differences between the optimized bulk properties predicted from the potentials and the values for these properties are discussed in terms of non-spherical nature of the electron density distribution. Empirical methods which incorporate these effects into interatomic potentials are briefly discussed.
Measurements of x-ray profiles and diffuse scattering from (111) and (100) oriented single crystal Niobium films implanted with Nitrogen to average levels of 5 and 0.5 atomic percent are discussed. Theoretical analysis of the asymmetric Bragg profiles are used to determine the strain profile in the implanted films. The measured strain profile results from two factors: (i) depth distribution of implants and knock-on damage and (it) elastic constraints. Residual elastic strains develop due to the constraints imposed by a sapphire substrate. Comparison of the diffraction results with theoretical predictions of TRIM indicates thde presence of measurable knock-on damage in the films. Huang and Stokes-Wilson scattering measurements made using synchrotron radiation at the ORNL beanmline, Brookhaven National Laboratory, were used to reveal the identity of defects formed during the knock-on process.
X-ray diffraction line profiles have been described in an earlier work published by the authors as a convolution of three functions: particle size broadening, and two types of non-uniform strain broadening. The strain coefficients used in this approach are simply related to the strain coefficients obtained by Krivoglaz et.al, using a theory based upon correlated dislocations. This connection enables us to determine the dislocation density and the ratio of the correlation range to the mean particle size. Results for cold worked fcc and bcc materials as well as highly imperfect sputtered films of Mo are considered. Dislocations are highly correlated in cold worked metals, whereas correlation is much lower in sputtered films deposited at low temperatures. In each case, the dislocation density is high. An analysis of wear debris consisting of cubic Zirconia gave the highest dislocation density and correlation. The close similarity between the early work on cold work filings and wear debris provides insights on wear mechanisms.
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