摘要:

Transmission electron microscopy observations of as-cast samples of an Al-Li-Cu icosahedral quasicrystal allowed us to demonstrate planar and line defects. By means of dark-field and weak-beam imaging techniques, planar defects were tentatively identified as π-type boundaries and low-angle subgrain boundaries, while line defects were found to be isolated dislocations. All the characteristics of the defects were determined by conducting extensive tilting experiments and performing conventional extinction contrast analyses. In particular, the isolated dislocations were found to have a Burgers vector aligned along a fivefold symmetry axis in physical space and to be screw in character.

ISSN:0950-0839    

DOI:10.1080/09500839508241608

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The construction of the γ surface requires the application of mechanical constraints in order that the energy of non-equilibrium fault configurations can be calculated. The choice of mechanical constraints is not unique and it is shown that γ surfaces constructed using different schemes can differ significantly. The measurement of planar fault energies using transmission electron microscopy techniques is also critically assessed. It is discovered that the constraining effects of the bounding partial dislocations can significantly modify the local fault energy. Experimentally measured fault energies should therefore be expected to differ from atomistically calculated values.

ISSN:0950-0839    

DOI:10.1080/09500839508241609

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In-situ heating experiments have been performed on thin foils of a single crystal with composition Fe-25·6 at .% Al. The isothermal evolution of antiphase boundaries (APBs) has been investigated by means of transmission electron microscopy. APBs play an important role in phase changes of long-range ordered alloys. The grown-in domains were found to have the DO3long-range order structure with randomly shaped APBs (swirl-shaped domains). Up to 400[ddot]C, no significant change in the domain and APB structure was observed. At 500[ddot]C, which is below the transition temperature (Tc≈ 550[ddot]C) from DO3to B2 coarsening of the domains has been analysed and dislocation motion was detected. The dislocations were analysed to have a Burgers vector of (a'0/4) (111) (a'0is the lattice parameter of the DO3lattice). The dislocations drag behind them APBs of B2 type. Reactions of the B2-type APBs with the DO3-type grown-in boundaries have been analysed. With increasing annealing time the density of the B2-type boundaries increases and the size of the domains becomes smaller. The associated thickening of the B2 domain walls has been imaged.

ISSN:0950-0839    

DOI:10.1080/09500839508241610

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The pinning energy due to the elastic interaction of a semicoherent Y2BaCuO5precipitate with the YBa2Cu3O7matrix is computed. This is achieved by setting up dislocation arrays at the interface. The elastic stresses generated by such arrays are integrated over a fluxoid volume to obtain the energy. It is seen that this elastic interaction energy makes an additive contribution to the totalJcvalue.

ISSN:0950-0839    

DOI:10.1080/09500839508241611

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

It is well known that overpressurized gas bubbles in solids will grow and equilibrate by acquisition of vacancies from nearby free surfaces, grain boundaries and other vacancy sources. Recently, Evans (1994,J. nucl. Mater.,210, 21) has suggested that the transfer of intragranular gas to grain boundaries in UO2may be substantially enhanced by gas bubble diffusion biased towards the grain boundaries by the flow of thermal vacancies from the boundaries into the vacancy-depleted grain interiors. To address this possibility, upper and lower bounds on the velocity of a gas bubble towards a vacancy source are determined by calculating the vacancy flux through an immobile bubble fixed with respect to the lattice, and the flux through a bubble moving with the host material respectively.

ISSN:0950-0839    

DOI:10.1080/09500839508241612

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Fluctuations in the stress field induced by a pair of infinite walls of uniform-randomly distributed dislocations which are coupled into dipoles are examined. Near each wall, the fluctuations decay as (x-a)−1/2, wherexis the distance from a wall and a is half the interwall spacing. At distances x much larger than the separation of the walls, stress fluctuations diminish asx−3/2.

ISSN:0950-0839    

DOI:10.1080/09500839508241613

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new description of the growth mechanism for the islands in submonolayer film coverage is presented which goes beyond the usual mean-field approximations. We show that the scaling behaviour observed during heterogeneous growth simulations is explained by the existence of capture zones for each island which remain unchanged throughout the evolution. These capture zones are in fact the Voronoi polygons for the island centres and so represent the departure of the local island environment from the average. The relevance and range of applicability of this description for homogeneous nucleation systems are discussed.

ISSN:0950-0839    

DOI:10.1080/09500839508241614

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Several deformed samples of a quasicrystalline phase T2 of an Al-Li-Cu alloy were studied by transmission electron microscopy. A quasicrystalline state, a modulated structure as well as a crystalline phase were observed by means of high-resolution techniques in the same region of a thin foil. As there is no evidence of boundaries to separate them, this implies that a continuous transformation from the quasicrystalline to crystalline states can occur in this alloy through structural modulation.

ISSN:0950-0839    

DOI:10.1080/09500839508241615

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor