摘要:

Transmission electron microscopy evidence is given of mechanisms. of annihilation that take place in the course of the room-temperature plastic deformation of Ni3Al. Under a load orientation that favours the operation of a single-slip system, the short dipoles that are present in foils sectioned parallel to the slip plane are identified as elongated closed superdislocation loops instead of dipoles terminating at free surfaces. The fact that dipoles are often aligned in rows such that one extremity of a given dipole corresponds with the closest extremity of the next dipole in the screw orientation is explained in terms of the cross-slip properties of Ni3Al-based alloys. The presence of two distinct categories of dipole rows is interpreted in terms of two different approach mechanisms. The formation of antiphase-boundary tubes-by direct annihilation of screw segments is questioned.

ISSN:0950-0839    

DOI:10.1080/09500839508240507

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Large single quasicrystals of the icosahedral Al-Pd-Mn phase have been grown using the Czochralski technique. Different experimental conditions which favour the growth of a large single grain have been tried. The best single grain obtained is 3 cm3in volume and has an overall mosaic spread of 0.06°, revealing the high quality of the sample. This represents an increase in volume of one order of magnitude, together with a decrease in the mosaic spread of the same amount when compared with our previous results using the Bridgman technique. The quasicrystallinity of the samples has been checked by various methods.

ISSN:0950-0839    

DOI:10.1080/09500839508240508

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We re-examine an experiment on the transient behaviour of Ni3Al which demonstrates that the anomalous flow stress is thermally reversible in the small-strain yielding regime. Further, we discuss the phenomenon within emerging kink-based models for the flow-stress anomaly in an effort to highlight critical aspects of these models which need further attention. Specimens from a Ni-22.9 at. % Al single crystal, having an orientation near (001), were pre-strained at 873 K and additionally strained at 300 K. The flow properties of this crystal were compared to those from a crystal strained at 300 K alone. Both crystals were examined using transmission electron microscopy. In the small-strain flow regime, the flow stress anomaly is reversible. Pre-straining at a higher temperature before room-temperature deformation shows little memory of the pre-straining during the yield transient, even though TEM observations show that many Kear-Wilsdorf locks are retained in the substructure. Microscopic examination also reveals the presence of a second dislocation structure, characteristic of the low-temperature deformation, in selected regions of thin foils from the pre-strained sample. This observation demands that some part of the substructure recovers extremely rapidly, if not instantly upon changing temperature and (or) unloading. We suggest that future models must consider the full behaviour of all dislocation loop characters and character junctions in accomplishing the thermally reversible transition from the elastic regime, through the microstrain regime, to flow.

ISSN:0950-0839    

DOI:10.1080/09500839508240509

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An improved multislice method is presented for performing dynamical calculations of high-energy electron diffraction and imaging in cases of inclined illumination. The method is shown by practical calculations to give results consistent with the traditional multislice method. The mathematical equivalency of the two methods for single-direction illumination cases is also given. However, there are some differences between the two methods for many-direction illumination cases such as in coherent convergent-beam electron diffraction (CBED). The improved multislice method may provide a more convenient expression for understanding coherent CBED problems and a more rapid algorithm for performing numerical calculations of the multislice approach in real space.

ISSN:0950-0839    

DOI:10.1080/09500839508240510

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The distributions of grain boundaries in hot-strained 80 wt% Ni-20 wt% Cr alloy have been determined with respect to their misorientation angles and the crystallographic axes, as well as to the reciprocal density σ of coincidence sites. It is established that the grain-boundary distribution in the alloy formed under high-temperature deformation differs from the stable grain-boundary spectrum typical of statically recrystallized materials that are prone to annealing twinning. There are also differences in the deformation behaviour of the samples with different spectra of grain boundaries. It is demonstrated that, in principle, it is possible to control the material properties through the grain-boundary distribution.

ISSN:0950-0839    

DOI:10.1080/09500839508240511

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

With the aim of improving ductility through structural modification, first-principles calculations have been carried out to study the effect of ternary additions of 3d transition metals on the structural stability of Ti3Al. It is found that all the ternary transition-metal additions substitute preferentially for Ti atoms in Ti3Al. Our calculations predict that V and Cr are potent β stabilizers, whereas Zr, Ni and Cu (with about 10 at. %) can stabilize Ti3Al into the L12structure. Both modified cubic structures can possibly provide improved ductility compared with that of Ti3Al with the hexagonal D019structure. It is also found that the electronic density of states at the Fermi level plays an important role in determining the structural stability.

ISSN:0950-0839    

DOI:10.1080/09500839508240512

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Large single decagonal quasicrystals of Al-Cu-Co-Ge alloy about 0.8 mm in diameter and 10 mm in length have been obtained by the directional growth technique. The single decagonal quasicrystals show decaprismatic morphology and their perfection was examined by electron diffraction. It was found that the perfection of these single quasicrystals depends on the growth rate during the directional solidification. A high density of mixed dislocations with Burgers vectors b parallel to the tenfold axis and stacking faults with displacement vectors R parallel to the tenfold axis were introduced into the quasicrystals during rapid directional growth. The effects of the temperature gradient and rate of growth on the defect formation during directional growth are discussed.

ISSN:0950-0839    

DOI:10.1080/09500839508240513

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The results of atomistic simulations based on the Born model of solids show that the crystalline-to-amorphous phase transition in α-quartz is the result of a dynamical instability. This work illustrates the potential of atomistic simulation to identify the atomic mechanisms for phase transformations. On applying high pressure to α-quartz, an acoustic vibrational mode became unstable, indicating supercell formation of size 3 × 3 × 1. Further increase in pressure resulted in the collapse of the crystalline structure to an amorphous solid displaying variations in bond length, bond angle and coordination number.

ISSN:0950-0839    

DOI:10.1080/09500839508240514

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The low-temperature icosahedral phase, which forms below approximately 700°C near the alloy composition Al71Pd21Mn8has been investigated by means of electron diffraction. Precise analysis of diffraction patterns indicates that the Bravais lattice of the low-temperature phase isPtype, different from the formerly proposedFtype. The six-dimensional lattice parameter of this newP-type lattice is determined to beaP= 20.881 Å by powder X-ray diffraction and is related to the high-temperatureF-type iscosahedral phase byaP= τaF, where τ andaFdenote the golden mean and the six-dimensional lattice parameter respectively of the high-temperature phase. The low-temperature transformation can be understood by a small breaking ofFsymmetry caused by superlattice ordering. A model of the required configuration of atomic surfaces (acceptance regions) is discussed by using the six-dimensional ‘diamond’ lattice. This diamond lattice has a lattice parameteraF2= 2aFand is related to the aboveP-lattice by a scaling transformation.

ISSN:0950-0839    

DOI:10.1080/09500839508240515

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We have used classical molecular dynamics to study the structure and dynamics of the (110) surface of CaF2. Rigid-ion simulations reveal that at high temperatures (but well below the melting point) cation mobility at the (110) surface leads to the adoption of a missing-row structure, with alternate (111), (111) and (110) microfacets along the [001] direction. This structure is preserved up to the melting point, and no surface melting occurs. Superionic anion diffusion occurs at the free surfaces at a lower temperature than the transition temperature found in bulk simulations. The inclusion of polarizability via the shell model lowers the melting point by over 15% and results in surface melting 50 K below the bulk melting point.

ISSN:0950-0839    

DOI:10.1080/09500839508240516

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor