M. H. Lewis

The defect structure and mechanical properties of spinel single crystals

Abstract The grown-in defect structure of non-stoichiometric magnesium aluminate spinels (of compositions between Al2O3/MgO = 3·5 and 1·5) has been analysed by electron microscopy. Dislocations in all crystals are composed of pairs of partial dislocations bounding a fault in the cation stacking sequence. Non-stoichiometric crystals with ⟨100⟩, ⟨110⟩ and ⟨111⟩ compression axes have deen deformed in the range 1300–1520°C. Measurements of flow stress for the three orientations together with observations of slip lines, birefringence patterns and the direct observation of dislocations by electron microscopy determine a principal glide plane of the type {110} and Burgers vector direction a/2⟨110⟩. Dislocation glide also occurs on {111} planes at higher resolved shear stresses such that τ111/τ110 = 1·5 for crystals of composition Al2O3/MgO ≈ 3·5. This ratio decreases with decreasing deviation from stoichiometry. Stoichiometric crystals with ⟨110⟩ compression axes undergo very small plastic strain before fracture...

A superlattice with monoclinic symmetry based on the compound V6C5

Abstract The ordered distribution of carbon vacancies in the compound V6C5 has been analysed by electron diffraction and dark-field electron microscopy. The dominant structure is that based on a superlattice with monoclinic symmetry having a lattice orientation relation with the disordered cubic matrix which may be summarized: The unit cell parameters, neglecting a distortion of the f.c.c. vanadium atom structure, are The vacancy distribution is related to that previously observed for this compound and found in the present investigation in minor fraction. The difference in the vacancy distribution is determined by the alternative stacking sequences of partially filled octahedral carbon atom layers. Hence one structure may be derived from the other by an ordering in the distribution of stacking faults which have been identified in the carbon sub-lattice.

Coincidence‐site‐lattice relations for MgO‐CdO interfaces

Abstract Electron microscope measurements of the angular distribution of small, cubic MgO and CdO crystals deposited on MgO {100} single‐crystal substrates show marked preferred orientations. The coincidence‐site‐lattice (CSL) theory for CdO/MgO interfaces is given and the measured distribution correlates well with CSL theory. The observed orientations are thought to be produced by the coherent rotation of the small crystals to orientations that have a local minimum in the interface energy.

A study of the vacancy distribution in non-stoichiometric spinels by magic-angle spinning NMR

Abstract A series of spinels of the general form MgO·xAl2O3 (x = 1·00 to x = 3·40) have been examined by magic-angle spinning (MAS) NMR to determine the vacancy distribution. It is found that the non-stoichiometric vacancies introduced with increasing x occupy the octahedral interstitial cation sites, rather than being randomly distributed or occupying tetrahedral sites. This study demonstrates that MAS-NMR can provide a convenient, readily interpretable method for examining structural problems in solids. A comparison is made with the earlier X-ray studies of the problem.

Precipitation in non-stoichiometric spinel crystals

Abstract Microstructural changes preceding the formation of the equilibrium (α-Al2O3) phase in non-stoichiometric magnesium-aluminium spinel have been examined by electron microscopy. Two intermediate phases have been identified which nucleate on paired partial dislocations in the spinel matrix. Both phases have monoclinic symmetry but differing morphologies. The first phase precipitates as thin plates with {113} habit and orientation relation {113} matrix ‖ {001} precipitate, ⟨110⟩ matrix ‖ ⟨010⟩ precipitate. The second phase grows as thin laths parallel to ⟨100⟩ matrix directions and orientation relation ⟨100⟩ matrix ‖ ⟨010⟩ procipitate, ⟨110⟩ matrix ‖ ⟨001⟩ procipitate. The laths coalesce on long ageing to produce composite plate shapes formed from laths in twin related orientation. Image contrast arises mainly from the mismatch between precipitate and matrix lattices and takes the form of interface fringes or matrix strain contrast. Thc extinction criterion for contrast is consistent with a lattice mi...

The ordered distribution of carbon atoms in titanium carbide

Abstract The combined techniques of selected-area electron diffraction and dark-field electron microscopy have been used to determine the ordered distribution of carbon atoms formed after low-temperature annealing in crystals with composition near Ti2C. The superstructure is based on a cubic lattice similar to that previously determined by neutron diffraction and is consistent with the antiphase boundary contrast observed in dark-field images.

Long-period order in vanadium carbide

Abstract Periodic arrangements of anti-phase boundaries (APBs) have been detected by electron diffraction and microscopy in long-range ordered crystals of vanadium carbide near to the composition V 6 C 5, The APBs are non-conservative and are believed to accommodate excess vacancies or carbon atoms in response to deviations in crystal composition from V 6 C 5. The atomic arrangement at non-conservative APBs has been deduced from diffraction contrast analysis of isolated APBs and their observed configurations in crystals containing non-periodic distributions. The observed variation in spacing of the long-periodic structure is consistent with analyses of the average crystal compositions. A mechanism of formation of the long-period structure has been deduced from observations on partially transformed crystals.

Cellular morphologies in rapidly solidified Al-Al2Cu and Al-Al3Ni eutectic alloys

Abstract The cellular structures found in impure specimens of Al-Al 2 Cu and Al-Al 3 Ni eutectic alloys solidified at 0.44 to 2.88 mm/sec have been investigated. In Al-Al 2 Cu elongated cells containing parallel traces of mismatch surfaces were found at the lowest solidification rate, but the traces were arranged more randomly and the cells became more equiaxed as the rate of solidification increased. In Al-Al 3 Ni the cells displayed a specific shape within each eutectic grain at the lowest solidification rate, but were less crystallographic in nature at the faster rates. The crystallographic orientation relation between eutectic phases has been determined for both eutectics and compared with data for the same systems solidified more slowly.

The crystal growth of transition metal interstitial compounds by a floating zone technique

Abstract A floating zone technique for the preparation of large high purity crystals of vanadium carbide, oxide and nitride with controlled stoichiometry is described. An essential feature of the process is the provision of a facility which enables growth under positive ambient gas pressures (up to 20 atm) to be carried out. Inert gas atmospheres are used to reduce vanadium vaporisation losses in vanadium carbide and oxide and so enable closer compositional control to be achieved, while the nitrides are grown under similar pressures of nitrogen and helium to reduce nitrogen losses. Zone levelling has enabled large single crystals of vanadium monocarbide (VC) with differing and closely controllable compositions to be produced. Vanadium monoxide crystals were grown over the whole cubic stoichiometry range but in the vanadium nitrogen system single phase cubic vanadium mononitride (VN) could not be produced and the microstructure of the alloy of highest nitrogen content consisted of a eutectic of the cubic VN and the hexagonal divanadium nitride (V 2 N) phases. Finally, in addition to the usual chemical and optical microstructural examination, electron microscopy and electron diffraction have been used to characterise the type and extent of the defect structures which are present in all three alloy systems studied.

Dislocation mechanisms for the nucleation of transformations in vanadium carbide

Abstract An electron microscope study has indicated that large intrinsic stacking faults are formed in the cubic structured vanadium carbide at compositions close to the VC–V2C phase boundary. It is proposed that these faults, bounded by Shockley partial dislocations, provide the nuclei for a transformation of the cubic carbide to the hexagonal V2C or ζ phases which occurs at slightly lower carbon concentrations.