Kevin M. Knowles

The crystallography of the martensitic transformation in equiatomic nickel-titanium

The phenomenological theory of martensitic transformations is applied to the various observed transformation modes in the shape memory alloy nickel-titanium. Since the martensitic phase has a crystal structure which can be considered to be a monoclinic distortion of the orthorhombic B19 structure of gold-cadmium and is twinned internally, a treatment based on the formulation of Lieberman. Wechsler and Read has been used. New experimental evidence is presented to show that one of the observed twinning modes in the martensite can be identified as a type II mode, with the shear direction η1 being derived from a cell edge of the parent B2 phase. The theoretical predictions for this mode and the mode where the twinning is type I with K1 = (111)M are found to be in satisfactory agreement with the available experimental results. A third twinning mode, of Type I with K1 = (001)M is shown to be unable to produce the lattice invariant shear necessary in the martensitic transformation.

Joining of engineering ceramics

Abstract Engineering ceramics such as alumina, zirconia, silicon nitride and silicon carbide can now be manufactured reliably with reproducible properties. As such, they are of increasing interest to industry, particularly for use in demanding environments, where their thermomechanical performance is of critical importance, with applications ranging from fuel cells to cutting tools. One aspect common to virtually all applications of engineering ceramics is that eventually they must be joined with another material, most usually a metal. The joining of engineering ceramics to metals is not always easy. There are two main considerations. The first consideration is the basic difference in atomic bonding: the ionic or covalent bonding of the ceramic, compared to the metallic bond. The second consideration is the mismatch in the coefficient of thermal expansion. In general, ceramics have a lower coefficient of thermal expansion than metals and, if high tensile forces are produced in the ceramic, either as a consequence of operating conditions or from the joining procedure itself, failure can occur. The plethora of joining processes available will be reviewed in this article, placing them in context from both an academic and commercial perspective. Comment will be made on research reporting advances on known technology, as well as introducing ‘newer’ technologies developed over the last 10 years. Finally, reviews and commentary will be made on the potential applications of the various joining processes in the commercial environment.

Characterisation of Zn3(VO4)2 phases in V2O5-doped ZnO varistors

Abstract Zinc oxide-rich ZnO–V 2 O 5 (ZV), ZnO–V 2 O 5 –MnO 2 (ZVM) and ZnO–V 2 O 5 –Sb 2 O 3 (ZVS) polycrystalline ceramics have been prepared for detailed microstructural and electrical characterisation. All samples exhibit non-linear current-voltage behaviour, with non-linear coefficients ranging from 5·0 for ZV to 16·7 for ZVM. The use of X-ray powder diffraction together with microstructural examination by transmission electron microscopy and a comparison of measured interplanar spacings with those quoted in the literature for α-, β- and γ-Zn 3 (VO 4 ) 2 , has shown evidence for the formation of β-Zn 3 (VO 4 ) 2 in ZV, γ-Zn 3 (VO 4 ) 2 in ZVM and α-Zn 3 (VO 4 ) 2 in ZVS. The Zn 3 (VO 4 ) 2 phases are found to exist as smaller grains embedded in ZnO grains or residing at triple junctions. Electron diffraction suggests that β-Zn 3 (VO 4 ) 2 has an orthorhombic A lattice, while γ-Zn 3 (VO 4 ) 2 has a monoclinic C lattice.

A high-resolution electron microscope study of nickel-titanium martensite

Abstract The martensitic phase of the shape-memory alloy equiatomic Ni─Ti has been examined by the high-resolution lattice-fringe imaging technique of transmission electron microscopy using axial illumination. This has, for the first time, shown evidence for the step and ledge structure of Type II twin interfaces, with the ledges being the nearest low-index rational plane to the irrational twin plane, as predicted in theoretical models of such interfaces. In addition, small areas with (001) martensite twins have been found which show features consistent with faulting on (001) planes and also occasionally features consistent with steps corresponding to twinning dislocations at the twin planes. However, simulations of the experimental two-dimensional lattice fringe images of Ni─Ti using a multislice approach to the dynamical theory of electron diffraction have demonstrated how difficult it is to correlate these images with any detailed picture at an atomistic level of either the twin interfaces or even, in ...

Evidence of hexagonal diamond in plasma-deposited carbon films

Hexagonal diamond grains of ∼30 nm diameter together with graphite and SiC are seen in predominantly amorphous carbon films deposited at low temperature on Si substrates from a CH4 plasma vapour source. The different crystalline phases are identified by grazing-angle X-ray diffraction which allows for substrate rotation and tilting to enable the 2θ peaks to be correlated with the angular displacements of specific planes. Electron energy-loss spectroscopy shows the chemical composition of the films to be predominantly carbon with traces of oxygen. Raman spectroscopy shows the peaks to be associated with amorphous carbon and graphite, together with a peak at 1170 cm−1 which is attributed to microcrystalline hexagonal diamond.

The dislocation geometry of interphase boundaries

Abstract Bilbys (1955) theory of surface dislocations has been applied to the case of a general interphase boundary where the lattice misfit is formally relieved by discrete arrays of dislocations. The approach developed here derives the line directions and array spacings of the dislocations for any matrix relating the two lattices on either side of the boundary. Examples are given of the application of the derived formulae to particular interphase boundary problems.

Dissociation of lattice dislocations in SrTiO3

Abstract Lattice dislocations within the grains of an experimental SrTiO3 internal boundary layer capacitor have been studied using weak-beam transmission electron microscopy and conventional two-beam imaging. The resulting images have been compared with simulated images for various possible models of dislocations in perovskites. The lattice dislocations observed in this study are shown to be of mixed character and to have 〈110〉 Burgers vectors. They are dissociated with a collinear splitting into two partial dislocations with ½〈110〉 Burgers vectors. Evidence has been found for both glide dissociation and climb dissociation. From the spacing of the partials, the stacking-fault energy in this doped SrTiO3 is estimated to be 145 ± 15mJm−2 for glide dissociation and 245 ± 30mJm−2 for climb dissociation.

Cross-sectional structure of tetrahedral amorphous carbon thin films

Abstract A cross-sectional transmission electron microscope study of the low density layers at the surface and at the substrate-film interface of tetrahedral amorphous carbon (ta-C) films grown on (001) silicon substrates is presented. Spatially resolved electron energy loss spectroscopy is used to determine the bonding and composition of a tetrahedral amorphous carbon film with nanometre spatial resolution. For a ta-C film grown with a substrate bias of − 300 V, an interfacial region approximately 5 nm wide is present in which the carbon is sp 2 bonded and is mixed with silicon and oxygen from the substrate. An sp 2 bonded layer observed at the surface of the film is 1.3 ± 0.3 nm thick and contains no detectable impurities. It is argued that the sp 2 bonded surface layer is intrinsic to the growth process, but that the sp 2 bonding in the interfacial layer at the substrate may be related to the presence of oxygen from the substrate.

The Application of Surface Dislocation Theory to the F.c.c.-B.c.c. Interface

The formal theory of surface dislocations has been applied to the f.c.c.-b.c.c. interfaces defined by (111)F || (110)B. With the Bain correspondence between the two lattices, various theoretical models and experimental results on these interfaces have been analyzed. The results of the analysis suggest that preferred interface orientations can be explained on the basis that they are those of minimum or near-minimum Burgers-vector contents. This concept leads to an improved criterion for comparing the elastic component of interfacial energies. The limitations of geometrical models for predicting low-energy interfaces are discussed.

The nature of the parent-martensite interface in titanium-manganese

Abstract A transmission electron microscope study has been made of the ‘zig-zag’ parent-martensite interfaces found between the b.c.c. parent phase and the internally twinned h.c.p. martensite phase in a Ti-5 wt% Mn alloy. The interfaces were found experimentally to have an orientation consistent with their being in the zone of the invariant line of the transformation for the Class A (α−, ω+) solution of the phenomenological theory. No evidence was found for the ‘steps’ or ‘ledges’ in the interfaces which would be expected if the interfaces are to be explained on the basis of small scale dilatations which make the lattice strain in the martensitic transformation an invariant plane strain. The orientation of the interfaces have been explained by a new model based on the surface dislocation approach. The model suggests that the occurrence of interface faceting in materials such as TiMn is a consequence of the individual twin-parent interfaces lowering their net Burgers vector contents. Predictions from this approach are found to be in good agreement with the present results and also previous experimental results. A plausible argument for this approach is presented using results from isotropic elasticity theory.