Partial Burgers Vectors Research Articles

The f.c.c. to h.c.p. martensitic phase transformation in CoNi studied by TEM and AFM methods

The thermally induced martensitic phase transformation from the high temperature (f.c.c.) to the low temperature (h.c.p.) phase was studied in a Co32% Ni single crystal by transmission electron microscopy (TEM), atomic force microscopy (AFM) and light microscopy with multiple beam interferometry (MBI). Quantitative analysis of the TEM results shows that the transformation takes place by consecutive glide of partial dislocations of the same Shockley partial Burgers vector on every other close packed plane. The tapering h.c.p. lamellae contain high shear strains and cause long range internal stresses that facilitate transformation induced plasticity. The AFM and MBI results show that the transformation shear strains are compensated on a mesoscopic scale. This indicates that the transformation induced stresses trigger the formation of new self-accommodating h.c.p. lamellae by an autocatalytic process. It should be pointed out that the reverse transformation (h.c.p. → f.c.c.) previously investigated in the same material showed the occurrence of a different transformation mechanism based on an atomistic compensation.

In situ TEM observations of dislocation processes in iron-aluminides with 25 and 30 at.% aluminium

Thin single crystal foils of long-range ordered binary iron—aluminium alloys with 25 and 30 at.% aluminium have been tensile tested inside of a transmission electron microscope. The motion of dislocations has been recorded under full load at temperatures ranging from 300 to 970 K. These experiments yield information pertaining to: i) the activated glide planes; ii) the mode of glide (viscous versus jerky and planar versus cross-slip); iii) the dissociation of perfect dislocations into partials and the magnitude of the partial Burgers vectors; and iv) the widths and specific energies of the antiphase boundaries between the partial dislocations. The results are discussed with a view of elucidating the well-known anomalous, positive temperature dependence of the critical resolved shear stress of these intermetallic compounds between 500 and 780 K.

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine

Widely dissociated dislocations have been observed in mantle olivine from the Erro-Tobbio peridotite in N.W. Italy. Analysis of diffraction contrast in transmission electron microscopy (TEM) indicates that the dissociation reaction involves the climb dissociation on (001) and {021} planes of b=[001] unit dislocations into partial dislocations with Burgers vectors approximately equal to 1/x 〈011〉. In the most extreme case a unit dislocation dissociates into four partials which bound three planar defects. The unusually wide dissociation and the greater extent of dissociation in olivine from amphibole-bearing rocks suggests that the dissociation is related to hydration. The occurrence of fluid inclusions along the dislocations confirms that the partials and planar defects are saturated with volatiles. Analysis of possible planar defect structures in Fo90 shows that; (i) the most likely partial Burgers vectors are b=〈0 3/11 1/4〉; (ii) two of the planar defects are cation-deficient and can be stabilised by segregation of H+ to produce (Mg, Fe) (OH)2 layers which are iso-structural with the OH-rich interlayer of the humite group minerals; (iii) the central planar defect is formed by removing a stoichiometric olivine (002) layer so does not produce any local chemical changes. The climb dissociation provides a possible mechanism for the transformation of olivine to a humite group mineral. OH-rich interlayers may nucleate on dislocations and extend into the crystal by climb resulting in a gradual increase of (Mg, Fe) (OH)2 content. The only addition of material required is hydrogen which can rapidly diffuse into olivine. If the dissociation is stable and occurs at high temperatures and pressures it may significantly influence the nature and kinetics of deformation mechanisms and the olivine — spinel shear transformation mechanism in hydrated olivine.

Dislocations in perovskites BaTiO3 and CaTiO3

Dislocations either grown-in or produced by plastic deformation have been studied in transmission electron microscopy (TEM) in two crystals with the perovskite structure, namely BaTiO3 (P4mm at room temperature) and CaTiO3 (Pcmn). Two distinct deformation processes have been used: (i) microindentation at room temperature and (ii) creep at high temperature for BaTiO3. It is found that for both materials the easy glide systems at room temperature are {110}PC 〈1\(\bar 1\)0〉PC where the subscript PC refers to the pseudo-cubic lattices of these crystal structures. The corre-sponding Burgers vector 〈110〉PC is not the shortest lattice translation but these glide systems are such that dislocation gliding does not involve bringing the various cations close to each other. In BaTiO3, after creep at elevated temperature, the above glide systems still are observed but dislocations with Burgers vectors {100}PC are also seen. Most often they are in climb configuration i.e. their line lies in a plane {100}PC perpendicular to their Burgers vector. Furthermore they are dissociated by climb in the above plane following a reaction like [010]PC→1/2[011]+1/2[01\(\bar 1\)] the two partial Burgers vectors being orthogonal. It is suggested that this climb dissociation is due to the precipitation of the supersaturated point defects when the samples are cooled down. The equilibrium concentration of these point defects as well as their mobility would be very large at high temperature thus allowing a high temperature deformation by dislocation climb rather than by dislocation glide as it is observed at room temperature.

Room-temperature deformation mechanisms and the defect structure of tungsten carbide

Defects in undeformed, annealed single crystals and in deformed single crystals of WC are characterized by transmission electron microscopy. Deformation is induced by indentation with a microhardness tester. High densities of stacking faults with atomic displacement vectors R = 1 6 〈11 2 ̄ 3〉 are found lying on {11̄00} planes near the indentations. Stacking faults with R = 1 6 〈11 2 ̄ 3〉 and undissociated dislocations with b = 〈0001〉, 1 3 〈11 2 ̄ 0〉, and 1 3 〈11 2 ̄ 3〉 are observed at distances greater than 2μm from an indentation. The Burgers vectors of the partial dislocations bounding one fault are found to be identical b p = 1 6 [11 2 ̄ 3 ̄ ] , resulting in a total Burgers vector for the extended dislocation b = 1 3 [11 2 ̄ 3 ̄ ] . (Stacking fault energy = 55 mJ/m 2). Stacking faults with R = 1 6 〈11 2 ̄ 3〉 are described in terms of the crystal structure of WC, illustrating several points: the coordination of carbon atoms by tungsten atoms is preserved at the fault, the number of bonds per atom which are broken during slip may be minimized, and the 1 6 〈11 2 ̄ 3〉 type of partial Burgers vector may be combined to form several types of total Burgers vector.

PLANAR DEFECTS IN PLASTICALLY DEFORMED SPINEL SINGLE CRYSTALS

Planar defects as observed when plastic deformation of nickel ferrite single crystals involves second phase production are studied. They lie within spinel matrix lamellae and are bounded by partial dislocations. Examples are studied, the determinations of displacement vectors and stabilization planes are compared with previous stacking fault energy calculations. From determination of partial Burgers vectors it seems possible that these defects could be related to partial dislocations involved in stress assisted second phase production.

Epitaxial Growth of Ice Crystals on the Muscovite Cleavage Plane and Their Relation to Partial Dislocations

The arrangement of oxygen atoms in hexagonal rings in crystals of ice and muscovite gives two possible alternatives of epitaxial connection between these two structures. Which one of these two alternatives is favored depends entirely on the location of potassium atoms in the cleavage plane of the muscovite structure. Since potassium atoms are displaced from the normal environment by the slip taking place along partial Burgers vectors, epitaxial growth of ice crystals on muscovite can be used for the determination of the slip directions and hence for the study of partial dislocations in muscovite crystals.

Twist boundaries in alumina-chromia ‘alloys’

Abstract A transmission electron microscope study has been made of twist boundaries in alumina, chromia and in alumina–chromia ‘alloys’. The angle of misorientation measured from the spacing of rotational Moiré patterns, from selected area electron diffraction and from Laue x-ray back reflections is consistent with that deduced from direct measurement of the network mesh diameter. The total and partial Burgers vectors have been identified. Measurements of specific stacking-fault energy (γ) have been obtained from the geometry of extended dislocation nodes for both the pure components and for various ‘alloy’ compositions.

Thermal Behavior of Stacking Faults in Graphite Platelets

Stacking faults were observed in graphite platelets by transmission electron microscopy over the temperature range 170° to 890°K. From measurements of fault widths w at various temperatures T, it was found that (1/w)(∂w/∂T)=+(2.6±5.2)×10−4(∘K)−1.This value was used to obtain the estimate (Kbp/γ)(∂/∂T)(γ/Kbp)=−(2.6±5.2)×10−4(∘K)−1,where γ is the stacking fault energy, K the elastic coefficient of the fault, and bp the magnitude of the partial Burgers vector. The essential temperature dependence of γ in graphite is of the same order as that of K, that is, (1/γ) (∂γ/∂T)∼(1/K) (∂K/∂T).