Structure and role of the interfacial layers in VC-rich WC-Co cermets

The microstructure of WC-Co composites sintered with VC or with a mixture of VC and Cr₃C₂ is investigated by several techniques in order to understand the grain growth inhibition process. In this work, using the high-resolution transmission electron microscopy and high-resolution energy-filtering electron microscopy, we are able to study on the atomic scale the microstructure and composition of the phases present in the alloys. A thin (VW)C _x layer less than 1 nm thick covering all surfaces of WC grains and thin (VW)C _x platelets embedded in the WC grains are evidenced. Microsteps are observed at the interface between Co and WC along the ?11amp;2macr;0?_WC directions. Small (VW)C _x precipitates lie on the (0001)_WC and {10amp;1macr;0} facets of these steps. On the (0001) surface of WC grains, other stacking sequences of the metal planes are sometimes observed and, in particular, the occurrence of the compound (VW)₂C is shown. Owing to these observations a grain growth inhibition mechanism is proposed.     

Segregations at antiphase boundaries in hexagonal close-packed superstructures. II

As was shown in the first part of this study [1], slip occurs in polycrystalline ordered Mg₃Cd, which has a DO₁₉ superstructure, along the {0001} basis planes, {1010} prismatic planes, and {1011} and {1012} pyramidal planes [2,3]. The formation of segregations at antiphase boundaries (APB's) in the {0001} basis and {1011} pyramidal planes was also examined there [1]. Segregations at APB's in planes are treated in this second part of the study. A zero-energy APB may form in the prismatic plane [4]. Segregation of atoms, on the other hand, is not allowed at such an APB according to the Gorskii-Bragg-Williams theory if correlation and interaction in the second and higher coordination spheres are not taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 62–70, May, 1969.The authors thank N. S. Golosov and N. V. Kozhemyakin for discussion of this study and valuable advice.     

Structural damage and its annealing response in neutron irradiated magnesium†

Abstract

Transmission electron microscopy and immersion density techniques were used to characterize the nature and extent of radiation damage in zone refined and commercial purity magnesium irradiated to 1.7 × 10²¹ neutrons/cm² (>0.1 MeV) at about 75°C. Swelling from radiation-induced voids was 1.9 and 0.7 per cent, respectively, for the high purity and commercial magnesium. This difference was caused by larger voids in the zone refined material. The voids had a non-equiaxed shape, being bounded by {0001}, {1010}, and {1011} and were flattened in ?0001?, but the larger voids were more equiaxed. There was pronounced clustering of voids on {0001} planes. The irradiation-induced dislocation structure was also clustered on the basal planes. This dislocation structure is deduced to consist of a complex tangle of 1/2 ?2023? partial dislocations and their associated stacking faults.

During postirradiation annealing, voids began to anneal out in 1 hr at 150°C and were eliminated in 1 hr at 320°C. The smaller voids as well as voids near grain boundaries disappeared first. Slow annealing of voids near some grain boundaries suggested that not all grain boundaries are equally good sinks for vacancies. Voids also became more equiaxed during annealing. Stacking faults were eliminated before the voids disappeared, and the dislocations farmed networks. Annealing treatments at 500 °C or higher caused the appearance of gas bubbles.     

Nanoscale determination of surface orientation and electrostatic properties of ZnO thin films

Scanning force microscopy related techniques are applied to study surface nanoscale properties. We show that nanogoniometry can be combined with local electrostatic measurements – electrostatic force microscopy and Kelvin probe microscopy – to identify surface planes families and to study their local electrical properties. The scanning force microscopy techniques employed are analyzed and the correct way of acquiring and interpreting data is discussed in detail. The experiments performed on ZnO films grown along the nonpolar [112̄0] direction show that these films completely facet into {101̄1} and {101̄1̄} planes, which follow a well defined pattern of surface potential along the [0001] direction. This pattern is explained in terms of the different ionic termination – Zn or O ions – of the exposed facets. Finally, the presence of inversion domain boundaries is discussed. PACS 68.47.Gh; 73.30.+y; 68.37.-d; 73.63.-b     

Mechanical properties of twist grain boundaries in Cu

In a previous paper we studied the Young's and shear modulus of a high-angle twist grain boundary (5) in Cu, using the EAM, and related it to the uniaxial strain derivatives of single crystals. In this paper, we discuss elastic properties of ten additional twist grain boundaries, from 8.8–43.6°. The monolayer Young's modulus at each boundary was calculated and found to be 20–50% higher than the bulk value for all eleven boundaries for both csl and type1 structures. The monolayer shear modulus at each boundary was calculated and found to be 93–98% lower than the bulk value for six grain boundaries with csl structure and found to decrease with increasing twist angle. The critical shear stress was also calculated for eleven boundaries with csl structure and found to roughly decrease with increasing twist angle.     

Morphology and microstructure of all-epitaxial ferroelectric tri-layered (Bi,La)4Ti3O12/Pb(Zr0.4Ti0.6)O3/ (Bi,La)4Ti3O12 thin films on SrTiO3(011)

The morphology and microstructure of all-epitaxial (Bi,La)₄Ti₃O₁₂/Pb(Zr_0.4Ti_0.6)O₃/(Bi,La)₄Ti₃O₁₂ (BLT/PZT/BLT) tri-layered ferroelectric films, grown on (011)-oriented SrTiO₃ (STO) substrates by pulsed laser deposition, are investigated by transmission electron microscopy (TEM). X-ray diffraction and electron diffraction patterns demonstrate that the epitaxial relationship between BLT, PZT and STO can be described as ; . Cross-sectional TEM images show that the growth rate of BLT is nearly two times that for PZT at the same growth conditions, and 90° ferroelectric domain boundaries lying on {110} planes are observed in the PZT layer. The 90° ferroelectric domains in the PZT layer extend up to 600 nm in length. Long domains penetrate into the neighboring columnar grain through the columnar grain boundary, whereas others are nucleating at the columnar grain boundaries. The roughness of the PZT/BLT interfaces appears to depend on the viewing direction, i.e., it is different for different azimuthal directions. Planar TEM investigations show that the grains in the top BLT layer have a rod-like morphology, preferentially growing along the [110]_BLT direction. The grain width is rather constant at about 90 nm, whereas the length of the grains varies from 150 to 625 nm. These morphological details point to the important role the crystal anisotropy of BLT plays for the growth and structure of the tri-layered films. PACS 81.15-z; 68.37.Lp; 77.84.-s     

Twin boundaries with 9R zone in Cu and Ag studied by quantitative HRTEM

When highly inclined against the {111} plane of the coherent twin boundary, 3 110 tilt boundaries in Cu or Ag have a complex structure. As the boundary plane approaches the symmetrical {211} orientation, the grain boundaries decompose into two phase boundaries. Between these phase boundaries the metal adopts a rhombohedral crystal structure, denoted as 9R. Not the {211}-oriented boundary, but a boundary inclined by 8° against {211} has the minimum energy in this family of grain boundaries with 9R zone. Using high resolution transmission electron microscopy, we have studied the atomistic structure of this special boundary. An iterative structure refinement based on quantitative image analysis reveals the atomistic structure of the grain boundary at a well-defined level of confidence. Comparing the refined grain boundary structure with a model obtained by molecular statics calculations exposes small, but significant discrepancies. These probably arise because in the model the stacking fault energy is too small and the short distance repulsion is too weak. Grain boundaries of equivalent geometry in Ag and Al exhibit different widths of the 9R zone. Experimental observations support a theory relating the equilibrium width of the 9R slab to the stacking fault energy and the elastic properties of the material.Presented at the Workshop on High-Voltage and High Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

Hardness and slip systems of orthorhombic hen egg-white lysozyme crystals

Hardness and slip systems by an indentation method were investigated on different habit planes of orthorhombic hen egg-white lysozyme (O-HEWL) crystals containing water. A dependence of the hardness on the water-evaporation time exhibits three stages as incubation, transition and saturated ones, as tetragonal (T)-HEWL crystals reported previously. The hardness values of (1 1 0), (0 1 0) and (0 1 1) habit planes of O-HEWL in the incubation stage or wet condition exhibits 6, 8 and 10 MPa, respectively. The hardness depends on indented planes but it is independent of the air-humidity and crystal volumes. These values correspond to the intrinsic hardness for O-HEWL crystals containing water. In the incubation stage, the slip traces are clearly observed around the indentation mark and the corresponding six kinds of slip systems are identified to be {0 1 1}<1 0 0>, {1 1 0}<1 1 0>, {0 1 1}<0 1 1>, {1 1 0}<0 0 1>, {1 0 0}<0 0 1> and {0 1 0}<0 0 1>.     

Surface modification of highly oriented pyrolytic graphite by reaction with atomic nitrogen at high temperatures

Dry etching of {0 0 0 1} basal planes of highly oriented pyrolytic graphite (HOPG) using active nitridation by nitrogen atoms was investigated at low pressures and high temperatures. The etching process produces channels at grain boundaries and pits whose shapes depend on the reaction temperature. For temperatures below 600 °C, the majority of pits are nearly circular, with a small fraction of hexagonal pits with rounded edges. For temperatures above 600 °C, the pits are almost exclusively hexagonal with straight edges. The Raman spectra of samples etched at 1000 °C show the D mode near 1360 cm⁻¹, which is absent in pristine HOPG. For deep hexagonal pits that penetrate many graphene layers, neither the surface number density of pits nor the width of pit size distribution changes substantially with the nitridation time, suggesting that these pits are initiated at a fixed number of extended defects intersecting {0 0 0 1} planes. Shallow pits that penetrate 1-2 graphene layers have a wide size distribution, which suggests that these pits are initiated on pristine graphene surfaces from lattice vacancies continually formed by N atoms. A similar wide size distribution of shallow hexagonal pits is observed in an n-layer graphene sample after N-atom etching.     

Modeling Anisotropic Single Crystal Growth Kinetics in Liquid Phase Sintered α-Al2O3

Single crystals of -alumina with {0001}, {11 and {01 2} orientations were embedded in a matrix of liquid phase sintered alumina to study -alumina single crystal growth in the presence of a calcium aluminosilicate liquid. Growth on the {11 0} and {01 2} surfaces was observed to occur by a diffusion-controlled mechanism while growth on the basal surface was controlled by two-dimensional nucleation. Growth models, which incorporate the change in matrix grain size, were derived and shown to fit the growth kinetics.     

The energy dependence of the {111} and {100} tilt grain boundaries on disorientation angle in Ni<Subscript>3</Subscript>Al

Dependence of the energy of {111} and {100} boundaries on tilt angle θ is investigated by the molecular dynamics method using intermetallic Ni₃Al as an example. It is shown that the energy (per single grain dislocation) of grain boundaries in the {111} planes is higher than that of the grains located in the {100} planes. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 33–35, November, 2007.