P.M. Bronsveld

Superlattice dislocations in the L12 ordered structure of Cu2NiZn

Abstract Dissociation criteria for superlattice dislocations on {111} planes have been applied to the ordering alloy Cu2NiZn. The calculations showed that the splitting of a superlattice dislocation into two superlattice Shockley partials, with Burgers vector of type 1/3a 0⟨211⟩, bounding a superlattice intrinsic stacking fault. (SISF), is energetically less favourable than the splitting inta two unit dislocations, with Burgers vector of type 1/2a 0⟨110⟩, separated by an antiphase boundary (APB). This theoretical prediction is in agreement with the type of superlattice dislocation observed in the electron microscope. It is rather difficult to predict a priori whether each unit dislocation is split into two Shockley partials separated by a complex stacking fault (CRF) because the cut-off parameters for the dislocation cores are not known with sufficient accuracy. The splitting into two Shockley partials could not be resolved in the electron microscope, which means that either the separation between the Sho...

HRTEM study of Co7W6 and its typical defect structure

The tetrahedrally close-packed structure of Co7W6 (m-phase) was studied by HRTEM. A mas- sive presence of random sub-unit cell twins was detected. These planar defects were characterized in detail and the twinning probability at potential mirror planes was established. High-precision image simulation of the bulk structure and defect region was performed, using a simulated evolution optimization strategy to refine the imaging conditions. Ab initio calculations were carried out in order to determine the energy involved in twinning, as well as the distortion induced in the structure. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Catalytic Graphitization of Wood‐Based Carbons with Alumina by Pulse Current Heating

Abstract Japanese cedar was preheated at 500°C and subsequently mixed with 40 µm Al2O3 particles. A pulse current heating method was used for a 5‐min carbonization step under a pressure of 50 MPa in order to promote the graphitization at temperatures between 2000 and 2200°C. The samples were analyzed in an analytical transmission electron microscope equipped with a GATAN Imaging Filter, in a high resolution transmission electron microscope and in a scanning electron microscope. Transformation into well‐ordered graphite could be enforced by the intermediate reaction of Al2O3 and carbon to plate‐like Al4C3. This latter compound dissociates under the proper CO pressure and temperature into Al vapor and solid graphite. The addition of Al2O3 and the pressurized heating device improve the graphitization in comparison with the effect of temperature alone. The electron microscopic observations are supported by XPS and XRD spectra.

Microstructural analysis of hot isostatically pressed Al-SiC

The difference between extruded and hot isostatically pressed (HIP) Al6061 both with a T6 final heat treatment and with a 30 wt.% SiC particulate reinforcement is one of densification. The higher density of the HIP material is not translated into a stronger material. The Mg2Si precipitation is favoured by the presence of the SiC reinforcement in both samples. HIP powder metallurgy material reveals clearly the original powder particle boundaries, in contrast to the more smeared-out stringers in the extruded material. Cleavage of SiC particles by neighbouring SiC particles is observed in the HIP samples, making them less ductile.

Formation of silicon carbide nanorods from wood-based carbons

Abstract Man‐made ceramic wood similar to petrified wood found in nature can be used at high temperature as the high oxidation rate of carbon above 500°C is suppressed by a 1 µm thin SiC coating similar to the shuttles heat shield. Possible applications are in the field of energy production, e.g., gas filters. In this paper, Japanese cedar was carbonized at 700°C, vacuum infiltrated with tetraethylorthosilicate (TEOS), and heated at 1,500°C for 30 min using a pulse current heating device. A FIB–SEM dual beam microscope combination was used to investigate the SiC/C composite micro‐texture. By milling away successive slices of material approximately 20 nm in thickness, it was possible to confirm that a reaction layer of SiC, about 60 nm in thickness, covers uniformly the surface of all cell walls. Furthermore, SiC nanorods were observed, growing more or less randomly in the charcoal pores. Cross‐sectional SEM images of the SiC nanorods showed that either hollow or solid structures could be formed, having diameters ranging from 100 to 500 nm. The SiC nanorod formation is being initiated by a catalytically operating SiC particle in the SiC coating. TEM analysis confirmed that they were grown along the [1 1 1] direction and indicated that an additional carbon layer covers the external surface of the rods making the rods more resistant against electron beam damage.

Effect of third elements on pseudoelastic behavior in Fe3Al single crystals

The pseudoelastic behavior of Fe3Al single crystals doped with an extra element (e.g. Ti, V, Cr, Mn, Co, Ni, Si, Ga, Ge) was investigated. In binary Fe-23.0at.%Al crystals with the D03 structure, 1/4[111] superpartial dislocations moved independently dragging the nearest-neighbor anti-phase boundaries (NNAPB) during loading. During unloading, the NNAPB pulled back the superpartials decreasing its energy resulting in a giant pseudoelasticity of which the recoverable strain is about 5 %. Addition of a third element significantly affected the pseudoelastic behavior of Fe3Al single crystals. Mn- or Ga-doped crystal demonstrated a giant pseudoelasticity. In particular, Ga-doping was found to be effective in the enhancement of the pseudoelasticity. On the other hand, the amount of strain recovery decreased upon doping of the other elements. The frictional stress of the superpartials, the back stress of the NNAPB and ordered domain structure in the crystals changed upon doping, which was closely related to the pseudoelastic behavior.

Evolution of nanaostructure of FeNi(Ti/Cr)N alloy during phase cycling

A possibility to manipulate the microstructure of cold rolled (down to less than 1% of the initial thickness) Fe94Ni4Ti2 and Fe93 Ni4 Cr3 foils via inter-phase cycling by nitriding and de-nitriding is investigated. The grains in the as-rolled material had a dominating (001)[110] texture and contained a complicated internal nanostructure due to a dislocation network. The interphase cycling α↔γ’-Fe4N and α↔ε-FexN (x~3) was investigated as a tool to change or destroy the texture. We observed that the α↔γ’-Fe4N phase transformations weaken but do not erase the texture. A stronger reduction of the texture was obtained in the α↔ε-cycling, where grains with a new orientation appear in XRD scans. During the α→γ’ phase transformation a lamella structure is formed which is coarsening with the number of cycles. It was found that the rates of the α↔γ’ and α↔ε interphase transitions were slowing down with the number of cycles. The observation is explained by an increase of kinetic barriers while removing/transforming the rolling-induced defects during the cycling.

X-ray stress analysis of neon implantation in laser-treated 304 stainless steel

Abstract A powerful laser beam focused on an austenitic stainless steel introduces a considerable amount of tensile stress in the first 5 μm as measured by X-ray diffraction. Microscopic analysis reveals a solidification structure 50 μm deep with a strongly oriented grain size of less than 5 μm induced by the high self-quenching rate. Subsequent implantation with a rare gas introduces a layer of pressurized gas bubbles at a depth of only 80 nm and a deformation layer as far down as 250 nm. Although the shear stress at the bubble-matrix interface transforms the metastable f.c.c. crystal structure locally into a martensitic b.c.c. phase, the actual transformed fraction is small. Mainly the implantation but also the transformation add in reducing the tensile stress by as much as 25% in total.

Effects of Cl+ and F+ implantation of oxidation-induced stacking faults in silicon

Three implantation effects were investigated in floating‐zone‐grown silicon: (a) the effect of Cl+ implantation resulting in the shrinkage of oxidation‐induced stacking faults; (b) the effect of F+ implantation giving rise to defaulting of the 1/3[111] Frank dislocations into 1/2[110] perfect dislocations due to the interaction with 1/6[112] Shockley partials; (c) the effect of a combined F+ and Cl+ implantation of dislocation motion. Notwithstanding the limited magnification of double‐crystal x‐ray topography it proves to be a valuable technique for determination of the effects of implantation, as removal of F; the oxidation layer is unnecessary for observation of the oxidation‐induced stacking faults. Moreover, the role of the oxide layer in the Si‐SiO2 interface can be followed more appropriately.

ANELASTIC RELAXATION IN AMORPHOUS PD39.5NI39.5P21

In order to study the structural relaxation in an amorphous Pd39.5Ni39.5P21 specimen we measured the isothermal change in the internal friction of the asquenched material with an inverted Ke-type torsion pendulum. This change reflects a reduction in the amount of free volume which is quenched-in during manufacturing. We also measured the anelastic relaxation behaviour of thoroughly annealed samples. At 498 K it turns out that both processes are governed by a spectrum of relaxation times with the anelastic relaxation process having its main peak at a value which is slightly larger than that found for the disappearance of free volume.