L. Levin

Controlled formation of surface layers by pack aluminization

Abstract Aluminum diffusion coatings on Armco iron were produced by a pack aluminization technique. Two different beds were applied: a high-activity bed containing pure aluminum as the coating source, and a low-activity bed containing Fe2Al5 as the source. In this work, the master alloy itself was also prepared by a pack aluminization procedure using aluminum and iron powders. The effect of the bed content on the coating was examined. With the low-activity bed, the desired FeAl was formed as the outermost coating layer. A set of complementary examination methods—XRD, SEM, EDX, RBS and CEMS—were employed, providing detailed information on the coating produced.

Nickel–aluminide coating of TiAl by a two-stage process

Abstract Nickel–aluminide protective coatings on the surface of TiAl alloys were produced by pack aluminization of preliminarily nickel-coated samples. The deposition of the nickel layer from a solution was carried out initially by an electroless, and subsequently by an electrolytic process, providing good adhesion of Ni to the TiAl alloy surface. Two bed regimes were used: a high-activity one (with pure aluminum as the coating source) and a low-activity one (with Fe2Al5 as the source). The effect of the bed composition and of the thickness of the preliminary Ni layer was examined. The layers were examined by optical microscopy, SEM with EDS and X-ray analysis. The coatings obtained with the high-activity bed contained Al-rich phases of low symmetry, making for high brittleness and poor adhesion of the surface layers; those obtained with the low-activity bed, with NiAl as the outermost layer, are oxidation resistant at high temperatures and have a ductile structure with good adhesive properties.

Time dependence of stress and hillock distributions during electromigration in thin metal film interconnections

Abstract Evolution and relaxation of stresses during electromigration (EM) in thin metal films are described by a diffusion-like equation with a sink term that accounts for stress relaxation by plastic flow. Plastic flow responsible for growth of hillocks is assumed to proceed by diffusional creep with a given threshold stress. Distribution of the compressive stresses along the interconnect line was derived for the Blech-Kinsbron test geometry. With these results, relations between EM rate, hillock formation rate and EM-induced internal stresses were developed.

The mechanisms of phase transformation in diffusion couples of the Cu-Si system

Abstract The mechanisms of diffusional transformation in a series of couples belonging to the Cu-Si system were studied. The couples consist of elemental materials or intermediate phases. For the Cu-Si couple it was found that the diffusion layer comprises the Cu 3 Si (η″) phase solely. For the Cu 5 Si(γ)-Si couple the diffusion zone contains the η″-phase subdivided into two morphologically different parts. In the cu-η″ and Cu-γ couples, formation of a narrow reaction zone with fragmentary structure took place. Models for processes occurring in each of the couples during diffusional annealing were put forward. It was shown that the dual microstructure of the η″-phase in the γ-Si couple can be attributed to different formation mechanisms on either side of the growing phases.

The origin of a new phase observed during quenching of a TiAl-2Fe alloy

Abstract Microstructure formation in the Ti–49.6at.% Al–1.9at.% Fe alloy after fast and slow cooling from 1200°C was examined and analyzed by X-ray diffraction, scanning and transmission electron microscopy combined with electron probe microanalysis. Formation of a previously unidentified τ′2 phase is reported and discussed. It has a tetragonal lattice with lattice parameters a=11.5 A, c=13.8 A. Possible orientation relationships between the τ′2 phase and the parent FeTi(Al) are [010]τ′2 ∣∣ [001]FeTi(Al) [100]τ′2 ∣∣ [110]FeTi(Al) and [001]τ′2 ∣∣ [ 1 10]FeTi(Al).

On the problem of high-rate reactive diffusion

Competitive mechanisms for fast diffusional growth of an interphase layer during the initial stage of reactive diffusion were analysed. The effect of fast growth is attributable to grain boundary diffusivity and/or accelerated volume diffusion due to vacancy supersaturation. In analysing the latter case, vacancy source movement and the space distribution of non-equilibrium vacancies were considered. The system parameters which determine the governing mechanism of the process were obtained. It was shown that if, due to vacancy supersaturation, bulk diffusion dominates, the process is intensified by the grain boundary diffusion.

Friction and wear of nitrogen-implanted glassy carbon

Abstract Scratch friction and wear behavior of a glassy carbon (GC) implanted with a high dose of nitrogen as studied when in contact with a steel ball and with diamond indenter. A sliding velocity of V = 0.2 mm s −1 and loads from 0.5 to 4 N during 50–300 cycles were applied in laboratory air (humidity ~50%). Smoother and more closely compacted surfaces than in the virgin state were obtained at doses of 1 × 10 16 –1 × 10 17 ions cm −2 , but surfaces became heavily blistered at higher ion dose levels (1–1.5 × 10 18 ions cm −2 ). It was shown that the enhancement of wear resistance is associated with the amorphization of the implanted layer, which leads to a rise in strength of implanted GC and to a decrease in the rate of crack propagation in a hostile environment. Crescent-shaped cracks were observed on the surface of virgin GC subjected to friction with a steel ball. At higher doses of implantation, steel wear particles adhered to the rough blistered surface in contact with the steel ball, and bubbles burst following contact with the diamond indenter. The implant dosage needed for maximum wear resistance was found to be in the vicinity of 1 × 10 16 ions cm −2 . The influence of the gradual change in the bonding mode with the of implantation dosage (from sp 2 , characteristic for graphite and GC, to sp 3 , characteristic for diamond) on the tribological properties of the carbon materials is discussed. The synergetic influence of implantation and environmental humidity was adduced to explain the observed behavior of GC in friction and wear.

A new model of the pest-like disintegration of intermetallics due to intergranular oxidation

Abstract A new diffusional model of disintegration of intermetallics during oxidation was developed. The model is concerned with intermetallics in which the pest phenomenon occurs as a result of intergranular oxidation. An example of such intermetallics is NbAl 3 . It was assumed that, in these cases, oxidation leads to an excess of vacancies at grain boundaries. The migration of the excess vacancies to the surface facilitates the diffusional growth of grain boundary cracks. The steady state vacancy concentration ahead of the crack tip was found from the diffusional equation compromising the vacancy sources and sinks. The rate of crack propagation was found through the vacancy flux to the crack tip. Two different steady state regimes of crack growth (fast and slow) were established, and the temperature of transition from one regime to the other was determined. The fast regime was related to the disintegration of intermetallics containing a substantial number of structural vacancies. The slow regime corresponds to the case when no disintegration occurs. The rate of diffusion crack propagation is only slighly affected by the gradients of the internal stresses and of the equilibrium vacancy concentration in the oxidation zone. The decisive role of the vacancy excess generation in the fast crack diffusion propagation was revealed. Activation energies of the process were estimated.

Driving force for grain boundary migration during electromigration: Effect of elastic anisotropy

Electromigration in thin metal films leads to internal stresses. Elastic crystal anisotropy may cause a distinct gradient of these stresses across the grain boundary if the adjacent grains have different orientations relative to the grain boundary and to the substrate. The stress gradient gives rise to a driving force for grain boundary migration. This driving force is proportional to the crystal anisotropy parameter and to the dilatation caused by electromigration. At typical values of the parameters the driving force is comparable to the driving force caused by the curvature when the radius of curvature is about 10{micro}m. The anisotropy driving force may cause low temperature recrystallization, especially in the regions where internal stresses are about the threshold value, or exceed it, and it may lead to an increase in the number of symmetrical grain boundaries during electromigration in the near-anode zone of interconnect lines.

Distribution of elements in the surface layer of amorphous AlLaNi ribbons

Abstract Amorphous ribbons of Al91La5Ni4 were produced by melt spinning. Ribbons 3 mm wide and 20 μm thick were obtained by rapid quenching of the melt in vacuo from 1273 K onto a Cu drum surface. The depth distribution of the elements in the surface layer of the specimen was studied by Auger electron spectroscopu combined with ion sputtering, and by electron probe microanalysis through varying the energy of the primary electrons. It was shown that the La and Ni contents of a surface layer several hundreds of nanometers thick are considerably lower than those in the bulk. La and Ni depletion was discovered on both faces (contacting and free) of the amorphous ribbons. The data obtained were explained on the basis of the concept of different surface activities of alloy components in the liquid state. In the process of melt spinning, a new surface is formed and the components in the near-surface layer are redistributed. As a result of the extremely limited duration of the quenching, there is insufficient time for the redistribution of the elements in the solid state. Therefore, the observed component depth distribution is the result of the “freezing” of the liquid-state near-surface layer.