A. Katsman

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.

Penetration of oxygen along grain boundaries during oxidation of alloys and intermetallics

Fast penetration of oxygen into grain boundaries and intergranular oxidation of β-NiAl has been observed. Since the solubility of oxygen in NiAl is virtually nil, special ways of oxygen ingress at grain boundaries have to be presumed. Selective intergranular oxidation of binary alloys and fast penetration of oxygen along grain boundaries were analyzed by computer simulation. Interdiffusion caused by consumption of the less-noble component by oxidation at the metal-oxide interface leads to deviation of the alloy composition from the original value. When the diffusivity of the less-noble component is higher than the diffusivity of the other component, a grain-boundary Kirkendall effect may lead to void-chain formation. Experimental evidence for this phenomenon is presented. The deviation in composition and void formation were considered as processes influencing the effective oxygen diffusivity. Both processes were found to allow penetration of oxygen as fast as grain-boundary interdiffusion occurs. In addition, oxygen penetration during intergranular internal oxidation when oxides form at voids beneath the metal-oxide interface was analyzed and treated as a self-propagating process. In this case, fast oxygen penetration is accompanied by fast internal oxide formation and fast displacement of the metal-oxide interface.

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.

Precipitation- and stress-influenced coarsening in Mg-based Mg–Zn–Sn–Y and Mg–Zn–Sn–Sb alloys

Extensive experimental research work has been carried out to investigate precipitation peculiarities in Mg–Zn–Sn-based alloys during aging at different temperatures. This in-depth research was conducted on Mg–4.4wt%Zn–4.0wt%Sn–0.6wt%Y and Mg–4.4wt%Zn–4.4wt%Sn–1.1wt%Sb using x-ray diffraction (XRD), transmission electron microscopy (TEM) including high-resolution TEM, and scanning electron microscopy (SEM) equipped with an energy-dispersive x-ray spectrometer (EDS). It was found that, first, a hexagonal close-packed (hcp)-MgZn 2 phase nucleates and grows in the form of needles having coherent interphase boundaries with α-Mg matrix. Then the face-centered cubic (fcc)-Mg 2 Sn-phase nucleates heterogeneously, mainly at the tips of MgZn 2 needles. A very certain mutual orientation of crystal lattices of MgZn 2 , Mg 2 Sn, and α-Mg matrix was revealed. The orientation of Mg 2 Sn precipitates is perpendicular to that of MgZn 2 needles. They grow in the form of plates parallel to the basal planes of α-Mg matrix. Two-phase T-like particles are very typical of alloys aged for 1 to 16 days at 175 to 225 °C. The width/length ratio of MgZn 2 needles inside T-like particles differs substantially from that found in single needles. The elastic/surface energy balance of needles and its influence on the morphology and coarsening behavior has been analyzed.

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.

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.

Microstructure and phase composition in a die cast Mg–Nd alloy containing Zn and Zr

Abstract The microstructure of the as-cast Mg-3.1Nd-0.45Zr-0.25Zn (wt.%) alloy was investigated. It consists of α-Mg matrix, a divorced intergranular phase and small Zr-containing particles. Scanning electron microscopy equipped with energy dispersive spectroscopy analysis on transmission electron microscopy samples was carried out in order to eliminate the Mg matrix contribution to the quantitative analysis results and to reveal the intergranular phase composition which was found to be Mg12–xZnxNd (x ∼ 0.31) (space group I4/mmm) with a bct crystal structure. The lattice parameters (a = 1.029 nm and c = 0.591 nm) were found using X-ray diffraction with the least squares refinement technique. The combination of different characterization techniques indicates that Mg atoms are substituted by Zn atoms without a change in the crystal structure.

Influence of Additional Elements (Si, Ti and B) on the Castability, Corrosion and Mechanical Properties of A201 Alloys

The additions of silicon, titanium and boron to the Al-4.97 wt% Cu-0.56 wt% Ag based alloys (A201) were used to improve the castability, mechanical properties and microstructure. The four alloys: A201, A201+Si, A201+Ti+B and A201+Si+Ti+B were investigated in the as-cast, solution treated at 550 °C for ~20 h and aged at 170 °C up to 32 days conditions. The effect of precipitation on the mechanical properties of the alloys were investigated by a combination of HRSEM, TEM and EDS. Addition of Si coarse the grain structure and changed the amount of the eutectic regions and affect the corrosion behavior. Additions of Ti and B refine the grain structure, produce the secondary phases which influenced the precipitation sequence then slowed down the age hardening process. Additions of Ti and B with Si affect the microstructure of A201 to contain more pores and also decrease the corrosion resistant.