R. Nowak

Factors affecting wettability and bond strength of solder joint couples

The paper discusses the scientific understanding of the role of interfacial phenomena in joining of dissimilar materials using liquid-phase-assisted processes. From the example of the Sn-alloy/Cu system, it is demonstrated that interaction in the liquid solder/substrate couples is accompanied by a number of complex interfacial reactions leading to significant changes in the structure and chemistry of interfaces (solder/substrate, solder/environment, substrate/environment) and remaining solder layer that finally influence the mechanical properties of solder joints. The experimental data on wetting behavior, interface characterization, and mechanical properties of different solder/metal substrate couples are analyzed in order to display the role of such factors as time and temperature of interaction, environment (protective atmosphere, flux), presence of oxide films on interfaces, alloying additions to a solder, formation of interfacial phases, and porosity.

Structural Characterization of Reaction Product Region in Al/MgO and Al/MgAl2O4 Systems

The reaction product region, formed between molten aluminium and MgO and MgAl2O4 single crystals of three different crystallographic orientations, was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectrometry (EDS). The Al/MgO and Al/MgAl2O4 couples were produced under ultra high vacuum at 800, 900 and 1000°C. The observations proved the redox reactions of Al with both MgO and MgAl2O4. Independently of crystallographic orientation of initial oxide single crystals, the reaction product region (RPR) was formed and it was built of oxide particles surrounded by continuous metallic phase. For Al/MgO couples, the RPR was composed of two layers, where in the first layer, the oxide phase was Al2O3 while in the second layer, the MgAl2O4 was identified. In the case of Al/MgAl2O4 couples, a single layer was distinguished and only the Al2O3 phase was recognized.

Interfacial interactions between liquid Ti–Al alloys and TiB2 ceramic

High-temperature interactions of liquid Ti and Ti74.3Al25.7, Ti49.3Al50.7, Ti26.5Al73.5 and Ti3.5Al96.5 (at.%) alloys with TiB2 ceramic are studied using the sessile drop technique. Molten Ti and Ti–Al alloys show a very good wetting of TiB2. A homogeneous TiB layer with a thickness of about 20 μm forms at the Ti/TiB2 interface. An approximately 200-μm-thick layer of densely packed TiB whiskers is observed at the interface of a Ti74.3Al25.7/TiB2 couple. The Ti49.3Al50.7 liquid penetrates the grain boundaries in the TiB2 ceramic and disconnects the grains in the near-interface region. The Ti26.5Al73.5 melt does not disconnect the TiB2 grains, but penetrates the grain boundaries over a distance of about 300 μm from the interface. Ti3.5Al96.5 just fills the pores in the ceramic up to a distance of about 120 μm from the interface.

Surface tension and density of Si-Ge melts

In this work, the surface tension and density of Si-Ge liquid alloys were determined by the pendant drop method. Over the range of measurements, both properties show a linear temperature dependence and a nonlinear concentration dependence. Indeed, the density decreases with increasing silicon content exhibiting positive deviation from ideality, while the surface tension increases and deviates negatively with respect to the ideal solution model. Taking into account the Si-Ge phase diagram, a simple lens type, the surface tension behavior of the Si-Ge liquid alloys was analyzed in the framework of the Quasi-Chemical Approximation for the Regular Solutions model. The new experimental results were compared with a few data available in the literature, obtained by the containerless method.

Sessile drop study of Gd-Ti monotectic alloys on ceramic substrates: phase transformations, wetting, and reactivity

Gd–Ti monotectic alloys were studied in a wide temperature range by the sessile drop technique. The eutectic and monotectic transformations in Gd–Ti alloys upon heating were in situ monitored by a high-speed and high-resolution CCD camera. Our findings suggest that the existing Gd–Ti phase diagram significantly underestimates the monotectic and binodal temperatures in this system. The wetting behavior and the high-temperature interaction of the Gd–Ti melts with yttria and yttria-stabilized zirconia substrates were investigated.

Microstructure Characteristics of the Reaction Product Region Formed due to the High Temperature Contact of Molten Aluminium and ZnO Single Crystal

Interface reactions between liquid aluminium and ZnO single crystal substrates of <1-100> orientation (at 1273 K under vacuum) were examined using scanning and transmission electron microscopy techniques. The substrates were subjected to the “pushing drop” tests when liquid is deposited from the capillary on the substrate surface and then, after appropriate contact time, it is pushed away. After short time of interaction with <1-100>ZnO substrate, three phases were detected: α-Al2O3, the alumina of unknown type and ZnAl2O4 spinel formed due to the solid state reaction between Al2O3 and ZnO.

Interactions between Superalloys and Mould Materials for Investment Casting of Turbine Blades

The need of increased efficiency of industrial gas turbines comes also through the improvement of the composition of superalloys (addition of new solutes) and of the manufacturing technologies involved in the investment casting process of the turbine blades. Thus, the knowledge of the interactions between the ceramic materials used for casting and the molten superalloys must be deepened in order to minimize the formation of internal defects, to improve the casting surface and to optimize finishing and casting operations. In this work, a study of the wetting behaviour of some Ni- or Co -based superalloys, used for the fabrication of turbine blades, has been performed with reference to the interactions of these alloys in the molten state with the silica-aluminate based ceramic materials forming the shell or the core in the casting process. Wettability tests have been performed by means of the sessile drop method at 1500°C; the characterization of the interfaces between the molten drop and the substrates has been made on solidified sessile drop samples by SEM/EDS analysis to check the final characteristics of the interfaces. The results are discussed in terms of chemical interactions in relation to the processing parameters and as a function of the surface and interfacial energetic properties of the systems.