Volume 93, Issue 6

Cu/Cu joints were produced in the temperature range 200 – 360 °C by the diffusion soldering process, using the In-Sn eutectic alloy as solder material. Two intermetallic phases (IPs) appeared in the interconnection zone, η (Cu-poor IP) and ζ (Cu-rich IP), but below 200 °C only η grew. Three mechanical tests were performed to characterize the mechanical behaviour of diffusion-soldered Cu/In – 48 Sn/Cu joints: the tensile and shear strengths and the (nanoindentation) hardness were measured. The results show that the ζ phase has better mechanical properties than the η phase. The hardness measurements indicate that the most brittle IP layer is the η layer. The η phase has two crystallization morphologies, involving a weak interface between the sublayers composed of relatively small grains and relatively large grains, respectively. The ultimate shear and tensile strengths show similar tendencies for all joints. The ultimate shear strength of the joints improves almost 3 times when the η phase is converted into the ζ phase by a solid-solid diffusion reaction process. The surface of the In– 48 Sn films was etched before the diffusion soldering to improve the mechanical properties. The resulting joints, if composed of a single ζ layer, can withstand stresses above 155 MPa.

The development of methods for the ultimate cast structure refinement is among major tasks of light metals metallurgy. The production of billets with the ultimately refined grain structure and, therefore, the improved plasticity extends the capabilities of continuous casting and hot deformation, allowing the production of defect-free billets and deformed semi-products. Moreover, the fine structure of a billet is inherited by the deformed metal and improves its resource characteristics. This paper summarizes the results of extended research performed in VILS during last 30 years, in particular in the field of the formation of the ultimately refined (non-dendritic) grain structure during casting light alloys with the aid of ultrasonic (cavitation) melt treatment.

The long-range diffusional phase transformation in multicomponent, multi-phase systems has been modelled based on the postulated principle of maximum dissipation rate of Gibbs energy. Use of the principle eliminates the need of a separate model for the interface velocity or assumptions about the interface composition. The principle is demonstrated for the austenite-to-ferrite transformation during cooling in an Fe-3% Cr-0.2% C alloy.

An investigation has been carried out to study the kinetics of growth of the intermetallics in the Cu–Sn system. The diffusion couples of Cu and Sn were prepared and annealed in the temperature range 367 – 510 °C. The Cu 3 Sn and Cu 4 Sn intermetallics were observed to form at temperatures above the peritectic reaction isotherm at 415 °C. The Cu 3 Sn and Cu 6 Sn 5 intermetallics formed in the diffusion couples below 415 °C. The growth distance of each intermetallic was measured as a function of time and temperature. The growth of the intermetallics follows the parabolic growth law. In general, the chemical diffusion of Sn in Cu has been observed to control the growth of the intermetallics. The diffusivity value is also close to the interdiffusion coefficient of Sn and Cu for the δ phase. The concentration –distance profile has clearly demonstrated the formation of these intermetallics in the diffusion couple experiment.

The atomic composition of a metastable β ″ phase precipitate, which is responsible for strengthening of Al –Mg – Si alloys in a precipitation-hardening process, was studied by means of Vickers microhardness tests, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) analyses. On the basis of the present TEM-EDX experiments and DSC measurements, our study revealed that the β ″ phase precipitate formed in Al –Mg – Si alloys has a high silicon concentration. It was also concluded that the precipitation behavior of the β ″ phase coincides with the precipitation occurring in the binary Al –Si alloy.

It is proposed to measure the interfacial tension of metastable immiscible alloys, such as Co–Cu or Fe – Cu, by the oscillating drop technique, using electromagnetic levitation. The undercooled melt of such alloys shows a miscibility gap and the levitated liquid sample forms a compound drop. The oscillations of compound drops are due to both, the surface tension of the outer liquid and the interfacial tension between the two liquids. By analysing the oscillation spectrum, these two quantities can be determined.

The initial oxidation of polycrystalline α-Fe and ε-Fe 2 N 1– x was investigated with X-ray photoelectron spectroscopy, ellipsometry and high-resolution transmission electron microscopy. Oxidation was performed at temperatures ranging from 300 to 600 K. The oxidation kinetics were described quantitatively with the coupled currents model according to Fromhold and Cook, adopting time-dependent work functions at the metal/oxide and oxide/oxygen interfaces. The evolution of the work functions of α-Fe could be related to the change of the oxide film composition, which evolved with increasing film thickness from approximately FeO to a composition close to Fe 3 O 4 . Upon oxidation of ε-Fe 2 N 1– x , the N atoms accumulated underneath the oxide film, which could lead to an N concentration larger than the maximum equilibrium solubility of nitrogen in ε-Fe 2 N 1 –x . This excess nitrogen hinders the oxidation and transport of Fe atoms and is associated with a negative charge, which led to a slower initial oxidation. Upon prolonged oxidation, the oxidation rate of ε-Fe 2 N 1 –x could be related to the presence of an Fe 1 – dO-like oxide or oxynitride in the oxide film.

This paper deals with the synthesis of MgAlON ceramics. The thermodynamic properties of this system were evaluated and the phase stability diagram of MgAlON was established. The synthesis of this material was successfully carried out by a careful choice of the experimental parameters in accordance with the phase stability diagram. The procedure adopted was to subject the starting materials to sinter under 25 MPa. A microstructure evaluation of the synthesised material was carried out. The mechanical properties of MgAlON were determined. The synthesised material was also subjected to X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy analyses. The crystallographic data of MgAlON were generated as a result of the present studies. A JCPSD file of MgAlON has been suggested.

The kinetics and morphology of the oxidation process of magnesium-aluminium oxynitride (MgAlON), aluminium oxynitride (AlON), O’SiAlON–ZrO 2 , and BN-ZCM have been studied in the temperature range 1373 – 1773 K. Oxidation experiments with powder and plate samples of the above materials have been carried out in air. MgAlON shows the best resistance to oxidation at lower temperatures (<1473 K), whereas at higher temperatures (~ 773 K), AlON shows the best resistance. O’SiAlON–ZrO 2 shows very good oxidation resistance up to 1673 K. But its oxidation rate increases strongly above 1673 K, presumably due to the formation of liquid phase. BN–ZCM has the poorest oxidation resistance due to the evaporation of B 2 O 3 . The activation energies for the chemical oxidation reaction of AlON, MgAlON, and O’SiAlON–ZrO 2 are 214, 330 and 260 kJ/mol, respectively. The overall diffusion activation energies for AlON, MgAlON, O’SiAlON–ZrO 2 and BN–ZCM are 227, 573, 367 and 289 kJ/mol, respectively.

Warm rolling experiments on AISI 1015 carbon steel were performed using a duo-reversible mill. The samples have been studied after warm rolling with three deformation ratios by light microscopy and scanning electron microscopy. During the warm rolling pearlite grains are subjected to two simultaneous processes: (i) deformation of the mosaic blocks followed by the separation of some blocks which then appear in the microstructure as fine lamellae pearlite grains, and (ii) deformation of the cementite lamellae which tend to a globular shape within the remaining grains. A slight variation of the hardness across the longitudinal section of the material was evidenced.

The elastic modulus and damping capacity of woodceramics have been investigated on a dynamic mechanical thermal analyzer. The experimental results reveal that the elastic moduli of woodceramics decrease with testing temperature. They are almost independent of vibration frequency, show different variation with strain amplitude for different woodceramics, and increase with sintering temperature.Woodceramics exhibits significant damping gains, especially at room temperature, higher strain amplitude, and higher vibration frequency. The values of damping capacity increase with vibration frequency and strain amplitude, decrease with testing temperature and sintering temperature. Defect damping, including point defect damping, interface damping, and micropores (microcracks) damping, is supposed to be the main contributor to the damping of woodceramics. At elevated temperatures, interface damping is likely to responsible for the majority of the damping of woodceramics.

Texture development during cold rolling of a unidirectionally solidified Al –Al 3 Ni alloy was investigated with emphasis on initial orientation, grain structure and the shape of second-phase dispersions. The shape of the Al 3 Ni second-phase dispersions was variously modified by heat treatment. The starting materials were cold rolled to 90 %reduction in two directions parallel (L-rolling) and perpendicular (T-rolling) to the casting direction. Depending on the initial orientation and grain structure against the rolling direction, different types of rolling textures were developed in the L- and T-rolled materials, respectively. Also, the intensity of the rolling textures in both the L- and T-rolled materials strongly depended on the shape of the Al 3 Ni dispersions. The observed rolling textures are discussed in association with the deformation zones formed around the Al 3 Ni dispersions.

The stability of M 7 C 3 carbides as reinforcement for A356 materials for tribological applications has been investigated. For this purpose, A356/M 7 C 3 , A356/SiC and A356/ M 7 C 3 /SiC composites were prepared by powder metallurgy and tested at room temperature against SAE 4620 steel ring and AISI 304 stainless steel counterfaces under loads of 10 – 150 N. For comparison, also unreinforced A356 specimens were processed and tested under the same conditions. The tribological behavior was evaluated by microstructural examination of the wear-effected zones and by weight loss measurements of the specimens and counterfaces. The wear behavior of A356/M 7 C 3 composite gave an excellent result as function of the applied load because the M 7 C 3 particles act as load-bearing elements due to their excellent bonding to the Al matrix, and their interfaces withtood the wear stresses even at the highest applied load. Moreover, the M 7 C 3 particles limited the incorporation of wear debris into the Al matrix and reduced the wear damage occasioned to the steel counterfaces compared to that of A356 Al alloy.

The microstructures of rapidly solidified Al-8 Ce and Al-20 Ce (wt.%) alloys have been characterized. The phases present in as-spun alloys are identified to be α-Al and α-Al 11 Ce 3 , identical to those in the ingot-like alloys. The microstructure of as-spun Al-8 Ce alloy is fully composed of irregular and elongated α-Al cells with intercellular α-Al 11 Ce 3 . In as-spun Al-20 Ce alloy, however, some zones comprise columnar and irregular α-Al dendrites with interdendritic α-Al 11 Ce 3 and other zones comprise irregular α-Al microdendrites with interdendritic α-Al 11 Ce 3 . Age hardening effect is not observed in the present alloys. The microhardness changes little with time when annealing below 300 °C but sharply decreases when annealing at 400 or 500 °C, indicating that the thermal stability of as-spun Al-8 Ce and Al-20 Ce alloys rapidly deteriorates above 300 °C.

The effect of quenching rate on the microstructure of the rapidly solidified Al –Sr alloys has been investigated using X-ray diffraction and transmission electron microscopy. The phases present in the melt-spun Al– 5 Sr and Al– 23 Sr (wt.%) alloys are identified to be α-Al and the equilibrium tetragonal Al 4 Sr. The microstructure composed of the α-Al cells with intercellular Al 4 Sr is dominant in the melt-spun Al– 5 Sr alloy quenched at 500 rpm. With increasing wheel speed to 1500 rpm, the areas fully comprising α-Al/Al 4 Sr eutectic are comparable to those comprising the α-Al cells with intercellular Al 4 Sr. Furthermore, there exist the following orientation relationships in the α-Al/Al 4 Sr eutectic: α-Al 〈100〉 || Al 4 Sr [001], α-Al {020} || Al 4 Sr (200) and α-Al {022} || Al 4 Sr (110). The Al 4 Sr dendrites with 90° branches, preferentially formed in the [110] direction, are dominant in the melt-spun Al– 23 Sr alloy quenched at 500 rpm. In the melt-spun Al– 23 Sr alloy quenched at 1500 rpm, however, the proportion of the Al 4 Sr dendrites with 45° branches becomes comparable to that of the dendrites with 90° branches.

Cr-containing steel panels have been exposed to marine atmosphere for 2 years. The phase constituents and the microstructure of the rust layer have been characterized using scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), Raman spectroscopy, and transmission electron microscopy (TEM). Results show that there are many cracks and pores in the rust layer on the surface of mild steel exposed for 2 years, resulting in a higher corrosion rate. But a compact rust layer with a phase constituent of alpha Cr substitute ferrite oxyhydroxide (α-Cr x Fe 1 –x OOH) has been formed on weathering steel panels with a Cr content more than 2 mass% exposed for 2 years. Furthermore, ion selective measurements showed that the rust layer of α-Cr x Fe 1– x OOH can suppress the penetration of corrosive anions, and accordingly weathering steel panels have a lower corrosion rate in the marine atmosphere.