Band 94, Heft 8

An atomistic Monte Carlo simulation of coherent precipitation on a body-centred cubic crystal lattice is presented. A binary system with atom types A and B is considered, while the ‘diffusion’ of the atoms is realized via a vacancy mechanism. Starting with a random distribution of 91% A atoms and 9% B atoms, the formation and growth of precipitates is simulated at a constant temperature of 773 K. As a result of the simulation, the precipitate radii distributions at different states of the system and the time evolution of the precipitate mean radii have been calculated. The results of the simulation are compared with the predictions of the classical Lifshitz – Slyozov–Wagner (LSW) theory. At the beginning of precipitation the mean radius R¯(t) $\bar{R}(t)$grows proportional to t 0.180 , while at later states the growth exponent approaches the classical value of 1/3. In order to reveal the growth exponent’s exact value, an even larger simulation is required, which remains as an interesting task for the future.

The Pd–Zr system has been critically assessed by means of the CALPHAD technique. The solution phases (liquid, body-centered cubic (Zr), face-centered cubic (Pd) and hexagonal close-packed (Zr)) were modeled with the Redlich – Kister equation. The intermetallic compounds Pd 3 Zr and γPdZr, which have a homogeneity range, were treated as the formulae (Pd, Zr) 3 (Pd, Zr) and (Pd, Va)(Pd, Zr) by a two-sublattice model with Pd and Zr or Pd and vacancies, denoted Va, on the first sublattice, Pd and Zr on the second one, respectively. Both compounds Pd 2 Zr and PdZr 2 having a tetragonal MoSi 2 -type structure were treated as one phase with the formula PdZr(Pd, Zr) by a three-sublattice model with Pd on the first sublattice, Zr on the second, and Pd and Zr on the third one, respectively. The others were treated as stoichiometric compounds. A set of self-consistent thermodynamic parameters of the Pd–Zr system was obtained.

The push –pull cyclic deformation of a copper bicrystal with components rotating gradually along the grain boundary (GB) was carried out under constant plastic strain amplitude at room temperature in air. The evolution of slip morphology and dislocation structure was observed by scanning electron microscopy electron channelling contrast technique. It was shown that slip morphology and dislocation structure in one of the grains changed gradually along the GB. This phenomenon can be understood based on the local irreversible rotation of crystals, which exists during the symmetrical push– pull loading. When the secondary slip was localized to form the secondary persistent slip bands, which interacted with the regular walls in the second type of deformation band, then the cell structure was formed. The second type of deformation band is one kind of sites for cell formation.

Alloys of the quasibinary section Cu –NiBe were experimentally investigated with differential thermal analysis, optical microscopy, electron microanalysis, transmission electron microscopy and X-ray diffraction. The construction of the quasibinary Cu–NiBe phase diagram was made based on the experimental results. The constitution of alloys of the whole section was studied along with the investigation of the microstructure and crystallographic relationship of the NiBe phase in aged alloys from the Cu-rich corner of the Cu–NiBe system.

The thermal stability of a Mg-8.6 wt.% Zr alloy processed by physical vapour deposition has been studied using differential scanning calorimetry and transmission electron microscopy. The alloy, in the as-deposited condition, is a solid solution of Zr in the Mg matrix. The microstructure is characterised by elongated grains oriented with their [0001] direction parallel to the growth direction. After DSC experiments, three exothermic transformations have been observed. The first one takes place above the deposition temperature and is related to the recovery phenomena due to stress relaxation generated during the growth process. The second transformation corresponds to the breakdown of the solid solution by the precipitation of Zr in the Mg matrix. This reaction takes place in two stages: (i) Zr plates are formed in the basal plane and (ii) these initial precipitates grow along the [0001] direction. An epitaxial relationship between those Zr precipitates and the Mg matrix of [0001] Zr || [0001] Mg and [11 2 0] Zr || [11 2 0] Mg has been obtained. The third exothermic transformation appears above 720 K and is caused by the strong oxidation of the deposit.

The development of the microstructure in Mg–Al –Ca alloys was investigated both in the as-cast condition and after solution heat treatment at 415 °C for 0.25 – 120 h, using secondary electron imaging, electron probe microanalysis and X-ray diffraction analysis in combination with light microscopy and image analysis. The main intermetallic compound in the Ca-containing alloys is Al 2 Ca, and the formation of the Mg 17 Al 12 phase is suppressed in the presence of Ca. The decomposition of Al 2 Ca during the solution treatment is more difficult than that of Mg 17 Al 12 , and undissolved Ca was still present in the form of stable Al 2 Ca even after 120 h. With increasing treatment time, the coarse Al 2 Ca phase, which was continuously distributed like a network around the α-Mg grains in the as-cast state, underwent thinning and necking in the initial stage. Then it became disconnected and fragmentized, and finally the fragmentized Al 2 Ca particles were spheroidized and fined due to the diffusion of Ca and Al along the Al 2 Ca/Mg interface. As a result, the continuous network-like Al 2 Ca phase turned into a dispersion of spherical particles. The fine Al 2 Ca particles were homogeneously distributed in the matrix, which can be very beneficial to creep resistance and mechanical properties of Ca-containing alloys.

The effect of Ti additions (0.01 – 0.20 wt.%) on the hardness, ultimate tensile strength, impact strength and creep properties of the gravity-cast Zn–Al –Cu–Mg alloy ZA-8 has been investigated. The Ti additions have no significant effect on the hardness. The ultimate tensile strength of the standard ZA-8 alloy has not been changed significantly with Ti additions up to 0.15 wt.% and then decreased with further increase in Ti content. The impact strength shows a maximum around 0.05 – 0.15 wt.% Ti. The lowest creep resistance is obtained at 0.01 wt.% Ti, and the highest value of creep resistance is reached for 0.20 wt.% Ti. Metallographic studies indicated that the addition of Ti has a profound effect on the microstructure. Also, Ti forms a complex-shaped intermetallic compound which was identified as TiZn 15 by X-ray diffraction. It is suggested that the TiZn 15 particles cause the increase in creep resistance and decrease in tensile strength.

This paper discusses the observations made pertaining to dry sliding wear characteristics of a Zn-based alloy at different sliding speeds and pressures. The wear rate increased with speed and pressure. The wear rate versus pressure plots attained two slopes wherein the slope was low up to a specific pressure followed by an increased slope at still higher pressures. This trend was observed at all the sliding speeds except the maximum; in the latter case one slope (identical to that of the higher slope at the lower speeds) only was noted. The seizure pressure was noted to decrease with speed. The temperature near the specimen surface increased with test duration, pressure and sliding speed. The rate of temperature increase was significantly high initially. This was followed by a lower rate of increase in temperature at longer test durations. In some cases, the rate of temperature increase became high once again towards the end of the tests. The influence of applied pressure and sliding speed on the temperature increase was practically identical to that on wear rate. Lower wear rate of the alloy agreed with less surface/subsurface damage and finer debris formation and vice versa.

The effect of rapid solidification on the microstructure of Al-5Sr, Al-10Sr and Al-23Sr (wt.%) alloys has been investigated using X-ray diffraction and transmission electron microscopy. The phases present in the melt-spun Al –Sr alloys are the equilibrium phases a-Al and tetragonal Al 4 Sr. The microstructure of the melt-spun Al –Sr alloys is quite different from that of the ingot-like alloys. Microstructure selection map is established for the rapidly solidified Al-rich Al –Sr alloys, in light of the microstructure together with the estimated cooling rates. The undercooling obtained through quenching at ≈ 9.5 × 10 5 K s –1 is sufficient for the Al-10Sr alloy but insufficient for the Al-23Sr alloy to completely suppress the formation of primary Al 4 Sr. Rapid solidification has a significant effect on the morphology of primary Al 4 Sr and the higher level of undercooling enhances the formation of the Al 4 Sr dendrites with 45° branches in the melt-spun Al-23Sr alloy.

The microstructure, the residual stresses and the texture of friction stir welds of 4 mm thick samples made of a 6013 Al –Mg– Si –Cu alloy in T4 state were investigated in order to study the influence of the friction stir welding process on the material and in order to determine the influence of different welding parameters. The results of the investigations reveal the influence of the process parameters (welding speed, tool diameter and rotational speed) on the weld properties such as the microstructure, the precipitation state, the hardness, the texture and the residual stresses. The formation of the residual stresses and their distribution is explained based on an existing, simple thermal model for the friction stir welding and on a classical model for the formation of the residual stresses in conventional fusion welding.

A study has been made to determine the influence of the carbon content on the kinetics of dynamic and static recrystallization during and after hot deformation of carbon steels. For this purpose, single- and double-hit hot compression experiments at various strain rates and temperatures together with the Avrami-type kinetics equation and Bergstrom approach have been utilized to investigate recrystallization behavior. The results show that the apparent activation energy of hot deformation decreases with increasing carbon content and this phenomenon results in a faster dynamic recrystallization at high temperatures and/or low strain rates. Also, increasing carbon content leads to a higher rate of static recrystallization at low strain rates while at high strain rates it causes a slower kinetics of static recrystallization.

The torsion behavior of an ultrahigh carbon steel containing 1.3 wt.% C has been studied at high strain rates (2 – 26 s –1 ) and high temperatures (900 – 1200 °C). Adiabatic heating strongly affects the shape of the stress– strain curves. Grain size measurements at strains of ε = 2 and ε = 5 reveal that the austenite grain size does not change between these two strains and is finer than the initial austenite grain size. This is an indication that a recrystallization process took place in the interval of strains between ε peak and ε = 2 and that the deformation state associated to the strains of ε = 2 and ε = 5 can be considered as steady state. Stress–strain rate relations have been analysed for ε peak , ε = 2 and ε = 5. It has been found that data obtained at ε peak correlate well with a lattice diffusion-controlled dislocation creep process. On the other hand, analysis of the stress –strain rate relations found at the steady state indicated that the deformation occurs by the contribution of two independent mechanisms: grain boundary sliding controlled by pipe diffusion and slip creep controlled by lattice diffusion.

The main objective of the present work was to study the low-velocity weight-drop impact damage behaviour of carbon – carbon silicon carbide (C/C– SiC) composites on electrical resistivity of the direct-current potential drop method. The results show that the electrical resistivity increases with increasing impact energy, which confirms that pre-microcracks of C/C– SiC composite grow very rapidly with a little amount of impact load and turn into a loss of conductivity of the material.

The paper deals with the depth determination of residual stress states from diffraction data. First an historical overview of the known approaches is given. Then we apply the approximate inverse method to this problem. This method is known to be very efficient and stable with respect to noise-contaminated data. It is even possible to prove convergence and it allows an error estimate of the calculated depth resolved residual stress profile.

We produced submicrocrystalline Cu using the equal channel angular pressing technique with four passes. The as-deformed samples exhibited a high yield strength > 400 MPa and a poor elongation before failure < 20%. The majority of the grain boundaries after equal channel angular pressing were of the low-angle type. A series of heat treatments in the temperature range 220 – 300 °C was performed and partly recrystallized microstructures with different fractions of recrystallized grains were obtained. The microstructures were analyzed by a combination of transmission electron microscopy, orientation image microscopy, light microscopy and scanning probe microscopy. The increase in ductility in partly recrystallized samples was accompanied by a decrease in the yield strength. It was suggested that to obtain a bulk nanocrystalline material which is both strong and ductile the majority of the grain boundaries should be of the high-angle type. It is shown that the scanning probe microscopy technique allows one to determine the microstructure of partly and fully recrystallized samples and to draw qualitative conclusions about the defects density and their distribution in the sample.