Andrzej Pawłowski

Superionic phase transition in (NH4)2SeO4·2NH4HSeO4 single crystal at 378 K

Abstract DTA, structural and electric conductivity investigations were made for (NH4)4H2(SeO4)3 single crystals. A high-temperature phase transition at 378 K to a superionic phase was found. The phase is characterized by a high electrical conductivity (∼4.10−3 Ω−1 cm−1) and a low activation energy (0.11 eV).

Practical aspects of discontinuous precipitation and dissolution

Abstract A method of grain refinement which does not require plastic deformation is discussed. The refinement is obtained by a carefully designed thermal treatment in which the discontinuous precipitation (DP) reaction is followed by a dissolution process. Two different mechanisms of refinement were identified based on the investigations performed on Al–22xa0at.% Zn and Ni–4xa0at.% Sn alloys. The first one is associated with holding the orientation relationship of the former DP process. Such a behaviour was observed in the Al–22xa0at.% Zn alloy in which the grains are refined by a factor of 7–8. If the small grains are created via recrystallisation within the area previously covered by a DP cell, like in Ni–4xa0at.% Sn alloy, a refinement factor of 40 or even more can be obtained.

Mechanism of early stages of discontinuous dissolution in AlZn alloys

Abstract The early stages of the discontinuous dissolution of lamellar precipitates has been studied in the alloys Al-15at.%Zn and Al-21.55at.%Zn. Optical microscopy, transmission electron microscopy and X-ray phase analysis have been applied. Three typical areas where cellular dissolution begins have been taken into consideration: the boundary between supersaturated solid solution and lamellar precipitates; the impingement of various cell areas; the primary grain boundary. It has been concluded that the period of incubation of the new α∼ phase nuclei is connected with the deviation from equilibrium state of the concentration within the α phase lamellae as well as with the intergacial free energy at the cell boundaries. It has been proposed that, for cellular dissolution, the period of incubation is governed by the volume diffusion opposite to the backward migration of the cells which is controlled by the diffusion at the cell boundaries. Displacements of dislocations towards the cell fronts, the pinning of these dislocations, and the formation of the transition phase on them, may be reasons for the “trace” of the backwards-migrated cell boundaries.

Microstructure evolution and electrical properties of yttria and magnesia stabilized zirconia

The paper contains the results of microstructure and chemical composition analyses of micrograins, which were formed on initial grain boundaries in ZrO2–Y2O3 and ZrO2–Y2O3–MgO systems. It has been found that the micrograins appear in the process of diffusion induced by grain boundary migration (DIGM). The observed processes can be described as both liquid film migration (LFM) and chemically induced grain boundary migration (CIGM). New micrograins had an increased content of Y2O3 and a cubic symmetry. Zirconia–yttria solid solutions with magnesia particulate addition showed an increased amount of migration nuclei and bigger size of new grains. However, no change in the chemical composition of the grains has been detected. The ionic conductivity measurements have shown that the activation energy (Ea) of conductivity at lower temperatures is connected to a DIGM-like process and to the distance of grain boundary migration. In the case of materials with dominating LFM process an increased grain boundary migration distance leads to a lowering of the activation energy of conductivity. Contrary to that, in the materials with dominating CIGM process an increase of migration zone causes increase of Ea values. The data obtained with respect to the type of DIGM process (LFM or CIGM) indicate that the grain boundary conductivity contribution increases with the CIGM distance.

Microstructure and phase composition of ZrO2-MgO structural ceramic after high-temperature deformation

Abstract The ZrO2–13xa0mol% MgO ceramic was investigated by using transmission electron microscopy (TEM) and X-ray diffraction after partial sub-eutectoid decomposition, and after high-temperature compression up to 10% at 900xa0°C. The sub-eutectoid decomposition led to the formation of ZrO2 tetragonal precipitates. The high-temperature deformation enhanced the transformation of the cubic ZrO2 (c-ZrO2) matrix to tetragonal ZrO2 (t-ZrO2), while existing tetragonal precipitates and part of the matrix were partially transformed to monoclinic ZrO2 (m-ZrO2). Numerous deformation bands composed of martensite plates were observed in the matrix.

Crystallographic and microchemical characterization of the early stages of eutectoid decomposition in MgO-partially stabilized ZrO2

Abstract The early stages of eutectoid decomposition in ZrO 2 -8 mol% and 11 mol% MgO were studied by means of analytical electron microscopy. The observed reaction products at the grain boundaries were monoclinic or tetragonal ZrO 2 phases and MgO precipitates of two different “cellular” morphologies. The application of energy dispersive X-ray microanalysis led to the detection of an abrupt change in the MgO content in the m-ZrO 2 phase, indicating that the eutectoid decomposition in ZrO 2 –MgO is controlled by interface rather than volume diffusion. The crystallographic relationships between the parent phase and decomposition products were also determined.

Influence of aging on discontinuous dissolution in Al-22at.% Zn alloy

Abstract The influence of prior aging and volume diffusion on the kinetic parameters of discontinuous dissolution were studied in al-21.55at.%Zn alloy. The following kinetic parameters were considered: the critical temperature of discontinuous dissolution T cr , the rate of discontinuous dissolution V d 1 , the diffusivity KD b,m λ at the boundary of receding cells, and the activation energy of discontinuous dissolution. A new method of calculation of KD b,m λ is proposed. It is concluded that T cr , V d 1 , and KD b,m λ are strongly affected by the conditions of prior aging. The estimated diffusivities for discontinuous dissolution are more than two orders of magnitude smaller than those for diffusion at stationary grain boundaries. This result is caused by volume diffusion ahead of the precipitation front and within α-phase lamellae. However, the discontinuous dissolution is still controlled by diffusion at the boundary of receding cells. The influence of aging temperature and volume diffusion on KD b,m λ values was found to be connected with the change in pre-exponential factor in the Arrhenius equation.

Diffusional penetration during diffusion-induced grain-boundary migration process in an Al-Zn couple

The existence of diffusion-induced grain-boundary migration (DIGM) has been re-examined by electron probe micro analysis and analytical electron microscopy in the aluminium substrate of Al-Zn diffusion couple annealed in the temperature range 395–535 K. The investigation revealed two basic kinds of DIGM: laminar and turbulent. The laminar kind occurs over the whole temperature range and is characterized by a small migration distance and large migration depth. The zinc enrichment at a sample surface is 4.0–5.0 wt% and gradually decreases with increasing depth. The turbulent kind is limited to annealing temperatures above 450 K. In this case, the width of the alloyed zone is much greater, close to the surface of sample and then dramatically decreases, showing a behaviour similar to the laminar morphology. The zinc content at the surface of sample is 8.0–9.0 wt%. The diffusivities of DIGM calculated based on Cahns equation agree well with the values of stationary grain boundary in diluted AlZn alloys. Evidence for the existence of DIGM was the asymmetry of the zinc profile with regard to the final position of the boundary. Microanalytical scan across the alloyed zone showed an abrupt change of the zinc concentration at the moving boundary. This suggests that the role of volume diffusion during DIGM is not so important and a considerable chemical contribution to the total driving force should exist.

The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable α′ phase was identified in the first stage of dissolution nucleating at the α-η boundaries. Interface dislocations at the α-α′ boundaries forming a perpendicular net in the 〈110〉 directions were identified as of screw character and of Burgers vector b=1/2a 〈110〉. During dissolution a change of shape of α′ precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 〈110〉 directions.

Studies on Al2O3 - ZrO2 Coatings Structure before and after Thermal Treatment

Phase transitions occurring in the Al2O3-ZrO2 composite as a result of plasma spraying and subsequent coating annealing are described. X-ray diffraction and transmission electron microscopy methods are applied to study these effects. Separate layers of amorphous, nano- and fine-crystalline structure with the columnar crystals areas have been observed. Thermal treatment causes formation of more ordered phases. Effect of partial stabilization of cubic and tetragonal zirconia due to the presence of alumina has been remarked.