Papers by Author: Jiří Čermák

Abstract: Diffusion of 65Zn in two commercial Mg-based alloys AZ91 and QE22 with short Saffil fibers was studied. Experiments were carried out in the temperature interval 648 – 728 K by serial sectioning method. The effective diffusion coefficients Deff were compared with 65Zn diffusion coefficients Dv obtained with the same alloys without Saffil fibers. The evaluation of the influence of the interface between the matrix and the fibers upon Deff was done and the zinc diffusion coefficient Di in the interface boundary matrix/Saffil was estimated. Unlike the Arrhenius-like behavior of volume diffusion in both alloys, it was observed that the temperature dependence of both Deff and Di was significantly concave in the measured temperature interval. This behavior was attributed to relaxation of thermo-elastic stresses in the composite induced by a large difference between coefficients of thermal expansion (CTE) of Saffil fibers and metal matrix. The maximum values of Deff and Di, respectively, lie close to 693 K, where CTE has a minimum.

Abstract: Self-diffusion of 110mAg has been investigated in fiber reinforced QE22 magnesium alloy matrix composite. Short Saffil fibers (97% -Al2O3 + 3% SiO2) were used as reinforcement. The diffusion measurements were carried out in the temperature interval 648 – 728 K by serial sectioning method. The volume diffusion coefficients Dv (alloy without reinforcement) and the effective diffusion coefficients Deff (alloy with reinforcement) were obtained by analysis of the penetration curves. The silver diffusion coefficient in the interface boundary matrix/Saffil Di was also estimated. The temperature dependence of volume diffusion coefficients Dv was compared with previous data measured using 65Zn in the same alloy and with literature data for Zn impurity diffusion in Mg single crystal. It was observed, that the temperature dependence of both Deff and Di was significantly non-linear in the measured temperature interval. This behavior supports previous observations with zinc diffusion in the same alloy.

Abstract: Kinetics of hydrogen desorption from Mg2NiH4 was studied. Experimental material was prepared by two techniques – by melting and casting and by ball-milling and compacting into pellets. Experimental materials were hydrogen charged at elevated temperature and pressure. The pellets were charged in two different regimes resulting in structures with high fraction of twinned low-temperature phase LT2 and with low fraction of LT2. It was made an attempt to measure diffusion coefficients of hydrogen and its temperature dependence both in high-temperature (HT) and in low-temperature (LT) phases of Mg2NiH4. The measurement was carried out in temperature interval from 449 K to 576 K by the volumetric method. It was found that the LT2 slows-down the desorption rate considerably.

Abstract: Diffusion of 65Zn in Mg-based alloy AZ91 with short Saffil fibres was studied by sectioning method in the temperature interval 648 – 728 K. The results were compared with those obtained in the measurements made with the same materials without the reinforcement. This enabled to assess the influence of the interfaces between the matrix and the fibres and to evaluate the diffusion characteristics of interface self-diffusion.

Abstract: In the temperature range 600–1000 °C, the effect of material purity on self-diffusion along grain boundaries has been studied in both the pure (Puratronic 99.9945%) and the technical (99.5%) nickel. The penetration profiles were measured by the serial sectioning method using the 63Ni radiotracer. The extensive electron backscatter diffraction (EBSD) analysis was performed on the same samples in order to reveal possible differences in microstructure induced by the impurity content. The obtained microstructure characteristics were further interpreted in terms of the coincidence site lattice (CSL) model.

Abstract: Grain boundary self-diffusion in both the cast and the cold-rolled Puratronic 4N5 nickel was studied in the temperature range from 600 °C to 1000 °C. The experiments were carried out with the samples pre-annealed at 1100 °C in comparison to the samples pre-annealed at intended individual diffusion temperatures. The relative grain orientation was analyzed on the same samples by means of electron backscatter diffraction (EBSD) and grain boundaries (GBs) were characterized in terms of the coincidence site lattice (CSL) model. Considering the non-linear Arrhenius temperature dependencies obtained for most specimens by using conventional method of profile evaluation in the B-type kinetics and the appearance of two high-diffusivity paths in diffusion profiles measured, a more suitable BB-type and AB-type diffusion models were applied for data evaluation.

Abstract: Coefficient of 65Zn heterodiffusion in Mg17Al12 intermetallic and in eutectic alloy Mg - 33.4 wt. % Al was measured in the temperature region 598 – 698 K using serial sectioning and residual activity methods. Diffusion coefficient of 65Zn in the intermetallic can be written as DI = 1.7 × 10-2 m2 s-1 exp (-155.0 kJ mol-1 / RT). At temperatures T ≥ 648 K, where the mean diffusion path was greater than the mean interlamellar distance in the eutectic, the effective diffusion coefficient Def = 2.7 × 10-2 m2 s-1 exp (-155.1 kJ mol-1 / RT) was evaluated. At two lower temperatures, the diffusion coefficients 65Zn in interphase boundaries were estimated: Db (623 K) = 1.6 × 10-12 m2 s-1 and Db (598 K) = 4.4 × 10-13 m2 s-1.

Abstract: Heterodiffusion of 65Zn in Mg, Mg – x Al (x = 1.77, 3.9 and 9 wt.% Al) and in the commercial AZ91 alloy was studied in the temperature interval 498 – 848 K by serial sectioning and residual activity methods. The concentration and temperature dependence of Zn bulk diffusion coefficient D is described by the relation D = exp (0.1 × cAl – 9.16) × exp (-125.8 kJ mol-1/RT) m2 s-1 (cAl – concentration of Al in wt.%). Zn grain boundary diffusivity P = s δ Db (s – segregation factor, δ - grain boundary width and Db – diffusion coefficient in grain boundary) was also determined and it was found that it obeys the Arrhenius law P = 7.2 × 10-15 × exp (-46 kJ mol-1/RT) m3 s-1.

Abstract: 99Mo tracer diffusion in Fe79-yMo8Cu1B12+y (y = 0, 3, 5) alloys was studied using serial sectioning method in the temperature range 573 – 773 K. Sputtering by Ar+ ions was applied as a sectioning technique. The measurement was carried out with the ribbon-like samples prepared by planar flow casting technique and crystallized before the diffusion experiment by a thermal shock in vacuum. The structure was formed by areas of three distinct types differing in chemical composition r = at. % Mo / at. % Fe: (i) r = 0.02 (crystallites of Fe, mean size d ~ 150 nm), (ii) r = 0.09 (relaxed phase; d ~ 150 nm) and (iii) r = 0.83 (boride phase; d ~ 30 nm). The measured profiles were linear in co-ordinates ln cMo vs. x1.5 (cMo – concentration of Mo in depth x), indicating that the diffusion of 99Mo was controlled by „type A-B” kinetics.

Abstract: In this paper, the results of measurements of chemical diffusion of C in weldments of Fe- C-Mn alloys with content of 0-15 % Mn, are reported. Alloys were cast in induction furnace under protective Ar atmosphere. They were hot-forged to rods of diameter 15 mm, machined down to diameter 12 mm, and cut into pieces of 4 mm thickness. These cylindrical samples were metallographically polished. Diffusion couples, welded by electrical impulse, were sealed together with Ti-chips, into evacuated silica ampoules. After that, they were annealed at 1073-1373 K for 96-1.5 hours, respectively. Measurement of C redistribution was done in a normal direction to welding interface using electron-scanning microscope Philips SEM-505WDS equipped with Microspec WDX-2A. Diffusion coefficients of C were obtained and the relations between manganese and carbon were found.