Papers by Author: Ivo Stloukal

Abstract: Diffusion of both titanium and nickel was measured in the near stoichiometric Ni-49.4at.%Ti alloy with the B2 ordered structure. The radiotracer technique and the 44Ti and 63Ni isotopes were applied in the temperature interval from 900 to 1300 K. The penetration profiles were determined by precision parallel grinding or by ion beam sputtering at larger and smaller penetration depths, respectively. Titanium and nickel diffusivities were found to follow linear Arrhenius dependencies with the pre-exponential factors of 2.710-7 and 4.710-9 m2/s and the activation enthalpies of 205 and 143 kJ/mol, respectively. A vacancy mediated diffusion mechanism is suggested to provide diffusion of both nickel and titanium in the compound NiTi.

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: 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: Gallium grain boundary (GB) diffusion of Ga in polycrystalline magnesium was investigated by radiotracer residual activity technique using 67Ga isotope. The diffusion measurements were carried out under conditions of Harrison’s B-type kinetics in temperature range 639 – 872 K. An approximate evaluation procedure was proposed to calculate both volume diffusion coefficient Dv and GB diffusivity P = s dDb (s is the segregation factor, d the GB width and Db the GB diffusion coefficient). The obtained results showed the following linear Arrhenius relationships: Dv = 1.2 × 10-4 × exp (–134.3 kJ mol-1/RT) m2 s-1 and P = 3.8 × 10-9 × exp (–94.9 kJ mol-1/RT) m3 s-1, where R is universal gas constant and T the temperature. Obtained results were compared with literature data on Mg self-diffusion and with In impurity diffusion in magnesium.