Ivo Stloukal

Short-circuit diffusion in NiAl alloys

Abstract The tracer grain boundary diffusivity, P, of nickel in polycrystalline NiB, Ni-9 at%AlB and in NiAlB intermetallic has been measured with boron concentrations xB up to 0.3 at% B. The measurements were done by the residual activity method in the temperature interval from 773 to 1073 K. It was observed that P decreases with growing concentration of boron in NiB and Ni-9 at% AlB. This behaviour is similar to the dependence of P on xB in Ni3Al, reported in our previous paper. In boron-doped NiAl intermetallic, on the other hand, P does not depend on xB.

Diffusion of Titanium and Nickel in B2 NiTi

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.

Phosphorus grain boundary diffusion and segregation in Fe-2.2 wt.% Si alloy

The grain boundary diffusion of 32 P in a polycrystalline Fe-2.2 wt.% Si alloy was measured with the radiotracer technique under the condition of the Harrison type-B regime (845-1173 K) and type-C regime (735-830 K). The triple product P = sδD gb (s denotes the segregation factor, δ the grain boundary width and D gb the grain boundary diffusion coefficient) was obtained in the B regime. The pre-exponential factor P 0 = (1.8 +3.2 -1.1 ) × 10 -10 m 3 s -1 and the activation enthalpy Q gb = 171.3 ± 8.3 kJ mol -1 were evaluated. The initial part of the penetration profiles measured in the B regime was used to calculate the volume diffusion coefficients D v . The resulting Arrhenius parameters of volume diffusion were Do = 10 +2600 -10 m 2 s -1 and Q v = 320 ± 45 kJmol -1 . The grain boundary diffusion coefficient D gb was directly measured in experiments in the C regime. The temperature dependence is characterised by the Arrhenius parameters D 0 gb = (7.9 +28.2 -6.2 ) × 10 -3 m 2 s -1 and H gb = 201.5± 9.9 kJ mol -1 . Combining both series of measurements, the segregation coefficient s was determined, resulting in the segregation enthalpy H s = -30 ± 10 kJ mol -1 and the pre-exponential factor so = 47 +159 -35 . The obtained results were compared with segregation data of P measured by Auger electron spectroscopy in α-Fe.

Diffusion of 65Zn in the Mg17Al12 intermetallic compound and in the Mg-33.4wt.% Al eutectic

Abstract The coefficient of 65Zn heterodiffusion in the Mg17Al12 intermetallic compound (-phase) and the eutectic alloy Mg-33.4wt.% Al was measured in the temperature region 598–698K using serial sectioning and residual activity methods. The diffusion coefficient of65Zn in the intermetallic can be written as DI=1.5×10−2m2s−1× exp(–154.3kJmol−1/RT). At temperatures T648K, where the mean diffusion path is greater than the mean interlamellar distance in the eutectic, the effective diffusion coefficient Deff=2.710−2m2s−1exp(–155.1kJmol−1/RT) was evaluated. At two lower temperatures, the diffusion coefficients 65Zn in interphase boundaries were estimated: Db (623K)=1.610−12m2s−1 and Db (598K)=4.4 10−13m2s−1. The specific (Mg)/ interfacial energy =140mJm−2 was assessed from measurement of the time dependence of interlamellar distance during isothermal anneal.

Zn Diffusion in Binary Base of Light Mg-Al Alloys

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.

Kinetics of Phase Transformations in Mg2Ni-H System

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.

Diffusion of Zinc in Two-Phase Mg-Al Alloy

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.

Silver Self-Diffusion in Al2O3/QE22 Magnesium Alloy Matrix Composite

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.

Diffusion of 65Zn in AZ91 and QE22 Alloys with Saffil Reinforcement-Influence of Interfaces Matrix/Saffil

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.

Influence of Interfaces upon the Self-Diffusion in AZ91 Alloy Reinforced by Saffil Short Fibres

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.