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Print version ISSN 1516-1439
Mat. Res. vol.15 no.5 São Carlos Sept./Oct. 2012 Epub Aug 30, 2012
Rafaella Martins Ribeiro; Luis Fernando Lemus; Dilson Silva dos Santos*
Programa de Engenharia Metalúrgica e de Materiais - PEMM, Coordenação de Programas de Pós-graduação em Engenharia, Instituto Alberto Luiz Coimbra de Pós-graduação e Pesquisa de Engenharia - COPPE, Universidade Federal do Rio de Janeiro - UFRJ, CP 68505, CEP 21941-972, Rio de Janeiro, RJ, Brazil
Samples of amorphous and partially crystallized Fe40Ni38Mo4B18 alloy were submitted to hydrogen gas permeation from 523 to 643 K. The hydrogen permeation curves exhibited a single sigmoidal shape, typical of tests where no hydride formation occurs. It was observed that the hydrogen diffusivity increases for the amorphous samples and partially crystallized alloy with the temperature increase. The hydrogen diffusion coefficient as a function of temperature was found to be D = 5.1 ± 0.5 × 10-12 exp (-11.0 ± 3.5/RT) (m2.s-1) for amorphous condition and D = 3.6 ± 0.5 × 10-11 exp (-19.8 ± 3.3/RT) (m2.s-1) for the partially crystallized condition. This suggests that the annihilation of defects in the amorphous structure and the crystalline phase precipitate contributes to the increase of the hydrogen diffusion.
Keywords: hydrogen diffusion, amorphous metallic alloy, crystallized alloy
Amorphous metallic alloy ribbons were exhaustively studied because of its capability to storing hydrogen and membrane application1-3. Regardless of the composition, amorphous alloys exhibit large amount of sites for hydrogen absorption, but many of this sites have high occupancy energy so the hydrogen diffusivity in these materials is always lower than the corresponding crystalline alloys. Another important remark concerning hydrogen in amorphous alloys is the ability to form hydride. Rodmacq et al.4, based on neutron diffraction experiments, reported the hydride formation in the CuTi alloy. Fagundes et al.5 studied Fe40Ni38Mo4B18 amorphous alloy, by electrochemical permeation tests, and observed the hydride formation. The hydride formation was attributed to the double sigmoidal shape exhibited in the hydrogen permeation curves despite the fact that the hydride could not be confirmed by X-ray diffraction (XRD) after hydrogenation because the amorphous structure did not change. In contrast to these results, the literature6,7 reported no occurrence of hydride in amorphous alloys which has strong ability to form hydride when crystalline.
The present work aims to studying the amorphous and partially crystallized Fe40Ni38Mo4B18 alloy by hydrogen gas permeation at high temperature.
2. Experimental Procedure
The alloy, made by melt-spinning with nominal composition Fe40Ni38Mo4B18 , was supplied by Allied Signal Co in the form of ribbons 25.4 mm wide and 25 µm thick. Isothermal heat treatment was carried out at 698 K during 30 minutes under an inert atmosphere to promote the partial crystallization of the sample. The crystallization temperature was chosen from the differential scanning calorimetry tests, DSC, performed previously8.
The XRD was performed in a Shimadzu XRD-6000 apparatus using Cu-Kα radiation, before and after the heat treatment to evaluate the occurrence of the sample crystallization.
Hydrogen gas permeation tests were performed from 523 to 643 K in a home apparatus with a double compartment cell that are separated by the sample. After vacuum and rinse with argon, 1 MPa of a constant hydrogen pressure was applied and in the opposite side the compartment was kept under ~10-7 torr of vacuum, using a turbo molecular pump. The hydrogen, that diffuses through the sample arrives in the opposite compartment is immediately released to the compartment due to the negative pressure, maintaining the hydrogen concentration always zero. The hydrogen flux is measured by using the mass flow meter Omega FMA-1600 A with a data collection interval of 100 ms. Based on this configuration, the initial and boundary conditions are given by:
- for t = 0 C0 = 0, 0 < x < L
- for t > 0 C0 = C1 and CL= 0
Applying these conditions to Fick's second law, it is possible to obtain the flux evolution as a function of time, JL(t), as shown in Equation 1, assuming that the diffusivity of hydrogen does not vary with the concentration increase.
where J∞ is the steady state flux, D is the hydrogen diffusion coefficient, L is the sample thickness and n = 1,2,3,....
The dependence of the diffusion coefficient with temperature is given by:
where D0 is the diffusion coefficient independent of temperature and Q is the driving force.
3. Results and Discussion
Figure 1 shows the XRD spectra of the alloy as received and after isothermal annealing, where the beginning of crystallization can be seen that occurs after the relaxation process of the sample. Du et al.8 performed in situ X-ray diffraction to study the Fe40Ni38Mo4B18 crystallization at 693 K for different annealing times, and the crystallization product was found to be Fe-Ni phase and (FeNiMo)23B6 phase appears after 48 hours of annealing.
The hydrogen permeation curves, obtained at 1 MPa from 523 to 643 K for amorphous and partially crystallized conditions, are shown in Figure 2. It can be seen that all curves exhibit a sigmoidal shape, indicating no hydride formation. Two aspects must be considered for this behavior: the increase of the temperature and the low hydrogen pressure used in the test (1 MPa). In general, the occurrence of hydrides is strongly affected by the increase of the temperature. At high temperatures, the transition of solid solution to hydride can occurs instantaneously i.e. without providing a pressure plateau, which depends on the alloy. These factors contribute to the increase of the hydrogen amount in the alloy.
The permeability of the studied alloy is about 10-9 mol.m-1s-1 Pa-0.5 at 643 K. This value is in agreement with the results obtained by Kim et al.9 for the alloy (Ni0.6Nb0.4)90Zr10 used in hydrogen filters, which has an alloying element with greater affinity to hydrogen than the alloy studied in this work.
Figure 3 shows the hydrogen diffusivity for the amorphous and partially crystallized conditions compared to the results previously obtained in electrochemical permeation tests5. The hydrogen diffusivity in the amorphous sample can be expressed as a function of temperature by the relation D = 5.1 ± 0.5 × 10-12 exp (-11.0 ± 3.5/RT) (m2.s-1) and D = 3.6 ± 0.5 × 10-11 exp (-19.8 ± 3.3/RT) (m2.s-1) for the partially crystallized sample.
It can be seen that the hydrogen diffusion in the sample after partial crystallization is slightly higher than in the amorphous structure. This occurs due to the mechanisms of annihilation of defects in conjunction with the appearance of ordered crystals embedded in an amorphous structure, which can increase diffusion rate.
The gap between the measurements of the electrochemical permeation refers to the inability to perform this test at temperatures above 353 K. In the same way for temperatures below 523 K, the hydrogen permeation through the alloy is very low.
It is important to note that the product of crystallization of Fe40Ni38Mo4B18 is fcc10 Fe-Ni phase, due to the high nickel content in this alloy. The diffusion capacity in fcc systems10 is much smaller when compared with bcc systems6. Finally, the values of D0 and Q obtained in this study are consistent with the diffusion in metallic systems which occurs via interstitial jump mechanisms or in the case of amorphous structure, the arrangements of short-range order11.
The hydrogen diffusivity in the Fe40Ni38Mo4B18 amorphous and partially crystallized alloy was determined at high temperature. The annihilation of structural defects and the partial ordering induced crystallization lead to an increased diffusivity of hydrogen. There was no evidence of changes in the permeation curves suggesting the formation of hydride in the temperature range and hydrogen pressure studied.
The authors acknowledge the CNPq, CAPES and FINEP for the financial support for this research.
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Received: December 7, 2011
Revised: June 26, 2012