Rafaella Martins Ribeiro

Hydrogen Absorption Study of Ti-based Alloys Performed by Melt-spinning

The hydrogen absorption and desorption of Ti53Zr27Ni20 icosahedral quasicrystal (ICQ) and Ti50Ni50 shape memory alloy (SMA) melt-spun ribbons was studied. Samples were exposed to hydrogen gas at 623 K and 4 MPa for 1000 minutes. The total capacity of hydrogen obtained for Ti53Zr27Ni20 and Ti50Ni50 was 3.2 and 2.4 wt. % respectively. The Thermal Desorption Spectrometry (TDS) of the hydrogenated alloys shows that both alloys start to desorb hydrogen around 750 K. X-ray diffraction (XRD) patterns, performed after hydrogenation, indicate a complete amorphization of the Ti53Zr27Ni20 i-phase alloy, while the Ti50Ni50 alloy remained crystalline after hydride formation.

Crystallization kinetics of the (Fe50Co50)73.5Ag1Nb3Si13.5B9 amorphous alloy

Abstract The crystallization kinetics of the (Fe50Co50)73.5Ag1Nb3Si13.5B9 amorphous alloy was studied using ferromagnetic resonance (FMR) measurements on samples annealed at 693 K for several periods of time and differential scanning calorimetry (DSC) measurements using several heating rates between 5 and 30 K/s. The increase of the FMR linewidth with annealing time is attributed to crystallization. Assuming a linear relation between the linewidth increase and the transformed volume fraction, it is possible to determine the Avrami exponent, n, from the FMR results: n=1.83±0.10. The DSC measurements show two exothermal peaks, suggesting a crystallization process involving more than one phase. The Avrami exponents calculated from the DSC measurements are n=1.80±0.10 for the first peak and n=1.50±0.10 for the second peak; these results are consistent with a diffusion-controlled process with a decreasing nucleation rate for the first peak and with a nucleation rate close to zero for the second peak. There is a reasonable agreement between the FMR results and the DSC results for the first peak. The activation energies are 353.5±2.8 and 424.9±10.2 kJ/mol for the first and second peaks, respectively.

INFLUÊNCIA DOS TRATAMENTOS TERMOMECÂNICOS NA ABSORÇÃO DE HIDROGÊNIO NO ZIRCÔNIO PURO

Rodrigo Vitorino da Silva1 Rafaella Martins Ribeiro2 Dilson Silva dos Santos3 Resumo Ligas a base de zircônio são utilizadas na indústria nuclear devido à suas boas propriedades mecânicas, baixa absorção de nêutrons e boa resistência à corrosão. No entanto, durante a operação do reator, o hidrogênio é absorvido pela liga, resultando na formação de hidretos. Para aumentar o tempo de operação desses componentes é necessário o desenvolvimento de novas ligas que retardem ou diminuam esse efeito. Este trabalho tem como objetivo avaliar a influência dos tratamentos termomecânicos no zircônio puro, através de testes que avaliem a corrosão, absorção e dessorção de hidrogênio nas condições laminada a frio e tratada termicamente. Análises de difração de raios-X indicam a existência de uma matriz Zr-α para as condições analisadas. Após a hidrogenação são formados ZrH e ZrH2. O zircônio puro encruado absorveu 1,23%p H. O zircônio puro laminado e tratado termicamente absorveu 0,85%p H. O resultado de DSC mostra que a decomposição dos hidretos ocorre em faixas de temperatura entre 318 e 368°C para a condição tratada termicamente. Para condição laminada, o resultado mostra uma transformação de fase entre 565 e 650°C. O conjunto de resultados permite concluir que a amostra tratada termicamente absorve menos hidrogênio, porém o início do processo é menor. A amostra tratada termicamente é menos resistente a corrosão e a velocidade da corrosão é menor. Palavras-chave: Zircônio; Hidrogênio; Laminado; Tratado termicamente.

Diffusion-Controlled Nanocrystallization of the Fe73.5Cu1Nb3Si15.5B7 Finemet Amorphous Alloy

Crystallization of the amorphous metallic alloy Fe73.5 Cu1Nb3 Si8.5 B14 was investigated by ferromagnetic resonance (FMR), small angle in situ X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC). Only one crystalline phase was observed by WAXS and only one peak was observed by DSC. The activation energies, calculated from FMR and DSC data, were 287 kJ.mol-1 and 313.4 kJ.mol-1, respectively. The values calculated for the Avrami exponent were 0.98 (FMR) and 1.4 (DSC). These values correspond to different mechanisms of nucleation and growth; however, the SAXS /WAXS results suggest that the dominant mechanisms are nucleation and growth of crystals from small dimensions.

Crystallization Kinetics of Fe-Based Amorphous Alloys with Addition of Ag-Y

Amorphous metallic alloys ribbons, with composition Fe73.5B9Si11.5Ag2Y4 and Fe73.5B9Si11.5Ag4Y2, namely FeBSi-Ag2Y4 and FeBSi-Ag4Y2 respectively, were submitted to nonisothermal heat treatment by using differential scanning calorimeter, DSC, under different heat rate to determine the crystallization kinetics of these alloys. It was observed, for each alloy, two peaks as a product of crystallization, which indicates two reactions of phase transformation corresponding to α-Fe and Fe3B respectively. The Avrami exponent, n, and also the activation energy for crystallization Ec for two alloys are very close, indicating the same behavior for the two compositions analyzed. For sample FeBSi-Ag2Y4 the n values obtained were 1.5± 0.1 (first peak) and 1.7± 0.1 (second peak), while for the FeBSi-Ag4Y2 the n values were 1.5±0.1 and 1.8±0.4 respectively. The results are consistent with a diffusion-controlled process with a decreasing nucleation rate, for all reactions. Introduction The ability to predict and control the crystallization of metallic glasses is often critical for the preparation and preservation of useful microstructures. The quantitative analysis of crystallization data can also provide valuable information about the magnitude and temperature dependence of nucleation and growth processes. The interest in the nanostructured alloys has recently been focused on control of the nanocrystalline structure which often induces superior mechanical and physical properties, such as good mechanical properties, soft magnetism, high catalytic properties and others. Several techniques have been used to study the crystallization of glasses and amorphous metallic alloys, such as ferromagnetic resonance (FMR) [1-2], small angle x-ray scattering [3], electrical resistivity [4] Mössbauer spectrometry [5] and differential scanning calorimetry (DSC) [6-7]. The last one when used on the non-isothermal module should be adapted for better represent the crystallization kinetics of the amorphous metallic alloys [8]. The aim of this work was to investigate the crystallization kinetics of two Fe-based amorphous metallic alloy, using a non-isothermal technique by means of DSC.