Papers by Author: Benilde F.O. Costa

Abstract: Perturbed Angular Correlation (PAC) measurements were performed at room temperature on samples of Hf7Ni10 and of Hf7Ni10 combined with TiV0.8Cr1.2, and at temperatures in the range from 17 K to 350 K in a concentrated hydride of the combined alloy. The PAC results show that the Hf7Ni10 intergranular phase becomes amorphous in the combination process. The results for the hydride composite show in the Hf7Ni10 phase the formation of a stable interstitial hydrogen state below 100 K and a smooth rearrangement of the hydrogen atoms position above 100 K,corresponding to a slow diffusion movemnet of a short-range nature. A second phase transition is observed at around 300 K, presumably to aphase precursor of the hydrogen desorption of the hydride.

Abstract: A coarse-grained tetragonal sigma phase Fe47.4V52.6 at% alloy was ground in vacuum in a vibratory mill. The sigma phase transforms into a bcc alpha phase. A concomitant partial amorphisation of the bcc phase occurs. The near-equiatomic FeV alloys are concluded to behave in the same way as the related FeCr alloys when they are ball-milled in vacuum in similar milling conditions. From Mössbauer spectra, it is concluded that about half of the iron atoms are contained in the amorphous phase in the stationary state. From X-ray diffraction patterns there is also a clear evidence of amorphisation, not seen in previously published studies.

Abstract: A β-FeSi2 sample was ball-milled for different periods in a vibratory ball-mill and studied by X-ray diffraction and Mössbauer spectroscopy. It transforms gradually with milling time into an α-FeSi2 phase.

Abstract: Elemental powder mixtures of Fe and Si were mechanically alloyed with a ball-mill. Mössbauer spectroscopy and X-ray diffraction were used to characterise the microstructural changes of these mixtures which are induced by high-energy ball-milling. Mössbauer spectra are discussed in terms of two main spectral components (corresponding to FeSi alloy and α-Fe) and the time dependence of the alloy formation follows a Johnson-Mehl type law. Calorimetry measurements show that the formed alloy is stable up to 800°C as no crystallisation or phase transformation peaks are observed. From X-ray diffraction, a crystallite size of 9 nm is obtained. Magnetic measurements at low temperature were carried out on the final alloy and the saturation magnetic moment at 4.2K is 0.44 μB/Fe.