Papers by Author: Maria do Carmo Silva

Abstract: In this work a 50Ni50Ti at % powder mixture, commercially pure, prepared by mechanical alloying in an attritor with the following conditions: the milling speed and the ball charge were 1500 rpm and 15:1 respectively. The milling time was 1h under a nitrogen atmosphere at room temperature. After milling it was determined the particles size distribution, the phases by Xray diffractions (XRD) and the powder morphology by scanning electron microscopy (SEM). The powders after milling were compacted and hot extruded at 600 °C with an extrusion ratio of 3 to 1 and characterized by evaluation the density and microstructural determination by optical microscopy. The obtained results are discussed to show that this route could be an alternative route to prepare the shape memory alloy.

Abstract: Silicon-iron alloys with silicon content about 6.5wt.%Si offer a great potential for applications aiming reduction of core loss in electric parts. Deposits of the Fe-6.5wt%Si alloy produced by spray forming were annealed at temperatures between 400 and 1300oC, during 1h in vacuum. The grain size has a great importance to the magnetic properties. In the present work, it was analyzed the influence of the metallography parameters in order to get the best accuracy to determine the grain size according to ASTM 112-96. Chemical composition, time and temperature of specific etchings were modified and tested in different conditions of observation in light microscopy. Bright field, polarized light and dark field were used in the samples aiming to measure the grain size. The best etchings were Nital 10% and Marshall, both at room temperature. The results of grain measurement are presented in their relationship with the magnetic properties.

Abstract: The effect of high energy milling on powders of a FeNi (50/50) alloy and a 316L stainless steel has been evaluated by means of X-Ray Diffraction (XRD). The average microstrain as function of the milling time (1/2h, 1h and 8h) was determined from XRD data. The displacement and broadening of the (XRD) peaks were used for estimate the stacking fault energy (SFE), using the method of Reed and Schramm. It was estimated SFE=79 mJ/m2 for the FeNi (50/50) alloy and SFE=14 mJ/m2 for the 316L stainless steel. The better experimental conditions for determining the SFE by XRD are discussed.

Abstract: NiTi alloys with equiatomic composition of NiTi have the highest technological interest for its potencial application in differents areas such as biomedical, naval, aerospace, nuclear, automobilist , robotic,etc. In this work , it was used a 50Ni50Ti at % powder mixture, comercially pure, prepared by mechanical alloying in a Attritor with the following conditions: the milling speed and the ball charge were 1500 rpm and 10:1 respectively. The milling time was 2,4,8 and 16h, under an argon atmosphere at room temperature. After milling it was determined the particle size distribution, the phases by X-ray diffractions (XRD) and the powder morphology by scanning electron microscopy (SEM). The milling promotes dissolution of Titanium in Nickel and continuous amorphization by increasing the milling time. After 16h milling the alloy was almost amorphous. The powders after milling were compacted and heat treated at high temperature and microstructural evolution was characterized. In the heat treated samples were detected different phases showing heterogeneity in the alloy. The detected phases were Ni3Ti, NiTi, Ni2Ti and Ni2Ti4O. Contamination by milling was detected in the powder after milling and in the heat treated samples.