Marcelo Falcão de Oliveira

Crystallization behavior of amorphous Al84Y9Ni5Co2 alloy

Abstract The crystallization behavior and microstructure development upon annealing amorphous Al 84 Y 9 Ni 5 Co 2 melt-spun ribbons have been investigated by a combination of differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Crystallization is shown to occur in three stages: (1) primary crystallization of α-Al; (2) formation of an unknown crystalline phase; (3) formation of Al 3 Y, Al 9 Co 2 and a phase similar to Al 16 Ni 3 Y. The morphology, EDS and diffraction patterns indicated that the two unknown phases, from the second and third stages, are different phases and both are quaternary compounds.

A new correlation between electronic parameters and glass forming ability of metallic alloys

A quantitative correlation between the glass forming ability and the electronic parameters of metallic alloys is presented. It is found that the critical cooling rate for glass formation (R c) correlates well with the average work function difference (Δφ) and the average electron density difference ( ) among the constituent elements of the investigated alloys. A correlation coefficient (R 2) of 0.77 was found for 68 alloys in 30 metallic systems, which is better than the previous proposed correlation between the glass forming ability and the average Pauling electronegativity difference.A quantitative correlation between the glass forming ability and the electronic parameters of metallic alloys is presented. It is found that the critical cooling rate for glass formation (R c) correlates well with the average work function difference (Δφ) and the average electron density difference ( ) among the constituent elements of the investigated alloys. A correlation coefficient (R 2) of 0.77 was found for 68 alloys in 30 metallic systems, which is better than the previous proposed correlation between the glass forming ability and the average Pauling electronegativity difference.

Corrosion resistance and glass forming ability of Fe47Co7Cr15M9Si5B15Y2 (M=Mo, Nb) amorphous alloys

In the present work the effect of substituting Mo with Nb on the glass forming ability and corrosion resistance of Fe-Co-Cr-M-Si-B-Y (M=Mo, Nb) ribbons with high corrosion resistance is investigated. The X-ray powder diffraction pattern indicates that the alloy containing both Nb and Mo presented greater glass forming ability than the alloy containing either of these elements separately. The results obtained indicate that Mo is more effective in enhancing corrosion resistance than the Nb in 4.0 M HCl solution. The alloy containing both Nb and Mo presented greater overall corrosion resistance than the alloy containing only one of these elements.

Corrosion behaviour of a dissimilar joint TIG weld between austenitic AISI 316L and ferritic AISI 444 stainless steels

Abstract The AISI 444 stainless steel (SS) has become an option to substitute the AISI 316L SS because of its low cost and satisfactory corrosion resistance. However, the use of AISI 444 alloy tubes in heat exchangers causes the welding of a dissimilar joint. The aim of this study was evaluate the corrosion resistance of the tube-to-tubesheet welded by a TIG process composed of AISI 316L and AISI 444. Preparation of samples was executed through replication of tube-to-tubesheet joints. In order to test the corrosion resistance of the welded joint, the following tests were applied: sensitisation, mass loss from room temperature up to 90 °C and electrochemical corrosion tests in 0.5 mol/L HCl and 0.5 mol/L H2SO4 electrolytes. The results have shown that the dissimilar joint suffers galvanic corrosion with increased degradation of the heat-affected zone of the AISI 444 tube. Nevertheless, the mechanisms of localised corrosion (pit and intergranular) were more active in the AISI 316L alloy. It is concluded that the dissimilar joint showed better corrosion resistance than the welded joint composed solely of AISI 316L at temperatures up to 70 °C, as the conditions observed in this work.

Crystallization Behavior of Amorphous Ti51.1Cu38.9Ni10.0 Alloy

A good glass former, Ti57.4Cu33.4Ni9.2, was selected using the topological instability criterion (lambda criterion) and the average electronegativity approach. The crystallization behavior and microstructural development of amorphous melt-spun ribbons of this new composition in response to heat treatment were investigated using a combination of differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results indicate that the crystallization of the Ti57.4Cu33.4Ni9.2 alloy takes place through of three exothermic reactions with the nucleation of TiCu, Ti2Cu and Ti2Ni. The kinetics of crystallization were investigated by DSC, and the kinetic parameters were determined using Kissinger’s method.

Resistência à corrosão de junta dissimilar soldada pelo processo TIG composta pelos aços inoxidáveis AISI 316L e AISI 444

O aco inoxidavel AISI 444 tornou-se uma opcao para substituir a liga AISI 316L devido ao seu menor custo e satisfatoria resistencia a corrosao. Entretanto, o uso da liga AISI 444 no feixe tubular de trocadores de calor acarreta na soldagem de uma junta dissimilar. O presente estudo teve por objetivo avaliar a resistencia a corrosao da junta tubo-espelho soldada pelo processo TIG composta pelas ligas AISI 316L e AISI 444. A manufatura das amostras consistiu em replicar o projeto da junta tubo-espelho de trocadores de calor. Realizou-se em juntas soldadas ensaios de sensitizacao, perda de massa por imersao desde a temperatura ambiente ate 90 oC, e ensaios eletroquimicos de polarizacao potenciodinâmica nos eletrolitos 0,5 mol/L de HCl e 0,5 mol/L de H2SO4. Os resultados mostraram que a junta dissimilar sofreu corrosao galvânica com maior degradacao na zona afetada pelo calor (ZAC) do tubo AISI 444. Porem, os mecanismos de corrosao localizada (pite e intergranular) demonstraram ser mais ativos para a liga AISI 316L. Conclui-se que a junta dissimilar apresentou melhor resistencia a corrosao do que a junta soldada composta unicamente pela liga AISI 316L em temperaturas de ate 70 oC, conforme as condicoes observadas neste trabalho.