Gilberto Carvalho Coelho

Liquidus projection of the Nb–Si–B system in the Nb-rich region

Alloys of the Nb–Si–B system have been evaluated aiming its use as high temperature structural materials. In this work the liquidus projection of the Nb–Si–B system in the Nb-rich region has been established based on the microstructural characterization of arc-melted alloys through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Six different primary solidification regions were observed: Nbss, NbB, T2 (aNb5Si3), Nb3Si, T1 (bNb5Si3) and D88. The following ternary invariant reactions are proposed to occur in the region of study: II1: L+NbB()Nbss+T2 ;I I 2: L+T1()Nb3Si+ T2 ;I I 3: L+Nb3Si()Nbss+T2; III1: L+NbB+D88()T2; III2: L+T1+D88()T2. # 2003 Published by Elsevier Science Ltd.

New data on phase equilibria in the Nb-rich region of the Nb-B system

The currently accepted Nb-B phase diagram shows Nbss (solid solution), Nb3B2, NbB, Nb5B6, Nb3B4, NbB2, B, and liquid L as the stable phases in this system. There is a general agreement in the literature about the stability of the NbB, Nb3B4, and NbB2 phases. However, the stability of Nb3B2, Nb5B6, and Nb2B3 phases is arguable. The aim of this work was to reevaluate the phase equilibria in the Nb-rich region (0-50at.% B) of the Nb-B system. The alloys were arc melted from high purity materials and heat-treated at 1700 °C under high vacuum. The samples were characterized by scanning electron microscopy/back-scattered electron image (SEM/BSE) and x-ray diffraction (XRD). The most important findings were: (1) no liquid formation was observed during heat-treatments of the alloys at 1700 °C; (2) the eutectic reaction in the Nb-rich region is L ↔ Nbss+ NbB with liquid eutectic composition close to 16 at.%B; and (3) the Nb3B2-phase is formed through the peritectoid reaction Nbss+ NbB ↔ Nb3B2. These results support the phase diagram proposed by Rudy [1969 Rud] for the Nb-rich region, which is not in agreement with the currently accepted Nb-B phase diagram.

Microstructural characterization of Nb–B–Si alloys with composition in the Nb−Nb5Si2B (T2-phase) vertical section

Multiphase alloys of the Me–B–Si (Me=refractory metal) systems have been considered for structural applications at high temperatures. In this work, the solidification pathway and phase stability of Nb-rich Nb–B–Si alloys were evaluated from arc-melted and heat-treated samples. The primary phases observed in the region of study were Nbss, T2-phase and D88-phase. The as-cast microstructure of all the alloys indicates formation of an Nbss+T2 eutectic in the last part to solidify. From the heat treatment of several alloys, the Nbss−T2 two-phase field was observed to exist at 1700 °C. Based on DTA experiments, the Nbss−T2 two-phase field should be stable up to 2150 °C.

Evaluation of the invariant reactions in the Ta-rich region of the Ta-B system

Invariant reactions in the Ta-rich side of the Ta-B system were evaluated. The alloys were arc melted from pure materials. Pellets prepared from powder mixtures of Ta and TaB were heat treated under vacuum at 1900 and 1950 °C. The microstructures of the alloys were characterized through scanning electron microscopy images and x-ray diffraction (XRD). The microstructural analyses confirmed the type of reactions occurring in Ta-rich region; however, the composition of the liquid phase should be altered: (a) L ↔ TaSS+Ta2B from 23 to 18 at.% B and (b) L+TaB↔Ta2B from 27 to 22.5 at.% B. The formation of Ta3B2 was confirmed as the understoichiometry of Ta2B. The decomposition of Ta2B occurs between 1900 and 1950 °C. The XRD data for alloys with composition between 34 and 50 at.% B indicate the existence of a new high-temperature phase.

Microstructural evidence of beryllium in commercial dental Ni-Cr alloys

The focus of this work was to determine microstructural features in commercial Ni-Cr alloys which could be used to identify indirectly the presence of beryllium. Thus, eight commercial alloys were characterized by chemical analysis, thermal analysis, X-ray diffraction (XRD), scanning electron microscopy - back-scattered electron images (SEM/BSE), energy-dispersive spectroscopy (EDS). The results indicate that the presence of beryllium can be inferred from microstructural analysis via XRD and SEM/BSE. The X-ray diffractograms of the beryllium-containing alloys showed clearly the existence of the NiBe intermetallic phase. SEM/BSE images of these alloys show a very characteristic eutectic microstructure which also indicates the presence of this element. These characteristics are not observed in the beryllium-free alloys.

Isothermal section of the Ti-Si-B system at 1250 ° C in the Ti-TiSi2-TiB2 region

region) of the ternary Ti-Si-B system at 1250 °C was determined from heat-treated alloys prepared via arc melting. Microstructural characterization has been carried out through scanning electron microscopy (SEM), X-ray diffraction (XRD) and wavelength dispersive spectrometry (WDS). The results have shown the stability of the near stoichiometric Ti

On Ti-18Si-6B and Ti-7.5Si-22.5B powder alloys prepared by high-energy ball milling and sintering

B composition. The present study concerns the preparation of titanium alloys that contain such phase mixed with α-titanium and other intermetallic phases. High-purity powders were initially processed in a planetary ball-mill under argon atmosphere with Ti-18Si-6B and Ti-7.5Si-22.5B at. (%) initial compositions. Variation of parameters such as rotary speed, time, and ball diameters were adopted. The as-milled powders were pressureless sintered and hot pressed. Both the as-milled and sintered materials were characterized by X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometry. Sintered samples have presented equilibrium structures formed mainly by the α-Ti+Ti

Mechanical Alloying and Sintering of Ni-Sn and Ni-Mg Powder Mixtures

Various intermetallics can be synthetized by mechanical alloying depending on alloy composition, type of mill, and milling parameters. This work discusses on the phase transformations during ball milling and subsequent sintering of Ni-Sn and Ni-Mg powder mixtures. The structural evaluation of as-milled and sintered samples was conducted by X-ray diffraction and scanning electron microscopy. Metastable phases were formed during milling of the Ni-Sn and Ni-Mg powder mixtures. The Ni3Sn and Ni3Sn2 compounds were produced after sintering from the as-milled Ni-25Sn and Ni-40Sn powders, respectively. Two-phase Ni2Mg+NiMg2 alloy was found after sintering of as-milled Ni-66Mg powders because the nominal Mg amount in starting powders was reduced due to its preferential cold welding during milling and/or evaporation during sintering.

Microstructure and Oxidation Resistance of Mechanically Alloyed and Sintered Ni-Nb and Ni-Nb-Ta Alloys

The present work reports on the microstructure and oxidation resistance of Ni-25Nb, Ni-20Nb-5Ta and Ni-15Nb-10Ta alloys produced by high-energy ball milling and subsequent sintering. The sintered samples were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry, and static oxidation tests. Homogeneous microstructures of the binary and ternary alloys indicated the major presence of the β-Ni3Nb compound as matrix, which dissolved large amounts of tantalum. Consequently, the β-Ni3Nb peaks moved toward the direction of smaller diffraction angles. Iron contamination lower than 6.7 at.-% was detected by EDS analysis, which were picked-up during the previous ball milling process. After the static oxidation tests (1100°C for 4 h) the sintered Ni-25Nb, Ni-20Nb-5Ta and Ni-15Nb-10Ta alloys presented mass gains of 31.5%, 30.5% and 28.8%, respectively. Despite the higher densification of the Ni-15Nb-10Ta alloy, the results suggested that the tantalum addition contributed to improve the oxidation resistance of the β-Ni3Nb compound.

AVALIAÇÃO DE MICRODUREZA DE LIGAS Hf–Si–B

O avanco tecnologico tem gerado demanda crescente por materiais que possam ser utilizados sob alta temperatura, o que inclui ligas intermetalicas MR–Si–B (MR = metal refratario) com estruturas multifasicas, que podem ser aplicados tambem em ambientes oxidantes. Assim, este trabalho teve por objetivo estudar a microdureza de ligas do sistema Hf–Si–B na secao isotermica a 1600oC, na regiao rica em Hf. Ligas Hf–Si–B foram produzidas com lâminas de Hf (min. 99,8%), Si (min. 99,998%) e B (min. 99,5%), em forno a arco e tratadas termicamente a 1600 oC sob atmosfera de argonio. As relacoes de fases nessas ligas foram previamente identificadas por difracao de raios-X e por contraste, em imagens obtidas via microscopia eletronica de varredura no modo eletrons retroespalhados. As ligas tiveram sua dureza analisada pelo metodo Vickers (microdureza) com carga de 0,2 kgf e tempo de aplicacao de 20 s . Os resultados, obtidos da media aritmetica de cinco medicoes realizadas para cada liga, mostram que as ligas estudadas apresentam elevada dureza.