Carlos Angelo Nunes

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.

Production, Microstructural Characterization and Mechanical Properties of As‐Cast Ti‐10Mo‐xNb Alloys

Beta titanium (Ti) alloys are one of the most promising groups of Ti alloys for biomedical applications. This work presents the production, microstructural characterization, and mechanical properties of as-cast Ti-10Mo-xNb (x = 0, 3, 6, 9, 20, and 30) alloys. They were produced via arc melting and characterized by scanning electron microscopy and X-ray diffraction. The density of each alloy was measured by the Archimedes method and the mechanical characterization was carried out by using the Vickers microhardness test and Youngs modulus measurements. The results show a near beta microstructure for niobium (Nb) contents lower or equal to 9 wt% while beta single-phase microstructure was obtained for higher Nb additions. The microhardness values decreased with the increase of Nb content in the alloys. The elastic modulus values of Ti-10Mo-3Nb and Ti-10Mo-20Nb alloys were lower than those of cp Ti and Ti-6Al-4V.

Nb–Ta alloys by aluminothermic reduction of Nb2O5/Ta2O5 mixtures and electron beam melting

Abstract The aluminothermic reduction (ATR) of Nb 2 O 5 followed by the electron beam melting (EBM) of ATR-electrodes has been successfully applied to the commercial production of pure niobium. In this work the technical viability for production of Nb–Ta alloys by the same route is discussed. We have calculated the heat balance for the ATR step for situations involving the use of either a pre-heated charge or a thermal booster substance (NaClO 3 ). The overall effects of using over-stoichiometric Al amounts in the ATR step are discussed. Microstructural and chemical analysis results as well as values of metallic yield on the production of a Nb-20 wt% Ta alloy ingot are presented. The limits for refining Nb–Ta alloys in terms of oxygen and nitrogen by melting under high vacuum are estimated by employing the metal (Nb, Ta)-gas (N 2 , O 2 , H 2 O) interaction equations available in the literature.

Maximisation of the ratio of microhardness to the Young's modulus of Ti-12Mo-13Nb alloy through microstructure changes.

Alloys for orthopaedic and dentistry applications require high mechanical strength and a low Youngs modulus to avoid stress shielding. Metastable β titanium alloys appear to fulfil these requirements. This study investigated the correlation of phases precipitated in a Ti-12Mo-13Nb alloy with changes in hardness and the Youngs modulus. The alloy was produced by arc melting under an argon atmosphere, after which, it was heat treated and cold forged. Two different routes of heat treatment were employed. Phase transformations were studied by employing X-ray diffraction and transmission electron microscopy. Property characterisation was based on Vickers microhardness tests and Youngs modulus measurements. The highest ratio of microhardness to the Youngs modulus was obtained using thermomechanical treatment, which consists of heating at 1000°C for 24h, water quenching, cold forging to reduce 80% of the area, and ageing at 500°C for 24h, where the final microstructure consisted of an α phase dispersed in a β matrix. The α phase appeared in two different forms: as fine lamellas (with 240±100 nm length) and massive particles of 200-500 nm size.

Phase Transformations during the Preparation of Ti6Si2B by High-Energy Ball Milling

Recently, it was identified the existence of a new intermetallic phase in the Ti-Si-B ternary system with atomic composition near Ti6Si2B. In the present work, we report on the phase transformations during the preparation of Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B (at.-%) powders in a planetary Fritsch P-5 ball mill from high-purity elemental powders. To understand the phase transformations, powder Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B samples milled for 90 h were vacuum heated at various temperatures. The starting materials and milled powders were characterized by means of X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopes, and differential scanning calorimetry (DSC). Results indicate that the Ti peaks widen and weaken with the increasing milling time and the silicon was practically dissolved into Ti and TiH2 lattices during milling and formed solid solutions in pre-alloyed Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B powders, respectively. The use of titanium hydride instead of titanium as starting material allowed accelerating the mechanical alloying process, i.e., the Ti6Si2B phase was formed during heating at lower temperatures than in case of titanium as starting material. As previously observed, the decomposition reaction of the titanium hydride occurred near 550C. Powder (TiH2)-22.2Si-11.1B sample milled for 90 h presented very fine particle size lower than 20 nm. The ternary Ti6Si2B phase was formed in powder Ti-22.2Si-11.1B samples after heat treatment. Traces of Ti and Ti5Si3 were also detected.

Analysis of nickel-niobium alloys by inductively coupled plasma optical emission spectrometry.

Inductively coupled plasma optical emission spectrometry (ICP-OES) was applied to the analysis of major and minor elements of Ni-Nb alloys obtained by aluminothermic reduction process. Digestion of samples was made using a mixture of HF+HNO(3). Minor and trace elements were determined without matrix separation. The precision for all constituents was <3%. Recoveries for the analyte-spiked samples were 95%.

The effect of excess Al and fabrication environment on the composition and microstructure of V–Al alloys

Abstract V–Al alloys have been used commercially to produce pure vanadium by electron beam melting (EBM), as well as to obtain Ti-based alloys (e.g., Ti–6Al–4V). Concerning the production of pure vanadium, earlier studies have indicated that V–Al alloys presenting Al contents in the range of 10–15 wt% are the most suitable for direct EBM. In this study, we have investigated the aluminothermic reduction (ATR) of V 2 O 5 in order to define process parameters for the production of V–Al alloys with composition in the previously mentioned range. The effect of excess Al and reactor atmosphere (air, argon) on the alloy composition (Al, O, N) and microstructure was studied. The produced alloys were characterized by chemical analysis (Al, O, N), scanning electron microscopy (SEM) (back-scattered electron image, BSEI), electron probe microanalysis (EPMA) (wavelength dispersive spectroscopy, WDX), X-ray diffraction (XRD) and microhardness measurements.

Mechanical Characterization of Ti–12mo–13nb Alloy for Biomedical Application Hot Swaged and Aged

Beta titanium alloys were developed for biomedical applications due to the combination of its mechanical properties including low elasticity modulus, high strength, fatigue resistance, good ductility and with excellent corrosion resistance. With this perspective a metastable beta titanium alloy Ti-12Mo-13Nb was developed with the replacement of both vanadium and aluminum from the traditional alloy Ti-6Al-4V. This paper presents the microstructure, mechanical properties of the Ti-12Mo-13Nb hot swaged and aged at 500 °C for 24 h under high vacuum and then water quenched. The alloy structure was characterized by X-ray diffraction and transmission electron microscopy. Tensile tests were carried out at room temperature. The results show a microstructure consisting of a fine dispersed α phase in a β matrix and good mechanical properties including low elastic modulus. The results indicate that Ti-12Mo-13Nb alloy can be a promising alternative for biomedical application.