Suchergebnisse

El artículo está disponible online en la web del editor (Elsevier) desde el 23 de junio de 2016. ; Wear and high temperature oxidation resistance of some titanium-based alloys needs to be enhanced, and this can beeffectively accomplished by surface treatment. Molybdenizing is a surface treatment where molybdenum is introducedinto the surface of titanium alloys causing the formation of wear-resistant surface layers containing molybdenum, whilealuminizing of titanium-based alloys has been reported to improve their high temperature oxidation properties. Whereaspack cementation and other surface modification methods have been used for molybdenizing or aluminizing of wroughtand/or cast pure titanium and titanium alloys, such surface treatments have not been reported on titanium alloys producedby powder metallurgy (PM). Also a critical understanding of the process parameters for simultaneous one stepmolybdeno-aluminizing of titanium alloys by pack cementation and the predominant mechanism for this process havenot been reported. The current research work describes the surface modification of titanium and Ti-6Al-4V prepared byPM by molybdeno-aluminizing and analyzes thermodynamic aspects of the deposition process. Similar coatings are alsodeposited to wrought Ti-6Al-4V and compared. Characterization of the coatings was carried out using scanning electronmicroscopy and x-ray diffraction. For both titanium and Ti-6Al-4V, the use of a powder pack containing ammoniumchloride as activator leads to the deposition of molybdenum and aluminium into the surface but also introduces nitrogencausing the formation of a thin titanium nitride layer. In addition, various titanium aluminides and mixed titanium aluminiumnitrides are formed. The appropriate conditions for molybdeno-aluminizing as well as the phases expected to beformed were successfully determined by thermodynamic equilibrium calculations. ; Regional Government of Madridthrough the project S2013/MIT-2862 (MULTIMAT-CHALLENGE-CM), and by the Ministry of Economy and Competitiveness of Spain through the project MAT2012/38650-C02-01

MAX phases exhibit excellent combination of ceramic and metallic-like properties. In this work, MAX phase Ti3SiC2 powder was synthesized starting-off with different combinations of elemental powders and carbides. The powders used were Ti, Si, C, SiC and TiC in different combinations, molar ratios and powder size. Powders were heat treated on a vacuum furnace for different times and temperatures for in situ production of the Ti3SiC2 MAX phase. High purity synthesized samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to identify and quantify the different phase constituents present. The main phase constituents in the powders produced were Ti3SiC2 and TiSi2. Up to 94% of Ti3SiC2 MAX phase was obtained using Ti:SiC:C as starting powders in a molar ratio of 3:1.5:0.5. Different phase constitution was observed on the surface and the centre of the samples. An optimal starting powder composition, molar ratio, heat treatment temperature and time is proposed for the formation of high purity Ti3SiC2 MAX phase. Selected mixture was studied thermodynamically and a reaction mechanism of formation of the MAX phase is proposed. ; Las fases MAX presentan una excelente combinación de propiedades cerámicas y metálicas. En este trabajo se sintetizaron polvos de fase MAX Ti3SiC2 a partir de diferentes combinaciones de polvos elementales y carburos. Los polvos utilizados fueron Ti, Si, C, SiC y TiC en diferentes combinaciones, proporciones molares y tamaño de partícula. Los polvos se sinterizaron en un horno de vacío a diferentes tiempos y temperaturas para la producción in situ de la fase MAX Ti3SiC2. Las muestras sintetizadas de alta pureza se analizaron mediante difracción de rayos X (XRD) y microscopía electrónica de barrido (SEM) para identificar y cuantificar las diferentes fases constituyentes presentes. Las principales fases constituyentes en los polvos producidos fueron de Ti3SiC2 y TiSi2. Se obtuvo hasta un 94% de fase MAX Ti3SiC2 utilizando como polvos de partida Ti:SiC:C en una proporción molar de 3:1,5:0,5. Se observó diferente formación de fase en la superficie y en el centro de las muestras. Finalmente, se propone una composición óptima de polvo de partida, relación molar, temperatura de sinterización y tiempo para la formación de la fase MAX de Ti3SiC2 de alta pureza. La mezcla seleccionada se estudió termodinámicamente y se propone un mecanismo de reacción para la formación de la fase MAX. ; The authors would like to thank the funding provided for this research by the Regional Government of Madrid (Dir. Gral. Universidades e Investigación) through the project P2018/NMT4411 (ADITIMAT-CM), the Spanish Government through the Ramón y Cajal contract RYC-2014-15014 and the project MAT2012-38650-C02-01.

The influence of carbon content on an iron-chromium cermet composite reinforced with Ti(C,N) (50 vol.%) has been studied. A thermodynamic simulation was performed using the ThermoCalc software to calculate the phase diagram of the composite. The results were validated by a thermal study performed using differential thermal analysis (DTA), and cermet samples with C percentages between 0 and 1 wt.% added to the steel matrix were prepared using a conventional powder metallurgy process. The sintered samples were characterised by measurements of density and hardness, microstructural analysis using scanning electron microscopy (SEM), microanalyses using energy dispersive X ray spectroscopy (EDX) and X-ray diffraction (XRD). The results obtained show a significant influence of the C percentage on the solidus temperature, which influences sintering behaviour, leading to changes in the Ti(C,N) particles' shape and composition, due to changes in the stoichiometry of the Ti(C,N). This influence is reflected in the cermet microstructure and hardness. The results are discussed with reference to the DTA and thermodynamic studies. ; The authors would like to acknowledge the financial support of the Spanish Ministry of Science and Innovation through R&D Project MAT2009-14448-C02 and the Regional Government of Madrid through the programme ESTRUMAT-CM (Ref. S2009/MAT-1585). ; Publicado

In this study, the oxidation behaviour of an iron matrix cermet containing 50 % vol. Ti(C,N) was investigated before and after heat treatment by oxidation tests performed in static air at temperatures between 500 °C and 1000 °C. The oxidation mechanism for this type of composite materials was established and it was found that the heat treated material presents lower mass gain than the as-sintered material at the early stages of the oxidation, due to the volatilization of oxides. The oxidation behaviour of the sintered cermet was compared to those obtained for two commercial materials used in the fabrication of cutting tools, a highspeed steel and a cemented carbide, revealing a higher oxidation resistance for the iron matrix cermet due to the formation of a self-protective layer during the oxidation process. ; The authors acknowledge the funding received for this work from the Spanish Government through the R&D projects MAT2009-14448-C02-02 and MAT2012-38650-C02-01 and the Regional Government of Madrid through the ESTRUMAT program (S-2009/MAT–1585).

In Press, Accepted Manuscript ; The aim of this work is the design, processing and characterization of a metal&-ceramic composite (cermet) with an iron &- aluminum alloy as metallic matrix and NbC as ceramic phase. The cermet is obtained from a composite powder containing in situ formed NbC-rich precipitates on an iron rich metal matrix. The starting powder was supplied by CBMM (Brazil), and it was produced by a synthesis process under development which introduces other metals such as Al, Si and Ti to the composition. Due to the in situ process, good bonding is expected between the carbide and the matrix. However it is necessary to study the processability of those complex particles and the transformations occurring during sintering to get the final microstructure. Thermodynamic studies by means of ThermoCalc® software were performed to predict the phases stable with the starting composition and also the influence of Fe and C additions. Samples were produced by uniaxial pressing and vacuum sintering (PS), and also by Field Assisted Hot Pressing (FAHP). The processing parameters for PS processing, that is, sintering temperature and time were based on themodynamic simulations by ThermoCalc® software together with thermal analysis. The powders were characterized by measuring density, particle size, carbon content and chemical composition; and consolidated samples by density and Vickers hardness. The microstructure and morphology of the powder and consolidated samples was analyzed by SEM. The addition of Fe and both C and Fe to the starting cermet composition provided good results as the final microstructure consisted essentially of NbC and Fe matrix. ; Spanish government, R&D project MAT2012-38650-C02-01 ; Publicado

MAX phases are a promising family of materials for several demanding, high-temperature applications and severe conditions. Their combination of metallic and ceramic properties makes MAX phases great candidates to be applied in energy production processes, such as high temperature heat exchangers for catalytic devices. For their successful application, however, the effect of the processing method on properties such as wear and mechanical behavior needs to be further established. In this work, the mechanical and wear properties of self-synthesized Ti3SiC2 and Cr2AlC MAX phase powders consolidated by different powder metallurgy routes are evaluated. Uniaxial pressing and sintering, cold isostatic pressing and sintering and hot pressing were explored as processing routes, and samples were characterized by analyzing microstructure, phase constitution and porosity. Wear behavior was studied by reciprocating-sliding tests, evaluating the wear rate by the loss of material and the wear mechanism. ; The authors would like to thank the funding provided for this research by the Regional Government of Madrid (Dra. Gral. Universidades e Investigación) through the project P2018/NMT4411 (ADITIMAT-CM), and the Spanish Government through the projects PID2019-106631GB-C43 and RTC2019-007049-4.

The use of titanium hydride as a raw material has been an attractive alternative for the production of titanium components produced by powder metallurgy, due to increased densification of Ti compacts, greater control of contamination and cost reduction of the raw materials. However, a significant amount of hydrogen that often remains on the samples could generate degradation of the mechanical properties. Therefore, understanding decomposition mechanisms is essential to promote the components' long life. Several studies on titanium hydride (TiH2) decomposition have been developed; nevertheless, few studies focus on the effect of the alloying elements on the dehydrogenation process. In this work, the effects of the addition of different amounts of Fe (5 and 7 wt. %) and Nb (12, 25, and 40 wt. %) as alloying elements were evaluated in detail. Results suggest that α→β transformation of Ti occurs below 800 °C; β phase can be observed at lower temperature than the expected according to the phase diagram. It was found that β phase transformation could take place during the intermediate stage of dehydrogenation. A mechanism was proposed for the effect of allying elements on the dehydrogenation process. ; This research was funded by the Regional Government of Madrid (program ADITIMAT-CM, ref. S2018/NMT-4411), and by the Ministry of Economy and Competitiveness of Spain (program MINECO, ref. PCIN-2016-123 project BIOHYB, and Ramón y Cajal contract RYC-2014-1504). ; Publicado

Proceeding of: World PM2016 Congress Proceedings. New materials and applications, biomedical applications ; MAX phases Ti2AlC and Ti3SiC2 foams with controlled porosity and pore size were produced using the space holder method. The foams were processed using water-leachable crystalline carbohydrate as space holder that involves: mixing, cold isostatic pressing, dissolution and sintering. Three combinations of volume percentage (20%-60%) and size distribution (250-1000 mum) of space holder were introduced during mixing. The foams were characterized and compared with the material without space holder. The characterization included: morphology (overall, open and closed porosity by Archimedes method) and gas permeability. Foams with porosity up to about 60 vol% and pore size distribution ranging from about 250 to 1000 mum were produced. Experimental porosity was compared to the theoretical expected porosity. The results show a bimodal porosity that can be customized by the sintering and the space holder. This study connects the processing parameters to the porosity created and allows control of porosity and pore size to produce tailor-made properties. ; The authors would like to thank the funding provided for this research by the Regional Government of Madrid- Dir. Gral. Universidades e Investigación, through the project S2013/MIT-2862 (MULTIMAT-CHALLENGE-CM), and by Spanish Government through Ramón y Cajal contract RYC-2014-15014 and the project MAT2012/38650-C02-01

This contribution focuses on modifying the composition and surface characteristics of powder metallurgy titanium substrates to improve their wear resistance and to reduce the surface Young's modulus, while maintaining the corrosion resistance and biocompatibility. The modification consists of Mo and TiN coatings produced by colloidal techniques which are deposited by electrophoretic deposition (EPD) or spraying onto green or sintered titanium substrates. The final surface structures are formed during a heat treatment, during which diffusion and consolidation of the layers take place. The surfaces are characterised by XRD, SEM, and micro-indentation; showing diffusion areas of between 40 and 85 microns with clear microstructural changes as well as higher hardness values. ; Funds provided by Spanish Government (program MINECO, ref. MAT2012-38650-C02-01) and Regional Government of Madrid (program MULTIMAT-CHALLENGE, ref. S2013/MIT-2862).

Proceeding of: World PM2016 Congress (New materials and applications: Biomedical applications) ; The aim of this work was the modification of the composition and surface microstructure of powder metallurgy titanium to improve the wear resistance and reduce the elastic modulus while maintaining the corrosion behavior, characteristics needed for biomedical applications. For this purpose, Mo and Nb coatings were produced by colloidal techniques. Stable aqueous suspensions were prepared from micro-sized powder of Mo and Nb particles, deposited onto the powder metallurgy titanium substrates (green or sintered). After a heat treatment to promote the diffusion and the consolidation of the layers, microstructural changes were obtained. In the case of green substrates, the co-sintering process provides a diffusion depth of 85-100 micron. In the as-sintered case, a uniform depth of 40-65 microns was reached. The surfaces were characterised by micro-hardness, corrosion and tribocorrosion testing, and the results showed that hardened surfaces presented lower tendency to corrosion both under static conditions and under sliding. ; Funds were provided by Spanish Government (programme MINECO, ref. MAT2012-38650-C02-01), Regional Government of Madrid (programme MULTIMAT-CHALLENGE, ref. S2013/MIT-2862) and Institute of Álvaro Alonso Barba (IAAB) for the research stay in CMEMS-UMINHO (University of Minho).

Invited keynotes paper from EUROPM2017, Milan. ; This work focuses on the evaluation of modified surfaces on Ti produced by powder metallurgy. These newly designed surface modifications are achieved by deposition and diffusion of a stable aqueous suspension prepared in one case from micro-sized Nb powder (Ti beta-stabilizer element) and in another case from Nb plus the addition of ammonium chloride, NH4Cl, (thermo-reactive diffusion process). Different design parameters such as diffusion element (Nb or Mo), state of the Ti substrate (green or sintered) and the treatment process (diffusion or thermo-reactive diffusion) lead to all the surface-modified materials, GreenTi-Nb, SintTi-Nb and Ti-Nb-NH4Cl, GreenTi-Mo, SintTi-Mo and Ti-Mo-NH4Cl. The modified Ti surfaces present a gradient in composition and microstructure (beta / alpha+beta / alpha phases) resulting in an improvement in some of their mechanical properties: (1) higher micro-hardness in all the modified materials and (2) lower elastic modulus (more similar to that of the human bone) in those without NH4Cl. ; The authors would like to thank the funding provided for this research by the Regional Government of Madrid (program MULTIMAT-CHALLENGE-CM, ref. S2013/MIT- 2862), and by the Ministry of Economy and Competitiveness of Spain (program MINECO, ref. PCIN-2016-123 and Ramón y Cajal contract RYC-2014-15014).