Molybdeno-Aluminizing of Powder Metallurgy and Wrought Ti and Ti-6Al-4Valloys by Pack Cementation process

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-C02-0

Influence of carbon content on the sinterability of an FeCr matrix cermet reinforced with TiCN

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).Publicad

Oxidation and corrosion protection by halide treatment of powder metallurgy Ti and Ti6Al4V alloy

The aim of this work is to study the effect of halide treatment in powder metallurgy Ti-6Al-4V alloy, in order to improve high temperature oxidation and electrochemical corrosion resistance. Halide treatment was performed by immersing samples in a powder mixture containing 3 wt% NH4Cl, at 200 degrees C and 950 degrees C. Oxidation resistance was assessed by exposure to 600 degrees C for 300 h. The microstructure and phase constitution of the formed oxide layers was characterized. Corrosion resistance in aircraft simulated environment was evaluated with electrochemical impedance spectroscopy. Notable improvement in oxidation and corrosion resistance was observed for samples treated at 200 degrees C.The authors wish to thank the Ministry of Education and Science for funding through R&D Project MAT2012-38650-C02-01, and the Community of Madrid for its funding through ESTRUMAT program (S-2009/MAT-1585)

Boro-aluminising of P91 steel by pack cementation for protection against steam oxidation

High performance alloys are often the materials used for various components exposed to high temperature environments. In many cases, protective coatings are applied in these alloys, providing higher corrosion and oxidation resistance, compared to the base material. This study investigates the feasibility to apply boro-aluminising treatment on P91 steel by pack cementation process, to increase the steel high temperature properties in oxidising and corrosive environments. Packs activated by AlCl₃, NH₄Cl and KBF₄ were used to carry out the coating deposition at a temperature of 715°C for 6 h. The coating formed was analysed by means of SEM and XRD, and the compounds formed were identified. Cyclic steam oxidation for a total of 1008 h at 650°C revealed an oxide scale of 50 mum for the uncoated P91 steel, while the coated steel shows practically no oxidation effect.Publicad

Estudio de la síntesis de polvos de la fase MAX Ti3SiC2 por sinterización sin presión

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

Oxidation and Corrosion Behavior of New Low-Cost Ti-7Fe-3Al and Ti-7Fe-5Cr Alloys from Titanium Hydride Powders

High production costs of Ti alloys usually hinders their use in industry sectors like the automotive and hence, low-cost titanium alloys could broaden titanium alloy usage. This work presents the study of three alloys— Ti-Fe, Ti-Fe-Al, and Ti-Fe-Cr—produced by powder metallurgy methods. The design of the compositions was aimed at reducing cost and enhance the oxidation and corrosion resistance while not decreasing the mechanical performance. The use of titanium hydride as raw material instead of Ti powder is highlighted as a key feature in the design and manufacturing procedure of the alloys. Introducing a dehydrogenation process during sintering favors the densification process while reducing the oxygen contamination and the production cost. There is a lack of studies focused on the implementation of affordable PM Ti alloys in high demanding environments. Therefore, a study of high temperature oxidation resistance and electrochemical behavior was performed

Cermets based on FeAl-NbC from composite powders: design of composition and processing

In Press, Accepted ManuscriptThe 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-C02-01Publicad

Influence of heat treatment on the high temperature oxidation mechanisms of an Fe-TiCN cermet

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)

The use of dolomite as foaming agent and its effect on the microstructure of aluminium metal foams—Comparison to titanium hydride

Proceedings of: 8th EUFOAM Conference. Borovets, Bulgaria, 14-16 July 2010.In this paper dolomite MgCa(CO₃)₂ a naturally occurring mineral was demonstrated to be an effective foaming and stabilizing agent for aluminium with several notable advantages relative to the currently used titanium hydride foaming agent. Characteristic cell structures and microstructural features of foams produced with a dolomite foaming agent are examined and the properties of dolomite based foams produced in a one step process are compared with those produced using titanium hydride based process. The most notable structural feature of dolomite based foams is a smaller cell size and thinner cell faces. Foaming with MgCa(CO₃)₂ also gives rise to a marked increase in the stability of molten foams with a large range of foaming temperatures possible, and an almost complete absence of melt drainage even with extended foaming times. Many of these properties are attributed to the cell surfaces being covered by a thin oxide film formed during the foaming process.Publicad

Green synthesis and characterization of silver nanoparticles produced using 'Arbutus Unedo' leaf extract

Metallic nanoparticles have received great attention from chemists, physicists, biologists and engineers who wish to use them for the development of a new generation of nanodevices. In the present study silver nanoparticles were synthesized from aqueous silver nitrate through a simple and eco-friendly route using leaf broth of Arbutus unedo, which acted as a reductant and stabilizer simultaneously. The aqueous silver ions when exposed to the leaf broth were reduced and stabilized over long periods of time resulting in the green synthesis of surface functionalized silver nanoparticles. The bio-reduced silver nanoparticles were appropriately characterized. The results revealed the formation of single crystalline Ag nanoparticles with a narrow size distribution for each sample. The particles, although discrete, were predominately coated with the organic leaf extract forming small aggregates, which makes them stable over long time periods and highly appropriate for coatings or biotechnology applications.Publicad