Papers by Author: Ana Sofia Ramos

Abstract: The aim of this work is to join -TiAl intermetallics to Ni based superalloys by solid state diffusion bonding. The surface of the -TiAl alloys and Ni superalloys to be joined was prepared by magnetron sputtering with a few microns thick Ni/Al reactive multilayer thin films with nanometric modulation periods. Sound joining without cracks or pores is achieved along the central region of the bond, especially at 800°C and when a 14 nm period Ni/Al film is used as filler material. During the diffusion bonding experiments interdiffusion and reaction inside the Ni/Al multilayer thin film and between the interlayer film and the base materials is promoted with the formation of intermetallic phases. The final reaction product in the multilayer films is the B2-NiAl intermetallic phase. The interfacial diffusion layers between the base materials and the multilayer films should correspond to: 3-NiTiAl and 4-Ni2TiAl phases from the -TiAl side; Ni-rich aluminide and -phase from the Inconel side. These intermetallic phases are responsible for the hardness increase observed on the diffusion layers.

Abstract: Joining nickel based superalloys to gamma-TiAl intermetallic alloys will contribute to a more efficient application of these advanced materials, particularly in extreme environments. In this study, Inconel alloy and gamma-TiAl are joined using as filler alternated nanolayer thin films deposited onto each base material. The nanolayers consisted in Ni/Al exothermic reactive multilayer thin films with periods of 5 and 14 nm deposited by d.c. magnetron sputtering in order to improve the adhesion to the substrates and to avoid the reaction between Ni and Al. Diffusion bonding experiments with multilayer coated alloys were performed under vacuum at 800°C by applying 50 MPa during 1h. Bonding was achieved in large areas of the centre of the joints where regions without cracks or pores were produced, especially when using multilayer thin films with a 14 nm modulation period.

Abstract: Successful solid state bonding of titanium aluminides requires the use of high temperature and pressure. In previous works, authors have demonstrated that the use of Ti/Al multilayer thin film as an interlayer, deposited by d.c. magnetron sputtering onto the joining surfaces, can effectively lower the bonding temperature. The enhanced diffusivity of these nanometric layers and the heat evolved by the formation of γ-TiAl improves the joinability of titanium aluminide by solid-state diffusion bonding. In the present work, further improvement of the process was pursued by doping the interlayer with 2.8 at.% of Ag; previous studies have confirmed that silver favours the transformation Ti+Al→γ-TiAl. The solid-state diffusion bonding experiments were performed in vacuum by applying 50 MPa at 900°C for 1 h. The effect of the third element on the microstructure and chemical composition along the bonding interface has been analyzed. Microstructural characterisation of the interface was performed by scanning and transmission electron microscopy. Chemical compositions were analysed by energy dispersive X-ray spectroscopy. No defects were observed at the interface and sound bonding was achieved between the interlayers and base γ-TiAl. The bonding interface shows a fine-grained microstructure, slightly coarser than the one formed at the same temperature with the undoped Ti/Al multilayer.

Abstract: As TiAl based alloys begin to approach maturity, the development of successful and cost effective joining methods will be required. The growing industrial interest in these materials, particularly in aerospace and automotive industry, led to an interesting challenge - how to joint parts and components in order to produce integrated and resistant structures. Diffusion bonding of materials produces components with thinner interfaces than other joining techniques do. The absence of abrupt microstructure discontinuity and the small deformation induced maximize joint strength. This work focuses on the joining of TiAl using a thin multilayer obtained by alternating nanometric layers of titanium and aluminium. The Ti/Al layers were deposited onto the γ-TiAl samples by DC magnetron sputtering. The interfaces of these diffusion bonded joints depend on processing and deposition conditions. In this work we describe the influence of bilayer thickness (period) and on microstructure and chemical composition of the joining interfaces.

Abstract: The optimisation of joining technologies is essential to the application of advanced materials in the design of parts and devices. The development of intermetallic compounds, as structural materials, inevitably requires a new approach to join these compounds to themselves or to other materials. Among different intermetallic classes, titanium aluminides are one of the most studied. However, the industrial application is far from being proportional to the research, due to different problems, where joining processes have an important role. The present paper highlights the state of art on joining γ-TiAl alloys. A review is presented with special emphasis on solid-state diffusion bonding process, because it seems to be the most suitable technique to produce high quality joints of advanced materials. The influence of the bonding conditions on the physical and mechanical properties of the joints is highlighted and the introduction of single or multiple interlayers to assist in the bonding process is discussed. A novel approach developed by the authors to the solid-state diffusion bonding of γ-TiAl alloys using Ti/Al multilayer thin films as bonding materials is proposed. The improvement of the solid-state diffusion bonding will induce sound joints at lower temperatures or pressures.

Abstract: Porous Ti-Nb alloys are promising candidates for biomedical applications. In the present study, alloy powders containing 60 wt-% Nb were prepared by high-energy milling of Nb, Ti, and/or TiH2 powders. The high-energy milling process was carried out in a planetary ball mill. The starting and as-milled materials were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). Elemental (Nb, and Ti) and TiH2 powder mixtures with composition Nb-40wt%Ti were mechanically alloyed for 2 to 30 h. The formation of a BCC Nb(Ti) solid solution by high-energy milling using elemental Ti powder to produce Nb-40Ti was observed after milling for 30 h. A HCP-Ti solid solution was formed after milling for 30 h due to the partial decomposition of titanium hydride powder mixture during high-energy milling.