Papers by Author: Filomena Viana

Abstract: Intermetallics and superalloys brazing development is a current topic owing the extending use of these alloys in industrial applications. In this work a γ-TiAl alloy was joined to Inconel 718 by active metal brazing, using Incusil-ABA as filler. Joining was performed at 730 °C, 830 °C and 930 °C, with a 10 min dwelling time. The interfaces were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). For all processing conditions, the reaction between the base materials and the braze alloy produced multilayered interfaces. For all processing temperatures tested (Ag), (Cu), AlNi₂Ti and AlCu₂Ti were identified at the interface. Raising the brazing temperature increased the thickness of the interface and coarsened its microstructure. The increase of the extension of the interface was essentially due to the growth of the reaction layers formed near each base material, which were found to be mainly composed of intermetallic compounds. The mechanical behavior of the joints, at room temperature, was assessed by microhardness and shear tests. For all processing conditions the hardness decreases from periphery towards the Ag-rich centre of the joints. Brazing at 730 °C for 10 min produced the joints with the highest average shear strength (228±83 MPa). SEM and EDS analysis of the fracture surfaces revealed that fracture of joints always occurred across the interface, preferentially through the hard layer, essentially composed of AlNi₂Ti, resulting from the reaction between Inconel 718 and the braze alloy.

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: Nanocrystalline metals demonstrate a broad range of fascinating mechanical properties at the nanoscale, namely a significant increase in hardness and superior yield stress. In this regard, understanding grain growth in nanocrystalline metals is crucial, particularly because nano size grains are characterized by a high curvature, which results in a high driving force for grain growth. In this work, the effect of annealing conditions on grain size of copper nanocrystalline thin films was investigated. The nanocrystalline copper thin films were first deposited by d.c. magnetron sputtering on a copper substrate. The specimens were then annealed in vacuum at 100, 300 and 500°C from 10 minutes to 5 hours. Transmission electron microscopy observations revealed that the as-deposited thin films have a bimodal grain size distribution; an average grain size of 43±2nm and the presence of nanotwins. Abnormal grain growth was observed for some samples annealed. Increasing the annealing time induced significant grain growth and promoted twin formation in the larger grains. Finally, the hardness of these nanocrystalline Cu thin films was determined using atomic force microscope. The relation between mechanical properties, annealing conditions and grain size was analyzed.

Abstract: A detail study focussing the microstructural evolution of the interfacial zone in the course of the processing of Ti-47Al-2Cr-2Nb joints using Tini 67 as filler alloy was carried out in this investigation. Experiments, aiming the understanding of the mechanisms that promote the melting of the braze alloy, were performed below the solidus temperature of the filler, at 750 and 900°C. Diffusion brazed samples were joined at 1000 and 1100°C, with no dwelling stage and subsequently quenched in water in order to frozen the microstructure formed at the bonding temperature. The interfaces were analysed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. In the course of the heating stage, several single phase layers were formed within the filler alloy due to the solid state interdiffusion of Ti and Ni atoms. At 900°C, the microstructure of the filler evolved form the initial Ti (α)/(Ni)/Ti/ (α) layers to a Ti (β)/Ti2Ni/TiNi/TiNi3/TiNi/Ti2Ni/Ti (β) layered microstructure. The filler alloy begun to melt due to the eutectic reaction between the contiguous layers composed of Ti (β) and Ti2Ni. After joining, the main phases detected at the interfaces were α2-Ti3Al, Ti-Ni-Al and Ti-Ni intermetallics. For joining at 1000°C, a substantial amount of residual filler (Ti2Ni and Ti (α) particles) was also detected at the central zone of the interface. No marked evidences of residual filler zones were noticed for joining at 1100°C; instead, a mixture α2-Ti3Al with Ti-Ni-Al or Ti2Ni intermetallics was detected at the centre of the interface.

Abstract: The heat treatment of γ-TiAl alloy (Ti-47Al-2Cr-2Nb (at.%)) diffusion brazed joints was investigated. Joining was performed using a Ti/Ni/Ti clad-laminated braze alloy foil at 1050 and 1150°C with a dwell time of 10 minutes. The joints were subsequently heat treated at 1250 and 1350°C for 240 and 30 minutes, respectively. The microstructure and the chemical composition of the interfaces were analysed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. Microhardness tests performed across the interface were used to roughly predict the mechanical behaviour of the as-diffusion brazed and of the heat treated joints. Diffusion brazing produced interfaces with two distinct layers essentially composed of α2-Ti3Al and of TiNiAl; γ-TiAl was also detected for joining at 1150°C. After heat treating, the as-diffusion brazed microstructure of the interface was completely replaced by a mixture essentially composed of γ-TiAl and α2–Ti3Al single phase grains and of (α2 + γ) lamellar grains. Microhardness tests showed that the hardness of the as-diffusion brazed interfaces, which ranges from 567 to 844 HV (15 gf), is significantly higher than that of the titanium aluminide alloy (272 HV). All post-joining heat treatments lowered substantially the hardness of the interface, as the hardness of the main phases detected at the interfacial zone after heat treating the joints is comprised between 296 and 414 HV.

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.