Papers by Author: Sonia Simões

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: 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.