Liliana I. Duarte

Thermodynamic re-assessment of Fe-Ti binary system

The Fe–Ti binary system was re-assessed using the CALPHAD method in order to improve the capability of being extrapolated to a ternary or higher-order system. Compared with previous assessments, the main focus was put on the thermodynamic description of the two intermetallic compounds Fe2Ti and FeTi. The C14_Laves phase Fe2Ti was described by the two-sublattice model, which is widely used at present. By checking the homogeneity range on the boundary of the ternary systems involving the binary, the phase boundary of this compound was further confirmed. The FeTi phase with a BCC_B2 crystal structure was treated as the ordered phase of the BCC_A2 phase and a unified Gibbs energy function was used to describe both the ordered and disordered phases. Reproduction of the specific heat capacities of these compounds was another aspect paid particular attention to. Comprehensive comparisons of the calculated and experimental results regarding the phase diagram and thermodynamic properties show a good agreement between them and prove the validity of the present thermodynamic description.

Joining of TiAl using a thin multilayer

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.

Determination of liquidus temperature in Ti-rich alloys of the Fe-Ni-Ti system obtained by DTA, electrical conductivity and XRD measurements

Abstract Despite its significant technical relevance, a number of uncertainties remain regarding the Fe–Ni–Ti ternary system. These concern phase relations, the solidus–liquidus range and the liquidus projection, especially in the Ti-rich corner. In this study the melting and solidification behaviour of Tirich Fe–Ni–Ti alloys was characterized using differential thermal analysis, electrical conductivity measurements and high-temperature X-ray diffraction. Combining these different methods allowed us to determine the liquidus temperatures more precisely than with differential thermal analysis measurements alone. The results were compared with calculated values using the ThermoCalc software package.

Bonding γ-TiAl Alloys Using Ti/Al Nanolayers Doped with Ag

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.

Tungsten Carbide Powder Inserts in Ductile Iron

In this work the surface of ductile iron is hardened by a tungsten carbide (WC) layer. The inserted layer is obtained by pouring liquid cast iron on a mixture formed by WC particles (with an average diameter of 6 micrometers) and sodium silicate (acting as a binder). Results show that a sound inserted layer with 2.5mm thickness was produced; the layer is compact, the interstices between particles (with dimensions close to some tenth micrometer) are filled with base metal and the continuity between the WC particles and the base metal is ensured by growth of tungsten rich carbides from the WC particles into the metallic matrix. The surface hardness of the inserted layer achieves Fig.s higher than 500VHN/4.9N.