A.S. Ramos

Joining of Superalloys to Intermetallics Using Nanolayers

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.

Structure and properties of sputtered TiAl-M (M = Ag, Cr) thin films

Abstract The aim of this work was to study the influence of two additional elements — silver and chromium — on the structure and mechanical properties of TiAl–M (M=Ag, Cr) thin films synthesised by sputtering. The films were studied in the as-deposited condition (metastable state) and after successive annealings performed in order to obtain the stable (γ-TiAl)-based intermetallic phase. The experimental techniques used in this work for chemical and structural characterisation were electron probe microanalysis, transmission electron microscopy/electron diffraction, X-ray diffraction and differential scanning calorimetry. The mechanical analysis consisted of the determination of hardness and ductility. To do this, a new tensile test procedure able to evaluate the ductility of thin films was developed. The results showed that, contrary to the as-deposited state, the addition of silver or chromium does not lead to a significant improvement in the mechanical properties, hardness and ductility of the heat-treated films (γ-TiAl structure). However, they did contribute to a better understanding of the role of silver and chromium on the structure of these titanium aluminides.

TEM and HRTEM characterization of TiAl diffusion bonds using Ni/Al nanolayers.

Diffusion bonding of TiAl alloys can be enhanced by the use of reactive nanolayer thin films as interlayers. Using these interlayers, it is possible to reduce the conventional bonding conditions (temperature, time, and pressure) and obtain sound and reliable joints. The microstructural characterization of the diffusion bond interfaces is a fundamental step toward understanding and identifying the bonding mechanisms and relating them to the strength of the joints. The interface of TiAl samples joined using Ni/Al nanolayers was characterized by transmission electron microscopy and scanning transmission electron microscopy. Microstructural characterization of the bond revealed that the interfaces consist of several thin layers of different composition and grain size (nanometric and micrometric). The bonding temperature (800, 900, or 1,000°C) determines the grain size and thickness of the layers present at the interface. Phase identification by high-resolution transmission electron microscopy combined with fast Fourier transform and electron energy-loss spectroscopy analyses reveals the presence of several intermetallic compounds: AlTiNi, NiAl, and Al2TiNi. For bonds produced at 800 and 900°C, nanometric grains of Ti were detected at the center of the interface.

Mechanical characterisation of γ-TiAl thin films obtained by two different sputtering routes

Abstract In this study (TiAl)-based films were magnetron sputtered using two different methods: two targets (Ti+Al) and a γ-TiAl target. In both cases, the as-deposited films had to be heat treated in order to obtain the intermetallic γ-TiAl. The effect of the addition of chromium on the structure and mechanical properties was studied. The films were submitted to ageing treatments and the resulting structures and mechanical properties were also studied. To get more insight into the films, the residual stresses were also evaluated. After heat treatment (HT), the films sputtered from two targets are constituted by a single γ-TiAl phase while in the films produced with one target it is possible to observe the presence of the α2-Ti3Al phase. Under annealing, the as-deposited compressive stresses give rise to low tensile stresses. The 18 h HT leads to a pronounced increase in hardness in the films obtained by using two targets. The hardness of the films produced using one target increases gradually with the HT holding time. In all cases, ductility and hardness exhibit inverse trends.

Reaction-Assisted Diffusion Bonding of Advanced Materials

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.

Phase Transformations during the Preparation of Ti6Si2B by High-Energy Ball Milling

Recently, it was identified the existence of a new intermetallic phase in the Ti-Si-B ternary system with atomic composition near Ti6Si2B. In the present work, we report on the phase transformations during the preparation of Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B (at.-%) powders in a planetary Fritsch P-5 ball mill from high-purity elemental powders. To understand the phase transformations, powder Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B samples milled for 90 h were vacuum heated at various temperatures. The starting materials and milled powders were characterized by means of X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopes, and differential scanning calorimetry (DSC). Results indicate that the Ti peaks widen and weaken with the increasing milling time and the silicon was practically dissolved into Ti and TiH2 lattices during milling and formed solid solutions in pre-alloyed Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B powders, respectively. The use of titanium hydride instead of titanium as starting material allowed accelerating the mechanical alloying process, i.e., the Ti6Si2B phase was formed during heating at lower temperatures than in case of titanium as starting material. As previously observed, the decomposition reaction of the titanium hydride occurred near 550C. Powder (TiH2)-22.2Si-11.1B sample milled for 90 h presented very fine particle size lower than 20 nm. The ternary Ti6Si2B phase was formed in powder Ti-22.2Si-11.1B samples after heat treatment. Traces of Ti and Ti5Si3 were also detected.

TEM characterization of As-deposited and annealed Ni/Al multilayer thin film.

Reactive multilayer thin films that undergo highly exothermic reactions are attractive choices for applications in ignition, propulsion, and joining systems. Ni/Al reactive multilayer thin films were deposited by dc magnetron sputtering with a period of 14 nm. The microstructure of the as-deposited and heat-treated Ni/Al multilayers was studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) in plan view and in cross section. The cross-section samples for TEM and STEM were prepared by focused ion beam lift-out technique. TEM analysis indicates that the as-deposited samples were composed of Ni and Al. High-resolution TEM images reveal the presence of NiAl in small localized regions. Microstructural characterization shows that heat treating at 450 and 700°C transforms the Ni/Al multilayered structure into equiaxed NiAl fine grains.

Characterisation of Modified Sputtered (TiAl)-Based Intermetallic Materials Doped with Silver and Chromium

TiAl-M (M = Ag, Cr) thin films were synthesised by sputtering and heat-treated in order to obtain the γ-TiAl phase. The results showed that the addition of silver or chromium does not lead to a significant improvement of the mechanical properties of the γ-TiAl structure. However, structural results obtained from similar materials produced by foundry confirm that sputtering might be used as a screening technique, in order to give helpful information for the future production of better performance bulk (titanium aluminide)-based materials. INTRODUCTION Different methodologies have been followed with a view to attempting to increase the ductility of the ordered phase γ-TiAl: (i) the use of the composition Ti-48Al, as a way of obtaining a two-phase structure formed by γ-TiAl together with a small amount of α2-Ti3Al [1,2] (ii) microstructural refinement by heat treatment [3], (iii) the addition of alloying elements in order to reduce the covalent degree of Ti-Al bonding [4-6] and finally (iv) the use of new processing techniques, such as mechanical alloying [7], rapid quenching [8] and sputtering followed by posterior annealing, to obtain the ordered γ-phase with controlled grain size [9-11]. The aim of this study is to use magnetron sputtering as a screening technique, to produce modified titanium aluminides with different contents of silver and chromium, which could be produced later as bulk materials using other techniques. EXPERIMENTAL DETAILS Ti-xAl (0≤x≤100at.%) and TiAl-xM (M=Ag, Cr and 0≤x≤10at.%) thin films (table I) with ≈ 3μm thickness were co-deposited by d.c. magnetron sputtering onto metallic substrates. The Ti-xAl films were obtained from a single target with foils of the other metal superimposed (titanium target with aluminium foils or vice-versa). The TiAl-M films were sputtered from two elemental targets – aluminium and titanium – onto which silver and chromium foils of differing sizes were placed. The samples were studied in their as-deposited state and after isothermal annealing at high temperatures and holding times in a hydrogenated argon atmosphere (5% H2). The chemical composition of the films was determined by electron probe microanalysis (EPMA). X-ray diffraction experiments were performed with Co-Kα radiation. Differential scanning calorimetry (DSC) measurements were carried out in a dynamic N2 atmosphere with 5% of H2. Films for TEM analysis were thinned on both sides by ion milling in an argon atmosphere. A 300kV TEM equipment was used. The hardness tests were carried out with loads of 70 and 300mN in an ultramicroindentation device with a Vickers indenter. A correction method [12] was applied during the calculation procedure. Table I – Chemical composition (at.%) of the (a) Ti-xAl and (b) TiAl-xM (M=Ag, Cr) thin films (a) (b) The ductility of the films was evaluated using a tensile sample with a geometry developed for this purpose [13]. The deformation gradient in the sample was determined measuring it in fifteen regions along the sample (each one 5mm long). The deformation was measured using a travelling microscope, with a accuracy of 1μm. After tensile test the films present cracks where the strain imposed exceeds their ductility, as seen in figure 1. Optical microscopy was used to define the boundary of the region where cracks appear. The strain attained is this region characterises the ductility of the film. Figure 1 Example of observations, by optical microscopy, after deformation, showing the development of cracks. Three different regions of a TiAl-Ag sample, annealed during 1 h, are shown: (a) 1%, (b) 2% and (c) 5% of deformation. (b) (a)

The influence of silver on the structure and mechanical properties of (TiAl)-based intermetallics

Abstract The influence of silver on the structure and mechanical properties of sputter-deposited Ti-Al-Ag films with an aluminium content close to 48 at.% and 0

Ni/Al Multilayers Produced by Accumulative Roll Bonding and Sputtering

Ni/Al multilayers are known to transform into NiAl in a highly exothermic and self-sustaining reaction. The fact that this reaction has a high heat release rate and can be triggered by an external impulse, are reasons why it has already attracted much research attention. There is a huge potential in the use of Ni/Al multilayers as a controllable and localized heat source for joining temperature-sensitive materials such as microelectronic components. The heat released and the phases resulting from the reaction of Ni and Al multilayers depend on the production methods, their composition, as well as the bilayer thickness and annealing conditions. The present research aims to explore the influence of these variables on the reaction of different multilayers, namely those produced by accumulative roll bonding (ARB) and sputtering. Structural evolution of Ni/Al multilayers with temperature was studied by differential scanning calorimetry, x-ray diffraction and scanning electron microscopy. Phase evolution, heat release rate and NiAl final grain size are controlled by the ignition method used to trigger the reaction of Ni and Al. The potential use of these multilayers in the diffusion bonding of TiAl was analyzed. The ARB multilayers allow the production of joints with higher strength than the joints produced with commercial multilayers (NanoFoil®) produced by sputtering. However, the formation of brittle intermetallic phases (Ni3Al, Ni2Al3 and NiAl3) compromises the mechanical properties of the joint.