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

Abstract

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)