Production of TiA14V + SiC fibers composite and its structural evolution after heat treatments [Preparazione del composito Ti6Al4V+SiC fibre e sua evoluzione strutturale dopo trattamenti termici]

A composite of Ti6A14V alloy reinforced with unidirectional SCS-6 SiC fibers has been fabricated by Hot Isostatic Pressing (HIP) at CSM (Centro Sviluppo Materiali). Fig.1 shows the inner structure of a SCS-6 fiber (left) and the composite cross-section (right). The stages of the preparation route are illustrated in fig.2: 1- preparation of the multilayered structure (5 layers of Ti6Al4V, 4 layers of SiC); 2- material cutting in its final shape (200 x 450 mm2); 3-material allocation in the mould, which is then evacuated (3x.10-6 mbar); 4- heating (T =890°C); 5- pressure application (Pmax= 1200 bar) for 30 minutes; 6- cooling down to room temperature; 7- composite extraction from the mould. The material has been studied by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM.) XRD patterns of the composite and of the matrix alloy (powder) have been recorded at different temperatures up to 600°C in argon atmosphere and at room temperature after the thermal cycle for investigating the material structural evolution. Moreover, XRD spectra of composite and alloy were recorded at room temperature after heat treatments of 1 hour at 100, 200, 300, 400, 500 and 600°C. The experiments have been performed by employing a hightemperature X-ray camera. A sketch of its heating system is shown in fig.3. Overall spectra (5° ÷ 45° 2_ angular range, steps of 0.05° and counting time of 2s/step) and precision peak profiles of the {100}, {002}, {101}, {102}, {110} and {103} reflections (steps of 0.005° and 10s/step) were collected using Mo-Kα radiation (λ=0.071 nm). The central peak positions have been determined by an iterative procedure. The strain (εhkl) affecting {hkl} planes was obtained by eq.(1), where d and d0 are the interplanar spacings of the composite and of a stress free sample (Ti6Al4V powder) respectively. Fig.4 shows precision profiles of the first three diffraction peaks of matrix alloy and composite, collected at increasing temperatures up to 600°C. In both cases the peaks shift to lower angles as temperature increases; interplanar spacings are reported in table 1. Fig 5 shows the hexagonal cell parameters (a,c) vs temperature of composite and powder. Composite values are smaller than the powder ones for every temperature. The εhkl trends vs. temperature are plotted in fig.6: higher the temperature higher the strain, the values are negative and different for each set of planes. Fig. 7 compares the spectra of composite and powder before and after the thermal cycle. The composite peaks result shifted towards higher angles whereas those of the powder in the opposite way. Angular peak positions and interplanar spacings of composite in as-prepared condition (I) and after thermal cycle up to 600° C (F) are reported in table 2. The XRD spectra of composite after heat treatments of 1 hour at different temperatures are shown fig.8: the peak positions progressively shift towards higher angles by increasing the treatment temperature. Metal-fiber interface in the as-prepared material and after heating of 1 hour at 600°C in air has been investigated by AFM (see micrographs in figg. 9 and 10). The AFM observations show that the interface is not seriously damaged by the treatment in air. The increase of cell parameters during heating in composite and alloy is due both to thermal expansion and gas absorption. After cooling the increase of a and c observed in the alloy with respect the original values is due to trappig of gas, in particular nitrogen and oxygen, in the lattice: gas solubility increases at higher temperature and a residual part remains in the lattice after cooling. On the contrary, composite lattice was found compressed after cooling: X-ray diffraction peaks shift towards higher angles. The composite exhibits the same behaviour after heating of 1 hour at temperatures ranging from 100 to 600°C. Being σz=0, the compression along z-axis, giving the strain εz measured by XRD, is due to a biaxial state of tensile stresses in the plane of the sample surface (see fig.11). The origin of residual stresses of different sign in alloy and composite is connected to the role played by the fibers, which constrain the lattice expansion following gas absorption. This explanation is supported by AFM showing the integrity of metal-fiber interface also after the heat treatment at 600°C in air. Another results is that different strains affect different sets of crystalline planes of the composite. Therefore, to minimize the residual state of stress following heat treatments in atmosphere containing N 2 and O2, texture control seems of the utmost importance.