a b s t r a c tThe structural stability of the single-crystal PWA 1483 superalloy has been investigated by internal friction (IF) and dynamic modulus measurements from room temperature to 1073 K. The examined samples were in the solubilized state.The vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations has been employed. Frequency was ∼350 Hz.IF spectra recorded in successive test runs on the same samples show a Q −1 maximum (M1) above 623 K, whose intensity and position change from one run to another; in correspondence with M1 the modulus undergoes a slow decrease followed by a sudden increase. Sometimes another maximum (M2) has been observed at lower temperature (∼523 K).After each run the values of the modulus and of Q −1 change indicating that a progressive irreversible transformation occurs. Damping phenomena have been attributed to the rearrangement of dislocation structures in the disordered matrix. This rearrangement modifies the density and the average distance of pinning points. This explanation is supported by transmission electron microscopy (TEM) observations.
The composite of Ti6Al4V alloy, reinforced by unidirectional SiC fibres coated with graphite, has been investigated through a new fabrication process, roll diffusion bonding (RDB), developed at the CSM laboratories. Microchemical characteristics of the interface between carbon (coating of SiC fibres) and metal (Ti6Al4V) have been investigated by means of XPS, multipoint AES and photoemission spectromicroscopy (Electron Spectroscopy for Chemical Analysis (ESCA) microscopy beamline at the ELETTRA synchrotron light source). The results obtained revealed that the composition of the fibre-metal interface is similar to that of the composites produced by conventional method of hot isostatic pressing (HIP).In the matrix, near to the fibres, the anomalous shape of Al 2p signal indicated the presence of C-Al atom pairs which are responsible for the anelastic characteristics of the composite.
Titanium-metal-matrix composites (Ti-MMC) are materials with very large specific resistance and potential operative temperature up to 800° C. At present these composites are produced by Hot Isostatic Pressing (HIP), a reliable but expensive manufacturing method. To cut production costs, Centro Sviluppo Materiali SpA (CSM) has developed and patented an experimental plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The experimental Roll Diffusion Bonding (RDB) pilot plant permits a reduction of process costs of about 40% with respect to the HIP process. This work reports the results of microstructural and mechanical examinations carried out on composites realized by RDB and HIP. The comparison shows that the fibre-matrix interface is stable in both the composites while the mechanical properties of RDB composite are better due to its smaller grain size and high dislocation density.
a b s t r a c tThe Ti6Al4V-SiC f composite, reinforced by unidirectional SiC fibres (SCS-6), has been investigated by internal friction (IF) and dynamic modulus measurements in the temperature range 300-1173 K. Experiments have been carried out by using a vibrating reed technique with electrostatic excitation and frequency modulation detection of flexural vibrations in the frequency range from 600 to 1800 Hz.For comparison the monolithic Ti6Al4V alloy has been submitted to the same experiments. The IF spectrum of the composite exhibits a relaxation peak at about 870 K superimposed to an exponentially increasing background. The peak, not present in the spectrum of matrix alloy, has activation energy H = 186 kJ mol −1 and relaxation time 0 = 2.3 × 10 −15 s. The peak has been explained by an interstitial-substitutional pair reorientation mechanism in the ␣ phase of Ti6Al4V matrix around the fibres.
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