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Title: Process-microstructure studies in TiAl/SiCf composites
Author: Sandhu, Suki
ISNI:       0000 0001 3551 1415
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1998
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The commercially available SCS6, DERA SM1140+ and SM1040 fibres with Nd/NdBx double barrier coatings which were designed in this thesis, were coated with Ti-48A1, Ti-48A1-2V and Ti-6A1-4V alloys using the Matrix Coated Fibre (MCF) process developed by SMC-DERAFarnborough and consolidated by HIP to produce intermetallic matrix composites (IMCs) with Vf=50% fibre reinforcement. The materials were examined by electron microscopy techniques including microanalysis and by Auger electron spectroscopy and nano-indentation. In the matrix coated fibres the coating consisted of a zone of fine equiaxed micro structure adjacent to the fibre surface, which evolved into a zone of coarse, long columnar grains which were perpendicular to the fibre axis. The coating comprised strata of material concentric to the fibre surface, in some cases exhibiting fine scale compositional banding. There was no evidence of a reaction between fibre and coating. Oxidation of Nd after deposition on the SM1040 fibres was prevented by a thin coating of the compliant layer with a Ti-alloy matrix. The matrix of the IMCs with SCS6 fibres exhibited compositional variations with high A1 content in the matrix near the fibres and decreasing with distance from the fibre. A 1μm thick reaction zone was formed at the matrix/fibre interface at the expense of the Si rich C coating of the fibre. Cracks were present in the matrix which consisted of the TiAl and Ti3Al phases, with some twinned gamma TiAl grains. AI2O3 particles were present in the region of the matrix with about 50 to 55 at% Al. Carbon and Si diffused in an opposite direction to Ti through the reaction zone, where it is suggested that TiCl-x, Ti5Si3 and Ti2AlC were formed. The matrix of IMCs with SM1140+ fibres contained the TiAl, Ti3Al phases and Al2O3 particles. TiAl was present mainly around the uniform reaction zone and AI2O3 was haphazardly distributed only in gamma TiAl regions of the matrix. No cracks were present in the matrix of this IMC, but the reaction zone was excessively thick. It is suggested that TiC1-x and Ti2AlC were formed in the reaction zone. The presence of AI2O3 particles in the composites with the SCS6 and SM1140+ fibres was attributed to high level of O present in the as-coated fibres and the composite after consolidation of the matrix coated fibres. In the IMCs with SM1040 fibres (uncoated SiC fibres) radial cracks were present in the matrix, some penetrating into the fibres. The matrix exhibited compositional variation and consisted of the TiAl and Ti3Al phases with no AI2O3 particles. Silicon had diffused in the matrix forming titanium silicide and probably titanium carbide at the interface. A new coating was designed and applied by PVD on the SM1040 fibre. The coating consisted of a Nd compliant layer and a NdBx diffusion barrier layer, of thicknesses 2 to 3 μm and 2 to 5 μm respectively. In the IMCs with SM1040 fibres coated with the Nd/NdBx double coatings extensive cracking was still present in a matrix consisting only of the TiAl and the Ti3Al phases. Across the matrix the Al content varied form 35 to 48 at% from one fibre to another, with the Ti3Al phase dominating the central areas between the fibres. These compositional variations were confirmed by the nano-indentation studies, which showed that the moduli values were lower in the TiAl regions and higher in the Ti3Al regions. Little Ti and Al had diffused into the reaction zone which was 3-10μm thick and was composed of TiB particles in a Nd2O3 matrix. These results suggested that the NdBx coating performed well as a diffusion barrier. However, the high incidence of matrix cracks suggested that the Nd layer did not behave well as a compliant layer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Titanium aluminide; Matrix coated fibres