Interface characterization of an SCS-6/Ti-22Al-23Nb composite

Samples of an SCS-6/Ti-22Al-23Nb composite have been annealed in a temperature range between 500{dollar}spcirc{dollar}C and 1050{dollar}spcirc{dollar}C for up to 2000 hours. Microstructural evolution of the reaction zone has been investigated using various TEM techniques including bright-field and dark-field imaging, electron diffraction and energy dispersive X-ray spectroscopy. In the as-fabricated and most of the annealed composites, two phases, i.e. (Ti,Nb)C and {dollar}rm(Ti,Nb)sb5(Si,Al)sb3,{dollar} have been found to locate in three separate layers: an inner layer containing both (Ti,Nb)C and {dollar}rm(Ti,Nb)sb5(Si,Al)sb3{dollar} fine grains, an intermediate (Ti,Nb)C layer and an outer {dollar}rm(Ti,Nb)sb5(Si,Al)sb3{dollar} layer. At 1050{dollar}spcirc{dollar}C, two additional phases, (Ti,Nb)Si and (Ti,Nb){dollar}sb3{dollar}(Al,Si)C, as well as (Ti,Nb){dollar}sb5{dollar}(Si,Al){dollar}sb3{dollar} and (Ti,Nb)C, all appear in a large intermediate layer with the components in inner and outer layers unchanged. Thermodynamically, the above phases have been shown to be stable at the temperatures investigated and would arise via interdiffusion and relevant chemical reactions. The {dollar}alphasb2{dollar} phase in the matrix has been seen to promote the overall growth of the reaction zone.; The growth of the reaction zone as well as the phase layers are diffusion controlled and follow parabolic laws. Two stages with different growth rates have been observed at 1050{dollar}spcirc{dollar}C and the reason for this phenomenon is due to the morphological transition of the reaction products which is further related to the diffusion path change. Various parameters of the interfacial growth kinetics have been calculated for the composite system and, compared with other matrix materials, the Ti-22Al-23Nb has less reactivity with the SCS-6 fiber.; The effect of the reaction zone on the composite fracture behavior has been evaluated by the three-point-bending test. No substantial decrease in the maximum fracture load {dollar}rm Psb{lcub}max{rcub}{dollar} has been observed until a 0.8 {dollar}mu{dollar}m interfacial reaction zone has been produced. However, the {dollar}rm Psb{lcub}max{rcub}{dollar} decreases rapidly thereafter and catastrophic failure of the composite occurs when the reaction zone is over 5 {dollar}mu{dollar}m. It has been estimated that the highest operating temperature of this composite is about 750{dollar}spcirc{dollar}C.; Innovative approaches have been developed to model the composite interface: the chemical vapor deposition (CVD) of a C/Si film on the matrix material and the direct immersion of matrix material in the mixtures of SiC and C ultrafine powders before exposing them to temperatures approximating those of composite fabrication. The reaction products in the model system prepared with the CVD thin film or the C/SiC powder mixture and their distribution sequence are very similar to those in the actual titanium aluminide composite interface. The results from this study indicate that the experimental simulation of the interfacial boundary composition so as to study its evolution and reactions is a promising technique.