Microstructure characteristics of burning products of Ti-V-Cr fireproof titanium alloy by frictional ignition

Titanium fire in the aero-engine is a typical accident caused by ignition and burning of titanium alloy, which leads to a huge damage. Some articles wrote it as to turn pale at the mention of titanium. Fireproof titanium alloy, a new type of structural titanium alloy with fireproof function, has been developed to prevent titanium from fire hazard and to ensure safe and reliable service of aero-engine. In view of the lack of clear understanding of the microscopic mechanisms found currently for the structural functionality of fireproof titanium alloys, in this work, using frictional ignition technology in oxygen-rich environment (friction oxygen concentration method), associated with in-situ observation, SEM, EDS and XRD analyses, the microstructure characteristics of burning products of Ti-V-Cr fireproof titanium alloys are investigated and the element distribution law associated with microscopic mechanism during combustion reaction process is disclosed. Results show that Ti-V-Cr fireproof titanium alloys produce dazzling white light during combustion, with the typical flame characteristics of metal combustion. The generated products after burning are mainly TiO2, V2O5 and Cr2O3 oxides, in the form of dispersive particles and dense continuous body. The dispersive particles are in regular spheric shape, with a size of 10-50 m; the dense continuous products after burning presents divisional feature. After the combustion lasts 18 s, four distinct zones form from the alloy matrix to the combustion surface and they are in the sequence of transitional zone, heat-affected zone fusion zone, and combustion zone, with sizes of 40-50, 200-210, 60-70, and 18-21 m respectively. Further, some small granular shaped bulges exist in the transitional zone, in some fixed directions; in the heat-affected zone, a large number of V-based solid solution and some Ti-based solid solution form, and the titanium containing V-based solid solution is much higher than the needle-like precipitation phase in the matrix. In the fusion zone, there are some V-based solid solutions in most of Ti-based solid solution; while, the combustion zone mainly contains the mixed oxides of Ti, V, and Cr. The V-based solid solution in the heat-affected zone reduces the diffusion rate of titanium to the fusion zone, slowing the preferential reaction between titanium and oxygen in the combustion zone; while the generated mixed oxides of TiO2, V2O5, Cr2O3, etc. in the combustion zone and the solution of oxygen in titanium in the fusion zone jointly prevent the diffusion of oxygen to the alloy matrix, thus the Ti-V-Cr fireproof titanium alloys can have excellent fireproof functions.