| Titanium and its alloys are broadly applied in various fields such as advanced aero-engines,chemistry,biology,and so on owing to their high thrust-weight ratio and excellent corrosion resistance.However,the so-called "titanium fire" occurs under high pressure and friction ignites unfavorable and rapid burning,which is hard to control and leads to catastrophic accidents.Due to combustion process is a kind of drastic and complex oxidation,this paper investigated oxidation process to act as a basis research of burning behavior and burning characteristics to reveal relationship between oxidation and combustion,which explains burning mechanism of burn resistant titanium alloys.In order to explain effect different element composition on burn resistance and how copper works during burning,Ti-V-Cr and Ti-Cu series burn resistant titanium alloys are researched to understand how copper work during burning.Durning oxidation process,oxidation kinetics show that Ti-25V-15 Cr alloy possesses the lowest oxidation rate,and oxidation rate of Ti-14 Cu is much lower than that of commercial non-burn resistant Ti-6Al-4V alloy which is used for comparative experiments.This indicates that Ti-14 Cu alloy has oxidation resistance.In addition,it is found that clearer widened grain boundaries in Ti-14 Cu alloy with the increase of temperature and net-structure is ultimately formed.The second part discusses burning behavior of cast Ti-25V-15 Cr and Ti-14 Cu alloys through oxidation rate studied in the first part to unveils the burning mechanisms of these alloys and discuss whether burn resistance of Cr and V can be replaced by Cu.It is found that V and Cr in Ti-25V-15 Cr alloy forms V2O5 and Cr2O3 oxides,which enhance the density of the oxide layer that impedes the diffusion of oxygen and decrease oxidation rate.For Ti-14 Cu alloy,a clear Cu-rich layer is formed at the interface between burning product zone and heat affected zone,which consumes oxygen by producing Cu-O compounds and impedes the reaction with Ti-matrix.Additionally,burned Ti-Cu alloy exhibits a globular microstructure with Cu-rich phases surrounded Ti when grains are spherical.Liquid phases penetrated along grain boundaries.Most importantly,based on globularization microstructure,the concentration of oxygen in copper phases is lower than that in Ti phases.Therefore,we can assume that Cu-rich phases have an impact on diffusion of oxygen,which further blocks oxygen diffuses towards substrate to enhance burn resistance.More energy is required for oxygen to transform from air to substrate durning burning process.As for Ti-6Al-4V alloy,it shows non-burn resistance with clear cracks on the surface after burning which offers channels for oxygen diffusion.Based on burn resistance of copper,burn resistance of Ti-14 Cu after semi-solid deformation is investigated.The results show that deformation temperature enhances burn resistance due to widened Cu-rich layer.This means that more energy and higher content are required for oxygen to get into substrate from air.This work has established a fundamental understanding of burning resistant mechanisms for titanium alloys.More importantly,it is found that Cu could endow titanium alloys with similar burn resistant capability as that of V or Cr,which opens a cost-effective avenue to design burn resistant titanium alloys.Moreover,the burning resistance of the alloy can be effectively tuned by controlling the temperature during the semi-solid forging process. |
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