| Titanium matrix composites(TMCs)have wide application prospects in the fields of aerospace,marine engineering,energy and chemical engineering due to their excellent mechanical properties such as low density,high specific strength and high specific stiffness.However,the applications of the TMCs at high temperatures have been seriously restricted due to their poor oxidation resistance and hot corrosion resistance.Surface coating is one of the most effective methods to improve the high temperature properties of TMCs without changing their mechanical properties.Based on the requirements of high temperature protective coating and the mechanism of alloying elements on high temperature properties of titanium alloys,the network structured Ti-Al coating and Ti-Al-Si coating were fabricated on the Ti Bw/Ti6Al4 V composites with network structure by short-time hot-dipping at high temperature and long-time heat treatment at low temperature.Then the microstructure of the coatings was analyzed.Effects of process parameters on microstructure of the coatings were discussed,and the high temperature oxidation behaviors and hot corrosion behaviors of the coatings were studied.The Ti-Al coating is mainly composed of the Ti Al3 phase,which has very good adhesion to the network structured Ti Bw/Ti6Al4 V composite substrate.The cyclic oxidation behaviors of the coating and Ti Bw/Ti6Al4 V composites indicate the Ti Al3 coating can significantly improve the high temperature oxidation resistance of the composites.This is mainly due to the loose and porous Ti O2 oxide scales were produced on the composites,while continuous and dense Al2O3 oxide scales were formed on the Ti Al3 coating during the high temperature oxidation process.There were no cracks existed in the oxidized coating.This is mainly due to the network structure of the coating can effectively improve the ability of the coating to suffer from stress.In addition,the reaction zone with layered structure of Ti Al and Ti Al2 was generated at the interface between the coating and composite substrate during the oxidation process,which is disadvantage to the long-term oxidation resistance of Ti Al3 coating.The interfacial reaction between the coating and composite substrate can be inhibited by adding the Si element into the Ti-Al coating.The microstructure of the Ti-Al-Si coating varies with the change of Si contents.Ti-Al-3Si coating is mainly composed of the outer thick Ti(Al,Si)3 layer and the inner thin Ti3 Al Si5 layer.Ti-Al-10 Si coating has similar microstructure to Ti-Al-3Si coating,but there are some lamellar Ti3 Al Si5 phases dispersed distributed on the top of Ti(Al,Si)3 layer.Ti-Al-12 Si coating consists of the Ti(Al,Si)3 phase and Ti3 Al Si5 phase.Ti-Al-20 Si coating is made up of the homogeneous Ti3 Al Si5 phase.All of the Ti-Al-Si coatings with different contents can remarkably improve the high temperature oxidation resistance of the network structured Ti Bw/Ti6Al4 V composites.Moreover,the Ti-Al-3Si coating has the highest oxidation resistance among them.The second is the Ti-Al-20 Si coating,and then the Ti-Al-10 Si coating,finally the Ti-Al-12 Si coating.The main reason can be attributed to that the Ti-Al-Si coating with different composition has different microstructure,and the high temperature oxidation resistance property is also different.However,the dense and continuous Al2O3 oxide scales with network structure can be formed on the Ti-Al-Si coating with different composition during the oxidation process.The existence of Si atoms in the coating can promote the formation of Al 2O3 oxide and suppress the formation of Ti O2 oxide,which can also refine the oxide particles to improve the ductile of oxide scales.In addition,the Ti5Si4 interfacial layer is formed between the Ti-Al-Si coating and the composite substrate during the oxidation process.This could act as barrier to retard the interfacial reaction between the Ti(Al,Si)3 coating and composite substrate,which can effectively improve the high temperature anti-oxidation life of the coating.During the hot corrosion process in Na Cl,25 wt.%Na Cl+75wt.%Na2SO4 and Na2SO4 deposited salts,both of Ti-Al-3Si coating and Ti-Al-20 Si coating could provide protection for the network structured Ti Bw/Ti6Al4 V composites.Ti-Al-3Si coating has the highest hot corrosion resistance in Na2SO4 deposited salts,and has the lowest hot corrosion resistance in Na Cl deposited salts.This is mainly due to the dense Al2O3 oxide scales can be formed in the outermost layer of the coating during the hot corrosion process,but the dissolution rate of Na Cl deposited salts for the dense oxide scales is fast,and the coating has serious internal oxidation in the Na Cl deposited salts.Ti-Al-20 Si coating has the highest hot corrosion resistance in Na2SO4 deposited salts,and has the lowest hot corrosion resistance in Na Cl and Na2SO4 deposited salts.This is mainly due to the coating could form the dense Al2O3 oxide scales in the beginning,and the oxide scales is continuously dissolved with the hot corrosion time extending,the dense oxide scales can't be formed due to the low Al contents in the coating.In Na Cl deposited salts,the reaction can generate volatile chloride and regarded as an autocatalytic process.The melting point of Na Cl and Na2SO4 deposited salts is relatively low,and the hot corrosion degree of the coating is the most serious. |
PDF Full Text Request