Study On Thermodynamics Analysis And Kinetics Mechanism For Ti-Al-C System

In this paper thermodynamics analysis and kinetics mechanism on Ti-Al-C system are discussed mainly.Ti₂AlC and Ti₃AlC₂ are both typical layered ternary material of MAX phase that combine the merits of both metals and ceramics.Their unique property lies on the fact that they have high fracture toughness and can be easily machined like metal.Furthermore,they have high electricity conductivity of metals and high strength and excellent oxidation-resistance of ceramics.However,there are many duality or temary phases of Ti-Al-C system and the formation mechanism of Ti-Al-C system is very complex.So this paper studies on thermodynamics analysis and kinetics mechanism on Ti-Al-C system including thermodynamics calculation,fabrication of material by pressureless sintering,kinetics equations and experimental of corrosion behavior for Ti-Al-C system.Apparatus used for analysis and detection include X-ray diffraction (XRD),scanning electron microscopy(SEM),thermal analysis instrument and electrochemistry workstation.The emphasis of this research is on thermodynamics calculation for Ti-Al-C system by programming in C⁺⁺Build language.The stability of intermediate and tendency of reactions can be judged by calculating standard formation Gibbs free energy of matters and standard reaction Gibbs free energy change of reactions on Ti-Al-C system at different temperature.The chemistry potential of members on Ti-Al-C system is determined using phase diagram,and reaction route is forecasted.This research also focuses on synthesis of Ti₂AlC and Ti₃AlC₂ by pressureless sintering in order to validate the result of thermodynamics calculation.Effects of different factors including temperature,sorts of the raw materials(elemental powder mixture of Ti,Al and active carbon or powder mixture of TiC,Ti and Al),and doping Si on the synthesis process are investigated.Research on the quantum chemistry calculation is based on structure character of Ti₂AlC and Ti₃AlC₂,and set up calculated models. According to the method of density function theory and discrete variation method (DFT-DVM),net charge of the atoms,and covalent bond orders of Ti₂AlC,Ti₃AlC₂ and doping Si series are calculated. Furthermore,kinetics equations of Ti-Al-C system are studied according to do thermal analysis on duality and trinity system.The activation energies corresponding to reactions are calculated through peak temperatures in DSC curves of different velocity. Combined to the results of thermodynamics calculation,the path of reaction is judged.Finally,we characterized corrosion properties in different acid or alkali solutions of MAX phase synthesized by our group partly.Results show that For Ti-Al-C system,it is possible that there are intermetals from elemental powers as follows:TiC,Al₄C₃,TiAl,Ti₃Al and TiAl₃.Their orders of stabilities in turn from strong to weak are TiC＞Al₄C₃＞TiAl₃＞TiAl＞Ti₃Al due to calculation results of their standard formation Gibbs free energy.So TiC is the most stable intermetal in Ti-Al-C system.On the other hand,Al₄C₃,TiAl,Ti₃Al and TiAl₃ will exist in a stage as some middle production,but they will react to become TiC by spontaneity,and they will disappear at last and be replaced by TiC.During the synthesis of Ti₂AlC and Ti₃AlC₂ in Ti-Al-C system,Al-Ti intermetallics are formed from Al melt and Ti after Al melts with temperature increasing,which is TiAl₃ firstly,and then reacts into TiAl with Ti,furthermore to form Ti₃Al.At the same time,C diffuses to Ti and form TiC.It will separate out Ti₂AlC or Ti₃AlC₂ when it dissolves into Ti-Al liquid phase.If the temperature of reactions is high enough,the tendency of formation Ti₃AlC₂ is more than that of Ti₂AlC.Results of pressureless sintering that although the synthesis productions from 2.0Ti/1.0Al/1.0C and 3.0Ti/1.1Al/1.8C are different,the same points following:Ti-Al intermetallics exist at 700℃-1100℃and they disappear at high temperature (1300℃-1500℃);Ti₂AlC and TiC are stable compounds which exist at different temperature stage,but Ti₃AlC₂ will not form at low temperature stage and it can form little at high one.The formation of Ti₃AlC₂ is easier from 3.0Ti/1.1Al/1.8C than 2.0Ti/1.0Al/1.0C.Moreover,doping TiC can avoid Al-Ti intermetallics.From results of quantum chemistry calculation that the ionic and covalent bonds in Ti₃AlC₂ are stronger than those in Ti₂AlC,especially ionic bonds between Ti and Al.Ti₂AlC doping Si has more obvious tendency to form a semiconductor than Ti₂AlC. It is also revealed reaction process of Ti-Al-C system by thermal analysis:Al melts at 660℃or so,Al(s)→Al(l),it's decalescence is about 34J·g^-1.When the temperature is above 660℃,the main exothermic reaction is between Ti and Al,Al(l)+3Ti(s)→Ti₃Al(s),it's activation energy equals to 36.385kJ·mol^-1.During it there are some other reactions as follows:Ti(s)+C(s)→TiC(s)and 4Al(l)+3C(s)→Al₄C₃(s).The latter two reactions are both little exothermic reaction.So for Ti-Al-C system the reaction between Ti and Al is given priority to the initial stage reaction,and the reaction between TiC and Ti-Al intermetallics makes Ti₂AlC and/or Ti₃AlC₂,viz TiC+ Al-Ti→Ti₂AlC and/or Ti₃AlC₂.It is shown from chemistry corrosion behavior that the corrosion rates of Ti₂AlC and Ti₃AlC₂ were low in all kind of acid and alkali.However,those of Ti₂AlN and Ti₃SiC₂ were nearly variational dramaticly immersion in special acid.The corrosion type of Ti₂AlC and Ti₃AlC₂ gave priority to intergranular corrosion in all kind of acid,in the case of Ti₂AlN was pitting corrosion;but Ti₃SiC₂ was no corrosion.The resistance of acid corrosion of the four materials in acid was much better than that in alkaliAt last,it is shown from electrochemistry properties that 312 phase of MAX is more stable than 211 in all solutions:Ti₃SiC₂ and Ti₃AlC₂ passivate in all solutions,while Ti₂AlC and Ti₂AlN undergo active dissolution in 1mol·dm^-3HCl with evolution of gas; corrosion current for Ti₃SiC₂ is less than those of Ti₃AlC₂,Ti₂AlC and Ti₂AlN,indicating the former is more stable.