Hot Deformation Behaviors And Microstructure Evolution Of (TiBw+Silicide)/TA15 Composites

Titanium alloys are ideal material in aerospace applications for they are light wighted,have high fracture strength,have well resistant to both high temperature and corrosive environments.By inducing reinforcement into titanium alloys,we get titanium matrix composites?TMCs?with higher strength-wight ratio and better performance under high temperature,but at the cost of severe decreasment in room temperature plasticity,thus limits the application of TMCs.TMCs with network-distributed reinforcement are a new kind of TMCs emerges in recent years.By controlling the distribution of reinforcement,network reinforced TMCs gains better high temperature performance with significant less plasticity loss.Si is an alloying element that widely used to enhance the hight temperature performance of titanium alloys and TMCs,but it also makes the hot working of the materials more difficult.The purpose of this thesis is to better direct the hot working process of TMCs with Si addition.Contutive equations and processing maps are calculated for TMCs with different amount of Si addition to characterize their hot deformation behaviors,the microstructures after deformation are observed and compared to analyse the influences on TMCs of Si addition.The deformation substructures and their interaction with reinforcements is studied in order to get a beter understanding of the roles that TiBw reinforcement and silicides plays in hot deformation of TMCs.Network-reinforced TiBw/TA15 TMCs with Si addition of 0.5wt.%and 1wt.%is prepared by powder metallurgy,along with unreinforced TA15 alloy,at the purpose of comparison.The raw powders of TA15,Si and Ti B2 are mixed with low energy ball milling to retain the sphere shape of TA15 powder.Ball milled powders are sintered under1300?for 1.5 hours.TiB whiskers in as-sintered material are network-distributed,the diameter of each net grid is about 100?m,the?phase and?phase in matrix forms a bascketwave-like structure,silicides are distributed on?/?interfaces,with the diameter of about 200 nm,TiBw significantly refines microstructure of TiBw/TA15 composite.Room temperature tensile test shows that the tensile strength of TMCs could reach over 1000 MPa,the presence of TiBw enhanced the tensile strength and the plasticity of TMCs at the same time.The addition of Si will enhance the tensile strength of TMCs,at the cost of plasticity.High temperature tensile test shows that the strength of TiBw/TA15 composite with 0.5wt.%and 1wt.%Si additinon could reach 550 MPa and680MPa under650?,respectively,which is significantly higher than TA15 alloy.Hot compression test is performed under the strain rate 1s^-1-0.001s^-1and the temperature 870?-1100?for both of TMCs.Constitutive equations and processing map is calculated for both materials based on obtained data in hot compression test.Higher Si addition leads to higher flow stress as well as higher temperature sensitivity.The strengthening effect of Si addition is more significant in?+?dual phase area than that in?single phase area.The deformation activation energy of TiBw/TA15 composites with0.5wt.%and 1wt.%Si addition in dual phase area are 594KJ/mol and 645KJ/mol respectively,and 338KJ/mol and 319KJ/mol in single phase area,respectively.It shows that the addition of Si significantly raises the deformation resistivity of TiBw/TA15 in dual phase area,but have little impact on that of single phase area.The processing map reaches maximum at 910?-950?,0.1s^-1-0.01s^-1for both materials,and have relatively high dissipation efficiency under 1020?,which indicates the ideal processing areas.The microstructure analysis shows that the main flow softening mechanism in dual phase area is the globularization of?laths and dynamic recrystallization which takes place on?/?boundaries and Ti Bw.The volume fraction of?phase decreases as the amount of Si increases,Si will precipitate on?/?boundaries after hot deformation.The microstructure of Ti Bw/TA15 becomes fully lath after deformation under single phase area.In the low temperature zone of single phase area,the flow softening mechanism is mainly dynamic recovery and dynamic recrystallization.Under higher temperature in sincle phase area,the flow softening is controlled by dynamic recovery,TiBw is observed to anchor the?grain boundaries and to provide dynamic recrystalization nucleus.Silicide completely diffused into TA15 matrix in single phase area.TEM analysis reveals high dislocation density around?/?phase boundaries and Ti B whiskers after dual phase deformation,where dynamic recrystallization is favoured.Silicides also favours to precipitate in areas with high fault density.The precipitated silicides will obstruct the movement of dislocations as well as that of recrystallization grain boudnaries.Silicides were rearely found in after?phase deformation,as well as dislocations.