Study On Microstructure And Deformation Mechanism Of Ti20Zr6.5Al4V Alloy

Traditional titanium alloys and zirconium alloys possess lower strengths,which limits their further application in other areas.For this reason,in recent years,researchers have developed a series of new TiZr-based alloys.Among these new alloys,Ti20Zr6.5Al4V alloy has a high strength while maintaining the high specific strength of conventional titanium alloys,and are expected to be used as structural materials for spacecraft.Researchers have conducted intensive research on Ti20Zr6.5Al4V alloy,covering many aspects.However,the research on the microstructure and deformation mechanism of the alloy is still insufficient,which is not conducive to the further performance of this excellent material.In this paper,Ti20Zr6.5Al4V alloy was treated by aging and annealing,respectively.And the effects of microstructure on mechanical properties under the different alloy states were investigated.And the mechanical properties difference of annealed Ti20Zr6.5Al4V alloy with excellent comprehensive mechanical properties in different directions were studied.Finally,the microstructure and deformation mechanism of Ti20Zr6.5Al4V alloy,as well as Ti6Al4V alloy and pure Zr which with component similarity and phase similarity to the alloy were studied and compared in the uniaxial tension and uniaxial compression deformation process.The results show that the Ti20Zr6.5Al4V alloy has an ultra-high tensile strength of1730 MPa and a low elongation of 2.3%after aging treatment at 450°C for 2 h;and after750°C,2 h annealing treatment the alloy possessed a tensile strength of 1235 MPa and an elongation of 10.8%.The alloy consists of a single?-phase,and dual(?+?)phase after aging and annealing treatments,respectively.The volume fractions of?phase and?phase are 87.3%and 12.7%,respectively.The aging treatment forms a fine lamellar primary?phase(?_p)and an ultrafine secondary?phase(?_s)which dispersed between?_p.This structure has strong fine grain strengthening and dispersion strengthening effect,thereby greatly enhancing the strength of the ageing alloy.Lamellar?laths with similar orientations precipitated along the prior-?grain boundaries cause stress concentration and lead to crack initiation and propagation,thereby affecting the ductility of the aged alloy.Zr is a?phase stablising element in annealed Ti20Zr6.5Al4V alloy.The hardness of?phase in the alloy is higher than that of the?phase.The annealing treatment forms a structure in which the?phase is uniformly distributed between?laths in the Ti20Zr6.5Al4V alloy,and this structure greatly increases the ratio of the?/?phase interface per unit area.The appearance of these phase interfaces plays a good role in pinning and hindering the movement of dislocations.In addition,?phase has better plastic deformation ability than?phase,which leads to an increase in the plasticity of the annealed alloy.At the same time,the growth of the grains during the annealing process and the appearance of the softer?phase lead to a decrease in the strength of the annealed alloy.The annealed Ti20Zr6.5Al4V alloy exhibits a peculiar difference in mechanical properties in different directions,that is,the tensile strength remains substantially unchanged,while the elongation changes drastically.During the tensile deformation process,?laths with similar orientation distributed along the prior-?grain boundary can significantly affect the initiation and propagation of cracks.The crack easily initiation at the prior-?grain boundary,extends along the prior-?grain boundary,and can pass through some aggregation zones of?laths with similar orientation.This crack initiation mechanism and expansion mechanism result in an elongation difference of the annealed20Zr alloy in different directions.When the annealed alloy undergoes plastic deformation during uniaxial tensile test and uniaxial compression,the deformation is performed by slip mode,and the formation of twins is suppressed.During tensile test,the pyramidal<a>slip system,pyramidal<c+a>slip system and prismatic slip system can be activated,and the basal slip system is suppressed.Both the pyramidal<a>slip system and basal slip system can be activated during uniaxial compression deformation,while the prismatic slip system and pyramidal<c+a>slip system are suppressed.The addition of Zr inhibits the formation of deformed twins,so that the plastic deformation of the annealed Ti20Zr6.5Al4V alloy can only be carried out by slip mode.The addition of Zr has an effect on the initiation of the slip system and promotes the initiation of the pyramidal<a>slip system.