Microstructures And Mechanical Properties Of Laves Phase Cr₂Nb/Ti Multiphase Alloys

Laves phase Cr₂ Nb,as an intermetallics with TCP structure,has been a potential high-temperature structural material because of its high melting point?1740??,excellent high-temperature strength,appreciable corrosion resistance and creep resistance.However,the engineering applications of Cr₂ Nb were restricted by its low facture toughness.Therefore,systematic research of ternary Cr₂Nb-20/30/40Ti?at.%?alloys was carried out by combining of alloy designing methods and manufacture techniques,to study the selection and competitive growth of phases,the mechanical behaviors of alloys,and the solidification characteristic and polytypic transition of Cr₂ Nb phase.The main contents and conclusions are as following:Laves phase?Cr,Ti?₂Nb was developed with various polymorphisms containing not only the stable C15 but also the metastable C36,5R,6H,and 7R in lamellar eutectic?Cr,Ti?₂Nb/b-Ti of arc-melted Cr₂Nb-40 Ti alloy.The preservation of metastable structures was generated from following two factors.One the one hand,solute Ti within?Cr,Ti?₂Nb was manifested to occupy the Cr sites,thus the synchroshear deformation and polytypic transition of?Cr,Ti?₂Nb was hampered.On the other hand,solute Ti increased the formation enthalpy of?Cr,Ti?₂Nb,the stability of?Cr,Ti?₂Nb can be decreased,leading to the formation of metastable structures.In constrast,the phase b-Ti in the eutectic?Cr,Ti?₂Nb/b-Ti prompted the polytypic transition of?Cr,Ti?₂Nb.Through calculating the lattice dismatches between b-Ti phase and various polytypic structures,the interface between b-Ti phase and C15-?Cr,Ti?₂Nb can be develoed as thermodynamics stable interface,being in favor of the transition of metastable polytypics to stable C15 structure.Thus,the polytypic transition of?Cr,Ti?₂Nb can be prompted.High-temperature heat treatment?1473K/48h?was effective in refining the grain size of Cr₂Nb-20/30 Ti alloys,in reducing the amount of?Cr,Ti?₂Nb phase,and in improving the morphology of microstructures.As a result,the deformation constraint of microstructures was relieved and the fracture toughness of alloys was increased.The crack bridging was found to be main toughening mechanism for these two alloys.And for Cr₂Nb-40 Ti alloy,the microstructure evolved to dispersive distribution of?Cr,Ti?₂Nb particle after heat treatment.The deformation of alloy was processed within three dimension space of microstructure.Thereby the deformation ability and facture toughness of alloy was improved.The main toughening mechanism evolved to crack deflection.Preferred growth behavior of Laves phase C15-?Cr,Ti?₂Nb was observed in directionally solidified Cr₂Nb-20/40 Ti alloys.The growth planes of C15-?Cr,Ti?₂Nb changed gradually from multiple planes of?220?,?311?,?222?,?422?to single plane of?220?,as the solidification rate increased continuously.Based on crystal structure of C15-?Cr,Ti?₂Nb,the atom arrangements of various planes were analyzed,and the bare bonding energies of these planes were calculated.It was found that the plane?220?was formed with superior growth dynamics,so it will be solidified preferentially during the process of directional solidification.Laser surface melting had great effects on the microstructures of Cr₂Nb-20/30/40 Ti alloys.With increasing the scanning speeds from 50 mm/s to 200 mm/s,the microstructure of Cr₂Nb-20 Ti alloy was composed of single phase?Cr,Ti?₂Nb,and the microstructure of Cr₂Nb-40 Ti alloy consisted of single phase b-Ti,while the microstructure of Cr₂Nb-30 Ti alloy evolved from eutectic?Cr,Ti?₂Nb/b-Ti at 50 mm/s to single phase b-Ti at 100²00 mm/s.Based on the Trivedi-Magnin-Kurz?TMK?eutectic growth model and the Kurz-Giovanola-Trivedi?KGT?dendritic growth model,with incorporating the non-equilibrium effect of rapid solidification,the selection and competitive growth of b-Ti,?Cr,Ti?₂Nb phases,and?Cr,Ti?₂Nb/b-Ti eutectic was analyzed.The results accounted very well for the evolutions of these microstructures.After the process of laser surface melting,the mechanical properties of alloys were improved significantly,because of refinement of grain size and evolution of solidified phases.The micro-hardness of alloys reached 9.1 GPa,lager than 8.7 GPa for Cr₂ Nb.The fracture toughness of alloys increased to 22.5 MPa×m1/2,16 times larger than single phase Cr₂ Nb.