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

Posted on:2017-01-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y L XueFull Text:PDF
GTID:1311330536951820Subject:Materials Processing Engineering
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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 MPaŚm1/2,16 times larger than single phase Cr2 Nb.
Keywords/Search Tags:Laves phase, Cr2Nb based alloys, directional solidification, laser surface melting, polytypic transition, microstructure evolution, mechanical properties
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