| Own to higher melting points,good high-temperature strength,excellent resistance to corrosion and oxidation,Pt-based alloys are widely used in the high-tech applications for instance microelectronics,aerospace,energy and chemical industries.They can withstand aggressive high temperature environment,thus are potential new high-temperature structural materials.Considering the importance of Pt in military and national defense fields,it is important to develop Pt-based superalloys with excellent properties.Pt is recyclable and its cost can be reduced by alloying with some cheaper metals,so it is drawing more and more researches on it.Current mature Pt-based alloys have remarkable advantage whether melting point or corrosion resistance.However,poor high-temperature strength and ductility limits their development.The ?/?' Pt-based alloys introduce L12 precipitates with higher melting points in the matrix,which significantly increases melting points of Pt alloys and strengthen their high-temperature mechanical properties.Therefore,they are promised as good materials of rocket nozzle.The ?/?' Pt-based superalloys introduce FCC-Pt with rich slip system as y-matrix,and Pt3Hf with higher melting point together with high strength as ?'-precipitates,which significantly improves the mechanical performance of Pt,but its strengthening mechanisms are still unclear.Based on the first-principles calculation,the strengthening and toughening mechanisms of the ?/?' Pt-based superalloys are investigated.The main work and results are as follows:(1)Based on the first-principles calculations,we study the effect of alloying atoms on the stacking fault energy of Pt.The stacking fault energy that we obtained in this work is 284mJ/m2,which has a good agreement with the value of experiment and relative literatures.It shows that apart from the element of Y,all alloying elements in our work increase the unstable stacking fault energies.From Rice's theory,the level of applied stress intensity factor required for dislocation nucleation is shown to be proportional to (?),the unstable stacking fault energy.So our results indicate that all researched alloying elements except Y can impede dislocation nucleation:Mo and Ru have strongest impeding effect,the second is Hf,Ir,Ru,Pd has little effect.Besides,the surface energy(ys)of materials is a parameter reflecting the ability of resisting cleavage fracture.The ratio of ?s to ?us is used to evaluate brittle vs ductile of materials.The smaller the ratio,the more brittle the materials are.Our results indicate that these alloying elements decrease the ductility of Pt as the order:Hf>Y>Mo>Ru>Rh>Ir>Pd.In order to reveal the mechanisms of alloying elements on the stacking fault energy of Pt,the size factor,valence difference,and the electronic structure of Pt and its alloys are analyzed.It shows that the weakened atomic bonding between Pt and X atom(where X is Pd,Y,Hf,Mo)results lower stacking fault energies,while enhanced atomic bonding between Pt and the X atom(where X is Ru,Ir,Rh)results higher stacking fault energies.(2)The deformation mechanisms of L12 Pt3Hf are investigated using the first-principle methods.By calculating the generalized stacking fault energies of three possible defects in L12 Pt3Hf,we obtained Superlatice Intrinsic Stacking Fault(SISF)energy of 111ml/m2,Anti-Phase Boundary(APB)energy of 804mJ/m2,and Complex Stacking Fault(CSF)energy of 929mJ/m2.From the GSF curves,it concludes that SISF and APB can exist stably in Pt3Hf,while CSF can not It has been observed that some alloys with L12 structure exhibit anomalous yield stress,which is related with the formation of CSF structure.Since the SISF energy is significant lower than APB energy in our research,it can be concluded that<110>superdislocations of Pt3Hf favor to dissociate into superpartials bounding SISF.So it can be predicted that L12 Pt3Hf will not display yield stress anomaly,but present normal behavior,which has a good agreement with experimental result. |
PDF Full Text Request