Effect Of Mo On Microstructure And Mechanical Properties Of IN718 Alloy

IN718 alloy is a precipitation hardened Ni-Fe-Cr base superalloy,which is strengthened mainly by ?" phase and partially by ?' phase.Due to its excellent comprehensive properties,IN718 alloy has been widely applied in aerospace,nuclear power,and oil industries.However,?' phase will be instable and then transform into ? phase when the long-term service temperature of IN718 alloy is higher than 650?,and thus the strength of IN718 alloy decreases markedly.In the past 30 years,a lot of work have carried out to improve the microstructure stability of IN718 alloy and intended to develop a new 718 type alloy used beyond 650?.In this study,IN71 8 alloy has been designed by increasing Al,Ti and reducing Nb within the composition specifications in order to obtain a compact morphology of ?"phase and ?' phase,in addition,and increasing Mo to strengthen the ? matrix.This study mainly focuses on the effects of Mo on the microstructure and mechanical properties of IN718 alloy.Moreover,the effect of Mo on the thermal stability of IN718 alloy after long-term aging at 680? is also investigated.And these effects are also analyzed which can be consulted for further improving the properties of IN718 alloy.Additionally,the microstructure and mechanical properties of IN718 alloy with an extra-high Mo content of 7.50%have been studied and optimized by adjusting heat treatments.And an optimum heat treatment has been selected to improve the mechanical properties of IN718 alloy with 7.50%Mo addition.The effects of Mo on the as-cast microstructure,dissolution of Laves phase and diffusion of Nb in IN718 alloy during homogenizing have been investigated.The results show that the concentration of Nb in the dendrite core is lowered with the increase of Mo addition,which definitely indicates that Mo increases the segregation of Nb during the solidification of IN718 alloy.In addition,Mo decreases the concentrations of Nb,Fe and Ni in Laves phase.As a result,the precipitation of Laves phase is greatly increased by the increase of Mo addition,which absorbs more residual liquid and thus reduces the area of segregation zone.The contents of Nb,Fe and Ni in Laves phase decreases,so Laves phase tends to continuously grow into large particle in IN718 alloy with higher Mo addition.Conversely,Laves phase is formed with the eutectic reaction of(Laves+?)in IN718 alloy with lower Mo addition.Mo slightly increases the melting point of Laves phase and prolongs the dissolution time of Laves phase.Moreover,Mo markedly increases the time of Nb distributed homogeneously in the as-cast microstructure,which indicates that Mo impedes the diffusion of Nb noticeably.Based on the melting temperature and dissolution of Laves phase,the homogenization treatment process of IN718 alloy with Mo addition is determined to be 1150? × 50h+1190? × 50h.After standard heat treatment,the effects of Mo on the microstructure and mechanical properties of IN718 alloy have been investigated.The results show that Mo inhibits the precipitation of ? phase on the grain boundary.?phase is inhibited thoroughly but Laves phase begins to precipitate on the grain boundaries instead when Mo addition is higher than 5.50%.The amount of Laves phase increases with the increase of Mo addition.Mo does not change the transgranular precipitation,namely both the disk-shaped ?" phase and spherical ?' phase still precipitate in the ? matrix grain.Mo has no obvious effect on the amount of the precipitation of ?" phase and ?' phase,but their sizes decrease with the increase of Mo addition.Mo slightly reduces the tensile strength of IN718 alloy at room temperature and yield strength at 680?.However,Mo obviously improves the stress rupture life of IN718 alloy at 680? and 725MPa,and has no obvious effect on the stress rupture ductility.During the long-term aging at 680?,the effects of Mo on the microstructure and mechanical properties and thermal stability of IN718 alloy have been studied.The results show that that Mo reduces the coarsening rate of ?" phase and ?' phase,inhibits the precipitation of ?-Cr phase.Therefore,Mo improves the stability of the tensile properties of IN718 alloy.In addition,Mo improves the ultimate tensile strength.During the aging at 680?,the stress rupture life of IN718 alloy decreases to some extent,but Mo can significantly prolong the life.When Mo addition is less than 5.50%,the creep mechanism of alloy is the formation of twins and stacking faults resulted from the shearing of ?" phase and ?' phase by dislocation.When Mo addition is raised to the level of 7.50%,the creep mechanism of alloy is only the formation of twins.The effects of Mo on the grain boundary precipitation and grain growth of IN718 alloy have also been studied.It has been found that ? phases precipitate at the grain boundaries of the alloys with the Mo addition within the scope of 2.80%?4.00%,while Laves phases precipitate at the grain boundaries when Mo addition is in the range of 5.50%?7.50%.The grain boundary precipitates effectively prevent the grain growth by pining the grain boundary,while the grains grow rapidly when the grain boundary precipitates dissolve at higher temperature.The grain growth mechanism of IN718 alloy with different Mo content is controlled by the self-diffusion and the activation energy value for grain growth is 210?260kJ/mol.The grain growth exponent reduces with the increasing Mo content.The microstructure and mechanical properties of IN718 alloy with 7.50%Mo addition have been investigated and optimized by adjusting heat treatments.As a comparison,a commercial IN718 alloy treated by standard heat treatment has also been analyzed.The results show that increasing the solution temperature from 960? to 1050? noticeably reduces the precipitation of Laves phase on the grain boundary and increasing the two stage aging temperatures markedly increases the sizes of ?" phase and ?'phase.The strengths of IN718 alloy with 7.50%Mo addition are increased by the appropriate precipitation of Laves phase on the grain boundary and increase of the sizes of ?" phase and ?' phase.By comparison,it is found that the tensile properties of IN718 alloy with 7.50%Mo addition at room temperature and 680? are significantly improved by following the heat treatment:1020?×1h,AC;750? × 8h,furnace cooled at 55?/h to 650?,650?×8h,AC.In addition,the stress rupture properties of IN71 8 alloy with 7.50%Mo addition at 680?/725MPa are kept unchanged.