Effect Of Phosphorus On The Deformation Behavior Of GH984G Alloy

The Ni-Fe based superalloy GH984G,with low cost,intellectual property rights,good creep rupture strength,excellent oxidation and corrosion resistance,has been regarded as a candidate material for 700? ultra-supercritical(USC)power plants application.Phosphorus microalloying is an important method to improve the creep rupture strength of GH984G alloy.However,the effect of phosphorus on creep property under low stress is still unknown.Therefore,it is necessary to investigate the effect of phosphorus on the microstructures and properties in GH984G alloy.According to the requirements for hot components of 700? USC power plants,this study investigates the effect and mechanism of phosphorus on hot deformation,tensile,impact and creep properties of GH984G alloy.The results may help the composition design and processing technology optimization of the phosphorus microalloying Ni-Fe based alloy.The effects of phosphorus on the hot deformation behavior in the homogenized,forged and hot-rolled GH984G alloy have been investigated.It is found that the optimal processing parameters of the homogenized alloy with different phosphorus contents all locate in the high temperature and low strain rate domain,but the optimal deformation temperature range reduces from 1050?-1200? to 1100?-1200? with the increase of phosphorus content.And the temperature range of high-temperature instability domain in the homogenized alloy extends from 1175?-1200? to 1100?-1200?with the increase of phosphorus content.The optimal deformation temperature and strain rate of the forged and hot-rolled alloys are both obtained in the vicinity of 1150?/1.0 s-1,which has almost no connection with phosphorus content.The phosphorus mainly influences the continuous dynamic recrystallization(CDRX)during hot deformation in the following two ways.Firstly,the phosphorus dissolving in matrix hinders the CDRX by inhibiting dislocation motion and acts as solute drag effect.Secondly,the phosphorus-addition increases the volume fraction and size of MC carbides.The MC carbides can increase the nucleation sites and promote the CDRX by particle stimulated nucleation mechanism.The fractions of CDRX nucleation of the homogenized,forged and hot-rolled alloy decrease in turn.Therefore,the effects of phosphorus on the hot deformation behavior decrease in turn in the alloy with different conditions.The hot deformation behavior in homogenized alloy is influenced by the solute drag and particle stimulated nucleation.The dynamic recrystallization(DRX)fraction decreases firstly and then increases with the rising of phosphorus content.The hot deformation of forged alloy is only influenced by particle stimulated nucleation and the DRX fraction increases with the rising of phosphorus content.However,the phosphorus-addition has almost no effect on the hot deformation behavior of hot-rolled alloy.In addition,the phosphorus microalloying can decrease the ratio of?3 twin boundaries after hot deformation when the DRX fraction is high.The impact toughness at room temperature of GH984G alloy after long-term thermal exposure at 650?-750? up to 10000 h decreases with the increase of phosphorus content.Phosphorus segregates at the grain boundaries and then twin boundaries with the increase of phosphorus content and the prolonging of thermal exposure.The excessive segregation of phosphorus at grain boundaries in the alloy with high phosphorus content decreases the grain boundary cohesion and then damages the impact toughness.In addition,the impact toughness decreases rapidly and then remains stable with the prolonging of thermal exposure,meanwhile,it rises with the increase of thermal exposure temperature.Nevertheless,the phosphorus content has slight effect on the tensile properties at 700? of GH984G alloy.The study also focuses on the interaction between phosphorus content and creep stress on the creep behavior.The beneficial effects of phosphorus on the creep properties and the optimal phosphorus content both reduce with the decrease of creep stress.The alloy with 0.04wt.%phosphorus has the longest creep rupture life under 700?/400 MPa.And the alloys with 0.02wt.%-0.04wt.%phosphorus possess the lowest minimum creep rate under 700?/250 MPa-350 MPa.However,the optimal phosphorus content reduces to 0.01wt.%under 700?/200 MPa.According to the results,it is speculated that the phosphorus will be of no beneficial effects on the creep properties below a critical creep stress.And the critical creep stress reduces with the decrease of phosphorus content.The critical creep stress is about 190 MPa in the alloy with 0.02wt.%-0.04wt.%phosphorus.Nevertheless,it reduces to about 130 MPa when the phosphorus content is 0.01wt.%.Therefore,the phosphorus content should not be higher than 0.01 wt.%in GH984G alloy,that is beneficial to the long-term application on the USC power plants.