PPB structure elimination,DRX nucleation mechanisms and grain growth behavior of the3rd-generation PM superalloy for manufacturing aviation components

Previous Particle Boundary(PPB),as the detrimental structure in Powder Metallurgy(PM)components,should be eliminated by subsequent hot process to improve the mechanical properties.The objective is to investigate the Dynamic Recrystallization(DRX)nucleation mechanisms and grain growth behavior of the 3rd-generation PM superalloy with PPB structure.Microstructure observation reveals that PPB decorated with(Ti,Ta,Nb)C carbides belongs to the discontinuous chain-like structure.The elimination of PPB networks can be achieved effectively via hot deformation due to the occurrence of DRX.Four different DRX nucleation mechanisms were proposed and discussed in detail according to the special microstructure characteristics of the PM superalloy.Firstly,local lattice rotations can be detected in the vicinity of(Ti,Ta,Nb)C carbides during hot deformation and thus PPB structure serves as the preferential nucleation sites for DRX grains via Particle-Stimulated Nucleation(PSN).Then,Discontinuous-DRX(DDRX)characterized by grain boundary bulging dominates the microstructure refinement and Continuous-DRX(CDRX)operated by subgrain rotation can be regarded as an important assistant mechanism.At last,the initial Σ3 boundaries would lose their twin characteristics owing to the crystal rotation and then transform into the general High Angle Grain Boundaries(HAGBs).The distorted twins provide additional DRX nucleation sites,viz.,twin-assisted nucleation.Particular attention was focused on the grain growth behavior of the PM superalloy in subsequent annealing process.The recrystallization temperature was determined to be about 1110.C and 1140.C can be considered as the critical temperature for grain growth.The findings would provide theoretical support for microstructure refinement of the 3rd-generation PM superalloy,which is of pivotal significance for improving the mechanical properties of aviation components.

Seasonal Interaction of River Water-Groundwater-Salt Lake Brine and Its Influence on Water-Salt Balance in the Nalenggele River Catchment in Qaidam Basin,NW China

Identifying interactions among river water,groundwater and salt lake brine is important for sustainable exploitation of brine mineral resources.In this study,we investigated the water exchange rate in dry and wet seasons,and assessed the influence of seasonal water exchange on brine concentration and crystallization process in the Nalenggele(NLGL)catchment of Qaidam Basin,China.The results show that the surface water infiltration and groundwater recharge rates in the wet season were 2.81×10^(-3)and 1.15×10^(-3)m^(3)/(s·m)in upstream,1.63×10^(-2)and 1.53×10^(-2)m^(3)/(s·m)in midstream,and 2.83×10^(-4)and 6.82×10^(-5)m^(3)/(s·m)in downstream,respectively;while their counterparts in the dry season were 9.81×10^(-4)and 5.05×10^(-4)m^(3)/(s·m)in upstream,and 8.34×10^(-3)and 7.78×10^(-3)m^(3)/(s·m)in midstream,respectively.The water exchange strongly influenced brine concentration and crystallinzation process,with brine chemistry belonging to 3K_(2)SO_(4)·Na_(2)SO_(4) and KCl types in wet and dry seasons,respectively.The strong water exchange in wet season destroyed the water-salt balance,while the low water exchange rate in dry season facilitated preparation of KCl products.