The coarsening behavior of γ' particles in a nickel-base superalloy FGH95 was investigated by means of experimental observations and growth kinetics calculations. The results show that when aging at 1000,1080 and 1140°C for different times,the relation of average particle size to time obeys the cube law ( a /2)3= kt,where k is 15.49 × 103,77.5 × 103 and 230.04 × 103 nm3/min,respectively. The particle size distributions are better fit to the LSW theoretical distributions when aging at 1000°C within 1440 min. The activation energy for γ' particles coarsening is determined to be 288.20±1.79 kJ/mol,which correlates well to the diffusion activation energies of Al,Ti,and Nb in the nickel matrix. This indicates that the coarsening of γ' particles is controlled by the diffusion of Al,Ti,and Nb in the nickel matrix. The coarsening kinetics of γ' particles in FGH95 is predicted as rt 3 = 1.04 × 10 16t exp[-(288200 ± 1790 )/RT].
The coarsening and ageing hardening behaviors of γ′ particles in superalloy GH742 have been investigated by field emission scanning electron microscopy （FESEM）, coarsening kinetics calculation and microhardness testing when ageing at 1 050, 950 and 900 ℃ for different time. The cube of γ′ particle size and ageing time follows a linear law as predicted by LSW theory. The particle size distributions give better fit to the LSW theoretical distribution. The activation energy for γ′ coarsening is accurately determined to have 245. 06±14.42 kJ/mol when considering the effect of temperature on the solution concentrations in matrix. Based on the activation energy, the coarsening kinetics of γ′ particle is predicted as r^-t^3=7.35×10^15 Ce/T exp -（245060±14420）/RT t.The microhardness studies indicate that microhardness decreases rapidly with increasing the ageing temperature and it has a maximum value corresponding to a critical particle size beyond which the microhardness increase stalls with increasing the ageing time.
LI Hong-yu SONG Xi-ping WANG Yan-li CHEN Guo-liang
The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD),reverse Monte Carlo (RMC),ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral,FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model,characterized by imperfect ordered packing (IOP),was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore,the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also ex-plored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity,then 2D periodicity,and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked.
The characteristics of γ′ precipitates in a superalloy quenched from 1050°C at different rates were investigated using field emission scanning electron microscope（FESEM）.When quenched from 1050°C, the size of primary aging γ′ precipitates has a small increase in the specimens that experienced iced-brine-quenching, oil-quenching, and air-cooling-quenching conditions and a drastic increase in the specimen that experienced a furnace-cooling-quenching condition.The cooling γ′ precipitates have unimodal distributions after quenching at the air-cooling rate and bimodal distributions after quenching at the furnace-cooling rate, but there are not these distributions in the specimens that experienced iced-brine-quenching and oil-quenching conditions.When aging at 760°C, the size of primary aging γ′ precipitates appears unaffected in the specimens that experienced iced-brine-quenching, oil-quenching, and air-cooling-quenching conditions.However, it has a drastic increase in the specimen that experienced a furnace-cooling-quenching condition, and it is interesting that the bigger cooling γ′ precipitates have a coalescence and octodendritic shape.The microhardness study indicates that the hardness has no variation in the specimens that experienced iced-brine-quenching, oil-quenching, and air-cooling-quenching conditions and has a drastic decrease in the specimens that experienced a furnace-cooling-quenching condition and obtains the minimum microhardness value 390.8 HV.
LI Hongyu, SONG Xiping, WANG Yanli, and CHEN Guoliang State Key Lab for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
