Microstructures And Mechanical Properties Of Ni-base Superalloy Fabricated By Friction Stir Processing

Friction stir processing(FSP)is a solid-state metalworking technique derived from friction stir welding(FSW),which can be used for the preparation of fine-grained ma-terials.This technique has been widely applied in low-melting-point materials,such as aluminum and magnesium alloys.However,its application in high-melting-point materials is restricted due to severe tool wear and narrow processing window.So far,researches on the FSW/P of superalloys were mainly focused on the alloys with low strength,but few works have been conducted on the superalloys with high strength.Moreover,as for FSW/P of superalloys,it is needed to build the relationship between the microstructural evolution,the tensile properties and related deformation mechanisms at high temperature.In this study,two superalloys with different volume fraction of ?'precipitates were selected:Ni-Fe based superalloys and Ni-Co based superalloys.The FSP of these superalloys were investigated comprehensively and the relationship be-tween microstructure and mechanical properties in the stir zone(SZ)were deeply ana-lyzed.The goals of this study are:(a)to determine the feasibility of the stir tool;(b)to explore the processing window for FSP of high-strength superalloys and obtain the optimal processing parameters;(c)to clarify the grain refinement mechanism during FSP;(d)to uncover the intrinsic connection between the microstructure and excellent strength-ductility synergy behavior and the Portevin-Le Chatelier(PLC)effects.Firstly,a Ni-Fe based superalloy with low volume fraction of ?' precipitates was selected to investigate the grain refinement mechanism during FSP and the effect of pro-cessing parameters on microstructure and mechanical properties.The results showed that the tensile yield strength of all FSP samples at 700? were enhanced clearly com-pared to the base material(BM).The tensile yield strength of FSP samples depended on the grain size.The FSP sample 400-50 had the finer grain size and higher strength,when other processing parameters were fixed,the optimal processing parameters were rotation rate of 400 rmp and traverse speed of 50 mm/min.The percentage of high-angle grain boundaries(HAGBs)of FSP samples was 58.1-65.1%and dislocation structures were observed in the SZ,indicating that the grain refinement mechanism of the Ni-Fe based superalloy during FSP was continuous dynamic recrystallization.After FSP,the tool was hardly worn,which can be concluded that the tool can be used for the FSW/P of superalloys.Secondly,in order to expand the application of FSP,a Ni-Co based superalloy with high volume fraction of ?' precipitates was selected to explore its processing window,reveal the grain refinement mechanism during FSP,and investigate the hardness and tensile properties of FSP samples at different temperatures.The processing window of FSP of Ni-Co based superalloys was narrow,and the suitable processing parameters were traverse speed of 50 mm/min and rotation rate of 400-700 rpm.In the SZ,the grain boundaries migrated from dislocation-free grain to the grain with high dislocation density and the bulging grain boundaries were observed,which proved that the grain refinement mechanism was discontinuous dynamic recrystallization.The microstruc-ture of fine grains without primary ?' was obtained in FSP samples,which was different from traditional metal materials which must rely on the pinning of primary to obtain a fine-grained structure.The BM and advanced wrought Ni-Co based superalloy were chosen for comparative experiments,the hardness,tensile strength,ductility and work-hardening ability of FSP samples were enhanced compared to the BM.Without the loss of ductility,70%higher yield strength were obtained in FSP samples and reached 1.3 GPa with the combined effects from fine grains and nanotwins.The combined strength-ening of nanotwins and precipitation led to higher yield strength of FSP samples than that of the advanced wrought Ni-Co based superalloy.The relationship between the tensile behavior and tensile deformation mechanisms was built.The results showed that the deformation microstructure of FSP sample after subjected to tensile deforma-tion at 400? was microtwins sweep through the ? matrix and ?' precipitates.The PLC effect was attributed to the interaction between solutes and defects like twins and mobile dislocations.The equicohesive temperature of the FSP samples was between 400 and 650?.At 650?,the material underwent intergranular fracture because of a large of straight grain boundaries were introduced through FSP,which was the reason of ductility drop.Thirdly,the effect of pre-processing heat treatment and processing parameters on microstructure and mechanical properties of friction stir processed Ni-Co based super-alloy were investigated.The experimental results showed that only the structure of grain boundaries and the percentage of the twin boundaries were influenced by the pre-processing heat treatment,but the other microstructures as well as tensile properties below equicohesive temperature showed no relationship with the pre-processing heat treatment.As the rotation rate changes,the grain size maintained a stable range and the tensile properties of FSP samples with different rotation rates were similar.Finally,the PLC effects of all Ni-Co based superalloys tensioned at 400? were analysed based on dynamic strain ageing theory.The sparse and high-stress drop saw-tooth of FSP samples were explored,and the influences of processing parameters and Co content on the PLC effect of FSP samples were studied.The fine grain,high den-sity nanotwins and high-hardness corrugated layers inhibited the dislocation,resulting in an increase in the mobile dislocation density.Meanwhile,solute atoms have more time to deflect towards the mobile dislocation,enhancing the interaction between the mobile dislocation and solute atoms.Therefore,the stress amplitudes in FSP samples were much higher than that of the BM.The decrease in the serration frequency of FSP samples was attributed to the dissolution of ?' precipitates and low density dislocation in fine grains.The high-amplitude serrations might be caused by a large number of stacking faults(SFs)or microtwins.With increasing the content of Co,the stacking fault energy of alloys decreased,resulting in the high density of SFs or microtwins,and thus increasing the stress amplitude.The PLC effects of FSP samples were influenced by the grain size,the percentage of twin boundaries and HAGBs which depended on the processing parameters.