Effects Of Solution Heat Treatments On Microstructure Of AM3 Single Crystal Superalloy

Nickel-based single crystal superalloys have been widely used in key hot-end parts of aviation engines and gas turbines due to their high temperature bearing capacity and superior mechanical properties.The remarkable feature of its composition change is the increase of the content of refractory elements,which improves the temperature bearing capacity and high temperature mechanical properties of the alloy.However,the addition of refractory elements has caused a series of problems such as poor structural stability,the increase of density and cost,which severely restricted the application of nickel-based single crystal superalloys in industry.In order to give full play to the performance potential of the existing superalloys,the effects of different heat treatment processes on the microstructure of the first-generation single crystal superalloy AM3 were investigated in the paper.By adjusting the solution treatment method and parameter,the effects of the three solution methods on the dendrite structure,?' phase,eutectic structure,carbide and element segregation were thoroughly studied.The main conclusions are as follows:(1)After stepwise heat treatment,the dendritic structures and eutectic structures were significantly reduced or even disappeared for the alloy with a carbon content of 0.045wt.%.Homogenization treatment contributed to the sufficient diffusion of Al,Ti and Ta,which could increase the incipient melting temperature of the alloy.However,the effect of multi-step homogenization on the increase of the incipient melting temperature of the alloy was limited.(2)During the stepwise solution treatment,with the increase of solution temperature and the extension of solution time,the segregation ratio of Co,Mo,Cr,W and Al was closer to 1 for the alloy with a carbon content of 0.045wt.%.However,there was no regular change for the segregation ratio of Ti with the extension of the solution time because of the existence of the incipient melting structure after heat treatment.The existence of residual dendrite structure made the segregation of elements be more serious after the ramp solution heat treatment.The remelting solution heat treatment used a higher solution temperature,which greatly reduced the degree of segregation of alloying elements.(3)The butterfly and irregular ?' phases,which were not uniform in arrangement and distribution in the as-cast for the alloy with a carbon content of 0.045wt.%.After the heat treatment,the precipitated ?' phases were transformed into cubic and stripe shape with more uniform arrangement and distribution due to the sufficient diffusion of the elements.For the one-step and two-step solution heat treatments,the size and volume fraction of the ?' phase gradually were increased with the increase of the solution temperature.The size and volume fraction of the ?' phase gradually were decreased for multi-step solution heat treatment,with the increase of solution temperature and the extension of solution time.(4)With the increasing of ramp solution temperature,the rate of annexation and growth between ?' phases was increased.The size and volume fraction of ?' phase of the alloy without carbon and the alloy with a carbon content of 0.150wt.%gradually were increased.At the same time,the increase of solution temperature increased the nucleation rate and precipitation kinetics of the ?' phase,and a large number of small ?' phases would be precipitated after solution treatment.Therefore,the size and volume fraction of ?' phase of the alloy with a carbon content of 0.045wt.%and the alloy with a carbon content of 0.085wt.%were increased first and then were decreased.(5)After the stepwise heat treatment,a wider matrix channel was formed,accompanied by a higher composition gradient,which was conducive to the coarsening of the alloy.Therefore,the size and volume fraction of the alloy with a carbon content of 0.045wt.%were the largest after the stepwise heat treatment.The low segregation of the alloy with a carbon content of 0.045wt.%after remelting solution heat treatment increased the degree of undercooling and the nucleation rate of ?' phase,which was beneficial to the precipitation of a large number of fine ?' phases.Therefore,the size and volume fraction of?' phase after remelting heat treatment were the smallest for the alloy with a carbon content of 0.045wt.%.(6)For alloys with different carbon contents,most of the skeleton-shaped and chinese-shaped carbides were decomposed into blocky,granular,and thin rod-like shapes after heat treatment,which were conducive to the improvement of the creep properties of the alloy.After remelting solution heat treatment,the carbide type of the alloy with a carbon content of 0.045wt.%was not changed and remained MC type.The carbide type of the alloy with a carbon content of 0.045wt.%were not changed after one-step solution heat treatment,which all were MC type riched in Ta.After two-step and multi-step solution heat treatment,some carbides were converted from MC type to M6C and M23C6 due to the increase of solution temperature and the extension of solution time.For ramp heat treatment,with the increasing of solution temperature,the carbide types of the alloy with a carbon content of 0.045wt.%and the alloy with a carbon content of 0.150wt.%were not changed,which were all MC(TaC,TiC)type carbides.During the ramp solution heat treatment with a peak temperature of above 1311?,some carbides of the alloy with a carbon content of 0.085wt.%were transformed into M6C type.