Investigation Of Microstructures And Mechanical Properties Of M951G Nickle-Base Superalloy

In this dissertation,the microstructure,tensile behavior,creep behavior,low cycle fatigue behavior and the corresponding deformation mechanisms,microstructural evolutions as well as fracture characteristics of the newly designed nickel-base superalloy M951G have been studied.Both the as-cast and heat-treated microstructures consist of ?' phase,y matrix,and MC type carbide.The heat-treated microstructure still maintains a pronounced dendritic structure.Two types of y ?' phases exist in the heat-treated microstructure:one is coarse cuboidal y'particles with an average edge length about 430 n,the other is spherical ?'particles of 65-80 nm mean diameter.The yield strength(YS)of M951G alloy progressively decreases from 20 ? to 600 ?,then gradually increases with the increase of temperature and reaches a peak at 800 ?.After that,it drops sharply.The UTS-temperature curve has the same trend as that of YS-temperature curve.UTS reaches the peak value at 700 ?.Plasticity and strength have an inverse dependence on temperature.Though the value of elongation is high,the intermediate temperature brittleness(ITB)behavior for M951G alloy at 700 ? is quite remarkable.The primary deformation mechanisms include shearing of the y' precipitates by a/2(101)dislocations below 600 ? and by-passing of ?'precipitates by a/2(101)matrix dislocations above 900 ?.At intermediate temperatures,it shows a transition from shearing to by passing.After creep deformations at 700 ? and 800 ?,M951G alloy maintains a well microstructure stability.Therefore,the effects of precipitate dimensional changes on the creep properties of M951G alloy at low and intermediate temperatures is very limited.The dominant deformation mechanisms at 700 ? and 800 ? are isolated stacking faults shearing the ?' precipitates,continuous stacking faults cutting through the y and ?' phases and few dislocations lying in the ? matrix channel.The fracture mode at low and intermediate temperatures is a mixed type of transgranular and intergranular,which is due to the remarkable stress concentration along the grain boundaries,broken carbides and eutectics and the variation of creep strain rate.After creep deformations at 1000 ?,the cubic ?'phase changes to rafted shape,and the spherical ?'phase is totally dissolved,and the width of y' channels is significantly increased.Degradation of microstructure at high temperatures is detrimental to the creep properties of M951G alloy.The dominant deformation mechanisms at 1000 0C are APBs shearing the rafted?' phase and climbing of dislocations in the y matrix channel.The fracture mode turnsto pure intergranular cracking at 1000 ?.The coarsening and interlinking of the micropores along the grain boundary lead to the occurrence of intergranular fracture.Therefore,900 ? is the transition temperature of deformation microstructures,fracture modes and dislocation configurations.The values of apparent stress exponents are calculated to be about 11.594 and 6 at 700 ? and 1000 ?,respectively.The changes of the deformation mechanisms,fracture modes and the degeneration of microstructures are supposed to be responsible for it.?tAPB,?SF are the CRSSs needed for the operation of the APB coupled dislocationpairs and stacking fault shearing of the ?' precipitates,respectively.?OB,?CL are the CRSSs for Orowan bowing and climbing process of dislocations in the y matrix channel.The CRSSs(critical resolved shear stresses)of different creep mechanisms show different dependencies of testing temperature.?APB,tOB and ?CLare decreased to varying degrees with the increase of testing temperature;on the contrary,there is a positive correlation between ?SF and testing temperature.The favorable deformation mechanism below 800 ? is shearing of the microstructures by stacking faults.While the favorable deformation mechanisms change to dislocations climbing in the y matrix channels when temperature above 900 ?.At intermediate temperatures(800-900 ?),both APBs and stacking faults can be observed in the ?' precipitates.Cyclic stress responses of M951G alloy under different testing conditions aresignificantly different.At 900 ?,the cyclic stress response curve shows a continuous cyclic softening during the whole cyclic deformation at relatively higher total strain amplitude of 1.2%.However,when the total strain amplitude varies from 0.5%to 1.0%,the specimens show an initial cyclic hardening at the beginning of the tests followed by a well-defined cyclic stability.At 1000 ?,under total strain amplitudes from 0.5%to 1.2%,the cyclic stress response curves show a slight cyclic softening at the beginning of cyclic deformation.When the total strain amplitude decreases to 0.4%,M951G alloy initially shows a period of cyclic hardening and then enters into a long period of cyclic stable stage.At low strain amplitude,the cracks mainly initiate at the oxidized carbides andeutectics along the grain boundaries on the sample surfaces and propagate intergranularly.At high strain amplitude,apart from the oxidized carbides and eutectics along the grain boundaries,casting micropores and residual eutectics in the grain interior also become the crack initiation sources.Fracture mechanism changes to a mixed mode of transgranular and intergranular.The average size of ?' precipitates during the low-cycle fatigue deformation has a close relationship with the change of interfacial flow stress difference.The fatigue life and cyclic stress of M951G alloy under the same total strain amplitude at 1000 ? are significantly lower than that at 900 ?,which is due to the degradation of microstructures,greater number of shearing dislocations in the ?'precipitates and higher degree of surface oxidation.