Microstructures And Properties Of A Nickel Base Single Crystal Superalloy

The high strength of single crystal (SX) superalloys was resulted from the variousstrengthening mechanisms and the effect of alloying elements. Therefore, it is difficult tomake clear the correlation between the chemical compositions, processing parameters andthe microstructure or mechanical properties. A Re-free, low cost, low density nickel-basesuperalloy DD265was directionally solidified (DS) by liquid metal cooling (LMC) methodin the present study. The processing of LMC casting, tensile behavior, creep properties andthe effect of long term annealing on the microstructure stability and mechanical propertieswere investigated.During DS casting by LMC method, more grains generated in the starter resulted inbetter crystal orientation. With the increase of withdraw rate finer microstructure wasgenerated, which means fine primary dendrite arm spacing and low volume fraction ofeutectics. More obvious changes were observed with the increase of the shell temperature.With the change of processing parameters almost no change of creep properties could bedetected, while the room temperature tensile properties were affected dramatically.Very fine microstructure could be obtained by LMC method, the primary dendritearm spacing changed to one half of that in alloys cast by HRS, so did the sizes of eutecticsand carbides. The incipient melting temperature was increased. A simpler solution heattreatment with lower solution temperature and shorter time was employed in alloys cast byLMC than HRS. During the solution heat treatment all the as-cast MC carbides dissolvedin alloys cast by LMC, while only small amount of carbides dissolved in alloys cast byHRS. This was probably due to the lower micro-segregation and finer carbides in LMCalloys. With the increase of temperature the yielding strength and ultimate tensile strengthshow the same trend, i.e. decreased a little bit, and then increased from around450oC, amaximum value was achieved at600oC then decreased slowly, after850oC they decreasedsignificantly. The elongation and percentage of area reduction show the opposite trends.DD265alloy shows the same creep performance as the typical1stgeneration SXsuperalloys. During low temperature creep three stages were shown, while during hightemperature creep only steady stage and accelerate stage were shown. The creep propertieswere affected by the crystal orientation, during975oC/255MPa creep the creep rupture lifedecreased in the sequence of [11]> near [001]-[11]>[001]> near [001]-[011]>[011].Schmidt factor of (111)[01] slip systems and the crystal rotation should be the mainreason.No TCP phases were observed in DD265alloys after2000h annealing at900oC and1000oC. After900oC annealing, the yielding strength and ultimate tensile strengthdecreased, increased and then decreased again with the increase of annealing time. After1000oC annealing both the yielding strength and the ultimate tensile strength decreasedwith the increase of annealing time. The creep properties show the same trend as the tensileproperties.