Microsegregation and Carbide of DD10 Master Alloy with Pouring Temperature AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY23100005

Chinese Library Classification Number:TG244+.1

Citation Information:

The elevated alloy content in superalloy master alloys results in significant microsegregation during the solidification process，potentially giving rise to the formation of sizable carbides and brittle eutectic phase. Nonetheless，eliminating the brittle phase from the master alloy of single-crystal superalloys during subsequent directional solidification is challenging，as it tends to be retained and can significantly degrade the product's performance，particularly in turbine blades. To address the challenge of microsegregation and other solidification defects in superalloys，an investigation into the impact of pouring temperature on microsegregation and carbide precipitation in DD10 （nickel-base single crystal superalloy） master alloy was conducted. Aiming to address the susceptibility of superalloys to solidification defects，such as microsegregation，a 1.3 kg DD10 master alloy ingot was produced in a laboratory vacuum induction furnace. Subsequently，a small sample was extracted from the ingot for analysis. Following grinding and polishing of the sample，scanning electron microscope （SEM） and energy dispersive X-ray spectroscopy （EDS） techniques were employed to observe and analyze the carbides present. Additionally，the sample underwent corrosion treatment using a microetching solution （comprising 76.45% HCl，7.21% H₂SO₄ and 16.34% CuSO₄）. Optical microscopy was utilized to examine the alloy's microstructure. Further analysis was conducted with electron probe microanalysis （EPMA），involving both surface scanning and point scanning of the samples at various pouring temperature to assess microsegregation within the ingot. The equilibrium distribution coefficient of alloying elements in superalloy DD10 was determined using the equilibrium module within the Thermo-Calc thermodynamic software. To comprehensively investigate the impact of cooling rate on microsegregation，it was considered in conjunction with the microsegregation model. The results indicated that DD10 master alloy exhibited a characteristic dendritic microstructure. As the pouring temperature decreased，secondary dendrite spacing decreased while the cooling rate increased. Al，Ta，and Ti exhibited positive segregation between dendrites，while Co，Cr，and W showed negative segregation within the dendrite core. Mo，on the other hand，exhibited relatively uniform distribution without a noticeable segregation tendency. The segregation of Ti，Ta，and W was particularly pronounced. As the pouring temperature decreased，the segregation of alloy elements initially decreased and then increased，with the lowest segregation observed at 1380 ℃. Significant improvement in the segregation of Ti，W，Cr，and Co was observed at a pouring temperature of 1380 ℃. Thermo-Calc thermodynamic software calculated the liquidus temperature of the DD10 master alloy during equilibrium solidification to be approximately 1341 ℃，with a solidus temperature of about 1293 ℃. During solidification，the γ phase precipitated initially，followed by the precipitation of MC carbides. At 1172 ℃，following solidification，the γ phase underwent a transformation known as γ'. Results from the microsegregation model calculations indicated that as the cooling rate increased，the concentrations of positive segregation elements Ti and Ta in the interdendritic regions decreased，while the concentrations of negative segregation elements W in the same regions increased，resulting in a weakening of microsegregation in the alloy. Nonetheless，the extent of microsegregation was influenced by secondary dendrite spacing and liquid fluidity. At a pouring temperature of 1380 ℃，the alloy exhibited a moderate secondary dendrite spacing，coupled with good liquid phase fluidity，resulting in the least degree of microsegregation. The findings revealed that carbides within the DD10 master alloy were predominantly located in the interdendritic regions. At a pouring temperature of 1360 ℃，numerous large-scale strip carbides，characterized by their richness in Ti and Ta （referred to as MC carbides），were observed. However，at temperatures of 1380 ℃ and 1400 ℃，predominantly massive carbides were present. Bulk carbides were categorized as either composite or pure carbides. Composite carbides primarily consisted of Ti （C，N） at the core，with peripheral carbides comprising MC carbides rich in Ti and Ta. Similarly，pure carbides also consisted of MC carbides rich in Ti and Ta. The size and area ratio of MC carbides exhibited an initial decrease followed by an increase with rising pouring temperature. At a pouring temperature of 1380 ℃，the MC carbides attained their minimum size，with an average size of 7.27 μm. Conversely，at 1360 ℃，the carbides reached their maximum size，averaging 10.28 μm. The average carbide size was 7.86 μm at 1400 ℃. As the pouring temperature decreased，the quantity of carbides diminished. This phenomenon was primarily associated with microsegregation and secondary dendrite spacing. Greater microsegregation resulted in more carbide precipitation，while smaller secondary dendrite spacing facilitated the formation of large-size long strip carbides. The formation of large-size massive carbides at higher pouring temperature and large-size strip carbides at lower pouring temperature adversely impacted the alloy's properties. In conclusion，when the pouring temperature was 1380 ℃，the microsegregation of the master alloy was at its minimum，while the precipitation amount and carbide size were optimized. Consequently，selecting the appropriate pouring temperature enhanced the reduction of microsegregation and optimized carbide formation in the master alloy.