Materials Science Forum Vols. 747-748

Abstract: sothermal compression tests for superalloy GH720Li were conducted to investigate the effect of hot deformation parameters on microstructure evolution. The changing characteristics of flow stress during hot deformation were also studied. The flow instability area was calculated based on Prasad Plastic Instability Criterion and the hot processing map was obtained. The results showed that dynamic recrystallization was the main softening mechanism of GH720Li alloy during hot deformation. The peak and steady stress decreased significantly with an increase of test temperature and a decrease of the strain rate. Finally, three unstable areas have been found from the processing map.

Abstract: A 700 advanced ultra-supercritical (A-USC) power plant technology project was initiated in China in the year of 2011. The highest temperature components in A-USC boiler are superheater and reheater tubes. The fire-side metal temperature can reach 750 (even higher). Based on the very long time service (30-40 years) these important high temperature tubes require 10⁵h long time stress rupture strength higher than 100MPa and the corrosion/oxidation layer loss less than 2mm for 2×10⁵h. The highest temperature components in 700 steam turbine are the buckets of different stages and require very long service time. At this severe condition only superalloys can fulfill these unusual requirements. This paper reviewed Fe-Ni and Ni-base superalloys from the view point of structure stability and long term mechanical properties and corrosion/oxidation resistance for 700 A-USC power plant materials selection.

Abstract: In order to improve the oxidation performance of IC32, the NiAlHfSi coating was deposited on the alloy by electron beam physical vapor deposition (EB-PVD) method. The oxidation resistance of the coated alloy at 1423 K was investigated. The microstructures of the samples before and after oxidation were examined by SEM, XRD and EPMA. It was found that the oxide scales compactly formed on the surface of the coating, and the oxidation mainly consisted of Al₂O₃ and NiAl₂O₄. Phase transformation occurred from β-NiAl to γ-Ni₃Al in the coating after oxidation for extend periods. The diffusion and oxidation of Mo were prevented effectively by the coating, which improved the oxidation performance of IC32 significantly.

Abstract: The as-cast Ni-W alloys with 15wt%W, 25wt%W and 30wt%W were annealed in hydrogen at 1100. The effect of the annealing time on the microstructure of Ni-W alloys was studied, and the phase constituents and microstructure of annealed Ni-W alloys were characterized by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that no any phase changed for Ni-15%W, Ni-25%W and Ni-30%W alloys annealed for 60 min, 90 min and 150 min, which were still consisted of single-phase Ni (W) solid solution. However, microstructure had a significant change after annealing. With increase of annealing time, the microstructure of Ni-15%W alloy became more uniform after annealing for 90 min, and the average grain size was 95μm, whereas the grain size of Ni-15%W alloy increased significantly after annealing for 150 min. For Ni-25%W and Ni-30%W, there was no obvious change on the grain size with increase of annealing time, and the amount of oxides at grain boundaries gradually reduced. After annealing for 150 min, the impurities at grain boundaries almost disappeared. Subsequently, the annealing at 1100 for 150 min was beneficial for the desired microstructure of Ni-25%W and Ni-30%W alloys.

Abstract: Oxide dispersion strengthened (ODS) materials have received extensive attention due to their excellent resistance to high temperature creep, fatigue, corrosion and radiation damage. In this paper, the ODS iron powder is prepared by co-precipitation. The oxide dispersion strengthened iron was prepared by spark plasma sintering (SPS) from the ODS iron powder. The microstructure and mechanical properties of the materials were investigated by Scanning Electron Microscope and micro-electronic universal tester. The results reveal the Al₂O₃ dispersion of ODS iron powder by co-precipitation is uniform and the mechanical properties are excellent.

Abstract: The third generation single crystal superalloy DD9 was processed with different withdrawal rates and the effect of withdrawal rate on the tensile and stress rupture properties of the alloy was investigated. The relation between the mechanical properties and microstructure of the alloy with different withdrawal rates was discussed. The results showed that the withdrawal rate had a little effect on the tensile properties at 25 of the alloy. The tensile strength at 980 and stress rupture life at 1120/140MPa of DD9 alloy all increased with the increasing of withdrawal rate. The γ precipitates of specimen tensile at 25 had a little extension in the stress orientation. The extension of γ precipitates in the stress orientation at 980 was much more than that at 25. The vertical γ matrix at 980 became thinner and horizontal γ matrix became thicker slightly. The γ precipitates of those had rafted in a direction transverse to the applied stress.

Abstract: Effects of Nb/Ti ratio on the microstructures of two experimental Ni-based cast superalloys of the Nb/Ti ratio 0.46 and 0.03 were investigated. The Nb/Ti ratio had a significant effect on the dendritic microstructures, especially the γ precipitate, of the two alloys under the as-cast condition. The amount and composition of primary MC carbides were closely related to the Nb/Ti ratio for that the Nb atoms have a stronger tendency to form the MC carbides than the Ti atoms. During thermal exposure, the primary MC carbides gradually degraded through two reactions: firstly MC + γ M₆C + γ and then MC + γ M₂₃C₆ + γ. The Nb/Ti ratio had little influence on the reaction formation but obvious influence on the decomposition degree of the primary MC carbides decomposition. In addition, the second M₆C and M₂₃C₆ carbides on the boundaries (GBs) gradually formed a continuous chain during thermal exposure. The γ-free zones appeared along the GBs as the growth of the second carbides and γ precipitates on the GBs when thermal exposure time last up to 3000 h.

Abstract: 9Cr-0.35wt.%Y₂O₃ oxide dispersion strengthened (ODS) ferritic/martensitic steels were prepared by mechanically alloying (MA) and spark plasma sintering (SPS). FE-SEM and TEM with X-ray energy spectrum (EDX) were employed to characterize the microstructural evolution and chemical composition before and after heat treatment. The tensile properties at room temperature were also investigated by electronic tensile test. The result shows that it is mainly of equiaxed ferrite microstructure by SPS with mean grain size of about 500nm. Dispersoids about 5-20nm which are enriched in Y, Ti and O uniformly distribute in the matrix. It exhibits a high relative density, ultimate tensile strength and yield strength of 99.5%, 1554MPa and 1430MPa, respectively. The microstructures are of slender lath martensitic after 10%NaCl water solution quenching, while after tempering at 750 which change into mainly equiaxed ferritic and a little residual ferrite. The ODS steel exhibits ultimate tensile strength, yield strength and total elongation of 1198MPa, 1006MPa and 12.8% after tempering, respectively.

Abstract: A new method was employed to synthesize nanosized Y₂O₃ dispersion strengthened Fe-base superalloy powders using self-propagating combustion. A concentrated solution of nitrates of iron and yttrium was used as oxidizer, malic acid as fuel to fabricate Fe₂O₃/Y₂O₃ powders at low temperature. Afterwards, uniform and fine Fe/2wt.%Y₂O₃ powders were obtained by selective reduction with hydrogen, on the surface of which dispersive Y₂O₃ particles of the mean size of 30 - 50 nm were observed.

Abstract: A low cost Ni-Fe-based wrought superalloy for 700 advance ultra-supercritical coal-fired power plants was developed. The stability of microstructure and mechanical properties of this alloy during long-term thermal exposure was investigated by SEM,TEM and tensile tests. The experimental results showed that the major precipitates in the alloy were spherical γ, MC and discrete M₂₃C₆ distributing along grain boundary after the long-term exposure at 700 and 750 and no harmful phases, such as σ phase and η phase, were found. However, after exposure at 800 up to 3000 h, small amount of lath-like η phase precipitated at grain boundary by consuming the surrounding γ. The η phase exhibited a fixed orientation relationship with the γ matrix. During thermal exposure γ coarsened with increasing the exposure time and exposure temperature. In addition, all major phases and their stability temperature ranges were calculated by JMatPro and these results were confirmed by the experimental results. The 700 tensile tests revealed that the alloy after exposure at 700 and 750 for 3000 h exhibited excellent ductility and strength. Therefore, the GH984G alloy possessed excellent stability of microstructure and mechanical properties between 700 and 750 up to 3000 h, and it is a promising material for 700 advance ultra-supercritical coal-fired power plants.