Materials Science Forum Vols. 747-748

Abstract: The influence of the re-melting times on chemical composition, microstructure, and mechanical properties of revert alloy K452 were systemically investigated. It was shown that the key factor in the engineering application was the control of gas content and porosities level in revert alloy. By an advancing technology in combination with the superheat treatment, adding a small amount of alloying elements such as C, Al, Ti, and using ceramic foam filters during vacuum induction melting, the composition and mechanical properties of the revert alloy in addition of 50% scrap were similar to that in the virgin alloy, which has successfully been used in applications for nozzle vanes of some new gas turbines.

Abstract: This discussion reviewed the occurrence of stress corrosion cracking (SCC) of alloys 182 and 82 weld metals in primary water (PWSCC) of pressurized water reactors (PWR) from both operating plants and laboratory experiments. Results from in-service experience showed that more than 340 Alloy 182/82 welds have sustained PWSCC. Most of these cases have been attributed to the presence of high residual stresses produced during the manufacture aside from the inherent tendency for Alloy 182/82 to sustain SCC. The affected welds were not subjected to a stress relief heat treatment with adjacent low alloy steel components. Results from laboratory studies indicated that time-to-cracking of Alloy 82 was a factor of 4 to 10 longer than that for Alloy 182. PWSCC depended strongly on the surface condition, surface residual stresses and surface cold work, which were consistent with the results of in-service failures. Improvements in the resistance of advanced weld metals, Alloys 152 and 52, to PWSCC were discussed.

Abstract: Recrystallization behavior after indentation loading and heat treatment at different temperatures of the as-cast and solution-treated alloy was investigated. The results showed that slip deformation occurred in the as-cast and heat treated samples after deformation and apparent slip line can be observed. Slip lines of heat treatment state was significantly more than that of the as-cast samples. Neither equiaxed nor cellular recrystallization occurred in the surface of the as-cast and solution-treated samples after heat treatment at 1120; Slip traces were examined in the vicinity of indentation and M₆C carbide precipitated on the slip line in heat-treated samples, while cellular recrystallization occurred in the internal part and MC carbides became the nucleation centers. Equiaxed recrystallization occurred simultaneously in samples of the two states with the heat treatment at 1310 after deformation and the recrystallization area increased with the processing time; after heat treatment with same time and temperature, recrystallization area of the cast samples were less than that of heat-treated samples.

Abstract: The effects of increasing atomic interchange potentials to the precipitation process and microstructure of Ni-Al-Cr alloy have been simulated based on the microscopic phase field theory. The first nearest neighbour atomic interchange potentials of Ni-Al-Cr alloys for L1₂ and D0₂₂ phase was calculated out according to the formula which were referenced on the relation equation between atomic interchange potentials and long range order parameters by Khachaturyan. The results indicated that Ni-Al (W_Ni-Al) and Ni-Cr (W_Ni-Cr) s first nearest neighbor atomic interaction potentials will increase linearly while the temperatures rose. Moreover W_Ni-Al increased but W_Ni-Cr decreased roughly linearly if Al atoms concentration rose, and conversely inversed. In addition, these atomic interchange potentials changing with temperature and concentration were in good agreement with earlier study.

Abstract: Through the spark plasma sintering technology (SPS), the Nb/Nb₅Si₃/Cr₂Nb mixed powders with different volume ratios of 7:1:2 and 7:2:1 were sintered into ingots. The effects of three powder mixing methods on phase constitution, microstructure and room-temperature mechanical properties of the ternary Nb/Nb₅Si₃/Cr₂Nb alloys were investigated. The result shows that the method of dry mixing without ball milling is advantageous for toughness, the increase of SPS temperature decreases the toughness whereas improves the hardness. The stiffening Nb₅Si₃ and Cr₂Nb phases tend to form a continuous network, resulting in decrease of toughness. It is interesting that when the fractions of the Nb₅Si₃ phase larger than that of the Cr₂Nb phases, these strengthening phases prefer to appear in the form of network, which is detrimental for toughness.

Abstract: 2 at.% and 4 at.% Mo were added to Co-9Al-9W-2Ta-0.02B alloy to replace W (hereafter referred to as the alloys of 2Mo and 4Mo, respectively; Mo-free alloy was referred to as the 0Mo alloy). The effects of Mo additions on the high temperature oxidation behavior of the alloys at 800 °C in air have been studied. The results indicated that, after oxidation in air at 800 °C for 100 h, the oxide film of the 0Mo alloy remained intact, but the cracking and spallation of the oxide film took place in the alloys of 2Mo and 4Mo. Oxidation kinetic curves revealed weight gain per unit area of the 0Mo alloy was 36.86 mg·cm^-2, which was lower than that of the alloys of 2Mo (65.16 mg·cm^-2) and 4Mo (48.54mg·cm^-2). These suggested that the 0Mo alloy displayed superior oxidation resistance compared to the alloys of 2Mo and 4Mo caused by the formation of volatile MoO₃ oxide, and sharp compressive stress formed in the outer layer during the oxidation. The oxide layer was composed of three layers of the Co₃O₄ + CoO outer layer, middle complex oxide layer containing Co, Al and W (Mo), inner Al₂O₃ layer and γ/Co₃W zone adhere to the γ/γ' substrate.

Abstract: The precipitation and dissolution behavior of δ phase at varied states including as cast, heat treated and thermal processed of GH4169G alloy, have been investigated. The results indicated that δ phase existed in the vicinity of Laves phase and carbides with mainly the rod-like, needle-like morphology, and lath-like morphology because of the segregation of Nb in the interdendritic area. The amount of the δ phase decreased with the increase of the annealing temperature, and the dissolution rate increased with the increase of the annealing temperature. After initial holding of 2 h at 1020°C, the residual δ phase fully dissolved at 1140°C for 20min for the as-cast ingot. The minimum holding time for δ phase dissolution at 1020°C was 2h.

Abstract: The interfacial reactions between ceramic moulds and DZ417G and DZ125 superalloys were investigated. The microstructure and composition of the interface region were observed by optical microscope, X-ray diffraction and scanning electron microscope with energy dispersive spectroscopy. The results showed that (Al_1-xCr_x)₂O₃ solid solution phase with pink color was formed from the dissolution of Cr₂O₃ and Al₂O₃ and vapour phase, which was transferred to the reaction surfaces. The reaction layer thicknesses of DZ417G and DZ125 alloys were about in the range of 40-50μm. The interface reaction product between DZ417G alloy and ceramic mould was TiO₂ and the product between DZ125 alloy and ceramic mould was HfO₂.

Abstract: The effects of hot isostatic pressing on the microstructures of a third generation single crystal Ni-based superalloy DD10 were investigated by using optical microscope (OM), scanning electron microscope (SEM), electron microprobe analyzer (EPMA). The results showed that the micropores in the interdendritic region were eliminated completely after hot isostatic pressing at 1320 and 150MPa. Meanwhile, the morphology of γ precipitates changed to be more cuboidal and the distribution of γ precipitates in both dendrite core and interdendritic region became more uniform after hot isostatic pressing. Hot isostatic pressing also promoted the homogenization of the composition between dendrite core and interdendritic region and the dendritic segregation of Re, W, Al and Ta was decreased.

Abstract: The effect of Ru addition (0 wt.%, 3 wt.%, 5 wt.%), on γ morphology, elemental segregation, microstructural stability under long-term thermal exposure at 980 and stress rupture properties at 1100°C/130MPa have been studied. The results showed that with the increase of Ru content, the γ/γ eutectic volume fraction and the dendrite arm spacing decreased gradually. The γ' phase mean size in three alloys decreased with the increase of the Ru content. On the other hand, the dendrite segregation of Al, Ta towards interdendrite area and ReW towards dendrite core area was alleviated gradually with the increase of the Ru content. The increase of Ru content from 0 wt.% to 5 wt.% pronouncedly enhanced the stress rupture properties by suppressing the precipitation of TCP phases effectively at high temperature.