Lan Zhang Zhou

Effect of Hafnium on the Microstructure and Creep Property of a Hot Corrosion Resistant Superalloy

The effects of the selective addition of Hafnium (Hf) on the grain boundary, phase, carbides and creep properties of experimented nickel superalloy after standard heat treatment and long-term exposure were investigated. Predicted by the Bayesian neural network, the creep life is prolonged with Hf content of 0-0.6 mass%, which is more effective at low stresses. The decrease of creep life of Hf free alloy after long term exposure was pronounced. Comparative study showed that the mainly small, coherent, blocky and closely spaced MC(2) and M23C6 carbides precipitated on the grain boundaries in the 0.4wt% Hf contained alloy, and that relatively larger, incoherent MC(1) carbides precipitated on the grain boundaries in the Hf free alloy. During long term thermal exposure, fine discrete M23C6 carbides decomposed from primary carbide, inducing a layer along the grain boundary, and the coarsening of grain boundary in Hf free alloy is more pronounced. At high stresses, the Hf-free alloy exhibited a stronger tendency of rafting than the 0.4Hf alloy, while the tendency of appearance of rafting was very similar at low stresses. However, Hf can render the alloy prone to the formation of σ phase, according to D-electrons method. Thus, the Hf content needs to be controlled to a suitable level.

Effect of Hot Isostatic Pressing on the Microstructure of K417G Cast Turbine Discs

The present study focused on the effect of hot isostatic pressing (HIP) treatment on the microstructure of K417G superalloy. The experimental results showed that after the HIP treatment the size and volume fraction of the porosities significantly decreased. In addition, the dendritic structure and γ/γ eutectics in the as-cast specimens became obscure after the HIP treatment due to the improvement of segregation. The γ′ phases in the dendrite core were smaller than those in the interdendritic region, whether in the as-cast or HIP specimens. The slow cooling at the end of the HIP treatment leads to the irregular morphology of the γ′ phases.

Thermal Stability and Tensile Properties of GH984G Alloy for A-USC Power Plants

GH984G is a new Ni-Fe-based alloy which has been designed for use as superheater, reheater, and header materials for boilers in 700°C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the tensile properties and microstructure evolutions of GH984G were investigated during long-term thermal exposure at 700 and 750°C up to 15000h. The results show that the major precipitates were MC, M23C6 and γ ́, and no detrimental TCP or GCP phase appeared during long-term thermal exposure. The morphology of γ ́ was spherical and had no obvious change, but the size increases. After thermal exposure at 700°C for 15000h, the 700°C yield strength slightly increased, but the yield strength decreased after thermal exposure at 750°C for 15000h. The variation of strength is attributed to the coarsening of γ ́ precipitation. The deformation mechanism is the moving dislocations shear γ ́ precipitates and the stacking faults form in γ ́ precipitates at room temperature and 700°C. At 750°C, the deformation mechanism is characterized by the formation of dislocation loops and tangles.

Effect of Nitrogen on Microstructure and Properties of a Cast Cobalt-Base Superalloy

The effect of the nitrogen on the microstructure and properties of a cast Co-based superalloy K640S was investigated with the different level ranged from 24 ppm to 85 ppm. The results indicate that almost all nitrogen exist in the form of nitride precipitation in the vacuum induction melting. It is found that high N level would extend the solidification window during the solidification process, and then increase the quantity of the carbide eutectic phases and led to the distribution in the network form of the eutectics in the interdendritic regions. High nitrogen content (more than 40 ppm) can reduced the mechanical properties of the testing alloy. So, it is suggested that the nitrogen in the recycled Co-based superalloys should be controlled at a suitable level in order to avoid the degradation of their properties.

Microstructure and Mechanical Properties of a Cobalt-Based Superalloy Used for Nuclear Power Station

The microstructure and mechanical properties of a Co-Cr-W-Mo superalloy used for the driving mechanism of nuclear power station were investigated. This alloy was strengthened by W and Mo elements and carbides for high hardness, and an excellent wear resistance was achieved by the high content of carbides. Also, this alloy was of excellent hot-corrosion resistance due to the addition of 29 wt. % Cr. The as-cast alloy was equiaxed, and consisted mainly of γ-Co solid solution, γ/M7C3 eutectics and Cr-rich M7C3 carbides. The Rockwell hardness was up to 37 - 40. The alloy presented high tensile strength, and the fractography of tensile ruptured specimens was characterized obviously by the interdendritic fracture.

Influence of Fe Content on Microstructure and Mechanical Properties of GH984 Alloy

GH984 is an economic alloy due to the elimination of Co and its containing more than 20% Fe. As a major constituent of GH984 alloy, the Fe reduces the cost, but promotes the precipitation of TCP phase. In order to get an optimum alloy with desired balance among structural stability, mechanical properties and cost, the influence of Fe content on microstructure and mechanical properties of GH984 alloy was investigated in this paper. The results showed that the Fe content had no obvious influence on the major precipitates which were spherical γ′, blocky MC and discrete M23C6 at grain boundary after standard heat treatment, except the γ/γ ́ misfit. However, during long-term thermal exposure at 750°C, the decrease of the Fe content retarded the precipitation of η phase and enhanced the structural stability. The Fe content had no obvious influence on the tensile strength and ductility. At condition of 700°C/400MPa, the rupture life had not evident change, but the elongation decreased, the fractural model exhibited mix-fracture model and the characteristics of intergranular fracture became obvious with increasing the Fe content. It can be concluded that the decrease of Fe content can improve the structural stability and ductility.

High Cycle Fatigue Behavior of a Wrought Nickel-Base Superalloy GH4698

Smooth and notched specimens of a wrought nickel-base superalloy GH4698 were subjected to high cycle fatigue (HCF) loading at room temperature and 650°C in ambient atmosphere. The experimental results show that the fatigue strength decreased with the increasing of temperature and cyclic stress, and fatigue limit of notched specimens was lower than that of smooth specimens. The fracture surface presents typical characteristics of fatigue fracture, where both stage I and stage II crack propagation can be found. For the smooth specimens, the single primary crack initiated on the surface or subsurface, while multiple primary cracks initiated on the surface for the notched specimens because of local stress concentration.

Investigation on Microstructure and Mechanical Properties of Directional Solidified DZ417G Alloy with Different Proportion of Recycling Alloy

The DZ417G alloy with different proportion of recycling material was fabricated by directional solidification, and its microstructure and mechanical properties were investigated. The results show that the microstructure of the directionally solidified alloy changes little, which indicates the recycling material has little effect of on the alloy. However, the ductility of the directional solidified alloy with different proportion of recycling material changes greatly. With the increase of the recycling materials, the mechanical properties of the alloy decrease firstly and then increase a little and decrease at last. But the strength of the alloy almost has no change. The alloy with 70%recycle material has the relative better mechanical properties.

Study on the Transformation of (α+β) Phase to β Phase in Deformed Ti-6Al-4V Alloy during the Heat Treatment

In the present paper the Ti-6Al-4V alloy with α and β phases was deformed by hot rolling and solution heat treated at different temperature for 30 minutes to investigate the transformation of (α+β) to β. The results show that the deformed alloy possesses the typical (α+β) dual-phases microstructure, when the alloy is treated below the 950 °C. With the increase of the heat treatment temperature the volume of α phase decreases significantly. When the temperature is higher than the 970 °C, almost no α phase is left in the alloy.

The Effect of Gallium on the Microstructure of Superalloy K444

In order to figure out whether Gallium works in the superalloy K444, its composition was modified by adding about 0.003 wt.% and 0.01 wt.% Ga, respectively. Microstructure of the modified superalloy K444 after standard heat treatment was investigated by using optical microscopy(OM),scanning electron microscopy(SEM) and energy dispersive spectroscopy (EDS). The primary dendritic distance of these alloys had no significant changes with the increasing of Ga content, and there were no significant changes for the grain size of the three alloys. The content of eutectic was most in the alloy containing 0.01wt.% Ga, and the morphology of eutectic was most irregular. The segregation of other elements was effected by Ga and the elements of Cr and Zr were segregated seriously in the alloy containing 0.01wt.% Ga.