Effect of Nb on δ Phase Precipitation and the Tensile Properties in Cast Alloy IN625

Cast alloy IN625 is a candidate Ni-base alloy for advanced ultra-supercritical (A-USC) power plants. The aim of this study was to investigate the influences of Nb on δ phase precipitation and the tensile properties in this alloy. The results show that the δ phase is easy to precipitate after long-term aging at 700 °C (the service temperature of A-USC power plants) and it is strongly affected by the content of Nb in the alloy. The strength of alloys after aging at 700 °C for 10,000 h increases with the increasing Nb content and also increases sharply when compared to that of as-heat-treated alloys. The increase in strength is found to be primarily associated with the precipitation of the δ phase

The microstructure evolution and its effect on the mechanical properties of a hot-corrosion resistant Ni-based superalloy during long-term thermal exposure

The microstructure evolution and its influence on the mechanical properties are investigated in a hot-corrosion resistant Ni-based superalloy during long-term thermal exposure. It is found that the tertiary gamma' phase disappears and the secondary gamma' phase coarsens and coalesces gradually, which acts as the main reason for the decreasing of strength at both room temperature and 900 degrees C. During exposure, the grain boundary coarsens from discontinuous to half-continuous and finally to continuous structure. The optimum half-continuous grain boundary structure composed of discrete M23C6 and M3B2 wrapped by gamma' film leads to the elongation peak at room temperature in the thermally exposed specimens. At 900 degrees C, the increase in the elongation is attributed to the much softer matrix and the formation of microvoids. The behavior of primary MC decomposition is a diffusion-controlled process. During exposure, various derivative phases including M23C6, gamma', eta, M6C and sigma sequentially form in the decomposed region. Primary MC decomposition and the precipitation of sigma phase have little effect on the mechanical properties due to their low volume fractions. (C) 2012 Elsevier Ltd. All rights reserved

The effect of Hafnium on the microstructure and tensile property in a Ni-based superalloy

The effects of Hafnium on the microstructure and tensile property of a Ni-based superalloy after solution heat treatment and long-term thermal exposure were investigated with field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), X-ray diffractometer (XRD). The results revealed reveal that Hf intensified intensifies the element segregation and promoted promotes the precipitation of MC carbide and honeycomb-like Ni5Hf phase in the as-cast alloy. The solid solution strengthening and second phase (MC) strengthening effects are both enhanced in the alloy doping 0.5 wt% Hf after solution heat treatment. Therefore, the tensile yield strength at 700 °C of the alloy with 0.5 wt% Hf is higher than that in the Hf-free alloy. Otherwise, a large amount of the low melting point Ni5Hf phases appear in the alloy with 0.8 wt% Hf. Ni5Hf phase is prone to become the crack initiation, which decreases diminishes the yield strength in the alloy with 0.8 wt% Hf. The addition of 0.5 wt% Hf can inhibit the formation of needle-like δ phase at grain boundary due to the decrease of grain boundary energy and diffusion rate at grain boundary. Therefore, the 700 °C tensile strength and ductility increase simultaneously in the alloy with 0.5 wt% Hf after thermal exposure at 700 °C

Effect of hot isostatic pressing on microstructure of cast gas-turbine vanes of K452 alloy

The effect of hot isostatic pressing (HIP) treatment on microstructure of gas-turbine vanes made of K452 alloy was investigated by OM, SEM and TEM. The results showed that HIP treatment played a great role in the porosity healing processing, where 80% of porosities were eliminated and the diameter of remnant porosities decreased to 10 μm. The healing mechanism of the porosities was consistent with existing theories of porosity closure based on vacancy diffusion. According to the result of the tensile test, the plasticity of the alloy was improved as the result of the elimination of the porosities and the improvement of dendritic segregation, while there was not an obvious improvement in the tensile strength after the programmed HIP process. In addition, HIP had a positive effect on narrowing down the dispersion of tensile properties

Microstructure and mechanical properties of NiAl-Cr(Mo)-Hf/Ho near-eutectic alloy prepared by suction casting

The microstructure and mechanical properties of 0.15 at.% Hf and 0.2 at.% Ho doped Ni-33Al-28Cr-6Mo near-eutectic alloys prepared by conventional casting and suction casting were investigated. The results reveal that the addition of Hf and Ho results in the formation of Ni(2)AlHf and Ni(2)Al(3)Ho phases, respectively, along the NiAl and Cr(Mo) phase interface in intercellular regions. Compared with the conventional-cast alloy, the microstructure of suction-cast alloy is well optimized, characterized by the thin interlamellar spacing, high proportion of eutectic cell area and fine homogeneously distributed Ni(2)AlHf and Ni(2)Al(3)Ho phases. Furthermore, the room temperature mechanical properties of the suction-cast alloy improve significantly, which can be attributed to fine microstructure, uniform distribution of the Ni(2)AlHf and Ni(2)Al(3)Ho phases and solid solubility extension

Primary MC decomposition and its effects on the rupture behaviors in hot-corrosion resistant Ni-based superalloy K444

The mechanism of primary MC decomposition and its influence on the rupture behaviors in the hot-corrosion resistant superalloy K444 are investigated by the combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that the blocky primary MC mainly distributes at interdendritic regions and grain boundaries after the standard heat treatment. During long-term thermal exposure, primary MC decomposes gradually, which can be summarized into three stages. Firstly, the primary MC reacts with the gamma matrix and produces the M23C6 particles and gamma' film, which can be described as MC + gamma -> M23C6 + gamma'. In the second stage, the un-decomposed primary MC arrests the Ni element and forms the M6C particles and eta phase, which can be expressed as MC + gamma -> M6C + eta. In the third stage, the remaining primary MC reacts with the diffused Ni element and forms the eta phase with M6C and M23C6 particles inside, which can be described as MC + gamma -> M6C + M23C6 + eta. The reaction of the second and third stages should be ascribed to the high (Ti + Nb + Hf)/Al ratio and segregated W, Mo and Cr element in the decomposed region. In addition, the platelet-like sigma phase is found in the third stage. The primary MC decomposition has different effects on the rupture behaviors between tensile and stress-rupture test. (C) 2012 Elsevier B.V. All rights reserved

Effects of Al and Ti on thermal expansion behavior of a Ni–Fe–Cr-based superalloy: A combined experimental and first-principle study

The effects of Al and Ti on the thermal expansion coefficient (TEC) of a Ni–Fe–Cr-based superalloy, for the application of advanced ultra-supercritical (A-USC) systems, were investigated by combining the experiments and first-principles calculations. The results imply that MC/M23C6 carbides and grain boundary seem to play a minor role in the TEC of alloys due to their relatively low proportions. Al and Ti elements increase the TEC of alloys when present in γ matrix, but they decrease the TEC value by taking part in the formation of γ′ phase. When the amount of γ′ phase is low, the TEC of alloys at 600–700 °C is dominated by γ matrix, and consequently shows an increasing or constant trend with the increase of Al + Ti content. Replacing Al element with Ti element decreases the TEC of alloys in two ways: directly by reducing the TECs of γ matrix and γ′ phase, and indirectly by increasing the amount of γ′ phase. These results can provide the theoretical basis and beneficial guidelines for the composition design and optimization of low-expansion superalloys

Microstructural Evolution and Mechanical Properties of a Ni-Based Alloy with High Boron Content for the Pre-Sintered Preform (PSP) Application

The pre-sintered preform (PSP) is an advanced technology for repairing the Ni-based superalloy blade in a turbine. In general, boron is added to the Ni-based superalloys in small quantities (1.0 wt.%) on the microstructure evolution and mechanical properties in Ni-based superalloys for the PSP application is rarely studied. The variety, composition and evolution of the precipitates during solution heat treatment in the alloy with high B content were determined by EBSD, EPMA and SEM. The results indicate that Cr, W and Mo-rich M5B3 type borides precipitate from the matrix and its area fraction reaches up to about 8%. The area fraction of boride decreases with the prolonging of solution time and the increase of temperature higher than 1120 °C. The borides nearly disappear after solution treatment at 1160 °C for 2 h. The redissolution of boride and eutectic results in the formation of B-rich area with low incipient melting (about 1189 °C). It can bond metallurgically with the blade under the melting point of the blade, which decreases the precipitation of harmful phases of the blade after PSP repairing. The microhardness within the grain in the PSP work-blank first decreases (lower than 1160 °C) and then increases (higher than 1185 °C) with the increase of solution heat treatment temperature due to the dissolving and precipitation of borides. The tensile strength of the combination of PSP work-blank and Mar-M247 matrix at room temperature after solution treatment is related to the area fraction of boride, incipient melting and the cohesion between PSP work-blank and Mar-M247 matrix

Study on the homogeneity of tilted dendritic structures in single crystal superalloys

Dendrites play a crucial role in the microstructure of single crystal superalloys, predominantly developing along the [001] orientation. The mechanical performance is greatly influenced by the uniform distribution of dendrites. During the solidification process, we frequently observe instances of dendrites deviating from the [001] orientation, resulting in tilted structures. These deviations give rise to both aligned and misaligned arrays within the transverse section. The study comprehensively examines the homogeneity of tilted dendritic structures. As solidification progresses, new dendrites within the aligned array tend to maintain a hexagonal structure. Simultaneously, the remaining metastable structures gradually transition into hexagonal structures. In contrast, various polygons mutually transform in misaligned array, resulting in a dynamic adjustment of their proportions. Consequently, the aligned array exhibits a higher proportion of hexagonal structures and a more uniform dendrite spacing compared to the misaligned array. Within the transitional region, an increase in heptagonal structures leads to heightened non-uniformity in dendrite spacing. The predominance of hexagonal structures can be attributed to their more uniform solute distribution, facilitated by the characteristics of the aligned array, which promote hexagonal structure formation by adjusting the solute field distribution. On the other hand, due to the random positioning of dendrites in the misaligned array, various stacking structures coexist in dynamic equilibrium. The research reveals an intrinsic relationship between macroscopic array patterns, stacking structures, dendrite spacing, and microscopic solute distribution. These findings provide a theoretical foundation for the production of high-quality single crystal dendritic structures and offer insights into their influence on material properties