Stress Rupture Properties Research Articles

Microstructure and Stress-Rupture Property of Large-Scale Complex Nickel-Based Single Crystal Casting

The microstructure and stress-rupture property of the large-scale complex single crystal (SX) casting DD10 were investigated in high-rate solidification process. It is found that the primary dendrite arm spacing (PDAS) does not increase monotonically with the height increase. When across the platform, the temperature gradient increases due to the effect of platform, and the corresponding PDAS decreases. The distribution of eutectic volume fraction in large-scale complex SX casting is affected by PDAS, solid back diffusion, and the development of high order dendrites. The eutectic volume fraction contained in the sample taken below the platform decreases with the height increase. While the eutectic volume fraction contained in the sample taken upper the platform increases gradually with the height increase. After heat treatment, most of the γ/γ′ eutectics are eliminated and the components are distributed uniformly. The similar stress rupture properties of the samples at different heights in the same direction are obtained.

Effects of Cr content on the microstructure and stress rupture property of a directionally solidified Ni-based superalloy during long-term thermal exposure

The effects of Cr addition on the microstructure and stress rupture property during long-term thermal exposure have been investigated in a directionally solidified Ni-based superalloy. The increment of Cr content resulting in the excessive precipitation of σ phase is the principal reason for the catastrophic reduction in the stress rupture properties. The amounts of rounded cuboidal γ′ and its coalescence increase, but the tendency of γ′ alignment decreases with the increment of Cr content, which leads to the reduction of the stress rupture property to some degree. The change of lattice misfit shows a good agreement with the change of γ′ morphology and the tendency of γ′ alignment. The change of γ′ shape is accompanied by the increase of the misfit dislocation density, which causes the rise of γ′ coalescence. The increase of Cr content can promote the formation of M23C6 or M6C in the vicinity of degenerating primary MC. Thus, the increased Cr content facilitates the decomposition of primary MC, which deteriorates the stress rupture properties. In addition, the increase of Cr content can promote the formation M23C6 and M6C along gain boundaries (GBs), facilitating the formation of a semicontinuous carbide chain to some degree and broadens GBs and thus promoting the crack nucleation at GBs, which plays a certain role in the degradation of the stress rupture properties.

Improved 1200 °C stress rupture property of single crystal superalloys by γ′-forming elements addition

The 1200°C/80MPa stress rupture properties of single crystal superalloys are pronouncedly improved by adding γ′-forming elements Al and Ta. The fraction of γ′-precipitate at high temperature enhances with Al and Ta added. This fraction change yields the increase in concentrations of γ-strengthening elements Mo and Re in γ-channel. It produces the elevated solid solution hardening of γ-matrix and the more negative two-phase lattice misfit. The property improvement is ascribed to the increased γ′ fraction and the elevated γ solid solution level, both induced by Al and Ta addition.

Influence of thermal exposure on microstructure and stress rupture properties of a new Re-containing single crystal Ni-based superalloy

In this study, the long-term thermal microstructural stability and related stress rupture lives of a new Re-containing Ni-based single-crystal superalloy, DD11, were investigated after high-temperature exposure for different lengths of time. The results show that the γ′ precipitates retained a cuboidal morphology and the γ′ size increased after short thermal exposure for 50 h at 1,070 °C. As the thermal exposure time was prolonged to 500 h, the cuboidal γ′ gradually changed into irregular raft-like morphology due to particles coalescence, and the morphology of the microstructure was almost unchanged after further thermal exposure up to 3,000 h. The stress rupture experiments at 1,070 °C and a tensile stress of 140 MPa showed that the rupture lives increased significantly after thermal exposure for 50 h and dropped dramatically with increasing exposure time up to 500 h but decreased slowly after exposure for more than 500 h. These results imply that stress rupture properties did not decrease when the γ′ remained cuboidal but degraded to different extents during the γ′ coarsening process. The coarsening of the γ′ precipitates and change in morphology were regarded as the main factors leading to the degradation of the stress rupture lives. This study provides fundamental information on the high-temperature long-term microstructural stability and mechanical performance, which will be of great help for DD11 alloy optimization and engineering aeroengine applications.

Coupling effects of tungsten and molybdenum on microstructure and stress-rupture properties of a nickel-base cast superalloy

In order to comprehensively understand the forming mechanism of abnormal phases solidified in a nickel-base cast superalloy with additives of tungsten and molybdenum, the coupling effects of W and Mo on the microstructure and stress-rupture properties were investigated in this paper. The results indicated that the precipitation of primary α-(W, Mo) phase depended tremendously on the amount of W and Mo addition. When the total amount of W and Mo was greater than 5.79 at%, α-(W, Mo) phase became easily precipitated in the alloy. With increasing of Mo/W ratio, the dendrite-like α-(W, Mo) phases were apt to convert into small bars or blocky-like phases at the vicinities of γ′/γ eutectic. The morphological changes of α-(W, Mo) phase can be interpreted as the non-equilibrium solidification of W and Mo in the alloy. Since the large sized α-(W, Mo) phase has detrimental effects on stress-rupture properties in as-cast conditions, secondary cracks may mainly initiate at and then propagate along the interfaces of brittle phases and soft matrix. During exposing at 1100 ℃ for 1000 h, the α-(W, Mo) phases transformed gradually into bigger and harder M6C carbide, which results in decreasing of stress-rupture properties of the alloy. Finally, the alloy with an addition of 14W-1Mo(wt%) maintained the longest stress lives at high temperatures and therefore it revealed the best microstructure stability after 1100 ℃/1000 h thermal exposure.

The effect of stress on primary MC carbides degeneration of Waspaloy during long term thermal exposure

Microstructure evolution during long-term thermal exposure and stress rupture in Waspaloy was investigated in detail. During thermal and stress exposure, γ′ phase and grain boundary M23C6 carbides kept microstructure stability. The primary MC carbides only experienced long time thermal exposure didn't decompose significantly while those experienced long time thermal and stress exposure degenerate severely. Decomposition of primary MC carbides is detrimental to the stress rupture properties of the alloy and it can be expressed as: MC + γ → M23C6 + γ′. After long term aging (650°C/5000h), the stress rupture specimen (815°C/325MPa) showed a mixed mode of deformation mechanism : particle shearing and precipitate bypassing.

Effect of minor additions on the microstructures and stress rupture properties of a directionally solidified Ni-based superalloy

The effects of C, Zr and Y/Ce on the microstructures and stress rupture properties of a directionally solidified Ni-based superalloys were investigated. The morphology of MC carbides does not change in as-cast alloys, as the C content is increased or the Zr is added. However, the Y/Ce additions promote the transformation of MC carbide morphology from blocky to script-like type. Compared with that in as-cast alloys, there is no obvious change in the morphologies of MC carbide or chemical compositions after heat treatment. The increasing of C content and the addition of Zr or Y/Ce significantly decreases and increases the amount of the γ/γ′ eutectic, respectively. After a solid solution treatment and subsequently a two-stage aging treatment, the γ′ phase is characteristic with two kinds of shapes and sizes, and the volume fraction of cuboidal γ′ phase is much larger than that of spherical γ′ phases. More MC carbide and residual eutectic result from the decreasing of cuboidal γ′ phase volume fraction. The decrease in volume fraction of cuboidal γ′ phase is principally responsible for the reduction of the stress rupture property. In addition, script-like carbide and residual eutectic remaining after heat treatment play the secondary role in the degradation of the stress rupture properties. The less regularly arranged cuboidal γ′ phase can damage the stress rupture life to some extent. The spherical γ′ phase re-dissolves into γ′ matrix widening the spacing between large cuboidal γ′ phase, and the addition of Zr inhibits grain boundary sliding and cracking; these two facts are beneficial to the rupture elongation.

Effect of carbon on the microstructure and stress rupture properties of a new Ni-Cr-W-Fe alloy for advanced ultra-supercritical power plants

This paper studied the effect of carbon on the microstructure and stress rupture properties of a new Ni-Cr-W-Fe alloy. With carbon content increasing from 0.008 wt% to 0.073 wt%, the volume fraction and average size of MC and M23C6 carbides increased significantly. The morphology of MC remained dispersed blocky shape while the morphology of M23C6 changed from continuous thin strip to granular. But no obvious variations were observed in the quantity, size and morphology of γ′ phase. During heat treatment, the hot-rolled grains grew up quickly and the grain growth rate of 0.008% C alloy was almost three times higher than that of 0.073% C alloy. The reason was that the MC carbides effectively inhibited the grain growth. After stress rupture test at 750 °C/300 MPa and 750 °C/400 MPa, the stress rupture life was in parabolic curve relationship with carbon content, 0.034 wt% and 0.045 wt% C alloys had higher stress rupture lives than other alloys. With the increase of carbon content, the ductility increased firstly and then reached relatively stable status, the fracture mode gradually varied from intergranular fracture to intergranular dimple mixed fracture. The change of stress rupture properties with carbon content was closely related to carbides and the grain size, which would be analyzed in this paper.

Influence of type and morphology of carbides on stress-rupture behavior of a cast cobalt-base superalloy

The relationships between the type and morphology of carbides and stress-rupture property have been studied in a cast cobalt-base superalloy. Different types and morphologies of carbides are achieved by varying the heat treatment conditions. After heat treatment at 1240 °C/4 h, the primary M7C3 decomposes into M23C6, and the formed M23C6 and primary MC partially dissolve into the matrix. After heat treatment at 1280 °C/4 h, the primary M7C3 melts, resulting in the formation of a highly developed lamellar structure of M23C6, and the primary MC mostly dissolves into the matrix. The heat treatment at 1100 °C/300 h gives rise to the carbide transformation of M7C3 → M23C6 and a profusion of secondary M23C6 carbide around M23C6 and MC. The samples in stress-rupture test at 980 °C for 83 MPa manifest the following rupture life: 1280 °C sample > 1240 °C sample > as-cast sample > 1100 °C sample. The longer rupture life of the samples of 1240 °C and 1280 °C is derived from the stable microstructure, dispersed MC carbide morphology and the supersaturated matrix. The shortest rupture life of the 1100 °C sample is mainly attributed to the overaging. Additionally, the carbide transformations of M7C3 → M23C6, MC → M23C6, M23C6 → M6C take place in the as-cast sample during creep exposure.

Stress-Rupture Strength, Structure, and Properties of Welded Joints of 15Kh1M1F and X10CrMoVNb9-1 (P91) Steels

Stress-rupture tests of transverse similar-metal and dissimilar-metal welds are simulated numerically. The dependence of the stress-rupture strength of P91 steel weldments on the relative width of the interlayer and the ratio between the creep rates of the parent metal and the weakened region in the heat-affected zone is identified and described. An upper bound estimate of the stress-rupture strength of a dissimilar-metal weld is obtained using a model of a weldment with two soft interlayers with identical properties. Achange of the point of rupture with reduction in the stress is found numerically and confirmed experimentally.

Microstructure Evolution and Stress Rupture Properties of DD6 Single Crystal Superalloy after Overheating

The second generation single crystal superalloy DD6 after standard heat treatment was respectively overheated at 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1320°C for 1h and air cooled. The effect of overheating on the microstructure and stress rupture properties at 980°C/250MPa of the alloy was investigated. The results showed that the size of γ′ phase was slightly increased overheating at 1100°C, 1150°C and 1200°C. The size of γ′ phase had a big increase and its size distribution was very uneven after overheating at 1250°C. The small part of γ′ phase has serrated γ′/γ phase surface as a result of un-completely solution and the irregular small γ′ phase was in the majority when overheated at 1300°C. While all the irregular small γ′ phase precipitated again after completely solution when overheated at 1320°C. There was no fine second γ′ phase in the γ matrix channel of the alloy after standard heat treatment and overheating at 1320°C. But the fine second γ′ phase precipitated in the γ matrix channel after overheating at every temperature of 1100 °C~1300°C. No obvious change of the stress rupture life was found after overheating at 1100°C, 1150°C, 1200°C and 1250°C. The stress rupture life considerably reduced after overheating at 1300°C, whereas slightly reduced after overheating at 1320°C. The appearance of the raft had almost no change after overheating at 1100°C. With increasing of overheating temperature from 1100°C to 1250°C, the length of raft became shorter and the width thickening. The γ phase formed the wavy raft after overheating at 1300°C and 1320°C and the thickness of latter was larger than that of the former. Finally, the relationship between the microstructural evolution and stress rupture properties of the alloy after overheating was discussed.

Effect of Low-Angle Boundary on Microstructure and Stress Rupture Property of a Third Generation Single Crystal Superalloy DD10

The microstructure of a third generation single crystal superalloy DD10 with 0°~15° grain boundary and stress rupture property at 980°C/280MPa and 1100°C/140MPa have been studied and compared in the present investigation. The results showed that the primary dendritic stems at either side of low angle boundary indicated angular differences as compared with structure of principal [001] crystal. After thermal treatment, the grain boundary changed from irregular chain structure of γ’ and γ to flat and thin γ layer. Under the condition of 980°C/280MPaand 1100°C/140MPa, stress rupture life of the DD10 alloy with 7° low angle boundary decreased little compared with [001] crystal. The rupture was non-intergranular fracture. The 9° low angle boundary did not have a remarkable effect on stress rupture property of the DD10 alloy.

Influences of W and Al on the Microstructure and Stress-Rupture Property of Re-Free Ni-Based Single Crystal Superalloys

The influence of W and Al on the solidus and liquidus temperatures, microstructure and stress-rupture property at 980 °C/250 MPa was investigated in three Re-free experimental Ni-based single crystal superalloys. The results indicated that the solidus temperature increased for 14.0 °C and 9.8 °C by adding 0.84 wt.% W only and adding 0.45 wt.% Al and 0.44 wt.% W to the base alloy, respectively. The γ′ morphology changed from nearly cuboidal in the base alloy to cuboidal by adding 0.45 wt.% Al and 0.44 wt.% W. The volume fraction of γ′ precipitates increased, while the γ channel width decreased after adding Al and W. The additions of Al and W improved the stress-rupture life at 980 °C/250 MPa because of higher γ′ volume fraction, narrower γ matrix channel and more complete rafting structure.

The effect of reheat treatment on microstructure and stress rupture property of a directionally solidified nickel-based superalloy after long-term thermal exposure

The recovery of microstructure and creep properties of a directionally solidified superalloy after long-term thermal exposure was investigated. After reheat treatment, successful restoration of γ′ microstructure is achieved including morphology, size and chemical composition. However, carbides degradations within interior grains and at grain boundaries are irreversible through the reheat treatment. Short and long-term creep rupture properties are effectively improved by the reheat treatment, but cannot be fully recovered compared to that of the original material. During prolonged thermal and creep exposure, grain boundary carbides degradation occurred rapidly in comparison with that in the original material, which is regarded as the main reason leading to the reduction of stress rupture lifetime.

Microstructure stability of a ru-containing nickel based single crystal superalloy

The microstructure evolution and stress rupture properties at 1100°C/140MPa of a Ru-containing single crystal superalloy after long term aging at 1100°C for 200h-1000h were investigated. The results showed that the γ′ phase rafting and the γ matrix channel broadening appeared after long term aging. The rafting degree enhanced with increasing of aging time. A small amount of TCP phase began to appear after aging for 200h. The volume fraction of TCP phase after aging for 1000h was only 0.1%, which indicated the alloy had excellent microstructure stability. The stress rupture life of the alloy at 1100°C/140MPa represented the decrease tendency with increasing of aging time. The γ′ rafting and TCP phase precipitation were the main factor for this degradation.

The influence of melt superheating treatment on the cast structure and stress rupture property of IN718C superalloy

Abstract A modified thermally controlled solidification using melt superheating treatment has been designed to an IN718C superalloy to achieve grain refinement and further improve mechanical properties. It is found that the applied melt treatment can refine grain size from 6420 to 89 μm and greatly reduce the amount of porosity from 15.22 to 0.015%. The morphology of the microstructure is evolved from dendritic to granular structure. Meanwhile, the melt superheating treatment has no obvious influence on the dendrite segregation of Cr and Fe, but reduce the dendrite segregation of Nb and Ti. The amount of Laves phase decreases with increasing of melt superheating temperature from 1380 to 1680 °C. Corresponding to this, the stress rupture property under 650 °C/620 MPa is significantly improved.

Effects of Heat Treatment on Surface Recrystallization and Stress Rupture Properties of a Fourth-Generation Single-Crystal Superalloy after Grit Blasting

The specimens of a fourth-generation single-crystal superalloy were grit-blasted and heat-treated in vacuum at 1100, 1150, 1200, 1250 and 1300 °C for 4 h, respectively. Then, the microstructure and the stress rupture properties of the recrystallized alloy were investigated at 1150 °C/120 MPa. The results showed that a cellular recrystallization occurred in the surface layer after heating at 1100, 1150 and 1200 °C for 4 h. An equiaxed recrystallization formed as the specimen was heat-treated at 1300 °C for 4 h, while a mixed recrystallization occurred in the specimen heat-treated at 1250 °C for 4 h. The recrystallized depth clearly increased with a rise of the heat treatment temperature. The stress rupture life continuously decreased with a rise of the heat treatment temperature up to 1250 °C. Although the overall stress rupture life reduced to different degrees, the stress rupture life of specimen after heat treatment at 1300 °C was relatively high and intermediate between those of specimens treated at 1150 and 1200 °C. The fact that the stress rupture life reduced to different degrees after heat treatment can be attributed to the recrystallization of the surface layer and to the microstructure evolution of the interior of the specimen. The small γ′ phase precipitated again after heat treatment at 1300 °C for 4 h. So, the stress rupture life was relatively longer than that after heat treatment at 1200 or 1250 °C although the equiaxed recrystallization formed in the surface layer.

On the precipitation hardening of the directionally solidified GTD-111 Ni-base superalloy: Microstructures and mechanical properties

Effects of precipitation hardening on microstructures and mechanical properties of the directionally-solidified (DS) Ni-base GTD-111 superalloy are investigated. DS specimens were subjected to different cycles of solution treatments at temperatures of 910°C, 980°C and 1050°C. The solidified and heat-treated microstructures along with the stress rupture properties were evaluated to investigate influence of the homogenization and second solution treatments. The results showed the homogenization treatment resulted in a more uniform distribution of the γ′ precipitates. It was shown that growth and preferred distribution of the γ′ precipitates occurred along the <100> direction in the primary solution treatment. More growth of γ′ precipitates during secondary solution treatment led to the splitting of the precipitates into a group of fine precipitates. The best stress rupture result was achieved in the specimen with the second solution treatment at 980°C for 2h followed by aging at 843°C for 24h due to the suitable size, volume fraction, and size distribution of the γ′ precipitates in the DS microstructure.

The Effect of Process Parameters in Interdendritic-Melt Solidification Control Technique on the Microstructure and Properties of a Ni-Base Superalloy

A novel technique called interdendritic-melt solidified control (IMSC) was developed to manufacture equiaxed superalloy in the efforts of reducing porosity. The effect of process parameter, such as withdrawal rate, of the IMSC on the porosities and mechanical properties of IN792 alloy was investigated, compared with conventional investment casting (CC) technique. The IMSC and CC samples were characterized by optical metallography and scanning electron microscopy. In addition, the minor phases, such as MC carbides and trace amount of η phase, were identified by synchrotron X-ray diffraction. The results indicate that proper withdrawal rate for IMSC can produce castings with much reduced porosity and higher stress rupture properties at elevated temperature compared to conventional investment casting. However, fast withdrawal rate will produce much severe porosity and lower mechanical property.

High Temperature Stress Rupture Anisotropy of a Ni-Based Single Crystal Superalloy

High temperature stress rupture anisotropies of a second generation Ni-base single crystal (SC) superalloy specimens with [001], [011] and [111] orientations under 900 °C/445 MPa and 1100 °C/100 MPa have been investigated in the present study, with attentions to the evolution of γ/γ′ microstructure observed by scanning electron microscopy and the dislocation configuration characterized by transmission electron microscopy in each oriented specimen. At 1100 °C/100 MPa as well as 900 °C/445 MPa, the single crystal superalloy exhibits obvious stress rupture anisotropic behavior. The [001] oriented specimen has the longest rupture lifetime at 900 °C/445 MPa, and the [111] oriented sample shows the best rupture strength at 1100 °C/100 MPa. While the [011] oriented specimen presents the worst rupture lifetime at each testing condition, its stress rupture property at 1100 °C/100 MPa is clearly improved, compared with 900 °C/445 MPa. The evident stress rupture anisotropy at 900 °C/445 MPa is mainly attributed to the distinctive movement way of dislocations in each oriented sample. Whereas, at 1100 °C/100 MPa, together with the individual dislocation configuration, the evolution of γ/γ′ microstructure in each orientation also plays a key role in the apparent stress rupture anisotropy.