Longer Creep Rupture Life Research Articles

Enhancing microstructural stability and creep properties by Ta addition in Ni-based single crystal superalloys

To investigate influences of Ta content on microstructural stability during thermal exposure at 900 °C (up to 3000 h) and creep properties at 980 °C/163 MPa in Ni-based SX superalloy, alloys with 7Ta, 8Ta and 9Ta (wt.%) were studied utilizing DSC, SEM, TEM, HR-XRD and APT. The results shows that Ta addition enhanced the partitioning behavior of alloying elements. Besides, lattice misfit was controlled by both alloy composition and temperature. Ta addition increased absolute misfit both room temperature and 980 °C. The difference was that the γ/γ′ lattice misfit changed from positive to negative at room temperature. The γ′ coarsening indicated that Ta addition reduced the effective diffusion coefficient and exacerbated W segregation at γ/γ′ interface, which was beneficial to improve the microstructural stability. Merging of several adjacent γ′ precipitates in preferred direction was observed to be delayed with increasing Ta content, which occurred at 500 h for 7Ta alloy, 800 h for 8Ta alloy and 1000 h for 9Ta alloy, respectively. Ta addition affected the diffusion kinetics and greatly prolonged the steady creep duration, and then resulted in longer creep rupture life. It was discussed based on the γ matrix channels, γ′ raft thickness, the perfection degree of γ′ rafts, residual γ′ precipitates, interfacial dislocation networks and solid solution strengthening.

Long-term creep behaviours and structural stabilities of austenitic heat-resistant stainless steels

ABSTRACT For heat resistant alloys, long-term structural stability at high temperatures is a critical issue for alloy design and applications. In this paper, the long-term creep behaviours and structural stabilities of six heat resistant high Ni alloys and austenitic stainless steels have been studied. The longest creep rupture life is up to 359 283 hours. High Ni and Cr alloys show a good combination of high creep and oxidation resistances. Precipitation of nano MX particles with a very low growth rate improves long-term creep resistance at high temperatures. Long-term stable multiple nanoprecipitates of MX, Cu-rich, Laves and M23C6 phases can greatly contribute to the creep strength. Low Ni austenitic stainless steels show comparatively low oxidation and creep resistances. It was first found that at 800°C, Cr2N could form in the low Ni steel with a long-term crept by the absorption of nitrogen from the air into the matrix.

Examinations of equations for creep rupture life with a large creep database on grade 91 steel

Equations for predicting creep rupture life of grade 91 steel have been revised officially in Japan and European Union. First, the revised equations are examined with a large creep rupture database on the steel. The database consists of 2066 data points, and its longest creep rupture life is 232,833 h crept at 600 °C. The equations deviate from data points of the database in a long-term region beyond 30,000 h. Decrease in activation energy for creep rupture life in the long-term region is the cause of the deviation. A multi-region analysis of the database can be a probable way of reducing the deviation. However, some modifications of the analysis are necessary when applying it to the large database containing data points of multiple heats. A trial of the modifications is made in the second part of this paper. The equations obtained through the trial can well represent all the data points including the ones in the long-term region.

Investigation of the microstructure and strength in G115 steel with the different concentration of tungsten during creep test

The microstructure and creep strength of three G115 specimens, 9 Cr-(2.3, 2.6, 3.0)W-3 Co (wt%), were investigated after creep test at 650 °C under 120 MPa using field emission transmission electron microscopy, field emission scanning electron microscopy, X-ray diffraction, and Ultra-high temperature confocal microscope, focusing on martensitic lath, precipitates, dislocation densities and the block boundaries. The results show that the coarsening rates of the laths and Laves phase particles increased with the increase in W concentration during creep test. The majority of cavities was nucleated at high angle boundaries where many Laves phase particles existed in the neck portions of the specimens. The types of the crack were different among the three steels: the cracks in 2.3 W G115 steel specimens mainly resembled “wedge crack”, while the cracks in 2.6 W and 3.0 W G115 steel specimens mainly resembled “flower crack”. The 2.3 W G115 steel has the longest creep rupture life.

Effect of creep phenomena on room-temperature tensile properties of cast & forged P91 steel

Abstract In present research work, the as-received cast & forged (C&F) P91 steel was subjected to the creep test at temperature of 620 °C and 650 °C for applied stress of 120 MPa. The room temperature tensile test was conducted after normalizing and tempering (N&T) treatment of the ruptured creep specimen. The standard tensile-test specimen was prepared from the gauge section of creep ruptured specimen. The N&T treatment was performed to restore the microstructure and mechanical properties of virgin P91 steel (N&T P91 steel). The microstructure of creep fractured specimen in ruptured state and N&T condition were characterized by using field-emission scanning electron microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDS). The fracture surface morphology of crept specimen and the tensile tested specimen was also studied. The effect of prior creep deformation on the mechanical strength was more significant in the sample with longer creep rupture life.

Effects of Cooling Rates after Solution Heat Treatment on the Creep Behavior of Directionally Solidified CM-247LC Superalloy

In this study, three cooling rates, namely, Argon cooling, air cooling and furnace cooling after solution heat treatment of directionally solidified CM-247LC superalloy were proposed to explore the high temperature/low stress (982°C/200MPa) creep behavior. Standard heat treatment schemes of DS CM-247LC superalloy are solution treatment at 1260°C for 2 hour, then first aging at 1079°C for 4 hour and followed by second aging at 871°C for 20 hour. Results show that Argon cooling specimen provided the longest creep rupture life, which exceeds that of the air cooling specimen around 40 hour; whereas the creep rupture life of furnace cooling specimen was the shortest one of 100 hour, which is shorter than that of the air cooling specimen around 40 hour. Rupture strains of all three specimens were almost identical around 20%. Microstructural differences of gamma prime morphology were observed to explain the differences of creep rupture life.

Influence of Element Re on Creep Behavior of Single Crystal Nickel-Based Superalloys at Intermediate Temperature

AbstractBy means of creep curves measurement and microstructure observation, an investigation has been made into the influence of the element Re on creep behaviors of single crystal nickel-base superalloy at intermediate temperature. Results show that, after fully heat treated, microstructure of the alloy consists of the cubic γ′ phase embedded coherent in the γ matrix. Comparing with Re-free superalloy, 4.5% Re alloy displays a better creep resistance and longer creep rupture life in the ranges of the applied stresses and temperatures. After crept for 425 h up to fracture at 760 °C/800 MPa, the γ′ phase in the alloy keeps still the regular cubical configuration, which is attributed to the effect of the element Re decreasing the diffusing rate of other elements during creep. The deformation features of the alloy during creep are that the dislocations move in the γ matrix channels and shear into the γ′ phase, the 〈110〉 super-dislocation shearing into the γ′ phase may be decomposed to form the configuration of (1/3)〈112〉 super-Shockleys partials plus the stacking fault, which may hinder dislocations movement and restrain the cross-slipping of dislocations. This is thought to be the main reason of the alloy having a better creep resistance. In the latter stage of creep, the crack is firstly initiated in the interface of γ′/γ phases and propagated along the interface vertical to the stress axis up to the occurrence of the creep fracture, which is thought to be the fracture mechanism of the alloy during creep.

Monotonic tension and creep behavior of single crystal CMSX-486 under combustion environment

The monotonic tension and creep loading behavior of single crystal nickel-base superalloy, CMSX-486 was investigated at 1093°C under ambient laboratory and simulated combustion environments. There was degradation in tension and creep performance under combustion environment. A reduction of about 30% in yield strength and ultimate tensile strength and 75% reduction in strain-to-failure were observed in combustion environment relative to laboratory environment. The creep rupture life reduced in combustion environment in comparison to laboratory environment. The creep rupture occurred at a lower strain level in the combustion environment when compared to laboratory environment. The laboratory environment tested specimens experienced higher % neck area reduction, exhibiting more ductility relative to the combustion tested specimens. The mass loss in combustion environment was about 3 times larger than in laboratory environment. The microstructure showed different distribution patterns of γ/γ′ phases. The combustion environment resulted in relatively more contiguous regions of γ phase which presented favorable condition for faster propagation of defects resulting in creep rupture over shorter time span. The subcritical crack growth regions on the fracture surface of the laboratory environment tested specimens were relatively much rougher with more number of craters/voids resulting in longer creep rupture life.

Effects of Ag particles content on properties of Sn0.7Cu solder

To improve properties of Sn0.7Cu solder, method of particles reinforced was employed. Effects of Ag particle contents (1, 3, 5, 7.5, and 10 vol.%) on spreadability, microstructure, shear strength and creep rupture life of Sn0.7Cu solders have been studied. The experimental results indicate that intermetallic compound (IMC) grows, Shear strength is increased and grains are fined with the increasing of Ag particles. When content of Ag particles is more than 5 vol.%, growth rate of IMC is increased significantly. When the content of Ag is 5 vol.%, the composite solder presents best spreadability and excellent creep rupture property which have maximum spreading area, minimum wetting angle and longest creep rupture life (about 22 times as long as that of Sn0.7Cu solder).

高Ｃｒフェライト系耐熱鋼における長時間クリープ強度に対するＲｅの効果

There has been a great demand for superior heat resistant steels in order to raise the thermal efficiency of fossil-fuel power plants and to reduce CO2 emission to the global environment. To this end, by using the d-electrons concept, 9–12% Cr ferritic steels were designed for use of a steam turbine rotor operated in the USC power plants at the steam temperature of 620 to 650°C. The crucial issue for the design is to suppress the deterioration of the long-term creep strength by alloying. First, the Re addition was found to give a beneficial effect on the creep strength of a 10%Cr–4%W steel. Then, the creep tests were performed with the six Re-free and 3.5% W ferritic steels to get an optimum Cr content in the range of 8.5 to 11.5%. As the result, it was found that an excess amount of Cr yielded a detrimental effect on the creep properties, and the 9% Cr steel was the best in view of the very long-term creep strength tested in the condition of 650°C, 98 MPa. Subsequently, a series of creep tests was conducted with the steels by fixing at 9% Cr but by varying the W content from 2 to 4% and the Re content from 0 to 0.5%. From the prolonged creep tests for more than 40,000 h, it was shown that the 9Cr–4 W–0.5Re steel had the longest creep rupture life among all the high Cr ferritic steels so far developed in the world.

Constitutive relations on creep for SnAgCuRE lead-free solder joints

Taking the most promising substitute of the Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the effect of rare earths (REs) on the creep performance of the Sn-3.8Ag-0.7Cu solder joints. The SnAgCu-0.1RE solder with the longest creep-rupture life was selected for subsequent research. Creep strain tests were conducted on Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints in the intermediate temperature range from 298 K to 398 K, corresponding to the homologous temperatures η=0.606, 0.687, 0.748, and 0.809 and η = 0.602, 0.683, 0.743, and 0.804, respectively, to acquire the relevant creep parameters, such as stress exponent and activation energy, which characterize the creep mechanisms. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established, demonstrating the dependence of steady-state creep rate on stress and temperature. By correcting the apparent creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep-activation energy is obtained. Results indicated that at low stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady-state creep rates of these two solder joints are both dominated by the rate of lattice self-diffusion. While at high stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation-pipe diffusion activation energy, so the steady-state creep rates are dominated by the rate of dislocation-pipe diffusion. At low stress, the best-fit stress exponents n of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are 6.9 and 8.2, respectively, and the true creep-activation energy of them both is close to that of lattice self-diffusion. At high stress, it equals 11.6 and 14.6 for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints, respectively, and the true creep-activation energy for both is close to that of the dislocation-pipe diffusion. Thus, under the condition of the experimental temperatures and stresses, the dislocation climbing mechanism serves as the controlling mechanism for creep deformation of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints. The creep values of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are both controlled by dislocation climbing. Dislocation glide and climb both contribute to creep deformation, but the controlling mechanism is dislocation climb. At low stress, dislocation climbing is dominated by the lattice self-diffusion process in the Sn matrix and dominated by the dislocation-pipe diffusion process at high stress.