Decrease In Creep Life Research Articles

Effects of phosphorus and boron on the γ″ precipitate evolution and creep properties of IN718 superalloy during long-term thermal exposure at 680 ℃

A study was conducted to investigate the effect of the combined addition of P and B on the γ″ precipitate and creep properties of IN718 superalloy under long-term thermal exposure at 680 ℃. Atomic probe tomography (APT) analysis and X-ray diffraction (XRD) results indicated that P could segregate in the γ″ precipitates in a substitutional way and then improve the stability of the γ″ precipitates when subjected to prolonged thermal exposure. No segregation of B was found in the γ″ precipitates. The addition of P and B significantly improves the creep properties of the IN718 superalloy during long-term thermal exposure at 680 ℃. By increasing the exposure time, the creep life of the high P and B alloy at 650 ℃/725 MPa initially increased, peaking at 500 h, and then decreased as the exposure time increased. However, the low P and B alloy shows a consistent decrease in creep life with increasing thermal exposure time. During a relatively short thermal exposure, the microstructural analysis indicated that the substantial enhancement in creep life of the high P and B alloy primarily stems from cross twins and Lomer-Cottrell (LC) locks. As the thermal exposure time increases, the more stable γ″ precipitates in the high P and B alloy, which directly improves the creep properties. The dominant deformation mechanism in the two alloys shifts from predominantly twin to a combination of twin, stacking fault (SF) and antiphase boundary-like (APB-like) shear γ″ precipitate with prolonged thermal exposure.

Variations in Creep Performance of P92 Steel Within Its Composition Range: Influence of N Solubility

Underspecified composition ranges often lead to alloys with unpredictable mechanical performance. To better understand the changes in microstructure and mechanical performance associated with variations of key elements, three versions of P92 are formulated within, or close to, the specified allowable for N, B, and C ranges. Chromium and Si are also varied to influence N solubility. Different service conditions (i.e., temperature and stress) are explored. It is observed that >80% decrease in creep life occurs at 625 °C and 155 MPa for the highest B and N containing alloy. Multiscale characterization reveals key changes due to the trace element variation. The high B and N containing alloy forms deleterious BN precipitates with morphology that promotes crack nucleation and damage accumulation, but this alloy additionally forms higher fractions of beneficial MX precipitates. The alloy with the lowest B and N concentrations but greater C content shows the best creep performance—a consequence of the refined M23C6 carbide precipitate population and the absence of large‐scale inclusions or BN precipitates. Calculations of creep activation energy reveal that the high B and N containing alloy is more prone to damage accumulation which causes an early onset of accelerating creep and greater minimum creep rate.

Effect of thermal exposure on the microstructure and creep properties of a fourth-generation Ni-based single crystal superalloy

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickel-based single crystal superalloy was investigated. The thermal exposure of samples after the full heat treatment was carried out at 1000 °C, 1100 °C and 1140 °C for 100 h and 200 h. The γ′ coarsening, γ′ rafting and γ channel widening were observed in samples after thermal exposure. When the thermal exposure time was constant, the morphology of γ′ phase in the alloy evolved significantly with increasing aging temperature. The interfacial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature. When the higher exposure temperature was used, enlargement of the defect pores was observed in samples, the microcracks were more likely to initiate and propagate at the corner of these pores. After aging at 1000 °C for 100 h, the creep life at 1140 °C/137 MPa was slightly longer than that of heat-treated sample, which could be attributed to the slightly coarsened γ′ phase, homogenization of refractor elements. In contrast, the creep life of sample exposed at 1140 °C for 100 h was greatly decreased. The decrease of creep life was dominated by the rafting of γ′ phase, the irregular interfacial dislocation networks as well as the enlargement of homogenization pores.

Partitioning of tramp elements Cu and Si in a Ni-based superalloy and their effect on creep properties

An alloy's processing history, including melting, remelting or the choice of stock material, affects its purity and eventually involves tramp element pickup or retention. In this investigation, variants of a novel Ni-based superalloy were manufactured with different levels of purity. The so-called low purity alloys contained 0.138 wt. % Cu and 0.019 wt. % Si while the Cu and Si levels were below x-ray fluorescence (XRF) detection limits of 0.003 and 0.010 wt. %, respectively, in the high purity ingots. Atom-probe tomography (APT) was carried out and revealed Si partitioning at the following interfaces: grain boundaries, MC carbide/γ, M3B2 boride/γ and M3B2 boride/γ′. Copper was found to primarily segregate to the γ′ precipitates. An average of 2.4 × decrease in creep life and 4.3 × decrease in creep ductility was measured in the low purity alloys, which was attributed to the embrittlement caused by Si segregation to grain boundaries. Furthermore, the positive effect of B on the creep properties was mitigated by the presence of Si. Thermodynamic predictions for the matrix and γ′ precipitate compositions represented the trends observed experimentally although the extent of preferential partitioning lacks accuracy. Monte Carlo simulations were performed to describe the partitioning of Cu and Si atoms to either γ or γ′ phases.

The effect of coarsening of γ′ precipitate on creep properties of Ni-based single crystal superalloys during long-term aging

The correlation between the coarsening of γ′ precipitate and creep properties during long-term aging at 1000 °C for 100–1000 h of a fourth-generation nickel-based single crystal superalloy was investigated. The average size of γ′ precipitate increased with the extension of aging time and the morphology of γ′ precipitate kept cuboidal shape after 1000 h. Experimental observation on the coarsening of γ′ phase demonstrated that the relationship between rt3−r03 and t was in accordance with the LSW theory and the coarsening rate k was about 5954 nm3/h. The creep life of aged samples at 1140 °C/137 MPa increased firstly and then decreased with the exposure time prolonging. The casting pores and creep pores were observed on the fracture surface of samples after creep rupture. Compared with the creep pores, the microcracks formed at the edge of casting pores had greater effect on the creep life. The creep properties of samples during aging were mostly affected by the size of γ′ phase. The promotion of creep life of aged sample was rationalized by the slightly grown γ′ phase, the diffusion of the refractory elements and the accumulation of dislocations in the γ channel. The decrease of creep life was predominantly affected by the seriously coarsened γ′ phase because the significant increase of the size of γ′ phase leaded to a decrease in the resistance of dislocation movement.

Correction to: Creep Property and Phase Stability of Sulfur-Doped Ni-Base Single-Crystal Superalloys and Effectiveness of CaO Desulfurization

The direct and complete recycling method for Ni-base superalloy is being developed and studied to reduce the material cost for cost-effective operation of gas turbine systems. Understanding the effect of sulfur contamination is important to determine allowable sulfur content after the recycling. However, in the case of single-crystal superalloys, this effect on material properties is not well known except for the detrimental effect on the oxidation resistance. In the present study, creep tests, aging tests, and cyclic oxidation tests have been performed on PWA1484 with varying sulfur content. The increasing sulfur content has been found to correlate with degradation of properties evaluated here. It is observed that the decrease in creep life in PWA1484 due to sulfur doping is primarily due to coarsening of the γ/γ′ interfacial dislocation network, increase in precipitation kinetics of topologically closed-packed phase, and decrease in oxidation resistance. For recycling purposes, a CaO crucible was used in the casting process, which successfully decreased the sulfur level in the alloy, and the resulting material showed comparable or even better properties in comparison to the low sulfur content material.

Structural and Mechanical Characterization of Service Exposed 2.25Cr–1Mo Steel

During long term high temperature service exposure of boiler tubes, the microstructural degradation of the boiler tube material occurs, which causes the reduction in creep resistance. In the present study, 1,93,756 h (22.12 years) service exposed boiler tube (superheater tube) was investigated for microstructure and mechanical properties (hardness, tensile, creep). During service, the tube was operating at temperature of 540 °C and 40 MPa stress level. XRD analysis of internal oxide scale of service exposed tube confirmed the formation of hematite and magnetite, resulting in the localized heating which accelerated the microstructural degradation. Comparative microstructural study with the unexposed material revealed the significant coarsening and spheroidization of carbides during service exposure. The hardness and yield strength of exposed tube material decreased by ~17 and 15 %, respectively due to agglomerations of carbides like M23C6, M2C and M6C at grain boundaries and decomposition of pearlite colonies in exposed tube material. Fractographic studies on the tensile sample of exposed material exhibited mixed mode of fracture. Significant decrease in creep life was observed as compared to unexposed material due to coarsening of precipitates and reduction in the dislocation density.

Effect of long-term thermal exposures on microstructures and mechanical properties of directionally solidified CM247LC alloy

A directionally solidified CM247LC alloy was exposed at 871 °C and 982 °C for 1000 h, 5000 h, and 10000 h under free stress in order to study the effect of microstructural degradation on the creep properties. None of the specimens exposed at temperatures up to 10000 h produced any kind of topologically close-packed phases because of the excellent phase stability of CM247LC alloy. The plate-like M6C carbide was formed only at exposure of 982 °C for 10000 h through a decomposition reaction between γ and MC. Moreover, an M23C6 carbide layer was observed between the M6C and the matrix. The exposure at 982°C for 5000 h and 10000 h had a spontaneous rafting of γ′ under free stress, while the exposure at 871 °C for 1000 h, 5000 h, and 10000 h had a non-rafted structure. The spontaneous rafted structure resulted in a drastic decrease in creep life. A 3-dimensional morphology of γ′ in the as-crept specimens, which were pre-exposed at 982 °C for 5000 h and 10000 h, had a non-rafted structure. This microstructural feature proves that the significant decrease in creep life of the specimen resulted from a loss of coherency between γ and γ′.

Determination of the ductile to brittle temperature transition of aluminide coatings and its influence on the mechanical behavior of coated specimens

The ductility of various coatings deposited by chemical vapor deposition, pack cementation and slurry processes on Fe- and Ni-based alloys was characterized by indentation at room temperature. A hot indentation apparatus has also been developed to more rapidly determine the ductile to brittle transition temperature of coated specimens. Creep testing has been conducted on bare and coated alloy 230 (NiCrW) specimens at 800 °C with a significant decrease in creep life observed. Based on the observed failure of coated 230 specimens, the impact of coating ductility on substrate creep properties is discussed.

Monte Carlo simulation of the effect of fiber characteristics on creep life of a fiber tow

Fatigue of reinforced ceramics at elevated temperatures was numerically evaluated with a fiber dominated, power-law creep model. A Monte Carlo simulation of fiber creep in a uniaxially loaded tow was used to examine the influence of fiber radius, elastic modulus, and strength on creep respose. The simulation permitted variation of both the average magnitude and dispersion of fiber characteristics while maintaining constant power-law creep parameters. A linear increase in creep life was predicted for an increase in mean fiber radius, and a linear decrease in creep life was predicted for an increase in the standard deviation of fiber radii. A linear increase in creep life was predicted for both an increase in mean fiber elastic modulus and standard deviation of elastic moduli. Characteristic fiber strength and Weibull modulus were predicted to have a significant effect on creep life of a SiC fiber low. An increase in either the characteristic strength or Weibull modulus was predicted to result in an increase in creep life.