High Temperature Creep Resistance Research Articles

Unveiling the Re effect in Ni-based single crystal superalloys

Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called “Re effect” have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic.

High temperature creep resistance of a thermally stable nanocrystalline Fe-5 at.% Zr steel

The application of nanocrystalline (NC) materials at high temperatures is challenging due to their poor thermal stability or low creep resistance. Here we report that a thermally stable NC Fe-5 at.% Zr steel produced by High-Pressure-Thermal-Compression sintering exhibits an excellent creep resistance (with a creep rate of 3.92 × 10−8 s−1 at 923 K and under the applied stress of 250 MPa). The excellent creep resistance is ascribed to its highly stable NC structure stabilized by nano-sized precipitates. Mechanical testing suggests that the creep of the NC Fe-5 at.% Zr steel is controlled by dislocation activities rather than diffusion dominated mechanisms.

Effect of laser shock peening on microstructure and residual stress of Ti<sub>2</sub>AlNb alloy

Ti2AlNb-based alloys are developed from conventional titanium aluminides (TiAl-based alloys and Ti3Al-based alloys). Comparing with TiAl and Ti3Al-based alloys, Ti2AlNb-based alloys have better mechanical properties of strength density ratio, high temperature fracture toughness and creep resistance. It is expected to replace traditional high temperature materials such as nickel-based alloys for the manufacture of aero-engine components. Laser shock peening (LSP) is an advanced surface modification technology which induces high amplitude, large depth residual compressive stress on the surface of materials. It can effectively enhance mechanical properties of material such as micro-hardness, fatigue resistance, high temperature oxidation performance. Compared to other surface modification technologies such as shot peening and rolling, LSP can not only induce deeper compress residual stress (1–2 mm), which is about five to ten times deeper than that of shot peening, but it can refine the grain or even form nanocrystals which plays a significant role in enhancing properties of material. For these reasons, this surface treatment technology has been widely used in the aircraft and nuclear energy industries. In this paper, the surface modification of Ti2AlNb alloy was carried out by laser shock peening, and the microstructure evolution, residual stress and the performance at high temperature were investigated. The results show that laser shock peening can significantly reduce the grain size of the surface of Ti2AlNb alloy. Dislocation lines, dislocation tangles and dislocation walls were generated after LSP. The micro-hardness is increased from 350 HV to 395 HV and the effect layer is about 1.2 mm. The residual compressive stress of −337 MPa is generated on the surface of the sample. In addition, residual stress is released obviously at high temperature. The residual stress on the surface of sample reduces to −65 MPa after heat treatment at 720°C for 4 h. The residual stress is almost completely released after heat treatment at 720°C for 10 h. However, the release rate of residual stress is slowed down at 600°C. The effect of laser shock peening is still presence after heat treatment for 100 h.

Site preference of V and its influence on the elastic properties in the boride series VxMo5–xSiB2 as studied by first principles density functional theory

Mo-Si-B alloys provide a significantly higher creep resistance at temperatures above 1100 °C compared with Nickel-based superalloys. However, the high density of this alloy is a drawback when used as a turbine blade material. One approach to reduce the density of Mo-Si-B alloys, without decreasing the high temperature creep resistance too much, is alloying with vanadium. With this procedure a density reduction from 9.60 g/cm³ in Mo-9Si-8B to 7.73 g/cm³ in Mo-40V-9Si-8B can be achieved. Generally, in Mo-ZV-9Si-8B alloys (Z = 10, 20, 30, 40 at.%), phases like VxMo5–xSiB2 (T2 crystal structure), VxMo3−xSi (A15) and VxMo1−x (B2) are present and interestingly, the following behavior can be observed after alloying with V: Rietveld refinement revealed that at least in the Mo-40V-9Si-8B alloy vanadium prefers the 4c Wyckoff position in T2-V3Mo2SiB2. The present work will explain the reason of this observation using first principles density functional theory (DFT) calculations and also demonstrate the influence of V and Mo occupations in the T2-VxMo5–xSiB2 phases on the elastic properties like bulk modulus, shear modulus, Young’s modulus and Vickers hardness with the help of DFT.

Effect of Helium on Dispersoid Evolution under Self-Ion Irradiation in A Dual-Phase 12Cr Oxide-Dispersion-Strengthened Alloy.

As one candidate alloy for future Generation IV and fusion reactors, a dual-phase 12Cr oxide-dispersion-strengthened (ODS) alloy was developed for high temperature strength and creep resistance and has shown good void swelling resistance under high damage self-ion irradiation at high temperature. However, the effect of helium and its combination with radiation damage on oxide dispersoid stability needs to be investigated. In this study, 120 keV energy helium was preloaded into specimens at doses of 1 × 1015 and 1 × 1016 ions/cm2 at room temperature, and 3.5 MeV Fe self-ions were sequentially implanted to reach 100 peak displacement-per-atom at 475 °C. He implantation alone in the control sample did not affect the dispersoid morphology. After Fe ion irradiation, a dramatic increase in density of coherent oxide dispersoids was observed at low He dose, but no such increase was observed at high He dose. The study suggests that helium bubbles act as sinks for nucleation of coherent oxide dispersoids, but dispersoid growth may become difficult if too many sinks are introduced, suggesting that a critical mass of trapping is required for stable dispersoid growth.

Crystal Plasticity Study of Heterogeneous Deformation Behavior in γ Matrix Channels during High Temperature Low Stress Creep of Single Crystal Superalloys

Abstract Single crystal nickel-based superalloys have high temperature creep resistance due to particle strengthening by high volume fraction of coherent γ′ precipitates distributed within nickel-based solid solution γ matrix. In the high temperature low stress regime, published experimental studies reveal that the creep deformation mechanism during the secondary stage is predominantly by dislocation glide in the γ matrix only, and that there is a preferential motion of dislocations in the matrix, oriented in a direction perpendicular to the stress. In this work, the crystal plasticity finite element method is employed to perform creep simulations on a representative volume element in the high temperature and low stress regime. A sine-hyperbolic–based material creep model was used for the matrix, while the precipitates are assumed to be elastic. A softening model incorporating the evolution of mobile dislocation density was used to capture the transition from secondary to tertiary creep. The predicted creep curves agree well with the published experimental measurements on single crystal superalloy CMSX-4. The simulations predict a higher creep strain distribution in the horizontal channel of the matrix (perpendicular to the applied stress) as compared with the vertical channel (horizontal to the applied stress). Local creep strain distributions in the channels were found to be greater than twice their average creep strain. The results provide key insights into the distribution of macroscopic creep strain in the local channels of the γ matrix to further aid in the microstructural design of creep-resistant superalloys.

In Situ Creep Behavior Characterization of Single Crystal Superalloy by UV-DIC at 980 °C

High temperature creep resistance is a critical characteristic of Ni-based single crystal (SX) superalloys. In this work, the creep behavior of a Ni-based SX superalloy was in situ characterized at 980 °C by ultraviolet (UV) imaging combined two-dimensional digital image correlation (DIC) in vacuum environment. The surface pattern was fabricated to maintain stable over 65 h at 980 °C. The pattern images captured by UV imaging were analyzed using mean gray value and the full-field strain map of creep deformation was obtained. A laser displacement senor (LDS) was employed for measuring the creep strain on the specimen for comparison. The creep deformation result shows a good agreement between DIC and LDS, the microstructure of the different creep areas on the specimens also demonstrate that the results of DIC are reliable. The in situ creep characterization by UV-DIC shows a great potential for investigating creep behaviors at high temperatures.

Effect of precipitate evolution on creep damage of reduced activation ferritic/martensitic steel friction stir welded joint

This paper reports on the precipitate evolution and its influence on the high temperature creep resistance of 9% Cr martensite ferritic steel FSW joint. The creep rupture time of the FSW joints welded by 400 rpm and 200 rpm are 859.29 h and 1173.8 h respectively, while that of BM is 3398 h. The creep fracture occurs in ICHAZ where a pronounced increase of coarse Laves phase is detected. Most of the Laves phase particles are large in size and mainly nucleate and grow rapidly at the micrograin boundaries adjoining the M23C6 particles where W, and other elements segregate. The large Laves phase particles will deteriorate the pinning effect of M23C6, weaken the solid solution strengthening effect and attribute to the increased cavity nucleation. In addition the size of Laves phase in ICHAZ is bigger than that in stir zone (SZ). These reasons explain why creep fracture occurs in the ICHAZ and the time to rupture will be shorter than base metal.

Precipitation behavior of Ti-45Al-3Fe-2Mo-0.5C intermetallics after creep tests at 750 °C

The addition of carbon can improve high temperature creep resistance of TiAl intermetallics. In this work, the precipitation behavior of Ti-45Al-3Fe-2Mo-0.5C during the creep process at 750 °C and 200 MPa has been investigated. The results show that the strain of C-doped TiAl is about 60% lower than that of the C-free TiAl at a creep time of 8 × 105 s. The precipitations, including β phase, Ti3AlC phase and Ti2AlC phases, occurred in the lamellar structure of Ti-45Al-3Fe-2Mo-0.5C after creep tests. The formation of both Ti3AlC and Ti2AlC carbides is promoted at dislocations during creep tests.

Preparation, mechanical properties, and diffuse reflectance of YAG continuous fibers and nanofibers

Yttrium-aluminum-garnet (YAG) ceramic fibers not only have good flexibility, a large aspect ratio, good continuity, light weight, and high strength but also a high melting point, stable physical and chemical properties, high-temperature creep resistance, and excellent optical properties of YAG itself. These properties allow YAG fibers to be used as structural reinforcement and as basic optical materials. In this study, a YAG precursor was selected by comprehensive analysis of the spinning properties of solutions synthesized from aluminum powder, AlCl3·6H2O, and Y(CH3COO)3·4H2O. YAG precursor continuous fibers and nanofibers were spun by extrusion-forming and electrospinning, respectively. The decomposition, crystallization, and microstructure of the fibers were analyzed by thermogravimetry-differential thermal analysis (TG-TDA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mechanical properties and diffuse reflectance of the YAG ceramic fibers were also determined. The results showed that the YAG precursor synthesized with an AlCl3·6H2O/Al powder molar ratio of 1:4 and Al/Y ratio of 3:5 had the best spinnability. The bundle of continuous fibers was 50 × 40 μm, and the length of the individual fibers was greater than 50 cm. The diameter of as-obtained YAG continuous fibers was approximately 23 μm. Both YAG continuous fibers and nanofibers showed good flexibility, uniform diameters, and high crystallinity. During the heat treatment process, loss of free water and crystalline water, decomposition and carbonization of organic matter, and crystallization and densification of the fibers occurred. YAG was directly crystallized from an amorphous material at 750 °C with a ceramic yield of 32.5%. The tensile stress and Young's modulus of the as-obtained YAG continuous ceramic fibers reached 851 MPa and 150 GPa, respectively. The average diffuse reflectance of the YAG nanofibers in the wavelength range of 700–2500 nm reached 92.7%.

Crystal Viscoplasticity Model of Molybdenum Including the Influence of Silicon in Solid Solution

Abstract Molybdenum (Mo)-based alloys offer higher temperature capability than nickel-based superalloys. The challenge of designing these alloys is tailoring the chemistry and microstructure to achieve high-temperature strength, creep and fatigue resistance, and oxidation resistance while maintaining lower-temperature ductility and damage tolerance. Structure-properties modeling tools can be used to identify optimum microstructures. However, one missing element in this tool set is a constitutive model for the more ductile α-Mo phase over the entire temperature range from room temperature to 1,400°C. A crystal viscoplasticity model is developed for α-Mo, including the influence of varying amounts of silicon (Si) in solid solution. The temperature, strain rate, and Si dependence of the deformation behavior needed to determine the model parameters are obtained from new compression experiments.

Microstructural evaluation of a Fe-12Cr nanostructured ferritic alloy designed for impurity sequestration

Fast reactor fuel cladding candidate materials require proficiency in extreme environments consisting of high temperatures and irradiation doses in excess of 150 displacements per atom (dpa). Nanostructured oxide dispersion strengthened (ODS) alloys have been developed extensively for this purpose due to their notable high temperature strength, creep resistance, and irradiation resistance. However, their properties can deteriorate if interstitial impurities such as C and N are not well controlled during the fabrication process. A new Fe-12Cr nanostructured ODS alloy OFRAC (Oak Ridge Fast Reactor Advanced Fuel Cladding) with solute additions of Mo, Ti, and Nb has been developed to provide the desired properties mentioned above while simultaneously sequestering impurities within the matrix. After extrusion at 850 °C, the as-extruded microstructure consists of an average 490 nm grain size and a high number density (6.8 × 1023 m-3) of 2.2 nm diameter (Y,Ti,O) nanoclusters distributed homogeneously in the matrix. Atom probe tomography investigations suggest non-stochiometric compositions for the smallest nanoclusters. In addition, a second population of nanometer scale (Nb,Ti) rich carbonitrides is also present in the microstructure that captures the potentially detrimental C and N impurity atoms present in the matrix. Atom probe tomography results indicate elemental segregation of Cr, Mo, and Nb to grain boundaries in the as-extruded material, consistent with previous investigations of solid solution strengthening by solute additions. The ability of OFRAC to sequester impurities introduced from the powder metallurgical approach to nanostructured ferritic alloy development, compounded with its beneficial mechanical properties, makes this alloy a competitive candidate for fast reactor applications.This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Effect of impurity antimony on the creep behavior of 2.25Cr-1Mo heat-resistant steel

Abstract The creep properties of 2.25Cr-1Mo steel samples, undoped and doped with Sb (0.05 wt%), are examined under different applied engineering stresses (150–400 MPa) and temperatures (773–873 K). The creep behavior follows a unified Monkman-Grant relation e m × t r 1.04 = 0.046 for both the undoped and Sb-doped steels, where e m is the minimum creep rate in h−1 and tr is the rupture time in h. Under the same experimental conditions, the rupture time of the undoped steel is much longer than that of the Sb-doped steel, and meanwhile the minimum creep rate of the former is apparently smaller. Moreover, the threshold stress for creep is much lower for the Sb-doped steel than for the undoped one. This indicates that the impurity element Sb severely lowers the high temperature creep resistance of 2.25Cr-1Mo steel. As shown by boundary microanalysis, there is apparent grain or subgrain boundary segregation of Sb, which could promote the boundary sliding and the nucleation of cavities or microcracks, thereby leading to the creep property deterioration of the steel.

Effect of Y2O3 doping on the high-temperature properties of magnesia aluminate spinel refractories

A series of low-creep magnesium aluminate spinel refractories doped with 0–3 wt% Y2O3 were prepared by sintering at 1750 °C. The physical properties, especially the high-temperature properties of the refractories, were investigated. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were applied to characterize the phases and microstructure of the refractories. The results indicated that yttrium aluminum garnet (YAG) could be in situ formed with the addition of Y2O3 in the spinel refractories. This stabilized the grain boundary of the spinel. The mechanical properties and high-temperature creep resistance of the spinel refractory could significantly be improved by the addition of Y2O3.

The Precipitation Behavior and Mechanical Properties of Long Term Serviced HR3C/T92 Dissimilar Joint

The dissimilar weld joint of HR3C steel and T92 steel is widely used in the ultra supercritical (USC) boiler due to the different service conditions of HR3C steel and T92 steel. It is important to study how serviced influenced the HR3C/T92 dissimilar joint. The HR3C/T92 dissimilar joint had already served 51020 h under 605 °C and 26.25 MPa. The microstructure and mechanical properties of the long term serviced HR3C/T92 dissimilar joint were tested by XRD, SEM, EPMA,TEM, the impact testing machine and the creep and stress rupture testing machine. The XRD results show that the weld is mainly γ-Fe, which is similar to the HR3C base metal, while the T92 base metal is mainly α-Fe. The SEM results indicate that serviced HR3C’s precipitation behavior is complicated, cause multiple precipitated phases precipitated during the service. Chain-liked M23C6precipitated along the grain boundary. M23C6phase cause the chromium depletion zone along the grain boundary. T92 base metal precipitated lots of carbide particles during the service. Those carbide particles are around 1 μm. The impact fracture of HR3C base metal is brittle fracture while the impact fracture of T92 base metal is ductile fracture. Moreover, the impact fracture of weld is mixed fracture. Chain-liked M23C6along the grain boundary is the main cause of the brittleness of serviced HR3C steel.The constant strain creep tests were proceeded under 650 °C, 700 °C and 750 °C. The high-temperature creep resistance of HR3C base metal is better than that of T92 base metal after service. The resistant to high-temperature creep of served joint is similar to that of serviced T92 base metal. With the increase of temperature, the creep resistance of the T92 base metal decreases the most. On the contrary, the creep resistance of the HR3C base metal shows the minimum reduction.

A Hierarchical Multiscale Modeling Investigation on the Behavior of Microtextured Regions in Ti-6242 α/β Processing

Ti-6242 is a near alpha titanium alloy, which has excellent high-temperature creep resistance and is widely used in jet engine compressors. This alloy is susceptible to creep fatigue failure under dwell loading below 473 K. The existence of microtextured regions (MTRs) contributes significantly to this fast crack propagation. Mechanical processing in the alpha + beta region has been employed to eliminate MTRs, but the efficiency depends significantly on the applied strain path. Previous investigations based on crystal plasticity finite element (CPFE) simulations have demonstrated the relationship between breakdown efficiency and loading direction. Therein, MTRs with regular geometry and pure initial orientation were used to isolate the effect of loading direction from initial microstructure. In this paper, the behavior of MTRs with realistic initial microstructure was investigated using a hierarchical multiscale modeling framework, and the microscale results were analyzed in detail to understand the behavior of MTRs under different loading conditions. It was shown that a hierarchical multiscale model with realistic initial microstructure at the microscale can reflect the influences from different strain paths, initial orientation distributions, and positions of the region simultaneously. The combined effect of initial orientation distribution and loading direction on the MTR breakdown efficiency is discussed in detail.

Recent research progress on CALPHAD-based ICME modeling of magnesium alloys

Magnesium alloys combine several favorable attributes, including low density, high strength-to-weight ratio, good damping capability and so on. However, their application is limited by their relatively poor high-temperature performance and creep resistance. These deficiencies can be alleviated by the alloying-induced solid solution and precipitation strengthening. The integrated computational materials engineering (ICME) approach based on the calculation of phase diagram (CALPHAD) software is shown to accurately quantify the effect of alloying elements on the strengthening precipitates. Thus, it is quite instrumental forin enhancing the alloy development process, alloy design, and manufacturing optimization. This paper summarizes the theoretical background of CALPHAD-based ICME, outlines the current research progress in this direction and provides some insights into future development.

Cyclic oxidation behavior of Fe-Cr modified slurry silicide coated Nb-18.7Si alloyed with Ti and Zr

The Nbss/Nb3Si-Nb5Si3 alloys exhibit good high-temperature strength and creep resistance, and are candidate high-temperature materials for hypersonic aerospace applications. However, their poor oxidation resistance necessitates the application of protective coatings.The present study examines the comparative cyclic oxidation behavior of uncoated and Fe-Cr modified slurry silicide coated Nb-18.7Si, Nb-18.7Si-5Ti and Nb-18.7Si-5Zr (in at.%) alloys in air at temperatures between 800–1400 °C. The uncoated samples showed extensive loose-scale formation from the initial stages of oxidation. On the other hand, the protective multi-phase oxide scale formed on the coating surface prevented dimensional degradation for at least 5–7 h at high temperatures of 1400 °C.

Phase Equilibria and Microhardness of As-Cast and Annealed Ni-Al-Os Alloys in Ni-Rich Region

In this paper, the isothermal section at 1273 K and liquidus projection of ternary Ni-Al-Os system in Ni-rich region were firstly measured by using 6 annealed and 6 as-cast Ni-Al-Os alloys with 65 at.% Ni in combination with x-ray diffraction, optical microscopy and electron probe microanalysis techniques. For the determined partial isothermal section at 1273 K, 2 single-phase, 4 two-phase and 2 three-phase regions were observed. The solubilities of Os in both γ and γ′ phases were also determined. For the proposed liquidus projection, four primary surfaces of γ′, γ, β and δ, and two invariant reactions were identified. Secondly, the microhardness of both as-cast and annealed alloys were measured. The evolution trend of microhardness in both as-cast and annealed alloys with Os addition generally increases first, and then decreases. Thirdly, the further comprehensive discussion on possible substitution of Re by Os in new-generation nickel-based single-crystal superalloys were performed in terms of strengthening degree, high-temperature creep resistance, and possibility for formation of harmful topologically close-packed phases. It was finally concluded that Os may be used as a new additional element to replace or partly replace Re in Ni-based single crystal superalloys.

WC-Co-Re cemented carbides: Structure, properties and potential applications

Cemented carbides of the WC-Co-Re system represent a new class of hard materials having a significantly increased Young's modulus, hot hardness and high temperature creep resistance. The WC-Co-Re phase diagram was evaluated and compared with the corresponding WC-Co phase diagram. Physical and mechanical properties of such composites were measured at room and elevated temperatures and compared with those of conventional WC-Co cemented carbides. Microstructures of the WC-Co-Re cemented carbides at different carbon contents, binder contents and WC grain sizes were examined. Rhenium being dissolved in the Co-based binder is found to be a very strong grain growth inhibitor with respect to WC coarsening during liquid-phase sintering. The Young's modulus, hot hardness and high temperature creep resistance of the WC-Co-Re cemented carbides are greater than those of conventional WC-Co cemented carbides. Due to their unique properties the WC-Co-Re materials can find applications in use in high-pressure high-temperature components for synthesis of diamond and c-BN and cutting Ni-based super alloys and other heat-generating workpiece materials.