Showing papers on "Creep published in 2011"

Gamma titanium aluminide alloys : science and technology

Abstract: Preface INTRODUCTION CONSTITUTION The Binary Ti-Al Phase Diagram Ternary and Multicomponent Alloy Systems THERMOPHYSICAL CONSTANTS Elastic and Thermal Properties Point Defects Diffusion PHASE TRANSFORMATIONS AND MICROSTRUCTURES Microstructure Formation on Solidification Solid State Transformations DEFORMATION BEHAVIOR OF SINGLE-PHASE ALLOYS Single-Phase Gamma(TiAl) Alloys Deformation Behavior of Single-Phase Alpha2(Ti3Al) Alloys Beta/B2 Phase Alloys DEFORMATION BEHAVIOR OF TWO-PHASE ALPHA(Ti3Al) + GAMMA(TiAl) ALLOYS Lamellar Microstructures Deformation Mechanisms, Contrasting Single-Phase and Two-Phase Alloys Generation of Dislocations and Mechanical Twins Glide Resistance and Dislocation Mobility Thermal and Athermal Stresses STRENGTHENING MECHANISMS Grain Refinement Work Hardening Solution Hardening Precipitation Hardening Optimized Nb-Bearing Alloys DEFORMATION BEHAVIOR OF ALLOYS WITH A MODULATED MICROSTRUCTURE Modulated Microstructures Misfitting Interfaces Mechanical Properties CREEP Design Margins and Failure Mechanisms General Creep Behavior The Steady-State or Minimum Creep Rate Effect of Microstructure Primary Creep Creep-Induced Degradation of Lamellar Structures Precipitation Effects Associated with the Alpha2 -> Gamma Phase Transformations Tertiary Creep Optimized Alloys, Effect of Alloy Composition and Processing Creep Properties of Alloys with a Modulated Microstructure FRACTURE BEHAVIOR Length Scales in the Fracture of TiAl Alloys Cleavage Fracture Crack-Tip Plasticity Fracture Toughness, Strength, and Ductility Fracture Behavior of Modulated Alloys Requirements for Ductility and Toughness Assessment of Property Variability FATIGUE Definitions The Stress-Life (S-N) Behavior HCF Effects of Temperature and Environment on the Cyclic Crack-Growth Resistance LCF Thermomechanical Fatigue and Creep Relaxation OXIDATION BEHAVIOR AND RELATED ISSUES Kinetics and Thermodynamics General Aspects Concerning Oxidation Summary ALLOY DESIGN Effect of Aluminum Content Important Alloying Elements - General Remarks Specific Alloy Systems Summary INGOT PRODUCTION AND COMPONENT CASTING Ingot Production Casting Summary POWDER METALLURGY Prealloyed Powder Technology Elemental-Powder Technology Mechanical Alloying WROUGHT PROCESSING Flow Behavior under Hot-Working Conditions Conversions of Microstructure Workabiliy and Primary Processing Texture Evolution Secondary Processing JOINING Diffusion Bonding Brazing and Other Joining Technologies SURFACE HARDENING Shot Peening and Roller Burnishing Residual Stresses, Microhardness, and Surface Roughness Surface Deformation Due to Shot Peening Phase Transformation, Recrystallization, and Amorphization Effect of Shot Peening on Fatigue Strength Thermal Stability of the Surface Hardening APPLICATIONS, COMPONENT ASSESSMENT, AND OUTLOOK Aerospace Automotive Outlook

Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate

Abstract: Laboratory tests are performed to investigate the effects of a new method of mixture proportioning on the creep and shrinkage characteristics of concrete made with recycled concrete aggregate (RCA). In this method, RCA is treated as a two component composite material consisting of residual mortar and natural aggregate; accordingly, when proportioning the concrete mixture, the relative amount and properties of each component are individually considered. The test variables include the mixture proportioning method, and the aggregate type. The results show that the amounts of creep and shrinkage in concretes made with coarse RCA, and proportioned by the new method, are comparable to, or even lower than, those in similar concretes made entirely with natural aggregates. Furthermore, it is demonstrated that by applying the proposed “residual mortar factor” to the existing ACI and CEB methods for calculating creep or shrinkage of conventional concrete, these methods could be also applied to predict the creep and shrinkage of RCA-concrete.

Deformation of Earth Materials : an Introduction to the Rheology of Solid Earth

Abstract: Part I. General Background: 1. Stress and strain 2. Thermodynamics 3. Phenomenological theory of deformation Part II. Materials Science of Deformation: 4. Elasticity 5. Crystalline defects 6. Experimental techniques in the study of plastic deformation 7. Brittle fracture, brittle-plastic transition 8. Diffusional creep 9. Dislocation creep 10. Effects of pressure and water 11. Physical mechanisms of seismic wave attenuation 12. Deformation of multi-phase materials 13. Grain size 14. Lattice preferred orientation 15. Effects of phase transformations 16. Stability and localization of deformation Part III. Geological and Geophysical Applications: 17. Composition and structure of Earth's interior 18. Time-dependent deformation of Earth and rheological structures 19. Inference of rheological structure of Earth from mineral physics 20. Heterogeneity of seismic wave velocities and its geodynamic significance 21. Seismic anisotropy and its geodynamic significance References Index.

Properties of magnesium alloys reinforced with nanoparticles and carbon nanotubes: a review

Abstract: Magnesium alloys suffer from only moderate high-temperature strength and creep resistance. Aluminium-free magnesium alloys for sand casting or alloys containing aluminium with expensive additional alloying elements may be in use, but only microparticle or microfibre-reinforced magnesium alloys really exhibit satisfactory creep strengths at temperatures up to 250 °C. Reinforcing magnesium alloys with ceramic nanoparticles could be a solution for preserving a low density while increasing the high-temperature performance. When produced using melting processes, nanoparticle-reinforced magnesium composites are expected to enjoy strengthening due to the grain refinement described in the Hall–Petch relation. When an isotropic distribution of nanoparticles is achieved, the composites are additionally expected to be Orowan-strengthened. In this review, a variety of ceramic materials, such as SiC, Al2O3, Y2O3, SiO2 and carbon nanotubes were investigated for reinforcement. Pure magnesium and various magnesium alloys were chosen as the matrix material and both powder metallurgical (PM) and melting processes were used for production of the composites. The mechanical properties of the composites were generally enhanced, compared to an unreinforced alloy; not only at room temperature, but also at elevated temperatures. In some cases an increase in strength in combination with increased ductility was also identified.

Dwell fatigue crack nucleation model based on crystal plasticity finite element simulations of polycrystalline titanium alloys

Abstract: In this paper a crystal plasticity-based crack nucleation model is developed for polycrystalline microstructures undergoing cyclic dwell loading. The fatigue crack nucleation model is developed for dual-phase titanium alloys admitting room temperature creep phenomenon. It is a non-local model that accounts for the cumulative effect of slip on multiple slip systems, and involves evolving mixed-mode stresses in the grain along with dislocation pileups in contiguous grains. Rate dependent, highly anisotropic behavior causes significant localized stress concentration that increases with loading cycles. The crystal plasticity finite element (CPFE) model uses rate and size-dependent anisotropic elasto-crystal plasticity constitutive model to account for these effects. Stress rise in the hard grain is a consequence of time-dependent load shedding in adjacent soft grains, and is the main cause of crack nucleation in the polycrystalline titanium microstructure. CPFE simulation results are post-processed to provide inputs to the crack nucleation model. The nucleation model is calibrated and satisfactorily validated using data available from acoustic microscopy experiments for monitoring crack evolution in dwell fatigue experiments.

The effect of Re and Ru on γ/γ′ microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys

Abstract: A new in-house designed series of Ni based superalloys with stepwise increased Re and Ru additions has been investigated, to systematically determine the influence of Re and Ru on γ/γ′-microstructure and high temperature creep properties. Improved creep resistance and thus also a higher alloy temperature capability of up to 87 K/at.% was found for additions of Re. Additions of Ru revealed a lower temperature capability improvement of up to 38 K/at.% for low Re-containing second generation alloys. However, in third and fourth generation alloys with higher Re-contents, no significant influence of Ru on creep rupture strength was observed. The creep properties are discussed with respect to the γ′-volume fraction, γ′-size and γ′-coarsening rate, as well as the γ/γ′-lattice misfit and the γ/γ′ partitioning coefficient of the different Re and Ru containing alloys. The presented data shows, that these microstructure parameters are strongly influenced by additions of Re, but only marginally by additions of Ru. A further influence on creep rupture strength is given by the solid solution hardening of the γ-matrix, which is discussed based on solid solution hardener concentrations either experimentally derived or calculated from ThermoCalc data.

Hot Deformation and Processing of Aluminum Alloys

Abstract: Aluminum and Its Alloys Introduction, History, and Applications Crystal Structure and Slip Behavior Starting Point for Analyzing Plastic Forming Alloy Development Alloy and Temper Designations Solidification, Segregation, and Constitutive Particles Aluminum Industry Organization Metal Forming and Deformation Modes Introduction to Metal Forming Industrial Processing Overview Computational Modeling and Simulation Mechanical: Elastic/Plastic Stress/Strain Mechanical: Constitutive Equations Microstructural Development Hot Work Testing Techniques Introduction to Testing Torsion Compression Tension Hot Rolling Extrusion Hot Working of Aluminum DRV, Historical Perspective Steady-State Flow Curves Constitutive Analysis Microstructure Evolution Crystal Rotations, Texture, and Strain-Induced Boundaries GB Serrations Geometric DRX: Grain Refining DRV DRX of Al Alloys GB Sliding and Migration Hot Ductility and Failure Mechanisms Hot Working of Dispersoid and Solute Alloys General Dispersoid Effects Al-Mn Alloys, Can Stock (3000 Series) Al-Fe and Al-Fe-Co Conductor Alloys Al-Si Eutectic Forging Alloys (4000 Series) Mechanical Alloying Rapidly Solidified Alloys Al-Mg Alloys (5000 Series) Al-Mg-Mn Alloys (5000 Series) Hot Ductility of Al-Mg and Al-Mg-Mn Alloys Precipitation Hardening Alloys Introduction: Precipitation Behavior Al-Mg-Si Alloys (6000 Series) Al-Cu-Mg Alloys (2000 Series) Al-Zn-Mg-Cu Alloys (7000 Series) Al-Li-XX Alloys (8000 Series, 2090) Aluminum Matrix Composites Introduction Varieties and Fabrication Hot Deformation Forging Experiments Comparative Extrusion Behavior Comparison of Hot Working of other Metals Face-Centered Cubic Metals (Low SFE) Body-Centered Cubic Metals HCP Metals Workability of Dual Phase Alloys of Ti, Zr, and Fe Summary: Hot Workability of Different Crystal Structures Creep: Strain Rates below 10-4 s-1 Introduction: Objectives and Description Five-Power-Law Creep Diffusional Creep Harper-Dorn Creep Three-Power Law Viscous Glide Creep Creep Behavior of Particle-Strengthened Alloys Creep Fracture: Grain Boundary Sliding Cold Working Introduction Fundamentals of Single-Crystal Plasticity Deformation of Polycrystals Development of Dislocation Substructure: Textures Influence of Solute and SFE Very High Strains: Cyclic Extrusion-Compression Very High Strains by Equal-Channel-Angular Pressing Comparison of Hot and Cold Working Models for Cold and Hot Working Static Restoration, Annealing Introduction SRV after Cold Working SRX after Cold Working SRV after Hot Working SRX after Hot Working SRV and SRX during Hot Forming Schedules Thermomechanical Processing Introduction to Objectives Strengthening Mechanisms Hot Working with Retained Substructure Hot Working with Static Recrystallization Cold Working and Annealing Product Textures Deformation, Precipitation, and Particles Powder Consolidation: Multiphase Materials Superplasticity Introduction to the Phenomenon Fundamentals of Superplasticity Classification of Processes for Refining Microstructure Heavy Cold or Warm Working and SRX Warm Working with cDRX in Superplastic Straining Severe Plastic Deformation at Room Temperature Extrusion Introduction to the Process Control Parameters Insights from FEM Substructures and Microstructures in Extrusions Extrusion Microstructures in Al-Mg and Al-Mg-Mn (5000 Series) Extrusion of Al-Mg-Si Alloys (6000 Series) Extrudability of Al-Zn-Mg Alloys (7000 Series, Cu Free) Extrudability of Al-Zn-Mg-Cu Alloys (7000 Series) Extrudability of Al-Cu-Mg Alloys (2000 Series) Surface Failure in Extrusion Extrudate Properties and Recrystallization Rolling Introduction: The Rolling Process Static Recovery and Recrystallization Hot and Warm Rolling Rolling, DRV, SRV, and SRX: Textures Rod Rolling Particle-Stabilized Wire Torsional Simulation of Rolling Rolling Simulation in Compression Modeling of Rolling Hot and Cold Forging Feedstock Preheat Forging Rate: Press Effects Shape and Fiber Control: Die Type Modeling: Shape and Structure Microstructure Development Properties Comparison to Semisolid Forming

Overview of Strategies for High-Temperature Creep and Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels

Abstract: A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the ~923 K to 1173 K (650 °C to 900 °C) temperature range due to the formation of a protective Al2O3 scale rather than the Cr2O3 scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe2(Mo,Nb)-Laves, Ni3Al-L12, etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA alloys are discussed relative to compositional and microstructural factors.

Aseismic sliding of active faults by pressure solution creep: Evidence from the San Andreas Fault Observatory at Depth

Abstract: Active faults in the upper crust can either slide steadily by aseismic creep, or abruptly causing earthquakes. Creep relaxes the stress and prevents large earthquakes from occurring. Identifying the mechanisms controlling creep, and their evolution with time and depth, represents a major challenge for predicting the behavior of active faults. Based on microstructural studies of rock samples collected from the San Andreas Fault Observatory at Depth (California), we propose that pressure solution creep, a pervasive deformation mechanism, can account for aseismic creep. Experimental data on minerals such as quartz and calcite are used to demonstrate that such creep mechanism can accommodate the documented 20 mm/yr aseismic displacement rate of the San Andreas fault creeping zone. We show how the interaction between fracturing and sealing controls the pressure solution rate, and discuss how such a stress-driven mass transfer process is localized along some segments of the fault.

9%Cr heat resistant steels: Alloy design, microstructure evolution and creep response at 650 °C

Abstract: In this work 9%Cr alloys were designed supported by computational thermodynamic methods. Two sets of alloys were produced: 9%Cr alloys with 0.1%C and 0.05%C and 9%Cr alloys containing ∼0.03% Ti with 0.1%C and 0.05%C (always wt%). Microstructure investigations showed good agreement with the predicted phases of the thermodynamic modeling. The volume fraction of precipitated M 23 C 6 carbides is directly related to the carbon content of the alloys. For Ti-containing alloys the precipitation of nano-sized Ti-rich MX carbonitrides was observed. The microstructure evolution (sub-grain and particle size) during creep at 650 °C/100 MPa was investigated by STEM-HAADF. The sub-grain size evolution and the coarsening of precipitates (MX carbonitrides, M 23 C 6 and Laves phase) were more pronounced for Ti-containing alloys. 9Cr alloys without Ti and with low carbon content presented the highest creep strength of all investigated alloys.

Comparison of various 9–12%Cr steels under fatigue and creep-fatigue loadings at high temperature

Abstract: The present article compares the cyclic behaviour of various 9–12%Cr steels, both commercial grades and optimized materials (in terms of creep strength). These materials were subjected to high temperature fatigue and creep-fatigue loadings. TEM examinations of the microstructure after cyclic loadings were also carried out. It appears that all the tempered ferritic–martensitic steels suffer from a cyclic softening effect linked to the coarsening of the subgrains and laths and to the decrease of the dislocation density. These changes of the microstructure lead to a drastic loss in creep strength for all the materials under study. However, due to a better precipitation state, several materials optimized for their creep strength still present a good creep resistance after cyclic softening. These results are discussed and compared to the literature in terms of the physical mechanisms responsible for cyclic and creep deformation at the microstructural scale.

Effect of serrated grain boundaries on the creep property of Inconel 718 superalloy

Abstract: This article presents an investigation of the effect of serrated grain boundaries on the high temperature creep behaviour of IN718 superalloy. XRD, DSC, SEM and TEM have been conducted to study the microstructures; experimental results have shown that the morphology of grain boundaries can be a function of heat treatment conditions. In addition, creep tests have been performed on samples with and without serrated grain boundaries under 650 °C/625 MPa. In conclusions, the formation of δ phase during the heat treatment can influence the morphology of grain boundaries, and an improvement of 400 h creep rupture life has been attributed to the serrated grain boundaries.

Comparison of creep rupture behaviour of type 316L(N) austenitic stainless steel joints welded by TIG and activated TIG welding processes

Abstract: Creep rupture behaviour of type 316L(N) austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIG (A-TIG) and multi-pass conventional TIG (MP-TIG) welding processes were studied at 923 K over a stress range of 160–280 MPa. Both the weld joints possessed lower creep rupture lives than the base metal. The A-TIG weld joint displayed higher rupture lives than the MP-TIG weld joint. Failure in the weld joints occurred in the weld metal. Progressive localization of creep deformation in the weld metal of both the joints led to the premature failure. Accumulation of creep deformation at higher rate was observed in the weld metal of the MP-TIG joint than in the A-TIG joint. Finer microstructural features and higher amount of δ-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIG weld joint. Orientation of the columnar grains and δ-ferrite was nearly transverse to the welding direction in the A-TIG joint, whereas it was towards short transverse to the welding direction in the MP-TIG weld joint. TEM investigation of creep exposed weld metal showed the extensive formation of M 23 C 6 carbides, σ-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIG joint. With creep exposure, the δ-ferrite transformed to σ and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIG weld joint than in the A-TIG weld joint.

Microstructural degradation of Gr.91 steel during creep under low stress

Abstract: Microstructural changes during creep at 600 °C under 70 MPa were investigated in the case of interrupted Gr.91 steel samples by taking into account the dislocation structure and Z-phase formation. The creep life monotonically increased with a decrease in the applied stress at each temperature considered in the study. However, the long-term creep life was shorter than that determined from the short-term creep data at 600 °C and 650 °C, meaning premature failure. The subgrain size gradually increased during creep up to 70,000 h, after which rapid subgrain coarsening occurred. Preferential recovery of the subgrain structure occurred around the prior-austenite grain boundary (PAGB) after 50,000 h and 70,000 h. After creep rupture, subgrain recovery was observed over the entire area of each sample. Z-phase formation was clearly visible for 30,000 h after creep. The number density of the MX particles gradually decreased after 30,000 h because of Z-phase formation. After creep rupture, the number density of the MX particles was almost the same as that of the Z-phase particles. During creep, the V content of the Z-phase gradually increased but the Nb content decreased. Changes in the chemical composition of the Z-phase occurred after a longer time in Gr.91 steel than in 12Cr steel.