Temperature Creep Deformation Research Articles

Fatigue assessment for tower bolts of floating offshore wind turbine considering preload loss due to ambient temperature creep

A framework for fatigue assessment for tower bolts of floating offshore wind turbine (FOWT) taking into account the impact of preload loss due to ambient temperature creep and plastic deformation is proposed. The ambient temperature creep of high strength steel is modeled in terms of Norton's theory. The residual preload of tower bolt is predicted by a detailed finite element (FE) model of the threaded bolt connection. Subsequently, the residual preload is applied to the FE model of the entire tower segment to achieve the stress range of the bolts. Fatigue life prediction is finally conducted using the Palmgren-Miner's (PM) rule based on S-N curves, with corrections by effects of mean stress and bolt sizes. A fatigue assessment for tower bolts of a 5 MW FOWT is then performed to demonstrate the capability of the proposed framework. Moreover, impact of initial preload, thread friction coefficients and flange gap on preload loss and fatigue life of tower bolts are discussed.

Effects of Ta on the high temperature creep behavior and deformation mechanism of a Ni-based single crystal superalloy

The effects of Ta on the high temperature creep behavior and deformation mechanism in a Ni-based single crystal (SX) superalloy were investigated at 1120 °C/137 MPa. Ta was regarded as an essential strengthening element of the γʹ phase in the alloy, and the results indicated that the increase of Ta prolonged the creep rupture life. In this work, the addition of Ta increased the volume fraction of the γʹ phase and narrowed the γ channels, making the dislocation bowing out at the primary stage of creep more difficult. As creep continued, with a lower effective diffusion coefficient of alloying elements and larger γʹ raft thickness, the alloy with 6.5 wt% Ta addition was considered to possess the lower dislocation climbing rate, which reduced the minimum creep rate of the alloys. Subsequently, it was noted that the density of superdislocations in γʹ phase was clearly reduced with the increase of Ta. This was supposed to be the elevated Ta content in the alloy, which increased the lattice misfit and led to the formation of a denser dislocation network, thereby enhancing the interfacial strengthening effect and effectively preventing the cutting of superdislocations. The Ta effect on high-temperature creep was systematically summarized in this study, which was an important guideline for further composition design and optimization of Ni-based SX superalloys.

Evaluation of room temperature creep deformation of in situ Fe-based bulk metallic glass nanocomposites by instrumented indentation

In the present work, Fe57Cr9Mo5B16P7C6 (at. %) in situ bulk metallic glass nanocomposites of varying crystallinity, i.e. 0 to 80 vol. %, were synthesized by spark plasma sintering. Room temperature indentation creep behaviour of these composites was evaluated with nanoindentation technique. Effect of crystalline phase content on overall creep behaviour of the BMG composites was evaluated. Creep resistance of the BMG composites was improved with increase in the amount of crystalline phases up to 40 vol. %. Improved creep resistance is attributed to the presence of nanocrystalline phases in amorphous matrix and lower defect concentration in the BMG composite samples. However, excess crystallinity imparted brittleness and resulted in poor creep performance of the BMG composites. Derived stress exponent values are significantly lesser for the BMG composite sample in comparison to the amorphous compact indicating the transition in the mode of plastic deformation from inhomogeneous to nearly homogeneous. Loading rate has more pronounced influence on final creep displacement of fully amorphous sample in comparison to that of BMG composite.

On the low temperature creep controlling mechanism in a high strength spring steel

Despite of the well-known fact that high strength steels are exhibiting low temperature creep deformation, its rate controlling mechanism is yet to be understood. The strain hardening theory and the exhaustion creep model were proposed almost seven decades ago to unravel the low temperature creep mechanism in a single phase homogeneous isotropic material. However, their applicability to low temperature creep deformation in a technical material like a modern high strength steel is still a matter of investigation owing to their nature of multi-phase, in-homogeneous, and an-isotropic behavior. The authors have grabbed this chance to experimentally validate the exhaustion creep model based on low temperature creep tests of the SAE 9254 spring steel. The ultimate tensile strength and yield strength σe of SAE 9254 were determined in a temperature range of 298K≤T≤353K at a constant strain rate of 2.5∙10−4s−1. Low temperature creep deformation behavior was studied at the above-mentioned temperatures T at a constant stress σ=1634MPa, and at stresses 1071MPa≤σ≤1634MPa at a constant temperature T=353K for a duration of 1hr, respectively, at each condition. The retained austenite quantity was determined prior and post low temperature creep testing by means of X-ray diffraction. A reworked exhaustion creep model well describes the stress σ and temperature T dependency of low temperature creep deformation behavior in SAE 9254. Consequently, our hypothesis of a low temperature creep rate controlling mechanism reads: Low temperature creep strain is predominantly contributed by dislocation glide in the retained austenite phase. The reduction in creep rate with time can then be attributed to the exhaustion of this specific deformation mechanism by strain induced martensitic transformation of the retained austenite.

Nucleation of superlattice intrinsic stacking faults via cross-slip in nickel-based superalloys

Superlattice intrinsic stacking faults (SISF) are the main culprit for the low temperature creep deformation of modern nickel-based superalloys used in jet engines. While these faults were identified over fifty years ago, their nucleation mechanism remains unclear. This work provides the first ever experimental evidence, via transmission electron microscopy, of a SISF nucleating from a cross-slip event in a polycrystalline alloy. Such an instance was identified in a grain with a near-〈001〉 tensile loading orientation. In the nucleation mechanism proposed, cross-slip allows the two dissimilar a2〈110〉 dislocations required to form a SISF to meet on adjacent planes at a precipitate interface. The concept of a nascent fault is introduced: the initial stacking fault that forms on a crystallographic plane and the dislocations of which continue to form coplanar faults as they glide away. This nucleation mechanism and the subsequent dislocation evolution are detailed taking into consideration the shear stresses on the individual Shockley partials and the full dislocations involved, as well as the stress orientation dependence of the energy barrier for cross-slip. These findings will guide future characterisation efforts in the field and inform the modelling of more realistic predictive models of creep behaviour.

High temperature creep deformation behavior of heat-treated (α + β)-Mg-9Li-4Al-1Zn alloy

In this study, the high temperature uniaxial tensile creep deformation behavior of (α + β)-Mg-9 mass%Li-4 mass%Al-1 mass%Zn (LAZ941) alloy was investigated. The creep strength of the LAZ941 alloy was significantly improved by heat treatment. The creep deformation behavior of the heat-treated LAZ941 alloy was unique. As the results of creep tests, it is found that the stress exponent of LAZ941 was about 3. The activation energies of creep at the early and middle stages of deformation were 114.7 and 84.6 kJ･mol −1 , respectively, indicating that the diffusing elements controlling the viscous glide of the dislocation changed during creep deformation. • The creep strength of the LAZ941 alloy is improved by heat treatment. • The time to reach 0.1 creep strain is about 9 times longer due to heat treatment. • The creep deformation behavior of heat-treated LAZ941 alloy is unique. • The activation energy of creep for LAZ941 alloy changes during deformation.

High temperature creep deformation of nanocrystalline diamond films

Abstract Diamond displays a combination of unique properties, including the highest hardness among materials, chemical inertness and high thermal conductivity. Therefore, nanocrystalline diamond films offer a huge potential for industrial applications. In fine-grained ceramics as well as metallic materials, high temperature creep deformation is dominated by grain-boundary-deformation mechanisms that become increasingly important with decreasing grain size. In this work we demonstrate that it is possible to inelastically deform nanocrystalline diamond films at elevated temperatures and stresses that are significantly lower than those reported for single-crystal diamond. The initial, isothermal, transient creep flow exhibits a logarithmic character, typical of creep in general. The isothermal steady state creep deformation, which follows transient creep, is analyzed using a physics-based model for grain boundary sliding rate controlled flow.

Fabrication of high-performance silica-based ceramic cores through selective laser sintering combined with vacuum infiltration

Silica-based ceramic cores have been widely used for manufacturing aero-engine hollow blades due to excellent mechanical properties and chemical stability. In this study, selective laser sintering (SLS) based on 3D printing was combined with vacuum infiltration (VI) to fabricate silica-based ceramic cores. The influence of laser processing parameters and infiltration times on the mechanical properties of the silica-based ceramics was investigated. The results showed that the optimal SLS processing parameters were determined as 11 W, 2000 mm/s and 100 µm for the laser power, scanning speed and hatch space, respectively. Meanwhile, as the infiltration time increased to 120 min, the room temperature flexural strength was gradually improved to 7.45 MPa, while the linear shrinkage was constantly reduced to 0.60%, all due to the enhanced α-cristobalite and mullite content. Furthermore, the high temperature creep deformation and flexural strength at 1550 ℃ of the silica-based ceramics prepared with the optimal SLS processing parameters and infiltration time were 0.31 mm and 15.04 MPa, which meets the need of ceramic cores. Consequently, high-performance silica-based ceramic cores can be fabricated by SLS-VI for the rapid manufacturing of hollow blades.

Influence of Ta on the intermediate temperature creep behavior of a single crystal superalloy

The influence of Ta on the intermediate temperature creep behavior and deformation mechanism in a Ni-based single crystal superalloy has been investigated at 760 oC and 750 MPa. The results indicate that there is a significant increasing of creep rupture life after the addition of Ta. Lattice misfit between γ and γ′ phase at 760 oC increases in the Ta-containing alloy. Microstructure showed that the formation of stacking faults in the γ matrix and denser dislocation network at the γ/γ′ interfaces caused by Ta addition are found during the primary creep stage. And a large amount of stacking fault locks which act as an important pattern in the strengthening generate in the γ′ precipitates, resulting in a lower steady state creep rate and longer steady state creep life in Ta-containing alloy. And possible formation mechanism of the stacking fault locks is also discussed.

Investigating Nanoindentation Creep Behavior of Pulsed-TIG Welded Inconel 718 and Commercially Pure Titanium Using a Vanadium Interlayer

In a dissimilar welded joint between Ni base alloys and titanium, creep failure is a potential concern as it could threaten to undermine the integrity of the joint. In this research, the mechanical heterogeneity of a Pulsed TIG welded joint between commercially pure titanium (CpTi) and Inconel 718 (IN718) with a vanadium (V) interlayer was studied through a nanoindentation technique with respect to hardness, elastic modulus, and ambient temperature creep deformation across all regions (fusion zones and interfaces, mainly composed of a dendritic morphology). According to the experimental results, a nanohardness of approximately 10 GPa was observed at the V/IN718 interface, which was almost 70% higher than that at the V/CpTi interface. This happened due to the formation of intermetallic compounds (IMCs) (e.g., Ti2Ni, NiV3, NiTi) and a (Ti, V) solid solution at the V/IN718 and V/CpTi interfaces, respectively. In addition, nanohardness at the V/IN718 interface was inhomogeneous as compared to that at the V/CpTi interface. Creep deformation behavior at the IN718 side was relatively higher than that at different regions on the CpTi side. The decreased plastic deformation or creep effect of the IMCs could be attributed to their higher hardness value. Compared to the base metals (CpTi and IN718), the IMCs exhibited a strain hardening effect. The calculated values of the creep stress exponent were found in the range of 1.51–3.52 and 2.52–4.15 in the V/CpTi and V/IN718 interfaces, respectively. Furthermore, the results indicated that the creep mechanism could have been due to diffusional creep and dislocation climb.

Fabrication and properties of silica-based ceramic cores modified by mullite powders with different particle sizes

Ceramic cores provide complex internal cavity structures in the investment precision castings of hollow blades for aero-engine. The silica/mullite ceramic cores were fabricated by hot injection molding, adding fused mullite powders with different particle sizes as mineralizers. The particle size of mullite mineralizers on the crystallization and properties of ceramic cores was investigated in the paper. The result shows that the mullite mineralizers promote the crystallization of cristobalite in ceramic cores, and finer mullite powders can further improve the heterogeneous nucleation, and generate larger quantities of cristobalite. Mullite mineralizers with large particle size can lead to low sintering degree, poor mechanical properties and creep resistance of ceramic cores. While the particle size of mullite mineralizers is too small, the core contains too much quartzite crystal phase, and the microcrack occurs during the phase transformation during cooling, which also causes the deterioration of the strength at room temperature and the high temperature deformation resistance ability. The ceramic cores with mullite powders in particle size of 19 μm show excellent comprehensive performance, with linear shrinkage of 0.65%, porosity of 31.8%, room temperature strength of 17.8 MPa, high temperature creep deformation of 0.4 mm, and leaching rate of 0.06 g/min.

Characterization of thermal aging effects on the creep performance of T91 using small punch creep testing

AbstractThis paper is concerned with the development and application of a small punch creep testing method for assessment of the effects of thermal aging and associated precipitate coarsening on the high temperature creep life and deformation behavior of a 9Cr ferritic–martensitic steel. The tests are conducted on P91 steel, for both aged (up to 1 year) and unaged specimens, at temperatures of 600°C. The unaged test results are shown to be consistent with previously published creep tensile test rupture life data in terms of Chakrabarty membrane stress. Aging is shown to have a significant, detrimental effect on creep life and deformation rate. The steady‐state displacement rate, time to failure, and displacement at failure data, for both aged and unaged, were successfully correlated via the modified Monkman–Grant relationship.

A hyper-reduction computational method for accelerated modeling of thermal cycling-induced plastic deformations

For materials under cyclic thermal loadings, temperature and strain rate-dependent creep deformation can occur due to the thermal expansion mismatch near material interfaces, leading to deterioration of fatigue life. Simulation of the nonlinear mechanical deformation processes of materials subjected to thermal cycling with high-fidelity numerical models often consume high computational costs due to material nonlinearities and long thermal loading period. To accelerate such thermal cycling simulation, an efficient reduced-order modeling framework is proposed. We adopt the reproducing kernel particle method (RKPM) to generate offline high-fidelity model snapshots, which are utilized to create low-dimensional surrogate models based on the proper orthogonal decomposition under Galerkin projection. Based on the stabilized conforming nodal integration technique, gradient smoothing at single integration point per integration cell is employed in conjunction with a least-squares stabilization for high computational efficiency. To further speed up the simulation, a hyper-reduction method is introduced to avoid the full domain integration during the elastoplastic online simulation. Numerical examples on modeling of thermal cycling-induced plastic deformation and thermal fatigue life prediction of a flip chip assembly are analyzed to demonstrate the effectiveness of the present hyper-reduced-order model in significant reduction of computational time while preserving desired accuracy.

Effect of Ta Content on High Temperature Creep Deformation Behaviors and Properties of PM Nickel Base Superalloys

Effect of Ta Content on High Temperature Creep Deformation Behaviors and Properties of PM Nickel Base Superalloys

Microstructures and Mechanical Properties of Al-Zn-Sn Bearing Alloys for High Performance Applications

The mechanical behavior and indentation creep of Al-20 wt% Zn bearing alloy has been modified with adding 0.2 wt%, 0.5 wt%, 1 wt%, 1.5 wt% Sn. These bearing alloys were prepared by melt spinning technique. The scanning electron microscopy (SEM) was used to study the morphology of the melt spun alloys and x-ray diffractometer (XRD) for the identification of the phases pre- sent in these melt-spun bearing alloys. The results show that the structure of Al80-x-Zn20-SnX (X = 0.2%, 0.5%, 1% and 1.5%) bearing alloys is characterized by the presence of α-Al of FCC structure and SnZn intermetallic compound of anorthic structure. The Al-20Zn-1.5Sn has a smaller crystallite size and grain size as indicated from X-ray and SEM analysis respectively, which leading to the enhancement of the mechanical properties. The mechanical properties and indentation creep of these bearing alloys were studied by tensile test machine and vickers indentation testing at room temperature, respectively. The Al-20Zn-1.5Sn has higher hardness value and creep resistance than other alloys. This was attributed to the strengthen effect of Sn as a strong solid solution element in Al-matrix. The stress exponent values in the range 2.4 - 4.2 indicate that the grain boundary sliding is the possible mechanism during room temperature creep deformation of melt-spun Al-Zn-Sn bearing alloys.

Correlation area size of excess dislocations of the same sign as a new mesoscopic characteristic of the dislocation substructure

The conception of correlation area size (CAS) of excess dislocations of the same sign (EDSS) as a new characteristic of deformation substructure of metals and alloys has been developed. CAS plays important role in elevated temperature creep deformation behavior of metals along with such substructure characteristics as subgrain size and misorientation angle. The main reasons of appearance and some features of CAS of EDSS have been considered. It is shown that CAS can be considered as feature determining the critical grain size. When grain size is less than CAS the creep rate becomes depending on grain size and mechanism changes from high temperature recovery to grain-boundary glide one.

Correlation between bonding, vacancy migration mechanisms, and creep in model binary and ternary hcp-Mg solid solutions

High temperature creep deformation of hcp-Mg alloys is dominated by dislocation climb driven by out-of-plane (OOP) vacancy migration. Past experiments and atomistic simulations have indicated that Zn addition reduces vacancy migration tendencies and improves creep resistance. Here, we have compared in-plane (IP) and out-of-plane (OOP) vacancy migration mechanisms in binary Mg–X (Ca, Y, and Gd) and ternary Mg–X (Ca, Y, and Gd)–Zn alloys using density functional theory based first principles computations. Irrespective of Zn addition, the migration barrier for OOP diffusion was consistently higher than IP in our prototype binary and ternary alloys. The presence of Zn in ternary systems, however, substantially increases the OOP activation barrier relative to binary alloys. The higher OOP barrier in Mg–X–Zn was attributed to favorable local relaxation, enhanced charge localization, higher interplanar bond stiffness, and greater s orbital electron occupancy in the peak saddle state. Combined, these factors restrict non-conservative dislocation climb by impeding out-of-plane vacancy movement and improve the creep resistance of ternary Mg–X (Ca, Y, and Gd)–Zn alloys.

The effects of misfit and diffusivity on γʹ rafting in Re and Ru containing Nickel based single crystal superalloys—details in thermodynamics and dynamics

Rafting is one of the prominent features of Nickel single crystal superalloys in high temperature creep deformation, which has significant influence on creep rupture property. Some detailed comparisons were carried out to illustrate the different effects of misfit and diffusion on the rafting behavior. Its thermodynamic and kinetic processes were also discussed in the present study. Firstly, comparing with the lattice misfit, atom diffusion shown more remarkable influence on γʹ rafting in the present alloy system. Second, the misfit stress release made Gibbs free energy of γ and γʹ phases move in the opposite direction, which provided driving force for the dissolution of γʹ in horizontal channel. Thirdly, external loading could not change the fraction of γʹ phase, the dissolved part of γʹ must re-precipitated in the adjacent vertical channel powered by rafting driving force. At last, the atoms migration by diffusion were mainly occurs in the disordered γ channel, the pipe diffusion along interfacial dislocations accelerated the efficiency of atom migration for rafting.

Three-dimensional modeling of creep deformation in magnesium reduction retorts

Metallic magnesium is produced through a reduction reaction inside thick-walled tubes called retorts. Retorts operate at 1200 °C with an internal vacuum that causes high temperature creep deformation and short lifespan. This paper predicts retort creep deformation via three-dimensional (3D) finite element simulation using creep model constants obtained by experimental tests. Simulation results were verified by comparing the ovality of cross-sections along the axial length of an experimentally failed retort with <10% error. The discrepancies are resulted from the crack formation by creep at the grain boundaries and further inside the enlarged grains accelerated by precipitation and internal oxidation. Results spotlighted the detrimental effect of a temperature gradient at the retort's cool end on localized deformation along its length. Optimization results indicated that retort wall thickness exerts more significant effect compared to the retort diameter on lifespan and economic performance (three times). Retort dimensions of 325 mm inner diameter and 35 mm wall thickness represent the most economical geometry.

Transformation super plasticity deformation of reduced activation ferritic/martensitic steel

Reduced Activation Ferritic/Martensitic (RAFM) steel is a promising candidate for the blanket structural materials of fusion reactors. One of the key issues in the use of RAFM for the blanket is its low workability. As a solution of this issue, innovative processing technology based on the transformation super plasticity (TSP) was proposed. In general, TSP is known as high temperature creep deformation which is induced by phase transformation. In this study, tensile tests under constant load were carried out with cyclic temperature variation for a RAFM steel to investigate the effect of cyclic temperature variations on elongation. The result of the tensile test under constant load for the RAFM steel with cyclic temperature variations exhibited a macro-elongation to 172%. The results of the tensile tests with varying loads showed the possibility of reducing the necessary time and enhancing the controllability for the formation of RAFM steel products using TSP.