Substrate Creep Research Articles

Implementation of a viscoplastic substrate creep model in the thermomechanical simulation of the WAAM process

This research work focusses on the implementation of a viscoplastic creep model in the thermomechanical simulation of the wire arc additive manufacturing (WAAM) process for Ti-6Al-4 V structures. Due to the characteristic layer by layer manufacturing within the WAAM process, viscoplastic material effects occur, which can be covered by implementing a creep model in the thermomechanical simulation. Experimental creep tests with a wide temperature, load and time range were carried out to examine short-term creep behaviour in particular. A Norton-Bailey creep law is used to accurately fit the experimental data and describe the base material’s creep behaviour. Subsequently, the fitted Norton-Bailey creep law was implemented in the thermomechanical simulation of the WAAM process. Finally, to determine the effect of creep on global distortion and local residual stress state in the substrate, simulations of a simplified linear, three-layer WAAM structure, with and without applying the implemented creep law, were carried out and compared to experimental data. The thermomechanical simulation with implemented creep model reveals a significant improvement in the numerical estimation of distortion and residual stress state in the substrate. The maximum distortion is reduced by about 13% and respectively the mean absolute percentage error between simulation and experiment decreases by about 34%. Additionally, the estimation accuracy with respect to the local residual stress state in the substrate improved by about 10%.

Damage evolution of a thermal barrier coating system with 3-dimensional periodic interface roughness: Effects of roughness depth, substrate creep strength and pre-oxidation

Damage evolution during thermocyclic loading of atmospheric plasma sprayed thermal barrier coating (TBC) systems with 3-dimensional (3-D) periodical quadratic, hexagonal and stochastical interface roughness was compared. Furthermore, effects of roughness depth, substrate creep strength and pre-oxidation were investigated. As substrates planar Fecralloy EisenChrom™ was used. The samples with periodical hexagonal roughness showed on average near cubic oxidation kinetics and faster oxidation at the roughness valleys than at the peaks. The coatings on all samples with 3-D quadratic roughness failed immediately after TBC-deposition. For the other profiles excessive segmentation and continuous crumbling of the TBCs occurred. However, the coatings mostly did not spall-off during 836 oxidation cycles. Thermography permitted to detect <1mm diameter spot-shaped initial delaminations (IDs) that formed during thermal cycling. With progressive cycling their number and area increased until they merge. The positions where IDs formed correlated with locally larger TBC thicknesses associated with the spray path during TBC deposition. Samples with pre-deposited oxide layer showed significant larger delamination areas after thermal cycling. Sample bending towards the TBC and deformation of the substrate below the TBC occurred, indicating stress relaxation. Despite the partially large delaminations, this relaxation led to the prevention of macroscopic TBC detachment and large area spallation till the end of the thermal cycling tests. Cross-sections of the 3-D periodic roughened samples revealed no effect of the roughness depth on the amount of cracking or on damage evolution after certain cycle numbers.

New insight into crack formation during corrosion of zirconium-based metal-oxide systems

Zirconium alloys are typically used in nuclear pressurized water reactors (PWR) as fuel cladding tubes due to their chemical stability and their mechanical strength at operating temperatures (≈300°C). However, the corrosion of Zr-based cladding tubes is one of the factors limiting the burn-off rate in PWRs. It is commonly accepted that the corrosion kinetics involves a periodic succession of growth where the oxide thickness varies parabolically with time. As the oxide thickens, a cracking structure forms. The oxide appears striated with periodic layers of cracks running parallel to the metal/oxide interface. This cracking structure has been experimentally related to the periodicity of the oxide growth. In the present work, a finite-element study is used to investigate the development of stresses in the oxide under the combined influence of molar volume expansion during oxide formation, metal/oxide interface geometry and metallic substrate creep. The generation of tensile stresses capable of initiating the cracks that are observed experimentally is explored.

Influence of Interface Roughness, Substrate and Oxide-Creep on Damage Evolution and Lifetime of Plasma Sprayed Zirconia-based Thermal Barrier Coatings

Abstract The stress state in thermal barrier coating (TBC) systems for gas turbine blades during thermal cycling is mainly governed by (i) the thermal and mechanical mismatch between the ZrO2-base topcoat and the Ni-base substrate alloy, (ii) the roughness profile at the interface between MCrAlY-bondcoat (BC) and TBC, and (iii) plastic deformation and creep in the BC and in the oxide scale growing at the interface between TBC and BC. In the present study, the influence of (ii) and (iii) on the thermal cycling lifetime was studied using a simplified model system consisting of (a) FeCrAlY substrates with different creep strength, i.e. a conventional and an oxide dispersion strengthened alloy, (b) oxide layers with different grain size, i.e. a coarse grained version applied by oxidation and a nanocrystalline version applied by sputtering and (c) a plasma sprayed ZrO2-TBC stabilized by 8% Y2O3. A trend of decreasing lifetime with increasing creep strength of the FeCrAlY was observed. Infrared pulse thermography analysis of delamination crack growth during thermal cycling tests showed that in case of low creep strength substrates, small delaminations grow continuously, link to each other and finally lead to spallation of the TBC, whereas in the case of high creep strength substrates fast crack growth and spallation occurs as soon as one delamination exceeds a diameter of approx. 2 mm. A fracture mechanics model for delamination crack growth is able to roughly describe this behavior. The influence of oxide creep seems to be less pronounced than that of substrate creep. However, the highest lifetimes were observed for the combination of Fecralloy substrate and nanocrystalline oxide layer. The influence of roughness on lifetime remains unclear due to a wide lifetime scatter.

Thermal cycling damage evolution of a thermal barrier coating and the influence of substrate creep, interface roughness and pre-oxidation

Abstract The influence of roughness profile shape, roughness depth, bond coat creep strength and pre-oxidation on the thermal cycling damage evolution and lifetime of a plasma-sprayed ZrO2 thermal barrier coating system was investigated. A simplified model system was used where FeCrAlY substrates simulated the bond coat. Substrate creep was varied by using the oxide dispersoid strengthened alloy MA956 and the conventional material Fecralloy. Stochastic 3- and periodic 2-dimensional roughness profiles were produced by sand blasting and high speed turning. Damage evolution is significantly influenced by substrate creep with a trend to higher lifetimes for the fast creeping substrate. Pre-oxidation has no influence. Lifetimes of the periodically profiled samples are up to 100 times lower than these of stochastically profiled samples. In the case of periodically profiled samples, the highest lifetime was reached for the highest roughness depth combined with local undercuttings in the roughness profile. For stochastically profiled samples the influence of roughness depth could not be determined due to the wide lifetime scatter.

Studies Regarding Corrosion Mechanisms in Zirconium Alloys

Abstract Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415°C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360°C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre- and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360°C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.

The influence of substrate creep on mesenchymal stem cell behaviour and phenotype

Human mesenchymal stem cells (hMSCs) are capable of probing and responding to the mechanical properties of their substrate. Although most biological and synthetic matrices are viscoelastic materials, previous studies have primarily focused upon substrate compressive modulus (rigidity), neglecting the relative contributions that the storage (elastic) and loss (viscous) moduli make to the summed compressive modulus. In this study we aimed to isolate and identify the effects of the viscous component of a substrate on hMSC behaviour. Using a polyacrlyamide gel system with constant compressive modulus and varying loss modulus we determined that changes to substrate loss modulus substantially affected hMSC morphology, proliferation and differentiation potential. In addition, we showed that the effect of substrate loss modulus on hMSC behaviour is due to a reduction in both passive and actively generated isometric cytoskeletal tension caused by the inherent creep of substrates with a high loss modulus. These findings highlight substrate creep, or more explicitly substrate loss modulus, as an important mechanical property of a biomaterial system that can be tailored to encourage the growth and differentiation of specific cell types.

Anisotropic TGO rumpling in EB-PVD thermal barrier coatings under in-phase thermomechanical loading

An electron beam physical vapor deposited (EB-PVD) Y 2O 3–ZrO 2 thermal barrier system has been tested under in-phase thermomechanical fatigue (TMF) conditions with thermal gradient in the through-thickness direction. Undulations in the thermally grown oxide (TGO) were observed to have clear anisotropic behavior with respect to the directions parallel and perpendicular to the loading axis. It was found that undulation wavelengths were nearly the same in both directions but the amplitude in the perpendicular direction was much larger than in the parallel direction. A recent model of TGO rumpling was adapted and used to analyze and explain the origins of the observed rumpling behavior under TMF conditions. Methods for deducing variation in the coefficient of thermal expansion with temperature and in the creep properties of the substrate from the experimental strain data are also presented in the course of the derivations. Model results show that tensile stress applied in the loading direction can overcome the compression occurring from lateral expansion during oxide formation, causing undulations to flatten; undulations perpendicular to the loading axis are unaffected. However, ratcheting in the strain cycle experienced by the substrate, which occurs naturally by substrate creep, is necessary for anisotropic rumpling under cyclic stress conditions. Model predictions for constant applied stress are also presented, demonstrating a reversal in the direction of undulation alignment under compression. A threshold stress is identified, in both tension and compression, sufficient to produce appreciable anisotropic rumpling. The model predictions provide a clear mechanism for the anisotropy and further evidence that the lateral expansion strain in the oxide is the driving force for oxide rumpling.

Stress Induced by Constrained Sintering of 3YSZ Films Measured by Substrate Creep

3YSZ green layers approximately 10 μm thick were screen printed onto 3YSZ substrates up to 300 μm in thickness. The stress induced by constrained sintering of the film (between 1150° and 1350°C) was measured by monitoring the bending displacement of vertical strips of bilayers using a long‐distance microscope. In order to deduce the stress it was first necessary to measure the creep properties of the substrates by monitoring the bending of horizontal beams under gravity. The creep strain rate of the 3YSZ substrates was linearly dependent on applied stress at the low stresses and strains involved in the present work. The creep viscosity appeared to increase with strain (time), which might be due to changes in grain‐boundary composition, and had higher activation energy at temperatures above approximately 1250°C. The magnitudes of the creep viscosities are in reasonable agreement with other creep data in the literature for 3YSZ. The in‐plane stress induced during constrained sintering of the 3YSZ films had a maximum value of approximately 3 MPa at 1200°C. This behavior is consistent with literature results reported for constrained sintering of bulk alumina. The stress induced by the constraint is of a similar order to the estimated sintering potential.

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.

Substrate creep on the fatigue life of a model dental multilayer structure

In this article, we investigated the effects of substrate creep on the fatigue life of a model dental multilayer structure, in which a top glass layer was bonded to a polycarbonate substrate through a dental adhesive. The top glass layers were ground using 120 or 600 grit sand papers before bonding to create different subsurface crack sizes and morphologies. The multilayer structures were tested under cyclic Hertzian contact loading to study crack growth and obtain fatigue life curves. The experiment results showed that the fatigue lives of the multilayer structures were impaired by increasing crack sizes in the subsurfaces. They were also significantly reduced by the substrate creep when tested at relatively low load levels, i.e. P(m) < 60 N (P(m) is the maximum magnitude of cyclic load). But at relatively high load levels, i.e. P(m) > 65 N, slow crack growth was the major failure mechanism. A modeling study was then carried out to explore the possible failure mechanisms over a range of load levels. It is found that fatigue life at relatively low load levels can be better estimated by considering the substrate creep effect.

Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.

An elastic/plastic analysis of the intrinsic stresses in chemical vapor deposited diamond films on silicon substrates

Diamond films were grown over Si substrate at 1223 K by the hot filament chemical vapor deposition (CVD) method using CH4/H2 gas mixtures (1%, 2%, 3% CH4,) and the intrinsic stresses in the film were deduced using the ex situ curvature method. After subtracting the curvature change during the cooling process, average stresses in the film during the CVD process, 〈σf〉, were calculated using the elastic/plastic analysis which treated the creep deformation of the substrate. The intrinsic stress kept increasing during the CVD process and was generally several times larger than the 〈σf〉 which tended to saturate around the film thickness (tf) of 10 μm. For thicker films, substrate creep became significant and the substrate stress was substantially relaxed by creep. Fraction of the creep strain with respect to the total strain at the film–substrate interface was around ∼1/3 when tf∼10 μm, and increased as large as 2/3 during the film deposition. The intrinsic stress was believed to arise from the grain growth during the CVD process, because the stress deduced from the actual grain size measurements agreed reasonably with calculated values from the above analysis. Later, the diamond film layer was removed by O2 electron cyclotron resonance etching, and the remaining curvatures of the substrate were compared with those deduced from the elastic/plastic analysis. Residual stresses in the substrate after the film removal were tensile near the interface and the substrate bottom but compressive in the middle.

Residual stress in plasma sprayed Y 2O 3PSZ coatings on piston heads

Yttria—partially-stabilized zirconia (Y—PSZ) thermal barrier coatings (TBCs) were deposited on Al alloy disks under different plasma spraying conditions, using both air plasma spraying (APS) and atmosphere and temperature controlled spraying (ATCS) processes. X-ray diffraction (XRD) was used to make a non-destructive residual stress analysis (RSA) on the surface of coatings, either as-sprayed and after a burner rig test. Regardless of the deposition process the surface of as-prepared coatings was always in a low tensile state (10–35 MPa), with an increases of the stress near the coating edge. The stress intensification in this region, where mechanical failure is supposed to start when the coated components are subjected to thermal and mechanical solicitations, was confirmed by a finite element modelling. The burner rig test validated this picture, showing an interfacial crack propagating from the border towards the center as the main failure mechanism. As a consequence of the thermal cycling, the surfaces stress changed from tensile to compressive. In particular, the stress was high (−150 MPa) in the central region still well-adherent after the test and decreasing towards the edge where the coating was partially detached. The role played by ceramic sintering, Al substrate creep, and Y—PSZ phase instability during the burner rig test, is also discussed.

Computer simulation of oxide scale cracking/healing processes under substrate creep at high temperatures and comparison of the results to experimental data

AbstractA computer programme is described which simulates the cracking and healing processes in protective oxide scales on heat‐resistant alloys under tensile straining at high temperatures. The main aim of the programme has been to calculate the critical strain rates for the maintenance of the protective effect of oxide scales after cracking by healing. The results from simulation calculations using input data for 3 heat‐resistant steels investigated earlier are compared to experimental results for these materials. The comparison reveals good agreement between calculated and experimental data concerning the critical strain rates for the maintenance of the protective effect by crack healing. The programme, thus, offers a tool for the theoretical estimation of critical strain rates when knowing some necessary mechanical data of the scale and the oxidation rate constant.

The influence of oxidation on the steady-state creep of iron at 823–1223 K

Abstract The influence of oxidation on the creep behaviour of high-purity iron at 823–1283 K at constant tensile stresses of 4–16 MN m−2 has been studied with a specially constructed ultra-high-vacuum unit. Steady-state creep was enhanced (oxidation-weakening) as the oxygen pressure was increased from 10−9 to 1 mbar. Oxidation-assisted vacancy injection is believed to enhance dislocation and point-defect mobility, especially if the adhering oxide film is repeatedly fractured and repaired during substrate creep. Oxidation-strengthening appears most likely at very low oxygen pressures where a strong, adherent film can impose a mechanical constraint to substrate creep and may provide a barrier to emergent dislocations. Oxidation was found to enhance the temperature effect on steady-state creep irrespective of the creep deformation mechanism in operation.