Dislocation Microstructure Research Articles

Microstructural complexity and dimensional changes in heavily irradiated zirconium

Using atomistic simulations based on the creation-relaxation algorithm, we explore the evolution of microstructure in irradiated zirconium over a broad range of radiation exposure. In agreement with experimental observations, we find that at relatively low temperatures, microstructure evolves towards an asymptotic dynamic steady state forming at doses close to 1 dpa. Simulations show the spontaneous formation of $a$-type interstitial dislocation loops, gradually transforming into an $a$-type extended dislocation network and giving rise to macroscopic anisotropic dimensional changes in a textured material. A fully developed $a$-type interstitial/vacancy and $c$-type vacancy dislocation microstructure corresponds to the highest degree of irradiation growth anisotropy and vanishingly small volumetric swelling of the material.

Dislocation microstructure and its influence on corrosion behavior in laser additively manufactured 316L stainless steel

The hierarchical nature of additively manufactured materials necessitates a multimodal approach for quantifying microstructural features and corresponding chemical heterogeneities that ultimately impact their properties and performance. In laser powder-bed fusion (L-PBF) 316L stainless steel, corrosion behavior has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. Here, we employ a suite of advanced synchrotron x-ray techniques and correlative transmission electron microscopy for the analysis of microstructure and chemical heterogeneities in L-PBF 316L as a function of printing speed. Our findings reveal an appreciable dislocation density consistent with the formation of a cellular dislocation microstructure in L-PBF 316L, which is correlated to spatial variations in the local Cr concentration and the formation of complex Mn 7 C 3 nanoinclusions. Cyclic voltammetry experiments reveal that relative to wrought 316L, the printed samples exhibit either a comparable or marginally reduced susceptibility to uniform corrosion but with an increased affinity for pitting particularly in the samples printed at the highest speed with the largest dislocation density. Given the spatial correlations between regions of high dislocation density and the formation of chemical heterogeneities known to degrade corrosion performance, our findings demonstrate the impact of the microstructural defect state and its variation with printing speed on the resistance of L-PBF 316L to uniform and localized corrosion.

Use of Multi-Modal Synchrotron Analytical Techniques Combined with Electrochemical Analysis to Understand Anomalous Localized and General Corrosion Behavior of Additively Manufactured 316L Stainless Steel

Additive manufacturing (AM) of alloys such as stainless steels has the potential to be a disruptive technology for design of complex structures which eliminates the potentially damaging effects of mechanical failure and localized corrosion associated with interfaces between separate parts (which can now be printed as part of a single assembly). In addition, increasingly complex geometries can be manufactured with nearly as much ease as simple ones, and new mechanisms for optimization and generative design can allow manufacturers to achieve significant material and weight savings. However, challenges remain, in particular due to the potential for material variability both between parts made with the same nominal build parameters and even within a single build, as the consistency in properties inherent in the commercial-scale forging of alloys to be machined into components is exchanged for the considerable advantages of on-site, distributed custom parts production. Our studies and related work by other groups indicates that this is clearly true in the case of corrosion susceptibility in AM alloys. By studying the relationship between build parameters and electrochemical properties, we propose that it will be possible to tailor alloys for enhanced corrosion properties.AM processes produce materials with hierarchical microstructures containing fusion boundaries at the macroscale, irregular grains and grain boundaries at the microscale, and subgrain dislocation structures at the nanoscale. In laser powder bed fusion (LPBF) formed 316L stainless steel structures, corrosion performance has been discussed in the context of chemical heterogeneities formed in the presence of these hierarchical microstructures. Furthermore, localized corrosion (pitting) is closely related to microstructural defects including dislocation structures, inclusions, and localized elemental segregation which has been found to occur during the very rapid heating and cooling which occurs during laser fusion AM processes.In this presentation, we explore multi-modal synchrotron X-ray techniques for quantifying hierarchical microstructures and their connection to the underlying chemical distribution in 316L stainless steel, as well as localized heterogeneities in the passive film structure. Our results show that the dislocation density depends on the printing conditions with implications for the chemical distribution at the nanoscale, which in turn may play a key role in inconsistent corrosion behavior. Furthermore, we discuss the correlation between this print-formed alloy surface microstructure (dislocation structures, Mn inclusions, and elemental segregation), and local heterogeneities in the passive film formed on LPDF 316L in Cl-containing media. These heterogeneities observed in the passive layer may in turn impact repassivation and other processes critical in localized cession phenomena.Samples formed at varying speeds using pulsed and continuous laser deposition are compared via cyclic polarization in 3.5% NaCl and 0.1M HCl solutions, as well as using Mott-Schottky analysis of passive layers formed in 0.1M HCl. Surface corrosion layers are characterized using laboratory-based X-ray photoelectron spectroscopy to determine the impact on passivity of the aforementioned microsegregation associated with process-induced microstructures. Corrosion rates and pit densities are then discussed to build a connection between printing conditions and corrosion performance vis-à-vis microstructural and chemical information from synchrotron measurements performed at Brookhaven National Laboratory (BNL), including high energy X-ray diffraction using the X-ray Powder Diffraction (XPD) beamline at the National Synchrotron Light Source-II (NSLS-II) which provided data on minor precipitate population(s), atomic structure and dislocation microstructures critical to corrosion behavior. In addition, heterogeneities in elemental composition data is provided by 2D X-ray fluorescence (XRF) and 2D X-ray Absorption Spectroscopy (XAS) nanometer resolution-mapping obtained at the Hard X-ray Nanoprobe (HXN) beamline at the NSLS-II and correlated with process-induced hierarchal structures via use of the microscopy facilities at the Center for Functional Nanomaterials (CFN). X-ray photoemission microspectroscopy/low energy electron microscopy (XPEEM/LEEM) conducted at the Electron Spectro-Microscopy beamline at the NSLS-II provides evidence that the passive layer itself contains inhomogeneities, including areas enriched in oxidized Cr and Mn, as well as microstructural boundaries, reflecting the inhomogeneities in the microstructure of the alloy surface beneath. Mott-Schottky analysis of the passive layer provides further evidence that these inhomogeneities, correlated with alloy microstructure, impact the semiconducting character (and hence protective properties) of the passive layer.By employing a combination of multi-modal synchrotron characterization techniques, electron microscopy, and baseline testing protocol combined with electrochemical polarization experiments, we are able to study the role of microstructure across multiple length scales on electrochemical properties and corrosion performance. Understanding the impact of microstructure and chemical heterogeneities on the susceptibility to pitting and intergranular attack will enable microstructurally-informed process optimization and materials design for enhancing the corrosion resistance of LPBF 316L stainless steel, with implications for AM and subsequent durability of alloys in general.

Dislocation density model and microstructure of 7A85 aluminum alloy during thermal deformation

Dislocation density model and microstructure of 7A85 aluminum alloy during thermal deformation

Evaluation of dislocation activities and accumulation in cold swaged CoCrFeMnNi high entropy alloy

The CoCrFeMnNi high entropy alloys (HEAs) performs large work strengthening with excellent deformability. The work strengthening and deformation mechanism are facilitated by dislocation activities and dislocation accumulation. In this study, the dislocation density and microstructure of the CoCrFeMnNi HEA were characterized by the neutron line profile analysis using convolution multiple whole profile (CMWP) method and EBSD in a comparison with a binary FeNi alloy. The CoCrFeMnNi HEA and FeNi alloy were plastically deformed by rotary swaging until 85% area reduction. The characteristics (e.g., low stacking fault energy, local variation with different atom species) rising from the high compositional complexity of the CoCrFeMnNi HEA cause different dislocation activities and levels of dislocation accumulation from the binary FeNi alloy. The dislocation density of the CoCrFeMnNi HEA continuously increased during the cold swaging and was significantly larger than the FeNi alloy. The larger dislocation accumulation of the CoCrFeMnNi HEA is facilitated by higher compositional complexity, extensive dislocation arrangement, and strong grain fragmentation. The increasing heterogeneity of dislocation distribution in the CoCrFeMnNi HEA was contributed by dislocation cell formation and increasing geometrically necessary dislocations (GNDs). The larger work strengthening in CoCrFeMnNi HEA is correlated with large total dislocation density during cold swaging.

Particle size effects on dislocation density, microstructure, and phase transformation for high-entropy alloy powders

Recrystallization and phase transformation reactions are driven by energy stored in the parent phase due to previous processing history. Grain boundaries, lattice strain, and dislocation networks may affect subsequent microstructural evolution, especially during metal powder consolidation processes. In this work, AlCrFe2Ni2 eutectic high-entropy alloy powders over a wide range of size distribution were used to investigate the correlation between dislocation density and microstructure as a function of particle size. Line profile analysis of the X-ray diffraction patterns of the as-received (quenched) samples shows that the dislocation density increases linearly with decreasing particle size. Based on microstructure analysis of the as-quenched and the annealed samples, it is found that the correlation is associated with the grain boundary length which increases with decreasing particle size, revealing that the key source of dislocation density is dislocations within the grain boundaries. The grain boundary energy acts as a driving force for the metastable to stable phase transformation of AlCrFe2Ni2, showing that the phase transformation kinetics is a function of particle size. This work shows a direct experimental observation and quantitative analysis that in metallic powder systems particle size is one of the key parameters which affects the dislocation density and the phase transformation kinetics most probably due to the different cooling rates achieved during powder production.

Orientation dependent hardening of {111} plate precipitate by parametric dislocation dynamics

The effects of the orientation, geometry of slip plane, existence of cross slip on the dislocation interaction with a {111} plate precipitate has been investigated by parametric dislocation dynamics based on Green's function method. The dislocation bypassing process is greatly influenced by the geometrical relation of the {111} precipitate and the dislocation Burgers vector, where the internal stress of the misfitting precipitate is found to be critical to the orientation dependent hardening behaviour of the {111} precipitation variants classified as Type-A (intersected by 0 deg.), Type-B (intersected by 60 deg.) and Type C variant (parallel to slip plane). The energy analysis reveals that the orientation dependent hardening in the simulated stress strain curves is attributed to the topological change in the dislocation microstructure influenced by the internal stress around {111} variants, where the cross slip enhances the increase of the dislocation self-energy. Hence, the simultaneous hardening mechanism by the misfit precipitates and dislocations is suggested.

Effect of size and orientation on stability of dislocation networks upon torsion loading and unloading in FCC metallic micropillars

At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, they exhibit strong anisotropic behaviors as the size of sample decreases to micron and nanometer length scales. In this study, three-dimensional dislocation dynamics (DD) simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal networks formed in [101] and [111] oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery.

Effect of Rolling Temperature and Subsequence Ageing on the Mechanical Properties and Microstructure Evolution of an Al-Cu-Li Alloy

The mechanical properties and microstructure evolution of an Al-Cu-Li alloy sheet processed via hot rolling (HR) (at 400 °C and 500 °C) or cryorolling (CR) (at −100 °C and −190 °C) and subsequence aging at 160 °C for 10 h were investigated. Before aging, the highest ultimate tensile strength of 502 MPa was achieved when the sheets were cryorolled at −190 °C, while the better ultimate tensile strength of 476 MPa and the best elongation rate of 11.1% was achieved simultaneously when the sheets were cryorolled at −100 °C. The refined grains and numerous uniform deformation-induced dislocations microstructures were responsible for the improved strength and enhanced ductility of the cryorolled sheets compared to that of the alloy processed by hot rolling with a low dislocation density zone (LDDZ) and high dislocation density zone (HDDZ). After aging at 160 °C for 10 h, the ultimate tensile strength further improved resulted from the greater precipitation strengthening, and the increased precipitates provided greater resistance to dislocations movement resulting in the increased ductility although the dislocation density decreased. The uniform dislocation microstructures in the cryorolled sheets provide numerous nucleation sites for the precipitates, leading to higher strength after aging.

The dislocation structure of slip bands in deformed high entropy alloy nanopillars

• Slip bands formed in compressed Al 0.1 CoCrFeNi nanopillars are examined by TEM. • A slip band consists primary and secondary dislocations with the latter mostly behind the slip band. • The role of dislocation pileup in front of the slip band is discussed. • A slip band model is proposed based on browns ellipsoidal slip band with supporting evidence of dislocation dipoles. Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sudden crystal slips. While large crystal slips can be easily observed on the surface of deformed crystals, unraveling the underlying dislocation interaction mechanisms, however, has been a longstanding challenge in the study of single-crystal plasticity. A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy (HEA) of Al 0.1 CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar. Here, we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip. Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations, secondary dislocations, and dislocation pileups. The results were compared with the previously proposed slip band models. The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars.

Dislocation electron tomography: A technique to characterize the dislocation microstructure evolution in zirconium alloys under irradiation

Diffraction-contrast electron tomography was used to analyse the 3D geometry of the dislocation microstructure in a zirconium alloy before and after ion irradiation. The material had been strained at room temperature prior to irradiation. After straining, the material exhibited mainly screw dislocations with <a> Burgers vectors. From the analysis of the habit plane of <a> dislocations with non-screw segments, it was deduced that they have glided mainly in the prismatic planes and to a lesser extent in the first order pyramidal planes. After irradiation, dislocation loops with <a> Burgers vectors were observed. It was shown that the loops are not pure edge and have their habit plane located around the planes {101¯0}, tilted up to 20° towards the planes (0001) and {112¯0}. Furthermore, it was proven that the initial screw dislocations have climbed under irradiation. Several dislocations were also found to have interacted with loops during climb. The climb of <a> dislocations under irradiation is an important mechanism that can explain part of the in-reactor deformation of zirconium alloys when subjected to simultaneous mechanical loading and irradiation. Interactions between dislocations and loops occurring during dislocation climb may also play a significant role on the in-reactor deformation of zirconium alloys.

Cell structure formation in a two-dimensional density-based dislocation dynamics model

Cellular patterns formed by self-organization of dislocations are a most conspicuous feature of dislocation microstructure evolution during plastic deformation. To elucidate the physical mechanisms underlying dislocation cell structure formation, we use a minimal model for the evolution of dislocation densities under load. By considering only two slip systems in a plane strain setting, we arrive at a model which is amenable to analytical stability analysis and numerical simulation. We use this model to establish analytical stability criteria for cell structures to emerge, to investigate the dynamics of the patterning process and establish the mechanism of pattern wavelength selection. This analysis demonstrates an intimate relationship between hardening and cell structure formation, which appears as an almost inevitable corollary to dislocation dominated strain hardening. Specific mechanisms such as cross slip, by contrast, turn out to be incidental to the formation of cellular patterns.

3D discrete dislocation dynamic investigations of persistent slip band formation in FCC metals under cyclical deformation

The formation of persistent slip bands (PSBs) at early stages of fatigue plays an important role in fatigue damage. In order to get a better understanding of the physics involved at the dislocation scale, 3D discrete dislocation dynamic simulations are set up in the case of an isolated surface grain of Cu cyclically loaded in single slip. Simulations reveal the complex transformation process of the dislocation microstructure from a homogeneous distribution into an organized microstructure made of slip bands mainly composed of dislocation dipoles and prismatic loops. These slip bands are the embryo of the PSBs observed for large number of cycles. The localization mechanism evidenced by the simulations is well explained from a stress field analysis which highlights the role of cross-slip. A saturation regime is reached after a few cycles during which dislocation microstructure, dislocation densities and height of extrusions/intrusions remain unchanged. Furthermore, the linear relationship between the volume fraction of slip bands and the imposed plastic strain amplitude is retrieved by the simulations and the stability of the slip bands is verified by decreasing the plastic strain amplitude after the saturation regime is reached. Finally, when compared to AISI 316L stainless steel, Cu shows a tendency to build more bands with relatively larger thicknesses which is attributed to the differences in the cross-slip probability.

The characteristics of lamellar bands developed in cold-rolled Ta[sbnd]10W alloy at low strains

Elongated lamellar bands parallel to rolling plane are usually formed in γ-fiber of TaW alloys at high strains. However, in this study, a type of lamellar bands oriented with α-fiber is formed in Ta10 W alloy cold-rolled to 5%. Meanwhile, island equiaxed grains with special orientations can be seen in the lamellar bands. Furthermore, the lamellar bands are not homogenously distributed in the whole samples and concentrated in some elongated-grain bands. Although the lamellar bands are composed of extending planar dislocation boundaries with one or two dislocation cells across, they cannot result from incremental rotation of microbands or microshear bands, since no shear bands appear in these lamellar bands at such low strain. Therefore, the lamellar bands can be initiated by the retained elongated lamellar boundaries after annealing. They show a persistent microstructural feature as the ratio of reduction increases. As the hardness and dislocation density of the persistent lamellar bands is quite different from that of other deformed microstructures, these bands are bad for plastic deformation process. Meanwhile, protrusions at high-angle lamellar boundaries migrating to the recovered lamellar bands are observed during annealing because of the inhomogeneous dislocation microstructure. The lamellar bands can be fully eliminated when the annealing temperature reaches 1500 °C.

Effect of microstructure, layer thickness, and interface behavior on the plasticity of accumulative roll bonded nanometallic laminates using dislocation dynamics simulations

This work aims to understand plastic behavior in different layers of accumulative roll bonded Fe/Al nanometallic laminates (NMLs) using dislocation dynamics simulations. Our simulations start with a synthetic microstructure obtained with an initial pre-deformation followed by a relaxation under zero stress, to represent defect content akin to these NMLs. Subsequently we observe their plastic responses under uniaxial stresses along the transverse (parallel to interface normal) and longitudinal (perpendicular to interface normal) directions for different layer thicknesses. Our simulations suggest significant influence of pre-deformation on the initial dislocation microstructure that also introduces significant plastic heterogeneity between the different layers in the NMLs. We also observe a higher plastic strain per dislocation for Fe layer than the Al layer, as well as, a higher co-deformation and plastic strain under longitudinal loading as compared to transverse loading. Finally comparing the plastic responses from a rigid and a glide interface description, we find that a glide interface doesn’t always account for a higher strain.

Data-mining of dislocation microstructures: concepts for coarse-graining of internal energies

Continuum models of dislocation plasticity require constitutive closure assumptions, e.g., by relating details of the dislocation microstructure to energy densities. Currently, there is no systematic way for deriving or extracting such information from reference simulations, such as discrete dislocation dynamics (DDD) or molecular dynamics. Here, a novel data-mining approach is proposed through which energy density data from systems of discrete dislocations can be extracted. Our approach relies on a systematic and controlled coarse-graining process and thereby is consistent with the length scale of interest. For data-mining, a range of different dislocation microstructures that were generated from 2D and 3D DDD simulations, are used. The analyses of the data sets result in energy density formulations as a function of various dislocation density fields. The proposed approach solves the long-standing problem of voxel-size dependent energy calculation during coarse graining of dislocation microstructures. Thus, it is crucial for any continuum dislocation dynamics simulation.

Application of electron tomography of dislocations in beam-sensitive quartz to the determination of strain components

In this study we apply electron tomography of dislocations to quartz with a view to assess whether the von Mises-Taylor criterion is satisfied or violated in a deformed crystal of quartz. We propose a method to perform electron tomography with few projected images which allows extension of this technique to beam-sensitive materials such as quartz. The 3D characterization of the dislocation microstructure allows the evaluation of contributions to dislocation mobility with no ambiguity. From the geometrical characteristics of the dislocations and their Burgers vectors, we show how to identify the non-zero components of the strain tensor. We show that in the quartz grain investigated, the von-Mises-Taylor criterion is satisfied thanks to the climb of 〈c + a〉 dislocations.

Evolution of dislocation microstructure in irradiated Zr alloys determined by X-ray peak profile analysis.

During neutron irradiation of metals, owing to the enhanced number of vacancies and interstitial atoms, the climb motion of dislocations becomes significant at room temperature, leading to a recrystallization of the material. Moreover, the vacancies and interstitial atoms tend to form prismatic dislocation loops that play a crucial role in the plastic properties of the materials. X-ray peak profile analysis is an efficient nondestructive method to determine the properties of dislocation microstructure. In the first half of this article, the foundation of the asymptotic peak broadening theory and the related restricted-moments peak-evaluation method is summarized. After this, the microstructural parameters obtained by X-ray peak profile analysis are reported for irradiated E110 and E110G Zr alloys used as cladding material in the nuclear industry.

Influence of non-homogeneous microstructure on hydrogen diffusion and trapping simulations near a crack tip in a welded joint

Hydrogen assisted fracture near welds is the result of a combination of microstructural changes and the accumulation of hydrogen. With the aim of predicting local hydrogen concentrations, hydrogen redistribution near a crack tip is simulated using a Boundary Layer approach and diffusion modelling is modified by trapping phenomena. The simulated non-homogeneous geometry includes layers that reproduce weld metal, heat affected zones and base metal of a 2.25Cr-1Mo steel; mechanical and diffusion properties have been extracted from references. The hydrogen transport model here considered involves a stress dependency that affects local concentrations; thus, the possible interaction between constraint effects associated to a graded material with hydrogen entry and transport is studied. Results show that the constraint effect is not significative for the loading and for the widths assigned to the weld and the heat affected zones (2.5 to 5 mm); however, for the HAZ-centred crack, a higher hydrostatic peak and the corresponding increase in lattice hydrogen are found. A two-type trapping process is also simulated to reproduce simultaneously the effect of dislocation trapping and microstructure delayed diffusion. Hydrogen is weakly trapped in dislocations and it is added to the lattice concentration to obtain a measure of diffusible hydrogen near a crack tip while effective diffusivity is strongly reduced by deeply trapped hydrogen. Differences between environmental or internal hydrogen sources are expected to be more accurately captured because stress-dependent boundary conditions have been implemented for hydrogen uptake.

Study on high temperature creep behavior of the accident-resistant cladding Fe–13Cr–4Al-1.85Mo-0.85Nb alloy

The Fe–13Cr–4Al-1.85Mo-0.85Nb alloy, as accident-resistant fuel cladding material in light water reactor (LWR), was prepared by a vacuum induction melting, forging and hot rolling process. In this study, the creep behaviors of the alloy at the temperature range of 400–450 °C were studied. The effects of rolling direction (RD), transverse direction (TD), creep temperatures and stresses on the creep properties were systematically investigated. The results show that the steady-state creep rates of the FeCrAl alloy increase with the increase of the loaded stress, which vary in the range of 8.81 × 10 −10 s −1 to 8.40 × 10 −8 s −1 in the RD and 8.30 × 10 −9 s −1 to 5.17 × 10 −7 s −1 in the TD. Based on the analysis of the real stress exponent n equaling to 5.0, the creep processes in the both RD and TD are controlled by dislocation climbing mechanism, as evidenced by the microstructure of dislocations tangled with precipitations after creep test. At 540 MPa and the temperatures of 400–450 °C, the creep activation energies in the RD and TD are 421.06 kJ mol −1 and 424.7 kJ mol −1 respectively. Before creep test, the dispersion of fine Mo-, Nb-, and Si-rich Laves phases is observed. After creep test, ‘spheroidization’ of these secondary Laves phases obviously occurs. EBSD analysis shows that the grains in the TD are smaller and equiaxed, while the grains in the RD are large and deformed. This is one of the reasons on the big differences of the creep rate of the alloy in the TD comparing with the RD. The average geometrically necessary dislocations (GNDs) of the cross-sectional structure of the alloy decrease after creep test in both the RD and TD. Moreover, the creep rate in the RD is about 1/10 of that in the TD, mainly due to the large elongated grains with less grain boundaries and higher GNDs in the RD. The obtained conclusions provide researchers and engineers with useful references for the engineering application of the FeCrAl alloy as accident-resistant fuel cladding material. • Creep process of the FeCrAl alloy is controlled by dislocation climbing mechanism. • Creep properties and reasons on anisotropy of creep rates of the alloy are revealed. • The creep rates of the alloy in the RD is around 1/10 of that in the TD. • GNDs of the cross-sectional structure of the alloy decrease during the creep process. • Creep activation energies in the RD and TD are 421.06 and 424.7 kJ mol −1 respectively.