Cyclic deformation behavior of austenitic Cr–Ni-steels in the VHCF regime: Part I – Experimental study

A stable (AISI 316L) and a metastable (AISI 304L) austenitic stainless steel were investigated with respect to their VHCF behavior. The focus of the paper lies on the investigation of the cyclic deformation behavior of the two materials at very low stress amplitudes. The 304L steel is characterized by a pronounced cyclic softening during its initial stage of cyclic deformation. In the course of the following loading cycles, a phase transformation (γ-austenite → α′-martensite), accompanied by volume expansion is associated with the reduction of the global plastic strain amplitude and induces compressive stresses in the near surface layer. As a consequence, the material shows no failure up to 10⁹ cycles at 240 MPa. In contrast, the type 316L steel has a higher stacking fault energy and the microstructure remains fully austenitic during cyclic deformation when analyzed by means of magneto-inductive methods. In this case, very localized plastic shear occurs and the slip band topography reveals the formation of pronounced intrusions. Microcracks initiate from these intrusions in the VHCF regime and samples failed also beyond 10⁷ cycles. This study presents a comparative investigation of the damage evolution – including dislocation morphology and phase transformations – during cyclic loading for both materials. The combined effect of the individual deformation mechanisms is investigated for both materials in the context of a microstructure-sensitive simulation discussed in Part II of this study.     

Effect of solid particle impact on light transmission of transparent ceramics: Role of the microstructure

Sand erosion was done on soda lime glass and transparent ceramics such as alumina and magnesium-aluminate spinel with different microstructures. Surface roughness and optical transmission were measured before and after erosion. The increase of surface roughness depends on both the hardness and grain size of the material. Nearly no surface degradation occurs on polycrystalline samples with HV3 > 15 GPa. The decrease of the real in-line transmittance (RIT) after sand blasting is linked to the increase of surface roughness. We have found that this RIT decrease is correlated to three parameters: incident light wavelength, nature of the material (mechanical properties like hardness) and material microstructure. The influence of these will be discussed. Finally, for all polycrystalline ceramics and single crystals, the RIT is only slightly or not altered after sand blasting either at IR or visible wavelengths.     

Gamma irradiation effect on optical and dielectric properties of potassium dihydrogen phosphate crystals

The effect of Co⁶⁰ gamma-ray irradiation on potassium dihydrogen phosphate crystals is investigated at doses ranging from 1 kGy to 100 kGy with different diagnostics, including UV–Vis absorption spectroscopy, fluorescence spectroscopy, DC electrical conductivity, positron annihilation lifetime spectroscopy and Doppler-broadening spectroscopy. The optical absorption spectra show a wide absorption band between 250 and 400 nm after γ-irradiation and its intensity increases with the increasing irradiation dose. The simulation of radiation defects show that this absorption is assigned to the formation of substitutional impurity defects due to Al, Mg ions substituting for K ions. The fluorescence peak at 355 nm blue shifts after irradiation. The fluorescence intensity is observed to increase with the increasing irradiation dose. The positron annihilation lifetime spectroscopy is used to probe the evolution of vacancy-type defects in potassium dihydrogen phosphate crystal. The variation of size and concentration of vacancy-type defects with the different irradiation dose is investigated. The Doppler-broadening spectroscopy gives direct evidence of the formation of irradiation-induced defects. The dc electrical conductivity of γ-irradiated potassium dihydrogen phosphate crystals increases with the increasing irradiation dose when the dose is less than 10 kGy, whereas keeps constant at high irradiation dose of 100 kGy. The increase of electrical conductivity is associated with the increase of the proton defect concentration in the crystal. A possible explanation about the change of proton defect concentration with irradiation dose is presented.     

Crystal plasticity modeling of damage accumulation in dissimilar Mg alloy bi-crystals under high-cycle fatigue

Damage accumulation in Mg AZ31–AZ80 alloy bi-crystals under fatigue loading at room temperature is studied using a modified version of the crystal plasticity finite element model of Abdolvand and Daymond. The model accounts for strain accommodation by both slip and tensile twinning, and is first shown to reasonably describe monotonic single crystal Mg experimental data from the literature. The high cycle fatigue behavior was then investigated in misoriented dissimilar alloy bi-crystals through stress-controlled simulations up to 1000 cycles. Nine different orientation combinations were simulated and the fatigue damage evolution, defined as the cumulative shear strain amplitude, were compared and analyzed. The bi-crystal geometry was used to simulate possible microstructure combinations occurring, for instance within an idealized friction stir weld. Findings suggest that when either of the alloy bi-crystal grains is oriented for basal slip, poor fatigue performance can occur by twinning or slip localization depending upon the neighboring orientation.     

Thermo-mechanical FE model with memory effect for 304L austenitic stainless steel presenting microstructure gradient

The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behavior at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X-ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a large dislocation microstructure gradient in correlation with hardening gradients due to machining. A memory effect, bound to the pre-hardening gradient, was introduced into an elasto–visco-plastic model in order to determine the stress and strain fields at the inner surface. The temperature evolution on the inner surface of the tubular specimen was first computed via a thermo-elastic model and then used for our thermo-mechanical simulations. Identification of the thermo-mechanical model parameters was based on the experimental stabilized cyclic tension–compression tests performed at 20 °C and 300 °C. A good agreement was obtained between numerical stabilized traction–compression cycle curves (with and without pre-straining) and experimental ones. This three dimensional simulation gave access to the evolution of the axial and tangential internal stresses and local strains during the tests. Numerical results showed: a decreasing of the tangential stress and stabilization after 40 cycles, whereas the axial stress showed weaker decreasing with the number of cycles. The results also pointed out a ratcheting and a slightly nonproportional loading at the inner surface. The computed mean stress and strain values of the stabilized cycle being far from the initial ones, they could be used to get the safety margins of standard design related to fatigue, as well as to get accurate loading conditions needed for the use of more advanced fatigue analysis and criteria.     

Anisotropic and temperature effects on mechanical properties of copper nanowires under tensile loading

Atomistic simulations are used to investigate the mechanical properties of copper nanowires (NWs) along 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 crystallographic orientations under tensile loading at different temperatures. The inter-atomic interactions are represented by employing embedded-atom potential. To identify the defects evolution and deformation mechanism, a centrosymmetry parameter is defined and implemented in the self-developed program. The simulations show that Cu NWs in different crystallographic orientations behave differently in elongation deformations. The stress–strain responses are followed by a particular discussion on yield mechanism of NWs from the standpoint of dislocation moving. Generally, the study on the incipient plastic deformation will be helpful to further understanding of the mechanical properties of nanomaterials. In addition, the Young’s modulus decreased linearly with the increase of temperature. The crystal structure is less stable at elevated temperatures.     

Phonon spectrum and related thermodynamic properties of bcc-Fe with an edge dislocation

The phonon spectrum and related thermodynamic properties of perfect bcc-Fe crystal and bcc-Fe with a [1 0 0] edge dislocation are calculated with the recursion method by using F-S N-body potential. For perfect bcc-Fe crystal, our calculated results of phonon spectrum, vibrational entropy, heat capacity and vibrational energy are in excellent agreement with the experiment and related calculation results. The configuration of [1 0 0] edge dislocation obtained by molecular dynamics relaxation shows that a string of interstices is formed along the dislocation line, thus the local vibrational densities of states (LDOS) of atoms in the dislocation core are quite different from the bulk phonon spectrum, and there exist some sharp peaks in certain frequencies. However, for the atoms away from the dislocation core, their LDOS's gradually approach the bulk spectrum. Meanwhile, the local vibrational energies of the dislocation core atoms are also evidently changed except for the atoms on the slip plane. These results indicate that the bondings between the atoms in dislocation core are greatly changed. Furthermore, it is also proved that the average vibrational entropy of bcc-Fe with a [1 0 0] edge dislocation is higher than that of perfect crystal.     

Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels

Austenitic stainless steels have been used for over 100 years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1 weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C + N: 0.65–1.2) and ratios (C/N: 0.13–∞) of C and N. Manson–Coffin analyses revealed distinct differences in the fatigue behaviour to CrNi-alloyed C + N steels investigated earlier. This contribution presents these differences and discusses them in relation to microstructural characteristics as well their alterations under cyclic loading.     

Dislocation density-based modeling of subsurface grain refinement with laser-induced shock compression

Laser shock peening (LSP) is an innovative surface treatment technique applied to improve the mechanical properties and surface microstructures of metallic components. This paper is concerned with prediction of the microstructural evolution of metallic components subjected to single or multiple LSP impacts. A numerical framework is developed to model the evolution of dislocation density and dislocation cell size using a dislocation density-based material model. It is shown that the developed model captures the essential features of the material mechanical behaviors and predicts that the total dislocation density reaches the order of 10¹⁴ m^?2 and a minimum dislocation cell size is below 250 nm for LSP of monocrystalline coppers using the laser energy density on the order of 500 GW/cm². It is further shown that the model is cable of predicting the material strengthening mechanism in terms of residual stress and microhardness of the LY2 aluminum alloy due to grain refinement in a LSP process with less laser energy densities on the order of several GW/cm².     

Synthesis of zeolite NaA at room temperature: The effect of synthesis parameters on crystal size and its size distribution

Zeolite NaA crystals were prepared by hydrothermal synthesis under room-temperature conditions. The products were characterized by XRD, SEM, IR and particle size analysis. Some influence parameters such as crystallization time, aging time, stirring speed, different sources of silicon and aluminum on the crystalline end products were studied. The results showed that crystallization time was a crucial factor for the final products, well-shaped crystals could be obtained at the crystallization time of 72 h. While further prolonging the crystallization time more, crystals continued to grow, along with the changes of crystal size distributions. The crystals obtained with the aging time of 1, 1.5, 2 and 3 h show the mean particle sizes of 368, 356, 338 and 314 nm, with the crystal size distribution ranges of 82–435, 70–441, 54–450 and 40–476 nm, respectively. Longer aging time leads to the mean particle size of crystal decrease. Whilst, the stirring speed affects the particle size distribution only slightly. Moreover, the aluminum source has much more obviously influence on the crystal phase of final product than the silicon source does in this system.     

Loading frequency and microstructure interactions in intergranular fatigue crack growth in a disk Ni-based superalloy

The paper examines the role of the loading frequency on the dwell fatigue crack growth mechanism in the super-solvus nickel-based superalloy, ME3. This is accomplished by carrying out a set of crack growth experiments in air and vacuum at three temperatures; 650 °C, 704 °C and 760 °C using a dwell loading cycle with hold time periods up to 7200 s imposed at the maximum load level. Results of these tests show that the transitional transgranular/intergranular loading frequency is 0.1 Hz, and are used to determine the apparent activation energy of the time-dependent crack growth process. Analysis of this energy in both air and vacuum showed that the intergranular cracking is governed by a mechanism involving grain boundary sliding. This mechanism is explained in terms of absorption of dissociated lattice dislocations into grain boundary dislocations. The gliding of these dislocations under shear loading is assumed to cause grain boundary sliding. A condition for this mechanism to occur, is that a critical minimum distance exists between slip bands impinging the affected grain boundary. This condition is examined by correlating the slip band spacing (SBS) and loading frequency using a model based on minimum strain energy accumulation within slip bands and that a unique configuration of number and spacing of bands exists for a given plastic strain. The model outcome expressed in terms of SBS as a function of loading frequency is supported by experimental measurements at both high and low loading frequencies. Results of the model show that a saturation of SBS, signifying a condition for intergranualr cracking, is reached at approximately 3 μm which is shown to coincide with the transitional loading frequency of 0.1 Hz.     

Atomistic simulations of fatigue crack growth and the associated fatigue crack tip stress evolution in magnesium single crystals

Using Large-scale Atomic Molecular Massively Parallel Simulator (LAMMPS), a classical molecular dynamics code, atomistic simulations were performed to investigate the fatigue crack growth rate and the evolution of the associated atomic stress fields near the crack tip during fatigue crack growth in magnesium single crystals. The interatomic bonds of atoms were described using the EAM potential. The specimens with initial edge cracks were subjected to uniaxial Mode I cyclic loading. For the sake of revealing the influence of the initial cracks’ crystal orientations, three different orientations were considered. The fatigue growth rate can be expressed by da/dN = cCTOD, where the values of constant c are determined by the atomistic simulations. Notably, the values of the constant c are much larger for magnesium single crystals than for FCC single crystals and vary widely from one orientation to another. The simulation results show that the evolution of atomic stress fields was highly dependent on the crystal orientations due to anisotropy and magnesium single crystals’ HCP structure. Interestingly, the von Mises stress or normal stress around the crack tip controlled the fatigue crack growth behaviors.     

Formability of friction stir processed low carbon steels used in shipbuilding

The stretch formability of a low carbon steel processed by friction stir processing (FSP) was studied under biaxial loading condition applied by a miniaturized Erichsen test. One-pass FSP decreased the ferritic grain size in the processed zone from 25 μm to about 3 μm, which also caused a remarkable increase in strength values without considerable decrease in formability under uniaxial loading. A coarse-grained (CG) sample before FSP reflected a moderate formability with an Erichsen index (EI) of 2.73 mm. FSP slightly decreased the stretch formability of the sample to 2.66 mm. However, FSP increased the required punch load (F_EI) due to the increased strength by grain refinement. FSP reduced considerably the roughness of the free surface of the biaxial stretched samples with reduced orange peel effect. The average roughness value (Ra) decreased from 2.90 in the CG sample down to about 0.65 μm in fine-grained (FG) sample after FSP. It can be concluded that the FG microstructure in low carbon steels sheets or plates used generally in shipbuilding provides a good balance between strength and formability.     

Zero-birefringence optical polymers by nano-birefringent crystals for liquid crystal displays

We report a method for compensating the birefringence of optical polymers by doping them with nanometer-size inorganic birefringent crystals. In this method, an inorganic birefringent crystal is chosen that has the opposite birefringence to the polymer and a rod like shape which is oriented when the polymer chains are oriented. As a result, the birefringence of the polymer is compensated by the opposing birefringence of the crystal. Positive orientational birefringence of poly(methylmethacrylate (MMA)-co-benzylmethacrylate (BzMA)) = 78/22 (wt/wt) was compensated by doping with 0.3 wt.% of the smaller strontium carbonate (SrCO₃) crystals with a length of about 200 nm and a width of about 20 nm. The birefringence of the copolymer containing SrCO₃ was almost zero with any draw ratio between 1.0 and 2.2. The polarization state was almost maintained when it passed through the film. On the other hand, we concluded that the size of the larger crystals (about 3.0 μm × about 300 nm) is too large to form an optically isotropic medium with the polymer. In spite of doping with 0.3 wt.% of the smaller SrCO₃ crystal, the transmittance of the doped film with a thickness of 30 μm was almost the same as the undoped one in the visible region. The increase in haze by doping with 0.3 wt.% of the smaller SrCO₃ crystal was 0.1%. Furthermore, the negative birefringence of PMMA was enhanced by the SrCO₃ crystal.     

Static and dynamic flexural strength of 99.5% alumina: Relation to surface roughness

The dynamic flexural strength of ceramics is an important property for all applications involving impact loading conditions. Therefore, this work reports a systematic comparison of static and dynamic flexural strength results for 99.5% commercial alumina, obtained using a recently reported adaptation of the 1-point impact experimental technique. Specimens of the same size and systematically varying surface roughness conditions were used in this study to assess the influence of the latter on the static and dynamic strength of this material. The investigated roughness levels ranged from 0.8 μm (coarse) to 0.05 μm (fine, polished). Under static loading, reducing the surface roughness causes a 10% increase in flexural strength for polished specimens. By contrast, the dynamic flexural strength is apparently not influenced by the surface roughness. A thorough microstructural and fractographic examination reveals the presence of bulk (surface) pore-like flaws that are not obliterated by the polishing process and therefore govern the failure process. It is suggested that strength improvements can be reached by suitable surface preparation, provided no bulk pores are native in the material, some of which are present on its surface. The identification of the role of surface flaws is expected to clarify the discrepancy found in the literature as to the influence of surface roughness on the mechanical properties of brittle materials.     

Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000 °C

An ultrasonic fatigue testing system capable of operating at temperatures up to 1000 °C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000 °C and loading frequency of approximately 20 kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000 °C for PWA 1484 occurred in a crystallographic manner on {1 1 1} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850 °C) with conventional servo-hydraulic loading frequencies (<100 Hz).     

The fatigue behavior and mechanism of FV520B-I with large surface roughness in a very high cycle regime

The fatigue properties of FV520B-I up to 10⁹ cycles when the surface roughness R_a ≈ 0.6 were tested and compared with two groups of previously obtained test results. The test results showed that the S-N curve continuously moved downward and the transition stress at which the crack origin changed from the surface to the subsurface decreased with an increase of surface roughness, and the conventional fatigue limit finally appeared. The initiation mechanism of subsurface cracks in a very high cycle fatigue regime was independent of surface roughness. The surface fatigue limit and the high cycle fatigue life were predicted by relevant models. The competition mechanism between surface cracking and subsurface cracking was further discussed.     

Relationship between mechanical properties and microstructural response of 6061-T6 aluminum alloy impacted at elevated temperatures

The high temperature impact properties and microstructural evolution of 6061-T6 aluminum alloy are investigated at temperatures ranging from 100 to 350 °C and strain rates ranging from 1 × 10³ to 5 × 10³ s⁻¹ using a compressive split-Hopkinson pressure bar (SHPB) system. It is found that the flow response and microstructural characteristics of 6061-T6 aluminum alloy are significantly dependent on the strain rate and temperature. The flow stress and strain rate sensitivity increase with increasing strain rate or decreasing temperature. Moreover, the temperature sensitivity increases with both increasing strain rate and increasing temperature. The flow stress–strain response of the present 6061-T6 alloy specimens can be adequately described by the Zerilli–Armstrong fcc model. The grain size and dislocation cell size increase significantly with a decreasing strain rate or an increasing temperature. The higher flow stress is the result of a smaller grain size and smaller dislocation cell size. The stacking fault energy of the deformed specimens has a value of 145.78 mJ/m².     

Beta tricalcium phosphate ceramics with controlled crystal orientation fabricated by application of external magnetic field during the slip casting process

Beta tricalcium phosphate (β-TCP) is a resorbable bioceramic that has hitherto been utilized in the medical field. Since it crystallizes in the anisotropic hexagonal system, properties such as chemical and physical ones are expected to depend on its crystal axis direction and/or on its crystal plane (anisotropy). Control of crystal orientation is thus important when used in polycrystalline form. Meanwhile, application of a strong magnetic field has been found to be a promising technique to control crystal orientation of anisotropic shape or structured crystals. In this work, we attempted to fabricate β-TCP ceramics with controlled crystal orientation by applying an external magnetic field during the slip casting process and subsequently sintering them at 1050 °C, below the β–α transition temperature. Application of a vertical magnetic field increased intensities of planes perpendicular to c-plane on the top surface, while a horizontal one with simultaneous mechanical mold rotation decreased it. These results indicated that crystal orientation of β-TCP ceramics were successfully controlled by the external magnetic field and together that the magnetic susceptibility of β-TCP is χ_c⊥ > χ_c//.     

Life time and cyclic slip of copper in the VHCF regime

Studies were performed on the fatigue properties of polycrystalline copper at very low cyclic stress amplitudes and very high numbers of cycles. The experiments were carried out with the time-saving ultrasonic fatigue technique. Thus, experiments up to more than 10¹⁰ cycles could be performed within reasonable testing time. Main result is that below the PSB threshold, irreversible cyclic slip along crystallographic planes still occurs, as evidenced by changes in the surface slip line pattern down to strain and stress amplitudes that are as low as approximately only half of the PSB threshold values. These values are named SB (slip band) thresholds. No fatigue limit in the conventional sense could be detected below 1 × 10¹⁰ cycles. Thus, a threshold value at 1 × 10¹⁰ cycles is defined (named fatigue life threshold in this study) which is roughly 50% higher than the PSB threshold. The question whether the SBs are actually PSBs or whether continued cycling below the PSB threshold would ultimately lead to PSB formation and finally failure after extremely long testing has to be answered in future studies.