Preferential Initiation Sites Research Articles

Mechanisms of Environmentally Induced Crack Initiation in Humid Air for New-Generation Al-Zn-Mg-Cu Alloys

Recent experience has shown that new-generation 7xxx-series alloys, that have a high Zn content and Zn/Mg ratios, have a greater susceptibility to hydrogen-environmental induced cracking (H-EIC) on exposure to humid air than more established materials, like AA7050. In this study, we report new evidence of the EIC initiation and crack growth behavior of two new-generation alloys, AA7085 and AA7449, when exposed to 50% humidity. In situ, time-lapse, optical imaging over large areas has enabled the exact initiation sites to be identified and investigated with high-resolution fractographic studies, providing evidence for the sequence and mechanisms of initiation and transition to sustained cracking. A consistent behavior was observed for both alloys. This has revealed that minute-scale corrosion reactions, involving highly localized condensed water, are necessary for initiation. The preferred initiation sites are metal ligaments between surface-connected pore clusters and/or intermetallic particles that are subjected to high-stress concentration and undergo mechanical damage with associated higher levels of local oxidation. The growth of short protocracks from these sites is a distinct stage and displays intermittent arrest markings evidenced by localized corrosion. In contrast, in humid air environments, long cracks in these alloys exhibited relatively constant, higher velocity, with extremely limited corrosion commensurate with oxidation of a free surface in this environment resulting in approximately 5 nm oxide layer.

3D Investigation of Damage During Strain‐Controlled Thermomechanical Fatigue of Cast Al–Si Alloys

The strain‐controlled thermomechanical fatigue behavior is investigated for three cast near‐eutectic Al–Si alloys with different Ni, Cu, and Mg contents. Synchrotron tomography and neutron diffraction experiments are used to correlate 3D microstructural features with damage initiation and evolution. The results show that the alloy with lower Cu, Ni, and Mg concentrations has up to 45% higher thermomechanical fatigue resistance for cooling/heating rates of 5 and 15 K s−1. In addition, this alloy also exhibits damage formation at later stages during thermomechanical fatigue and slower damage accumulation compared to other alloys. This difference in behavior is a consequence of its higher ductility, which is a result of the lower volume fraction and global interconnectivity of the 3D hybrid networks formed by Si and intermetallics and the absence of large primary Si clusters which act as preferred crack initiation sites during the early stages of thermomechanical fatigue.

Comparison of Metastable Pitting Behavior of Austenitic and Duplex Stainless Steels under U‐Bending Deformation

The metastable pitting behavior of austenitic stainless steel (304 SS) and duplex stainless steel (2205 DSS) under U‐bending deformation is comparatively investigated. This study highlights the effects of differences in inclusions and matrix between 304 SS and 2205 DSS with regard to the metastable pitting behavior under tensile and compressive deformation. The results show that the pit initiation frequency of 304 SS increases in the order: tensile < as‐received < compressive; in contrast, the initiation frequency of 2205 DSS increases in the order: as‐received < tensile < compressive. In addition, tensile and compressive deformation change the preferential initiation sites of metastable pitting in 2205 DSS. It is proposed that the types of transient signals can be comprehensively and objectively distinguished by the two parameters (t g/t rep) and [(I p/Q g):(I p/Q rep)]. The correlation relationship between transient signals and pitting morphologies is clarified by cluster analysis. Both tensile and compressive deformations promote the growth of a few metastable pits, thus increasing the transition probability from metastability to stability. Overall, both tensile and compressive deformations are detrimental to the pitting corrosion resistance of the experimental steels.

Morganella Phage Mecenats66 Utilizes an Evolutionarily Distinct Subtype of Headful Genome Packaging with a Preferred Packaging Initiation Site.

Both recognized species from the genus Morganella (M. morganii and M. psychrotolerans) are Gram-negative facultative anaerobic rod-shaped bacteria that have been documented as sometimes being implicated in human disease. Complete genomes of seven Morganella-infecting phages are publicly available today. Here, we report on the genomic characterization of an insect associated Morganella sp. phage, which we named Mecenats66, isolated from dead worker honeybees. Phage Mecenats66 was propagated, purified, and subjected to whole-genome sequencing with subsequent complete genome annotation. After the genome de novo assembly, it was noted that Mecenats66 might employ a headful packaging with a preferred packaging initiation site, although its terminase amino acid sequence did not fall within any of the currently recognized headful packaging subtype employing phage (that had their packaging strategy experimentally verified) with clusters on a terminase sequence phylogenetic tree. The in silico predicted packaging strategy was verified experimentally, validating the packaging initiation site and suggesting that Mecenats66 represents an evolutionarily distinct headful genome packaging with a preferred packaging initiation site strategy subtype. These findings can possibly be attributed to several of the phages already found within the public biological sequence repositories and could aid newly isolated phage packaging strategy predictions in the future.

Multiple Cis-acting Polypyrimidine Tract Elements Regulate a Cooperative Mechanism for Triticum Mosaic Virus Internal Ribosomal Entry Site Activity.

Diverse elements within the 5′ untranslated region of an mRNA can influence the translation efficiency at the main AUG codon. We previously identified a core picornaviral like Y16X11-AUG motif with 16-nt polypyrimidine CU tract separated by an 11-nt spacer sequence from the 13th AUG codon, which is recognized as the preferred initiation site within the Triticum mosaic virus (TriMV) internal ribosome entry site (IRES) element. The motif is proposed to function as an internal ribosomal landing site at the designated start codon. Here, we exposed the cooperative role of multiple CU-rich segments flanking the TriMV YX-AUG motif to reach and drive internal initiation of translation at the preferred start site. We propose that these auxiliary domains may enhance the ribosome capacity and their delivery at proximity of the correct initiation site. These polypyrimidine tracts can be modulated with a cryptic AUG in a position-dependent manner to replace the native YX-AUG motif, and thus uncovering a new layer of control of start codon selection. In line with these observations, mass spectrometry analysis of proteins directly interacting with translationally impaired TriMV IRES mutants that bear these motifs indicated an enrichment in 40S and 60S ribosomal related proteins, revealing a new function of polypyrimidine tracts to regulate IRES-driven translation. Accessibility of these RNA regions for in trans interaction was validated by SHAPE analysis of the entire TriMV leader sequence and supported by the ability of anti-sense oligonucleotides designed to block the CU tracts accessibility to impair IRES activity. This is the first evidence that defines the core modular domains required for ribosomal recruitment and start codon selection in a complex, multi-AUG viral 5′ UTR for translation in plants.

Ti-6Al-4V lattices obtained by SLM: characterisation of the heterogeneous high cycle fatigue behaviour of thin walls.

Architectured materials are attracting increasing interest since a few years, achieving numerous excellent specific properties compared to fully dense materials. The transport industry sector see periodic assemblies of elementary cells, called lattices, as a solution to lighten structures. This is made possible by the development of Additive Manufacturing technologies (AM). Among the numerous advantages of AM, these processes allow to create a wide range of porous cell topologies, and to tailor their complex geometry with the desired properties. However, understanding their fatigue properties is essential to validate their long-term use in load-bearing parts.The FA process plays an important role in fatigue life, as it generates residual stresses, heterogeneous microstructure, as well as surface and volume defects (roughness, gaz porosities, lack of fusion porosities), which are preferred sites for fatigue crack initiation. The fatigue life of these structures also depends strongly on their geometry, which, due to their complexity, induces scale and stress gradient effects.Recent studies have recognize thin-walls TPMS (Triply Periodic Minimal Surfaces) structures as the most promising structures for fatigue resistance, especially the gyroid geometry, compared to conventional strut-lattices.The first aim of this work is to characterise the major parameters influencing fatigue life at the lattice scale. Roughness, microstructure will be discribed on a lattice specimen, and topology effect on stress distribution will be numerically quantified. In order to understand the influence of these parameters, and identify the critical region for fatigue life, this work focuses on the local thin-wall scale. Here, the aim is to characterise the high cycle (HCF) fatigue heterogeneous response of Ti-6Al-4V thin-walls manufactured, characterising localy the HCF behaviour of a TPMS lattice structure (R=0.1; uniaxial tension loading, N=1.106, f=40Hz).

Solute segregation assisted grain boundary precipitation and its impact to ductility of a precipitation-hardenable magnesium alloy

Ag and Ca atoms segregate to grain boundaries (GBs) in a solution treated state, which assists the earlier formation of GB precipitates compared to the grain interior during the early stage of aging. Prolonged aging leads to the growth of GB precipitates, which act as preferred initiation sites for microcracks.

A study on the transport and interaction between blood flow and low-density-lipoprotein in near-wall regions of blood vessels

Differences in the dynamics and transport of blood make certain regions of the arterial network the preferred sites for initiation and formation of arterial diseases like stenosis and aneurysms. Understanding of such arterial diseases is directly linked to critical hemodynamic parameters such as the wall shear stress (WSS). The present work generalises the influence of WSS on the concentration of LDL that was observed in an earlier study. To this end, a wide variety of simplified flow domain, inspired by the near-wall regions of aneurysms and stenosis, are constructed and analyzed. The effects of pulsatile inflow condition, rheology of blood and curvature of the wall on the correlation between WSS and LDL concentration are investigated. It is demonstrated that the time-scale of variation of lumen-surface-concentration (LSC) of LDL is larger than a single cardiac cycle. As a consequence, the time-average values of WSS are sufficient to locate the regions of higher LSC. This idea is strengthened by making use of simplified flow domain that generates moving stagnation point. Further, it was observed that the rheology of the blood and curvature of the wall does not affect the observed correlation between the WSS and LDL concentration.

IASCC of neutron irradiated 316 stainless steel to 125 dpa

The irradiated microstructure and irradiation-assisted stress corrosion cracking (IASCC) susceptibility were determined for neutron-irradiated cold-worked (CW) 316 stainless steel tested in 320 °C simulated PWR primary water with various damage levels of 46.9, 67.4, and 125.4 dpa. The size and density of dislocation loops and precipitates were found to vary little over the dose range 46.9–125.4 dpa and dislocation loops were found to control the irradiation hardening. The %IG in constant extension rate tensile (CERT) tests and the stress to initiate a crack in four-point bend tests were consistent in establishing that the IASCC susceptibility was essentially unchanged across the dpa range. GB oxidization was observed prior to crack initiation, and localized deformation and triple junctions where local stresses were high were preferred crack initiation sites. IASCC cracks initiated preferentially on grain boundaries oriented nearly normal to the applied stress. An apparent threshold stress threshold for IASCC was determined to be ~45% of the irradiated yield strength for neutron-irradiated CW 316 SS between 46.9 and 125.4 dpa.

Wear and corrosion behavior of Mg-based alloy reinforced with TiC and ZrC particles

Abstract In this contribution, particle sizes of TiC (13 and 93 μm) and volume fractions of ZrC (5 and 10 vol.-%) with respect to reinforcement particles were varied to investigate the effects on the microstructure, hardness, density, wear and corrosion properties of AZ91 Mg matrix alloy. Experimental results revealed that the hardness, density and sliding wear performance of AZ91 alloy were markedly improved by the addition of carbide particles. Predominant wear and corrosion mechanisms were identified considering the size and volume fraction of the carbides. The composite sample comprising fine TiC particles (13 μm) exhibited the highest wear resistance at the same volume fraction as the coarse particles. Moreover, coarse ZrC particles with a low volume fraction (5 vol.-%) provided an enhanced wear resistance beyond that of the 10 vol.-% ZrC particles. Considering all the investigated composites, the corrosion resistance of the composites deteriorated with the increasing volume fraction and size of the carbide particles. Electrochemical measurements of the 0,5M NaCl solution revealed that increasing carbide particle size and volume fraction leads to lower corrosion resistance due to the formation of more cathodic areas which are preferred sites for the initiation of pitting corrosion.

Fatigue Behavior of Additively Manufactured IN718 with Columnar Grains

Fatigue failure of polycrystalline materials is often dominated by crack initiation processes, which are strongly influenced by salient features existing in the microstructure. Herein, a crystal plasticity framework based on the finite element method is used to quantitatively assess the mechanistic drivers existing in columnar IN718 polycrystals for fatigue crack nucleation. To that end, microstructurally differing polycrystalline samples are subjected to strain‐controlled cyclic loading at ambient conditions to elucidate the microstructural mechanisms of fatigue damage. 2D grain structures obtained from the cellular automata (CA) simulations of the additive manufacturing of IN718 are utilized to construct representative 3D microstructures for use in computational analyses. A criterion related to the stored energy density (SED) is used to predict the scatter in fatigue crack nucleation life. This criterion presents a unique and consistent approach toward predicting fatigue crack initiation at the microstructural scale. It is demonstrated that SED is necessary and sufficient to drive crack nucleation in low cycle fatigue. The results show that grain boundaries are the preferred crack initiation sites.

Crystallographic analysis of fatigue fracture initiation in 8Ni-0.1C martensitic steel

The present paper investigated characteristics of fatigue fracture behavior, particularly initiation stage of fatigue fracture (Stage I), in an as-quenched martensitic steel from microstructural and crystallographic points of view. The detailed crystallographic orientation analysis using EBSD revealed that block boundaries in lath martensite structure were the most preferential initiation sites for fatigue cracks. We found that incompatibility of plastic strains between adjacent blocks was the origin for the formation of initial fatigue cracks at block boundaries. Moreover, plastic deformation along {011} slip planes also played an important role on the transgranular crack propagation.

A novel DNA primase-helicase pair encoded by SCCmec elements.

Mobile genetic elements (MGEs) are a rich source of new enzymes, and conversely, understanding the activities of MGE-encoded proteins can elucidate MGE function. Here, we biochemically characterize three proteins encoded by a conserved operon carried by the Staphylococcal Cassette Chromosome (SCCmec), an MGE that confers methicillin resistance to Staphylococcus aureus, creating MRSA strains. The first of these proteins, CCPol, is an active A-family DNA polymerase. The middle protein, MP, binds tightly to CCPol and confers upon it the ability to synthesize DNA primers de novo. The CCPol-MP complex is therefore a unique primase-polymerase enzyme unrelated to either known primase family. The third protein, Cch2, is a 3'-to-5' helicase. Cch2 additionally binds specifically to a dsDNA sequence downstream of its gene that is also a preferred initiation site for priming by CCPol-MP. Taken together, our results suggest that this is a functional replication module for SCCmec.

The metastable pitting corrosion of 2205 duplex stainless steel under bending deformation

The effects of tensile and compressive deformation introduced by the bending deformation on metastable pitting corrosion of 2205 duplex stainless steel were studied by microstructure characterization and electrochemical tests. The results demonstrated that both tensile and compressive deformation resulted in severer plastic strains in ferrite than that in austenite, caused the inclusions debonding from matrix and the surrounding micro-cracks. The decrease in critical pitting temperature and pitting potential provides the degradation evidence of the deformed steel. Compared with the as-received samples, bending deformation promoted the initiation frequency and growth rate of metastable pits. It also revealed that higher metastable pitting initiation rate and pit growth under compressive deformation than tensile deformation. The preferential initiation sites and transients’ behavior of metastable pitting were discussed herein.

Holding time influence on creep behavior of transient liquid phase bonded joints of Hastelloy X

Transient liquid phase (TLP) bonding using various holding times (5, 80, 320, and 640 min) was conducted on Hastelloy X using an Ni–Cr–B–Si–Fe filler metal at 1070 °C. High temperature stress rupture (creep) tests were performed on the TLP bonded joints at 816 °C under a stress of 110 MPa. The 320 min holding time sample had the highest rupture life (95.9 ks). The presence of uniformly distributed borides along the joint grain boundaries was determined to be the primary reason for improved creep properties. These particles were observed using electron backscattered diffraction (EBSD). The large values of creep strain observed in the 640 min holding time sample were due to a decrease in the number of grain boundary borides present in the joint area. An investigation on the creep fracture surfaces revealed a transition of the failure mechanism from transgranular to intergranular with increasing holding time. The fracture surface and elemental map were studied using electron probe microanalysis (EPMA); results showed that cracks initiated at the interface of eutectic components in samples with lower holding times (5 and 80 min), while the hard and brittle grain boundary borides available in the joint were the preferred sites for crack initiation in the 320 min sample.

Effects of heterogeneity and coarse secondary phases on mechanical properties of 7050-T7451 aluminum alloy friction stir welding joint

The heterogeneous properties of different zones in the 7050-T7451 aluminum alloy butt joints produced by friction stir welding (FSW) are investigated by hardness test and miniature tensile test of characteristic zones. The correlation between the mechanical properties and the microstructure evolution as well as coarse secondary phases is explored by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The miniature tensile results demonstrate that the yield and ultimate strength of various zones of the joint (HAZ, TMAZ and SZ) have different degrees of decrease compared with the base material (BM). The dissolution of precipitation phases and the decrease of dislocation density result into the noticeable decrease of strength in the HAZ. The dislocation strengthening in the TMAZ and grain-boundary strengthening in the SZ can compensate the strength loss to some extent. The TEM analysis shows that the coarse secondary phases in the 7050 aluminum alloy FSW joint comprise mainly of Al23CuFe4, Al7Cu2Fe, Al2CuMg and Mg2Si with the size ranging from 500 nm to 10 μm. The influence on fracture from large to small is Al23CuFe4, Al2CuMg, Mg2Si and Al7Cu2Fe. The coarse secondary phases are the preferred crack initiation site, and the crack tends to propagate along the path where the coarse secondary phases are dense, which becomes pronounced as the strength decreases.

Effect of grain refinement on the hydrogen embrittlement of 304 austenitic stainless steel

The effect of grain size (in the range from 4 μm to 12 μm) on the hydrogen embrittlement (HE) of 304 austenitic stainless steel (ASS) was studied. HE susceptibility result shows that HE resistance increases with grain refinement. Electron backscattered diffraction kernel average misorientation (EBSD-KAM) mapping shows that the strain localization can be mitigated by grain refinement. Hence, strain localization sites which act as highways for hydrogen diffusion and preferred crack initiation sites can be reduced along with grain refinement, leading to a high HE resistance. Meanwhile, grain size shows no influence on the strain induced martensite (SIM) transformation during the hydrogen charging slow strain tensile test (SSRT). Hence, the SIM formed during hydrogen charging SSRT is not responsible for the different HE resistance of 304 ASSs with various grain sizes. Hydrogen diffusion is supposed to be controlled by a competition between short-circuit diffusion along random grain boundary (RGB) and hydrogen trapping at dislocations, leading to a maximum hydrogen diffusion coefficient in the 304 ASS with an average grain size of 8 μm.

Quasi-continuous-wave laser surface melting of aluminium alloy: Precipitate morphology, solute segregation and corrosion resistance

A quasi-continuous-wave laser surface melting is performed on 2219 aluminium alloys to improve the corrosion resistance. The fast cooling rate and the multi-directional solidification behavior of the process transform large and long-chained constituent particles into fine and isolated circular precipitates with alleviated solute segregation. The improved corrosion resistance is due to the formation of low-flaw-density passive film, the reduction of the driving force for corrosion and the improvement of the re-passivation capability of the modified surface. The large and long-chained constituent particles are the preferred sites for corrosion initiation and propagation.

The Triticum Mosaic Virus Internal Ribosome Entry Site Relies on a Picornavirus-Like YX-AUG Motif To Designate the Preferred Translation Initiation Site and To Likely Target the 18S rRNA.

Several viruses encode an internal ribosome entry site (IRES) at the 5' end of their RNA, which, unlike most cellular mRNAs, initiates translation in the absence of a 5' m7GpppG cap. Here, we report a uniquely regulated translation enhancer found in the 739-nucelotide (nt) sequence of the Triticum mosaic virus (TriMV) leader sequence that distinguishes the preferred initiation site from a plethora of IRES-encoded AUG triplets. Through deletion mutations of the TriMV 5' untranslated region (UTR), we show that the TriMV 5' UTR encodes a cis-acting picornaviral Y16-X11-AUG-like motif with a 16-nt polypyrimidine CU-tract (Y16), at a precise, 11-nt distance (X11) from the preferred 13th AUG. Phylogenetic analyses indicate that this motif is conserved among potyviral leader sequences with multiple AUGs. Consistent with a broadly conserved mechanism, the motif could be functionally replaced with known picornavirus YX-AUG motifs and is predicted to function as target sites for the 18S rRNA by direct base pairing. Accordingly, mutations that disrupted overall complementarity to the 18S rRNA markedly reduced TriMV IRES activity, as did the delivery of antisense oligonucleotides designed to block YX-AUG accessibility. To our knowledge, this is the first report of a plant viral IRES YX-AUG motif, and our findings suggest that a conserved mechanism regulates translation for multiple economically important plant and animal positive single-stranded RNA viruses.IMPORTANCE Uncapped viral RNAs often rely on their 5' leader sequences to initiate translation, and the Triticum mosaic virus (TriMV) devotes an astonishing 7% of its genome to directing ribosomes to the correct AUG. Here we uncover a novel mechanism by which a TriMV cis-regulatory element controls cap-independent translation. The upstream region of the functional AUG contains a 16-nt polypyrimidine tract located 11 nt from the initiation site. Based on functional redundancy with similar motifs derived from human picornaviruses, the motif is likely to operate by directing ribosome targeting through base pairing with 18S rRNA. Our results provide the first report of a broad-spectrum mechanism regulating translation initiation for both plant- and animal-hosted picornaviruses.

SCC evaluation of a 2297 Al-Li alloy rolled plate using the slow-strain rate technique

The stress corrosion cracking (SCC) susceptibility of 2297 Al-Li alloy in 1 M NaCl + 0.01 M H2O2 solution (CP solution) and 1 M NaCl + 0.01 M H2O2 + 0.6 M Na2SO4 solution (CPS solution) was investigated by slow-strain rate tests at various strain rates ranging from 10−5 s−1 to 10−7 s−1. The roles of H2O2 and SO42− in the corrosion process were estimated by potentiodynamic polarization and electrochemical impedance spectroscopy. 2297 Al-Li alloy does not fracture ascribed to SCC in CP solution, while it undergoes SCC in CPS solution. In CPS solution, with a decreasing strain rate from 10−5 s−1 to 10−7 s−1, the SCC susceptibility firstly rises and then declines exhibiting a peak value at a strain rate of 10−6 s−1. H2O2 promotes the active dissolution while SO42− lowers the corrosion rate. The SCC fracture is associated with a decline in the dissolution rate of the crack tip by SO42−, which leads to stress concentration. In CPS solution, a reduction in the local dissolution rate of the crack tip leads to stress concentration, resulting in SCC fracture. As the preferred initiation site for a crack, pits also show a noteworthy effect on SCC of 2297 Al-Li alloy.