A series of model steels containing copper levels up to 0.48wt%, nickel up to 0.22wt% and silicon levels of 0.52wt% were oxidised in air at 1050 and 1150??C, and in a CO2-N2 mixture at 1250??C for times of up to 3 hours. The scaling kinetics were measured and the behaviour of copper-rich phase formation at the scale/metal interface was investigated. When oxidised at 1050/1150??C, significant quantities of copper-rich phase were observed for most model steels. The relatively high oxidation rate under these conditions led to the rapid development of a copper-rich layer with little copper diffusing into the metal. However, when oxidised at 1250??C, the copper-rich phase did not form for a significant amount of time; and for some model steels, not at all. This was attributed to the considerably lower oxidation rate and the fact that more copper was found to have diffused into the metal. Alloying additions of nickel and silicon were found to be beneficial in reducing the amount of copper-rich phase measured at the scale/metal interface under the conditions investigated at 1150??C and 1250??C. This occurred because nickel and silicon addition promoted the occlusion of copper-rich phase into the scale. Copper enrichment during oxidation was modelled using a numerical description of the diffusion processes involved. Predictions of the time for commencement of copper-rich phase formation at 1250??C were in close agreement with observation. Agreement between predicted and observed copper-rich layer thickness was less successful under conditions where occlusion was significant, and the measured thickness varied non-uniformly with time. The cracking susceptibility of the model steels was examined using a hot compression test. Oxidation was performed in air at 1050, 1150 and 1250??C and most specimens were compressed at 1050??C. The amount of cracking was found to increase with the amount of copper-rich phase precipitated at the scale/metal interface during oxidation. In general, nickel addition reduced the amount of cracking at all temperatures; and under some conditions prevented cracking altogether. Silicon reduced or completely suppressed cracking when the subscale formed was liquid. The beneficial effects of nickel and silicon addition were attributed to their effect of promoting copper occlusion.

In this licentiate thesis the work done in the TURBO POWER project Influence of high temperature hold times on the fatigue life of nickel-based superalloys will be presented. The overall objective of this project is to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the nickel-based superalloy Inconel 718. Firstly, the constitutive behaviour of the material has been been studied, where focus has been placed on trying to describe the mean stress relaxation and initial softening of the material under intermediate temperatures. Secondly, the fatigue crack propagation behaviour under high temperature hold times has been studied. Focus has here been placed on investigating the main fatigue crack propagation phenomena with the aim of setting up a basis for fatigue crack propagation modelling. This thesis is divided into two parts. The first part describes the general framework, including basic constitutive and fatigue crack propagation behaviour as well as a theoretical background for the constitutive modelling of mean stress relaxation. This framework is then used in the second part, which consists of the four included papers.

Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19093号 / エネ博第317号 / 新制||エネ||64 / 32044 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当

Το τεχνολογικό προβλήμα της υποβάθμισης της δομικής ακεραιότητας “γηρασκόντων” αεροσκαφών αποτελεί σήμερα αντικείμενο έρευνας αιχμής τόσο για τις αεροπορικές βιομηχανίες και τους διεθνείς και εθνικούς οργανισμούς ασφάλειας πτήσεων, όσο και για την επιστημονική κοινότητα. Οι μέχρι σήμερα προσπάθειες εστιάζονται κυρίως στην μελέτη της επίδρασης των καταστάσεων πολλαπλής βλάβης και ευρείας έκτασης βλάβης κόπωσης στην δομική ακεραιότητα των γηρασμένων αεροσκαφών. Σε πολλές όμως περιπτώσεις εκτός από καταστάσεις πολλαπλής βλάβης και ευρείας έκτασης βλάβη κόπωσης, παρατηρούνται επίσης εκτεταμένα προβλήματα διάβρωσης παρά την καλή προστασία που γενικά προσφέρουν οι χρησιμοποιούμενες σήμερα μέθοδοι αντιδιαβρωτικής προστασίας. Πρόσφατες έρευνες έδειξαν ότι η προκαλούμενη διάβρωση δεν προκαλεί μόνο μείωση της διατομής των υλικών αλλά και ψαθυροποίηση λόγω υδρογόνου η οποία υποβαθμίζει την δυσθραυστότητα του υλικού και την ικανότητα του να αποταμιεύσει μηχανική ενέργεια πρίν την θραύση. Στην παρούσα διατριβή: - Παρουσιάζονται επιπρόσθετα αποδεικτικά στοιχεία για την ψαθυροποίηση λόγω υδρογόνου που προκαλείται από την διάβρωση στο κράμα αλουμινίου 2024, με βάση πειράματα εφελκυσμού σε διαβρωμένα και αδιάβρωτα δοκίμια του κράματος. Τα αποτελέσματα των δοκιμών εφελκυσμού υποστηρίζονται από εκτενή μεταλλογραφική και στερεοσκοπική μελέτη, ανάλυση των επιφανειών θραύσης καθώς επίσης και από μετρήσεις του εκλυόμενου υδρογόνου. Επιπρόσθετα μελετάται η επίδραση του πάχους των ελασμάτων στην μηχανική συμπεριφορά σε εφελκυσμό των διαβρωμένων δοκιμίων. - Επιβεβαιώνεται η αντιδιαβρωτική προστασία που παρέχει στο κράμα 2024 η επικάλυψη καθαρού αλουμινίου (Alcladding), αλλά επιπλέον, διαπιστώνεται ότι η επικάλυψη προσφέρει επίσης προστασία από την ψαθυροποίηση λόγω υδρογόνου που συνοδεύει την διάβρωση. - Η κύρια συμβολή της εργασίας έγκειται στην παροχή δεδομένων ότι η τοπική επικάλυψη (local Alcladding) της επιφάνειας των δοκιμίων με καθαρό αλουμίνιο σε περιορισμένα ποσοστά της επιφάνειας αυτών, αρκεί για μια αποτελεσματική προστασία τόσο έναντι της βλάβης διάβρωσης όσο και έναντι της προκαλούμενης ψαθυροποίησης λόγω διάχυσης και παγίδευσης υδρογόνου στο εσωτερικό του κράματος. - Τέλος, εισάγεται μεθοδολογία για την εκτίμηση της επίδρασης της διάβρωσης και της εξ’ αυτής προκαλούμενης ψαθυροποίησης του υλικού σε φαινόμενα κόπωσης του κράματος 2024 μέσω της τροποποίησης του “Χάρτη Βλάβης Κόπωσης”, ώστε ο τελευταίος να μπορεί να αξιοποιηθεί για την περίπτωση διαβρωμένων δοκιμίων. / A possible integrity loss represents a not tolerable scenario for aging aircraft structures. To face the mentioned technological and scientific problem essential efforts have been undertaken by the scientific community as well as the aircraft industries and the international and national flight safety organizations. Nowadays research focuses to the study of the effects of widespread fatigue damage (WFD) and multiple site damage (MSD) scenarios on the structural integrity of the aging aircrafts. However, in numerous cases, additionally to WFD and MSD, extensive corrosion problems have been observed. Recent investigations have shown that the corrosion attack does not cause only a reduction of the cross-section of the structural member as well as a possible onset of fatigue cracks, but also a corrosion induced hydrogen embrittlement which reduces the fracture toughness of the material and its ability to store mechanical energy before fracture. In the present thesis: - Evidence is presented for a corrosion-induced hydrogen embrittlement of the alloy 2024 also in the absence of mechanical loads. A parametric study including series of tensile tests carried out on both corroded and uncorroded 2024 aluminum alloy specimens has been performed. The tensile tests results are supported by an extensive metallographic and stereoscopic study, analysis of the fracture surfaces, as well as hydrogen measurements. The effect of the sheet thickness on the tensile behaviour of corroded aluminum alloy 2024 specimens has been investigated, as well. - The corrosion protection offered by the aluminum coating (Alcladding) on the substrate alloy 2024 is confirmed. Additionally, evidence is provided on the protection offered by the aluminum coating against hydrogen embrittlement that accompanies corrosion. - The main contribution of the thesis is the provision of data for the case of local coating on the specimen surface, indicate that aluminum coating in limited percentages on the specimen surface for the alloy 2024 is sufficient for an efficient protection against corrosion damage, as well as against the induced embrittlement stem from the diffusion and trapping of hydrogen in the material interior. - Finally, a methodology is introduced for the estimation of the effect of corrosion and the corrosion-induced hydrogen embrittlement on the fatigue behaviour of the aluminum alloy 2024, through the modification of the Fatigue Damage Map (FDM), in order to utilize it for the case of corroded members.

Zirconium alloys are susceptible to engineering problems associated with the uptake of hydrogen throughout their design lifetime in nuclear reactors. Understanding of hydrogen embrittlement associated with the precipitation of brittle hydride phases and a sub-critical crack growth mechanism known as Delayed Hydride Cracking (DHC) is required to provide the engineering justifications for safe reactor operation. The nature of bulk zirconium hydrides at low concentrations (< 100 wt. ppm) is subject to several contradictory descriptions in the literature associated with the stability and metastability of γ-phase zirconium hydride. Due to the differing volume expansions (12-17%) and crystallography between γ and δ hydride phases, it is suggested that the matrix yield strength may have an effect on the phase stability. The present work indicated that although yield strength can shift the phase stability, other factors such as microstructure and phase distribution can be as or more important. This suggests that small material differences are the reason for the literature discrepancies. DHC is characterised by the repeated precipitation, growth, fracture of brittle hydride phases and subsequent crack arrest in the ductile metal. DHC growth is associated primarily the ability of hydrogen to diffuse under a stress induced chemical potential towards a stress raiser. Knowledge of the factors controlling DHC are paramount in being able to appropriately describe DHC for engineering purposes. Most studies characterise DHC upon cooling to the test temperature. DHC upon heating has not been extensively studied and the mechanism by which it occurs is somewhat controversial in the literature. This work shows that previous thermo-mechanical processing of hydrided zirconium can have a significant effect on the dissolution behaviour of the bulk hydride upon heating. DHC tests with γ-quenched, furnace cooled-δ and reoriented bulk hydrides upon heating and DHC upon cooling suggest that the amount of hydrogen in solution is the primary factor controlling the occurrence of DHC and consistent with the postulation that the stress induced chemical potential is the driving force for DHC. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-01-24 06:14:14.152

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No description available.

Stress corrosion cracking (SCC) from the external surface of a buried pipeline is a serious matter and can cause significant economic and environmental losses. Despite of many research works which have been done on the understanding of crack initiation and propagation mechanisms, these mechanisms are still being debated. This research studied the crack growth behaviour of different pipeline steels including two types of X65, one X52 and one X80 pipeline steels in near-neutral pH environments. Crack growth behaviour of all steels has been found to be consistent with that of true corrosion fatigue. Crack growth rates were correlated with (K)2Kmax/f0.1. It was revealed that cracking behaviour of pipeline steels in near neutral pH environments is material dependent. Highest crack growth rate was seen in the steel which highest amount of hydrogen atoms could be generated and stored in its microstructure to contribute in cracking procedure due to hydrogen embrittlement effect. / Materials Engineering

The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition, their mechanical properties and the microstructures of these steels varied considerably in order to gather information for the discussion of the metallurgical influences under Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC) conditions. In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation was carried out to fifteen commercial steels. The experiments reproduced the Stress Corrosion Cracking condition at which commercial rock bolts had failed in Australians mines. Due to the selected materials, stress and electrolyte condition it is expected that Hydrogen Embrittlement (HE) will affect the steel failure. The approach was to use the Linearly Increasing Stress Test (LIST) and exposing the sample to a dilute pH 2.1-sulphate solution, in accordance with prior studies. Stress Corrosion Cracking was evaluated by analysing the decrease in tensile strength, loss of ductility and fractography observed using Scanning Electron Microscopy (SEM). The initial series of test to the fifteen steels were performed at the free corrosion potential (f.c.p.) vs. Ag/AgCl. From this initial test only five steels (AISI 1008, AISI 4140, AISI 4145H, pipeline X-65 and X-70) did not show Stress Corrosion Cracking features. These five steel were tested in accordance with the Linearly Increased Stress Test (LIST) in the dilute pH 2.1 sulphate solution at different electronegative applied potential to minimum value of -1500mV. The experimental procedure tried to reproduce the Stress Corrosion Cracking condition to identify the most aggressive condition the steel is able to support before failing due to Stress Corrosion Cracking to then compare the theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. The investigation compared the well-known theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. Surprisingly, the experimental result did not always agree with the theory.