Cracks In Oxide Layer Research Articles

Hafnium silicate formation during oxidation of a permeable silicon + HfO2 powder composite system

Silicon bond coats, applied to silicon carbide composites used in gas turbine engines as a part of an environmental barrier coating (EBC) system, arrest the transport of oxidizing species to the SiC composite by forming a SiO2 thermally grown oxide. Above 1200°C, a crystalline ß-cristobalite SiO2 phase is formed that subsequently undergoes a large thermal contraction during its ß→α-cristobalite transformation upon cooling, leading to oxide layer cracking and eventual EBC delamination. A model system of silicon particles in a highly porous HfO2 particle matrix is used to investigate the rate and mechanism by which ß-cristobalite SiO2 is dynamically converted to hafnium orthosilicate (HfSiO4; Hafnon) when oxidizing silicon is in contact with m-HfO2. The study compares the net rate of SiO2 formation in the model silicon + HfO2 system with that on only silicon particles, and finds that the thickness of the SiO2 layer that forms during high temperature (1250–1316°C) oxidation can be greatly reduced by the hafnon formation reaction. The study also investigates the mechanism by which hafnon forms, and reports a SiO2 rumpling process when its outward (radial) growth is locally constrained by HfO2 particles. Since hafnon is phase stable over the temperature range of EBC interest, and has a thermal expansion coefficient similar to silicon, the dynamic conversion of SiO2 to hafnon provides a promising approach for controlling the delamination of EBC systems that use silicon bond coats.

Effect of REs (Y, Nd) addition on high temperature oxidation kinetics, oxide layer characteristic and activation energy of AZ80 alloy

Abstract The oxidation behaviors of AZ80, AZ80-0.32 Y and AZ80–0.38 Nd (wt.%) alloys were researched at 413 °C, 420 °C, 427 °C and 433 °C for up to 6 h in air environment via a high precision analytical balance, a laser confocal microscope, differential scanning calorimeter (DSC) analysis, X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) observation, and X-ray photoelectron spectroscopy (XPS) analysis. The results show that the weight gain and oxidation rate of AZ80 are reduced significantly, the initiation form and propagation of cracks in oxide layer are changed. Compact and protective oxide layer forms on alloy surface with Y or Nd addition. And the activation energies of AZ80, AZ80-0.32Y and AZ80-0.38Nd alloys calculated via Arrhenius equation are 82.556 kJ/mol, 177.148 kJ/mol and 136.738 kJ/mol, respectively.

Recent understanding of the oxidation and burning of magnesium alloys

This paper reviews the following for magnesium (Mg) alloys: (a) fundamentals of oxidation and burning; (b) influence of alloying on oxidation; (c) theoretical models for improved oxidation resistance through alloying with calcium (Ca), beryllium (Be), strontium (Sr) and rare earth elements; (d) recent research on the improved oxidation resistance of beryllium-containing magnesium alloys; and (e) the recent proposed oxide reinforcement model. The oxidation resistance of magnesium alloys is closely related to the protective incubation period, which is mainly controlled by the crack resistance of the initially formed surface oxide layer. Cracking of the initially formed surface oxide facilitates the vapor phase diffusion of magnesium and results in catastrophic oxidation and eventual burning of magnesium. According to the oxide reinforcement model, an oxide layer with increased mechanical strength and hardness (such as the (Mg,Be)O layer on beryllium-containing alloys) can better withstand the internal tensile stress arising during the oxidation process and therefore can delay oxide layer cracking and extend the oxidation incubation period. This model provides guidance on developing new oxidation-resistant magnesium alloys.

Effect of Applied Stress on T91 Steel Performance in Liquid Lead at 400 °C.

The environmental performance of structural materials (e.g., corrosion and environmental assisted cracking) is one of the critical issues being addressed in the development of Generation IV fast reactors. This work aims to support the study of the performance of the ferritic-martensitic steel T91 in liquid lead, under close to operation conditions, in order to assess its suitability for an application in lead fast reactors. T91 steel was tested in air and liquid lead at 400 °C using static and slow loading regimes. Applied stresses were chosen to be slightly above the yield strength in order to evaluate the threshold stress and strain for crack initiation. Three-point bending static exposure and constant extension tests of tapered specimens were performed. Post-test surfaces and cross sections of specimens were observed using scanning electron microscopy techniques in order to detect cracks and to analyze oxide layers. The effect of strain rate of the oxide layer cracking was observed. In conclusion, T91 was not susceptible to liquid metal embrittlement, a special case of environmentally assisted cracking under the testing conditions. The cracking conditions are discussed based on previous experience.

Improved oxidation resistance of Mg-9Al-1Zn alloy microalloyed with 60 wt ppm Be attributed to the formation of a more protective (Mg,Be)O surface oxide

The oxidation resistance of Mg-9Al-1Zn alloy (AZ91) was significantly improved through microalloying with 60 ppm Be. Be reinforced the initially formed oxide layer on the alloy surface by (i) the formation of a (Mg,Be)O solid solution due to the Be enrichment; and (ii) the refined gains of the (Mg,Be)O. During high-temperature oxidation, the reinforced (Mg,Be)O layer provided sufficient strength to withstand the internal stresses and the Mg vaporisation, and therefore delayed the oxide layer cracking and debonding from the substrate. In addition, the initial oxidation rates of AZ91 and Be-containing AZ91 were closely related to surface roughness.

The influence of phase angle, strain range and peak cycle temperature on the TMF crack initiation behaviour and damage mechanisms of the nickel-based superalloy, RR1000

Thermo-mechanical fatigue (TMF) tests including 0°, 90°, −90°, 45° −135° and −180°, phasing (φ) between mechanical loading and temperature were undertaken on a polycrystalline nickel-based superalloy, RR1000. Mechanical loading was employed through strain control whilst 300–700°C and 300–750°C thermal cycles were achieved with induction heating and forced air cooling. Mechanical strain ranges from 0.7 to 1.4% were employed. Results show that, for the strain ranges tested, TMF life is significantly affected by the employed phase angle. Furthermore the strain range and peak cycle temperature used has a substantial influence on the significance of dominant damage mechanisms, and resultant life. Various metallographic examination techniques have outlined that the dominant damage mechanisms are creep deformation at higher temperatures and early cracking of oxide layers at lower temperatures.

A new insight into high-temperature oxidation mechanism of super-austenitic stainless steel S32654 in air

High temperature oxidation behavior and mechanisms for super-austenitic stainless steel S32654 were investigated at 900, 1000 and 1200 °C in air. The oxide layers were characterized by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), electron-probe microanalysis (EPMA), x-ray diffraction (XRD) and transmission electron spectroscopy (TEM). At 900 °C, oxidation follows parabolic law within the initial 10 h, and the external solid MnMoO4 molybdate oxide layer and the internal spinel oxide layer prevent oxygen or metal cations diffusion. The parabolic law then changes to a linear law due to the molten MnMoO4-MoO3 molybdate electrochemical reaction and oxide layer cracking. At 1000 and 1200 °C, oxidation follows linear law. The nitrogen in air plays a very important role in accelerating oxidation. The synergistic effects of the molten MnMoO4-MoO3 molybdate electrochemical reaction and discontinuous Cr2N precipitation strongly promote catastrophic oxidation.

Exploitation Load Impact on Electrochemical Properties of Shipbuilding 7xxx Series Aluminum Alloy Joints Prepared with TIG and FSW Methods

The paper presents the results of investigations on mechanical properties and electrochemical potential distribution within arc welded (TIG) and friction stir welded (FSW) joints subjected to slow strain rate tests. The materials investigated were high-strength 7xxx series (7020 and its modification 7020M) aluminum alloys intended for shipbuilding. The objectives were joint quality assessment and comparison of the advantages of new FSW method with the traditional TIG methods commonly utilized in shipbuilding industry. Joint quality was evaluated based on mechanical investigations, hardness distribution tests and simultaneous electrochemical corrosion potential measurements at various locations within the welded joints.Initiation of corrosion processes on TIG and FSW joints was identified as a radical decrease in corrosion potential related to load followed by oxide layer cracking. Arc welded (TIG) joints of 7xxx series alloys undergo corrosion at lower values of tensile load applied as compared to the FSW joints. Superior mechanical properties and higher corrosion resistance of the FSW joints make this technology well-suited for joining high-strength 7xxx series alloys.

Lifetime, Cyclic Deformation and Damage Behaviour of MAR-M247 LC under Thermo-Mechanical Fatigue Loading with 0°, 180°, -90° and +90° Phase Shift between Strain and Temperature

Thermo-mechanical fatigue tests under 0° (in-phase), +180° (out-of-phase), +90° (clockwise diamond) and -90° (counterclockwise diamond) phase shift between mechanical loading and temperature were conducted on nickel-based superalloy Mar-M247 LC. Tests were carried out under total strain control with a temperature range of 100 – 850°C and a heating and cooling rate of 5K/s. Mechanical strain amplitudes were 0.3 to 0.6% with a strain ratio of Rɛ = -1. Results show, that for the strain amplitudes tested, lifetime varies significantly with strain-temperature phase. In-phase loading gives the shortest lives followed by out-of-phase and the diamond-phase loadings which show comparable lifetimes. Metallographic examination indicates that the dominant damage mechanisms are creep damage at higher temperatures and early cracking of oxide layers at lower temperatures. Both mechanisms occur primarily under tensile stress. Hence, the portion of each mechanism varies with the phase angle.

Nugget formation and growth during resistance spot welding of aluminium alloy 5182

Surface interaction at the worksheet/worksheet interface during resistance spot welding of aluminium alloy 5182 with spherical tip electrodes was investigated. Oxide layer cracking and nugget formation were focused. Both experimental work and finite element analysis were employed to explain the contact behaviour at this interface. It was found that sheet separation and thus bending occurred during the squeezing phase of the resistance spot welding process and suggested a profound influence on nugget formation. The sheet separation caused enlarged and aligned cracks in the surface oxide layers which led to a good metal‐to‐metal contact near the periphery of the faying surface. High current densities which occurred at the beginning of the current phase caused significant heat generation in this zone. Consequently, the melting at the faying surface started near the periphery and moved in towards the central zone of the contact region to produce a ‘doughnut shaped’ nugget with a filled‐in but thin central region.On a étudié l'interaction de la surface à l'interface feuille de travail‐feuille de travail lors du soudage par points par résistance de l'alliage d'aluminium 5182 avec des électrodes à bout sphérique. On s'est concentré sur la fracture de la couche d'oxyde et sur la formation du noyau. On a utilisé tant le travail expérimental que l'analyse par éléments finis pour expliquer le comportement de contact à cette interface. On a trouvé que la séparation de la feuille, et donc le pliage, se produisait lors de la phase de compression du procédé de soudage par points par résistance, suggérant une influence profonde sur la formation du noyau. La séparation de la feuille résultait en fissures agrandies et alignées dans les couches d'oxyde de la surface, ce qui amenait un bon contact de métal à métal près de la périphérie de l'aire de contact. Des densités élevées de courant, qui se produisaient au début de la phase de courant, résultaient en un dégagement important de chaleur dans cette zone. Conséquemment, la fonte de l'aire de contact commençait près de la périphérie et se déplaçait vers la zone centrale de la région de contact, produisant un noyau en forme d'anneau avec une région centrale remplie, mais mince.

Concomitant Channel Cracking and Interfacial Delamination in Polymer/Oxide Nano Hybrid Permeation Barriers in Flexible Electronics

ABSTRACTPolymer/oxide nano hybrid multilayer permeation barriers are emerging as a promising solution to the stringent barrier requirement of flexible electronics. Yet the mechanical failure of the multilayer permeation barriers could be fatal to their barrier performance. We study two co-evolving failure mechanisms of the multilayer permeation barriers under tension, namely, the cracking of the inorganic oxide layer and the delamination along the oxide-organic interface, using computational modeling. An effective driving force for the oxide layer cracking is determined, which decreases as the oxide-organic interfacial adhesion increases.

Failure mechanics of organic–inorganic multilayer permeation barriers in flexible electronics

Organic–inorganic multilayer permeation barriers are emerging as a promising solution to the stringent barrier requirement of flexible electronics. Yet the mechanical failure of the multilayer permeation barriers could be fatal to their barrier performance. We study two co-evolving failure mechanisms of the multilayer permeation barriers under tension, namely, the cracking of the inorganic oxide layer and the delamination along the oxide–organic interface, using computational modeling. An effective driving force for the oxide layer cracking is determined, which decreases as the oxide–organic interfacial adhesion increases. Emerging from the study is a simple but effective design to enhance the deformability of multilayer permeation barriers by applying a thin protective coating. Further studies show that strong adhesion of the coating-oxide interface is crucial for the coating performance. The results from this study provide quantitative guidance for the material selection and structural optimization of organic–inorganic multilayer permeation barriers of high mechanical durability.

Ｎｉ基単結晶超合金ＣＭＳＸ‐２の高温疲労における表面酸化層割れ誘起き裂の発生メカニズムに関する検討

Ｎｉ基単結晶超合金ＣＭＳＸ‐２の高温疲労における表面酸化層割れ誘起き裂の発生メカニズムに関する検討

The effect of droplet sprinkling on the oxidation kinetics of zirconium cladding in steam

The influence of oxide layer cracking on the acceleration of Zr cladding oxidation in steam was investigated. Cracking occurs due to temperature gradients, which arise during cooling of the cladding surface by droplet jets. Experimental results on characteristics of heat removal and the temperature gradients induced are presented for different regimes of sprinkling. A model for the calculation of mechanical stresses under the experimental conditions was developed in frame of the theory of envelopes. It was shown that the stresses are sufficiently high to induce cracking of the outer oxide layer. Experimental investigations of oxidation kinetics of Zr-1%Nb cladding were carried out under the conditions of surface cooling by droplet jets. The results of these experiments confirmed that the reaction of oxidation can be strongly accelerated by sprinkling.

Cracking and spalling of protective oxide layers

This paper discusses the role of thermal stresses on the mechanical integrity of oxide layers. It is shown that the processes of cracking and spalling of oxides differ depending on the stress state (tensile or compressive) and on the relative strengths of oxide and oxide-metal interface. Under tensile conditions, through-thickness cracks develop from pre-existing defects in the oxide layer and these generate shear stresses along the interface which may result in decohesion. Under compressive conditions, spallation may result either from the growth of a tensile, wedge crack along the interface or by buckling and cracking of the oxide layer. Initial results of a finite element analysis are provided for both mechanisms.