Impressions and Conclusions of the Conference on Mechanical Properties and adherence of scale layers and their influence on the oxidation of metals

Review of the papers presented to the Conference and of discussions. The topics dealt with included the various ways in which growing oxide scales can deform to accommodate their growth stresses. According to that plastic deformation occurs by the following mechanisms: dislocation glide, dislocation climb, Herring-Nabarro stress-assisted diffusion creep, grain-boundary sliding, mechanical twinning and viscous flow of amorphous oxides. The deformation depends on the scale configuration and the stress system; many oxide systems are rather plastic at high temperatures, but pores, defect doping elements and second phase inclusions play important parts, too. It is generally accepted that the evidence for some plastic deformation in growing oxide scales is very strong, the most favoured mechanism at high temperatures being diffusion controlled creep associated with grain boundary sliding. Factors influencing adhesion of scales include electrostatic forces at the metalloxide interface, interface irregularities, stress/strain interactions between oxide and metal, stress relaxation in metal and oxide, and the presence of stress raisers such as voids, second phase particles and pores at the metal/oxide interface. The necessity of developing new measuring methods is pointed out, too.     

Microscopic Approach of Adhesion and Wetting of Liquid Metal on Solid Ionocovalent Oxide Surface

The adhesion and wetting of non-reactive liquid metals with solid ionocovalent oxides are studied on thebasis of the experimental work of adhesion W data obtained with the sessile drop method.An analysis of theexperimental W values of different liquid metals on various solid oxides is first performed to evidence the de-pendence of the work of adhesion of a metal/oxide system on the electron density of the metal and on thethermodynamic stability of the oxide.An electronic model is then proposed to describe the microscopic mech-anism of metal-oxide interactions.Based on the model,the contact angle and the work of adhesion of differentliquid metals on various solid oxides can be interpreted and estimated,and their correlations to the variousphysical quantities of the oxides can be easily deduced.The basic consideration of the model is that the adhe-sion between a metal and an oxide is assured by the electron transfer from the metal into the oxide valenceband which is not completely filled of electrons at high temperatures,and is enhanced when this electron trans-fer at the metal/oxide interface is intensified.The influence of interface defects on the wetting and adhesion issuggested and discussed.     

Effect of geometry on the performance of model metal matrix composite joints and transitions

The role of geometry in the mechanical performance of various configurations of joints and transitions in metal matrix composites (MMCs) was investigated. Three types of model joints were manufactured by pressure infiltration of molten Al-4.5% Mg into preforms of continuous polycrystalline alumina fibers. This method of fabrication allows metal continuity to be achieved throughout the joint region, creating composite/monolith interfaces free of the gross defects that commonly limit joint strength. Test results indicate that changes in joint configuration affect the level of plastic flow at the composite/monolith interfaces, and suggest that increasing the level of plastic constraint in these regions enhances performance. The factors controlling interface behavior in the various joint configurations were investigated with finite element techniques, based on constitutive behavior measured experimentally. Using these methods, the evolution of the stress state which develops at the composite/monolith interfaces was probed, providing insight into the interplay between the state of stress and the failure mechanisms that limit interface performance. With this in mind, the relationship between key components of stress and the mechanical performance of the experimental specimens is discussed with respect to debonding and void growth at the composite/monolith interface.     

Localized corrosion (pitting): A model of passivity breakdown including the role of the oxide layer nanostructure

A model of passivity breakdown including the role of the inter-granular boundaries of the barrier oxide layer on the redistribution of the potential at the metal/oxide/electrolyte interfaces in the passive state is presented. Different mechanisms of breakdown at the oxide grain boundaries are considered, depending on which interface governs the potential drop: (i) local thinning and dissolution of the oxide layer, (ii) metal voiding or (iii) particle growth at the metal/oxide interface followed by rupture of the barrier layer. The role of chloride ions is discussed in each case. The key experimental observations made at the nanometre scale and validating the model are discussed.     

Effect of Nanocrystallization on Sulfur Segregation in Fe–Cr–Al Alloy during Oxidation at 1000°C

From the 1980’s a theory named “the sulfur effect” has been applied to explain the scale adhesion and the reactive-element effect (REE) during high-temperature oxidation. It claims that the bond between the oxide scale and the metal substrate is intrinsically strong and that impurity sulfur in the metal segregates at the oxide scale/substrate interface and weakens the bond, and that REs getter the sulfur impurity and prevent it from segregating to the interface. In the present study, a cast polycrystalline sulfur-containing Fe–25Cr–5Al-1S (wt.%) alloy and its magnetron-sputtered nanocrystalline coating were oxidized at 1000°C, and the specimens were examined by XRD and SEM. The scale formed on the cast alloy was cracked and detached from the substrate even after isothermal exposure, and obvious sulfur enrichment was detectable at the scale/substrate interface. While, the scale formed on the nanocrystalline coating was very adherent after 100 cycles oxidation. Here, sulfur was preferentially distributed in the outer scale and internal oxides rather than at the scale/substrate interface. These results provide evidence that nanocrystallization can prevent sulfur segregation at the scale/substrate interface, hence enhance scale adhesion.     

On the influence of metal lattice diffusion on oxidation of metals and alloys

The influence of metal lattice diffusion on oxidation kinetics is discussed for two simple cases: (i) a pure metal, where vacancies generated at the scalemetal interface diffuse to sinks within the metal; and (ii) a binary alloy of metals A and B, with A forming the more stable oxide. In the first case it is shown that vacancy effects are generally negligible. Analyses suggesting the contrary have failed to replace atom concentration gradients by the more appropriate chemical potential gradients. For the alloy, Wagner's condition for breakdown of A oxide is confirmed. It is shown that growth of A oxide cannot be controlled by diffusion of A in the metal, if B atoms can react at the scale-metal interface; scale-breakdown intervenes.     

Investigation of Adhesion and Fracture Toughness of Thermally Grown Oxide Scales by Interface Indentation Test

HIGH TEMPERATURE ALLOYS rely on theformation of a dense layer against oxidation.Howeverthe crack,spallation and detachment of oxide layeunder growth stress and thermal stress fromtemperature difference cause the failure of theprotective layer.Therefore,high temperature materialsare required forming a good oxide scale with highstrength and high interface bonding strength.Anappropriate measuring method and evaluation fooxide/metal interfacial adhesion is of great importanceto understand …     

The complementary nature of x-ray photoelectron spectroscopy and angle-resolved x-ray diffraction part II: Analysis of oxides on dental alloys

X-ray photoelectron spectroscopy (XPS) and angle-resolved x-ray diffraction (ARXRD) were used to analyze the oxide layer on three palladium-gallium-based dental casting alloys. The oxide layers were approximately 10 Μm thick. The use of the techniques helped to determine which mechanism was responsible for oxide formation—either (a) oxide layer growth via diffusion of oxygen through the scale to the metal, causing the scale to grow at the metal-oxide interface, or (b) an oxide layer formed by metal ions diffusing through the scale to the surface and reacting with oxygen, causing the scale to grow at the oxide-air interface. The oxide growth mechanisms were correlated to previous layer adhesion results determined with biaxial flexure testing.     

Influence of ceramic–metal interface adhesion on crack growth resistance of ZrO2–Nb ceramic matrix composites

Yttria-stabilized zirconia strengthened with lamellar flaky-shape Nb metal particles was obtained by hot-pressing at 1500 °C for 1 h. The ZrO₂–Nb interface has been studied by atomistic, first-principles calculations and by high-resolution transmission electron microscopy. The influence of the ceramic–metal interface on the crack growth resistance has been investigated. Crack growth is shown to occur with a rising resistance, governed by intact metal ligaments in the crack wake. Crack extension occurs by a combination of plastic deformation on the metal particles and interface debonding. The connection between the interface adhesion and this microstructural toughening mechanism has been evaluated.     

Modelling the influence of reactive elements on the work of adhesion between a thermally grown oxide and a bond coat alloy

The durability of thermal barrier coating systems is primarily determined by the degree of adhesion between the thermally grown oxide (TGO) and the bond coat. Failure of the TBC is often the result of delamination at this interface. Adhesion can be improved by the addition of reactive elements (RE) to the bond coat alloy. REs include oxide forming elements such as Y, Zr and Hf. The so‐called reactive element effect has been attributed to a direct improvement of the bonding between the TGO and the bond coat. A macroscopic atom model has been developed to allow the work of adhesion between two compounds (e.g. an oxide and a metal compound) to be estimated. By calculating the work of adhesion across a number of different interfaces, the influence of reactive elements and impurities present in the substrate can be assessed. It has been found that the REs have a limited direct influence on the work of adhesion and can even result in a weaker interface. A large reduction in the work of adhesion is calculated when S and C are present at the interface. REs have a high affinity for both S and C. This indicates that the RE effect is primarily that of impurity scavenging, preventing diffusion of impurities to the interface. A number of experiments are reported, which demonstrate the RE effect and support the modelling results.     

Adhesion between Nonreactive Liquid Metals and Transition Metal Carbides

On the basis of the experimental work of adhesion(W)data,the adhesion between transition metal car-bides and pure liquid metals which do not react with carbides is studied.In view of great scattering of the ex-perimental values of W,a critical analysis of these results is performed.The selected W values for 9copper/carbide systems and 6 metal/TiC systems are used to discuss the various suggestions concerning themechanism of adhesion and to evidence the role of the valence electrons of the both carbide and metal on theinteractions between metals and carbides.The interactions between a metal and a carbide are essentially metal-lic interactions,resulting from the overlapping of the valence electrons at the metal/carbide interface.     

Evidence for Vacancy Injection during the Oxidation of Iron

Iron discs have been oxidised at 890 ℃ on one side only, with the other side protected by an inert gas. The scale-metal adhesion was very good. Initially, scale-metal adhesion was maintained by the scale relaxing towards the metal but after a time which depended upon the initial metal thickness, oxide relaxation ceased and the inert face moved towards the scale-metal interface. When the face which was normally inert was covered with a non-growing oxide layer, the scale-metal adhesion deteriorated. The results show that vacancies which were produced by oxidation were annihilated within the metal, that the inert face played a part and that the scale-metal interface is not a good sink for vacancies.     

焊缝金属氧化物夹杂的尺寸及分布辅助外电场控制法

提出了一种在埋弧自动焊熔渣-熔池间随焊施加一辅助外电场,利用外电场控制熔体内部离子流进而控制焊缝氧化物夹杂尺寸及其分布的方法,并以直流电源提供的电压作为外电场开展了初步试验研究.结果表明,以钨材作外电场的阴极向液态熔渣内部补充大量的自由电子可极大地改善液态熔渣的导电性能,有效地诱导熔体内部形成稳定的定向离子流;外电场作用于熔渣-熔池通过使熔体内部复合粒子的进一步电离,正负离子形成互为逆向的离子流并在界面产生电化学反应以减少氧化结合的机会,因而有利于夹杂物的细化及弥散分布;同时外电场也可促使熔池内部自由氧离子电迁移至渣-金界面,并在界面优先与熔渣中氧化物形成元素结合进入熔渣从而削弱熔渣对熔池金属的氧化作用,使焊缝金属的氧含量显著降低.     

薄板铝合金激光深熔焊熔池流动数值模拟

针对薄板铝合金激光焊接过程,采用有限体积法开展熔池流动研究.建立了三维焊接熔池流动数学模型,并采用高斯旋转体热源表征激光束的热作用.在考虑与不考虑表面张力作用下,分别计算获得了焊接温度场、熔池流场和熔池形貌.基于计算结果,分析了温度场云纹图、熔池焊接热循环曲线、熔池速度场分布多视图.最后进行相同参数下的激光焊接试验,基于观察获得的焊接接头形貌,综合分析了模拟结果和试验结果.结果表明,所建立的模型和模拟方法是合理可行的.同时考虑Marangoni对流作用所计算得到的熔池和焊缝几何形状更加接近实际焊接接头.     

Influence of Pt on the metal-oxide interface during high temperature oxidation of NiAl bulk materials

The purpose of the investigation was to study the mechanisms improving oxide scale adherence in Pt-free and Pt-rich β-NiAl materials during high temperature oxidation. Theoretical density-functional theory (DFT) calculations were used to calculate the work of separation of the β-NiAl(1 1 1)/α-Al₂O₃(0001) metal-oxide interfaces, in pure NiAl and Pt-rich NiAl materials. The experimental work was focused on the studies of the metal-oxide interface, its development and morphology. Based on the theoretical and experimental results a complementary picture is presented for a better understanding of the Pt effect on the oxidation and oxide adhesion. It is shown that the interfacial bonding is decreased with addition of Pt to β-NiAl. The beneficial effect of Pt on the adhesion energy is attributed to the enhancement of contact areas between the oxide and the metal. The influence of Pt on the diffusion of Al and the formation of interfacial voids is also discussed.     

Self-Repairing Metal Oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O₂ gas near 20 mbar. Information on the position of oxide growth has been gained from the ¹⁸O/SIMS technique. Rates of O₂ dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O₂. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O₂. An increased rate of O₂ dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.     

Mechanical aspects of the rare-earth effect

In order to gain a better understanding of the reactive-element effect (REE), the improvement of the oxidation behavior of chromia- or alumina-scale-forming alloys by the addition of small amounts of elements with higher affinity to oxygen than the scale-forming element, it is necessary to clearly distinguish between isothermal oxidation and the behavior of the metal/oxide composite system during cooling. An approach is presented based on fracture-mechanical considerations which correlates critical differential strain between scale and substrate, fracture toughness of the metal/scale interface, scale thickness, defect size and interfacial amplitude. This approach allows a quantitative assessment of the REE for scale adhesion, and although the necessary experimental data are yet lacking, it describes the reported REE in a qualitatively correct manner.     

Interface friction and deformation kinematics in metal compression with sticking friction

Friction at the die-metal interface results in nonuniform deformation in metal compression. With sticking friction, the plastic flow mechanism becomes extremely complex, and the force and energy requirements are higher compared to frictional compression. Earlier research, which had some limitations, enables evaluation of the forming load in metal compression involving sticking friction. In the present article, critical analysis has been made regarding frictional behavior at the real contact areas to propose a more realistic estimation of frictional constraints. For this purpose, available inferences in specific areas of metal forming, concerning fundamental concepts of friction and the influence of interface shear stress, bulk deformation, etc., on frictional behavior, have been used and modified as required. Also, a rational deformation kinematics has been proposed, assuming velocity discontinuity at every point within the flow field. The proposed estimation of friction and plastic flow kinematics yields results in line with the published experimental findings and the corresponding slip-line field solutions.     

Ultrasonic testing of adhesive bonds of thin metal sheets

This article discusses the use of pulse-echo ultrasonic testing for the inspection of adhesive bonds between thin metal sheets (0.8 mm). The method is based on the measurement of the reflection coefficient at the metal/adhesive interface. After describing briefly the physical aspects of the phenomenon, an index is defined to detect defective zones of the joint (both for the lack of adhesive and for insufficient adhesion); the influence of the experimental variables (transducer frequency, coupling medium and contact force) on the measurement is discussed. The analysis shows that coupling medium and transducer frequency do not influence the results and that the method is robust with respect to the variation of contact force. By means of a control experiment it is shown that the statistical distribution of the index corresponding to good and defective adhesion zones are sufficiently separate to be distinguished. Finally, a procedure based on the statistical theory of decisions is proposed to evaluate the integrity of the joint under test.     

Study of plastic deformation and interface friction process for ultrasonic welding

Ultrasonic spot welding (USW) is attracting increasing attentions in joining of dissimilar metals. Previous experimental results show that material plastic deformation and interface friction plays key role for joint formation process. In the present study, experimental and theoretical analysis was carried out to study USW of copper and aluminium. Evolution of interface friction and surface deformation was obtained quantitatively. Meanwhile, theoretical analysis was proposed to research material flow and contact behaviour between faying interfaces of specimens. Conclusions can be made that interface friction resulted in tremendous plastic deformation, which was beneficial for joint formation. Growth rate of plastic deformation decreased as welding proceeds. Compression stress and cyclical shear stress induced by ultrasonic vibration determined direction and degree of material flow.