Morphology and Frictional Characteristics Under Electrical Currents of Al_2O_3/Cu Composites Prepared by Internal Oxidation

Two Al2O3/Cu composites containing 0.24 wt.% Al2O3 and 0.60 wt.% Al2O3 separately are prepared by internal oxidation. Effects of sliding speed and pressure on the frictional characteristics of the composites and copper against brass are investigated and compared. The changes in morphology of the sliding surface and subsurface are examined with scanning electron microscope （SEM） and energy dispersive X-ray spectrum （EDS）. The results show that the wear resistance of the Al2O3/Cu composites is superior to that of copper under the same conditions, Under a given electrical current, the wear rate of Al2O3/Cu composites decreases as the Al2O3-content increases, However, the wear rates of the Al2O3/Cu composites and copper increase as the sliding speed and pressure increase under dry sliding condition. The main wear mechanisms for Al2O3/Cu composites are of abrasion and adhesion; for copper, it is adhesion, although wear by oxidation and electrical erosion can also be observed as the speed and pressure rise.

INTERNAL OXIDATION KINETICS ＆DIFFUSION MECHANISM OF OXYGEN IN Cu－Al SINTERED ALLOY

In this paper, internal oxidation kinetics and the diffusion mechanism of oxygen in sintered Cu-Al alloy have been studied by means of scanning electron microscopy,transmission electron microscopy and heat-weighting scales. The results show that the kinetic curves obey the parabolic law in the initial stage and are linear in the late stage.The oxygen atoms are mainly transported in the from of grain boundary diffusion and volume diffusion in the process of internal oxidation. The plastic deformation of Cu matrix around oxide particles increases the probability of [O] atoms moving in accordance with dislocation diffusion mechanism. Meanwhile, the activation energy and diffusion coefficient in the process of internal oxidation are evaluated by means of kinetic method so as to obtain the velocity equations of reacting interface moving in different stage.

Internal oxidation of hot-rolled ultra-high strength steel and its effect on the surface quality of cold-rolled sheets

In this study,the scale and internal oxidation of hot-rolled ultra-high strength steel sheets were characterized.It was found that both the formation of the scale and the internal oxidation of Si and Mn depended on the coiling temperature and position of the steel sample on the strip coil.At a relatively high coiling temperature,a large amount of internal oxidation was observed on the samples cut from the middle of the coil.The depth of the internal oxidation zone exceeded 10 μm and a thin iron layer covering the scale was observed in some cases.Pickling and cold-rolling experiments were conducted on selected samples.Scale pickling was found to be greatly delayed by the formation of an iron layer,which frequently resulted in under-pickled defects.In addition,pickling of the entire internal oxidation zone was difficult,except at the grain boundaries,where the degree of internal Si and Mn oxidation was enriched.The surface of the cold-rolled steel sheet was ruined by the remaining oxidation zone in the subsurface of the pickled steel.The internal oxidation of hot-rolled ultra-high strength steel must be precisely controlled to improve the subsequent surface quality of cold-rolled steel.

Internal Oxidation of Some Pd-40Ag-1RE Alloys

The internal oxidation behavior of Pd-40Ag-1RE (RE = Sm, Eu, Gd, Y) alloy wires has been studied in air at 800 °C 1200°C. The dependence of internal oxidation depth ξ on reaction time t can be expressed as ξ = Ktn, the reaction index n = 0.50 approximately 0.75. The higher the oxidation temperature is, the larger the n value is. It means that the internal oxidation of wire sampls at high temperature deviates from parabola rule observed on plate sample. The activation energies of the studied alloys range over 120 approximately 160 kJ/mol. The internal oxidation process is controlled by the bulk diffusion of oxygen.

INTERNAL OXIDATION BEHAVIOR OF Pd-40Ag-1M(M=Zr,Y)ALLOYS

The internal oxidation behavior of Pd-40Ag-1M(M=Zr,Y)alloy wires has been studied in air at 800—1200℃.The relationship between the internal oxidation depth ξ and the reaction time t can be expressed as ξ= Kt^n,where n=0.5—0.75.The higher the temperature,the larger the value of n is.The active elements Zr and Y show different internal oxidation characters.For the alloys eontaining Zr,the oxidation rate along the grain boundaries is about twice as high as that in grains,and“lateral oxidation”exists along the grain boundaries.For the alloys containing Y,the oxidation rates in grains and along the grain boundaries are roughly the same,and there is no“lateral oxidation”along the grain bounda- ries.The activation energies of both alloys are in the range of 120—150kJ/mol.Some prop- erties for oxidized alloys were studied.The mechanisms of the internal oxidation were dis- cussed.

Kinetics of Internal Oxidation of Cu-Al Alloy Plate

The kinetics of internal oxidation of dilut e Cu-Al alloys, containing up to 2.214% molar fraction Al, was investigated over the temperature range of 1023K to 1273K, and the depth of internal oxidation was measured by microscopies. A modified rate equation was derived to describe the kinetics of internal oxidation of Cu-Al alloy plate.Based on the derived equati on, the permeability of oxygen in solid copper was obtained from the internal ox idation measurements. The experimental results show that the depth of the intern al oxidation is a parabolic function of time, there is no evidence for preferent ial diffusion along grain boundaries and an outer layer of pure copper was forme d on the external surface of samples.

Relationship between powder characteristic and internal oxidation of Al in Cu-Al pre-alloyed powders

Since Cu-Al powder characteristics have important effects on the preparation of Cu/Al2O3 composite, the apparent activation energy of Al internal oxidation reaction in Cu-Al pre-alloyed powders with different characteristics was calculated in the present investigation. The microstructure and properties of the synthesized Cu/Al2O3 were studied. The results show that high-energy milling can obviously promote internal oxidation of Al in Cu-Al powders in the same solid solubility. At the same milling conditions and internal oxidation parameters, the solid solution of Al in Cu either in low or high amount will result in the poor microstructure and properties of the Cu/Al2O3 composite. Subsequently, when high-energy milling and internal oxidation are synchronously used to prepare the Cu/Al2O3 composite, there should be an appropriate solubility and milling effect for the pre-alloyed powders.

Kinetic Equation for Internal Oxidation of Cu-Al Alloy Spheres

The kinetics of internal oxidation of Cu-Al alloy spheres, containing up to 2.214% mole fraction Al was investigated in the temperature range 1 023 K to 1 273 K, and the depth of internal oxidation was measured in the microscopy. A kinetic equation was derived to describe the internal oxidation of Cu-A1 alloy spheres, which was checked experimentally by means of oxidation depth measurements. The results show that the derived equation is exact enough to describe the kinetics of internal oxidation of Cu-Al alloy spheres. Based on this equation and the oxidation depth measurements, the permeability of oxygen in solid copper has been obtained. Investigation also shows that in the process of internal oxidation, there is no evidence for preferential diffusion along grain boundaries.

Kinetic equation for internal oxidation of Cu-Al alloy cylinders

The kinetics of internal oxidation of Cu-Al alloy cylinders, containing up to 2.214mol% Al, were investigated in the temperature range of 1023 K to 1273 K, and the depth of internal oxidation was measured in the microscopy. A kinetic equation was derived to describe the internal oxidation of Cu-Al alloy cylinders. For the internal oxidation of Cu-Al alloys employed in the synthesis of alumina dispersion strengthened copper, the kinetic equation can be simplified. The derived equation was checked experimentally by means of oxidation depth measurements and the results show that the derived equation is exact enough to describe the kinetics of internal oxidation of Cu-Al alloy cylinders. Based on this equation and the oxidation depth measurements, the permeability of oxygen in solid copper was obtained. Investigation also shows that there is no evidence for preferential diffusion along grain boundaries in the process of internal oxidation.

Isothermal oxidation behavior of Nb-bearing austenitic cast steels at 950℃

The oxidation behaviors of three austenitic cast steels with different morphologies of primary carbides at 950℃ in air were investigated using scanning electron microscopy,energy dispersive spectroscopy,and focused ion beam/transmission electron microscopy.Their oxidation kinetics followed a logarithmic law,and the oxidation rate can be significantly decreased as long as a continuous silica layer formed at the scale/substrate interface.When the local Si concentration was inadequate,internal oxidation occurred beneath the oxide scale.The spallation of oxides during cooling can be inhibited with the formation of internal oxidation,owing to the reduced mismatch stress between the oxide scale and the substrate.The“Chinese-script”primary Nb(C,N)was superior to the dispersed primary Nb(C,N)in suppressing the oxidation penetration in the interdendritic region by supplying a high density of quick-diffusion Cr channels.In addition,the innermost and outermost oxidation layers were enriched with Cr,whereas the Cr evaporation in the outermost layer was significant when the water vapor concentration in the environment was high enough.These findings further the understanding regarding the oxidation behavior of austenitic cast steels and will promote the alloy development for exhaust components.

Isothermal oxidation behavior and mechanism of a nickel-based superalloy at 1000°C

The oxidation behavior of a nickel-based superalloy at 1000°C in air was investigated through X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy analysis. A series of oxides, including external oxide scales（Cr_2O_3,（TiO_2 ＋ Mn Cr_2O_4）） and internal oxides（Al_2O_3,Ti N）, were formed on the surface or sub-surface of the substrate at 1000°C in experimental still air. The oxidation resistance of the alloy was dependent on the stability of the surface oxide layer. The continuity and density of the protective Cr_2O_3 scale were affected by minor alloying elements such as Ti and Mn. The outermost oxide scale was composed of TiO_2 rutile and Mn Cr_2O_4 spinel, and the growth of TiO_2 particles was controlled by the outer diffusion of Ti ions through the pre-existing oxide layer. Severe internal oxidation occurred beneath the external oxide scale, consuming Al and Ti of the strength phase γ′（Ni_3（Al,Ti）） and thereby severely deteriorating the surface mechanical properties. The depth of the internal oxidation region was approximately 35 μm after exposure to experimental air at 1000°C for 80 h.

Fabrication of an in-situ nanoAl_2O_3/Cu composite with high strength and high electric conductivity

A heat-resistant dispersion-strengthening nano-Al_2O_3/Cu composite with highstrength and high electric conductivity was fabricated in a multiplex medium. The internaloxidation product, microstructures and properties of the composite, and the process flow weresystematically studied. It is confirmed that this new technique simplifies the process and improvesthe properties of the composite. X-ray analysis indicates that the alumina particles formed duringinternal oxidation consist of a large mount of gamma-Al_2O_3 and a certain amount of theta-Al_2O_3and alpha-Al_2O_3. TEM observation shows that the obtained gamma-Al_2O_3 nano-particles areuniformly distributed in the copper grains; their mean size and space between particles are 7 runand 30 nm, respectively. The main properties of the composite with 50 percent cold deformation areas follows: the electric conductivity is 51 MS/m (87 percent IACS), sigma_b = 628 MPa, and thehardness is HRB86. After annealing at 1273 K, all or most of the above properties remain, and themicrostructures are still dependent on elongated fiber-form grains.

Effect of Rare Earth on Void Band of Diffusion Layer and Properties of Aluminized Steel

The effects of the addition of rare earth （RE） elements on the void band in the diffusion layer, and the re sistances to both oxidation and spalling of aluminized steel were investigated through high temperature oxidation and spalling tests. The results showed that RE had significant effects on the void band in the diffusion layer and the properties of aluminized steel. After diffusion treatment, a considerable number of the voids between the middle layer and transitional layer of pure aluminized coating, aggregated into wavy-line-shaped void bands parallel to the outer surface. For the RE added aluminized coating, only a few voids aggregated into intermittent block shapes. During high temperature oxidation at 800 ℃ for 200 h, the wavy void band of pure aluminized coating aggregated further into a linear crack parallel to the outer surface, and the internal oxidation occurred within them; the open cracks perpendicular to the surface penetrated through the diffusion layer. For the RE added aluminized coating, only a few voids aggregated into intermittent meniscus shapes. During cyclic spalling tests, the peeling, spallation, and pulver ulent cracking occurred along the void band in the diffusion layer of pure aluminized coating, but only a little spallation occurred in the diffusion layer of the RE-added aluminized coating, in which cracks perpendicular to the surface were much smaller than those of pure aluminized coating and did not penetrate through the diffusion layer. It is evident that RE addition can restrain the formation and aggregation of voids and subsequently improve the resistances to oxidation and spalling. The mechanism of the RE effect on the void band in the diffusion layer is also discussed.

Characteristics of alumina particles in dispersion-strengthened copper alloys

Two types of alumina dispersion-strengthened copper（ADSC） alloys were fabricated by a novel in-situ reactive synthesis（IRS） and a traditional internal oxidation（IO） process. The features of alumina dispersoids in these ADSC alloys were investigated by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. It is found that nano-sized γ-Al2O3 particles of approximately 10 nm in diameter are homogeneously distributed in the IRS-ADSC composites. Meanwhile, larger-sized, mixed crystal structure alumina with rod-shaped morphology is embedded in the IO-ADSC alloy. The IRS-ADSC composites can obtain better mechanical and physical properties than the IO-ADSC composites; the tensile strength of the IRS-ADSC alloy can reach 570 MPa at room temperature, its electrical conductivity is 85% IACS, and the Rockwell hardness can reach 86 HRB.

Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr–Mn–N Austenitic Stainless Steels

The effect of intergranular precipitation on the internal oxidation behavior of Cr–Mn–N austenitic steels at 1000 °C in dry air atmosphere was investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction analysis. The results show that intergranular M23C6 carbide morphologies play an important role on the internal oxidation behavior of Cr–Mn–N steels. During the period of the oxidation, both discontinuous chain-shaped and continuous film-shaped intergranular M23C6 carbides precipitated along the grain boundaries. Internal oxides of silica preferentially intruded into the matrix along grain boundaries with discontinuous M23C6 carbide particles, while silica was obviously restricted at the interfaces between the external scale and matrix on the occasion of continuous film-shaped M23C6 carbides. It is seemed that reasonable microstructure could improve the oxidation resistance of Cr–Mn–N steels.

PHASE BOUNDARY OF α_2 PRECIPITATION IN Ti-Al-La TERNARY SYSTEM

The phase boundary,of α_2 precipitation at 600℃ in Ti-AI-La ternary system has been de- termined by TEM examination,which shows three kinds of phase regions,namely the single α phase region,the transitional phase region where there is superlattice reflection but no α_2 precipitation,and the α+α_2 region.A point of maximum aluminium content was found to exist on the α/(α+α_2)boundary and the track of the boundary was explained in terms of the internal oxidation of lanthanum and the electron concentration rule for α_2 formation.

Processing,Properties and Microstructures of Ag-ZnO Electrical Contact Composites

New powder metallurgy processing routes were designed to manufacture Ag ZnO electrical contact composites. Their physical properties, electrical contact properties and microstructures were investigated. By modelling tests, it is shown that the requirements of commercial use were met. It is proved that Ag ZnO composites could be used to substitute toxic Ag CdO on large load electrical contactors.

High temperature oxidation behavior of Ni-based superalloy GH586 in air

In this paper, the isothermal oxidation kinetics and oxidation behavior of GH586 superalloy from 800 to 1000℃ were investigated. The oxide scale morphologies of the surfaces and the cross sections after oxidation were characterized by means of X-ray diffraction （XRD） and scanning electron microscope （SEM） equipped with energy-dispersive spectroscopy （EDS）. The results show that the growth of the oxide scales on the surface of superalloy GH586 obeys a parabolic law with the activa- tion energy of 241.4 kJ.mo1-1 from 800 to 1000℃ The dense oxide scale formed at 800℃ is mainly composed of Cr203, NiCr204 and a small amount of TiO2. At 900℃, the oxide scale is divided into two layers： the outer layer with multiple cracks is mainly composed of ＂Cr203 and TiO2, while the inner is a layer of dense Cr203. Under the oxide scale, aluminum-rich oxides along the grain bound- aries are generated by the internal oxidation. At 1000 ℃for 100 h, cracks throughout the whole oxide film accel- erate the oxidation rate of Ni-based superalloy GH586 and large blocks of TiO2 in the oxide scale are generated, resulting in the spallation of oxide scale.

Oxidation Behavior of K4750 Alloy at Temperatures Between 750°C and 1000°C

This study investigated the oxidation behavior of a new casting Ni-based superalloy K4750 at 750°C-1000°C in air for 100 h-1000 h by isothermal oxidation tests.The oxidation-kinetic curves were plotted by the static discontinuous weight gain method.Observation and identification of oxidation products were conducted using scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),electron probe micro-analysis(EPMA)and X-ray diffraction(XRD).X-ray photoelectron spectrometer(XPS)was also used to analyze the chemical state of elements and the distribution in depth.The results showed that the oxidation-kinetic curves of K4750 alloy basically obeyed the parabolic law.The average oxidation rate below 900°C was less than 0.1 g/m 2·h which meant the alloy was at a complete anti-oxidation grade,and the alloy was at an anti-oxidation grade at 1000°C.The predominant surface oxide was Cr_(2)O_(3),and a double layer structure of the oxide scale was observed at all tested temperatures as time increased.The outer oxide layer contained continuous Cr_(2)O_(3)and a small amount of oxides mixed TiO2 and NiCr2O4,while the inner oxide layer was composed with Al_(2)O_(3).The oxidation process could be interpreted by the competitive oxidation of different elements.The diffusion rate of Ti through Cr_(2)O_(3)layer increased with increasing temperature,and thus the generation of TiO2 was advantageous.The dispersed TiO2 reaching a certain amount destroyed the continuity of the surface oxide layer,which accounted for the reduction of oxidation resistance of K4750 alloy at high temperatures.

Effect of Temperature on Scale Morphology of Fe-1.5Si Alloy

Because of the effect of silicon on the formation of oxide scale, red scale is the main surface defect of hot rolled Fe-Si plate, making the scale difficult for descaling compared with carbon steel. Thermogravimetric analyzer （TGA） is used to simulate isothermal oxidation process of Fe-1.5Si alloy for 60 min under air condition, and the temperature range is from 700 to 1 200 ℃. Electron probe microanalysis （EPMA） is used to observe cross-sectional scale morphology and analyze elemental distribution of the scale. Relational graph of temperature, scale thickness and scale structure is obtained. It is found that scale structure （outer Fe oxide layer＋inner FeO/Fe2SiO4 layer＋internal Si oxide precipitates） is almost unchanged with temperature except at 1000 and 1 200 ℃. At 1000 ℃ internal Si ox- ide precipitates cannot be found at the subsurface of the alloy, and at 1200 ℃ FeO/Fe2SiO4 not only forms a layer as usual but also penetrates into the outer Fe oxide layer deeply.