Search

This work highlights a new process for direct manufacturing of ductile iron that is based on the innovative combined technology of quenching and partitioning, and low-temperature-transformation of nano bainite as used for high strength steels. In this process, a commercial unalloyed ductile iron is austenitized at 890 °C for 20 min, then rapidly quenched to 180 °C for 5 s, and finally partitioned at 220 °C for 240 min. A maximum tensile strength of more than 1600 MPa, a hardness of 55 HRC at an elongation in excess of 5%, and the number of repeat tensile fatigue failure of 2.5 × 104 cycles at a stress amplitude of 600 MPa, are achieved. This is comparable to those of a high strength carbon alloy steel. These properties are due to the synergistic strengthening and toughening effects of a multiphase structure comprising tempered martensite, bainitic ferrite and retained austenite in the matrix, and a spherical graphite morphology. This work provides a guidance on how to produce unique steel-like ductile iron with a high level of strength and acceptable toughness.

An unalloyed ductile cast iron with a multiphase structure is designed by a novel austempering process. The designed austempering treatment consists of initial rapid quenching to 180°C after austenizing at 890°C for 20min, and finally austempering at 220°C for 240min. A multiphase structure comprising lenticular/needle-like prior martensite, fine needle bainitic ferrite and film retained austenite is obtained. The excellent mechanical properties, with a tensile strength of 1530MPa and an elongation of 3.1% can be achieved by controlling the matrix microstructure of 12% prior martensite, 15% retained austenite with 1.64% carbon content, and 73% bainitic ferrite. This is mainly attributed to prior marteniste which can promote refinement of multiphase colonies.

High nitrogen stainless steel (HNSS) is becoming an increasingly important engineering material because of its excellent corrosion resistance and good mechanical properties. A hot rolled Cr19Mn19Mo2N0.7 HNSS, having a microstructure consisting of banded ferrite and dotted/dendritic nitrides dispersed in the austenite matrix, was solution treated at 1160oC for various times. It is shown that the nitride can be dissolved, and the shape of ferrite can be changed by the solution treatment. The microhardness of both austenite and ferrite decreases with increasing solution treatment time. This is attributed to an increased grain size and a homogeneous distribution of alloying elements. The results are helpful in controlling the thermomechanical processing of HNSSs in commercial practice.

Austempered ductile iron (ADI) has been widely used in various industries due to its excellent combination of high strength, ductility and good wear resistance. The tensile behavior of an unalloyed commercial ADI with a multiphase structure designed by a novel multi-step austempering treatment is investigated. The developed austempering process consists of austenitizing at 890°C for 20min, then initial rapid quenching to 180°C, and isothermal holding at 190, 220, 250°Cfor 120min, and finally air cooling to room temperature. The optimum mechanical properties with an ultimate tensile strength of 1350MPa, a yield strength of 1090MPa, as well as an elongation of 3.5% is achieved at 220°C. This is attributed to a synergistic strengthening effect of multiphase structure including a prior martensite with fine needle bainitic ferrite and film retained austenite.

A commercial ductile iron is treated by a novel austempering process to obtain a good combination of strength and ductility. The samples are austenitised at 890°C for 10 min, then quenched into patented quenching liquid, and austempered in an electric furnace at 220°C for 5, 10, 30, 60, 240 and 600 min, respectively, finally air cooled. The bending test and the tensile test are conducted and microstructural features are analysed on the austempered ductile iron. The optimum mechanical property is achieved at 220°C for 240 min. Main reason for high strength and ductility is the formation of a fine structure consisting of multiple phases of pre-formed martensite and lath bainitic ferrite with film retained austenite.

Ferritic nitrocarburizing offers excellent wear, scuffing, corrosion and fatigue resistance by producing a thin compound layer and diffusion zone containing e (Fe2-3(C, N)), γ′ (Fe4N), cementite (Fe3C) and various alloy carbides and nitrides on the material surface. It is a widely accepted surface treatment process that results in smaller distortion than carburizing and carbonitriding processes. However this smaller distortion has to be further reduced to prevent the performance issues, out of tolerance distortion and post grinding work hours/cost in an automotive component. A numerical model has been developed to calculate the nitrogen and carbon composition profiles of SAE 1010 torque converter pistons during nitrocarburizing treatment. The nitrogen composition profiles are modeled against the part thickness to predict distortion. Experimental composition profiles of torque converter pistons are reported for model validation and to determine the effect of nitrogen composition on distortion. The surface residual stresses predicted by the model are also compared to the measured residual stresses. The correlations between nitrocarburizing temperature, distortion and composition profiles are discussed using the modeling results. CITATION: Manivannan, M., Stoilov, V., and Northwood, D., "Ferritic Nitrocarburizing of SAE 1010 Plain Carbon Steel Parts," SAE Int. J. Mater. Manf. 8(2):2015, doi:10.4271/2015-01-0601. 2015-01-0601 Published 04/14/2015 Copyright © 2015 SAE International doi:10.4271/2015-01-0601 saematman.saejournals.org Downloaded from SAE International by Madhavan Manivannan, Tuesday, March 03, 2015

The effect of unidirectional surface roughness on the corrosion behaviour of nickel in 0.5 M H 2 SO 4 solution was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization techniques. The surfaces, both before and after corrosion, were characterized by scanning electron microscopy, profi lometry for roughness and energy dispersive spectroscopy for oxygen content. The results were compared with those for patterned samples consisting of an array of holes. For the unidirectional surface roughness samples, an increase in roughness gave rise to an increase in corrosion rate, refl ecting the decreased ability to form a stable passive fi lm. The patterned samples showed a higher corrosion resistance, which is attributed to a different corrosion protection mechanism, namely heterogeneous wetting.

The effects of heat treatment on distortion, residual stress, and retained austenite were compared for case-carburized 4320 steel, in both the austempered and quench-and-tempered condition. Navy C-ring samples were used to quantify both size and shape distortions, as well as residual stress. The austempering heat treatment produced less distortion and a higher surface residual stress. Both hoop and axial stresses were measured; the difference between them was less than seven percent in all cases. Depth profiles were obtained for residual stress and retained austenite from representative C-ring samples for the austempered and quench-and-tempered heat treatment conditions. Austempering maintained a compressive residual stress to greater depths than quench-and-tempering. Quench-and-tempering also resulted in lower retained austenite amounts immediately beneath the surface. However, for both heat treatments, the retained austenite content was approximately one percent at depths greater than 0.5 mm.

In the plasma electrolytic oxidation (PEO) coating of light metal alloys, changing the electrical parameters and electrolytic composition can change the discharge behaviour and, ultimately, the thickness, surface morphology and porosity of the coating. In the present study a combination of cathodic and anodic current pulses with suitable Ton and Toff periods were used to control the porosity and other structural defects of PEO coatings of an AM60B magnesium alloy. In order to investigate the effect of a current mode on the plasma discharge behaviour and coating microstructure during the PEO treatment of magnesium alloy, the emission intensities of six different spectral lines from the plasma species were recorded simultaneously as a function of both time and current mode using optical emission spectroscopy (OES) system. The fluctuations in signal intensities and temperature during the coating process reflect differences in location of both the discharge initiation, and discharge type. The coating surface morphology and microstructure that are obtained can be linked to the plasma discharge behavior. These results are discussed in relation to the discharge behaviour, and how such changes in discharge behaviour relate to the coating mechanisms.

Magnesium alloys are considered one of the more promising materials for future use in many engineering applications. However, due to their high chemical and electrochemical reactivity, magnesium alloys have poor corrosion resistance in aqueous environments. Improving their corrosion resistance by coating can greatly extend their application. One promising coating method is plasma electrolytic oxidation (PEO). The nature of the coating formed, and the ultimate corrosion performance depends on the both the processing parameters (electrolyte, current density, current mode, processing time) and specific Mg-alloy substrate. In the present study, PEO coatings were produced on three different Mg-alloys (AJ62, AM60B and AZ91D) using different processing parameters. Scanning electron microscopy was used to characterize the coatings. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) in an aqueous 3.5% NaCl solution. Relationships are drawn between PEO processing parameters, substrate composition and corrosion performance. Electrochemical impedance spectroscopy data indicate that the bipolar PEO coated AZ91D Mg alloy demonstrates a higher corrosion resistance when compared to coated AM60B, AJ62 and pure Mg.

To further our understanding of the plasma electrolytic oxidation (PEO) process, and to aid in the optimization of the process, it is important to identify the mechanisms of coating formation. In the present work, coatings up to 110 μm thick were produced on an AJ62 Mg-alloy substrate using the PEO process. Optical emission spectroscopy (OES) was employed to follow the microdischarges and substrate and electrolyte elements present in the plasma discharge during the coating growth, and to determine plasma electron temperatures. During PEO processing of magnesium, some of the metal cations are transferred outwards from the substrate and react with anions to form ceramic coatings. Also, due to the high electric field in the discharge channels, oxygen anions transfer toward the magnesium substrate and react with Mg 2+ cations to form a ceramic coating. In PEO process, the ceramic coating grows inwards to the alloy substrate and outwards to the coating surface simultaneously. The total coating thickness variation compared with the geometrical dimensions of the uncoated and coated samples were investigated. For the coating growth, there are three simultaneous processes taking place, namely the electrochemical reactions, the plasma chemical reactions and thermal diffusion. Oxygen diffusion occurring during PEO processing is discussed in terms of coating growth mechanisms.

The application of magnesium alloys in the automotive industry has been limited by their inferior corrosion and wear properties. PEO (plasma electrolytic oxidation) is an electrochemical process that uses a non-hazardous aqueous electrolyte to oxidize the metal surfaces to form oxide coatings which impart high corrosion and wear resistance. In this study we investigated the effect of current mode (unipolar, bipolar or hybrid (combination of both)) on the corrosion and wear properties of PEO coatings formed on an AM60B magnesium alloy (mass fraction: Al 5.6–6.4%, Mn 0.26–0.4%, Zn ≤ 0.2%, balance Mg). Changing the current mode produces changes in the PEO process characteristics, including the breakdown voltage and discharge events, both in terms of discharge intensity and density. The morphology and microstructure of the coatings were investigated using scanning electron microscopy (SEM). PEO-coated materials tested using a pin-on-disk tribometer under dry sliding conditions show a higher coefficient of friction (COF) but a lower wear rate than the uncoated alloy. It was also found that the coatings formed using a bipolar or hybrid (unipolar + bipolar) current mode showed lower COF values and wear rates due to their modified coating microstructure and surface morphology. Inclined impact–sliding wear tests, which allow the simulation of a combination of impact fatigue failure and sliding wear failure under repetitive high local loads, also demonstrated the superior tribological properties of coatings produced using either a bipolar or a hybrid current mode.

Plasma electrolytic oxidation (PEO) is an electrochemical process uses an environmentally-friendly aqueous electrolyte to oxidize the metal surfaces to form ceramic oxide coatings which impart a high corrosion and wear resistance. In this study we investigated the effect of current mode on the properties of PEO coatings formed on an AM60B magnesium alloy. Unipolar, bipolar and hybrid (combination of both) current modes were used in this work. Optical emission spectroscopy (OES) was employed to investigate the PEO plasma. Coatings prepared using a hybrid current mode had a dense inner layer with fewer defects. The order of the applied current modes (unipolar and bipolar modes) had a significant effect on the coating morphology and corrosion resistance. The effects of the negative current part of the pulses on the coating were studied by using a unipolar current mode with different pulse timings. Coatings produced with a longer off-time had more porosity, and hence lower corrosion resistance. The corrosion protection efficiency of these coatings are compared to those produced by PEO coatings on a AJ62 Mg-alloy.

In this study, we have used PEO (plasma electrolytic oxidation) for the production of oxide coatings on a Ti–6Al–4V alloy at two different current modes, namely pulsed unipolar and bipolar. Optical emission spectroscopy (OES) in the visible and near UV band (280–800 nm) was used to characterize the PEO plasma. The emission spectra were recorded and the plasma temperature profile versus processing time was constructed using a line intensity ratios method. The aim of this work was to study the effect of the process parameters, including current mode and pulse duration time, on the plasma characteristics, surface morphology and microstructure and corrosion resistance of oxides grown on Ti–6Al–4V by PEO process. Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD) were used to study the coating microstructure, morphology and phase composition. The corrosion resistance of the coated and uncoated samples was examined by potentiodynamic polarization in a 3.5% NaCl solution. It was found that the plasma temperature profiles are significantly influenced by changing the current mode from unipolar to bipolar. The strongest discharges that are initiated at the interface between the substrate and the coating can be reduced or eliminated by using a bipolar current mode. This produces a thinner, denser and more corrosion-resistant coating.

Magnesium alloys are increasingly being used as lightweight materials in the automotive, defense, electronics, biomaterial and aerospace industries. However, their inherently poor corrosion and wear resistance have, so far, limited their application. Plasma electrolytic oxidation (PEO) in an environmentally friendly aluminates electrolyte has been used to produce oxide coatings with thicknesses of similar to 80 mu m on an AJ62 magnesium alloy. Optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285 nm-800 nm) was employed to characterize the PEO plasma. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the coated materials, and potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution were used to determine the corrosion behavior. It was found that the plasma discharge behavior significantly influenced the microstructure and the morphology of the oxide coatings and, hence the corrosion resistance. The corrosion resistance of the coated alloy was increased by changing the current mode from unipolar to bipolar, where the strong plasma discharges had been reduced or eliminated. (C) 2011 Elsevier B.V. All rights reserved.

Carbonitriding is a widely accepted method of heat treatment used by North American manufacturers for plain carbon steel. While the process imparts a hard, wear resistant case, it is also associated with both size and shape distortion, which can be problematic for components in tight fitting assemblies. This research compares the effects of the carbonitriding and nitrocarburising processes on SAE 1010 steel with respect to residual stress and distortion. Results indicate that the two processes develop residual compressive stresses and are associated with both size and shape distortion. Although the magnitude of the compressive stresses in nitrocarburised steels is lower, this process was noted to give rise to overall smaller dimensional changes than the carbonitriding process. The findings from this study are applied to a manufacturing application involving the surface treatment of a thin shelled automotive component in a light loading application.

A Plasma Electrolytic Oxidation (PEO) process was used to produce thin oxide coatings on a Zr–2.5 wt% Nb alloy. Effects of current density on surface morphologies and wear properties of PEO coatings were investigated and compared to the uncoated substrate and a commercially used black oxide coating. Corrosion properties at ambient and high temperature/pressure conditions were studied using potentiodynamic polarization tests and autoclave tests, respectively. Up to 30-day autoclave experiments were carried out in an aqueous condition of 300 °C and 10 MPa in 0.05 M LiOH solutions. It was found that most of the micro-pores which were produced during the PEO treatment were closed after the autoclave experiments. PEO coatings had larger weight gains in the first 10 exposure days than the black oxide coating. However, after 10 days, the corrosion rate of black oxide coating accelerated and exhibited a similar weight gain to PEO coatings after 30 days. PEO coatings prepared at low current densities had lower weight gains. Although the black oxide coating exhibited a good corrosion resistance, it had a much lower wear resistance than the PEO coatings. Compared with the uncoated substrate, all PEO coatings had a higher corrosion resistance, lower weight gain during autoclave tests and better wear resistance.

In this study, the contact resistance (CR) and electrochemical properties of TiN, CrN and TiAlN electron beam physical vapor deposition (EBPVD) coatings and their stainless steel 316L (SS316L) substrate were investigated in a simulated proton exchange membrane (PEM) fuel cell environment. The potentiodynamic polarization corrosion tests were conducted at 70 °C in 1 M H 2 SO 4 purged with either O 2 or H 2 , and the potentiostatic corrosion tests were performed under both simulated cathodic (+0.6 V vs. Ag/AgCl reference electrode purged with O 2 ) and anodic conditions (−0.1 V vs. Ag/AgCl reference electrode purged with H 2 ) for a long period (4 h). SEM was used to observe the surface morphologies of the samples after corrosion testing. All the TiN-, TiAlN- and CrN-coated SS316L showed a lower CR than the uncoated SS316L. While the corrosion performance of the coatings was dependent on the cathodic and anodic conditions, the CrN coating exhibited a higher (in the anodic environment) or similar (in the cathodic environment) corrosion resistance to the uncoated SS316L. Thus, the CrN-coated SS316L could potentially be used as a bipolar plate material in the PEM fuel cell environment. Although the EBPVD process greatly reduced number of pinholes in the coatings compared to other plasma enhanced reactive evaporations, future research efforts should be directed to eliminate the pinholes in the coatings for long-term durability in fuel cell applications.

Dissolution of secondary phases during thermal treatment in cast magnesium alloys influences their engineering properties. In this study, a kinetic model based on a Kissinger-type method has been developed for describing dissolution of secondary phases in the high pressure die cast magnesium alloy AM50 during a thermally activated heating process. Also, differential scanning calorimetry (DSC) was effectively used for investigating the dissolution kinetics of secondary phases in the AM50 alloy. By fitting a kinetic model to the DSC results, the activation energy of the dissolution of the secondary phases can be determined. In parallel, the microstructure of the alloy was analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). It was found that the distribution of secondary phases and the concentration of alloying elements both at the grain boundaries and in the grains play an important role in the solid-state transformation kinetics of die cast magnesium alloy AM50.

Carbonitriding is a metallurgical surface modification technique that is widely used in the automotive industry to increase surface hardness and wear resistance. Given the problems associated with carbonitriding, such as dimensional distortion, oxidation and non-uniform surface hardness, nitrocarburizing has been proposed as an alternative heat treatment method to improve the surface characteristics. The major advantages of ferritic nitrocarburizing are the minimal dimensional changes and distortion due to the low process temperature at which no phase transformations occur. This increases productivity and product quality, and decreases costs. The focus of this study was to determine the effects of carbonitriding and ferritic nitrocarburizing processes on the dimensional changes and residual stresses in a steel used for automotive applications. Navy C-ring specimens and prototype stamped parts made from SAE 1010 plain carbon steel were used in the testing. Gas, vacuum and ion ferritic nitrocarburizing processes with different heat treatment parameters were investigated. X-ray diffraction techniques were used for the residual stresses evaluation and surface phase analysis of the specimens.

Magnesium is subject to severe galvanic corrosion attack in a commercially available engine coolant, when in contact with stainless steel and an Al alloy. Thin oxide coatings with different coating thicknesses were produced using a Plasma Electrolytic Oxidation (PEO) process to protect Mg against the corrosion attack. The galvanic corrosion properties of the PEO coatings were investigated using immersion corrosion testing. The test results showed that the anti-corrosion properties of the PEO coatings increased with increasing coating thickness. PEO coatings with a thickness of several micrometers effectively prevented the galvanic corrosion of Mg, without significantly reducing the heat conductivity of the Mg.

A series of casting experiments were carried out with Mg–5.0 wt.% Al–2.0 wt.% Ca (AC52) alloy with different cooling rates from 0.5 to 65.0 °C/s. A significant refinement of the grain size of the alloy had been achieved at higher cooling rates of 20.0–65.0 °C/s. The decrease in cooling rates resulted in continuous coarsening of the eutectic phases from nanoscale particle to microscale needle within the primary grain of the alloys. EDS results showed that there were the higher solute content of alloying elements (Al and Ca) within the primary grain of the alloys resulting from the quenching effect of fast cooling. The nanoscale creep resistance of the as-cast AC52 alloys increased with an increase in cooling rate except that the alloy cast at 20 °C/s showed the highest creep resistance. The creep curves indicated that the interaction between the dramatically increased dislocations and the surrounding solute and dispersed fine eutectics caused a high plastic penetration depth in the primary stage after loading. Then creep deformation changed from the high stress exponent regime ( n ≈ 7) to the low stress exponent regime ( n ≈ 3) at a threshold stress (∼140 MPa) controlled by the dislocation and twining controlled mechanisms. The creep behavior was correlated with the microstructure including solute content of alloying elements and dispersed eutectic phases within the primary grain to explain the underlying creep mechanisms.

A physical vapor deposition (PVD) TiN coating has been used to increase the corrosion resistance of two stainless steel materials for bipolar plate application in proton exchange membrane fuel cells (PEMFCs). Our earlier studies had shown that a TiN coating on SS410 and SS316L increased the corrosion resistance of SS410 and SS316L significantly. In this study, we examine how the substrate affects the corrosion of TiN-coated stainless steel in the simulated anode and cathode environments. Potentiodynamic and contact resistance test results show that the polarization resistance and contact resistance of TiN-coated SS410 and TiN-coated SS316L are almost the same. However, in the simulated anode condition, the corrosion current density of TiN-coated SS410 is positive and the corrosion current density of TiN-coated SS316L is negative. Inductively coupled plasma optical emission spectrometry (ICP-OES) test results also show that the metal ion concentration is much higher for TiN-coated SS410 at the anode side. At the cathode side, the potentiostatic and ICP-OES tests show that the corrosion of TiN-coated SS410 and TiN-coated SS316L are in the same range. Therefore, the substrate has an effect on corrosion in the simulated anode working conditions of PEMFCs. In order to be the suitable bipolar plate materials, both the coating and substrate need to have a higher corrosion resistance.

In proton exchange membrane fuel cells (PEMFCs), the bipolar plates supply the reactant gases through the flow channels to the electrodes and serve the purpose of electrochemically connecting one cell to another in the electrochemical cell stack. Requirements of the bipolar plate material are: high values of electronic conductivity; high values of thermal conductivity; high mechanical strength; impermeability to reactant gases; resistance to corrosion; and low cost of automated production. Metallic materials meet many of these requirements but the challenge has been in obtaining the required corrosion resistance. In the paper, six metallic materials were investigated as potential bipolar plate materials. The results showed that the corrosion rates were too high even for the most corrosion resistant metals (SS316L and grade 2 Ti), and that coatings would be required.

The effects of Ca additions (0.5–2.0 wt.%) on the microstructure and the microhardness of an as-cast Mg–5.0 wt.% Al alloy have been investigated. The coarse microstructure of the base alloy can be refined through adding Ca. DSC and TEM results showed that, as Ca additions increased up to 1.5 wt.% Ca, the β-Mg17Al12 phase was completely replaced by a (Al, Mg)2Ca phase. The Vickers microhardness of the as-cast Mg–Al–Ca alloys increased with increasing Ca content. Tests on the Mg–5.0Al–2.0Ca (wt.%) alloy showed an indentation size effect, which was well described by Meyer's Law.

The microstructure of magnesium alloys containing aluminum (AM50) and calcium (AC51 and AC52) cast by high-pressure die casting (DC) or permanent mold casting (PM) were characterized by SEM and TEM, and the influence of these microstructural features on the local mechanical properties was investigated. The presence of (Al, Mg)2Ca phase was identified in PM AC51 and AC52 alloys, which has a high melting temperature, was thermally more stable than the Mg17Al12 phase. A nanoindentation technique using a Berkovich indenter was used to characterize the nano-scale mechanical behavior of the α-Mg grains of the alloys at room temperature. The results of nanoindentation testing show that the hardness and composite modulus of the alloys vary with penetration depth. Ca addition to the alloy gives rise to an increase in the hardness and modulus, which can be attributed to solid solution strengthening. A regression analysis of the experimental nanoindentation data reveals a linear correlation between the contact stiffness and the penetration depth for all the tested alloys. The established correlations were used to calculate the continuous hardness and the composite modulus. Both the experimental and calculated data show that hardness values exhibit peak load dependence, i.e. indentation size effect (ISE) on all the tested alloys, while composite modulus was insensitive to peak load. Upon reaching a peak value of hardness at the beginning of the indentation test, the hardness of all the tested alloys decreases considerably by six times from around 1.2 to 0.2 GPa with increasing the penetration depth during the course of testing. However, the composite modulus of the alloy is much less dependent on the penetration depth only changing from 35 to 25 GPa during the tests. The comparison of the calculated results with experimental measurements indicates that they are in good agreement.

Mg-Al-Ca alloys with 1wt.% and 2 wt.% Ca additions (AC51 and AC52) were cast by the Permanent Mold technique. The microstructures of the as-cast Mg-Al-Ca alloys were observed by SEM with EDS analysis. The secondary phases were mainly precipitated along the grain boundaries and exhibited a continuous network microstructure for the AC52 alloy and a divorced microstructure for the AC51 alloy. EDS microanalysis showed that the solute (Ca) content in the grains of the AC52 alloy is higher than that in the AC51. A three-sided pyramidal (Berkovich) diamond indenter was used to characterize the local nano-creep behavior at room temperature within the α-Mg in grains. The nano-creep results showed that the AC52 alloy has better creep resistance than the AC51 alloy at all loads at room temperature. The creep exponent n, obtained from the indentation creep data, changes from 6.3 to 3.0 for AC51 alloy and from 6.6 to 3.2 for AC52 alloy at a critical stress (132 MPa for the AC51 and 145 MPa for the AC52). The transition in creep behavior at higher stresses is associated with a change in the deformation mechanisms.

Increasing attention is being paid to the use of metallic materials as a replacement for non-porous graphite in bipolar plates (BPs) for polymer exchange membrane (PEM) fuel cells. The main aim of the present study was to investigate how O2 and H2 affect the corrosion behavior of 316L stainless steel in simulated anode and cathode environments of a PEM fuel cell. Open circuit potential (OCP) measurements together with potentiodynamic and potentiostatic tests were performed to investigate the corrosion behavior of SS316L in the O2- and H2-containing environments. Optical microscopy and scanning electron microscopy (SEM) were used to characterize the morphology of the corroded surface and the corrosion products. Inductively coupled plasma optical emission spectrometer (ICP-OES) was used to determine the levels of metal ions in solution after corrosion. The OCP and potentiodynamic tests showed that less corrosion of SS316L took place in a simulated cathode environment because it was easier to passivate SS316L in an O2-containing environment. However, the potentiostatic tests showed that there is less corrosion in a simulated anode environment, because H+ ions are reduced, and the resulting negative current can provide partial cathodic protection to the SS316L.

Corrosion performances of several metallic materials (Al6061 and Al319 alloys, 304 stainless steel and grey cast iron) in the ethanol-gasoline alternative fuels were investigated. Cyclic potentiodynamic polarization tests were used to study their corrosion behavior. Anodizing and plasma electrolytic oxidation (PEO) techniques were used to produce oxide coatings on the Al6061 and Al319 alloys, and the corrosion properties of these coatings in the alternative fuel environments were also evaluated. The results showed that, the 304 stainless steel, Al6061 and the coating materials are compatible with the alternative fuels. The oxide coatings on both Al alloys provided effective corrosion protection in the alternative fuel environments.

Bipolar plates are a key component of PEM fuel cells, and they constitute a large portion of the weight and total cost of a fuel cell stack. In order to reduce both the cost and weight of the bipolar plates, considerable attention is being paid to developing metallic bipolar plates to replace non-porous graphite. In this study, TiN was coated on a martensitic stainless steel (SS410) using a PVD technology (plasma enhanced reactive evaporation) to increase the corrosion resistance of the base metal. XRD, SEM, EIS and potentiodynamic tests were used to characterize the TiN-coated SS410. In order to investigate the suitability of these coated materials as the anodes and cathodes in a PEMFC, potentiostatic tests were conducted under both simulated cathode and anode conditions. The simulated anode environment was - 0.1 V vs SCE purged with H 2 and the simulated cathode environment was 0.6 V vs SCE purged with O 2 . The dense TiN coatings formed on SS410 much improved the corrosion resistance of SS410 and, thus, these coated materials could potentially be used in PEMFCs as a bipolar plate material provided they also satisfy the other physical and mechanical property requirements such as interfacial contact resistance, light weight, high mechanical strength and manufacturability.

SAE 8620 steel is typically used in the carburized condition for powertrain applications in the automotive industry, e.g. gears, roller bearings, camshafts. Such steels always contain retained austenite to varying degrees in the as-hardened and also in the tempered microstructures. As well as retained austenite, heat treatment can produce residual stresses, which lead to distortion (size and shape). The intent of this study was to investigate the effect of heat treatment parameters on the amount of retained austenite, residual stress and distortion in carburized SAE 8620 steel. A specially designed specimen, the Navy C-ring, was used for this study. The steel was first normalized prior to machining the Navy C-ring specimens. The specimens were then heat treated by carburizing at 927°C or 954°C (1700°F or 1750°F) at four levels of carbon potential (0.9, 1.0, 1.1, 1.2) followed by oil quenching and tempering at either 149°C or 177°C (300°F or 350°F). The distortion of the C-ring was evaluated by dimensional measurements of the inner diameter, outer diameter, gap width and thickness for size distortion, as well as flatness, cylindricity and roundness for shape distortion. X-ray diffraction (XRD) techniques were used to determine the residual stress and the amount of retained austenite. The amount of retained austenite was also measured by optical metallography. The amount of retained austenite and the residual stress increased with increasing carburizing temperature and carbon potential and decreased upon tempering. There was not a significant further reduction in the amount of retained austenite and residual stress when the tempering temperature was increased from 149°C to 177°C. Distortion was influenced by both the amount of retained austenite and the magnitude of the residual stress. With increasing retained austenite/residual stress, the distortion became more serious. Based on the distortion data for 3 parameters (OD, gap width and flatness) for the quenched and tempered specimens, the amount of retained austenite for minimum distortion was approximately 25%.

In order to reduce the cost, weight and volume of the bipolar plates, considerable attention is being paid to developing metallic bipolar plates to replace the non-porous graphite bipolar plates that are in current use. However, metals are prone to corrosion in the proton exchange membrane (PEM) fuel cell environments, which decreases the ionic conductivity of the membrane and lowers the overall performance of the fuel cells. In this study, TiN was coated on SS316L using a physical vapor deposition (PVD) technology (plasma enhanced reactive evaporation) to increase the corrosion resistance of the base SS316L. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods were used to characterize the TiN-coated SS316L. XRD showed that the TiN coating had a face-centered-cubic (fcc) structure. Potentiodynamic tests and electrochemical impedance tests showed that the corrosion resistance of SS316L was significantly increased in 0.5 M H2SO4 at 70 °C by coating with TiN. In order to investigate the suitability of these coated materials as cathodes and anodes in a PEMFC, potentiostatic tests were conducted under both simulated cathode and anode conditions. The simulated anode environment was −0.1 V versus SCE purged with H2 and the simulated cathode environment was 0.6 V versus SCE purged with O2. In the simulated anode conditions, the corrosion current of TiN-coated SS316L is −4 × 10−5 A cm−2, which is lower than that of the uncoated SS316L (about −1 × 10−6 A cm−2). In the simulated cathode conditions, the corrosion current of TiN-coated SS316L is increased to 2.5 × 10−5 A cm−2, which is higher than that of the uncoated SS316L (about 5 × 10−6 A cm−2). This is because pitting corrosion had taken place on the TiN-coated specimen.

A number of studies have been carried out on the use of electrolytic plasma oxidation (EPO) technology for depositing ceramic coatings on magnesium alloys for corrosion protection. However, very few have focused on the optimization of the EPO process parameters. In this study, a design of experiment (DOE) technique, the Taguchi method, has been used to optimize the electrolytic plasma oxidation for the corrosion protection of magnesium alloy AM50. The experimental design consisted of four factors (treatment time, current density, and KOH and NaAlO 2 concentrations), with three levels of each factor. Potentiodynamic polarization measurements were conducted to determine the corrosion resistance of the coated samples. An analysis of the mean of signal-to-noise (S/N) ratio indicates that the corrosion resistance of EPO-coated AM50 alloy is influenced significantly by the levels in the Taguchi orthogonal array. The optimized coating parameters for corrosion resistance are 12 min treatment time, 12 A/cm 2 current density, 0.9 g/l KOH, 15.0 g/l NaAlO 2 . The percentage contribution of each factor is determined by the analysis of variance (ANOVA). The results show that the concentration of KOH is the most significant factor affecting the corrosion resistance of the coatings.

The effects of Ni foil thickness on the microstructure and tensile properties of reaction synthesized multilayer composites have been systematically investigated. Multilayer composites have been fabricated by reaction annealing of their foil laminates consisting of 10 μm thick Al foils and 20, 50 and 80 μm thick Ni foils. Ni3Al multilayer composites prepared from 20 μm Ni foils were very brittle. Ni/Ni3Al multilayer composites prepared from 50 μm Ni foils exhibited significant work-hardening and behaved more like a ductile alloy, rather than a composite, with an ultimate tensile strength of 1050 MPa and elongations of >18%. The composites fabricated from 80 μm Ni foils, which contain Ni3Al precipitates, had a low yield strength and ultimate tensile strength (about 160 and 470 MPa, respectively), and had a very good capacity for plastic deformation (>34% elongation). The dislocations in Ni and Ni3Al layers can slide through the Ni3Al/Ni interfaces, with the result that the Ni and the Ni3Al layers in the composites can cooperatively deform during tensile testing. Delaminated interfaces containing Al2O3 inclusions further promote the capability of the Ni3Al layers for plastic deformation. As a result, the Ni/Ni3Al multilayer composites exhibit good tensile strengths and a high ductility. The existence of Ni3Al precipitates in the Ni layers inhibits cross-slip of dislocations, which results in dislocation networks in the Ni layers which have a preferred orientation.

Full-density nickel/aluminide multilayer composites were successfully fabricated by reaction synthesis using Ni and Al foils. The Al is the dominant diffusing species, and Ni2Al3 is the first phase formed during reaction between the Ni and Al foils at 620 °C. The reaction mechanism is such that only Al atoms can diffuse through the Al2O3 films formed on the Al foils, and the Al atoms then transform into Ni2Al3 by reaction with Ni. The Ni and Ni3Al layers in the composites can therefore cooperatively deform through slipping of dislocations in the Ni and Ni3Al layers through the Ni3Al/Ni interfaces. Thus, Ni/Ni3Al multilayer composites exhibit high ultimate tensile strengths and large elongations (1050 MPa and 18.2 pct).

Unidirectional surface roughness of varying magnitudes were created on both nickel and mild steel by grinding on SiC papers with grit sizes from G60 (roughest) to G1200 (smoothest) and the corrosion resistance in 0.5M H2SO4 solution was determined using a potentiodynamic polarization technique. A different trend of corrosion rate versus roughness was seen for the active-passive metal (nickel) and non-active-passive metal (mild steel). For nickel there was an increase in corrosion rate with increasing roughness, whereas for mild steel the corrosion rate decreased with increasing surface roughness. Furthermore, through a detailed examination of the surface before and after corrosion using techniques including profilometry, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), it was established that different corrosion mechanisms were operative for nickel and mild steel. For both metals, the smaller grit sizes produced a rougher surface with wider and deeper grooves. In the case of nickel, the higher roughness provided a greater contact area between the corrosive medium and metal and there was trapping of the corrosive ions in the deep grooves. Both of these factors would lead to an increase in corrosion rate. Also, for the smoother nickel surfaces, it is easier to form a stable passive film. For mild steel, which does not form a passive film, corrosion rates are generally much higher than for nickel. For the rougher surfaces with the deeper grooves, the corrosion product, FeSO4, can fill the grooves thereby acting as a barrier to further ingress of the corrosive ions to the un-corroded metal. WIT Transactions on Engineering Sciences, Vol 90, www.witpress.com, ISSN 1743-3533 (on-line) © 2015 WIT Press doi:10.2495/MC150321 Materials Characterisation VII 355

Due to their high chemical reactivity, relatively low melting point and low hardness, magnesium and magnesium alloys have relatively poor corrosion and wear resistance. Since both corrosion and wear are surface phenomena, a number of surface engineering techniques have been used to improve corrosion and wear performance. Whilst some surface hardening/strengthening methods have led to improvements in wear properties, they have not, in themselves, significantly improved the corrosion performance. Plasma electrolytic oxidation (PEO) has the potential to produce hard, compact oxide coatings that are well adhered to the magnesium alloy substrate. Such coatings can provide both improved wear and corrosion resistance. In this paper we describe how by changing the PEO processing parameters (substrate alloy; electrolyte; current or voltage; processing time) we can change the nature of the PEO oxide coatings (thickness; microstructure; porosity; phase content; composition) which, in turn, effects the corrosion and wear performance. All PEO-coatings have a three-layer structure with a porous outer layer, and intermediate dense layer and a thin inner dense layer. From a corrosion aspect, the performance of coatings is determined by the time taken for corrosion to initiate since this is much shorter than time taken for the coating to degrade. For PEO-coated Mg-alloys, this initiation time is primarily determined by the thickness, porosity and phase content of the inner dense layer at the coating/substrate interface. With respect to tribological properties, the coefficient of friction (COF) in dry sliding wear increases with increasing surface roughness of the PEO coatings. The wear rate is primarily determined by the thickness and hardness of the intermediate dense layer. Surface and Contact Mechanics including Tribology XII 163 WIT Transactions on Engineering Sciences, Vol 91, www.witpress.com, ISSN 1743-3533 (on-line) © 2015 WIT Press doi:10.2495/SECM150151 Coatings containing less porosity and higher spinel-phase (MgAl2O4, Mg2SiO4) content are harder and more wear-resistant.

Electrolytic plasma oxidation (EPO) has been considered as a new technique to form ceramic coatings on magnesium alloys for corrosion protection. In this study, a magnesium alloy, AM50, was treated by EPO, and the Taguchi experimental analysis method was used to systematically investigate the effects of four factors (treatment time, current density, and concentration of KOH and NaAlO2) with three levels on the thickness and corrosion resistance of the coatings. It was found that treatment time was a major factor affecting the thickness, while the KOH concentration had the most significant effect on the corrosion resistance of the coatings. The phase compositions, thickness and uniformity of the coatings were responsible for the differences of the corrosion resistance of the coatings. With an increase of the KOH concentration, the MgO phase was enriched, and the MgAl2O4 phase was reduced in the coatings. On the other hand, increasing the NaAlO2 concentration resulted in an enrichment of the MgAl2O4 in the coatings. It was also found that the coatings grew towards both their substrates and their external surface, and unevenly distributed sparks lead to non-uniformity in coating thickness.

In this study, aluminum oxide coatings were deposited on Al–Si alloy substrates to produce hard and corrosion protective films using three different techniques: hard anodizing (HA), anodizing (A), and modified anodizing (MA). Potentiodynamic polarization tests were conducted to assess the corrosion resistance of the coatings. A microhardness tester was used to measure the coatings hardness. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were used to investigate the coating microstructure and chemical composition both before and after corrosion. It was found that the modified anodizing, an environmentally friendly coating method, could produce a hard oxide coating with good corrosion protection for the Al–Si alloy.

The role of heat treatment on the erosion and corrosion behavior of 52100 steel was investigated in terms of: (i) electrochemical parameter measurements, (ii) identification of microstrain and strain energy induced by erosion, and (iii) the characterization of synergistic effect between erosion and corrosion. Heat-treatment both thermodynamically and kinetically affects the electrochemical reaction of eroded 52100 steel. Based on both the polarization resistance and corrosion rate measurements, it was found that a tempering treatment gives rise to the slowest corrosion kinetics. For both the annealed and tempered specimens, where the microstrain is created solely from mechanical deformation during erosion, there is a reasonable agreement between the calculated corrosion rate based on a mechano-electrochemical effect and the experimentally measured corrosion rate. However, for the quenched specimens in which a high level of microstrain is generated on quenching, there is a significant difference between the calculated (from mechano-electrochemical effect) and experimentally measured corrosion rates.

Electrolytic plasma processing (EPP) is an emerging, environmentally friendly, surface engineering technique. In this study, we have utilized the EPP technique to deposit silicate and phosphate coatings on magnesium for both corrosion and erosion protection. Potentiodynamic polarization measurements were used to investigate the corrosion properties of the coated samples. A stirring device was also used for corrosion and erosion testing. Coated and uncoated samples were immersed in a 3.5 wt.% NaCl solution with SiO2 sand in suspension and rotated at a given speed. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis were used to study surface morphology and chemical composition of the coatings before and after corrosion–erosion testing.

CO2 laser beam (LB) welding is conducted on 7075-T6 aluminum alloy sheets at two different welding speeds and compared with gas tungsten arc (GTA) welding. The mechanical and microstructural characteristics of the welds are evaluated using tensile tests, hardness tests, optical microscopy and energy dispersive X-ray spectroscopy (EDS). Results indicate that both the hardness and tensile strength of LB welds are higher than those of GTA welds. It is shown that the hardness value of the softened region in LB welds is in the fusion zone (FZ), whereas that in the heat affected zone (HAZ) of the GTA weld. Tensile strengths of LB welds after the post-weld artificial aging treatment at 120 °C for 26 h are improved but cannot be equivalent to those of base metal because the softened FZ of the LB weld is not completely recovered after post-weld artificial treatment.

The characteristics and mechanisms of the mechano-electrochemical effect were investigated both theoretically and experimentally for two steels (AISI 1020 and 52100) in various heat-treated conditions subjected to sequential erosion and corrosion. A mathematical equation is developed to describe the mechano-electrochemical effect in which the corrosion rate is exponentially related to the change of corrosion potential and increased stored strain energy. The agreement between calculated and measured corrosion rate is good for the annealed AISI 1020 steel and both the annealed and the tempered AISI 52100 steels, in which the microstrain and stored strain energy originate only from erosion.

In this paper, a review is given of new technologies designed to improve the performance of advanced Ni/MH and Ni/Zn batteries. The typical reaction process of the Zn electrode, and a special Zn electrode manufacturing process, provide useful methods for improving advanced Ni/Zn batteries. The reaction mechanism of the Zn electrode, which is related to the charge- and mass-transfer reactions, is different from that of the MH electrode and nickel hydroxide electrode. Thus, it is necessary to add wetting and binding materials in the pasted Zn electrode. The stability of the Zn electrode is dependent on these wetting and binding materials. A Zn electrode with a long cycle lifetime has been developed for an experimental Ni/Zn cell (1.2 A h ) . The experimental cell has been cycled over 120 times.