Mechanical properties of an Al91-Mn6-Nd3 nanostructured alloy CHICOT, D; VODA, M; DECOOPMAN, X ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
08/2011, Letnik:
528, Številka:
22-23
Journal Article
Recenzirano
A methodology for obtaining a nanocrystalline Al91-Mn6-Nd3 alloy and characterizing its mechanical properties is presented. The fabrication procedure, based on the melt spinning method, allowed the ...preparation of thin ribbons of a arrow right 430 mu m in thickness. The hardness and elastic modulus have been determined by classical and instrumented indentation tests considering the influence of the ribbon thickness on the indentation measurements. The Martens hardness has been calculated by means of instrumented indentation tests, for which the fixing support of the sample has no influence on the indentation measurement. The results have been subsequently compared with those obtained from classical Vickers hardness measurements, for which some composite hardness models have been applied in order to separate the contribution of the fixing support to the hardness measurement. It has been concluded that both methodologies lead to similar values for the mechanical properties of the alloy, i.e. a arrow right 43.4GPa for the macrohardness and a arrow right 43.8MPam1/2 for the hardness length-scale factor of the material. The elastic modulus of the material, resulting from the analysis of the unloading indentation curves, has been found to be a arrow right 4131GPa.
The objective of the present work is to determine the influence of the heat treatment on the corrosion resistance of a Cr3C2-NiCr coating of 450 mum thickness, deposited by a vacuum plasma spray ...process (VPS) on a steel substrate. The post-heat treatment of the as-deposited coating was carried out in Ar at 400 deg C and 800 deg C, respectively. The coatings were characterized by means of an electron probe micro analyzer (EPMA) with wavelength dispersive X-ray spectrometers (WDS). It was found that no significant changes were produced as a consequence of the heat treatment carried out at 400 deg C. Therefore, the corrosion experiments were conducted for the substrate, the as-deposited coating and the post-heat treated coating at 800 deg C. Potentiodynamic polarization showed that the annealed coating at 800 deg C has a better corrosion resistance than the as-deposited coating. The corrosion current density (Icorr) of this coating was approximately 3 and 4 times smaller than that corresponding to the as-deposited coating and steel substrate, respectively. This significant improvement of the corrosion behavior of the post-heat treated coating is mainly due to both the microstructural changes that take place in the coating and the diffusion of Ni into Fe at the coating-substrate interface, which ensures the presence of a metallurgical bond.
Vacuum plasma spraying (VPS) was used to spray a Cr3C2-NiCr coating of 150, 300 and 450 mum in thickness onto a plain carbon steel substrate, employing a commercially available Cr20Ni9.5C powder. The ...splat microstructures observed in the coating were found to consist of a NiCr matrix with a predominant Cr3C2 phase, besides Cr7C3 and Cr2O3. The adhesion of the coating to the substrate was evaluated by means of interfacial indentation techniques. It has been found that the interfacial toughness value changes from 7.6 to 10.1 MPa m1/2 when the thickness increases from 150 to 450 mum. Also, it has been found that the parameter Kcao, determined by linear regression from the Kca versus 1 / e2 curve by means of the interfacial indentation model advanced by Chicot et al., has a value of 9.8 MPa m1/2.
▶ A constitutive description of the AA7075-T6 aluminum alloy has been formulated. ▶ Tests were conducted between 123 and 298K, at constant and variable speeds. ▶ The description is a simplified form ...of the mechanical threshold stress model. ▶ The formulation separates the different contributions to the current flow stress. ▶ Changes in the ductility of the material are dictated by the changes in temperature.
The present investigation has been conducted in order to develop a rational approach able to describe the changes in flow stress of AA7075-T6 aluminum alloy with deformation temperature and strain rate, when this material is deformed at temperatures in the range of 123–298K at strain rates in the range of 4×10−4 to 5×10−2s−1. The constitutive formulation that has been advanced to accomplish these objectives represents a simplified form of the mechanical threshold stress (flow stress at 0K) model developed at Los Alamos National Laboratory (Los Alamos, New Mexico, USA). Thus, it is assumed that the current flow stress of the material arises from both athermal and thermal barriers to dislocation motion. In the present case, the effect of three thermal barriers has been considered: solid solution, precipitation hardening and work-hardening. The first two effects do not evolve during plastic deformation, whereas the last one is considered as an evolutionary component of the flow stress. Such an evolution is described by means of the hardening law earlier advanced by Estrin and Mecking (1984) 20. The law is implemented in differential form and is integrated numerically in order to update the changes in strain rate that occur during tensile tests carried out both at constant and variable crosshead speed. The extrapolation of the hardening components from 0K to finite temperatures is accomplished by means of the model earlier advanced by Kocks (1976) 19. The results illustrate that the constitutive formulation developed in this way is able to describe quite accurately both the flow stress and work-hardening rate of the material, as well as temperature and strain rate history effects that are present when deformation conditions change in the course of plastic deformation. The evaluation of the ductility of the alloy indicates that the changes in this property are mainly determined by deformation temperature rather by strain rate. When deformation temperature decreases from 298 to 123K, ductility also decreases from ∼35 to 24%. However, despite these relatively small variations, significant changes in the fracture morphology could be observed on the fracture surfaces of the examined specimens, with the predominance of a mixed ductile-brittle mechanism at lower temperatures.
▶ A constitutive description of low C-Mn steel under hot-working conditions has been developed. ▶ Tests were conducted between 850°C and 1150°C, at strain rates of 0.25–25s−1. ▶ The description is ...based on a simplified form of the Mechanical Threshold Stress Model. ▶ The formulation only considers the interaction of dislocations with thermal barriers. ▶ Work-softening and work-hardening transients can be easily predicted during deformation.
The present investigation has been conducted in order to develop a constitutive description for a low C-Mn steel employed in structural applications, when this material is deformed under hot-working conditions. The set of equations that encompass the constitutive description of the steel are based on a simplified form of the Mechanical Threshold Stress Model. Accordingly, it has been shown that the flow stress of the material can be considered to arise from two different contributions: solid solution and precipitation hardening, which do not evolve in the course of plastic deformation and work-hardening, the only evolving component of the flow stress. The evolution of work-hardening has been introduced in the model by means of the work-hardening law proposed by Estrin and Mecking, in differential form. Also, the temperature and strain rate dependence of the contributions of solid solution and precipitation hardening to the flow stress, as well as that of the saturation stress in the work-hardening law, are described by means of the model earlier advanced by Kocks. The constitutive formulation encompasses seven material parameters which have been determined employing the experimental data corresponding to the strain interval where the material only exhibits positive work-hardening. It is able to predict accurately both work-hardening and work-softening transients when changes in deformation conditions occur. Furthermore, it can be easily implemented in any advanced computational tool employed for the analysis of hot-working processes of this material.
The evaluation of the elastic response of coated systems under indentation loading represents a crucial issue, which determines the behavior of such systems under tribological applications. Although ...a number of models have been proposed in the literature for the description of the change in the composite modulus with indentation depth, as well as for the determination of the elastic modulus of monolayer coatings, only few works address the analysis of multilayer coatings. The present work proposes a general methodology, which allows the modification and extension of the models employed in the analysis of monolayer coatings, for the study of the elastic response of multilayer coatings. For this purpose, a number of models have been examined, including those proposed by Gao et al., Menčík et al., Perriot and Barthel, Antunes et al., Korsunsky and Constantinescu, Doerner and Nix, Bec et al. and Bull. The foundation of the advanced formalism is the physically-based concept proposed by Iost et al. for the computation of the volume fraction of each layer in the coating and therefore, of its contribution to the global elastic response under indentation. The modified models are further employed in the analysis of a coated system composed of a 2024-T6 aluminum alloy substrate coated with a multilayer coating of DLC/CrC/CNiPCr/NiP of approximately 54 μm in thickness, as well as, a set of experimental data reported by Bull for a bilayer coated system. It has been shown that the different models analyzed are able to provide a satisfactory description of the experimental data, although the quality of the fit depends on the number of material parameters involved in each model. The mean square error of the fit is employed for conducting a comparison between the models.
The stress corrosion cracking resistance of two API X65 microalloyed steels, with different microstructures has been evaluated both at room temperature and at 55 °C, by means of Linear Elastic ...Fracture Mechanics (LEFM) specimens known as MWOL (Modified WOL specimen), under constant displacement. The material with a ferritic–pearlitic microstructure showed crack tip dissolution, a transgranular crack growth mode and some evidence of Hydrogen Induced Cracking (HIC). At room temperature, besides crack propagation, decohesion between grain boundaries was also observed, as well as some cavitation at the front of the crack tip. The material with martensitic microstructure tested at room temperature showed a mixed transgranular-intergranular crack growth mode, without cavitation at the crack tip front. At 55 °C, the crack growth path observed was transgranular, with cracks longer than those observed at room temperature, indicating that some Hydrogen mechanisms, probably thermally activated, could be present in addition to the anodic dissolution mechanisms.
•Modified elastic fracture mechanics specimens to see and monitor cracking.•Study at two temperatures to simulate working conditions in the field.•Study of classical microstructure and recent martensitic microstructure used in the field.•Results demonstrated cracking susceptibilities and different mechanisms of HE.•Use of polarization curves to prove continuous anodic corrosion in the sour media at both temperatures.
The present investigation has been conducted in order to study the effect of the deposition of a ZrN coating, of ∼3
μm in thickness, on the static mechanical properties and fatigue behavior of a ...7075-T6 aluminum alloy substrate. It has been determined that the coating deposition process gives rise to a significant decrease in such properties, which is not fully compensated for the presence of the film. When fatigue tests were carried out in a 3
wt.% NaCl solution at low alternating stresses, the ZrN film partially compensated for the decrease in fatigue properties of the coated substrate. Extensive delamination of the coating from the substrate was observed under the action of cyclic stresses greater than approximately 220
MPa. Below this stress and in the presence of NaCl, the behavior of the coated material approached that of the uncoated alloy, which highlighted the good corrosion resistance of the ZrN coating and its ability to protect the substrate when it remained adhered to the latter.
This paper reports the results obtained from the study of friction and sliding wear in two corrosive solutions of an a-C:H coating deposited on 316L stainless against an alumina ball, employed as ...static counterpart. Calculations of the values of the von Mises stresses developed at the coating–substrate interface, as soon as the ball touches the coated sample, and how this state of stress influences the response of the coated system under the corrosion environment, are presented and discussed. The results obtained from these calculations, as well as from the experiments conducted in the present research, are compared with other experiments published in the literature, where a-C:H coatings deposited on different substrates and with different coating architectures were tested in similar corrosive media. It has been determined that in those systems, where the von Mises stress in the coating, found in the vicinity of the interface, exceeds the threshold value of approximately 370MPa, coating failure with spallation will take place, regardless of the substrate nature on which this coating has been deposited. From this analysis it has been concluded that the coating yield strength is of utmost importance in conferring the a-C:H coated system the required stability in a corrosive solution.
•The effect of two corrosive solutions on the sliding wear of an a-C:H coating has been evaluated.•The morphological features of the wear tracks obtained by means of SEM–EDS have been analyzed.•The calculation of the von Mises stress values at the coating–substrate interface has been performed.•The determination of a threshold stress value for the occurrence of coating spallation has been carried out.•It has been established that the a-C:H coating stability in a corrosive solution depends on the coating's yield strength.
The fatigue behavior of a SAE 4340 steel, coated with a 50% WC–10Co–4Cr/50% Colmonoy 88 deposit, by high velocity oxygen fuel (HVOF) thermal spray, has been investigated. The change in the maximum ...alternating stress with the number of cycles to fracture has been described by means of the relationship advanced by Stromeyer. A fractographic analysis has been carried out on some representative fracture surfaces, by means of scanning electron microscopy (SEM) techniques. The mechanical properties of the coating were characterized by means of nanoindentation tests. The results indicate that the coating is highly heterogeneous. Its deposition gives rise to a decrease in the fatigue strength of the substrate of ∼30%, in comparison with the uncoated substrate. The decrease in fatigue strength is due to the presence of stress concentrators at the substrate–coating interface, as well as the intrinsic characteristics of the coating.
► We investigated the fatigue behavior of a 4340 steel coated with a WC–10Co–4Cr/Colmonoy 88 deposit. ► The coating was deposited by HVOF thermal spray after prior grit blasting and it is very heterogeneous. ► The mechanical properties of the coating were characterized by means of nanoindentation tests. ► The coating deposited gives rise to a significant decrease in the fatigue strength of the substrate. ► The decrease in fatigue strength is due to the presence of stress concentrators at the substrate–coating interface.
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