Xian-Cheng Zhang

Microstructure and Sliding Wear Resistance of Laser Cladded WC/Ni Composite Coatings with Different Contents of WC Particle

The aim of this article was to address the effect of WC content on the microstructure, microhardness, and sliding wear resistance of laser cladded WC/Ni composite coatings. The content of WC particle in the feed powder varied in the range of 0-80 wt.%. Experimental results showed that the laser cladded coatings exhibited homogeneous microstructure without pores or cracks. By comparing with the 45# steel substrate, the microhardness of WC/Ni composite coatings was relatively high. The microhardness of coating increased with increasing the content of WC particles. The wear resistance of WC/Ni composite coatings was strongly dependent on the content of WC particle and their microstructure. When the WC content was lower than 40 wt.% in the feed powder, the wear rate of the coatings decreased with increasing WC content. The two-body abrasive wear was identified as the main wear mechanisms. For the coatings with WC content higher than 40 wt.% in the feed powder, their wear rate increased with increasing WC content. The three-body abrasive wear and fatigue wear were the main failures. The coating with 40 wt.% WC in the feed powder exhibited the best wear resistance.

Residual stresses within oxide layers due to lateral growth strain and creep strain: Analytical modeling

Deflection test has been widely used to estimate residual stresses within an oxide layer during isothermal oxidation of one surface of an alloy or metal. Deflection models so far developed have considered elastic, plastic, and creep model to predict stresses produced from curvature of the metal/oxide interface. However, none of the models have considered growth stress. During oxidation, when new oxide forms along grain boundaries lying perpendicular to the interface, it will generate a lateral growth strain. When this strain is constrained by the underlying metal, it will generate growth stress in the oxide. This lateral growth strain combined with creep strain of the both oxide and metal will then provide the realistic approach to deal with stresses in oxide/metal composite. Considering these situations an analytical model has been developed to determine residual stresses for the metal/oxide composite of the deflection test. The stress value in both metal and oxide is calculated for notional mechanical p...

Residual stresses in the elastoplastic multilayer thin film structures: The cases of Si/Al bilayer and Si/Al/SiO2 trilayer structures

A theoretical model was developed to predict the thermal residual stresses within the elastoplastic multilayer thin film structures. The plastic deformation of one of the films was considered. Special analyses were made on the bilayer structures, i.e., a film overlaid on a substrate, as they are more of practical interest. Closed-form solutions were derived to estimate the residual stresses in the films and curvature of the multilayer film structure, and the relationship between the temperature difference and the thickness of the plastic zone. The cases of Si/Al bilayer and Si/Al/SiO2 multilayer structures were studied to illustrate the implementation of this model. In these structures, Al film was assumed to be plastically deformed. Results showed that, for both structures, there was a linear relationship existing between the thickness of the plastic zone in Al film and the temperature difference. SiO2 layer deposited on the aluminum film had an obvious influence on the critical temperature at which Al f...

Effect of laser power on the microstructure and mechanical properties of TiN/Ti 3 Al composite coatings on Ti6Al4V

Laser nitriding is one of the effective techniques to improve the surface properties of titanium alloys and has potential application in the life extension of last-stage steam turbine blades. However, cracking of surface coating is a common problem due to heat concentration in laser nitriding process. Conventionally, the cracks can be avoided through heat treatment, which may have an important influence on the mechanical properties of coating. Crack-free TiN/Ti3Al IMC coatings on Ti6Al4V are prepared by plasma spraying and laser nitriding. The microstructures, phase constitutes and compositions of the coating are observed and analyzed with scanning electron microscopy(SEM), X-ray diffraction(XRD) and X-ray energy-dispersive spectroscopy(EDS). Microhardness, elastic modulus, fracture toughness of the coating are measured. The results show that the crack- and pore-free IMC coatings can be made through the proposed method; with increasing laser power, the amount and density of TiN phase in the coating first increased and then decreased, leading to the similar trend of microhardness and elastic modulus and the reverse trend of fracture toughness of the coating. Both the average microhardness and elastic modulus of the coating increase three times higher than those of the substrate. The volume fraction of the TiN reinforced phase in composite can be controlled by varying the laser power and the cracking problem in laser nitriding process is successfully solved.

Analytical modeling of stress and strain of symmetrically oxidized metal

During high temperature isothermal oxidation, a plane of oxide molecules is inserted in each oxide grain boundary. These oxide scales grow laterally and thicken in a columnar microstructure to generate a lateral growth strain. But as the oxide scales are constrained by the underlying metal, the metal constraint produces a net in plane compressive stress in the oxide scales. To balance a compressive stress in the oxide scales, a net in plane tensile stress is generated in the metal, which results creep elongation of an oxide/metal composite. Based on this mechanism, the present paper provides a simplified modeling approach to predict an evolution of the stresses and strains in the oxide/metal composite considering the lateral growth strain of the oxide and creep strain of the metal and oxide. Oxide stress variation due to the creep properties of the oxide/metal composite such as creep indexes (m and n) and creep constants (Aox and Am) is provided for the notional mechanical properties. Oxide stress variati...

Analysis on multiple cracking in film/substrate systems with residual stresses

The multiple cracks might be initiated in the film on a substrate due to the presence of residual stress. In this paper, the multiple film cracking in the film/substrate systems with residual stress was analyzed. First, a relatively simplified model with the closed solutions considering the edge effect was developed to predict the residual stress within the film segment. Some obvious relationships between the material properties and the dimensions of the film and substrate and the stress distribution in the film could be reflected by using this model. By comparing the analytical results with those from the existing analytical models, finite element analysis, and the existing experimental measurements, it could be concluded that the present analytical model was very rigorous. Then, the energy criterion was adopted to perform the analysis on multiple film cracking on the basis of the knowledge on the residual stress distributions. The closed-form solution for the critical misfit strain for the crack initiat...

Multiple film cracking in film/substrate systems with mismatch strain and applied strain

Multiple cracking of brittle films deposited on the relatively soft substrate was commonly used in different applications. In the present paper, multiple film cracking in the film/substrate systems was analyzed. The system was subjected to the combination of the residual stresses due to the mismatch strain between the film and substrate and the applied stress due to the unidirectional strain on the substrate. First, a simply analytical model was developed to derive the closed-form solutions for the residual stress distribution and redistribution due to the edge effects in the film segment. Second, the energy and strength criteria were adopted to perform the analysis on the multiple film cracking on the basis of the knowledge on the residual stress distributions. Third, the case of SiOx film/polyethylene terephthalate substrate system was used to illustrate the implementation of this model. The relationship between the fracture energy as well as the film strength and the film thickness was obtained. The pr...

Effect of Stress Ratio on the Fatigue Crack Propagation Behavior of the Nickel-based GH4169 Alloy

The fatigue crack growth behavior of the newly developed GH4169 nickel-based alloy at a maximum stress of 700xa0MPa and different stress ratios was investigated in the present work employing the specimens with a single micro-notch at a frequency of 129xa0Hz at room temperature. The results demonstrate a typical three-stage process of fatigue crack propagation processing from the microstructurally small crack (MSC) stage to the physically small crack (PSC) stage, and finally to the long crack stage. The crack growth rate in the MSC stage is relatively high, while the crack growth rate in the PSC stage is relatively low. A linear function of crack-tip reversible plastic zone size was proposed to predict the crack growth rate, indicating an adequate prediction solution.

Gradient Elastic–Plastic Properties of Expanded Austenite Layer in 316L Stainless Steel

The gradient mechanical properties, variation of stress with strain and surface cracking behavior of expanded austenite developed on 316L austenitic stainless steel were investigated by nanoindentation tests, X-ray residual stress analysis and scanning electron microscope observation in four-point bending tests. The results show that the plastic properties of the carburizing layer including true initial yield strengths and strain hardening exponents increase significantly from substrate to surface, while the true elastic modulus just improves slightly. Due to the onset of plastic flow, the residual stresses are almost equivalent to the true initial yield strengths from surface to the depth of ~xa010xa0μm. The results of four-point bending tests show that surface stress increases linearly with the increase in strain until the strain reaches ~xa01.0%, after that the plastic yield happens. The expanded austenite surface layer is brittle, and the cracks will be created at the strain of ~xa01.4%. The cracking stress is about ~xa02.4 GPa.

Creep-fatigue life prediction through multiple regression analyses

The aim of this paper was to propose a creep-fatigue life prediction model based on multiple regression analysis of the existing experimental data. Three different factors including temperature, hold time, and total strain range, were taken into consideration in this model. The prediction capacity of the present model was verified through comparison between predicted results and existing experimental data of different materials with different testing conditions. A good agreement was found between the predicted results and the experimental data.