A discrete-element based model of elastic-plastic materials with non-ideal plasticity and with an account of both cohesive and adhesive interactions inside the material is developed and verified. Based on this model, a detailed study of factors controlling the modes of adhesive wear is performed. Depending on the material and loading parameters, we observed three main modes of wear: slipping, plastic grinding, cleavage, and breakaway. We find that occurrence of a particular mode is determined by the combination of two dimensionless material parameters: (1) the ratio of the adhesive stress to the pure shear strength of the material, and (2) sensitivity parameter of material shear strength to local pressure. The case study map of asperity wear modes in the space of these parameters has been constructed. Results of this study further develop the findings of the widely discussed studies by the groups of J.-F. Molinari and L. Pastewka.
Breakaway dj crack
In this mode, surface interaction begins from slipping in the contact plane. Сontact area of asperities increases during sliding, thus tangential reaction force at the contact surface increases correspondingly. This force leads to asperities bending so that a frontal part of an asperity exerts tensile strain (stress distributions in the asperities are shown in Supplementary Materials, section 5). This leads to a tear crack formation in a bottom part of an asperity adjacent to the bulk of a main body.
The most intriguing result is that the magnitude of the adhesion force between separated surfaces, along with the stress of plastic yielding and shear strength, controls the mode of fracture and thus the wear mode of asperities of different scales. Further, we observed and described a great variety of modes of wear, which were not described so far. However, all of them can be considered as modifications or transition phases of the fundamental limiting cases of plastic smoothing and breakaway which were described already by Rabinowicz. The results of the study are important for understanding the mechanisms of wear in tribological systems of different nature from traditional engineering tribological pairs to biological joints and tectonic faults.
Therefore, sulfides were formed at the oxide layer and along the grain boundaries of the inner alloy in Figure 14b and d. The Cr-rich sulfides at the grain boundaries promote the outward diffusion of Cr and increase the scaling rate, leading to the rapid breakaway corrosion [73]. The above result shows that the Cr-rich sulfides produced by impurity SO2 appear to be detrimental to the oxidation of the alloy. There were other reports suggesting that adding 1%O2 into 550 C/1 bar Ar-50%CO2-0.5%SO2 gas could prevent the formation of sulfides in P92 steel [92]. This is because the addition of O2 reduces the activity of sulfur. Furthermore, another work reported that nickel-base alloys were more resistant to vulcanization than iron-base alloys [24,27].
For Si-containing alloy, there is a competition between segregations of S and Si on the alloy surface. As shown in Figure 15, there is significant S segregated on the surface of Fe9Cr0.5Si alloy in the temperature range of 550 to 900 C. With the increase in temperature, the segregation of S increases while the segregation of Si decreases. This indicates that Si and S repel each other at the surface, and the repulsion is strong at high temperatures. Hence, the Fe9Cr0.5Si alloy cannot form a silica layer as an additional diffusion barrier in 811 C/1 bar Ar-20%CO2-20%H2O-0.5%SO2 gas [93]. Without a silica layer, the formation of the chromia scale on the surface of Fe9Cr0.5Si alloy is not stable in this condition, which will lead to quick breakaway corrosion. It has also been reported that the presence of SO2 gas caused spallation for commercial steels containing high levels of Si [24]. Moreover, it has been shown that impurity SO2 was more corrosive at a lower temperature [94]. This suggests that the effect of impurity SO2 on oxidation is sensitive to changes in exposure conditions or alloy composition.
The impurity SO2 accelerates the oxidation rate due to the formation of Cr-rich sulfides at the grain boundary or inhibition of the formation of silica layer. However, the presence of SO2 can partially promote the formation of the chromia layer on Fe9Cr and Fe9Cr0.5Si model alloys in wet CO2 (Figure 16). This is a combined effect of impurity SO2 and impurity H2O. Additionally, in complex CO2 environments where both O2 and H2O exist, the presence of impurity SO2 could accelerate the occurrence of breakaway corrosion in commercial chromia-forming steels (347H, 304H, 310S, E-Brite) [24]. However, under the same condition, it reduced the scaling rate of IOZ and impeded the Fe-rich oxide spallation on FMs Grade 91 [24]. This may be because the strong adsorption of SO2 at the grain boundary seems to impede the diffusion of oxygen and cause low growth stress. This may also imply that impurity SO2 seems more inclined to destroy the chromia layer but not the Fe-rich oxide layer in this complicated environment.
Results of TEM analyses of propagating crack region in S-CO2 crept specimen about 5 mm away from the fracture region; (a) STEM micrograph and EDS mapping image; (b) enlarged micrograph of area indicated in (a) and SAD patterns of grains adjacent to crack [129]. Reproduced with permission from Elsevier.
More importantly, research shows that the oxidation or carbonization of the grain boundary will weaken the strength of the grain boundary, causing the failure mode to gradually change from transgranular fracture to intergranular fracture. Hence, grain boundary oxidation or carbonization can be the cause of crack initiation or propagation. This implies that the corrosion behaviors of grain boundaries can be the key to understanding the mechanism of crack initiation or propagation. This is because an understanding of preferential intergranular oxidation is necessary to predict intergranular stress corrosion cracking. However, there are quite few reports on the grain boundary corrosion behavior at present and there are many unresolved issues. Firstly, the grain boundary corrosion behavior is drastically different from the intragranular corrosion behavior and should be studied through detailed microstructure analysis. Secondly, the effect of grain boundary type on grain boundary corrosion behavior should be investigated to identify the most vulnerable grain boundary type. Lastly, the effects of solute segregation and grain boundary precipitation on grain boundary corrosion behavior should be surveyed. Resolving these issues can provide valuable guidance for subsequent work, such as grain boundary engineering, the development of new materials and modeling the failure of materials.
Furthermore, in most studies regarding the degradation of a mechanical property, the changes in the mechanical properties of alloys are evaluated after exposure to high temperature S-CO2. These results cannot represent the actual situation during plant operation because the tensile samples do not bear the loading during S-CO2 corrosion. Therefore, alloys are needed to be tested for stress corrosion, such as creep, crack growth, fatigue and crack initiation tests in a high temperature S-CO2 environment. It is crucial to understand the crack initiation mechanism and crack propagation mechanism of an alloy, which provide the basis to evaluate the long-term performance of materials under the synergistic actions of chemical and mechanical processes in S-CO2 cycle power generation systems. 2ff7e9595c
Comments