China
Africa
Explore chapters and articles related to this topic
Ceramics and the Mechanical Properties of Ceramic Coating Materials
Published in Yichun Zhou, Li Yang, Yongli Huang, Micro- and MacroMechanical Properties of Materials, 2013
Yichun Zhou, Li Yang, Yongli Huang
The grain boundary of polycrystallines is the distortion area of the crystal lattice, which has defects and traps. It is also the microuneven zone of chemical compositions, and collects impurities easily, forming the glass phase or microcrystal phase. Moreover, the grain boundary is the place where stress concentration occurs; therefore, sliding between adjacent grains is an important microscopic process of ceramic high-temperature creep. The cause of grain boundary sliding is plastic flow. When the grain boundary contains a Newtonian liquid, or liquid-like second phase materials, the diffusion coefficient of the liquid-phase grain boundary is related to the thermal activation of the second-phase material. If the liquid-phase layer has an appropriate thickness, and the extent of grain irregularity on both sides of the grain boundary does not hinder the shear process, the creep rate has the characteristics of Newtonian viscosity and is subject to the control of the grain boundary separation rate under tensile stress. If the extent of grain boundary irregularity is more serious, and the grain boundary layer thickness is thinner, the creep rate is a nonNewtonian viscous flow, and the creep rate should be related to the formation of holes at the grain boundary, and the crack growth at the three-intersection-point area. Therefore, plastic flow is only a partial reason for grain boundary sliding. This type of grain boundary sliding mechanism is due to the slip and climb motion of dislocations along the grain boundaries, or near the grain boundary, under high-temperature conditions.
Creep Behavior Investigation by Molecular Dynamics (MD) Simulation
Published in Snehanshu Pal, Bankim Chandra Ray, Molecular Dynamics Simulation of Nanostructured Materials, 2020
Snehanshu Pal, Bankim Chandra Ray
Grain boundary sliding is a phenomenon involving the movement of individual grains sliding over each other along their common GB [38]. High ductility of metals and superplasticity behavior of NC materials can be explained via GB sliding. During the deformation process, the elongation of the grains, avoiding the formation of cavities at the triple junction, is transpired by GB sliding. Grain growth at high temperature transpires usually via the diffusion of atoms along GBs. Nevertheless, grain growth can also be induced via grain rotation arising from GB sliding [39]. Grain boundary sliding is usually prominent in NC metals with grain size less than 10 nm.
Effects of severe-strain-induced defects on the mechanical response of two kinds of high-angle grain boundaries
Published in Philosophical Magazine, 2020
Lisa Neier, K. A. Padmanabhan, Sergiy Divinski, Gerhard Wilde
The finding that the introduction of basic shear units leads to an increase of the interface stiffness, i.e. it decreases the curvature-driven motion of the boundary has considerable significance for severe plastic deformation, wherein significant grain refinement is seen. A recent publication [54] presents an admirable overview of the microstructural evolution accompanying severe plastic deformation. The grains obtained are much smaller than a micrometer in size. Most of the grains are separated by high-angle grain boundaries, similar to the ones discussed in this paper. Grain boundary sliding, diffusion and grain rotation are considered to be the dominant deformation processes. Grain rotation which increases with decreasing grain size, is attributed to grain boundary misorientation, unbalanced shear stresses at the boundary and anisotropy of grain boundary energies. Dislocations emitted from the grain boundary change from unit dislocations to partial dislocations as the grain size goes down into the nanometer range [55]. In principle, the additional defects introduced by heavy deformation should raise the stored grain boundary energy and provide sufficient driving force for boundary migration and grain growth. However, particle and precipitate pinning as well as structural pinning mechanisms can significantly retard grain boundary migration. Elaborating on the latter mechanism, Ref. [56] have noted that a certain type of triple junctions, known as Y junctions that involve 1–3 high-angle grain boundaries, can retard boundary motion because of their morphology and the surrounding microstructure. This, in turn, will reduce grain growth. Both the ∑27 and ∑41 boundaries considered here qualify to be a part of the Y junctions. In this study it is shown that the introduction of basic shear units in such boundaries by SPD significantly inhibits grain growth, which is consistent with the experimental observations in Ref. [56].
Evaluation of liquid metal embrittlement-free weld by micro-plasma arc welding of Zn-coated interstitial free steel
Published in Welding International, 2023
Christopher Jose Chittilappilly, Swarup Bag, Arnab Karani
The high-temperature (HT) tensile tests are performed at 600 °C, 700 °C, and 725 °C both on GA-coated and uncoated specimens. These high-temperature tensile tests simulate the high temperature and external stress state where LME might occur. The stress-strain graph of hot tension tests is shown in Figure 12. It is observed that at 600 °C, the nature of stress-strain diagrams of both bare and Zn-coated IF steel are similar. The bare specimen shows a higher tensile strength value, and the coated specimen shows a higher elongation. The strain-hardening effect is evident for a bare sheet that limits the elongation or strain-softening zone after ultimate tensile strength. However, the strain-softening effect is much more apparent for coated IF steel, where the strain-hardening effect is minimal. For an LME-free weld, the critical amount of liquid Zn does not reach the grain boundary. The minimum amount of liquid Zn at the grain boundary probably creates the grain boundary sliding that usually occurs at ∼0.4 times the melting point temperature of steel. Grain boundary sliding also facilitates the dislocation movement, brings a limited amount of strain-hardening effect, and otherwise extends the strain-softening behavior after reaching the maximum strength value. However, the strength of the bare sheet extends due to the strain hardening effect in the absence of any liquid Zn at 600 °C. The overall area under the stress-strain diagram for both cases is almost similar. It seems both materials have similar toughness. The stress-strain diagram of the HT tensile test performed at 700 °C is quite different. For the bare specimen, the maximum strength is 30.8 MPa compared to a maximum strength of 38.8 MPa in the case of the coated specimen. The bare specimen shows higher elongation compared to the coated specimen. A reduction of 16.7% is observed. The overall area under the stress-strain diagram is almost equal in both cases. Here, high temperature plays a role in deciding the mechanical properties than the presence of Zn coating. With further temperature increases, the strength of both bare and coated steel is reduced. It is evident that as temperature increases, the whole level of the stress-strain graph drops. This implies that the tensile strength decreases as temperature increases. The tensile strength of GA-IF steel was 128.8 MPa at 600 °C, which reduces to 38.5 MPa at 700 °C. A drop of about 69% is observed here. In-room temperature conditions, the tensile strength was observed to be 266.7 MPa. The other noticeable effect is elongation or strain percentage. As temperature increases, the percentage of elongation also increases. Elongation is observed at a 62.3% rise in percentage as the temperature increases from 600 °C to 700 °C. Table 4 reports the difference in strength and elongations between bare and coated steel.