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An Introduction to Materials
Published in Paul J. Hazell, Armour, 2023
Both hardness and tensile (strength) tests result in plastic deformation of the sample. The mechanisms of plasticity are quite complex, and a readable explanation is provided by Callister (2007). The theory is largely based on the fact that crystals possess defects, and it is these defects that can move through a process called slip. These defects are known as dislocations. In an arrangement of atoms for a particular crystal, a dislocation is a defect about which there is a misalignment of atoms. Slip occurs because the stress that is applied in the tensile, compression or hardness tests (or indeed any stress for that matter) is transferred to the individual crystals in that material. This stress causes the movement of dislocations resulting in distortion of the crystal. The crystallographic plane along which this occurs is called the slip plane. This is the preferred plane for dislocation movement. For a particular crystal structure, the slip plane is the plane with the densest packing of atoms.
Manufacturing of 3D Print Biocompatible Shape Memory Alloys
Published in Ajit Behera, Tuan Anh Nguyen, Ram K. Gupta, Smart 3D Nanoprinting, 2023
Dislocations are generally visible with the help of TEM (transmission electron microscope) images. The dislocations inside SLM-fabricated NiTi alloy are highly dense and twisted kinds of dislocations. The average density of dislocation was observed to be in the range of 1011 cm2. Furthermore, there is an observation of straight dislocations and long flat kinks when viewed with higher magnification. In certain cases, small spiral-shaped dislocations are also observed. In the case of metals with a high concentration of vacancies, there are helical dislocations; this type of dislocations are generally observed in quench steels and aluminum alloys. The thermal motion of dislocation, along with the movement of bending dislocation in opposite direction, leads to the formation of helical dislocations. When AM manufactured NiTi shows helical dislocations, this is a clear indication of a higher concentration of vacancies. The dislocation formed in AM manufactured NiTi forms wavy or bowed morphologies as the dislocations tend to align themselves to minimize their energy [39]. The dislocation reaction happening during the formation of SMA by additive manufacturing can be known by investigating more on the bending dislocations. Methods like double beam analysis are used to investigate bending dislocations.
Mechanical System Failure
Published in Seong-woo Woo, Design of Mechanical Systems Based on Statistics, 2021
If a mechanical product (or a part) is subjected to a repeated stress (or loads), permanent deformation (or failure) happens at early stage. In material science, a dislocation is a linear crystallographic defect that possesses a sudden change in the positioning of atoms. The movement of dislocations (or slip) allows atoms to slide over each other at low stress levels. Slip happens on planes that have the highest planer density of atoms and in the direction with the highest linear density of atoms. In other words, slip happens in the directions in which the atoms are packed with little space since this requires the lowest quantity of energy. Therefore, they can slip past each other with force. The slip flow relies upon the repetitive structure of the crystal, which allows the atoms to shear away from their original neighbors. It, therefore, passes along the face and joins up with the atom of new crystals.
Microstructural changes and their influence on corrosion post-annealing treatment of copper and AISI 5140 steel in 3.5 wt% NaCl medium
Published in Cogent Engineering, 2023
Jilna Jomy, Sathyashankara Sharma, P.R. Prabhu, Pavan Hiremath, Deepa Prabhu
There is a decrease in the microhardness value for both AISI 5140 steel and copper post-annealing as seen in Table 3. This phenomenon can be described by the Hall-Petch effect. During the plastic deformation of the material, dislocations in the crystal lattice can move through the material. This dislocation gets piled up near the grain boundaries, which will cause the development of repulsive stress within the grain boundary. This stress causes a decrease in the energetic barrier for diffusion across boundaries, which leads to further deformation in the material causing higher yield strength. Since smaller grains have larger grain boundaries, it will lead to higher strength of the material (Yu et al., 2015). From the XRD, it is clear that annealing causes an increase in the grain size, thus causing a decrease in the strength of both the metals.
Effects of molybdenum on hot deformation behavior and microstructural evolution of Fe40Mn40Co10Cr10C0.5 high entropy alloys
Published in Science and Technology of Advanced Materials, 2023
Marzieh Ebrahimian, Mohsen Saboktakin Rizi, Sun Ig Hong, Jeoung Han Kim
The grain boundaries act as strong barriers for dislocation motion. With the accumulation or pile-up of dislocations near the grain boundaries, the Cr and C atoms can be absorbed owing to the high binding enthalpy between the elements and dislocations, resulting in the precipitation at the grain boundaries. The XRD and EBSD phase maps revealed that the addition of Mo increased the volume fraction of carbides in the Fe38.3Mn40Co10Cr10C0.5Mo1.7 alloy, compared to the Fe39.5Mn40Co10Cr10C0.5 HEA, which can be attributed to the decrease in enthalpy of formation of the Cr23C6 carbides. The substitution of Mo in Cr 8c-sites of Cr23C6 carbides enhanced their stability [57,58]. Furthermore, the phase map of the Fe38.3Mn40Co10Cr10C0.5Mo1.7 HEA in Figure 8(b4) indicates the existence of equiaxed martensite adjacent to the recrystallized fine grains during hot deformation. The lower values (130 J/mol) of the Mo-added alloy increased the metastability of the fcc phase by a thermal diffusion process and promoted martensite formation in the Fe38.3Mn40Co10Cr10C0.5Mo1.7 HEA at 800°C. This suggests the occurrence of discontinuous DRX through the thermally/shear-induced reversion of hcp ε-martensite to fcc during hot compression.
Development of TiCN-Co-Cr3C2-Si3N4-based cermets with improved hardness and toughness for cutting tool applications
Published in Powder Metallurgy, 2023
Balasivanandha Prabu Shanmugavel, Sri Harini Senthil Kumar, Chellammal Nandhini Aruna, Madhi Varshini Ramesh
The cracks that emanated from the edges of the indentation zones were measured and the toughness of the SN00, SN05 and SN10 are estimated to be 6.79 ± 0.61 MPa√m, 7.23 ± 0.45 MPa√m and 9.35 ± 3.10 MPa√m respectively using the Shetty’s Equation. Figure 3(a–c) shows the SEM images of the indentation zones for the SN00, SN05 and SN10 cermets respectively. The solid solution formed due to the reaction between TiCN and other ceramics formed in the rim structure, increased the fracture toughness. This process leads to an increase in the strength of the material, due to the introduction of a small amount of dissimilar atoms into the lattice of the material. This creates strain in the lattice, which serves as an obstacle for the movement of dislocation; the defects in the crystal structure are responsible for the deformation and plasticity in the metal. The material becomes tougher when it is more difficult for the crack to propagate. The fracture toughness was maximum for SN10 due to the presence of more secondary phases (in the rim structure) which acted as a medium for crack energy dissipation.