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Imperfections and Diffusion
Published in Yip-Wah Chung, Monica Kapoor, Introduction to Materials Science and Engineering, 2022
An edge dislocation is created when an extra plane of atoms is inserted in the middle of the crystal as shown in Figure 2.6. By convention, a positive (negative) dislocation has an extra plane of atoms in the upper (lower) plane. For a positive dislocation, atoms in the upper plane are under compression, whereas atoms in the lower plane are under tension. When given sufficient mobility, impurity atoms with atomic size different from the host tend to collect at dislocations. Impurity atoms segregate to the core of edge dislocations in order to reduce the elastic strain energy. Recall that for positive edge dislocations, atoms in the upper plane are under compression. Segregation of smaller impurity atoms to the upper plane reduces the amount of compression and hence the elastic strain energy. Applying the same argument shows that larger impurity atoms favor segregation to the lower plane of a positive edge dislocation. The region around impurity atoms trapped at the dislocation core is known as the Cottrell atmosphere.
Diffusion
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
In this case an atmosphere of carbon atoms is established below the slip plane of the dislocation, termed the Cottrell atmosphere, which is responsible for the appearance of yield point effects and strain aging in steels. These effects will be discussed in detail in Chapter 8.
Constitutive modeling of dynamic strain aging in commercially pure bcc metals
Published in Mechanics of Advanced Materials and Structures, 2023
In the case of dynamic strain aging (DSA), the average waiting time becomes equal to the relaxation time () required by the interstitial atoms to diffuse over the average width of the obstacles [2] through pipe diffusion [3]. During this time, a Cottrell atmosphere develops, which creates a pinning effect on the dislocations. Further movement of these pinned dislocations requires a higher stress, resulting in a visible increase in the strength of the metal, commonly known as DSA [4]. This is typically regarded as an abnormal response that has been documented in numerous studies [5–12]. Developing a constitutive model with wide applicability and the capability to capture complex material responses is the primary goal in material science research. A successful model should be able to obtain accurate model constants with a limited amount of experimental data while capturing both static and dynamic material responses. Therefore, many studies have been conducted on various metals and alloys with different crystal structures to understand their thermomechanical responses under a wide range of strain rates and temperatures [13–20]
Effect of powder oxygen content on the microstructure and properties of Co–Cr dental alloys fabricated by selective laser melting
Published in Powder Metallurgy, 2018
Wei Zhang, Yujia Li, Songhao Hu, Mingyang Zhang, Songhai Huang, Tianyun He, Yong Liu, Yuchang Wang
Figure 6 shows a comparison of the tensile and yield strengths for the Co–Cr alloy SLM specimens of the two groups and a traditional casting and milling specimen. Figure 7 shows the results for Rockwell hardness and elongation for the #1 and #2 alloys and comparing them with ASTM F75. Figure 8 displays the stress–strain curves of the #1 and #2 alloys. The three figures together reveal that the tensile and yield strengths of the Co–Cr alloys of the two groups are 1285 ± 15 vs. 1273 ± 20 MPa (#1) and 1165 ± 5 vs. 1190 ± 15 MPa (#2), respectively. It can be observed that while the tensile strength was virtually unchanged, the yield strength of the #2 alloy slightly increased. This is related to either the pinning effect of oxygen or the blocking effect of fine chromia on dislocations of the interstitial atoms. The pinning of dislocations by Cottrell atmosphere or second-phase particles will be overcome by increasing external load. This results in an increase in the yield strength. But due to a limited solubility of interstitial atoms and a small amount of second-phase particles, the strengthening effect is not significant [21]. There is no significant difference in Rockwell hardness between the Group #1 and #2 alloys on the XY surface and Z profile. The mean hardness values are 41.33 and 41.37 HRC, respectively, indicating an insignificant difference. However, the tensile strength, yield strength and hardness of the Group #1 and #2 alloys are significantly higher than those of the traditional cast and milled Co–Cr alloy or ASTM standard.