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Selected Case Studies of Ternary Systems
Published in D. R. F. West, N. Saunders, Ternary Phase Diagrams in Materials Science, 2020
The past half century has seen the development of a wide-ranging group of industrially important titanium based alloys, providing a variety of combinations of properties including relatively low density and good strength. The aerospace industry has been one of the most significant beneficiaries from the availability of titanium alloys, while major applications have also been found in many other fields. Titanium shows an allotropic transformation from the high temperature, bcc β form to the low temperature cph α form at 882.5°C and the behaviour and design of titanium alloys is based on the effect of alloying additions on this transformation. Of the wide range of elements relevant to the alloying of titanium, oxygen is always present, entering into interstitial solid solution in both the a and β phases. Nitrogen and carbon, which are present as impurity elements, also enter interstitial solution. All CP (commercial purity) grades of titanium depend on alloying by oxygen to provide strength. Also many commercial alloys have some oxygen added to increase strength. However, to avoid reduction of ductility, the oxygen content must be carefully controlled during processing; typically the aim is to limit the oxygen content to < ~1200 ppm.
Selected Case Studies of Ternary Systems
Published in D. R. F. West, N. Saunders, Ternary Phase Diagrams in Materials Science, 2017
The past half century has seen the development of a wide-ranging group of industrially important titanium based alloys, providing a variety of combinations of properties including relatively low density and good strength. The aerospace industry has been one of the most significant beneficiaries from the availability of titanium alloys, while major applications have also been found in many other fields. Titanium shows an allotropic transformation from the high temperature, bcc β form to the low temperature cph α form at 882.5°C and the behaviour and design of titanium alloys is based on the effect of alloying additions on this transformation. Of the wide range of elements relevant to the alloying of titanium, oxygen is always present, entering into interstitial solid solution in both the α and β phases. Nitrogen and carbon, which are present as impurity elements, also enter interstitial solution. All CP (commercial purity) grades of titanium depend on alloying by oxygen to provide strength. Also many commercial alloys have some oxygen added to increase strength. However, to avoid reduction of ductility, the oxygen content must be carefully controlled during processing; typically the aim is to limit the oxygen content to < ~1200 ppm.
Deep Level Traps in GaN Epilayer and LED
Published in Zhe Chuan Feng, Handbook of Solid-State Lighting and LEDs, 2017
Xuan Sang Nguyen, Soo Jin Chua
Point defects are created by isolated atoms, and where they interact with one another they are called complexes. Point defects include vacancies, interstitial atoms, substitutional impurities, antisite defects, and their complexes. Vacancy is a missing atom at a lattice site. Interstitial is formed by an atom in between lattice sites. It is possible to have a self-interstitial, that is, interstitials formed by atoms of the lattice. A substitutional impurity is an impurity atom C replacing a host atom A and is identified by the nomenclature CA. Antisite defects occur only in compound semiconductors. Considering the compound semiconductor formed by two elements A and B, an antisite defect is formed when an atom B occupies a site that should have been occupied by atom A. Complexes are combinations of some point defects such as a vacancy–interstitial pair denoted by VA − IA.
Research progress of diamond/aluminum composite interface design
Published in Functional Diamond, 2022
Zengkai Jiao, Huiyuan Kang, Bo Zhou, Aolong Kang, Xi Wang, Haichao Li, Zhiming Yu, Li Ma, Kechao Zhou, Qiuping Wei
In addition, the bonding strength between diamond and W needs to be designed. Therefore, Song [24] carried out a series of first-principles calculations to predict the diffusion parameters of carbon in tungsten and evaluated the effect of temperature on them. It is found that carbon atoms tend to occupy the interstitial position of octahedron rather than that of the tetrahedron, and the minimum energy path of carbon atom diffusion is through the interstitial position of a tetrahedron. Yi Huaixing [25] used the generalized gradient approximation in density functional theory to optimize the three structures of tungsten carbide crystals and obtained the stable configuration with the lowest energy. On this basis, he calculated its thermodynamic properties under high temperature and pressure. The results show that the pressure has a great influence on the volume ratio V/V0, the heat capacity is close to the Dulong-Petit limit at high temperature, the thermal expansion coefficient α increases slowly above 600 K at a given pressure, the Debye temperature theta ΘD is much less affected by temperature than by pressure, and the Green-Eisen coefficient γ varies greatly under low pressure.
Structure evolution of vacancy-hydrogen complexes in a nickel-based single-crystal superalloy
Published in Philosophical Magazine Letters, 2022
Xiao-Zhi Tang, Xiao-Tong Li, Ya-Fang Guo
Finally, we examine the MEPs of the decomposition of a complex. For simplicity, in Figure 4, only one hydrogen atom diffuses away and the other hydrogen atoms stay put. Results show that for all three complexes, decompositions at the MDN centre (black dot solid lines) need higher energy than that at the dislocation node (red dot solid lines). The red dot solid lines have humps because the excess volume of the dislocation core allows the diffusing hydrogen atom to stay at a tetrahedron interstitial site during the decomposition. This is a metastable transition state. Regardless of the position of the complex, the MEPs in Figure 4 all indicate that decomposition is not energetically favourable. After decomposition, the system energy rises about 0.5–1.0 eV. This is because other hydrogen atoms are ‘fixed’ at their original sites in the calculation for simplicity. In reality they are expected to find more relaxed positions to release the lattice distortion. So, a decomposition can be witnessed in a structure evolution, just as Figure 2 shows.
First-principles calculations to investigate thermodynamic and mechanical behaviors of molybdenum-lanthanum alloy
Published in Journal of Nuclear Science and Technology, 2023
Lu Wang, Kun Jie Yang, Chenguang Liu, Yue-Lin Liu
We further investigate structural character and thermodynamic stability of La at three positions including tetrahedron interstitial site (TIS), octahedron interstitial site (OIS), and substitution site (SS) in Mo, as shown in Figure. 1(a–c). Figure 2 presents the solution energies of La at these three sites. The solution energies are 10.57 and 11.19 eV for La at TIS and OIS, respectively. Although the solution energy of TIS-La is lower than that of OIS-La, these two energies are relatively higher and endothermic in Mo. So larger solution energies are mainly originated from that both TIS and OIS cannot provide effective space to hold one La atom. In general, the SS should be the preferred position for the alloying element La (as well as other alloying elements) since the SS possesses the larger space compared to these two interstitial sites. As expected, there is indeed a lower solution energy of 2.86 eV for La atom at SS in comparison with both TIS and OIS, as shown in Figure 2. This indicates that the dissolution of La at SS is energetically favorable in Mo. For comparison, three mixed dumbbell structures including <100>, <110> and <111> are also explored, as given in Fig. 1(d–f). One can find that their solution energies are much higher than that of La at SS, as shown in Figure 2. Comparing with each other, the <111> mixed dumbbell is more stable than both <100> and <110> mixed dumbbells, which is in agreement with the previous study [34] that Muzyk et al. calculated the solution energies of W-V and W-Ta mixed dumbbells in W and found that the <111> mixed dumbbell is also the most stable configuration.