Preparation and Characterization of Magnesia-based Powder Added With Transition Metal

The solid-phase reaction method for preparing forsterite (Mg2SiO4) using MgO and SiO2 powders has the disadvantages of high reaction temperature, long reaction time, and inhomogeneous reaction depending on the particle size of MgO. Therefore, MgO-based powders with a high reactivity were synthesized using a coprecipitation method with substitutional elements (Mn or Ni), and the effects of processing parameters on synthesizing MgO-based binary composition powders were investigated through the particle characteristics. The crystal structure continuously changed with the contents of the substitutional element, showing the same trend as the atomistic simulation results. The MgO-based powders showed higher reactivity than the conventional MgO powder, which could be confirmed in the particle characteristics, such as particle size and crystallinity, obtained in a short reaction time, and at a relatively low temperature. The optimum composition ratio in the binary composition powder for forming the Mg2SiO4 depended on the type of substitutional element, and the reaction mechanism was identified based on the particle characteristics.

Ab Initio Study of the Combined Effects of Alloying Elements and H on Grain Boundary Cohesion in Ferritic Steels

Hydrogen enhanced decohesion is expected to play a major role in ferritic steels, especially at grain boundaries. Here, we address the effects of some common alloying elements C, V, Cr, and Mn on the H segregation behaviour and the decohesion mechanism at a Σ 5 ( 310 ) [ 001 ] 36.9 ∘ grain boundary in bcc Fe using spin polarized density functional theory calculations. We find that V, Cr, and Mn enhance grain boundary cohesion. Furthermore, all elements have an influence on the segregation energies of the interstitial elements as well as on these elements’ impact on grain boundary cohesion. V slightly promotes segregation of the cohesion enhancing element C. However, none of the elements increase the cohesion enhancing effect of C and reduce the detrimental effect of H on interfacial cohesion at the same time. At an interface which is co-segregated with C, H, and a substitutional element, C and H show only weak interaction, and the highest work of separation is obtained when the substitute is Mn.

Interdiffusion Databanks of γ, γ′ and β Phases in NiAl-Based Ternary Systems

Advanced modern gas-turbine engines strongly rely on high-temperature thermal barrier coatings (TBCs) for the improved efficiency and power. Interdiffusion between the bond coat and the underlying Ni-based superalloy is one key factor limiting the lifetime of TBCs. In order to assist the engineering-oriented lifetime assessment and even design new TBCs, reliable composition- and temperature-dependent interdiffusivity databanks for γ, γ′ and β phases in different types of bond coats and Ni-based superalloys are the prerequisite. This chapter starts from a very brief introduction of the state-of-art experimental techniques and calculation methods for interdiffusivity determination in ternary systems. After that, the status of the interdiffusion databanks of γ, γ′ and β phases in NiAl-based ternary systems is then summarized, with a special focus on the demonstration of interdiffusivity data measured by means of single-phase diffusion couple/multiple techniques in combination with Matano-Kirkaldy method or numerical inverse method. Several typical results for NiAl-based γ, γ′ and β phases are also given. Finally, two examples of successful applications of the available interdiffusion databanks of ternary NiAl-based γ, γ′ and β phases are presented. One lies in the Re-substitutional element searching in potential new-generation Ni-based superalloys, while the other is the phase-field modeling of interdiffusion microstructure in ternary mode NiAlCr-based TBCs without/with the effect of temperature gradient.

The Solute Drag Model to Calculate Grain Growth Rate at High Temperatures in Carbon Steels

Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modeled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

Defect Control and Defect Engineering of Transition-metal Silicides

The microstructure, defect structure and thermoelectric properties of two different semiconducting transition-metal silicides, ReSi1.75 and Ru2Si3 upon alloying with a substitutional element with a valence electron number different from that of the constituent metal have been investigated in order to see if the crystal and defect structures of these silicides and thereby their physical properties can be controlled through defect engineering according to the valence electron counting rule. The Si vacancy concentration and its arrangement can be successfully controlled in ReSi1.75 while the relative magnitude of the metal and silicon subcell dimensions in the chimney-ladder structures can be successfully controlled in Ru2Si3. As a result, the improvement in the thermoelectric properties and the p- to n-type conduction transition are successfully achieved respectively for these semiconducting transition-metal silicides.

Creep Deformation of Ta Modified Gamma Prime Single Crystals

The role of substitutional element alloying of single phase γ' has become of primary interest to alloy designers who would like to exploit its low density and excellent oxidation resistance. Current γ' alloys have not shown sufficient strength to be useful in a creep limited environment. In order to maximize the potential of single phase γ' alloys and to more fully understand the creep strengthening mechanisms in two phase Ni-base superalloys, it has become necessary to clarify the role of Al-substitution elements. Ta is a potent strengthening element in γ' as well as imparting beneficial surface stability to superalloys; its effect on the creep properties of Ni3Al is the subject of this paper. The 1300°C isotherm of the Ni-Al-Ta system was determined in order to establish the γ' single phase field. Comrpositions were fabricated having chemistries which systematically varied both the Al:Ta ratio at Ni=75% and Ni:(AI+Ta) ratio at Ta=6%. Creep tests were conducted on <001> oriented single crystals at 760, 871 and 982°C. Electron microscopy was used to characterize the nature of slip deformation, confirm phase purity and to determine the existence of tetragonal distortions in these crystals. In this manner the strengthening due to Ta was examined in the absence of grain boundary effects. These γ' mono—crystals did not display classical creep response. Incubation creep was observed in all of the specimens tested. Surprisingly, the maximum incubation time was found to occur in the high ratio Ni:(Al+Ta) compounds, where less than 0.5% creep strain was obtained after 200 hours at stress. After incubation, either tertiary creep leading to failure, or apparently classic primary, secondary and tertiary creep ensued. In addition extremely long elongations, to 85%, were measured.