First principles study on the phase stability and mechanical properties of MoSi2 alloyed with Al,Mg and Ge

The phase stability, mechanical properties and electronic structure of C11_b and C40 MoSi₂ with alloying elements Al, Mg and Ge were systematically investigated using first principles methods. The calculated lattice constants and elastic constants of C11_b and C40 MoSi₂ are in good agreement with the previous results. It is found that there is a phase transition from C11_b to C40 when the concentrations of Al and Mg reach ∼7 at.% and ∼6 at.%, respectively. Based on the elastic constants, the anisotropy, ductility, hardness and melting temperatures are presented for MoSi₂ with alloying elements. For C11_b, the ductility will be enhanced by increasing the concentrations of Al or Mg. Simultaneously, hardness will be reduced by the increasing of Al or Mg. Ge have a reverse effects. For C40, the ductility is reduced weakly by Al or Mg. In addition, the effects of substitution of Mo by Nb with Si substitution of Si by Al, Mg and Ge are also investigated. Nb and Mg codoping can improve the ductility of MoSi₂. Finally, the density of states is used to analysis the effects of alloying elements on the mechanical properties, and the results are in consistent with the predictions based on elastic constants.     

Atomic-scale microstructure and elastic properties of quaternary Zr–Al–Si–C ceramics

Transmission electron microscopy characterizations and elastic properties of two quaternary carbides, i.e. Zr₂(Al(Si))₄C₅ and Zr₃(Al(Si))₄C₆ are reported. The space group and atomic-scale microstructures of both compounds were determined using a combination of selected area electron diffraction, convergent beam electron diffraction, high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. In addition, the combined experimental and theoretical studies on elastic properties for Zr₂(Al(Si))₄C₅ are presented. A full set of second-order elastic constants, bulk modulus, shear modulus, and Young’s modulus were calculated using first-principles calculations. Both experimental and theoretical works demonstrated that quaternary Zr–Al–Si–C ceramics possess close elastic properties to ZrC. Furthermore, Zr₂(Al(Si))₄C₅ retained a high Young’s modulus up to about 1580 °C, which can be attributed to its comparable activation energy of lattice drag process to that of ZrC.     

Microstructure and performance of Al-Si alloy with high Si content by high temperature diffusion treatment

The Al-Si alloy with high Si content was prepared by pressure infiltration.Microstructure observation shows that three-dimensional structure(3D-structure) is obtained from irregular sharp Si particles via high temperature diffusion treatment(HTDT).Flat Si-Al interfaces transform to smooth curves,and Si phases precipitate in Al and Si-Al interface.The bonding of Si-Al interface is improved by HTDT,which improves the mechanical performance of Al-Si alloy.The bending strength of 3D-Al-Si alloy increases by 6% compared with that of Al-Si alloy,but the elastic modulus changes a little.The coefficient of thermal expansion(CTE) of the 3D-Al-Si alloy is 7.7×10-6/°C from 20 °C to 100 °C,which decreases by 7% compared with that of Al-Si alloy.However,HTDT has little effect on the thermal conductivity of Al-Si alloy.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

Plastic deformation of single crystals of VSi2 and TaSi2 with the C40 structure

The deformation behavior of (0001)1 10 basal slip in single crystals of VSi₂ and TaSi₂ with the C40 structure has been investigated in the temperature range from room temperature to 1500 °C in compression. Plastic flow is observed only above 400 and 600 °C for VSi₂ and TaSi₂, respectively. The critical resolved shear stress (CRSS) for basal slip decreases with increasing temperature but exhibits a moderate peak around 1100 and 1400 °C for VSi₂ and TaSi₂ respectively. Dislocations with b = 1/31 10 gliding on (0001) basal planes both in VSi₂ and in TaSi₂ are observed to dissociate into two identical partials with b = 1/61 10 involving a stacking fault. High-resolution transmission electron microscopy of 1/31 10 dislocations indicates that basal slip both in VSi₂ and in TaSi₂ occurs through a conventional single shearing mechanism, unlike in isostructural CrSi₂ and Mo(Si,Al)₂ in which basal slip occurs through a synchroshear mechanism.     

First-principles density functional calculations of physical properties of orthorhombic Au2Al crystal

The study attempts to perform a systematical investigation of the thermodynamic, mechanical and electronic properties of orthorhombic Au₂Al crystal by using first-principles calculations incorporated with a quasi-harmonic Debye model. In addition, their temperature, hydrostatic pressure and direction dependences are also addressed. The investigation begins with evaluation of the equilibrated lattice constants and elastic constants of Au₂Al single crystal. Next, the mechanical features of the single crystal, such as ductile-brittle characteristic and elastic anisotropy, are assessed based on the Cauchy pressures, shear anisotropy factors and directional Young's modulus. Alternatively, the pressure-dependence of polycrystalline mechanical properties of Au₂Al, including bulk, shear and Young's moduli, and ductility, brittleness and microhardness characteristics are also estimated. Furthermore, the study also characterizes the temperature-dependence of thermodynamic properties of Au₂Al single crystal, namely, Debye temperature and heat capacity. At last, electronic characteristic analysis is carried out to predict the electronic band structures and density of states profiles of the crystal.The calculation results indicate that Au₂Al crystal is an elastically anisotropic material at zero pressure and a highly ductile material with low stiffness. In addition, the Young's moduli of the crystal would be markedly enhanced with the increase of the hydrostatic pressure. It is also found that the heat capacity of Au₂Al at low temperature strictly sticks to the Debye T³ law.     

中碳结构纲P-C法表面改性层耐热性的研究

为了提高中碳结构钢在各种腐蚀性介质中的耐蚀性 ,应用 P- C(Pack- Cementation)法进行复合表面涂覆 ,获得了 Cr- Al- Si复合表面改性层。研究了涂覆温度及时间对涂层表面成分的影响 ,用电子探针及能谱测定了涂层的表面成分及 Cr- Al- Si在涂层中的浓度分布 ,用 X射线测定了涂层中的相结构 ,确定了最佳的涂覆工艺参数。将涂覆后的中碳结构钢试样在空气介质中进行高温氧化试验 ,结果表明 ,P- C法涂覆形成的 Cr- Al- Si复合表面改性层在空气介质中具有优良的抗高温氧化性能。     

Self-propagating high temperature synthesis of MoSi2 matrix composites

MoSi2 is presently regarded as the most important material for electrical heating and as one with huge potential for high temperature structural uses. MoSi2 and MoSi2 matrix composites were prepared by self-propagating high temperature synthesis （SHS）. Pure MoSi2 was obtained and a compound of MoSi2 and WSi2was synthesized in the form of predominant solid solution （Mo,W）Si2. By adding aluminum of 5.5 at.% to Mo-Si, the crystal structure of MoSi2 changed into a mixture of tetragonal Cllb MoSi2and hexagonal C40 Mo（Si,Al）2. The （Mo,W）Si2-Mo（Si,Al）2-W（Si,Al）2 composite materials were synthesized by adding aluminum of 5.5 at.% to Mo-W-Si. However, if the amount of the added aluminum was not larger than 2.5 at.%, it did not have any significant effect. SHS is an effective technology for synthesis of MoSi2 and MoSi2 matrix composites.     

Interdiffusion and reaction between Zr and Al alloys from 425° to 625 °C

Zirconium has recently garnered attention for use as a diffusion barrier between U–Mo nuclear fuels and Al cladding alloys. Interdiffusion and reactions between Zr and Al, Al-2 wt.% Si, Al-5 wt.% Si or AA6061 were investigated using solid-to-solid diffusion couples annealed in the temperature range of 425° to 625 °C. In the binary Al and Zr system, the Al₃Zr and Al₂Zr phases were identified, and the activation energy for the growth of the Al₃Zr phase was determined to be 347 kJ/mol. Negligible diffusional interactions were observed for diffusion couples between Zr vs. Al-2 wt.% Si, Al-5 wt.% Si and AA6061 annealed at or below 475 °C. In diffusion couples with the binary Al–Si alloys at 560 °C, a significant variation in the development of the phase constituents was observed including the thick τ₁ (Al₅SiZr₂) with Si content up to 12 at.%, and thin layers of (Si,Al)₂Zr, (Al,Si)₃Zr, Al₃SiZr₂ and Al₂Zr phases. The use of AA6061 as a terminal alloy resulted in the development of both τ₁ (Al₅SiZr₂) and (Al,Si)₃Zr phases with a very thin layer of (Al,Si)₂Zr. At 560 °C, with increasing Si content in the Al–Si alloy, an increase in the overall rate of diffusional interaction was observed; however, the diffusional interaction of Zr in contact with multicomponent AA6061 with 0.4–0.8 wt.% Si was most rapid.