Qiang Yao

First-Principles Investigation of Phase Stability, Elastic and Thermodynamic Properties in L1 2 Co 3 (Al,Mo,Ta) Phase

First-principles calculations have been performed to investigate the phase stability, elastic, and thermodynamic properties of Co3(Al,Mo,Ta) with the L12 structure. Calculated elastic constants showed that Co3(Al,Mo,Ta) is mechanically stable and possesses intrinsic ductility. Young’s and shear moduli of polycrystalline Co3(Al,Mo,Ta) were calculated using the Voigt-Reuss-Hill approach. It was found that the shear and Young’s moduli of Co3(Al,Mo,Ta) were smaller than those of Co3(Al,W). States density indicated the existence of covalent-like bonding in Co3(Al,Mo,Ta). Temperature-dependent thermodynamic properties of Co3(Al,Mo,Ta) could be described satisfactorily using the Debye-Grüneisen approach, including entropy, enthalpy, heat capacity and linear thermal expansion coefficient, showing their significant temperature dependences. Furthermore the obtained data could be employed in the modeling of thermodynamic and mechanical properties of Co-based alloys to enable the design of high temperature alloys.

Microstructure and Thermal Properties of SiC/Al Composites for Electronic Packaging

The SiC particles reinforced aluminum matrix composites for electronic packaging was prepared by using pressureless infiltration. The microstructures and thermal properties were investigated. The results shown that the composites were free of porosity, the SiC particles were distributed uniformly and which interfaces were well controlled. The bending strength was less affected by different loading speeds, the experimental results shown that the average bending strength was more than 360 MPa. By studying the fracture surface of bending specimens, the SiC particles were brittle cleavage fracture and the matrix was ductile tough fracture. The thermal performance test results indicated that the coefficient of linear expansion increased first and then decreasing with the temperature rising, and the maximum value is 8.25 × 10−6 K−1 near 300 °C. The thermal conductivity decreased gradually, when the temperature is 25 °C, the maximum thermal conductivity was 225.7 W m−1 K−1, and the density was 2.95 g/cm3, meet the requirements of electronic packaging materials.

First-Principles Investigation of the Site Occupancy and Elastic Properties of Y in γ-Co3(Al,W)

Site occupancy of Y in the γ′-Co3(Al,W) was predicted theoretically by first-principles calculations based on density functional theory. By computing total energy as a function of applied strain, the elastic constants of quaternary Co3(Al,W) were also predicted. The results suggest that Y preferentially occupies the W sites in Co3(Al,W). The calculation of heat of formation shows that the occupancy of Y on the W sites decreases the phase stability of Co3(Al,W). The theoretical calculation also shows that the L12 Co24Al4W3Y compound is ductile in nature.