Pengfei Guo

First-principles calculation on β-SiC(111)/α-WC(0001) interface

The α-WC(0001) surface and β-SiC(111)/α-WC(0001) interface were studied by first-principles calculation based on density functional theory. It is demonstrated that the α-WC(0001) surface models with more than nine atom-layers exhibit bulk-like interior, wherein the surface relaxations localized within the top three layers are well converged. Twenty-four specific geometry models of SiC/WC interface structures with different terminations and stacking sites were chosen. The calculated work of adhesion and interface energy suggest that the most stable interface structure has the C-C bonding across the interface, yielding the largest work of adhesion and the lowest interface energy. Moreover, the top-site stacking sequence is preferable for the C/C-terminated interface. The effects of the interface on the electronic structures of the C/C-terminated interfaces are mainly localized within the first and second layers of the interface. Calculations of the work of adhesion and interface energy provide theoretical evidence that the mechanical failure may initiate at the interface or in SiC but not in WC.

HAZ Liquation Cracking Mechanism of IN-738LC Superalloy Prepared by Laser Solid Forming

The heat-affected zone (HAZ) liquation cracking mechanism of nickel-based superalloys with high (Al+Ti) content during the laser solid forming (LSF) process was investigated via laser remelting of an as-deposited IN-738LC superalloy. Microstructural HAZ analysis revealed that cracks consistently propagated from the HAZ to the remelting zone along the grain boundary (GB). The formation of a liquid film during GB liquation was mainly owing to localized melting of the semicontinuous γ-γ′ eutectic distributed along the GB. The solute segregation behavior of the IN-738LC alloy during LSF was analyzed using the Giovanola–Kurz model and Scheil models, revealing that a significant enrichment of γ-γ′ eutectic-forming elements in the residual liquid at the final stage of solidification (solid fraction ~ 0.87) in the molten pool was the main cause of semicontinuous γ-γ′ eutectic formation along the GB. Further, a B enrichment at the GB was identified in LSF-fabricated IN-738LC, which promoted cracking by lowering the GB liquation temperature and promoting wetting of the GB by the liquid film. Unlike the phenomenon observed in the welding of cast IN-738LC, the coherence between the γ′ phases and the γ matrix in LSF-fabricated IN-738LC can suppress the occurrence of constitutional liquation of the γ′ phase. To understand the interaction between the thermal stress and the liquid film in the LSF process, thermal stress as a cracking driving force was also estimated based on the measurement of the residual stress from the substrate to the remelting zone of the IN-738LC deposit by the Vickers micro-indentation method.