Metal matrix ceramic composites (MMCs) are widely applied materials in surface engineering due to their high hardness and excellent wear resistance. Recently, various MMCs have been successfully fabricated by a promising method named direct laser deposition. In this work, nickel-based hard surface coatings reinforced with multiple in-situ phases were deposited by wide-band laser. The strengthened phases were synthesized by varied content of Ti and B₄C precursor powders. The microstructure evolution, phase constitution and mechanical properties of the designed coatings were investigated. Results indicated the B₄C were decomposed and free C and B atoms were released in a molten pool. Multiple secondary phases such as TiC, Cr₇C₃, Cr₂₃C₆, TiB and CrB were in-situ synthesized. As the content of precursor Ti and B₄C powders increased, the microstructure of the laser-clad coatings was greatly refined due to the plentiful in-situ phases. Mechanical properties of the coatings revealed the maximum elastic modulus and microhardness reached 247 and 7.18 GPa in the experiment group. Friction tests indicated the average friction coefficient of optimized coating was about 0.50.

The Cu-rich phase is a high-efficiency and ultra-stable precipitation-strengthening phase and has been widely used in many steels and alloys, especially in heat-resistant steels and alloys. Creep damage is accompanied with the coarsening of the second phase. In the present work, the calculation of phase diagrams (CALPHAD) method and elastic–plastic mechanics are coupled with the phase field (PF) approach to investigate the growth behavior and the accompanying stress/strain field evolution of nano-sized Cu-rich precipitates in an Fe-Cr-Ni-Cu medium-entropy alloy. The results show that creep strain is intensified with the coarsening of Cu-rich particles. The simulated size of Cu-rich particles is in good agreement previous experimental reports. The plastic strain tends to shear the Cu-rich phase when they are relatively fine (~11 nm) are bypassed by the plastic strain due to the enhancing stress concentration around the interface, and the creep strain is rapidly aggravated with the growth of Cu-rich particles. The coarsening of Cu-rich particles will be retarded by the adjacent particles due to the overlapping of the diffusion zone, and hence the creep strain was reduced when crept for the same time. The retard effect will vanish when their distance is sufficiently long (~>60 nm). When the size of the Cu-rich particles is identical, the creep strain will be mitigated with elongation of the distance between two Cu-rich particles.

Pipe to elbow welds are usually identified as the weakest parts in the pipeline system of ultra-supercritical boilers due to the structural induced stress arising from internal steam pressure, and the constraint of supports and hangers. The finite element (FE) method has been applied to investigate the effect of structural induced stress on creep evolution in pipe to elbow welds. The results show that compressive axial structural induced stress can significantly increase the creep strain near the pipe’s outer surface. In contrast, the creep strain near the pipe’s inner surface is clearly accelerated by tensile axial structural induced stress. Compared with free deformation conditions in the pipe ends, when subject to a compressive axial structural induced stress under −30 MPa, the equivalent creep strain in the fine-grained heat affected zone (FGHAZ) at the 12:00 position on the outer surface increases by about 13.7 times. In the case of a 30 MPa tensile axial structural induced stress, the equivalent creep strain increases by about 83.3% in the FGHAZ at the 12:00 position on the inner surface. The maximum creep strain of the pipe to elbow weld in the ultra-supercritical boiler after creep for 5000 h is 1.9% and located at the 10:30 position in the FGHAZ on the pipe’s outer surface, which makes it the weakest part of the welded joint. The location of a crack in a pipe to elbow weld after running for 20,000 h is in agreement with the simulation results.