[en] Multi-layer Co-based alloy (HMSP 2537) were deposited on Ni-based superalloy plate with a TJ-TL-T5000 type CO₂ laser. Sections of such coatings were examined to reveal their microstructure and phase composition using optical microscope, scanning electron microscope (SEM), and X-ray diffractometer (XRD). The hardness and wear resistance of the coatings were tested. The results showed that the prime phase (γ-Co dendrite) and other phases, such as CrNi, Co₇W₆, and Cr₂₃C₆ existed in the coatings. Dendrite or cellular microstructures were observed perpendicular to the interface, and coarsening microstructures were obtained as more layers deposited. Dendrite paralleling to laser scan speed was also found near the top surface of the last layer. Fine microstructures of γ-Co dendrite and lamellar eutectic in inter-dendritic regions strengthened the coatings. Microhardness and wear resistance of the coatings were much higher than that of substrate but slightly decreased with layers increased

[en] Hardfacing coatings, made of Co-Cr-W-Ni-Si alloy + 20% SiCp, deposited by laser cladding on IF steel is introduced. Cross-section of such coatings has been examined to reveal their microstructure using optical microscope, scanning electron microscope (SEM) and X-ray diffractometer (XRD). MM-200 type wear tester is used to examine wear resistance of the coatings. The results showed that SiCp is dissolved completely during laser cladding process under this conditions, the primary phase γ-Co dendrite and Si₂W, CoWSi, Cr₃Si, CoSi₂ formed by C, Si reacting with other elements existed in the coatings. There existed some crystallization morphologies in different regions, such as planar (at the interface), followed cellular and dendrite crystallization from interface to the surface. The direction of solidification changes from one direction perpendicular to interface to multi-directions at the central and upper regions of the clad. The results also showed that the wear resistance of the clad improved by adding SiCp

[en] Micron-size Ni-base alloy (NBA) powders were mixed with both 1.5 wt.% (hereinafter %) micron-size CeO₂ (m-CeO₂) and also 1.5% and 3.0% nano-size CeO₂ (n- CeO₂) powders. These mixtures were coated on low-carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on the microstructures, phases and electrochemical corrosion of the coatings upon the addition of m- and n- CeO₂ powders to NBA (m- and n- CeO₂ /NBA) have been investigated. The results showed that a smooth coating was prepared under suitable processing parameters (P= 2.0 kW, V= 180 mm min^-1) by adding 1.5% n- CeO₂. In addition to the primary phases of γ-Ni, Cr₂₃ C₆ and Ni₃ B in the Ni-base alloy coating, CeNi₃ was formed in Ni-base alloy coatings with both n- CeO₂ and m-CeO₂ particles, and CeNi₅ appeared in the coating upon decreasing the size of CeO₂ particles. Well-developed dendrites were observed in the Ni-base alloy coating; directional dendrites grew at the interface in the coating upon the addition of m-CeO₂, whereas fine and multioriented dendrites grew upon decreasing the size of CeO₂ particles to the nanoscale. Actinomorphic dendrites and compact equiaxed dendrites grew from the interface to near the surface upon increasing the content of n- CeO₂ from 1.5 to 3.0%. In strongly acidic HNO₃ solution, the severe corrosion of dendrites occurred and there were many corrosion pits in the Ni-base alloy coating; intercrystalline corrosion also has a dominant role upon the addition of m-CeO₂, whereas uniform corrosion occurs in the coating as the size of CeO₂ particles is decreased to nanoscale

[en] In present paper, the relationship between fatigue and wear of three kinds of pearlite wheel steel samples is investigated by using rolling wear tester under 0.2% slip ratio condition. The results show that the weight loss, the surface hardness and the thickness of plastic deformation layer of three kinds of wheel samples are increased as the increase of cycles during rolling wear process. The weight loss of the G1 wheel sample is the highest owing to the existence of a large amount of proeutectoid ferrite (PF)grains. The correlation between weight loss and fatigue is competitive. High weight loss of the G1 wheel sample can hinder the formation of fatigue wear cracks. As a result, the G1 wheel sample surface changes to smooth after wear. The low weight loss of the G2 wheel sample and the G3 wheel sample cannot hinder the production of fatigue wear cracks during wear process. At the late wear stage, the wheel sample surface produces many fatigue wear cracks, and the wear mechanism becomes fatigue wear. Fatigue wear cracks, which are the rolling contact fatigue (RCF) crack source, can accelerate the RCF failure of wheel samples. (paper)

[en] A FeCoNiCrCuTiMoAlSiB_0.5 high-entropy alloy (HEA) comprised of a lath-like martensite phase was fabricated by laser cladding. The alloy combines attractive properties including hardness (11.6 GPa), elastic constant (187.1 GPa), fracture toughness (50.9 MPa m^0.5) and softening resistance (up to 900 °C). The nucleation of the martensite phase is co-triggered by laser rapid solidification and interstitial boron solute, owing to the improved lattice strain energy. The designed strengthening by martensite and interstitial solutes may enhance both hardness and toughness in other rapidly solidified HEAs

[en] The disorder-order transformation of three-dimensional Ni-Al-X (X = Ti, Cr, Co) ternary alloys is simulated on the atomic scale by a cellular automata (CA) method. The interaction of atom pairs and the total energy of the system are calculated using Lennard-Jones potential. The variables to be optimized in yielding a minimized energy were found to include the atomic positions, the (long- and short-range) ordering parameters, the volume fraction of the γ' (L1₂) phase, and the configuration energy dependence on the number of CA steps. The results are consistent with the ordering kinetics and the atomic arrangement found by a cluster variation method (CVM)

[en] Highlights: • New medium-entropy high speed steel coatings exhibit excellent hot wear resistance • Secondary hardening due to coherent nano-sized M₂C contributes to the high hardness • The secondary hardening is enhanced with the increasing C content • The C content needs to be optimized for high hardness and good hot wear resistance Novel Fe₆₈(MoWCrVCoNiAlCu)₃₂ (at.%) medium-entropy high-speed steel (ME-HSS) coatings, containing various carbon contents from 0.65 to 1.05 wt%, are prepared by laser rapid solidification. The newly prepared ME-HSS coatings are characterized by a hard martensitic matrix enhanced by secondary hardening, and specifically by coherent nano-sized M₂C. The secondary hardening effect is enhanced with the increasing carbon content. The high amount of alloying elements in ME-HSS coatings results in excellent oxidative wear resistance, without leading to serious compositional segregation and coarsening of carbides.

[en] Composite coatings, made of nano-Al₂O₃/cobalt-based alloy, produced by a 5-kW CO₂ laser on Ni-based superalloy were investigated. The coatings were examined to reveal their microstructure using optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction instrument. The results showed that some equilibrium or non-equilibrium phases, such as γ-Co, Cr₂₃C₆, CoAl₂O₄, Al₂O₃ and ε-Co existed in the coatings. Fine and short dendritic microstructure and columnar to equiaxed transition occurred by adding nano-Al₂O₃ particle. With the increase of nano-materials (1%, mass fraction), fully equiaxed crystallization appeared. These were contributed to that nano-Al₂O₃ particles acted as new nucleation site and rapid solidification of the melted pool. The results also showed inhomogeneous dispersion of nano-Al₂O₃ that resulted in the formation of ε-Co phase in the coatings. The sub-microstructure of the clad was stacking fault. The mechanism of formation of equiaxed grains was also analyzed

[en] The morphology, topology and kinetics of normal grain growth in two-dimension were studied by computer simulation using a cellular automata (Canada) model based on the lowest energy principle. The thermodynamic energy that follows Maxwell-Boltzmann statistics has been introduced into this model for the calculation of energy change. The transition that can reduce the system energy to the lowest level is chosen to occur when there is more than one possible transition direction. The simulation results show that the kinetics of normal grain growth follows the Burke equation with the growth exponent m = 2. The analysis of topology further indicates that normal grain growth can be simulated fairly well by the present CA model. The vanishing of grains with different number of sides is discussed in the simulation

[en] The temperature of dynamic strain aging (DSA) regime in P91 steel is between 523 K and 773 K. The activation energy (Q) for onset of DSA is 73 kJ/mol, while that for finale of DSA is 202 kJ/mol. Two main Internal friction (IF) speaks were observed, Snoek and SKK with the activation energy of 67.9 kJ/mol and 121 kJ/mol, respectively. IF shows that activation energy of 73 kJ/mol is equal to that of C atom body diffusion in α-Fe, and 202 kJ/mol is equal to binding energy between C atoms and moving dislocations. These results confirm that the mechanism of DSA can be explained by the diffusion of C atoms and pinning between C and moving dislocation. These investigations indicate that DSA in P91 steel is resulted from C atom diffusion, instead of Cr or Mo atoms.