Shusuo Li

Microstructure and tribological properties of laser clad CaF2/Al2O3 self-lubrication wear-resistant ceramic matrix composite coatings

Abstract Self-lubrication wear-resistant CaF 2 /Al 2 O 3 ceramic matrix composite coatings were fabricated on substrates of Al 2 O 3 by laser cladding CaF 2 –Al 2 O 3 mixed powder blends. Compared with laser clad monolithic Al 2 O 3 coatings, the CaF 2 /Al 2 O 3 coating has much superior wear resistance and noticeably lower friction coefficient under dry sliding wear test conditions.

Dislocation network with pair-coupling structure in {111} γ/γ' interface of Ni-based single crystal superalloy.

The γ/γ′ interface dislocation network is reported to improve the high temperature creep resistance of single crystal superalloys and is usually found to deposit in {001} interface. In this work, a new type of dislocation network was found in {111} γ/γ′ interface at a single crystal model superalloy crept at 1100 °C/100 MPa. The dislocations in the network are screw with Burgers vectors of 1/2 a<110> and most interestingly, they exhibit a pair-coupling structure. Further investigation indicates that the formation of {111} interface dislocation network occurs when the γ′ raft structure begins to degrade by the dislocations cutting into the rafted γ′ through the interface. In this condition, the pair-coupling structure is established by the dislocations gliding in a single {111} plane of γ′, in order to remove the anti-phase boundary in γ′; these dislocations also act as diffusion channels for dissolving of the γ′ particle that is unstable under the interfacial stress from lattice misfit, which leads to the formation of {111}-type zigzag interface. The formation of this network arises as a consequence of more negative misfit, low-alloying γ′ particle and proper test conditions of temperature and stress.

Surface recrystallization of a Ni3Al based single crystal superalloy at different annealing temperature and blasting pressure

The effects of annealing temperature and grit blasting pressure on the recrystallization behavior of a Ni3Al based single crystal superalloy were studied in this work. The results show that the precipitation of the Y-NiMo phase occurs at 900 and 1000 °C, which precedes recrystallization. The initial recrystallization temperature was between 1000 and 1100 °C. Cellular recrystallization was formed at 1100 and 1200 °C, which consisted of large columnar γ′ and fine γ + γ′. The dendrite arm closed to the interdendritic region may act as nucleation sites during initial recrystallization by a particle simulated nucleation mechanism at 1280 °C. The size of the grains first turned large and then became small upon the pressure while the recrystallization depth increased all the time.

Rotating bending fatigue property of the Ni3Al-based single crystal superalloy IC6SX at 900°C

The high cycle fatigue behavior of a Ni3Al base single crystal superalloy IC6SX has been investigated at 900°C in this work. The specimens used for the fatigue tests were prepared by screw selection crystal method in a directional solidification furnace. The rotating bending fatigue tests were carried out at 900°Cin air, the stress ratio of R(σmax/σmin) was -1, and the rotating speed of the fatigue tests was 6500r/min(108Hz). The stress-fatigue cycle life (S-Nf) curve was obtained based on the fatigue tests, and the fracture surfaces were examined using scanning electron microscopy (SEM). It has been found that the median fatigue strength is 457.5MPa and the safety fatigue strength is 413.93MPa. And the fatigue fracture was composed of three different characteristic regions.

Research on the creep mechanism of a Ni3Al-based single crystal superalloy IC6SX under 980°C/205MPa

The Ni3Al-base single crystal superalloy IC6SX used in this work was prepared by spiral grain selection method. And the creep behaviors of the IC6SX alloys was studied systematically under 980°C/205MPa. The microstructure evolution, movement of dislocations, formation of the dislocation networks and dislocation configuration for the alloy during the creep process were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that the creep performance of the single crystal alloy IC6SX is excellent under the condition of 980°C/205MPa. The results showed the creep mechanism included dislocation slipping and diffusion for Ni3Al-based Single Crystal Superalloy IC6SX under 980°C/205MPa.

Effects of Ce and Dy on the Cyclic Oxidation Behavior of a Ni-Based Single Crystal Superalloy

To understand the role of rare earth elements in influencing high-temperature oxidation behavior of the Ni-based single crystal superalloy, a high Mo-containing nickel-based single crystal superalloy and the derivative alloy modified by Ce and Dy were designed. The results of oxidation tests conducted at 1100 °C indicated that doping 0.022 wt% Ce or 0.042 wt% Dy significantly improved the oxidation resistance of the alloy by reducing the oxide scale growth rate and enhancing the scale adhesion. The positive effects are probably because the doped trace of Dy or Ce can combine with O2− more quickly and reduce the oxygen contents on the oxide surface which decreases the oxidation rate. At the same time, it decreases the thickness of oxide scale which results in the reduction of the elastic strain energy and the weaker tendency to spallation of the oxide scale. But too high concentration of Dy or Ce was deleterious because its high oxidation rate results in much more defects which induce cracks easily.

Interdiffusion Behavior at Interface between Nialhfsicoatings and Ni3Al Based Superalloy Substrates

NiAlHf coatings doped with 1 at. % Si and 2 at. % Si were designed and prepared on surface of a Ni3Al based single crystal superalloy IC32 by electron beam physical vapor deposition (EB-PVD). The effect of the coatings on the oxidation resistance of alloy and performances at high temperature were investigated. The results have shown that the presence of Si could retard the inter-diffusion of elements at the surface between NiAlHf coatings and superalloy substrates and promote the formation of R-Ni(MoRe) phase. Meanwhile NiAlHf coating doped with 1 at. % Si had a close hardness with that of the substrate alloy, which has a smooth match between coating and substrate in mechanical properties. So the NiAlHf coating doped with Si will have superior oxidation performances in thermal cycle.