Shijie Zhu

Monotonic tension, fatigue and creep behavior of SiC-fiber-reinforced SiC-matrix composites: a review

The monotonic tension, fatigue and creep behaviour of SiC-fiber-reinforced SiC-matrix composites (SiC/SiC) has been reviewed. Although the short-term properties of SiC/SiC at high temperatures are very desirable, fatigue and creep resistance at high temperatures in argon was much lower than at room temperature. Enhanced SiC/SiC exhibits excellent fatigue and creep properties in air, but the mechanisms are not well understood. The present Hi-Nicalon/SiC has similar properties to enhanced SiC/SiC, but at higher cost. Improvement of Hi-Nicalon/SiC therefore seems necessary for the development of a high-performance SiC/SiC material.

Effect of thermal exposure on stress distribution in TGO layer of EB-PVD TBC

Abstract The stress distributions in thermally grown oxide (TGO) layer of electron beam enhanced physical vapor deposited thermal barrier coating (EB-PVD TBC) before and after thermal exposure are measured by photo-stimulated luminescence spectrum. It is found that the stress in the TGO layer in original state is an order of ~3 GPa. It increases from 3.0–3.9 GPa with the increase of heat exposure time from 0–100 h. The stress distribution in the direction of thickness in the TGO layer is not uniform. The stress near ceramic top coat is smaller than that close to metal bond coat. The stress at thickness imperfections in the TGO near the bond coat is smaller than that in the regular TGO.

Fatigue behavior of Al18B4O33 whisker-framework reinforced Al matrix composites at high temperatures

The aluminum borate porous ceramic with whisker-framework structure was in situ synthesized and used to manufacture aluminum matrix composites by squeeze casting. The processing technique for the reinforcement was not only non-harmful to the environment and humans, but also relatively cost-effective compared to SiC whiskers. The microstructures were observed and analyzed. The aluminum borate whiskers are uniformly distributed and randomly oriented. Fatigue behavior of the composites was investigated at temperatures from room temperature to 723 K. Fatigue life increased with an increase in volume fraction of aluminum borates. The fatigue life was correlated with stress and temperature. The damage mechanisms of fatigue are cracking of primary Si and the whiskers at room temperature and cavitation in the matrix at high temperatures.

Tensile damage evolution behavior in plasma-sprayed thermal barrier coating system

Cracking and fracture behavior in a plasma-sprayed thermal barrier coating system were evaluated using a sandwiched specimen under uni-axial tensile stress combined with an in situ microscope observation. It was found that initial crack initiation and transverse crack propagation occurred in the top coat layer. The number of transverse cracks increased rapidly and then saturated with an increase in tensile strain. After the saturation of the multiple cracks, the cracks passed through the interface between the top coat and bond coat, and propagated in the bond coat. Finally, decohesion occurred at the bond coat/substrate interface when the transverse cracks reached the interface. A model was proposed to describe the fracture processes.

Effect of stress ratio on fatigue crack growth in TiAl intermetallics at room and elevated temperatures

The fatigue crack growth behavior of γ-based titanium aluminides (TiAl) with a fine duplex structure and lamellar structure has been investigated by scanning electron microscope (SEM) in situ observation in vacuum at 750°C and room temperature. For the duplex structured material the fatigue crack growth rates are dominated by the maximum stress intensity, particularly at 750°C. The threshold stress intensity range for fatigue crack growth at 750°C is lower than that at room temperature for any corresponding stress ratio. The fatigue crack growth rate at 750°C is affected by creep deformation in front of the crack tip. The severe crack blunting occurs when the stress ratio is 0.5. For the lamellar structured material the scatter of fatigue crack growth data is very large. Small cracks propagate at the stress intensity range below the threshold for long fatigue crack growth. The effects of microstructure on fatigue crack growth are discussed.

Optimizing the Electrical Energy Conversion Cycle of Dielectric Elastomer Generators

A strategy to control the electrical charge is developed to achieve high energy density of soft dielectric elastomer generators for energy harvesting. The strategy is analytically shown and experimentally demonstrated to produce the highest energy density ever reported for a soft generator.

Creep and fatigue behavior of SiC fiber reinforced SiC composite at high temperatures

Abstract Tensile creep and tension-tension fatigue of SiC fiber reinforced SiC composite were investigated in argon at 1000 and 1300°C. Time-dependent (creep) strains under cyclic loading are much larger than those under constant load. However, the minimum creep strain rates under cyclic loading are similar to those under constant load at 1000°C and lower than those under constant load at 1300°C. All the data of the minimum creep strain rates versus time to rupture under both cyclic loading and constant load fall on the same line, i.e., the relations fit Monkman—Grant relationship. This means that creep controls the rupture life under cyclic loadings at high temperatures. At high stresses, the creep crack propagation paths are similar to fatigue crack propagation. However, at low stresses, more 0° fibers were broken in the way of flush with the matrix under creep load than that under fatigue load.

High temperature creep behaviour of an Al-8.5Fe-1.3V-1.7Si alloy reinforced with silicon carbide particulates

Abstract The creep behaviour of an Al-8.5Fe-1.3V-1.7Si (the 8009Al type, all numbers indicate wt.%) alloy reinforced with 15 vol.% silicon carbide particulates — the Al-8.5Fe-1.3V-1.7Si-15SiCp composite — is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behaviour is observed to be associated with the true threshold stress that decreases more strongly with increasing temperature than the shear modulus of aluminium. The minimum creep strain rate is controlled by the lattice diffusion in the composite matrix, and the true stress exponent is close to 5. The results are compared with those obtained investigating the creep behaviour of an unreinforced Al-8.5Fe-1.3V-1.7Si alloy in the same temperature range. The creep strength of the composite as characterised by the minimum creep strain rate is found to be up to six orders of magnitude higher in the composite than in the alloy. This creep strengthening is attributed to a much higher true threshold stress in the composite than in the alloy, which is primarily due to finely dispersed alumina particles appearing in the composite matrix as a result of composite fabrication. The creep behaviour is interpreted in terms of athermal detachment of dislocations from interacting particles admitting a temperature dependence of the relaxation factor that characterises the strength of dislocation/particle interaction.

Creep behaviour of an oxide dispersion strengthened Al-5mg alloy reinforced by silicon carbide particulates : an oxide dispersion strengthened Al-5Mg-30SiCp composite

Abstract The creep behaviour of oxide dispersion strengthened (ODS) Al–5%Mg alloy reinforced by 30vol.% silicon carbide particulates – an ODS Al–5Mg–30SiC p composite — is investigated at three temperatures ranging from 623 to 723 K. The creep is associated with a true threshold stress depending on temperature more strongly than the shear modulus of the matrix solid solution. At applied stresses only slightly higher than the true threshold stresses, the apparent stress exponent m c reaches values as high as ∼85 and the apparent activation energy Q c values as high as ∼2000 kJ mol −1 at 673 K. However, the true stress exponent is close to five and the minimum creep strain rate is controlled by the lattice diffusion in the composite matrix. The difference between the apparent and the true stress exponents as well as between the apparent and the true activation energies of creep is attributed to the strong temperature dependence of the threshold stress. The origin of the true threshold stress and its temperature dependence is discussed. Alloying the aluminium matrix with 5% Mg is shown to have only a slight effect on the creep strength of the ODS Al–30SiC p composite.

Disappearance of the true threshold creep behaviour of an ODS Al–30SiCp composite at high temperatures

Abstract The creep behaviour of an oxide dispersion strengthened (ODS) Al–30SiC p composite has been investigated in a broad temperature interval ranging from 623 to 798 K. At temperatures up to 723 K true threshold creep behaviour occurs, while at temperatures above 723 K no true threshold stress has been found. Accepting the creep model of Rosler and Arzt (Acta Met. Mater. A150 (1992) 21), the creep behaviour of this composite at temperatures ranging from 748 to 798 K is interpreted in terms of the thermally activated detachment of dislocations from small alumina particles in the composite matrix. The disappearance of the true threshold stress at temperatures above 723 K is then due to a transition from athermal to thermally activated detachment of dislocations from alumina particles.