Effects Of High Temperature Creep Research Articles

Sintering polycrystalline silicon carbide composite ceramics with ultra-high hardness under high pressure

Silicon carbide (SiC) is an important structural ceramic material, exhibiting exceptional comprehensive properties that are unmatched by metals and other structural materials. In this study, a combination of α-SiC micron powder and β-SiC nanopowder was utilized as precursor materials for high pressure and high temperature (HPHT) sintering. Under a pressure of 5.0 GPa, polycrystalline SiC samples with mixed grain size were sintered within a temperature range from 1000 to 1700 °C, and compared with SiC samples sintered from single micron powder under identical temperature and pressure conditions. The microstructures of the two sets of SiC samples were observed using scanning electron microscopy. Additionally, the stress states and strengthening mechanisms among SiC grains with mixed grain size under HPHT were further analyzed in conjunction with X-ray diffraction results. The polycrystalline SiC composite ceramics sintered at 1700 °C exhibited superior mechanical and thermal properties, achieving a Vickers hardness of 35.2 GPa that is 23.5 % higher than that obtained by conventional spark plasma sintering and even surpassing the hardness of single crystal SiC, demonstrating thermal stability up to 1405 °C in air environment. Transmission electron microscopy was employed to analyze defects and plastic deformation in these samples. The study suggests that the primary strengthening mechanisms of the sintered polycrystalline SiC composite ceramics under HPHT include the increase in micro defects induced by in-situ plastic deformation at elevated temperatures and the effects of high-temperature creep. This study provides new insights into the HPHT sintering of hard materials.

Two-way coupled CFD fire and thermomechanical FE analyses of a self-supporting sandwich panel façade system

A self-supporting sandwich panel façade system under fire is studied. First, a thermomechanical FE model, which comprises the complete façade system, and incorporates material degradation and geometrical nonlinear behaviour except for the insulation material and connections, is loaded by a fire temperature curve. Eurocode design rules then predict the screw connections of a sandwich panel will fail in shear. Secondly, an existing programme, FDS-2-Abaqus, is extended to allow its two-way coupled analyses, in which CFD fire simulations are updated for changes in the thermomechanical FE model, to be applicable to the façade system. Again, these simulations show the shear failure of the screws. Parameter studies show differences in system behaviour for improved screw properties; a fuel-controlled vs. ventilation-controlled fire; and different panel thicknesses. Interestingly, as thermal bowing of the panel retards screw failure, and thicker sandwich panels bow less than thinner panels, thicker façade panels will decrease failure time. This and other insights obtained, and the predicted failure of two tiny but critical screws within 80 s, as compared to 150 min lasting sandwich panels in standard fire tests, stresses the need to study complete systems under realistic fires, rather than to study individual components in standard fire tests. Future research will focus on detailed FE models of the connections; full-scale fire experiments; CFD temperature measurement points at the façade outside; detailed modelling of the insulation; and the effects of high temperature creep.

Effect of high temperature creep on the fire response of restrained steel beams

At room temperature, and at service load levels, creep has little effect on the performance of steel structures. However, under fire conditions, creep becomes a dominant factor and influences fire resistance of steel members. Under fire conditions, significant forces develop in restrained steel beams and these forces induce high stresses in the steel section. The extent of creep deformations is affected by magnitude and rate of development of stress and temperature in steel. In this paper, the effect of high temperature creep on fire response of restrained beams is investigated. Current high temperature creep models are compared. Finite element model created in ANSYS was validated by comparing the predictions with fire test data. The validated model was applied to investigate the effect of load level, heating rate, fire scenario and fire induced axial restraint on the extent of creep deformations. Results from the parametric study indicate that the influence of high temperature creep increases with the increase in axial restraint, heating rate, and load level. Generally, neglecting high-temperature creep effect stiffens the structural response and leads to reduced deflections but larger restraint forces. Therefore, neglecting high temperature creep in fire resistance analysis of steel structures can lead to unconservative predictions.

Stress relaxation during the growth of 3C-SiC∕Si thin films

In this work the authors study the strain of 3C-SiC thin films grown on (001) on-axis silicon substrates. They use ex situ wafer curvature measurements to monitor the residual strain of silicon carbide film. At high temperature creep effects take place and modify the intrinsic strain of silicon carbide film. From the time and temperature dependences of these effects, they determine the creep exponent and the creep activation energy for 3C-SiC. Obtained values of N=2.6±0.3 and Q=5.6±1.0eV are similar to those reported in literature for hexagonal polytypes of silicon carbide.

Characterization of Nanoscale Clusters in Ods Iron-Based Alloys

The high temperature application of iron-based ferritic alloys is limited by their rapid decrease of yield strength at temperatures approaching 1000°C. It has been shown that mechanical-alloying (MA) to produce oxide dispersion-strengthened (ODS) ferritic alloys improves their high temperature mechanical properties. Prior characterization of such materials has shown that in certain as-processed alloys the original yttria oxide particles are replaced by nanoscale (2-5 nm diameter) clusters containing Ti, Y and O. As a result of the high density of these fine clusters, dislocation pinning produced a ∼10-fold increase in dislocation density relative to similar ODS materials not exhibiting the nanoscale clustering. The improved creep resistance of the clustered material was attributed both to the higher dislocation density, additional dislocation pinning and resistance to recovery during creep. The current work examines clustering in a related alloy, as well as the effects of high temperature creep on the stability of such clusters.

Effects of high temperature creep on magnetic properties of steels (pipeline inspection)

An investigation was performed into detection of creep damage in the welded area of high-pressure, high-temperature steam pipelines. The measurements involved the use of a portable magnetic inspection device called the Magnescope. Results indicated a decline in the coercivity and remanence in creep-damaged pipes, probably due to segregation of impurities occurring during creep. The orientation of the inspection head with respect to the weld line affects the ability to detect creep damage. Measurements taken parallel to the weld line showed greater differences in the coercivity and remanence between the damaged and undamaged specimens. Measurements taken perpendicular to the weld line did not show large differences. >

Effects of creep on the microstructural features of an oxide-dispersion-strengthened superalloy

The effects of high temperature creep on the microstructure and the grain boundaries of the oxide-dispersion-strengthened superalloy MA6000 are investigated by means of transmission electron microscopy. Rafting of the γ′ phase is observed in the elongated grains, whereas in the small non-recrystallized grains γ′ rafting does not occur. Dislocations form networks around γ′ precipitates only in particular areas of the coarse grains where the oxide particles are absent. In the rest of the areas, where oxide dispersion is homogeneous, dislocations are pinned to the hard oxide particles instead of accumulating at γ-γ′ interfaces. Dislocation networks around the γ′ phase are not observed in the small grains. There is no detectable influence of high temperature creep on the grain boundaries of the material. The majority of them are of the small angle type and exhibit a very disordered morphology. Large angle grain boundaries are randomly oriented and they are mainly decorated by chromium carbides.