Xufei Fang

In situ observation and measurement of composites subjected to extremely high temperature

In this work, we develop an instrument to study the ablation and oxidation process of materials such as C/SiC (carbon fiber reinforced silicon carbide composites) and ultra-high temperature ceramic in extremely high temperature environment. The instrument is integrated with high speed cameras with filtering lens, infrared thermometers and water vapor generator for image capture, temperature measurement, and humid atmosphere, respectively. The ablation process and thermal shock as well as the temperature on both sides of the specimen can be in situ monitored. The results show clearly the dynamic ablation and liquid oxide flowing. In addition, we develop an algorithm for the post-processing of the captured images to obtain the deformation of the specimens, in order to better understand the behavior of the specimen subjected to high temperature.

Wrinkles formation and evolution of nanoribbons with finite length on elastomeric substrate

We report in situ observation of wrinkles formation and evolution of Si nanoribbons with finite length on elastomeric substrate via white light interferometer. The wrinkle originates from the middle of the nanoribbon, propagates symmetrically to the two ends, and finally reaches the stable configuration. The wavelength and amplitude will increase abruptly when the released strain exceeds the critical value. The interface interaction between Si nanoribbons and elastomeric substrate plays the key role for wrinkles formation.

Surface evolution at nanoscale during oxidation: A competing mechanism between local curvature effect and stress effect

The process of surface evolution of a chemically etched stepped structure at nanoscale during oxidation at 600 °C is in situ and real time observed using scanning probe microscope, which is integrated in a nanoindentation equipment for high temperature test. Experimental results reveal that this curved stepped structure becomes flat after being oxidized for a short period of time. However, after a longer time of oxidation, it is observed that the originally flat surface near the stepped structure becomes rough. Analysis shows that such a surface evolution is attributed to the competition between the nanoscale curvature effect (related to surface energy) and the stress developed in the oxide film during oxidation (related to strain energy). It is demonstrated that both the surface energy and strain energy can modify the surface chemical potential, which acts as the driving force of the surface diffusion of oxygen and further affects the oxide formation on the surface.

Measurements for displacement and deformation at high temperature by using edge detection of digital image

In this work, we propose a structural deformation measuring method based on structural feature processing (straight line/edge detection) of the recorded digital images for specimens subjected to a high-temperature environment. Both radiation light and oxidation at high temperatures challenge the optics-based measurements. The images of a rectangular piece of copper specimen are obtained by using a bandpass filtering method at high temperatures, then all the edges are detected by using an edge detection operator, and then a Hough transform is conducted to search the straight edges for the calculation of deformation. Especially, due to the severe oxidation, a special seed strategy is adopted to reduce the oxidation effect and obtain an accurate result. For validation, the structural thermal deformation and the values of coefficients of thermal expansion for the copper specimen are measured and compared with data in the literature. The results reveal that the proposed method is accurate to measure the deformation of the structures at high temperatures.

The Temperature-Dependent Strength of Metals: Theory and Experimental Validation

In this work, we propose a strength theory as a function of temperature and state of stresses for metals. Based on the fracture in the hydrostatic stress, we derived a generalized strength model, in which the fracture strength decreases almost linearly with the increasing of the temperature. Furthermore this generalized strength model was extended to the general state of stresses by replacing the equivalent hydrostatic stresses with the temperature effect based on the general thermodynamics principles. Molecular dynamics (MD) simulation was also conducted to simulate the fracture evolution at high temperature and to explain the mechanism of temperature-dependent strength at atomic scale. The proposed model was also verified by experiment of Mo-10Cu alloy at elevated temperature.

Temperature-Dependent Modulus of Metals Based on Lattice Vibration Theory

Fundamentally understanding the temperature-dependent modulus is the key issue for materials serving in high temperature environments. This paper proposes a model based on lattice vibration theory to predict the temperature-dependent modulus with respect to isothermal and isentropic assumption. The thermal vibration free energy is expressed as a function of the two independent scalars from the strain tensor and temperature. By using the Einstein theory, we present the analytical expression for the temperaturedependent Young’s modulus, bulk modulus, shear modulus, and Poisson’s ratio. The theoretical prediction agrees well with the experimental data. The proposed model is further degenerated to Wachtman’s empirical equation and provides the physical meaning to the parameters in Wachtman’s equation. [DOI: 10.1115/1.4025417]

Modification of the mechanism for stress-aided grain boundary oxidation ahead of cracks

In this work, we consider the diffusion mechanisms for different metal elements during oxidation and extend the stress–diffusion coupled model for stress-aided grain boundary oxidation ahead of cracks (Evans et al. in Scr Mater 69:179–182, 2013) to a more general situation including both outward and inward diffusion at the crack tip during oxidation. The analyses show that the transformation stress generated due to oxide formation near the crack tip could in principle promote the growth of Cr2O3 oxide at the crack tip in the intrusion direction but has no enhancement effect on NiO oxide in the extrusion direction with respect to the original position of the crack tip.Graphical abstract

Curvature effect on the surface topography evolution during oxidation at small scale

We use high temperature scanning probe microscopy (SPM) to in situ and real time characterize the evolution of surface topography of metals during oxidation. A nanoindentation method was used to create nanoindents as markers to pinpoint the locations where the evolution of the surface topography was studied. The SPM images reveal that during oxidation, the originally sharp tip of the indented pits exhibits a chamfering and flattening effect, suggesting that the tip curvature affects the surface topography evolution at the nanoscale/sub-microscale during the oxidation process. A model is proposed to explain the experimental result by considering the surface diffusion as well as the curvature effect.