Practical method to determine the effective zero-point of indentation depth for continuous stiffness measurement nanoindentation test with Berkovich tip.

The zero-point of indentation depth in nanoindentation or depth-sensing instrumented indentation tests should be precisely set to evaluate the indentation hardness and indentation elastic modulus of materials to be tested, especially at shallow depths. A critical contact stiffness value has been widely used to determine the zero-point in nanoindentation tests with a Berkovich tip using the continuous stiffness measurement (CSM) method. However, this criterion occasionally gives an inadequate zero-point owing to the surface roughness of materials, the vibration of the testing system, and the flaws of the CSM method at shallow depth. This study proposes a practical method to determine the effective zero-point of indentation depth, which was obtained linearly at the zero-point of contact stiffness and extrapolated from the depth-dependent contact stiffness values, except for those at initially unstable contact depths. The proposed method enables nanoindentation tests to obtain a constant indentation elastic modulus and low deviation of nanoindentation hardness of homogenously fused silica and metallic materials, which provides an efficient way to obtain more accurate test data.

Irradiation-induced ß to α SiC transformation at low temperature.

We observed that ß-SiC, neutron irradiated to 9 dpa (displacements per atom) at ≈1440 °C, began transforming to α-SiC, with radiation-induced Frank dislocation loops serving as the apparent nucleation sites. 1440 °C is a far lower temperature than usual ß â†’ α phase transformations in SiC. SiC is considered for applications in advanced nuclear systems, as well as for electronic or spintronic applications requiring ion irradiation processing. ß-SiC, preferred for nuclear applications, is metastable and undergoes a phase transformation at high temperatures (typically 2000 °C and above). Nuclear reactor concepts are not expected to reach the very high temperatures for thermal transformation. However, our results indicate incipient ß â†’ α phase transformation, in the form of small (~5-10 nm) pockets of α-SiC forming in the ß matrix. In service transformation could degrade structural stability and fuel integrity for SiC-based materials operated in this regime. However, engineering this transformation deliberately using ion irradiation could enable new electronic applications.

Application of FIB with micro pick-up to microstructural study of irradiated SiC/SiC composites.

The combination of focused ion beam (FIB) micro-processing and the lift-out technique using the micro pick-up system was applied to the preparation of TEM specimens of irradiated SiC/SiC composites. The deformation caused by microstructural evolution was observed in the pyrolitic carbon interphase, and several helium bubbles and cavities were detected in the CVI matrix.

Microstructural analysis of the deformation range under nano-indentation in beta-SiC.

As a powerful tool to analyse microstructural evolution under irradiation and the interaction of moving dislocations with radiation-induced defects, a focused ion beam (FIB) method was applied to ion-irradiated SiC followed by the nano-indentation test. An FIB method has excellent capability to prepare thin foils from the area of interest with a high accuracy of location and wide flexibility in the sampling direction. These advantages are demonstrated in the application to Si ion-irradiated SiC by transmission electron microscope observation.