Xiaodong Mao

Engineering design of the HELIMAK device

The HELIMAK experiment will provide a simplified magnetic geometry for study of magnetized turbulence, and control of some experimental parameters including flows. By allowing current to flow between the top and bottom boundaries, this magnetic configuration has equilibrium. Because the configuration also matches that found in tokamaks when initial ionization is produced by radiation at the electron cyclotron frequency, there is a sound experimental basis. HELIMAK would provide a test of the flow-shear stabilization mechanism. The HELIMAK device would consist of a vacuum chamber (VC), a toroidal field magnets system and a vertical field magnets system with supports structure. The magnet system and the VC of HELIMAK have been calculated and designed in detail.

A new method for fast statistical measurement of interfacial misfit strain around nano-scale semi-coherent particles

An innovative diffraction contrast imaging method derived from Ashby–Brown contrast is presented for fast and statistical measurement of interfacial misfit strain around nano-sized semi-coherent particles. A correlation between nano-scale interfacial misfit strain and the fringe-like transmission electron microscopy (TEM) contrast lines was set up, by which interfacial misfit strain could be obtained by measuring the inter-spacings between the so-called “no-contrast” lines. The mechanism of the measurement lies on the symmetry of the strain field at specific orientations around semi-coherent particles due to the presence of misfit dislocations, which induces “no-contrast” lines on TEM images under proper two-beam conditions. A much lower average lattice misfit strain of 4%, rather than 9.45% expected from lattice misfit between matrix and precipitate crystals, was revealed along (02)Matrix in austenitic oxide dispersion strengthened steel by this method, and was confirmed by geometric phase analysis (GPA) on high resolution transmission electron microscopy (HRTEM) images. This diffraction contrast imaging method is especially suitable for measuring misfit strain around particles that are smaller than 10 nm, and is expected to bridge the gap between nano-scale interfacial structure and mechanical properties of materials that are strengthened by semi-coherent nano-particles.