1. Processing, Microstructure and Mechanical Properties of Ultrafine Grained and Nanocrystalline Materials: Metals and alloys with ultrafine grained (UFG, grain size between 1000 nm and 100 nm) and nanocrystalline (NC, grain size below 100 nm) microstructures exhibit a number of novel properties. The most famous is the Hall-Petch relation which predicts that the strength of a metal is proportional to the inverse square root of its grain size. Recently, investigators have uncovered many new behaviors of UFG and NC metals. For example, strategies to improve the ductility of UFG/NC metals have been proposed and exploited. It has also been observed that the strain rate effect on the mechanical strength of UFG/NC metals has some unique trend depending on the lattice structure of the metals. Such observations is bringing out new ways to the production of UFG/NC metals and alloys with desired properties. In Dr. Wei’s research group, UFG/NC metals and alloys are processed by various technical routes, including powder metallurgy (high-energy ball milling followed by consolidation), severe plastic deformation (SPD), etc. Microstructure engineering for the processing of multi-phase UFG/NC alloys is of particular interest to his group.
2. Dynamic Behavior of Materials: Many materials applications pose the need for knowledge about the materials mechanical behavior under high rate (dynamic loading). Quasi-static loading means the strain rate is within the range of 10-4 s-1-100 s-1; dynamic loading means the strain rate is larger than this range. A well defined technique for the evaluation of the dynamic properties of materials is the Kolsky Bar (or Split-Hopkinson Pressure Bar–SHPB) systems. Dr. Wei’s lab has a conventional SHPB system and a miniature SHPB the bar diameter of which is 5 mm. The lab also has a high-resolution high-speed photographing system with a maximum framing rate at 106 fps (frames per second). The high-speed photographing system is synchronized with the SHPB system can allows the recording of the dynamic deformation and failure processes of the specimens.
3. Probing of Mechanical Properties at Micro/Nano Scale: Quest for scaling towards increasingly smaller devices requires knowledge of the materials behavior at micrometer and nanometer scales. Assumptions that small scale materials behavior being comparable to large scale properties have been proven dubious, inaccurate or even completely erroneous. In light of this, direct probing of mechanical properties of materials at micro/nanometer scale is necessary. Ultimately, it will be necessary to match the volume of the measurement to that of significance for function in a particular application. Conventionally, people have been using nano-indenter, for example, to investigate the mechanical properties of thin films, to examine the indenter size effect, to evaluate the incipient plasticity associated with single crystal metals, to study the phase transformation of diamond cubic semiconductors such as Si and Ge, and so on. However, it is not possible to use conventional nano-indentation to acquire experimental data with regard to the constitutive behavior of small scale materials systems. Dr. Wei’s group is working on using a modified nano-instrumentation to probe the stress-strain behavior of materials at the micro/nano scale. With collaboration with his colleagues domestic and abroad, he is also working on such issues using micro-tension on samples fabricated by focused ion beam (FIB) technique.
4. Fabrication and Characterization of Thin Films: Thin films or coatings are used in many cutting edge technologies. Dr. Wei’s group is working on processing and characterization of advanced thin film materials such as super-hard diamond-like carbon coatings. They use pulsed-laser deposition (PLD) to produce the coatings on various kinds of substrates, and measure their mechanical properties via nano-indentation. One contribution from Dr. Wei’s work is successful relief of the large internal compressive stress in such super-hard coating introduced during coating deposition.
5. Materials Characterization Using Modern Techniques: Dr. Wei’s group is using state-of-the-art techniques for the characterization of materials. Such techniques include transmission electron microscopy (TEM), X-ray diffraction (XRD), etc. High-resolution TEM is used to observe atomic-level structures of materials, such as grain boundary structures, dislocations, etc.