In-situ Tem Study Of The Plastic Deformation Behaviors Of Body-centered Cubic Structure Mo Single-crystal Nanowire

The plastic deformation behaviors of metals are controlled by dislocationactivities. The plastic deformation mechanisms of nano materials are different fromtheir bulk counterparts, which can be attributed to the difference of dislocationactivities. Many studies have been carried out on the plastic deformation behaviors offace-centered-cubic (FCC) metals and some mechanisms, such as dislocationstarvation/exhaustion mechanism, source truncation mechanism and weakest linktheory, have been proposed. For body-centered-cubic (BCC) metals, although manyexperiments and simulations have been carried out to study their plastic deformationbehaviors, there is no well established deformation mechanism because of thecomplex dislocation behaviors that arise from the atomic arrangement. In this thesis,the tensile tests of Mo single-crystal nanowires were conducted in transmissionelectron microscopy (TEM) by using a self-developed technique. The microstructureevolution process of the nanowires was in-situ recorded during the tensiledeformation with TEM CCD camera. The experiment process and main findings aresummarized below:(1) The preparation of Mo single-crystal nanowires. First, make the bulk Mosingle-crystal into TEM sample with the thinness smaller than~80nm throughmechanical method and twin-jet electropolishing. Then cut the sample with focus ionbeam (FIB) into nanowires of150~400nm in width,1~2μm in length. The lengthdirection of nanowires should be controlled to about <110> direction.(2) The in-situ TEM observations show that homogeneous elongation of Monanowires is impact by the length vs. width ratio. For the nanowires with small valuesof length vs. width ratio, it exhibits as large as~156%strain. For the nanowires withlarge value of length vs. width ratio, it exhibits of~23%strain, which is higher thanthat of their bulk counterparts (~8%). It indicated that the strain ability of the BCCmetals is significantly enhanced as the size is reduced to nanosize.(3) Different from the bulk Mo single-crystal, in which dislocations interact andentangle with each other before move to the surface. The in-situ TEM observationsshow that the dislocations nucleate and move very fast, then escaped from the surface.There is no obvious change for the dislocation density in the nanowires during thedeformation, which results from the balance between dislocation nucleation andannihilation. These dislocation activities lead the Mo nanowires exhibit ultra-largeplasticity ability. (4) The microstructure of the crack region of the nanowires was studied withselected area diffraction, high resolution TEM (HRTEM) and fast Fouriertransformation (FFT). The lattice distortion, low angle grain boundaries formation,and BCC to FCC transformation were detected.