No abstract

Lightweight magnesium alloys are attractive as structural materials for improving energy efficiency in applications such as weight reduction of transportation vehicles. One major obstacle for widespread applications is the limited ductility of magnesium, which has been attributed to 〈c+a〉 dislocations failing to accommodate plastic strain. We demonstrate, using in situ transmission electron microscope mechanical testing, that 〈c+a〉 dislocations of various characters can accommodate considerable plasticity through gliding on pyramidal planes. We found that submicrometer-size magnesium samples exhibit high plasticity that is far greater than for their bulk counterparts. Small crystal size usually brings high stress, which in turn activates more 〈c+a〉 dislocations in magnesium to accommodate plasticity, leading to both high strength and good plasticity.

Boundaries created through basal-prismatic transformation in submicron-sized single crystal magnesium have been investigated systematically using in situ transmission electron microscopy. We found that these boundaries not only deviated significantly from the twin plane associated with f1 0 1 2g twin, but also possessed a non-planar morphology. After the sample was thinned to be less than 90 nm, aberration-corrected scanning transmission electron microscopy observation found that the basic components of these boundaries are actually terrace-like basal-prismatic interfaces.

The initiation and propagation of a crack has been investigated in a (1 6 10) [110] oriented Ni 3 Al alloy single crystal by in situ TEM and using tension deformation at room temperature. The result has shown that the macropropagating direction of the crack was parallel to the tensile axis and the crack followed a zigzag path. Trace analysis indicated that the slip on (11 6 1) and ( 111) were activated during the crack propagating process. Calculations show that for this orientation of Ni 3 Al crystal, the stress concentration arising from the dislocation pile-up decided the choice of secondary slip systems and the macrodirection of the crack.MST/4843

In situ transmission-electron-microscopy (TEM) tensile testing has been a powerful tool for revealing the underlying physical mechanism when materials are subjected to a stress [1][2][3][4][5]. With this technique, the dynamics microstructure evolution of the materials can be recorded in a nano or even atomic scale. However, all the commercial in situ TEM tensile holders available so far suffer from the absence of quantitative ability and the complexity in sample preparation. Consequently, the potential exploration ability of in situ TEM tensile holders has been hindered substantially. Supported by Department of Energy (DOE) Small Business Innovation Research (SBIR) program, recently we have developed a new tensile device for operation inside a TEM which not only yields quantitative load-displacement data concomitant with real time images of the microstructural behavior, but also simplifies the sample preparation procedure essentially.In this work, we report the current progress in the application of this quantitative in situ TEM tensile device for measuring the mechanical properties of 1D nanostructures (silicon nanowires etc). Fig. 1 is the TEM images taken after the fracture of silicon nanowire. It was surprised to see that apparent plasticity has occurred even for nano wire with a diameter of 250 nm. As shown in Fig. 1a, three zones can be identified along the residual nano wire based on the contrast. Zone I corresponds to area without plastic deformation. Zone III represents area that has experienced heavy plastic deformation. Zone II, bounded by red and green dash line, indicates gradually decrease of plastic deformation from Zone 3 to Zone 1. The insets at the upper left corner and lower right corner are corresponding selected area different patterns from zone III and zone I, respectively. Besides the slight relative change of the density of second diffraction spots (Marked by white arrow in the insets of Fig. 1 a), there are no indication for either amorphous or polycrystallization. Fig. 1 b is the magnified dark field image of zone II and III shown in Fig. 23a. Dislocation like contrast can be identified clearly. The findings indicate new deformation mechanism for silicon nano wires which has been seen as classic brittle material in bulk form.