Tensile ductility and necking of metallic glass(2)

 

Methods

The first step in making the small samples was to prepare, using FIB, a thin plate with dimensions of 100 nm (thickness)300 nm. To reduce the possible damage to the sample surfaces caused by the Ga ions, the desired dimensions were reached with gradually reduced FIB beam currents, and the current density used for the final trim was about 50 pA. The energy-dispersive X-ray spectroscopy results showed that the Ga was incorporated in the sample surface at a low concentration of <1%, and TEM examination confirmed that FIB processing did not affect the amorphous nature of the samples.

In situ tensile straining experiments were carried out at room temperature using an FEI Tecnai F30 TEM operating at 300 kV with a Gatan Model 654 single-tilt straining holder. The deformation behaviour, sample morphologies and the final fracture were recorded using digital videotaping in the microscope. The strains were applied by applying intermittent displacement pulses manually through a trigger switch that activated a motor in the straining holder. All of the samples and platelets were being pulled at the same time, but our design of multiple samples and platelets at various locations and with different sizes made it possible for the deformation events (for example, necking, fracture and so on) not to start (and finish) all at the same time.

The entire deformation process for each sample took about several minutes, but our e-beam illumination and videotaping was carried out for about one minute at a time, rather than continuously for each sample. In other words, the beam was alternately focused on different gauge sections and platelets while the others continued to deform without being observed or exposed to the beam. For a given sample, its deformation mode could not be controlled by beam effects, because much of its elongation occurred when the e-beam was moved to observe other samples one after another. The beam-heating effects should also be small because the temperature rise was estimated to be insignificant as our specimen was part of a large metal piece (Fig. 1) with a good thermal conductivity33 (20 W mK-1). We stopped straining the sample several times to quickly take bright-field TEM images, diffraction patterns and HRTEM images, without unloading the stress.

The strain rate was estimated to be typically about 100 nm) are relatively thick and rounded, reducing the unwanted stress concentration. In comparison, the perforated locations obtained by ion-milling thinning in previous in situ TEM tensile studies of metallic glasses34 had very thin edges, uncontrollable geometries and stress states, and the highly concentrated forces usually caused immediate and rapid cracking.

 

Acknowledgements

Financial support from the National Nature Science Foundation of China (Grant Nos 50125103, 50671104 and 50625103)  are gratefully acknowledged. The authors were also part of the MANS research team, supported in part by CAS.

Competing interests statement:

The authors declare no competing financial interests.

Received 11 June 2007; Accepted 18 July 2007; Published online 19 August 2007.

 

References

 

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