Direct observation of shear–induced nanocrystal attachment and coalescence in CuZr-based metallic glasses: TEM investigation
Introduction
Owing to their attractive properties such as extremely high strength, high hardness, large elastic limits, and excellent superplastic formability [1], [2], [3], [4], the metallic glasses (MGs) constitute actually a new class of engineering material [5]. However, these materials exhibit a poor macroscopic plasticity and generally fail in a catastrophic manner which has severely limited their engineering applications. Several methods have been applied to overcome this problem, leading to the development of Metallic-glass-matrix composites with a dispersion of micro- or nano-scaled second phase which are able to suppress shear failure and enhance plasticity by forming multiple shear bands [6], [7], [8].
Since metallic glasses alloys have not been in thermodynamic equilibrium, supplying such alloys with mechanical energy in the form of deformation can promote various phase transformation routes. Chen, He and Shiflet [9] and later Kim et al [10] reported crystallization in a bent and in indented amorphous alloy ribbon by TEM, and we and others have reported crystal nucleation and/or growth during compressive deformation [11], [12], [13], [14]. More recently, S. Pauly et al [15], [16] reported a new deformation mechanism of CuZr-based bulk MGs promoted by polymorphic precipitation of B2–CuZr nanoparticles which has experienced phase transformation and twinning. All these phase transformation routes are intended to compete with the initiation and propagation of shear bands leading to work hardening and ductility enhancement.
We present the results of unique in situ observations, inside a transmission electron microscope (TEM), of nanocrystals formation in CuZr based metallic glass and subsequent attachment and coalescence of these nanocrystals during progressive tensile straining in TEM. These structural observations could establish a new self-toughening mechanism of CuZr-based bulk MGs underlying shear-induced coalescence of nanoparticles in deformed region.
Section snippets
Experimental procedures
Binary Cu50Zr50 was used as a model material owing to its low brittleness and considerable plastic strain (≈50% in compression [14]) which permits the better observation of nanoparticles formation and their subsequent phase transformations [17], [18]. The minor addition of Fe, which is being immiscible with Cu, was expected to promote the formation of larger number of crystallites. The ingot of (Cu50Zr50)98Fe02 was then prepared by arc melting high purity Zr (99.9%), Cu (99.99%), Fe (99.8%)
Results and discussions
Fig. 2 illustrates the initiation of a shear deformation zone (indicated by arrows) at low magnification with the thickness of 15–20 nm formed in front of the microcrack tip. The direction of the initiated microcrack was found to be perpendicular to the loading direction. The shear bands initiate from the thin region of a TEM sample and propagate to the thick region. A few bright parts suggest the presence of nanocrystallines of 3–5 nm size.
Upon further straining, TEM images of Fig. 3 show that
Conclusion
The deformation mechanism of CuZr-based metallic glasses and its phase transformations have been investigated by in-situ straining test in TEM. We found that the strain induces in situ nanocrystallization and subsequent attachment and coalescence of these nanocrystals. The shear-induced precipitation and coalescence of nanocrystals in shear bands is more prominent for a set of favoured factors such a low shear rate and low viscosity in shear bands. This novel deformation mechanism, which is
Acknowledgement
K.H. gladly acknowledges a European Marie Curie Post-Doc fellowship in the frame-work of the RTN Network ‘‘Ductile BMG Composites’’ coordinated by ARY.
The authors are very glad to dedicate this paper to the memory of Professor Alain Reza Yavari (one of the authors) who passed away recently and whom we sorely miss.
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