Correlation between structural changes and atomic motion during physical ageing in metallic glasses

20-01-2016

Metallic glasses display outstanding thermal, mechanical and chemical properties, which make them forefront−materials for technological applications in many diverse fields such as medicine, environmental science and engineering. Their widespread use is, however, limited by their lack of stability over time due to physical ageing. By combining dynamical and structural synchrotron techniques, researchers have directly connected microscopic structural mechanisms and atomic motion in metallic glasses for the first time, providing a unique broader view of their complexity at the atomic level.

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Stability and control of the material properties are key parameters for proper exploitation of the diverse technologically relevant features of metallic glasses [1,2]. To date, these requirements are still not fulfilled due to the strong physical ageing of metallic glasses which leads to a spontaneous temporal evolution of any functional physical property due to relaxation processes intrinsic to the glassy state.

A detailed understanding of the microscopic mechanisms ruling physical ageing is therefore fundamental to overcome this limitation, but requires the atomistic description of the structural and dynamical changes occurring within a metallic glass. This has yet to be discovered due to limitations in both experiments and numerical simulations.

Using high-resolution high-energy X-ray diffraction (XRD) at beamline ID15B (now ID15A) and X-ray photon correlation spectroscopy (XPCS) at beamline ID10, we have provided an overview and an atomistic understanding of the complexity of ageing in a metallic glass by connecting microscopic structural re-arrangements and atomic motion for the first time. We found that the atomic scale is dominated by the interplay between stress-releasing rearrangements leading to density inhomogeneity annihilation and fast dynamical regimes of ageing and medium range ordering processes, not affecting density, related to a more localised atomic motion. The evolution between these regimes is probably associated with a ductile-to-brittle transition.

The recent use of XPCS to follow the atomic motion in hard materials [3,4] has revolutionised the investigation of relaxation and ageing, giving access to their study at the atomic scale. Studies of metallic glasses have revealed an unexpected dynamics, characterised by fast temporal evolution of the atomic motion and stationary regime of ageing [4], even if macroscopically metallic glasses display a steady ageing for all properties [2]. This complex dynamics has been related to the presence of internal stresses in the material most likely due to microscopic elastic heterogeneities and atomistic density inhomogeneities [5], and suggests the existence of a complex mechanism ruling the atomic dynamics, unreported in previous macroscopic studies or in any current theory for glasses [2].

To clarify the unique atomic motion of metallic glasses, we have investigated the microscopic structural and dynamical changes occurring in a rapidly quenched Pd77Si16.5Cu6.5 metallic glass (glass transition temperature Tg = 625 K), during successive annealing below Tg.

Figure 1a shows structural relaxation times τα obtained with XPCS during the whole thermal protocol as a function of the time elapsed from the beginning of the heating protocol. This parameter represents the characteristic time for the dynamics at the probed length-scale. For T < 513 K, τα increases exponentially during the isotherm signalling a non-stationary dynamics, while it abruptly decreases during T changes due to the increasing thermal motion. The exponential increase corresponds to the fast ageing regime observed also in other hyper-quenched metallic glasses [2], which can be described by the relation τα (ta,T)~exp(ta/ τ*), with an almost constant growth rate parameter τ*~6000 s.  At T = 513 K, the ageing abruptly stops even if the system is well below Tg: the glass enters a stationary regime where ta remains constant at least on the probed experimental timescale.

Figure 1b reports the corresponding volume evolution as tracked by the position change of the first sharp diffraction peak (FSDP) measured by XRD. During isotherms, the volume decreases slightly, most likely corresponding to the annihilation of residual density inhomogeneity. As temperature increases, the total isothermal densification decreases. Simultaneously to the onset of the stationary regime in dynamics, at T = 513 K, volume reduction stops, corresponding to the full structural defect annihilation. This suggests that structural defects annihilation is the process responsible for the fast ageing.

Time and temperature evolution of the structural relaxation time measured and relative volume change

Figure 1. (a) Time and temperature evolution of the structural relaxation time measured with XPCS as a function of t-t0, where t0 is the time corresponding to the beginning of the heating protocol. Symbols: blue empty circles: 393 K; green filled circles: 433 K; red upward triangles: 453 K; cyan downward triangles:473 K; magenta squares: 493 K;black diamonds: 513 K. (b) Relative volume change, reported as a function of t-t0. Each temperature ramp (x) is followed by an isotherm (same symbols as in (a)). Inset: zoom of the curve at 433 K reported as a function of time from the beginning of the isotherm, together with the best fit to an exponential law.

As shown in Figure 2a, a continuous narrowing of the FSDP takes place, exhibiting ageing itself and going on even when there is no further densification. This narrowing can be taken as an indicator of an increasing medium range order, for r ≥ 6 Å [6].

Both volume and width can be described by an exponential law, with characteristic times τν and τΓ, respectively. While each of them has a different T dependence from the dynamical time for ageing τ*, the agreement between τ* and their average is impressive (Figure 2b), suggesting that the fast dynamical ageing is due to both processes: a structural defect annihilation and a medium range ordering, as far as they affect density.

Temporal evolution of the relative change of the width of the FSDP and characteristic time for ageing

Figure 2. (a) Temporal evolution of the relative change of the width of the FSDP. Same symbols as in Figure 1a.  (b) Characteristic time for ageing as obtained from the volume relaxation (blue squares), the narrowing of the FSDP (blue circles) and from XPCS data (black dots). The average of τν and τΓ is also reported (red circles), called tageing.

Once the structural defects are completely annihilated, no more density changes can take place, thus XPCS does not see ageing and we enter the stationary regime: no dynamical ageing is observed anymore at the atomic scale even if the macroscopic observables measured in conventional studies still evolve continuously with time toward the equilibrium liquid value. In this regime, the dynamical measurements indicate that other stresses still exist, most likely related to a frustration in the repetition and ordering of the atomic clusters.

These results provide a direct connection between dynamical and structural microscopic evolutions in metallic glasses which is fundamental for developing a microscopic theory for ageing and ultimately designing new amorphous materials with improved stability.

Finally, the understanding of the ageing in metallic glasses opens also the way to the comprehension of similar mechanisms in complex systems such as jammed soft materials and many biological systems, as glasses are often considered as archetypes of out-of-equilibrium systems.

 

Principal publication and authors
Unveiling the structural arrangements responsible for the atomic dynamics in metallic glasses during physical aging, V.M. Giordano (a) and B. Ruta (b), Nature Communications 7, 10344 (2016); Doi: 10.1038/ncomms10344.
(a) Institute of Light and Matter, UMR5306 Université Lyon 1-CNRS, Université de Lyon, Villeurbanne, (France)
(b) ESRF

 

References
[1] Y. Zhang, W.H. Wang and A.L. Greer, Nat. Mat. 5, 857 (2006).
[2] W.H. Wang, Prog. Mater. Sci. 57, 487 (2012).
[3] M. Leitner et al., Nat. Mat. 8, 717 (2009).
[4] B. Ruta et al., Phys. Rev. Lett. 109, 165701 (2012).
[5] H. Wagner et al., Nat. Mater. 10, 439 (2011); J.C. Ye et al., Nat. Mater. 9, 619 (2010).
[6] D. Ma and A.D. Stoica and X.-L. Wang, Nat. Mater. 8, 30 (2009).

 

Top image: Time-resolved evolution of the dynamics measured with XPCS. During the fast ageing, the atomic motion slows down with time as signalled by the broadening of the reddish diagonal due to a densification process (sketch). When ageing is absent (inset), the dynamics is stationary and the atomic movements do not affect the density.