# Enhancing thermal stability and mechanical properties in fully-amorphous ZrCu nanolaminates

Andrea Brognara, Chanwon Jung, Cristiano Poltronieri, Philippe Djemia,
Philipp Watermeyer, Gerhard Dehm, Matteo Ghidelli, James P Best

► To cite this version:

Andrea Brognara, Chanwon Jung, Cristiano Poltronieri, Philippe Djemia, Philipp Watermeyer, et al. Enhancing thermal stability and mechanical properties in fully-amorphous ZrCu nanolaminates. Acta Materialia, 2026, 303, pp.121681. 10.1016/j.actamat.2025.121681. hal-05359389

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Submitted on 11 Nov 2025

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Full length article

# Enhancing thermal stability and mechanical properties in fully-amorphous ZrCu nanolaminates

Andrea Brognara<sup>a,†</sup>, Chanwon Jung<sup>a,1 †</sup>, Cristiano Poltronieri<sup>b,2 †</sup>, Philippe Djemia<sup>b</sup>,
Philipp Watermeyer<sup>a</sup>, Gerhard Dehm<sup>a, * †</sup>, Matteo Ghidelli<sup>b, * †</sup>, James P. Best<sup>a †</sup>

ARTICLE INFO

Keywords:

Metallic glass

Fully amorphous nanolaminates

Thermal treatment

Structural relaxation

Crystallization

Mechanical properties

A B S T R A C T

The synthesis of thermally stable metallic glasses (MGs) which are mechanically robust and tunable at elevated temperatures remains a key research challenge. While most strategies focus on complex MG compositions with enhanced resistance to devitrification, we demonstrate that a simple microstructural approach – based on the fabrication of fully amorphous nanolaminates (NLs) – can significantly extend thermal stability even for binary ZrCu system, while retaining strong resistance to deformation. Moreover, we investigated annealing-induced atomic structural modifications (relaxation or partial crystallization). Annealing treatments up to 330 °C (T < T_g, the glass transition temperature) maintain the amorphous structure, while inducing atomic structural relaxation and free volume annihilation, as revealed by HRTEM and the reduction in corrugations on fracture surfaces. Moreover, we report strong layer intermixing with chemical interdiffusion, delaying the onset of crystallization and resulting in superior thermal stability with a fully amorphous structure up to 420 °C, significantly above the crystallization temperature of the individual layers (respectively, 320 and 360 °C for Zr24Cu76/Zr61Cu39). Annealing at T > T_g (~420 °C) results in a completely homogenous structure without NL features with partial crystallization, forming Cu-Zr-based intermetallic and Zr-oxide phases. We show an increase in elastic modulus and hardness for higher temperatures due to structural relaxation and nanocrystal formation, with a maximum hardness (7.6 ± 0.2 GPa) obtained at 420 °C for 60 mins annealing, ~20% above the deposited condition. These results highlight the potential of microstructural tailoring of MGs to enhance thermal stability and tailor the mechanical properties, which can be beneficial for advanced material applications.

1. Introduction

The synthesis of thermally stable metallic glasses (MGs) with high and tunable mechanical properties at progressively higher temperatures is a key research challenge [1,2]. Specifically, the development of strong, fully amorphous materials that effectively inhibit and delay crystallization is crucial for enhancing their resistance to devitrification, making them ideal for applications requiring long-term structural/thermal integrity in combination with the resistance to mechanical deformation [1,2]. In the literature, this approach is often carried out with the fabrication of progressively complex MG compositions (up to 5–6 metallic elements) especially in the bulk form, stabilizing the glass phase and resulting in higher glass (T_g) and crystallization (T_x) temperatures [3].

A similar approach is adopted for thin films, with the synthesis of thin film metallic glasses (TFMGs) with tailored composition (i.e. by combinatorial sputtering) with strong local order and large thermal stability [4]. However, this approach provides limited control on the mechanical properties and deformation behavior, which is mainly dictated by the strength of atomic bonds and the nature of the local order, resulting in higher/lower stresses necessary for the nucleation and propagation of shear bands (SBs), which eventually cause a macroscopically brittle behavior with a limited plastic deformation [2, 5].

Recently, it has been reported that the control of local heterogeneities can mitigate such catastrophic behavior in MGs, affecting the nucleation and propagation of SBs [6–8]. One common strategy is to

<footnote><sup>a</sup> Max Planck Institute for Sustainable Materials, Max-Planck-Str. 1, 40237 Düsseldorf, Germany</footnote>

<footnote><sup>b</sup> Laboratoire des Sciences des Procédés et des Matériaux (LSPM), CNRS, Université Sorbonne Paris Nord, 93430 Villetaneuse, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: dehm@mpie.de (G. Dehm), matteo.ghidelli@lspm.cnrs.fr (M. Ghidelli).</footnote>

<footnote n="1">¹ Present address: Department of Materials Science and Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, 48513 Busan, Republic of Korea.</footnote>

<footnote n="2">² Present address: Carl Zeiss S.p.A., Via Varesina 162, 20156 Milan, Italy.</footnote>
perform annealing treatments, thereby modifying the mechanical properties and deformation behavior [9,10]. The effects vary depending on whether the annealing is performed below or above the glass transition temperature ($T_g$). Annealing at $T < T_g$ induces local atomic rearrangements known as structural relaxation, which result in a variation of the short range order both in terms of the occupied atomic sites and their chemistry [11]. Additionally, annealing at $T < T_g$ are also known to cause the annihilation of free volume content naturally present within MGs atomic structure, therefore leading to a local densification phenomena [12,13]. These processes enhance mechanical properties such as hardness ($H$) and fracture strength [12,14–16]. For example, annealing a ZrCu-based BMG at $T < T_g$ increased $H$ from ~6.5 up to 7.3 GPa and elastic modulus ($E$) from around 113 up to 131 GPa [12]. Similar results are obtained for $Zr_{40}Cu_{60}$ TFMG showing an increment of $H$ and $E$ from 6.1 up to 6.5 GPa and from 100 up to 109 GPa, respectively [14]. However, this reduction of free volume also limits the plastic deformability of metallic glasses [5,17] due to reduced nucleation of fertile sites for shear bands (SBs) activation, resulting in lower fracture strain [10, 17,18].

In contrast, annealing at $T > T_g$ can cause the nucleation of nanocrystals within the amorphous matrix. These nanocrystals can act as barriers against SB propagation, enhancing hardness and potentially increasing plasticity in ZrCu-based alloys [19,20]. For instance, annealing ZrCu-based BMGs above $T_g$ increased fracture limit during compression from ~2 % up to ~10 % plastic strain [19]. Moreover, molecular dynamics (MD) simulations have shown that the nucleation of nanocrystals softer than the glass matrix can induce a toughening effect, which is more pronounced for relatively small crystallites < 2 nm [21]. Nevertheless, other literature studies reports embrittlement phenomena as a result of the precipitation of harder nanocrystals with intermetallic compositions within the amorphous matrix, which can induce embrittlement due to the development of detrimental stress fields in the material [10,18].

Alternative approaches to improve the mechanical properties and control the deformation behavior in MGs deal with the synthesis of nanostructured MGs [22–25], and fully amorphous or amorphous/crystalline nanolaminates (NLs) [26–30]. Specifically, the presence of regions with different local chemistry and atomic order, the formation of nanoclusters as well as the high density of interfaces between nanolayers with large chemical/structural contrast [31–33], hinder the formation of catastrophic single SB events, while enhancing mechanical properties [34–37]. In this context, annealing treatments can be exploited to further tune the degree of heterogeneity by introducing nanocrystals and promoting interface diffusion, representing an effective strategy to tailor and boost the mechanical properties, while mitigating SB localization. As an example, $Sc_{75}Fe_{25}$ nanoglasses exhibited up to ~10 % plastic deformability during micro-compression and increased yield stress from ~1.2 up to ~1.6 GPa after annealing at 200 °C, due to the presence of high free volume at glass-glass interfaces [15]. Moreover, cluster-assembled ZrCu/O TFMGs reported lower thermal stability than their compact counterparts, while annealing treatment at $T > T_g$ led to an increment in $E$ and $H$ up to 180 and 14 GPa, respectively, together with the development of compressive residual stresses reaching ~1.5 GPa at 420 °C [38].

However, a general experimental strategy toward the synthesis of strong and thermally stable TFMGs (i.e. with high $T_g$ and $T_x$) is missing, together with the investigation of the effect of thermal treatments on the structure and mechanical properties of heterogeneous glassy materials. This requires in-depth investigations especially focusing on relaxation/crystallization phenomena, the effect of nanointerfaces and chemical heterogeneities on thermal stability and mechanical properties, requiring atomic and microscale characterization techniques.

In this context, we investigate the effect of nanoarchitecture on thermal stability and mechanical properties of fully amorphous nanolaminates (NLs), evaluating the resistance against crystallization (vs its monolithic constituents), the annealing-induced atomic and structural

changes, the formation of partial crystallization and the relationship with the mechanical properties. Moreover, our approach focuses on the synthesis of fully amorphous $Zr_{24}Cu_{76}/Zr_{61}Cu_{39}$ NLs as “model” materials whose layer constituents – namely the binary ZrCu – are known for the limited thermal stability ($T_x$ usually < 360 °C) and limited possibility to tailor the mechanical properties [31,39,40]. We select a bilayer period ($\Lambda$) equal to 50 nm, because of the larger plasticity (up to 16 % strain in compression [21]) originated by the high density of nanointerfaces compared to the brittle NL with $\Lambda$ = 200 nm. Additionally, the relatively larger layer thickness (compared to NL with $\Lambda$ = 25 nm) enable an easier to evaluate morphological, structural, and chemical evolutions. The atomic- and micro-structural evolution are explored through X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), and atom probe tomography (APT), unraveling structural evolution for different annealing temperatures, while revealing exceptional thermal stability (up to 420 °C) as a result of atomic layer intermixing prior to crystallization way above the monolithic constituents. In parallel, we investigated the mechanical properties by nanoindentation and Brillouin light scattering as compared with the individual constituents and reporting increment of elastic modulus and hardness increasing up to respectively ~10 % and ~20 % higher than for the NL in the as deposited condition and even exceeding the hardness value of the hardest component (i.e. $Zr_{24}Cu_{76}$), for the NL with composite amorphous-crystalline nanostructure, as a result of structural relaxation and partial/total crystallization in combination with the presence of nanointerfaces.

## 2. Materials and methods

### 2.1. Synthesis of fully amorphous nanolaminates (NLs)

ZrCu amorphous nanolaminates (NLs) were synthesized by magnetron sputtering, as detailed in Ref. [28]. NLs were deposited on Si substrate coated with a $Si_3N_4$ 300 nm-thick film to avoid diffusion during annealing treatments. The composition of the individual layers (i.e. $Zr_{24}Cu_{76}$ and $Zr_{61}Cu_{39}$, at. %) was controlled by adjusting the power ratios applied to the pure Cu and Zr targets [31]. A fully automated deposition system controlled the sputtering sequence, enabling alternating deposition of 25 nm-thick layers (bilayer period, $\Lambda$ equal to 50 nm) to build up a total film thickness of 3 µm. The $Zr_{24}Cu_{76}$ layer was always in contact with the substrate, whereas the $Zr_{61}Cu_{39}$ layer was always positioned at the top surface of the NLs.

### 2.2. Annealing treatments

Annealing treatments were carried out at various target temperatures in order to investigate the evolution of structural and mechanical properties. Specifically, the primary objectives were: (i) To anneal the NL at sufficiently low temperature to avoid crystallization ($T < T_g$) to assess the presence of intermixing between the nanolayers. (ii) To induce partial or total crystallization ($T > T_g$) to determine if the crystallites could act as additional barriers against SB propagation and how they influence the mechanical behavior within a NL structure [12,19,20]. To reach these aims, annealing treatments were performed at various temperatures ranging from 330 to 450 °C. The annealing was conducted in a high vacuum ($5 \times 10^{-3}$ Pa), with both the heating and cooling ramps set equal to 10 °C/min. The annealing time was 60 min for all temperatures, with the addition of 80 and 120 min for annealing at 420 °C to achieve complete crystallization.

### 2.3. Structural characterization

The thickness and fracture morphology of the NLs after mechanical cleavage (before and after annealing) were investigated by scanning electron microscopy (SEM, Zeiss Auriga dual-beam). In addition, the annealing-induced morphology evolution of the NL has been further
investigated by a dual-beam SEM using a Ga focused ion beam (FIB) to prepare cross-sectional cuts of the specimens. Structural analysis of the films was conducted using grazing incidence X-ray diffraction (GIXRD) with Cu Kα radiation (λ = 0.154 nm), employing a Rigaku SmartLab 9 kW diffractometer. Measurements were carried out at 45 kV and 200 mA, with XRD data collected over a 2θ range of 20°–100° in continuous scan mode, with a step size of 0.01°, and a scanning speed of 0.75°/min.

Thermal stability and the onset of crystallization were assessed by acquiring in situ XRD spectra while increasing the temperature from Troom to 600 °C [31]. To prevent thermally activated interdiffusion and Zr-Si reactions, the films were deposited on Si (100) wafers coated with an amorphous SiNx diffusion barrier layer. The heating rate was set to 30 °C/min, and diffractograms were recorded at constant temperatures every 30 °C with holding times of ~20 min. The in situ XRD heating was conducted in a He atmosphere (1.35 × 10⁸ Pa) to limit oxidation. These measurements were performed using the same setup as for the as-deposited films but in Bragg-Brentano geometry (θ/2θ scan axis) with a reduced range of 25°–90°, scanned in continuous mode with a step size of 0.01° and a scanning speed of 3°/min. An offset of 3° was applied to avoid interference from the Si substrate signal.

Transmission electron microscopy (TEM) investigations were carried out on selected annealing samples to study the evolution of atomic structure, crystallization phenomena in connection with XRD and SEM results. The TEM cross-section lamella specimens were prepared using a Thermo Fisher Scios 2 dual-beam focused ion beam (FIB)-SEM from the coated area. The transmission electron microscopic investigations were performed on an image-aberration-corrected Titan Themis 80–300 microscope (Thermo Fisher Scientific) at an acceleration voltage of 300 kV. TEM-diffraction images were obtained with a camera length of 380 mm. The STEM investigations were carried out with a camera length of 300 mm, a semi-convergence angle of 17.7 mrad, and a probe current of 217 pA. Energy dispersive X-ray (EDX) analyses were conducted using the SuperX EDS detector system.

APT was used to investigate the chemical profile of NLs and the fluctuations within SBs, using volumes of material deformed by nanoindentation. The APT specimens were prepared using a Thermo Fisher Scios 2 dual-beam FIB-SEM, following the fabrication protocol in Ref. [41]. APT analyses were performed using a local electrode atom probe (CAMECA LEAP 5000 XS) in pulsed laser mode at a specimen base temperature of 50 K. A laser pulse energy of 40 pJ, a detection rate of 1 %, and a laser pulse frequency of 125 kHz were used. Data reconstruction and analyses were carried out with the AP Suite 6.3 software (CAMECA Instruments).

## 2.4. Mechanical analyses

Nanoindentation measurements were conducted using a continuous stiffness measurement (CSM) configuration under load control with a fixed load rate set to 0.05 s⁻¹. A KLA G200 Nanoindenter DCM head equipped with a diamond Berkovich tip (Synton-MDP, Switzerland) was used, following an experimental procedure similar to that described in Ref. [31]. Hardness (H) and elastic modulus (E) values were extracted using the Oliver and Pharr method [42], for depths below approximately 10 % of the film thickness to avoid the influence of the Si (100) substrate. Brillouin light scattering (BLS) experiments have been carried out using an identical methodology to that reported in Ref. [28], accounting for NLs anisotropy. For both the as-deposited sample and the sample annealed at 330 °C, the elastic modulus was determined from the shear modulus using a Poisson's ratio of 0.38, while no signal was obtained for samples annealed >400 °C.

# 3. Results and discussion

## 3.1. Effect of nanolayering on thermal stability and structural evolution

The evolution of the NL amorphous structure as a function of

temperature was characterized through in situ XRD heating, as already performed on monolithic ZrCu TFMGs in Ref. [31]. Fig. 1a shows the XRD diffractograms of the NL at increasing temperatures. At Troom, the amorphous peak of the NL corresponds to the superposition of the peaks of the individual Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ layers (with maxima at 41.0° and 36.6° respectively), which constitute the NL stack (see Fig. S1 in the Supplementary Information). The amorphous peak remains stable up to 270 °C; however, at 300 °C, the two superimposed peaks merge into a single amorphous peak that remains visible up to 420 °C. From 450 °C onwards, the NL shows a fully crystalline structure consisting of several Cu-Zr intermetallics (i.e. Cu₁₀Zr₇, Cu₈Zr₃, CuZr₂) and Zr-oxide (ZrO₂) phases (see Fig. 1a). A detailed crystalline phase identification, taking into account all intermetallic and oxides phases is reported in Figure S2 of the supplementary.

The merging of the two superimposed amorphous peaks at 300 °C is associated with the evolution of the local atomic structure, suggesting chemical intermixing between the layers before crystallization occurs [43]. The structural evolution of the NL is confirmed by the shift towards higher 2θ of the position of the maximum of the merged amorphous peak between 300 °C and 420 °C (i.e. from 38.4 up to 38.7°, Fig. 1b) and the decrease of the full width at half maximum (FWHM) from ~8.6 to ~7° (Fig. 1c). This results in a decrease in the average atomic distances (d) of ~0.02 Å, calculated using the Ehrenfest equation (2d·sin(θ) = 1.23λ) as in Refs. [31,44], with a more compact atomic structure as a result of structural relaxation [16,45]. The position of the maximum of the NL merged peak at 300 °C (i.e. 38.4°) falls halfway between those of the Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ monolayer constituents (i.e. 41.0° and 36.6°, respectively) and corresponds to the peak position of an ideal monolithic film with an average composition between those of the layers: approximately Zr₅₀Cu₅₀ [31,46]. The value of FWHM also provides valuable information on the density of local amorphous atomic structures, with a larger FWHM suggesting the presence of higher number of different atomic packing configurations [47]. This has been closely related by Li et al. [47] to the glass forming ability of the material, stating that a larger FWHM, indicates the presence of a larger degree of disorder in the glass atomic structure and consequently also a larger resistance against formation of crystalline phases. The continuous decrease in the NL FWHM observable in Fig 1, therefore suggests a structural evolution towards a progressively more ordered atomic structure following the increase of temperature, until finally crystallization of the amorphous matrix occurs above 420 °C.

Remarkably, the NL exhibits higher resistance to crystallization compared to its monolithic counterparts (i.e. Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ TFMGs), reaching a crystallization temperature (Tₓ) of at least 390 °C (see Fig. 1a,b). Although the first crystalline peaks likely corresponding to Cu₁₀Zr₇ and ZrO₂ phases start to appear at 360 °C, the NL structure remains amorphous up to 420 °C, while the Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ monolithic constituents show a completely crystallized structure already at Tₓ of 390 °C, as reported in Fig. 1b and Ref. [31].

The increased thermal stability of the NL is related to atomic intermixing phenomena, which delay the crystallization process, as reported in Ref. [43]. Initially, the heterogeneous structure with numerous interfaces activates diffusion processes, reducing the composition gradient between the Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ layers, which evolve towards an average Zr₅₀Cu₅₀ composition. Secondly, Zr₅₀Cu₅₀ is known to have a greater thermal stability against crystallization [31,48,49], further delaying the onset of crystallization. The combination of these factors explains the greater resistance to crystallization of the NL compared to its individual components.

## 3.2. Effect of isothermal annealing on structure and morphology

Based on in situ XRD heating results from Fig. 1a, isothermal annealing experiments were performed, enabling the investigation of atomic structure and phase evolution at different temperatures. The selected temperature range starts at 330 °C, which induces structural
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Structural evolution during in situ XRD heating. (a) At $T_{\text{room}}$, the NL exhibits an amorphous peak resulting from the superposition of its two constituent layers, $Zr_{24}Cu_{76}$ and $Zr_{61}Cu_{39}$. Starting from 300 °C, the two peaks merge due to chemical intermixing. The first crystallization peaks appear at 360 °C, but the NL maintains its amorphous structure up to 420 °C, with full crystallization occurring at 450 °C. The evolution of the amorphous peak position (b) and FWHM (c) of the NL after the peaks merge (300 °C) indicates structural and chemical changes in the NL before crystallization at temperature $T_x$. The NL exhibits greater resistance to crystallization compared to the monolithic compositions [31].</figcaption></figure>

evolution while avoiding crystallization, and gradually increases up to 450 °C to induce crystallization, while evaluating the process kinetics. The XRD diffractograms of the annealed NL at different temperatures are shown in Fig. 2.

Annealing at 330 °C for 60 min successfully induced structural evolution without crystallization (Fig. 2). The XRD diffractogram maintained an amorphous structure with no evident crystalline phases, though the merging of the two amorphous peaks suggests structural changes. This indicates that 330 °C is sufficiently below the $T_g$ of the NL, aligning with in situ heating XRD analysis (Fig. 1). To achieve a composite nanocrystalline-amorphous structure and investigate the NL crystallization process and kinetics, annealing treatments were conducted at $T > 330$ °C to exceed $T_g$.

At 420 °C for 60 min, the NL structure remained primarily amorphous, but few peaks appeared (Fig. 2), corresponding to crystalline

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Evolution of XRD diffractograms of the NLs at different annealing temperatures and durations. The sample annealed at 330 °C remains in an amorphous phase without crystallization. However, the amorphous peak evolves from the superimposed shape of the as-deposited condition into a single peak. In contrast, annealing at 420 °C for 60 min shows the appearance of the first crystalline peaks mostly of oxides at lower 2θ (highlighted by the arrows), while still retaining an amorphous matrix. Samples annealed for longer times and at $T \ge 420$ °C are completely crystalline.</figcaption></figure>

$Cu_{10}Zr_7$, $Cu_8Zr_3$, and $ZrO_2$ phases, in agreement with the in situ XRD observation in Fig. 1. This suggests that annealing at 420 °C for 1 h promoted a partial crystallization of the NL's components, which were reported to show signs of crystallization at 330 and 360 °C for the monolithic constituents, namely $Zr_{24}Cu_{76}$ and $Zr_{61}Cu_{39}$ [31]. However, chemical interdiffusion at the interfaces slowed down the crystallization process as reported in Ref. [43] and as observed with in situ XRD (Fig. 1), allowing the NL to retain amorphous phase even at this temperature.

Annealing at 420 °C for 80 and 120 min resulted in full crystallization of the films (Fig. 2). Specifically, the XRD diffractograms show the presence of intermetallic $Cu_{10}Zr_7$, $Cu_8Zr_3$, $CuZr_2$ phases together with $ZrO_2$, in agreement with Fig. 1. Further annealing at 450 °C produced a diffractogram identical to those of samples annealed at 420 °C for 80 and 120 min, indicating that full crystallization already occurred. A detailed peak identification of all crystalline phases is reported in Figure S3 of the supplementary.

To further investigate the atomic and nanoscale structural evolution, Fig. 3 reports high resolution transmission electron microscopy (HRTEM) bright field experiments for NL annealed at 330 °C, 1 h (i.e. prior to crystallization) and for NL annealed at 420 °C, 1 h in which partial crystallization has occurred (Fig. 2). For NL annealed at 330 °C, 1 h (Fig. 3a) a fully amorphous NL structure is still visible, highlighting the separation between the $Zr_{24}Cu_{76}$ and $Zr_{61}Cu_{39}$ sublayers. The position of the first diffracted halo (inset of Fig. 3a) is in agreement with the XRD analysis of Figs. 1,2, indicating a highly packed local order and the evolution of the global composition toward the equiatomic $Zr_{50}Cu_{50}$ composition. Moreover, chemical intermixing phenomena across the layers are shown in Fig. 3c,d as well as in the compositional profile reported in Fig. S4c and more precisely by atom probe tomography (APT) measurements (Fig. 6), in agreement with XRD analyses (Figs. 1,2). Finally, in Fig. 3a inhomogeneous contrast with darker regions with average size <30 nm can be observed. This is even more evident when samples are imaged by high-angle annular dark-field STEM (HAADF-STEM), reporting brighter regions indicating a higher density and higher packed structure (Fig. S4a). Such regions are fully amorphous and results from local densification phenomena prior to crystallization. This local densification phenomena has been already observed in the literature as in Ref. [16] dealing with post-thermal annealing experiments on $ZrCuAl$ TFMGs, and it indicates a marked evolution of local order within the amorphous structure prior to crystallization as a result of atomic
<figure><img src="image_5.png" /><figcaption>Fig. 3. (a,b) Bright-field TEM images of NL annealed, respectively, at 330 °C, 1 h and at 420 °C, 1 h. A nanolayer structure is visible in (a) with darker regions corresponding to local densifications, while a uniform structure with dispersion of denser crystalline precipitates is present in (b). (c) STEM/EDX mapping shows the Zr distribution. (d) STEM/EDX analysis indicates the Cu enrichment also at the round features.</figcaption></figure>

redistribution. In our case, this is even more accentuated within a NL structure in which the strong chemical gradient between layers promotes atomic diffusion.

Fig. 3b show the bright field HRTEM images of NL annealed at 420 °C, 1 h. The NL structure completely disappears as a result of the complete chemical intermixing between constituent layers, in agreement with the XRD analyses (Figs. 1,2). Moreover, the formation of a denser crystalline phase embedded in an amorphous matrix is visible with darker regions (average size <50 nm) and the diffraction spots in the SEAD (inset of Fig. 3b). Such crystallites are also visible as brighter regions in the HAADF images (Fig. S4b). The evaluation of the diffraction pattern reveals that the measured lattice spacing fits very well with the orthorhombic intermetallic Cu₁₀Zr₇ phase, in agreement with what is observed by XRD (Figs. 1,2). Finally, the EDX mapping reveals a complete intermixing of the nanolayers with an average composition Zr₃₇Cu₆₃ in agreement with XRD analyses, with a negligible (<0.5 %at.) presence of O, in agreement with APT results, Fig. 6.

The morphology and fracture surface of the NLs under different annealing conditions were also examined. Fig. 4 and 5 present the SEM and FIB cross-section images of samples with amorphous and completely crystallized structures. The as-deposited sample displays typical corrugations of MG fracture surfaces [5] with an average width of ~150 nm (Fig. 4a). After annealing at 330 and 420 °C for 60 min (Fig. 4c,e), the NLs still exhibit corrugations, but these are more uniformly distributed and have a smaller average width of ~60 nm, indicating extremely localized plasticity associated with (brittle) SB propagation [5,50]. The average corrugation width is reported to correlate with the glass toughness [51–53]. A reduction in corrugation width is associated with a decrease in free volume content and a reduction of fracture toughness and ductility, as a result of annealing [10,17,18,54].

Despite the changes in fracture morphologies for samples annealed at 330 and 420 °C for 60 min, the presence of nanolayers becomes difficult to distinguish (Fig. 4c,e). FIB cross-section cuts revealed that the layer structure remains intact after annealing at 330 °C (Fig. 4d), but not after

<figure><img src="image_6.png" /><figcaption>Fig. 4. SEM images of the fracture surfaces (left) and FIB cross-sections (right) of the as-deposited NL (a,b), and after annealing at (c,d) 330 °C for 60 min, and (e,f) 420 °C for 60 min. The as-deposited NL shows the presence of nanolayers (b) and corrugations, highlighted by the yellow arrows in (a). After annealing, the fracture surfaces do not show nanolayers (c,e), and the corrugations have a smaller width and spacing. For the sample annealed at 330 °C, the FIB cross-section (d) reveals that the layer structure is still intact, whereas for the sample annealed at 420 °C (f), it is not possible to distinguish any layers.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. SEM images of the fracture surfaces (left) and FIB cross-sections (right) of the NLs with a completely crystalline structure after annealing at (a,b) 420 °C for 80 min, (c,d) 420 °C for 120 min, and (e,f) 450 °C for 60 min.</figcaption></figure>

annealing at 420 °C for 60 min, both in agreement with TEM observations, Fig. 3.

Fig. 5 shows that the fracture surfaces of samples annealed at 420 °C for 80 min and 120 min and at 450 °C for 60 min are largely homogeneous, with FIB cross-sections revealing the presence of crystalline domains, and no evidence of nanolayers, consistent with their fully crystalline XRD diffractograms (Fig. 2).

APT analysis was conducted to investigate the evolution of the NL structure after annealing and verify chemical intermixing between the layers after annealing at 330 °C, complementing the in situ XRD and TEM analysis. Fig. 6 compares the APT analyses of the NL in the as-deposited condition and after annealing at 330 °C for 60 min. The 1D concentration profile of the annealed NL (Fig. 6d) shows a significant change in the composition of the layers compared to the as-deposited sample (Fig. 6c and Fig. S5 of the Supplementary InformZation). The content of Cu changed from ~70 and ~30 at. % for the layers of the as-deposited film, to ~56 and 45 at. % after annealing (with balance Zr), in agreement with TEM observations, Fig. S4.

Experimental insight into the intermixing mechanism was obtained from APT analysis. Since 3D reconstructions from APT can contain artifacts arising from field evaporation differences caused by local electric-field heterogeneity, we rescaled the layer thickness based on the known 50 nm periodicity of the alternating Zr-rich and Cu-rich layers, which remained constant after annealing. Two rescaled elemental profiles were overlaid along the deposition direction (see Figure below), revealing clear differences between them. In the as-deposited state, both layers exhibited identical thicknesses; however, after annealing, the Zr-rich layer became thicker than the Cu-rich layer. We observed an increase of approximately 5 nm in the Zr-rich layer thickness after annealing at 330 °C, accompanied by a corresponding reduction in the Cu-rich layer thickness. This observation is consistent with our STEM results shown in Fig. 3a and with the faster interdiffusion of Cu in the binary Cu–Zr system [55], which proceeds via collective atomic motion and hopping processes [56].

Thermodynamically, the intermixing is driven by the negative enthalpy of mixing ($H_{mix}$) of the Cu–Zr system. Calorimetry data at 1100 °C show a minimum in $H_{mix}$ near 45 at. % Cu (≈ –16.1 kJ/mol) [57]. The average $H_{mix}$ for the as-deposited Cu$_{70}$Zr$_{30}$/Cu$_{30}$Zr$_{70}$ multilayer is approximately –15.0 kJ mol$^{-1}$, whereas after annealing the

compositions evolve toward Cu$_{55}$Zr$_{45}$/Cu$_{45}$Zr$_{55}$ with an average $H_{mix}$ of –15.8 kJ mol$^{-1}$. This indicates that the intermixed state is thermodynamically more stable, providing the necessary driving force for diffusion-driven intermixing during annealing and thereby enhancing the thermal stability of the layered system.

Oxygen content analysis indicated no significant oxidation during annealing (Fig. 6e,f). The concentration of O for both samples is higher in the Zr-rich layers due to the higher oxygen affinity of Zr and reaches a maximum value of ~0.5 at. %. These results are in agreement with the HRTEM and STEM-EDX observations (Fig. 3a,c), indicating a progressively homogenous and equiatomic composition due to significant chemical intermixing occurring at 330 °C, while not disrupting the NL structure. Similarly, the O content is maintained low even after annealing and not affecting the structural evolution. Indeed, comparable values of O content between as deposited and annealed sample, also suggests that this might be related with APT sample preparation, rather than with oxidation processes during annealing.

The evolution of the amorphous NL peak position and FWHM during in situ XRD heating suggests that thermally activated interdiffusion and atomic rearrangement processes continue with increasing annealing temperature or duration (Fig. 1), aligning with TEM and APT data. The average atomic distance (d) for the XRD and SEAD analyses is comparable to a monolithic TFMG with ~Zr$_{50}$Cu$_{50}$ composition [31]. A Zr$_{50}$Cu$_{50}$ composition corresponds to the average of the layers before annealing, and it shows a higher thermal stability due to a lower relative value of the formation enthalpy $\Delta H$ [48,49]. Therefore, the evolution of the layer chemistry towards compositions with higher crystallization resistance results in an increment of thermal stability of the amorphous phase NLs, as observed in Figs. 1,3. This supports the hypothesis that diffusion increases the thermal stability of the amorphous phase of the NL compared to its monolithic layer references. Finally, further annealing at 420 °C for 60 min caused additional intermixing together with partial crystallization, as observed in XRD and TEM and by the lack of visible layers in SEM micrographs. Finally, once the chemical intermixing between the layers is completed the temperatures are so high that induce a very fast crystallization kinetics. This can be observed by comparing the annealing at 420 °C at 60 and 80 min, respectively, in which the NL transform form partly crystalline with an amorphous phase to a completely fully crystalline state.

A quantitative analysis of the diffusion dynamics and crystallization process is difficult to carry out especially due to the small amount of mass (<µg) for thin films. However, based on literature analysis of Cu$_{56}$Zr$_{44}$ and Cu$_{59}$Zr$_{41}$ amorphous multilayers, we can point out that the diffusion dynamics is primarily driven by the fast Cu transport with interdiffusivities equal to $1.7 \times 10^{-19} \text{ m}^2 \text{ s}^{-1}$ for Cu$_{56}$Zr$_{44}$ and $3.4 \times 10^{-19} \text{ m}^2 \text{ s}^{-1}$ for Cu$_{59}$Zr$_{41}$ at 673 K, although a systematic variation of annealing time and temperature has not been carried out [55]. Similar phenomena can be expected of our multilayers with a favored diffusion of Cu within Cu-rich layer, while dedicated study will be needed. The crystallization process has been studied in [58] for bulk CuZr reporting and activation energy of ~450 kJ/mol, while highlighting the formation of Cu$_{10}$Zr$_{7}$ and CuZr$_{2}$ in agreement with our results.

### 3.3. Effect of annealing temperature on mechanical properties

Fig. 7 shows the evolution of the elastic modulus (E) and hardness (H) for the NL extracted by nanoindentation and Brillouin light scattering (BLS) as a function of annealing temperature and duration. Fig. S6 of the Supplementary Information presents the evolution of properties versus nanoindentation depth. Both E and H values increased with the annealing temperature, while the NL remained fully and partially amorphous (i.e., annealing at 330 and 420 °C for 60 min, respectively). Compared to the as deposited state after annealing at 330 °C for 60 min (i.e. fully amorphous structure) the E and H show an increment of respectively ~9% and ~6% in value, while after annealing at 420 °C for 60 min (i.e. partially amorphous/crystalline structure) the E and H reach
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Comparison of APT results for (a,c,e) as-deposited NL and (b,d,f) annealed at 330 °C for 60 min. 3D atomic maps of (a) as-deposited and (b) annealed NLs showing that layers remain visible even after thermal treatment. A shear band is captured in the volume of (a). The 1D concentration profile scans (indicated by color gradient arrows) reveal differences in the compositions of the NL layers between the (c) as-deposited and (d) annealed samples, indicating that interdiffusion has occurred between the layers. Oxygen concentration profiles show a relatively low average content of ~0.5 at. %, both (e) before and (f) after annealing.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Evolution of elastic modulus (E) and hardness (H) as a function of annealing temperature and time for the 3 µm thick NLs. The dotted region signifies the range between the E and H values of the as-deposited Zr<sub>24</sub>Cu<sub>76</sub> and Zr<sub>61</sub>Cu<sub>39</sub> monolithic references, respectively, from Ref. [31]. BLS values are also highlighted using half-filled circular symbols.</figcaption></figure>

<caption>**Table 1**</caption>

Comparison of nanoindentation properties for the Zr<sub>24</sub>Cu<sub>76</sub>/Zr<sub>61</sub>Cu<sub>39</sub> NL of the present study and that of a monolithic Zr<sub>60</sub>Cu<sub>40</sub> TFMG from Ref. [14], reporting their evolution for the as-deposited condition, fully amorphous (annealing *T* < *T*<sub>g</sub>) and partially amorphous/crystallized (annealing *T* > *T*<sub>g</sub>). E values from BLS are included as *E*<sub>BLS</sub> for comparison. * value taken from Ref. [62] on the same material.

<table><thead><tr><th rowspan="2"></th><th colspan="3">Nanolaminate (NL), this work</th><th colspan="3">Monolithic Zr<sub>60</sub>Cu<sub>40</sub> from Ref. [14]</th></tr><tr><th>as-deposited</th><th><i>T</i> &lt; <i>T</i><sub>g</sub><br>(330 °C)</th><th><i>T</i> &gt; <i>T</i><sub>g</sub><br>(420 °C)</th><th>as-deposited</th><th><i>T</i> &lt; <i>T</i><sub>g</sub><br>(280 °C)</th><th><i>T</i> &gt; <i>T</i><sub>g</sub><br>(380 °C)</th></tr></thead><tbody><tr><td><i>E</i> (GPa)</td><td>98 ± 1</td><td>107 ± 1</td><td>108 ± 2</td><td>100 ± 1</td><td>109 ± 1</td><td>115 ± 2</td></tr><tr><td><i>E</i><sub>BLS</sub><br>(GPa)</td><td>87 ± 4</td><td>101 ± 4</td><td>–</td><td>70 ± 1 *</td><td>–</td><td>–</td></tr><tr><td><i>H</i> (GPa)</td><td>6.4 ± 0.1</td><td>6.8 ± 0.1</td><td>7.6 ± 0.2</td><td>6.1 ± 0.1</td><td>6.5 ± 0.1</td><td>6.6 ± 0.1</td></tr></tbody></table>

values respectively ~10 % and ~19 % higher (see Table 1).

The BLS measurements of *E* show similar trends as observed between the as-deposited sample and the annealed one at 330 °C. On the other hand, the *H* value for the partially-amorphous sample annealed at 420 °C for 60 mins reached a higher value (i.e. ~7.6 GPa) than that of the individual Zr<sub>24</sub>Cu<sub>76</sub> (i.e. ~7.0 GPa) and Zr<sub>61</sub>Cu<sub>39</sub> (i.e. ~4.5 GPa) sub-layer constituents. Fully crystallized specimens (after annealing at 420
°C for 80 min) showed a further dramatic increment of *E* and *H* (i.e. respectively of ~35 % and ~42 %, compared to the NL in the as deposited condition) which then remained constant upon increases in annealing time or temperature. This is due to the increased *E* and *H* as a result of the formation of intermetallic phases (Cu₁₀Zr₇, Cu₈Zr₃, CuZr₂) in the crystallized sample, in agreement with XRD and TEM observations, Figs. 1–3. This confirms that full crystallization was achieved, as also suggested by the lack of significant differences in XRD diffractograms (Fig. 2) and cross-section morphologies (Fig. 5) for samples annealed at 420 °C for 80 and 120 min and 450 °C for 60 min.

The increase in mechanical properties of the films that remained fully or partially amorphous is related to local chemical and structural rearrangements. As previously mentioned, the mechanical properties of TFMGs are closely related to their composition [31–33 39]. However, in this case, the increase in *E* and *H* values cannot be attributed solely to changes in layer composition. This is because during annealing, the films tend towards an average Zr₅₀Cu₅₀ composition, which would show *E* and *H* values between those of the Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ monolithic references determined in Ref. [31], not higher as reported in Fig. 7, which suggests structural differences occurring from thermal treatment.

Therefore, the increase in mechanical properties must be related to other factors during annealing, including free volume annihilation or the nucleation of crystallites [12,15,16]. The hypothesis of the annihilation of free volume during annealing at 330 °C is supported by the presence of corrugations with a smaller width on the NL's fracture surface compared to the as-deposited NL (see Fig. 4), as well as by TEM observations (Fig. 3a) highlighting the formation of denser regions. Both annihilation of free volume and local densification in MGs are linked to a transition towards a more densely packed atomic configuration, resulting in higher stiffness [13,31,39]. Additionally, free volume reduction decreases the available sites for SB nucleation, increasing resistance to plastic flow and consequently inducing higher hardness [10,12,15,16]. Furthermore, the presence of nanocrystals within the amorphous matrix in MGs can increase hardness by acting as obstacles to SB propagation and increasing resistance to deformation [12,59,60]. Moreover, the values of *E* and *H* for the fully amorphous NL are above the rule of mixture, indicating a marked effect of interfaces on the mechanical behavior as observed for as deposited fully amorphous NL in Ref. [21].

Finally, these results can be compared with those of Apreutesei et al. [14] who performed annealing on monolithic Zr₆₀Cu₄₀ TFMGs at both *T* < *T*<sub>g</sub> and *T* > *T*<sub>g</sub>. Similarly, annealing at *T*<sub>g</sub> induced structural relaxation without causing crystallization, while partial crystallization occurred when *T* > *T*<sub>g</sub>, as reported for the ZrCu NL (Fig. 2). The thermal treatments and structural modifications led to increased *E* and *H* values for both the Cu-rich monolithic film and the NLs, as shown in Table 1. When the NL and monolithic samples had comparable structural conditions – namely as-deposited (i.e., fully amorphous), fully amorphous after annealing at *T*<sub>g</sub>, and partially crystalline after annealing at *T* > *T*<sub>g</sub> – they exhibited very similar *E* values while the NL always had slightly higher *H* values (i.e., 6.6 GPa and 7.6 GPa for the monolithic film and the NL, respectively). These results can be put into context, considering that the mechanical properties of ZrCu TFMGs increase with their Cu content (at. %) [31,39,61], given that after annealing the NL tends towards an average composition of ~ Zr₅₀Cu₅₀ (see Fig. 6). This demonstrates how exploiting the heterogeneous nanostructure of fully amorphous NLs, it is possible to obtain films with greater hardness, elastic modulus, and thermal resistance to crystallization compared to monolithic TFMGs, highlighting that simple control over amorphous nanolayer architecture can play a significant role on the thermal stability and mechanical properties.

## 4. Conclusions

In this study, we demonstrate that the synthesis of fully amorphous nanolaminates (NL) is an effective strategy to enhance both the thermal stability and mechanical properties of thin film metallic glasses

(TFMGs), even for simple binary systems such as ZrCu. Moreover, we discuss in detail the effects of thermal treatments in such NLs and how they affect the atomic structural changes and the mechanical properties as a result of free volume annihilation, local densification and/or partial crystallization. The following conclusions can be drawn:

### *Structural evolution and mechanical properties*

* Annealing induced a gradual evolution in the NL's amorphous structure and chemistry, with progressive increase in elastic properties and hardness with increasing annealing temperatures. Up to 330 °C, the structure remained X-ray amorphous, but underwent through significative structural relaxation as indicated by the evolution of the amorphous peak shape (FWHM, and maximum position). Moreover, TEM observations highlighted the presence of regions with locally increased density, while the reduced width of corrugations on the fracture surface consequent to annealing below *T*<sub>g</sub>, speaks the occurrence of free volume annihilation.

* Annealing caused intermixing between different layers and a variation in their individual compositions. The atomic composition in the NLs tended towards Zr₅₀Cu₅₀ after annealing at 330 °C. The intermixing also influences the mechanical properties with average *E* and *H* values above the results-of-mixture. The presence of nano-interfaces affect the hardness compared to monolithic counterparts, while the densification enhances the modulus.

* Annealing above *T*<sub>g</sub> (420 °C for 60 min) induced partial crystallization of the amorphous structure, with the presence of both Cu-Zr intermetallic and Zr-oxide phases, which further increased *E* and *H* by acting as barriers to shear band propagation.

### *Thermal Stability*

* The NLs exhibited higher thermal stability against crystallization compared to their single components. The amorphous phase remained present up to 420 °C, whereas monolithic Zr₂₄Cu₇₆ and Zr₆₁Cu₃₉ films fully crystallized at 390 °C.

* The increased thermal stability is attributed to interdiffusion phenomena occurring at lower temperatures (from 300 °C), caused by the heterogeneous structure and interfaces of the NLs, which delay the onset of crystallization. The evolution of the average composition toward Zr₅₀Cu₅₀, which has greater resistance against crystallization due to the strong enthalpy of mixing, also plays a role.

* Once intermixing between the layers is complete, the temperature is sufficiently high to trigger rapid crystallization kinetics. This is evident when comparing annealing at 420 °C for 60 and 80 min, respectively: the nanolaminates evolve from a partially crystalline structure with residual amorphous regions to a fully crystalline state.

Overall, we demonstrate how the synthesis of fully amorphous NL can be an effective strategy to enhance both thermal stability and mechanical behavior even for model ZrCu MGs. Moreover, we shed light on the effects of annealing on structural and mechanical properties for complex fully amorphous NLs emphasizing the connection between local densification, free volume annihilation, chemical intermixing and (partial) crystallization with the mechanical behavior. Overall, our insights extend the potential of MGs for structural applications, providing guidelines to develop robust and thermally stable glasses, which can resist to devitrification, while enhancing mechanical properties. Finally, it could be imagined that further control could be obtained by varying diffusion lengths via the bi-layer period, or tuning of the layer chemistry.

## Data availability

Data will be made available on request.
CRediT authorship contribution statement

**Andrea Brognara:** Writing – review & editing, Methodology, Investigation, Data curation. **Chanwon Jung:** Writing – review & editing, Methodology, Data curation. **Cristiano Poltronieri:** Writing – review & editing, Investigation, Data curation. **Philippe Djemia:** Writing – review & editing, Investigation. **Philipp Watermeyer:** Writing – review & editing, Methodology, Investigation, Data curation. **Gerhard Dehm:** Writing – review & editing, Validation, Supervision, Funding acquisition. **Matteo Ghidelli:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Conceptualization. **James P. Best:** Writing – review & editing, Writing – original draft, Supervision, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

All the authors acknowledge Benjamin Breitbach for assistance with setting-up the XRD measurements. Dr. A. Brognara acknowledges the KSB Stiftung (funding No. 3.2023.14) for the funds supplied to support the project. Dr. M. Ghidelli acknowledges the financial support of the ANR MICRO-HEAs (Grant Agreement No. ANR-21-CE08-0003-01), the ANR "EGLASS" (Grant agreement no. ANR-22-CE92-0026-01) and the ANR "Super-Glasses" (ANR-24-CE91-0002-02). Moreover, he acknowledges the support from the Partenariats Hubert Curien (PHC) PROCOPE 2021 project "New-Glasses" (Grant No. 46735ZG) financing the cooperation between LSPM-MPISusMat. Dr. J.P. Best acknowledges the German side of this scheme supported through the Deutsche Akademi-sche Austauschdienst (DAAD) program "Programme des projektbezo-genen Personenaustauschs (PPP)" (Project-ID: 57561649) financed by the Bundesministerium für Bildung und Forschung (BMBF). C. Jung is grateful for financial support from the Alexander von Humboldt Foun-dation. G. Dehm gratefully acknowledges support by the ERC Advanced Grant "GB Correlate" (Grant Agreement No. 787446).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.actamat.2025.121681.

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