Bulk metallic glasses (BMGs) exhibit many excellent properties, such as high strength, corrosion resistance and maximum elastic limit ect, which are different from the crystalline counterparts due to their long-term disordered atomic arrangement [[1], [2], [3]]. Unfortunately, BMGs usually show poor macroscopic plastic deformation after yielding at ambient temperature, which results from highly localized shear banding as well as sheer softening and limits their wide application as engineering materials [[4], [5], [6]]. To solve this problem, several post treatment methods including surface mechanical attrition treatment (SMAT) [7], notching [8,9], rapid defect-printing (RDP) [10] and ion irradiation [11] treatment have been reported. Except the methods mentioned above, other attempts which aim to increase the heterogeneity of the amorphous materials or to generate small amounts of micrometer-sized [[16], [17], [18]] as well as nano-scale crystallines [[19], [20], [21]] embedded in amorphous matrixes have been carried out such as designing composition [12], annealing treatment [13], nitrogen additions [14] and severe plastic deformation [15]. The scientific connotation of all above technologies is to active as more as nucleation of shear bands or to hinder the propagation of shear banding.

As a typical mode of external force, ultrasound brings about strong forced vibration effect of atoms and/or molecules, and nonlinear effects such as acoustic cavitation and streaming in liquids, which modify the microstructures and properties of various materials [[22], [23], [24]]. Specifically, the introduction of ultrasonic energy into metallic glasses may increase their heterogeneity in atomic rearrangement and even lead to the formation of crystallites. For example, Wang et al. [25] have proved that ultrasonic resonance may modulate the inhomogeneity of metallic glasses, which could tailor the rejuvenated zones and improve its mechanical properties. Ichitsubo et al. [26] reported that the MHz frequency ultrasonic vibration lead to partial crystallization in Pd-based bulk metallic glass when it is annealed below glass transition temperature. However, they have not concerned the corresponding variation in mechanical properties. Ma et al. [27] used ultrasonic surface modification method to treat Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₁₀ BMG, and showed that its fracture strength and strain were enhanced in the three-point bending experiment. However, the above mentioned performance boosts only confines to its surface rather than the whole bulk metallic glass. Now an urgent question makes one fall into account whether ultrasound can overcome the dilemma of plasticity-toughening trade-off for BMGs?

In this work, we apply intensive 20 kHz ultrasound excitation to Zr_46.75Cu_46.75Al_6.5 BMG and investigate its effect on the microstructure evolution and the mechanical properties of this BMG. We found that Cu₁₀Zr₇ nanocrystals can be formed after ultrasonic vibration. This evolution of nanocrystalline precipitation leads to an increase of compressive plasticity at room temperature combined with an augmentation of yield strength. The possible toughening mechanism induced by ultrasound excitation is deeply discussed based on the experimental results.

Ingot of Zr_46.75Cu_46.75Al_6.5 alloy (composite is given in atomic percentage) was prepared from high purity Zr (99.99%), Cu (99.999%) and Al (99.9999%) elements, and melted by the arc-melting under a Ti-gathered argon atmosphere. Each master alloy was remelted at least four times to ensure its chemical homogeneity, and then the cylinder metallic glass samples were fabricated with the dimension of 2 mm in diameter and 80 mm in length by a copper mold suction casting technique. The ultrasonic excitation device is consisted of two parts: a KNbO₃ piezoelectric transducer with a resonant frequency of 20 kHz and a Φ20 × 500 mm emitter. The Zr_46.75Cu_46.75Al_6.5 BMG with a size of Φ2 × 10 mm was laid down on a platform and fixed right under the end face of the emitter by glue. The emitter was then moved down by a precise displacement controller until its end face was tangent to the side surface of the cylinder sample. Afterwards, the ultrasonic transducer was turned on and the longitudinal ultrasonic wave with amplitude about 15 μm is introduced into Zr_46.75Cu_46.75Al_6.5 BMG by 2 h.

The X-ray diffraction (XRD) was used to confirm the glassy nature by CuKα radiation produced by a Bruker D8 advance X-ray diffractometer. Differential scanning calorimetry (DSC) test was carried out by a Netzsch 404C differential scanning calorimeter. Each sample had a mass of 20 mg under high purity of argon atmosphere at a constant scan rate of 20 K/min. Thermodynamic parameters were determined based on the obtained DSC curves. Microstructures of the alloy specimens were characterized by a Tecnai G2 F30 transmission electronic microscopy (TEM). The compressive mechanical property was determined by an Instron 5562 testing machine and the compression samples were 4 mm in height and 2 mm in diameter with an initial loading strain rate of 1 × 10^-4 s^-1. The fracture and lateral morphology were characterized by a FEI Nova field emission scanning electron microscopy (FESEM). Nanoindentation experiments were carried out by using Hysitron Nanoindentation System TI 950 with a spherical tip of 2.2 μm radius. The load-displacement curves were measured in an 8 × 8 dot matrix for each sample with the maximum load of 3 mN and a constant strain rate of 0.2 s^-1.

The XRD patterns in Fig. 1(a) shows that both of the as-cast sample and the one after 2 h ultrasonic vibration (UST-2 h) have the broad diffraction halos, indicating their typical amorphous structure. However, it can’t exclude whether a small amount of fine nanocrystals are formed in these amorphous alloys considering the accuracy of XRD measurement. DSC experiments were performed and thermodynamic parameters of the as-cast and UST Zr_46.75Cu_46.75Al_6.5 samples were carried in detail. As presented in Fig. 1(b), their glass transition temperature, T_g, is determined to be 693 and 687 K, and the crystallization events for the as-cast and UST-2 h samples occur at 767 and 763 K, respectively. The corresponding parameters, glass transition temperature T_g, crystallization temperature T_x and crystallization peak temperature T_p of the two samples are similar to each other. But it is noticeable that the crystallization enthalpies of the two samples have an obvious drop from 53.6 to 48.9 J/g, which is about 10% decrease after ultrasonic excitation, indicating the possibility of crystallization on amorphous matrix during ultrasonic excitation.

To gain deep insight into the effect of ultrasonic excitation on the microstructure of Zr_46.75Cu_46.75Al_6.5 BMGs, TEM/HRTEM studies are performed. As for the as-cast sample, a uniform contrast with no obvious lattice fringes can be found from the TEM image shown in Fig. 2(a), and the corresponding SAED pattern presented in the inset consists of a broad diffraction halo, indicating the typical feature of amorphous structure. This is displayed more clearly with an isotropic maze pattern via HRTEM in Fig. 2(b). After ultrasonic vibration, a number of dark “band-like” regions with visible size of 20-50 nm precipitate uniformly from the amorphous matrix as presented in Fig. 2(c). The different size of nanocrystals results from the heterogeneity of BMG and its diverse respond to ultrasound. The diffused SAED ring pattern shown in the insert is superimposed with a few diffraction spots. The ordered cluster can be clearly seen in the enlargement in Fig. 2(d), a mixture of nanocrystallites distributes on amorphous matrix. The Fast Fourier Transformation (FFT) pattern of the marked yellow frame (Fig. 2(e)) shown in Fig. 2(f) consists of sharp diffraction spots of planes along the [-41-15] zone axis together with a diffuse halo, revealing the existence of Cu₁₀Zr₇ phase. (Detailed calculation information was listed in supplementary materials). The formation of such Cu₁₀Zr₇ nanocrystals has been also reported by pervious literature [28,29]. By using Reverse Monte Carlo (RMC) simulation technology and Molecular Dynamics (MD) simulation technology, Kaban et al. [29] simulated the topological and chemical short-range orders of Zr_47.5Cu_47.5Al₅ amorphous alloy and found Cu₁₀Zr₇, CuZr₂ and B2 CuZr phase precipitated in turn. Here, the research object Zr_46.75Cu_46.75Al_6.5 has the same atomic ratio of Cu and Zr as Zr_47.5Cu_47.5Al₅, and it is therefore understandable that Cu₁₀Zr₇ phase is preferentially precipitates in Zr_46.75Cu_46.75Al_6.5 BMG excited by ultrasonic excitation.

In order to investigate the effect of ultrasound vibration on the mechanical properties, uniaxial compression tests were performed at ambient temperature. Fig. 3(a) presents the typical compressive stress-strain curves for the as-cast and UST-2 h samples. The blue line stands for the as-cast compressive stress-stain curve, showing brittle manner with nearly zero plastic strain before failure. The yield strength σ_y, which is determined by the first plateau in the curve, is about 1457 MPa, and the ultimate compression stress is measured to be 1855 MPa. After ultrasonic vibration, the yielding stress is increased to 1579 MPa with the highest plastic strain of 3.9%, and the ultimate compression stress also displays a slight increase up to 2047 MPa. These results suggest that a combination of optimum ductility and strength can be achieved by ultrasound excitation for Zr_46.75Cu_46.75Al_6.5 BMG. Fig. 3(b) and (c) present the typical morphologies of the lateral surface for the two groups of samples mentioned above. Initially, only two straight and well separated shear bands can be observed near the fracture surface in the as-cast sample. By contrast, a conspicuous number of shear bands including the secondary shear bands were formed around the whole sample subjected to ultrasonic vibration. It is evident that the formation of such shear bands with high density and extensive distribution excited by ultrasound accounts for the enhanced compressive plasticity of Zr_46.75Cu_46.75Al_6.5 BMG.

Micro-mechanical properties related to the structural feature of Zr_46.75Cu_46.75Al_6.5 BMG are also studied by nanoindentation tests. Fig. 4(a) shows two typical displacement-load curves for the as-cast and UST-2 h samples. It is evident that the critical load for the first sharp pop-in event (indicated by arrows), which refers to the initial yield due to the formation of shear bands becomes higher after ultrasound treatment. Fig. 4(b) shows cumulative distribution of the load probability at the first yield point on the displacement-load curve. The probability distribution curve after ultrasonic vibration leans to a higher value direction overall, which agrees well with the typical results presented in Fig. 4(a), suggesting that a higher critical stress is needed to trigger the first shear band event in Zr_46.75Cu_46.75Al_6.5 BMG after ultrasonic vibration. This nanoindentation result coincides with the increase of yield strength in bulk compression test.

From the above results of structural features as well as macro- and micro mechanical properties, it is summarized that the dilemma of plasticity-toughening trade-off for BMGs can be overcame by ultrasound excitation. The volume fraction, size and distribution of crystallites are vital to the mechanical properties of crystal-amorphous composition [30,31]. Therefore, the formation of Cu₁₀Zr₇ nanocrystals may take the main responsibilities for the enhancement of mechanical property. The improvement of plasticity of BMGs induced by the existence of a small amount of Cu₁₀Zr₇ nanocrystals may lie in the following three aspects. Firstly, Cu₁₀Zr₇ nanocrystals with high strength can impede the extending of shear bands and cracks. The size of nanocrystallines (20-50 nm) is much wider than the size of shear bands about 10 nm [32]. In this case, these tiny nanocrystals with high stress located along the shear bands played as barriers, hindering future propagation of the primary shear bands and impeding the evolution of cracks. Secondly, due to stress concentration, new shear bands tend to be nucleated, growth and proliferation at the interfaces between the matrix and nanocrystallines. The newly formed secondary shear bands together with the wing-like shear bands work as a net, which can be extended and mixed with others to sustain the ever-growing strain during compression deformation. Thirdly, the growth of nanocrystals results in an increase of viscosity [33], which changes the direction of shear deformation to micro-zones with relatively low crystallization fraction [34] and makes the deformation more homogenous. As a result, the regenerative shear band branches delays the expansion of the main shear bands, prompting atoms to undergo sufficient and stable rheological changes. Thus, the combined actions mentioned above avoid catastrophic fracture at room temperature. On the other hand, the dispersion of Cu₁₀Zr₇ on the amorphous matrix, which serves as a reinforcing phase playing like “pinning”, decelerates the propagation of shear bands. Hence it needs a higher stress in order to activate new shear bands. Nanocrystallines contribute to the promotion both in yielding stress and ultimate compression stress.

In conclusion, we found that ultrasound facilitates the atomic rearrangement and leads to the formation of Cu₁₀Zr₇ nanocrystals dispersed on glassy matrix of Zr_46.75Cu_46.75Al_6.5 alloy, which leads to the promotion in both compressive plasticity and yield strength. Our work provides an easy-procurable, non-destructive, green environmental protection and low cost technical way of toughening BMGs by overcoming the strength-ductility trade-off dilemma.

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.10.035.