The effects of ultrasonic treatment on the structure and properties of Zr-based bulk metallic glasses at room temperature were investigated by nanoindentation, in situ high-pressure synchrotron diffraction and differential scanning calorimetry (DSC). Results show that the glassy characteristics of the samples were stable up to 31GPa. Furthermore, the hardness, bulk modulus, and distribution of reduced hardness improved after ultrasonic treatment. The DSC results indicate that ultrasonic treatment increased the thermal stability of the BMG. This study reveals that ultrasonic treatment can be used as an effective way to design BMG-based materials with selective properties.
Key words: ultrasonic treatment; nanoindentation; free volume; bulk metallic glasses
1. Introduction
Metallic glasses are relative newcomers in the glassy family [1-6], and are of fundamental interest and technological importance worldwide because they offer attractive benefits, combining some of the desirable mechanical, magnetic, and chemical properties of crystalline alloys and the formability of oxide glasses. Among the various kinds of bulk metallic glasses, Zr-based [7-9] alloys extensively used in commercial applications because they contain less rare and less noble metals. Additionally, Zr-based alloys also have excellent mechanical properties and high glass formability. However, the structure and properties of BMG are very sensitive to external fields. Ultrasonic treatment (UST) has been used to investigate the structural evolution and properties of various BMGs. Ichitsubo et al [10-13] studied the Zr- and Pd-based BMGs under ultrasonic vibrations and found that crystallization can be accelerated; Wang et al [14] and Keryvin et al [15] studied the equation of state and thermal stability of BMGs by the ultrasonic method.
In this work，the Zr55Al10Ni5Cu30 [16] bulk metallic glass (BMG) was subjected to ultrasonic waves at room temperature using an ultrasonic device, and the spatial nanohardness distribution of the samples were measured by nanoindentation. The compression behavior of the samples was investigated by in situ high-pressure synchrotron diffraction. The effects of UST on the thermodynamic properties of Zr55Al10Ni5Cu30 BMG were also analyzed by differential scanning calorimetry (DSC). This study provides valuable information about the structure and properties of the samples.
2 Experiments
Master alloy with a nominal composition of Zr55Al10Ni5Cu3 was prepared by arc melting a mixture of pure Zr, Al, Ni, and Cu (purity higher than 99.9%) under a Ti-gettered high-purity argon atmosphere. The ingots were remelted in a quartz tube by induction heating followed by injection into copper molds. The amorphous nature as well as the homogeneity of the samples was ascertained by D/MAX-RB X-ray diffraction (Fig. 1(a)). UST was conducted on a JXD-02 system at a frequency of 20 KHz for 48 h. The sample was identified by X-ray diffraction to be at fully glass phase after UST, as shown in Fig. 1(b).
The sample for nanoindentation was cut into dimensions of 10 - 10 - 3 mm3 and carefully polished to smoothen the surface. The indentations were carried out over a square area of 40-40 μm2 for the samples in a nanoindenter Hysitron TriboIndenter with a diamond Berkovich tip having a 260-nm radius of curvature. The constant contact force and the amplitude of the sinusoidal force were 0.2 and 0.25 μN, respectively. All the indentations were programmed to penetrate the same depth, viz., 100nm, and the spacing between adjacent indentations was 2 μm. The applied loading/unloading rate was 200 μN/s.
Powder was prepared for the pressure experiments. Samples were loaded into the 150-μm hole of a T301 stainless steel gasket in a membrane-type DAC with diamond-culet sizes of 400 μm. Ruby grains were loaded with the samples for pressure determination. A 16:3:1 methanol-ethanol- water mixture was used as a pressure- transmitting medium. The monochromatic X-ray had a spot beam of 80-80 μm and a wavelength ofλ=0.6199 Å. More experimental details can be found in Ref. [17.18].
DSC measurements were conducted under a high-purity argon atmosphere on a STA449C/DSC (NETZSCH Instruments Co., Ltd.) at different heating rates.
3. Results
The surface smoothness was examined by scanning electron microscopy. Only indistinct scratches that had minimal influence on the result of nanoindentation results were found on the sample surface. Given the geometry and dimension of the probe tip, an indentation impression of ~800 nm remained. Although the shear bands usually develop around indentations, they rarely extended the strain to an outer area as large as the impression itself [19,20]. Thus, the size of the entire strained zone of an indentation observed in this study was ~1.6 μm in this study. Therefore, a distance of 2 μm between two neighboring indentations was chosen to prevent the overlapping of neighboring strained zones. The hardness and the elastic modulus can be obtained via the nanoindentation. The average nanohardness of the specimens increase from ~7.22 GPa to ~8.26 GPa. The average elastic modulus increase from ~113.78 GPa to ~123.5GPa after UST. Ratios of nanohardness to elastic modulus (H/E) were 0.0634 and 0.0669 for the alloys untreated and treated respectively. These results indicated that the hardness, elastic modulus, and mechanical properties of the BMGs significantly improved after UST.
To gain a better understanding of the variation in hardness of the Zr55Al10Ni5Cu30 BMGs, contour maps of the spatial distribution of the nanohardness of Zr55Al10Ni5Cu30 BMGs over a 40-40 μm2 square area before and after UST were obtained, as shown in [Fig.2 (a) and (b), respectively]. The average hardness of the material is evidently increased after UST. To a certain extent, these maps also demonstrated that the amorphous alloy was inhomogeneous at the microscale.
The relative nanohardness was introduced to minimize the effect of scratches on the absolute value. To compare the spatial hardness distributions of the specimens before and after UST, reduced hardnesswas determined as ，where is th nanohardness value. Histograms (a), and (b), in Fig.3 characterize the nanohardness distribution of the Zr55Al10Ni5Cu30 BMGs before and after ultrasonic treatment, respectively. was distributed mainly between 0.9 and 1.115 for Zr55Al10Ni5Cu30 BMG, and between 0.85 and 1.125 for ultrasonically treated sample. The results indicate that the distribution of the reduced nanoindentation hardness for Zr55Al10Ni5Cu30 BMGs expanded after UST compared with the sample that was not subjected to UST.
The stability of Zr55Al10Ni5Cu30 BMG under high pressure was investigated by in situ high-pressure synchrotron diffraction. Fig. 4 shows the selected synchrotron radiation X-ray diffraction spectra of Zr55Al10Ni5Cu30 BMG under different pressures. With increased pressure, the broad diffusive amorphous halo visibly shifted to a high angle, which shows the compression behavior of bulk amorphous alloys. No new diffraction peaks were detected from the curves up to 31.19 GPa, indicating that the structure of the Zr55Al10Ni5Cu30 BMG was stable at room temperature.
The effects of UST on the thermal stability of Zr55Al10Ni5Cu30 BMG were investigated by DSC measurements at different heating rates (10, 20, 30, 40 K/min). Subsequently, the characteristic peak temperatures were derived from the DSC measurements. The activation energy for various processes in the present amorphous alloy can be estimated by the following Kissinger equation [22]:
where is the heating rate; Tθ represents the characteristic temperatures such as Tg, Tx, and Tp (crystallization peak temperature); and R denotes the gas constant. By plotting versus 1/T for the samples, (Fig. 5), the related effective activation energies were obtained. These energies are summarized in Table 1.The effective activation energy of BMG significantly increased after UST, which indicates the thermal stability of Zr55Al10Ni5Cu30 BMG was enhanced.
4 Discussions
The defects of amorphous alloy are often described by the free volume model, in which the inhomogeneity of the local distribution of excess free volume together with density fluctuation in amorphous materials results in inhomogeneous nanohardness. According to Turnbull and Cohen's theory [23], the increase in the free volume in BMGs can increase the interatomic distance and weakens the atomic bonding energy. Thus, the low concentration in the free volume after UST can results in a shorter interatomic distance and more closed packed structure, leading to higher hardness and elastic modulus [24]. When the samples were treated with ultrasonic waves, the atoms in the partially softened region of the amorphous alloy exhibited diffusions and rearrangements. The free volumes were annihilated and nanohardness increased. In the very few regions, the free volumes gathered with the diffusions and rearrangements of the atoms, thereby decreasing nanohardness. The annihilation and coalescence of free volumes induced a broader distribution of reduced hardness, indicating that UST enhanced the plasticity of the samples to a certain extent. Furthermore, Ruitenberg [25] pointed out that the effective activation energy obtained from Kissinger plots was approximately equal to
The first contribution (Q) is equal to the activation energy for the migration of a structural defect. The second contribution is related to the equilibrium defect concentration. Although the former does not change because of the ultrasonic process, the latter increases as defect concentration decreases, whereas the free volume concentration decreased and approached the equilibrium value with the annihilation of free volumes in the ultrasonic process. Thus, the hardness and the thermal stability have improved after UST.
5 Conclusions
In this study, the effects of UST on the structure and properties of Zr-based BMG were investigated. The Zr55Al10Ni5Cu30 BMGs remained stable up to 31 GPa after UST. The nanoindentation and in situ high-pressure synchrotron diffraction reveals that the hardness and bulk modulus increased, and that the distribution of reduced hardness expanded. The non-isothermal crystallization kinetic behaviors were also discussed and UST significantly improved thermal stability. The results demonstrate that UST can cause the rearrangement of atoms, resulting in the annihilation of free volume and the structural relaxation of BMGs. The mechanical and thermal properties were considered related to the changes in free volume and structure.
Acknowledgments
This work was supported by the National Basical Reseach Program of China (Grant No. 2010CB731600) and National Science Foundation of China (Grant No. 50731005/50821001).
