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Cooling rate effects on the microstructure and phase formation in Zr51Cu20.7Ni12Al16.3 bulk metallic glass

Abstract A detailed transmission electron microscope (TEM) study has been conducted to investigate the microstructures of the Zr51Cu20.7Ni12Al16.3 metallic glass formed at different cooling rates. It has been found that the most competitive crystalline phase to the amorphous structure is an oxygen-stabilized FCC NiZr2-type phase, which in turn acts as the leading phase to trigger the formation of other crystalline phases in the slow-cooled alloy.


Introduction
The study of in situ formed crystalline phases is important for bulk metallic glasses (BMGs) for two main reasons: Firstly, the knowledge of how and when crystalline phases form is vital to achieve defined composites (amorphous matrix with primary crystalline phases), which show greatly improved mechanical properties, such as plasticity; examples are shown in Zr-based [1] and Ti-based [2] BMGs. Secondly, knowledge of the crystalline phases competing with the desired amorphous structure is essential for designing the alloy compositions so as to obtain better BMG candidates.
The formation of crystalline phases in BMGs can be affected by several factors, namely the cooling rate [3], the composition (including impurity) [4,5] and the processing conditions [6,7]. Among them, the cooling rate is believed to be the most decisive one. For examples, Zhang et al. [8] showed that different cooling rates could result in significant different crystalline phases and/or microstructures, from a unique amorphous structure to complicated composites, in a La-based BMG-forming alloy; and Loser et al. [9] studied the cooling rate effects on the microstructure development of a Zr-Nb-Cu-Ni-Al BMGforming alloy.
In this study, we investigate microstructures of a Zr 51 Cu 20.7 Ni 12 Al 16.3 BMG [10] prepared by different cooling rates. Since traces of oxygen (500 ppm) were detected in the studied alloy system, the relationship between the impurity and the phase formation is discussed based on the microstructural analysis.

Experimental procedure
Rod-shaped samples of F3, F5 and F10 mm with the nominal composition of Zr 51 Cu 20.7 Ni 12 Al 16.3 were prepared by non-consumed arc melting and the copper mould casting technique under the protection of argon atmosphere. Based on a rough relationship between the cooling rate [r (K/s)] and the diameter (d) of the rod-shaped samples, i.e. r$10/d 2 [11], the cooling rates of these three alloys can be estimated as 100, 40 and 10 K/s for the F3, F5 and F10 mm alloys, respectively. Thermal analysis was performed on a NETZSCH STA 409 CD with a heating rate of 0.167 K/s. The X-ray diffraction (XRD) experiments were conducted on a D/max IIIA X-ray diffractometer with a Cu-K a target operating at 40 kV and 60 mA. Microstructure investigations using scanning electron microscope (SEM) and transmission electron microscope (TEM) were carried out in a Philips XL30 (operated at 20 keV), a JEOL 6460 LV (operated at 20 kV) and a Tecnai F20 (operated at 200 kV), respectively. Energy dispersive spectrometers (EDS) attached to the SEM and TEM were used to provide compositional information. TEM specimens were prepared by the tripod technique, finished by a low-angle (o61) and a low-energy (3.5 keV) ion-beam thinning to avoid the possible induced crystallization during the TEM specimen preparation. In order to determine the lattice parameters of crystalline phases accurately when using the selected area electron diffraction (SAED), pure gold nanoparticles were sputtered on the top of the TEM specimens in order to calibrate the camera length of the TEM. Oxygen measurement was performed using a Leco TC-436 type oxygen/nitrogen determinator, which detected a 500 ppm content of oxygen in all alloys. Fig. 1 shows XRD patterns of all three alloys. The XRD pattern taken from the F3 mm alloy shows a broad diffuse halo with no other diffraction peaks, indicating that the amorphous structure is dominating the F3 mm alloy. For the F5 mm alloy, small peaks superimposed on the diffuse halo can be seen, suggesting that a certain degree of crystallization has taken place. The calculation of the reduced crystallization enthalpy, obtained from the thermal analysis (V f ¼ [DH a ÀDH c ]/DH a , where V f is the volume fraction of the crystalline phase(s), DH a is the crystallization enthalpy of the pure amorphous alloy which is determined via measuring the exothermal peak of the DSC curve and DH c is the crystallization enthalpy of the partial crystallized alloy, see [12] for details) shown in Fig. 2, determines an approximately 12% volume fraction occupied by the crystalline phase(s).

XRD and thermal analyses
Compared with the F3 and F5 mm alloys, the F10 mm alloy is well crystallized as evidenced by two key facts. Firstly, the shape of diffraction peaks shown in Fig. 1 indicates that crystalline phases have been well developed in the slow cooled solidification process. Secondly, although the thermal analysis detects a change in the heat flow around the temperature of 520 K (see Fig. 2), the exothermic peak for the amorphous phase's crystallization is indistinct, indicating that the alloy is occupied by the crystalline structure dominantly. show a homogeneous microstructure for the F3 mm alloy, a microstructure of crystalline phase(s) with the matrix for the F5 mm alloy, and a well-crystallized microstructure containing several phases for the F10 mm alloy, respectively. The SEM results fit well with the XRD and the thermal analysis results. As can be seen from Fig. 3(b), the crystalline particles show the same contrast in the BSE image, meaning that, in the F5 mm alloy, these particles have a unique composition. Fig. 3(d) shows typical EDS point analyses from a particle and the amorphous matrix. It is clearly shown that Al is enriched in this primary crystalline phase with depleted Zr and Ni contents. It should be noted that a small amount of oxygen ($3 at%) has been detected in this crystalline phase, implying that the oxygen was involved in the formation of this phase during the solidification process.

SEM analysis
Since there are many phases in the F10 mm alloy as evidenced in Fig. 3(c), an EDS line analysis was performed over several of these crystalline phases [as shown in Fig. 3(e)]. Their relevant compositions are presented in Fig. 3(f). It can be noted that Al and Zr elemental variations can be observable, suggesting that a long-range diffusion took place during the solidification process.

TEM characterization
To understand the detailed microstructures, particular to determine the crystalline phases, TEM investigation was performed on all alloys. Fig. 4 shows a low-magnification bright-field image with an inset SEAD pattern taken from the F3 mm alloy, confirming the amorphous nature of this alloy. Fig. 5 shows the TEM investigation of the crystalline phase found in the F5 mm alloy. Fig. 5(a) is a typical bright-field image of the crystalline phase, showing the well-faceted nature of this phase. Through the series tilting technique, a series of SAED patterns titled along a dense diffraction row was obtained, as shown in Fig. 5(b). Using the projection method given by Ref. [13], a reciprocal lattice can be reconstructed as schematically shown in Fig. 5(c), in which the tilting angles between SAED patterns are given in both Figs. 5(b) and (c). Based on this analysis, the zone axes of these SAED patterns can be indexed and, from that, the structure of the primary crystalline phase can be determined to be face centered cubic (FCC) with a lattice parameter of a ¼ 1.21 nm.
When the same TEM analysis is applied to the F10 mm alloy, the case becomes complicated. Detailed SAED and EDS analysis suggests there exist five crystalline phases in this alloy. For the convenience of discussion, these phases are denoted as a, b, g, d and e, and Table 1 lists their averaged compositions.   Fig. 6(a) is a bright-field image taken from a typical region containing all five crystalline phases. Through reconstructing the reciprocal lattice from a series of SAED patterns for the a phase [as shown in Fig. 6(b)], its structure can be determined to be FCC with a lattice parameter of a ¼ 1.23 nm. It is noted that the lattice parameter of this phase is slightly larger than the one found in the F5 mm alloy and is very close to the FCC NiZr 2 structure (a ¼ 1.227 nm, space group Fd3m, a crystalline phase often found in Zr-based BMGs [14,15]). TEM EDS analysis showed $2 at% oxygen in this phase. Fig. 6(c) is a series of SEAD patterns taken from a particle of the b phase, which confirms that its structure also belongs to an FCC structure with a lattice parameter of a ¼ 0.74 nm. This structure is close to the Al 1.7 Ni 0.3 Zr structure (a Cu 2 Mg-type FCC structure, a ¼ 0.738 nm). Such a crystalline phase was believed to be one of the continuous annealing products in the Zr 52.5 Cu 27 Ni 8 Al 10 Ti 2.5 alloy [16].

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For the g phase, similar tilting experiment was carried out and results are shown in Fig. 6(d). Based on this, a tetragonal structure with lattice parameters of a ¼ 0.74 and c ¼ 0.66 nm can be determined. This structure is close to the intermetallic Zr 3 Al 2 phase (a ¼ 0.762 and c ¼ 0.699 nm, space group P4 2 /mnm), which was found in the Zr 60 Al 10 Cu 30Àx Pd x (x ¼ 0,10) alloys [17].
Extensive TEM investigation suggested that the d and e phases have a rhombohedral structure and a monoclinic structure, respectively. Fig. 7(a) shows a typical bright-field image of the d phase with an inset SAED pattern. The lattice parameters of this phase have been determined to be a ¼ 0.54 nm and c ¼ 2.91 nm. Fig. 7(b) shows a typical bright-field image of the e phase with an inset SAED pattern. The lattice parameters of the e phase are determined to be a ¼ 0.95, b ¼ 0.55, c ¼ 1.02 nm and b ¼ 721. Detailed determination of these two phases can be found in Ref. [18].

Discussion
Taking all the experimental results into account, following solidification sequences together with in situ formation of crystalline phases under different cooling rates (due to different diameters of rod samples) can be suggested as follows: For the high cooling rate (in the case of the F3 mm alloy), a single amorphous structure results.  As mentioned earlier, the primary crystalline phase found in the F5 mm alloy has a lattice parameter of a ¼ 1.21 nm. This phase is similar to several reported crystalline phases, namely NiZr 2 (a ¼ 1.227 nm) [14], Zr 4 Ni 2 O (a ¼ 1.2197 nm) [19,20], Zr 4 Cu 2 O (a ¼ 1.227 nm) [19] and Al 5 Ni 3 Zr 2 (a ¼ 1.208 nm) [21]. Since oxygen has been detected in this phase, we believe that it should be the oxygen-stabilized phase-NiZr 2 or Zr 4 Ni 2 O [14,19,22,23]. Since this primary phase forms in the F5 mm alloy, we expect it should be the most active crystalline phase to compete with the amorphous structure. The reason of the slight difference in lattice parameters between our oxygenstabilized phase and the standard NiZr 2 phase is possibly because a considerable amount of Cu and Al have been found in our case.

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In the slow-cooled (F10 mm) alloy, the lattice parameter of the NiZr 2 -type phase changes from the original a ¼ 1.21 nm in the F5 mm alloy to a ¼ 1.23 nm. This may be due to a higher Zr content found in this alloy and Zr having the largest atomic radius among the constitutional elements (r Zr ¼ 0.216, r Cu ¼ 0.157, r Ni ¼ 0.162, r Al ¼ 0.182 and r O ¼ 0.165 nm). Since the NiZr 2 -type phase appeared in the F5 mm alloy as the most competitive phase to the amorphous structure, it is believed that, in the slow-cooled alloy, this phase would be a leading crystalline phase. As a result of its formation, significant element diffusion takes place and results in localized composition fluctuation. This, in turn, breaks the balance of the composition for the amorphous structure and triggers the formation of other crystalline phases.
EDS linear analysis of element distribution shown in Fig. 3(f) suggests that Al and Zr are the most active elements in the slow-cooled solidification process, which in turn result in some Al-rich/Zr-poor regions or Al-poor/Zrrich regions. Consequently, Al and Zr elements are believed to play the key role in the in situ crystallization process.

Conclusions
Based on the experiments and discussion outlined above, the following conclusions can be made: (a) In the medium cooling rate (F5 mm) alloy, an oxygencontaining NiZr 2 -type FCC structure (a ¼ 1.21 nm) has been determined to be the primary crystalline phase. In the slow-cooled (F10 mm) alloy, five crystalline phases have been identified. They are the FCC NiZr 2 -type phase (a ¼ 1.23 nm), the FCC Al 1.7 Ni 0.3 Zr-type phase (a ¼ 0.74 nm), the primitive tetragonal Zr 3 Al 2 -type phase (a ¼ 0.74 and c ¼ 0.66 nm), the monoclinic phase (a ¼ 0.95, b ¼ 0.55, c ¼ 1.02 nm and b ¼ 721) and the rhombohedral phase (a ¼ 0.54 and c ¼ 2.91 nm). (b) The oxygen-stabilized NiZr 2 -type crystalline phase is believed to be the most competitive phase to the amorphous structure, so that this phase acts as the leading crystalline phase to trigger the formation of other crystalline phases in the slow-cooled (F10 mm) alloy. (c) It has been observed that a long-range diffusion of Zr and Al in the slow-cooled (F10 mm) alloy, which is believed to play the key role in the in situ crystallization process.