Thermal Stability and the Matrix Induced Brittleness in a Ti-based Bulk Metallic Glass Composite

Crystallization kinetics and nanocrystalline induced brittleness in an in-situ dendrites reinforced Ti44Zr20Nb12Cu5Be19 bulk metallic glass (BMG) composites were investigated. The activated energy of the present Ti-based metallic glass matrix is obtained to be about 201 KJ/mol. As the annealing temperature rises, the annihilation of free volume is believed to cause the increase of hardness and the decrease of plasticity. Brittle fracture occurs after the precipitation of nanocrystalline, which can be ascribed to the formation of the microcracks in the matrix during deformation.


Introduction
As a kind of potential advanced engineering materials, bulk metallic glasses (BMGs) process lots of unique properties related to the special amorphous structure, such as high strength, high hardness, large elastic limit 1,2 .However, catastrophic brittleness always occurs in BMGs due to the highly localized deformation 3,4 .In recent years, In-situ dendrites reinforced BMG composites have been successfully synthesized in Ti- [5,6] , Zr- [7,8] , Cu- [9] , La- [10] based BMGs, etc.The existing of these soft second phase in the amorphous matrix can effectively harmonize plastic deformation and stimulate the multiplication of shear bands by impeding over-propagation of single shear bands 10 .Great improvement on plasticity has been obtained in these BMG composites.However, as a metastable state in the thermodynamic, nanocrystalline has expected to take place at certain temperature in the glass matrix 11,12 , which will also greatly influence their properties.Up to now, the thermal stability and effects of pre-existing crystalline phase in the BMG composite are still far from clarification.Investigating the crystallization behavior of the matrix in BMG composite is very important for their extensive applications and long-term service.Therefore, thermal stability of the glass matrix and the crystalline induced brittleness in Ti 44 Zr 20 Nb 12 Cu 5 Be 19 (Ti44) BMG composites were investigated in the present work.The aim of the present work is to further uncover the deformation mechanism of these in-situ BMG composites.

Experimental Procedure
The ingots of Ti-based BMG composites with the nominal composition of Ti 44 Zr 20 Nb 12 Cu 5 Be 19 were prepared by arc melting elements (Ti, Zr, Nb, Cu, Be > 99.9% purity) in an argon atmosphere.Cylindrical samples with 3mm in diameter and 60 mm in length were obtained by a copper mould casting method.Kinetics of crystallization in the amorphous matrix was studied via the differential scanning calorimetry (DSC, NETZSCH 404C) under a flowing high-purity argon gas protection at the heating rates from 5 K/min to 40 K/min.To change the microstructure of the glass matrix, vacuum annealing treatments were performed at 653 K, 673 K, and 723 K for 15 minutes, respectively.The X-ray diffraction (DX-2700) and High-resolution transmission electron microscope (HRTEM, Tecnai G2 F30) was used to identify the struture evolution before and after annealing.The hardness was tested on the Vickers microhardness tester (Duramin-A300) with 1N maxim load and the dwell period of 10s.Quasi-static tests were conducted on the MTS SANS CMT5105 universal testing machine by using rod-like samples of Φ 3 mm × 6 mm.After fracture, the fracture surfaces were observed by scanning electron microscope (SEM,VEGA3 TESCAN).

Results and Discussion
Figure 1a displays the microstructure of the present in-situ dendrites reinforced BMG composite.It can be seen that dendritic second phase (dark contrast) homogenously embeds in the continuous matrix (white contrast).The volume fraction of the second phase is about 44% by image analysis.The XRD pattern of the crystalline peaks overlapped on the broad diffuse-scattering amorphous pattern in Figure 1b further confirms that the present composites consist of the Ti (Zr, Nb) solution and the amorphous matrix.
DSC curves of the present BMG composite at different heat rates are shown in Figure 2. Obvious exothermal peaks associated with the crystalline behavior of the glass matrix can be observed.It's also noted that with the increase of heat rates, both the onsets of crystallization, T x and the peak of crystallization, T p shift from the low to the high temperature in Figure 2, indicating an obvious dynamic effect on the crystalline of the glass matrix.To evaluate the stability of the present BMG composite, activation energy is calculated, which represents the barrier potential needed to be passed when the matrix translates from the disordered to the ordered state.Namely, higher activation energy, more stable of the matrix.Here, Kissinger method 13 is used to calculate the activation energy.

Thermal Stability and the Matrix Induced Brittleness in a Ti-based Bulk Metallic Glass Composite
Where, β and T c represent the heat rates and characteristic temperature, respectively.E c denotes the activation energy, R is the gas constant and A is a constant.Then, the apparent crystallization activation energy, E p , for the present BMG composite can be obtained as the slope of ln(β/T p 2 ) vs 1/T p curve, as shown in Figure 3. Table 1 summarizes the activation energy of the present BMG composite and some other Ti-based monolithic BMGs.It can be obvious seen that the BMG composite has the lowest activation energy.That implies that they will be more vulnerable to suffer from crystallization.Differing from the monolithic BMGs, which is more homogenous in the structure, the precipitated dendrites in matrix increase the heterogeneity in the matrix by formation of interface between the dendrites and the matrix.These regions could provide more nucleation sites and contribute to the nanocrystalline process.Therefore, the lower activation energy of the present BMG composite could be attributed to the special structure of BMG composite.Furthermore, because of the low thermal stability of the BMG composite, it's necessary to investigate   the effect of matrix evolution on the mechanical properties of the present BMG composite.Figure 4 is the XRD patterns of the present BMG composites after annealing at different temperature.When annealing below the crystalline temperature T x , the XRD pattern of the present BMG composite shows no obvious differences compared with the as-casted sample.However, after annealing above T x , obvious peaks corresponding to the nanocrystalline can be clearly seen in Figure 4, implying that nanocrystallization occurs in the matrix.
To further clarify the microstructure of samples annealed at different temperature, HRTEM images are exhibited in Figure 5.As shown in Figure 5a and Figure 5b, maze-like disordered structure without any lattice fringes corresponding to nanocrystallization can be seen in both the as-casted and annealed sample at 673 K, which indicates that the matrix still has an amorphous structure.However, lattice pattern induced by nanocrystalization can be clearly seen in sample annealed at 723K in Figure 5c, in accordance with the XRD pattern in Figure 4.
Figure 6 shows the variation of the hardness with the annealing temperature.With the increase of annealing temperature, the hardness of the present BMG composites increases.Although no obvious differences can be observed in Figure 5a, b, the hardness of samples annealed at 673K is still higher than that of as-casted sample, which can be attributed to annihilation of free volume in the matrix 12 .After the precipitation of nanocrystalline in the matrix at 673 K, the hardness increases dramatically to 465 Hv.
Ambient mechanical properties of samples with different structure are shown in Figure 7.The as-casted samples possess good plasticity (~ 11.5%).After annealed at 653 K and 673 K, the plasticity of the present BMG composites decreases to about 8.6% and 8.8%, respectively.However, brittle fracture occurs after annealed at 723 K. Mechanical properties of the present BMG composites are listed in Table 2.
Figure 8 displays the morphology of fractured samples annealed at different temperature.Both the as-casted and annealed samples below T x show shear failure with fracture angles smaller than 45 o , as marked in Figure 8a, b.The corresponding fracture surfaces are shown in Figure 8b, d,      respectively.Lots of melting layers can also be observed on their fracture surfaces, indicating the large temperature rising due to the release of elastic energy after fracture.However, after the precipitation of nanocrystals, as shown in Figure 8e, a cleavage fracture surface can be observed.
The magnified image of fracture surface (Figure 8f) shows lots of micro cracks, corresponding to the brittleness of the present BMG composite after annealed at 673 K.
For BMG composites, the plasticity will be determined by both the matrix and the dendrites.Compared with the melting point of dendrites, the annealing temperature is only about 0.2T m 18 .Thus, it's reasonable to deduce that the matrix causes the plasticity reduction of the present BMG composite after annealing.To illustrate the variation of mechanical properties, according to the STZ model 19 , the plasticity of metallic glass is initiated by stress induced cooperative rearrangement of small groups of atoms called shear transformation zones (STZs), which is believed to consist of a free volume site with immediate adjacent atoms 20 .Therefore, higher free volumes concentration promote the shear bands more easily to be generated in the glass matrix 21,22 .Further, more shear bands initiating in the matrix will results in the improvement of the whole plasticity.However, after annealing below T x , the increase of hardness obviously indicates the annihilation of free volume.With respect to the as-casted samples, BMG composites annealed at 673K involve fewer free volumes.Thus, as shown in Figure 7, lower plasticity is obtained as a consequence.When annealed above T x , the precipitation of brittle nanocrystals will further deteriorate the plasticity by generating microcracks at a rather low externally applied stress 23 , as shown in Figure 8f.The fast propagation of microcracks propagates in the matrix and causes the brittle fracture of the present BMG composite.

Conclusion
Thermal stability and nanocrystalline induced brittleness of in an in situ Ti-based bulk metallic glass composite (BMG composite) were investigated.The activation energy of the present BMG composite is determined to be 201KJ/mol.With the increase of annealing below crystallization temperature, the free volume in the matrix decreases and causes the increase of hardness of present BMG composite, as well as the decrease of plasticity from 11.5% to 8.8%.Brittle fracture occurs after the precipitation of nanocrystals above T x and the fast propagation of microcracks due to nanocrystals is believed to be responsible for the cleavage fracture of the present BMG composites.

Figure 1 .
Figure 1.Microstructure of the present BMG composite (a) and the corresponding XRD pattern (b).

Figure 2 .
Figure 2. Continuous heating DSC curves of the present BMG composites at different heating rates.

Figure 4 .
Figure 4. XRD patterns of the present BMG composite after annealing at different temperature.

Figure 6 .
Figure 6.Variation of the hardness with the annealing temperature.

Figure 7 .
Figure 7. Mechanical properties of the as-casted and annealed samples of Ti44 BMG composites at room temperature.

Figure 8 .
Figure 8. Fractographs of Ti44 BMG composite with different initial conditions,as casted (a and b), annealed at 673K(c and d) and annealed at 723K (e and f).

Table 2 .
Summary of the mechanical properties of as-cast and annealed samples at room temperature.