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Materials Science and Technology (MS&T) 2010 October 17-21, 2010, Houston, Texas · Copyright © 2010 MS&T'10® Amorphous Materials: Common Issues within Science and Technology

Evolution of microstructure and mechanical properties of Zr-based in-situ bulk metallic glass matrix composite under the Bridgman solidification J.L. Cheng1, G. Chen1*, H.W. Xu1 1 Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094 , P R China *Corresponding author: Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China. Tel./fax: +86 25 84315159. E-mail address: gchen@mail.njust.edu.cn

Keywords: composites, mechanical properties, microstructure evolution

Abstract The evolution of the microstructure in Zr-based in-situ bulk metallic glass matrix composites was investigated by the Bridgman solidification. By adjusting the withdrawal velocity, volume fraction and length scales of the precipitated phase can be controlled. Moreover, we have demonstrated that the mechanical properties of the composites are closely related to the microstructure of the BMG-matrix composites.

When the decrease in withdrawal velocity, volume fraction and characteristic size scale of β-Zr phase particles increases obviously, and result in significant increase in the plasticity of the composites. But, when the withdrawal velocity is slow enough to form brittle eutectic phases which are harmful to the ductility of BMG matrix composites, the values of plastic strain significantly decrease.

1. Introduction Recently, to improve plasticity of bulk metallic glasses (BMGs) at the room temperature, a series of in-situ BMG matrix composites with large plasticity were developed [1-8]. The remarkable ductility of these composites is interpreted by the effect of the soft and ductile secondary phases, which dispersed in the BMG matrix and stabilized against the shear localization and propagation of shear bands. However, effects of the volume, morphology and size of reinforcing phase on the mechanical properties of in-situ BMG matrix composites are unclear. In this study, we apply the Bridgman solidification to acquire Zr-based in-situ composites with different volume, morphology and size of β-Zr phase, and reveal the relationship between microstructure and plasticity for Zr-based in-situ composites.

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2. Experimental Ingots of nominal compositions Zr39.6Ti33.9Nb7.6Cu6.4Be12.5 were prepared by arc melting a mixture of Zr, Ti, Nb, Cu and Be with purity higher than 99.9 wt % under a Ti-gettered argon atmosphere. These ingots were cast into a rod-shaped with a 5mm diameter by low pressure casting, and then Bridgman growth (details are described in reference [9]) was carried out for these samples with 0.46-8.0 mm/s growth velocities (V), the temperature gradient (G) is about 17 K/mm.

Differential scanning calorimetry (DSC) was applied to analyze the thermal properties of samples at a heating rate of 20 K/min. Transverse sections of the rods were examined by X-ray diffraction (XRD) to analyze the phase structure of samples.

Morphology observations were carried out by optical microscopy (OM). The volume fraction, size and shape factor of crystalline phase were estimated from OM images using computer software.

The uniaxial compression tests were performed on 3 mm in diameter and 6 mm length cylindical specimens using an Instron-8801 testing machine at room temperature with an engineering strain rate of 5×10-4 s-1. The ends of specimens were polished carefully to ensure parallelism. At least three specimens for mechanical testing were measured to ensure that the results are reproducible. The fracture surfaces and the lateral surfaces of the deformed samples were examined by scanning electron microscopy (SEM).

In order to characterize microstructural evolution of Zr-based in-situ bulk metallic glass matrix, average equivalent diameter ( Deq) were used to estimate the size scale of reinforcing phase particles. Deq can be calculated by Eqs. (1): N

∑ Deq =

4A / π

i =1

N

(1)

where A is the area of reinforcing phase particles, and N is the number of particles.

3. Results and discussions 3.1 Microstructural evolution of the Zr-based in-situ bulk metallic glass matrix composite under the Bridgman solidification The X-ray diffraction patterns of the composites in Fig. 1 are obtained by the Bridgman solidification with withdrawal velocities of 2.0, 1.0, and 0.46 mm/s. As shown in Fig.1a and b, only sharp diffraction peaks from the body-center cubic (bcc) β-Zr solid solution superimposed on a broad scattering hump characteristic of an amorphous phase, no other crystalline phase peaks are detected. When the withdrawal velocity decreases to 0.46 mm/s, as shown in Fig. 1c, it is identified that beside diffraction peaks of β-Zr phase, other crystalline phase peaks are also detected.

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Figure 1 X-ray diffractograms of Zr39.6Ti33.9Nb7.6Cu6.4Be12.5 subject to different withdrawal velocity under Bridgman solidification

Figure 2 OM micrographs of Zr39.6Ti33.9Nb7.6Cu6.4Be12.5 subjected to Bridgman solidification at withdrawal velocities of (a) 8.0 mm/s, (b) 4.0 mm/s, (c) 2.0 mm/s, (d) 1.0 mm/s, (e) 0.46 mm/s, respectively and (f) their corresponding DSC curves

The microstructure graphs and DSC traces of the composites subject to different withdrawal velocity are shown in Fig. 2. It can be seen that only dendritic phase homogeneously dispersed in the glass matrix in Figs. 2(a)-(d). When the withdrawal velocity decreases to 0.46 mm/s, some dark eutectic cells (marked by arrows in Fig.

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2(e)) are also embedded in the bulk glass matrix. These are consistent with the results of XRD patterns of the composites. As shown in Fig. 2(f), the glass transition temperature (Tg) and crystallization temperature (Tx) are clearly observed, it further confirms that the composite matrix is an amorphous phase.

Figure 3 Average equivalent diameter (Deq) of β-Zr phase as a function of withdrawal velocity for Zr-based BMG matrix composites under Bridgman solidification

The relationships between the volume fraction, Deq of β-Zr phase and the withdrawal velocity for the Zr-based BMG matrix composites under Bridgman solidification are summarized in Table 1. It shows that the values of β-Zr phase volume fraction increase from 42% to 58% when the withdrawal velocities decrease from 8.00 to 1.00 mm/s, and then maintain constant. It indicates that 1 mm/s is slow enough to reach the quasi-equilibrium state [10]. Figure 3 shows that Deq of β-Zr phase particles as a function of the withdrawal velocities for the Zr-based BMG matrix composites. As shown in Fig. 3, Deq of β-Zr phase increase from 3.7 to 16.3 µm when the withdrawal velocities decrease from 8.00 to 0.46 mm/s. This is due to the solidification is not only a thermodynamics process, but also a dynamics process.

With the decrease in withdrawal velocities, the nucleation and growth of β-Zr phase will be performed at a lower supercooling and higher temperature, results in a lower nucleation rate and higher growth rate.

Table 1 Volume fraction, average equivalent diameter (Deq), yielding stress (σy) and plastic strain (εp) of Zr-based BMG matrix composites subject to different withdrawal velocity under Bridgman solidification Withdraw

Volume fraction

Average equivalent

velocities

of β-Zr phase

diameter (Deq)

(mm/s)

(%) ±3

(µm) ±0.5

8.0

42

4.0

σy (MPa)

εp (%)

3.7

1290

2.3

48

6.1

1250

10.5

2.0

54

11.5

1170

20.6

1.0

58

13.2

1120

22.9

0.46

58

16.3

1070

7.3

3.2 Mechanical properties of Zr-based in-situ bulk metallic glass matrix composite under the Bridgman solidification

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Figure 4 shows typical room temperature compression stress–strain curves of the composites subject to different withdrawal velocity under Bridgman solidification.

For the sample of 8 mm/s, the yielding occurs at 1290 MPa and 2.1% elastic strain, and the ultimate compression stress reaches 1490 MPa with 2.3% plastic strain. For the sample of 4 mm/s, the yielding stress decreases slightly, but the plastic strain increases to 10.5%. All the compression properties of these samples are summarized in Table 1.

Figure 4 The compressive engineering stress-strain curves for Zr-based BMG matrix composites subject to different withdrawal velocity under Bridgman solidification

Figure 5 Plastic strain (εBpB) as a function of H withdraw velocities for Zr-based BMG matrix composites under Bridgman solidification

Figure 5 shows that the compression plasticity as a function of the withdrawal velocities for the Zr-based BMG matrix composites. As can be seen, when the withdrawal velocity is higher than 1 mm/s, the compression plastic strain increases rapidly with the decrease in withdrawal velocity. It should be attributed to a more volume fraction and larger characteristic size scale of precipitated β-Zr phase.

Hofmann et al. [11] proposed a special microstructural length scale L to suppress shear bands unstable propagation for Zr-based in-situ bulk metallic glass matrix composite. Stabilization requires that L≈RP [11]. Rp is the plastic zone size and correlated with (Kc/σy )2, where Kc and σy are the fracture toughness and yielding strength of the materials, respectively. For Zr-based BMGs, the values of Rp are estimated to 60 µm [12] and 200 µm [11], respectively. Therefore, with the decrease in withdrawal velocity, characteristic size scale of β-Zr phase particles increases

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obviously, and results in significant increase in the plasticity of the composites. But, when the withdrawal velocities decrease from 1 to 0.46 mm/s, the values of plastic strain significantly decrease from 22.9 to 7.3%. This is due to some brittle eutectic phases are precipitated at 0.46 mm/s, which are harmful to the ductility of BMG matrix composites.

Conclusions A series of the BMG-matrix composites with the nominal composition of Zr

39.6

Ti

33.9

Nb Cu Be 7.6

6.4

12.5

are synthesized by the Bridgman solidification. The

microstructures of the composites, such as volume fraction and length scales of the precipitated phase, can be controlled by adjusting the withdrawal velocity. Moreover, the mechanical properties closely related with the microstructures, the plasticity of the BMG-matrix composites can be optimized by tailing the withdrawal velocity.

Acknowledgements This work was supported by the National Natural Sciences Foundation of China (Grant No. 50431030 and 50871054), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20093219110035), the Natural Science Foundation of Jiangsu Province (Grant No. BK2007213) and NUST Research Funding (Grant No. 2010010).

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