Isothermal crystallization in supercooled liquid state for Ca50Mg22.5Cu27.5 metallic glass

doi:10.1016/j.jallcom.2010.03.225

Journal of Alloys and Compounds

Volume 504, Supplement 1, August 2010, Pages S247-S250

https://doi.org/10.1016/j.jallcom.2010.03.225 Get rights and content

Abstract

The crystallization behavior of Ca₅₀Mg_22.5Cu_27.5 metallic glass in supercooled liquid has been examined using a technique of differential thermal analysis. Crystallization mechanism of the alloys during isothermal annealing in supercooled liquid was analyzed using the Johnson–Mehl–Avrami (JMA) equation. The experimental data of the isothermal crystallization was interpreted by the JMA model. The Avrami exponents for the ribbon and bulk alloys are 2.5 ± 0.5 and 2.3 ± 0.3, respectively. Both values of the Avrami exponent suggested that the crystal growth is controlled by a long-range diffusion process. The scaling law for crystallization of the alloys was also examined.

Introduction

Metallic glasses with a supercooled liquid have been fabricated for Mg–Ce–Ni [1], Al–La–Ni [2], Zr–Al–Ni [3] systems etc. The supercooled liquid state, which is a metastable state, changes finally into a stable crystalline structure. This transition is due to the lower energy level of crystalline state than that of supercooled liquid state below the melting temperature (T_m). For improving the stability of supercooled liquid state for metallic glasses, it is important to clear the crystallization mechanism in supercooled liquid state for metallic glasses. The time dependences of crystal nucleation and growth in supercooled liquid state for the metallic glasses have been analyzed from experimental data of differential thermal analysis (DTA) or differential scanning calorimetry (DSC) using Johnson–Mehl–Avrami (JMA) equation [4], [5], [6], [7], [8], [9]. It has been reported that the crystallization mechanism for Zr₆₅Cu_27.5Al_7.5, Pd_42.5Cu₃₀Ni_7.5P₂₀ and Mg₆₅Cu₂₅Y₁₀ metallic glasses depends on temperature [4], [5], [9]. These results disagree with those expected from the JMA model.

Recently, novel metallic glasses of Ca–Mg–Cu alloys have been developed [10], [11]. One of the thermal characteristics for Ca–Mg–Cu metallic glasses is that the alloy exhibits significantly lower glass transition temperature (T_g) than those of many metallic glasses. The T_g, crystallization temperature (T_x) and supercooled liquid region (ΔT_x = T_x − T_g) for Ca₅₇Mg₁₉Cu₂₄ rod alloy with a diameter of 4 mm have been reported to be 404 K, 440 K and 36 K, respectively [10]. The Ca–Mg–Cu alloys with the essential characteristic have a potential for practical applications. The Ca–Mg–Cu alloys in the supercooled liquid state can be deformed to a desired shape using plastic molds. However, the crystallization of supercooled liquid arises for the deformation processes in supercooled liquid state. Hence, it is important to investigate the crystallization kinetics of the Ca–Mg–Cu alloys for industrial applications of the alloys.

In this paper, the crystallization mechanism of Ca₅₀Mg_22.5Cu_27.5 metallic glass in supercooled liquid region was investigated using an analyzing technique of the JMA model. The kinetics of crystallization for the ribbon and bulk alloys has been measured by a mean of DTA. The universality of crystal growth for crystallization in supercooled liquid for metallic glasses also was evaluated.

Section snippets

Experimental procedures

A master-alloy ingot of Ca₅₀Mg_22.5Cu_27.5 was prepared from pure Ca, Mg and Cu metals by an induction melting method in an argon atmosphere. The Ca₅₀Mg_22.5Cu_27.5 ribbon and bulk metallic glasses were fabricated by melt spinning and copper-mold casting methods in an argon gas atmosphere, respectively. The bulk metallic glass fabricated is a plate of 10 mm width and 4 mm thickness. The amorphous structure of the samples was examined by X-ray diffraction with Cu Kα radiation. Temperature and time

Results and discussion

Fig. 1 shows the X-ray diffraction patterns for the Ca₅₀Mg_22.5Cu_27.5 ribbon and bulk metallic glasses. The broadened diffused-diffraction peak for the alloys is the typical characterization of amorphous structure. Fig. 2 shows the DTA curves for the Ca₅₀Mg_22.5Cu_27.5 ribbon and bulk metallic glasses. The thermal stability associated with crystallization for the samples was measured by DTA under a flowing argon atmosphere with a heating rate of 40 K/min. The thermal parameters of Ca₅₀Mg_22.5Cu_27.5

Conclusion

The isothermal crystallization of Ca₅₀Mg_22.5Cu_27.5 metallic glass in supercooled liquid has been investigated using a technique of differential thermal analysis. The crystallization mechanism of the Ca₅₀Mg_22.5Cu_27.5 ribbon and bulk alloys in supercooled liquid was analyzed using the Johnson–Mehl–Avrami (JMA) formula. The Avrami exponents for the ribbon and bulk alloys are 2.5 ± 0.5 and 2.3 ± 0.3, respectively. The experimental Avrami exponents were agreement with that predicted by the JMA model.

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Isothermal crystallization in supercooled liquid state for Ca50Mg22.5Cu27.5 metallic glass

Abstract

Introduction

Section snippets

Experimental procedures

Results and discussion

Conclusion

Mater. Charact.

Mater. Sci. Eng. B

Mater. Sci. Eng. B

J. Alloys Compd.

J. Non-Cryst. Solids

J. Alloys Compd.

Jpn. J. Appl. Phys.

Isothermal crystallization in supercooled liquid state for Ca₅₀Mg_22.5Cu_27.5 metallic glass