Nonisothermal crystallization kinetics, fragility and thermodynamics of Ti 20 Zr 20 Cu 20 Ni 20 Be 20 high entropy bulk
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Xin Wang School of Materials Science and Engineering, Hebei University of Technology, Hongqiao District, Tianjin 300130, China (Received 1 May 2015; accepted 3 August 2015)
The nonisothermal crystallization kinetics, fragility, and thermodynamics of Ti20Zr20Cu20Ni20Be20 high entropy bulk metallic glass (HE-BMG) have been investigated by differential scanning calorimetry. The activation energies for the glass transition and crystallization events were determined by Kissinger and Ozawa methods. The value of local Avrami exponent is less than 1.5 in most cases for all the three crystallization events, indicating that the major crystallization mechanism is diffusion-controlled growth of pre-existing nuclei. The local activation energy is stable during the whole crystallization process and this further confirms that the crystallization occurs through a single mechanism. Ti20Zr20Cu20Ni20Be20 alloy can be classified into “strong glass formers” according to the estimated fragility index and also shows a relatively low value of Gibbs free energy difference. However, compared with Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG, the glass-forming ability of Ti20Zr20Cu20Ni20Be20 HE-BMG is much lower and the related reasons have been discussed.
I. INTRODUCTION
Contributing Editor: Jörg Löffler Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2015.253
derived from the existing BMG system with superior glass-forming ability (GFA). It is well known that many Zr–Ti–Cu–Ni–Be or Ti–Zr–Cu–Ni–Be quinary alloys possess superior glass-forming ability.14–16 In this sense, the equiatomic and near-equiatomic alloys in Ti–Zr–Cu– Ni–Be system are probably good glass formers. Based on the above conception, a Ti20Zr20Cu20Ni20Be20 HE-BMG with a critical size of 3 mm has been successfully developed.13 Compared with other glass forming alloys in the same system, the glass-forming ability of Ti20 Zr 20 Cu 20Ni 20 Be20 HE-BMG is much lower. For understanding the thermal stability and crystallization mechanism, it is meaningful to investigate the crystallization kinetics and thermodynamic properties of BMGs. Many papers have been published on the crystallization kinetics of BMGs, including Zr-based, 17 Ti-based,18 Cu-based,19 Fe-based, 20 Mg-based,21 Ni-based,22 Ca-based,23 Al-based,24 and Ce-based BMGs,25 etc. However, very little information is available to date for HE-BMGs. In this study, the nonisothermal kinetics, kinetic fragility, and thermodynamic characteristics of Ti20Zr20Cu20Ni20Be20 HE-BMG have been systematically studied by differential scanning calorimetry (DSC). Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG, which is a typical representative of conventional BMGs with good glass-forming ability in Ti–Zr–Cu–Ni–Be quinary system, was chosen as a reference. Our research is an attempt to get a better idea of the characteristics of HE-BMGs.
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Ó Materials Research Society 2015
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High entropy alloys (HEAs) and bulk metallic glasses (BMGs)4,5 are both potential candidate materials for
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