Kinetics analysis of the isothermal crystallization of Cu₅₅Zr₄₅ metallic glass by differential scanning calorimetry

Abstract

The crystallization kinetics of Cu55Zr45 (at%) glassy alloy is studied under isothermal condition using differential scanning calorimetry (DSC). The plot of correlation between the crystallized volume fraction α and annealing time t shows a sigmoid‐type curve, which is steeper with higher annealing temperature. Furthermore, in isothermal crystallization condition, local activation energy Eα values, determined using the Arrhenius equation, range from 181.1 to 187.8 kJ/mol, which is nearly a constant. The local Avrami exponent n(α) values, obtained by the Johnson‐Mehl‐Avrami equation, which range from 2.2 to 4.0 at different annealing temeperatures, which indicates that the crystallization mechanism is diffusion‐controlled transformation. Moreover, n(α) becomes greater with increasing annealing temperature, which indicates that annealing temperature can affect nucleation rate and growth type.

Study on isothermal crystallization kinetics of Zr55.7Cu22.4Ni7.2Al14.7 bulk amorphous alloy

“…Each curve has a similar trend with respect to the crystallization volume fraction x , indicating that the n ( x ) does not vary much under the influence of different temperatures. At the same temperature, n ( x ) varies significantly with x increasing, which exposes that the crystallization mechanism of bulk amorphous alloy is different at different crystallization stages 20 .

Figure 4 Curves between local Avrami exponent and crystallization volume fraction.

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“…At the end of crystallization, the nucleation rate drops quickly with the annealing time increases. The amorphous sample structure is almost transformed into a crystalline structure 20 , 26 , 27 .…”

“…The Avrami exponent values are distributed between 1.8 and 2.2, indicating that the crystallization mechanism is similar at different temperatures 29 . The reaction rate constant k increases with rising temperature, revealing that the alloy is easier to crystallize at higher temperatures, due to the fact that the diffusion ability of internal atoms increases with rising temperature, and the diffusion of atoms facilitates structural ordering, so the crystallization reaction rate of bulk amorphous alloy increases with rising temperature 20 .…”

“…b = 1, 2, 3 respectively represent one-dimensional, two-dimensional, and three-dimensional growth. p = 0.5 indicates the growth of diffusion-controlled; p = 1.0 indicates the growth of interface-controlled 20 .…”

“…The improvement of thermoplastic processing performances by inhibiting the crack expansion and avoiding crystallization during thermoplastic processing of Zr-based bulk amorphous alloy 16 , 17 , and sought to enhance the thermal stability of amorphous alloys by adding a small amount of rare earth elements to improve the product properties after thermoplastic processing 18 , 19 . The thermal stability, crystallization kinetics of amorphous alloys are related to thermoplastic forming ability and hot workability of bulk amorphous alloys 20 – 23 . Although there has been a lot of research work in this area, there are still many issues that are still unclear.…”