Static and dynamic crystallization in Mg–Cu–Y bulk metallic glass
Introduction
Bulk metallic glasses (BMGs) based on magnesium have attracted considerable interest due to their potential use as structural or functional materials [1], [2], [3], [4], [5]. These amorphous materials can be generated as large diameter castings which equates to a high glass forming ability (GFA). For example, Inoue et al. [2] produced an amorphous Mg65Cu25Y10 alloy with a thickness of 4 mm by conventional mould casting. More recently, Ma and co-workers [6] have demonstrated a marked improvement in GFA of this alloy by slightly altering the base composition and found a critical diameter of 9 mm in an off-eutectic Mg58Cu30.5Y11.5 alloy.
A major concern with most types of BMG is their restricted use at elevated temperature due to their inherent thermodynamic instability which leads to rapid crystallization and a drastic change in properties [7], [8], [9], [10]. On heating, various reactions are known to occur with the first associated with a change from glassy behavior to that of a supercooled liquid (SCL) (termed the glass transition temperature; Tg). This is followed by one or more consecutive crystallization reactions, although various workers have found for certain types of BMG (see e.g. [11], [12], [13]), that phase separation occurs prior to the onset of primary crystallization (Tx). The temperature interval Tx−Tg is generally regarded as the SCL region with the extent of this region affected mainly by alloy type, composition and heating rate [7], [8], [9], [10], [11]. This region is an important feature of the BMG as it often allows it to be superplastically deformed to very large strains [14], [15], [16], [17].
The static crystallization behavior under both anisothermal and isothermal conditions have been reported for many types of BMG including Mg-base alloys [2], [18], [19], [20], [21], [22], [23]. However, there are only a few investigations into the relationship between static and dynamic crystallization; the latter is known to occur during deformation in the SCL region and is argued to result in accelerated crystallization [24], [25], [26], [27], [28], [29]. Such studies on deformation-induced crystallization have been carried out mainly on Zr-base alloys under both uniaxial tension [25] and compression [26], [27], [28]. The aim of the present work is to investigate the static crystallization behavior of Mg65Cu25Y10 and compare this behavior with dynamic crystallization during tensile straining in the SCL region. This mode of deformation was chosen as it is a more appropriate testing method than compression for investigating the ductility of BMGs as large strains (>1000%) are achievable (see e.g. [14]). Tensile testing also provides valuable information on crystallization behavior since any given sample contains both a deformed and undeformed zone. This information, in addition to understanding plastic instability (necking etc.) during straining are important parameters for assessing the effect of both flow conditions and crystallization in the SCL region on the formability of these materials [29].
Section snippets
Alloy preparation
A Mg65Cu25Y10 amorphous alloy was prepared from high-purity Mg (99.8 wt%), Cu (99.99 wt%) and Y (99.999 wt%) by a three-stage melting and casting procedure [30]: (i) small buttons of Cu71.5Y28.5 master alloy were prepared by arc melting under the protection of varigon (5 vol.% H2 in Ar) gas mixture; (ii) a balanced mixture of Cu71.5Y28.5 and Mg was then melted using an electrical resistance furnace in a BN-coated stainless steel crucible at 850 °C under argon gas of 99.997 vol.% purity, and (iii) the
Starting material and anisothermal annealing behavior
The degree of crystallization of the as-cast material was determined both by XRD and TEM. It was found that the starting alloy was amorphous (see e.g. Fig. 6a) which is consistent with previous work on comparable alloy systems where the critical diameter for obtaining a fully amorphous structure by injection casting is ∼4 mm [2], [6], [30]. Fig. 1 shows a typical DSC profile of the BMG during heating at a rate of 20 °C/min where there is one exothermic reaction (melting) and three other
Static crystallization
The static crystallization behavior of an amorphous Mg65Cu25Y10 alloy is similar to other studies on a range of BMGs [2], [18], [19], [20], [21], [22], [23]. The characteristic transformation temperatures (see e.g. Fig. 2) and the extent of the SCL region are affected by heating rate which is a well-known phenomenon [8]. This behavior allows the calculation of the activation energy for any given reaction using the well-known Kissinger equation [33]:where β is heating rate
Concluding summary
The crystallization behavior of Mg65Cu25Y10 bulk metallic glass (BMG) under various thermal and mechanical conditions was investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). It was found that the kinetics of both anisothermal and isothermal crystallization were adequately represented by the Kissinger and KJMA relations, respectively. The apparent activation energy for crystallization during heating was 1.44 eV which is
Acknowledgement
The authors would like to thank Professor Oleg Ostrovski for provision of laboratory facilities and the Australian Research Council for partial funding of this work.
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