Static and dynamic crystallization in Mg–Cu–Y bulk metallic glass

Abstract

The static and dynamic crystallization behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. 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 was calculated to be 139 kJ/mol; this value is close to the self diffusion of Mg in both a crystalline and non-crystalline matrix. The Avrami exponent was found to vary from 2.2 to 2.5 with increasing annealing temperature which implies that, at high annealing temperatures, nucleation occurs at a constant rate accompanied by diffusion-controlled growth of spherical grains. Tensile straining in the supercooled liquid region indicated that crystallization is slightly accelerated compared with static crystallization; this phenomenon was found to adversely affect the ductility of the alloy.

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 Mg₅₈Cu_30.5Y_11.5 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; T_g). 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 (T_x). The temperature interval T_x−T_g 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].