Influence of Ag and Co additions on glass-forming ability, thermal and mechanical properties of Cu–Zr–Al bulk metallic glasses

Abstract

In this study, the effect different amounts of Ag and Co have on thermal stability, phase formation and mechanical properties of the glass-forming alloys (Cu_0.5Zr_0.5)_95−x−yAl₅Ag_xCo_y (2≤x≤5 and 0≤y≤2) and Cu_47.5−yZr_47.5Al₅Co_y (0≤y≤2) was investigated, respectively. The addition of Ag increases the glass-forming ability (GFA) of the alloys by enhancing the stability of the undercooled melt and by simultaneously reducing the stability of the B2 CuZr high-temperature phase. Co instead, has the opposite effect. It leads to an increased thermal stability of the B2 phase, as well as a decreased stability of the undercooled melt and, consequently, lowers the GFA of the alloys. Moreover, a metastable big cube phase precipitates in some alloys stabilized by oxygen, which is introduced during casting. There is a strong interdependence between the phase formation and the tendency of the present alloys to vitrify. This correlation can be captured by the K-parameter (Song et al. 2011 [1]) calculated from the respective transformation temperatures. The plastic strain of the present ternary, quaternary and quinary alloys reflects the GFA as well: the higher the tendency to form a glass, the smaller the plastic strain. Once B2 crystals precipitate in the glass, the plasticity is enhanced significantly.

Introduction

Bulk metallic glasses (BMGs) are a relative young class of materials [2], [3], [4]. They show many unusual properties, especially when compared to their crystalline counterparts. For instance, they have a comparatively low Young's modulus and a large elastic limit [4]. Combined with yield strengths, which approach the theoretical limit, metallic glasses have the capability to store a rather large amount of elastic energy, i.e. they exhibit a large resilience [2], [4]. Their high hardness results in an improved wear resistance [3] and above their glass transition temperature (T_g) metallic glasses can be thermoplastically formed and near-net-shaping becomes feasible [4]. The major drawback, however, is the intrinsic macroscopic brittleness of monolithic metallic glasses due to the autocatalytic strain localization in shear bands [3], [4]. At the early stages of plastic deformation the irreversible strain is solely accommodated in so-called shear transformation zones that percolate and eventually form a shear band, which inevitably leads to fracture on further deformation [5]. To circumvent the limited ductility of bulk metallic glasses, composites consisting of ductile crystals embedded in a glassy matrix have been developed. The crystals hinder the formation and propagation of shear bands and thus can prevent premature failure [3], [5], [6]. The crystalline phase can precipitate in situ in the glassy matrix during casting, for example, when the composition and the cooling conditions are chosen properly [6]. Especially those bulk metallic glass matrix composites have been subject to intense research, in which the crystalline phase exhibits a shape-memory effect [1], [7], [8], [9], [10]. One family of these alloys is derived from the binary Cu₅₀Zr₅₀ alloy [11], [12]. These shape-memory bulk metallic glass matrix composites show an enhanced damage tolerance and next to an improved plastic strain they exhibit work hardening [7], [8], [9], [10].

In order to understand and improve the evolution of the composite microstructures and with it the resulting mechanical properties, it is important to identify and understand the impact of additional alloying elements. Next to the casting parameters, the microstructure evolution upon quenching is mainly influenced by the composition of the alloy [2], [4], [6]. In the case of CuZr-based alloys, certain alloying elements like Al [13], [14], [15] and Ag [16], [17], or even both [18], [19], [20], [21] increase the glass-forming ability (GFA). The GFA enhancement has been explained by the topological stabilization of atomic clusters in the glass (increase in packing density) due to differing atomic sizes [2]. But more recent ab initio simulations have shown that only Ag decreases the packing density of the cluster, whereas Al, instead, increases it [22]. The enhanced stability of the clusters in CuZr has thus been attributed to the electronic stabilization of the glass by Al or Ag [22].

However, the constituent elements of the alloy also affect the stability of the crystalline phases, which form in these alloy systems. One example is the role Co plays with regard to the thermal stability of the B2 CuZr shape-memory phase, which is only thermodynamically stable at elevated temperatures [23], [24], [25]. When a binary CuZr melt is cooled, the B2 phase precipitates at temperatures below 1208 K and if the system is given enough time to equilibrate, it decomposes into the thermodynamically stable Cu₁₀Zr₇ and CuZr₂ compounds below 988 K [26]. The addition of Co is known to enhance the stability of the B2 CuZr phase significantly [8], [23], [24], [27]. This might be due to the fact that B2 CoZr is stable even at room temperature [28], [29]. Both B2 phases have similar lattice constants (CuZr: 0.3259 nm [25], CoZr: 0.3197 nm [29]) and 5 at% Co are sufficient to alter the crystallization sequence on heating in Cu–Zr–Co glasses by stabilizing the B2 phase [23], [24]. At a Co content of at least 10 at% 3 mm rods solely contain the B2 (Cu, Co)Zr phase [24]. If the CuZr-base alloy is a good glass-former, e.g. Cu₄₆Zr₄₆Al₈, even rods with a diameter of up to 2 mm vitrify after the addition of 4 at% Co [30]. These reports indicate that CuZr-based glass-formers react quite sensitively to the addition of Al, Ag and Co. To quantify the tendency of a CuZr-based glass-former to form a shape-memory BMG composite, the K-parameter has been introduced [1]. It captures the competition between vitrification, formation of the B2 phase and the low-temperature equilibrium phases (Cu₁₀Zr₇ and CuZr₂) [1]. Next to the phase formation also the mechanical properties are strongly affected by these alloying elements [8], [13], [14], [16], [17], [18], [19], [20], [21], [31], [32], [33]. The interrelation between alloy composition, phase selection and mechanical properties is an important aspect to be understood if such materials shall be used in applications in the future.

The aim of this study hence is to simultaneously add Ag as well as Co to a Cu_47.5Zr_47.5Al₅ glass-forming alloy and to investigate (i) how the GFA depends on the stability of the B2 phase and (ii) whether the effects of Co and Ag add or rather compensate each other. Furthermore, the relationship between the GFA and the mechanical properties as well as the impact of B2 crystals on improving the plasticity is addressed.