Improvement of mechanical properties in MgCuYNdAg bulk metallic glasses with adding Mo particles

https://doi.org/10.1016/j.jallcom.2010.04.066Get rights and content

Abstract

In this study, the Mg58Cu29.5Y6Nd5Ag1.5 bulk metallic glasses (BMGs) with superior glass forming ability are made by Cu-mold casting and injection casting processes. Critical rod size of the Mg58Cu29.5Y6Nd5Ag1.5 BMGs (base alloys) reached ∼4 mm in diameter. The results of Vickers indentation and compression tests of Mg58Cu29.5Y6Nd5Ag1.5 alloy reveal that the mechanical performance of the BMG base alloy is brittle. The highest fracture toughness and plastic strain of the base alloy are only ∼9 MPa m1/2 and ∼0%, respectively. SEM observation is followed after the mechanical properties tests. It showed that the dominant patterns of the fracture surface are the featureless mirror-like and river-like regions. For enhancing the mechanical properties of the base alloys, porous Mo particles are added to form Mg58Cu29.5Y6Nd5Ag1.5–Mo BMG composites (BMGCs). It is found that the fracture toughness and the fracture strength of the 30 vol.% Mo particles reinforced Mg58Cu29.5Y6Nd5Ag1.5 BMGCs is promoted to ∼50 MPa m1/2 and ∼1100 MPa. The observation of the fracture surface of Mg58Cu29.5Y6Nd5Ag1.5–30 vol.% Mo BMG composite via the compression test reveals that the shear bands and cracks propagation is resisted by the dispersion of Mo particles and secondary shear bands are formed during the plastic deformation. It is also observed that the fracture surface is occupied by vein-like pattern, instead of mirror-like region. Moreover, the plastic strain of the Mg58Cu29.5Y6Nd5Ag1.5–30 vol.% Mo BMG composite increased to ∼29%.

Introduction

As compared with the traditional crystalline alloys, bulk metallic glasses with the short-range or random atomic ordering have been paid worldwide attentions due to their unique mechanical properties, such as high compressive fracture strength, hardness, and good corrosion resistance [1], [2], [3], [4]. Among bulk metallic glasses, the lightweight Mg-based alloys (the specific weight for Mg is 1.7 g/cm3), such as Mg–Cu–RE–TM (RE: rare earth metal, such as Y, Gd, Nd and Tb; TM: transition metal) BMGs [5], [6], [7], [8], [9], with high glass forming ability (GFA) and large supercooled liquid region are of interest for the resource shortage and environment contamination problems. However the brittle fracture behavior and low plasticity of BMGs at room temperature limited their potential in application, especially for the Mg-based alloys. Hence, numerous efforts have been made to solve the low ductility problems of Mg-based BMGs. Recently, the most attractive and effective way to enhance the room-temperature plasticity of BMGs were focused on the ex situ method. To improve the mechanical properties of BMGs by the ex situ method, bulk metallic glass composites (BMGCs) are synthesized by the introducing of various metallic or ceramic second phase particles (e.g., TiB2 [10], ZrO2 [11], WC [12], SiC [13], Nb [14], Mo [15], and Ti [16]) into the BMG matrix alloys. Many kinds of Mg-based BMGs have been used to form the bulk metallic glass composites by following this idea. Previous studies revealed that the dispersion of micrometer or nanometer size crystalline particles in the amorphous matrix leads to the serrated flow of shear bands and the retarded crack propagation. Hence, the remarkable macroscopic plasticity promotions of the BMGCs were observed. For example, the Mg65Cu25Gd10 BMGC with adding 40 vol.% Ti exhibits ∼40% plastic strain [16]. It is suggested that the mechanical properties of BMGCs increased with increasing volume fraction of second phase particles. Besides, it has also been reported that the yield strength or the glass forming ability of BMGCs may decrease depending on the category of the reinforcements.

In order to explore the effects of the adding of second phase particles on the mechanical properties of BMGs, the Mg58Cu29.5Y6Nd5Ag1.5 alloy with high glass forming ability was selected as the base material in this research. The micro-addition of Ag, which has a large negative heat of mixing with Mg and Cu, had also been certified to be effective to promote GFA and thermal stability of Mg-based BMGs [15]. Porous Mo particles with large amount, 20, 25 and 30 vol.%, were doped in the base alloys. The selection of Mo as the reinforced particles in BMG matrix was based on the following reason: Mo particle is stable and insoluble in the Mg-based BMG matrix and it exhibits high melting temperature and elastic modulus (near to the value of 2890 K and 327 GPa, respectively) [15]. The effects of the dispersion strengthening mechanism on the mechanical property improvement of the brittle Mg-based BMGCs are discussed in this paper.

Section snippets

Experimental procedures

Alloy ingots with the selected composition Mg58Cu29.5Y6Nd5Ag1.5 were made from elemental metals with purity ∼99.9 at.%. First, the CuYNdAg ingot was prealloyed by the arc melting method under a Ti-gettered argon atmosphere. Then the CuYNdAg intermediate alloy was re-melted with the Mg pieces to obtain the base material by the induction melting process. Afterwards, the reinforced particles were mixed with the melted base alloy by using a mechanical stirring process to develop the Mg58Cu29.5Y6Nd5Ag

Results and discussions

Fig. 1 shows the DSC traces of Mg58Cu29.5Y6Nd5Ag1.5 BMG base alloy and BMGCs with 20, 25 and 30 vol.% of Mo particles, and the basic thermal properties extracted from the DSC scanning curves are summarized in Table 1. The glass forming ability (GFA) parameters, Trg(=Tg/Tl), γ(=Tx/(Tg + Tl)) and γm(=(2Tx  Tg)/Tl), are also estimated and listed in Table 1. Slightly decreasing of Tx with the increasing of Mo content, and so was the supercooled liquid region (ΔTx) of Mg-based BMGCs. However, the Mg58Cu

Conclusions

The Mg58Cu29.5Y6Nd5Ag1.5 BMGCs with containing 30 vol.% Mo shows a high plastic strain and fracture strength of 29% and 1124 MPa, respectively. The micro-addition of Ag and the induction of reinforced Mo particles constitute the notable plasticity improvement of MgCuYNd BMGs. Mechanical properties of Mg58Cu29.5Y6Nd5Ag1.5 BMGCs increased with increasing Mo content. Due to the insoluble characteristic of Mo in the Mg-based alloy, the addition of Mo particles does not cause critical change of

Acknowledgements

The authors are gratefully acknowledge the sponsorship by National Science Council of Taiwan (R.O.C.) under the project no. NSC 95-2221-E-214-016-MY3 and the Micro- and Nano-Structure Analysis Lab in I-Shou university.

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