Fabrication of Bulk Metallic Glass Composite with High Magnesium Content

Abstract

In this work, in-situ bulk metallic glass composites in the Mg-Ni-Gd ternary system with high Mg content (>80 at.%) that contain a ductile Mg-rich crystalline phase were produced by copper mould casting. The stability and devitrification process of the amorphous matrix in the composite structure have been studied. The precipitation of crystalline phases from the amorphous matrix and the volume fraction of these phases were found to be dependent on cooling rate and alloy stoichiometry. Introduction Bulk metallic glasses (BMGs) based on lightweight metals exhibit low densities and high strengths and may potentially serve as future high specific-strength materials. There are a number of Mg-based BMGs discovered in multicomponent alloy systems that exhibit yield strength values about 3-5 times higher compared with those for conventional crystalline Mg alloys [1-3]. However, their low ductility and fracture toughness inhibit their use as lightweight structural materials. The plasticity of Mg-based BMGs can be enhanced significantly by incorporating soft crystalline phases in the high-strength amorphous matrix [4-6]. Metallic glass/composite structures can be produced by different methods including the ex-situ introduction of insoluble particles, or by in-situ methods including annealing an amorphous alloy or adjusting the cooling rate to precipitate a crystalline phase from the amorphous bulk during solidification. In this work, the latter was employed to produce an amorphous-crystalline composite based on the ternary Mg-Ni-Gd glass forming system. The aim is to study the dependence of alloy microstructure on cooling rate and devitrification processes of the amorphous matrix phase in these Mg-based BMG composites. Experimental procedure High purity elements Mg (99.85 wt.%), Ni (99.9 wt.%) and Gd (99.9 wt.%) were used to prepare the nominal alloy composition in this work. A Ni-Gd master alloy was prepared for nominal alloy composition using a Buehler MAM-1 arc melting machine in a Ti-gettered Ar atmosphere on a water-cooled copper hearth. The remaining balance of Mg was alloyed with the Ni-Gd master alloy and remelted several times to ensure homogeneity using an induction furnace under a vacuum-purged circulating argon (99.998 wt.%) atmosphere. On the final melt cycle, the alloy was heated above its liquidus temperature, cooled to the desired casting temperature and cast directly into a naturally cooled wedge-shaped copper mould generating a range of cooling rates in a single casting and, hence, a range of different phases and microstructures throughout the wedge sample. The phases present in the samples was determined by X-ray diffraction (XRD) using a Philips X’Pert Pro MRD X-ray diffractometer fitted with a micro-capillary tube with a 0.5 mm focus size using a CuK radiation source. Analysis of the as-cast microstructures was carried out on a Hitachi S3400 scanning electron microscope (SEM). Various thermal analyses for the samples were carried out using a Netzch 404 differential scanning calorimeter (DSC). Results and discussion Based on work reported by Park et al. [1] the Mg-rich region in the glass-forming Mg-Ni-Gd ternary system was selected for composite composition design. In order to precipitate the soft Mg-rich phase from the amorphous matrix/supercooled liquid a Mg concentration of 84 at.% was selected. A wedge-shaped sample of the Mg84Ni10Gd6 alloy composition was produced by copper mould gravity casting. The microstructure and phase evolution of samples taken from various wedge thicknesses were found to vary significantly. Formation of a single glassy phase was observed up to a wedge thickness of 0.7 mm indicating that glass forming ability (GFA) of the Mg84Ni10Gd6 alloy has been deteriorated when compared to that of the known Mg75Ni15Gd10 system [1]. The amorphous nature of the alloy was confirmed by the XRD trace (Fig. 1a) showing a broad diffraction halo. A combination of amorphous and multiple crystalline phases is present in the samples at thicknesses of 0.8-2.5 mm; these are indicated in the XRD pattern for this sample range by the broad underlying halo and crystalline phase reflections corresponding to Mg-rich (α-Mg) and Mg5Gd phases (Fig. 1b&c). Crystalline hcp-Mg, Mg5Gd and Mg2Ni were the predominant crystalline phases that formed, as shown in Fig. 1c. At larger thicknesses (> 2.5 mm) the microstructure was found to be completely crystalline. 20 25 30 35 40 45 50 55 60 65 70 75 80 x Mg84Ni10Gd6 Mg5Gd Mg Mg2Ni a) Z=0.5 mm