Abstract
Metallic glass powder of the composition Al86Ni6Y4.5Co2La1.5was consolidated into 10 mm diameter samples by spark plasma sintering (SPS) at different temperatures under an applied pressure of 200 MPa or 600 MPa. The heating rate and isothermal holding time were fixed at 40°C/min and 2 min, respectively. Fully dense bulk metallic glasses (BMGs) free of particle-particle interface oxides and nano-crystallization were fabricated under 600 MPa. In contrast, residual oxides were detected at particle-particle interfaces (enriched in both Al and O) when fabricated under a pressure of 200 MPa, indicating the incomplete removal of the oxide surface layers during SPS at a low pressure. Transmission electron microscopy (TEM) revealed noticeable nano-crystallization of face-centered cubic (fcc) Al close to such interfaces. Applying a high pressure played a key role in facilitating the removal of the oxide surface layers and therefore full densification of the Al86Ni6Y4.5Co2La1.5metallic glass powder without nano-crystallization. It is proposed that applied high pressure, as an external force, assisted in the breakdown of surface oxide layers that enveloped the powder particles in the early stage of sintering. This, together with the electrical discharge during SPS, may have benefitted the viscous flow of metallic glasses during sintering.
Highlights
Metallic glasses (MGs) have been investigated for decades due to their intrinsically unique physical and chemical properties [1]
Under an applied pressure of 200 MPa, the density of the bulk metallic glasses (BMGs) increased with increasing sintering temperature from 248.5∘C to 278.5∘C
Further increasing the sintering temperature to 308.5∘C, which is above the Tg of the alloy and above the peak temperature for the first crystallization stage of the Al86Ni6Y4.5Co2La1.5 MG powder [8], resulted in little increase in the sintered density
Summary
Metallic glasses (MGs) have been investigated for decades due to their intrinsically unique physical and chemical properties [1]. Al-based MGs are promising advanced materials which have attracted increasing attention for their ultrahigh specific strength and relatively low cost compared with most other MGs [2] Due to their low glass forming ability (GFA), fabrication of Al-based BMGs through a conventional cooling process from liquid has proved to be challenging [3,4,5]. Dense Ti-, Ni-, Cu-, and Fe-based BMGs with >10 mm diameters have been fabricated using SPS [9,10,11,12] These MGs have much higher glass transition temperatures (Tg) [1, 3] compared to Al-based MGs and can be readily consolidated at high sintering temperatures without nano-crystallization. It is essential to remove this surface oxide layer to enable viscous
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