Elsevier

Intermetallics

Volume 39, August 2013, Pages 69-73
Intermetallics

Abnormal crystallization in Al86Ni6Y4.5Co2La1.5 metallic glass induced by spark plasma sintering

https://doi.org/10.1016/j.intermet.2013.03.013Get rights and content

Highlights

  • Abnormal crystallization below glass transition temperature was observed during SPS.

  • A crystalline phase Al5Co2 was identified using XRD and TEM.

  • Crystallization was affected by the current effect and the pressure applied during SPS.

Abstract

Abnormal crystallization was observed in Al86Ni6Y4.5Co2La1.5 metallic glass powder at about 20 °C below its glass transition temperature (Tg, 271.5 °C) during spark plasma sintering (SPS). The crystallization product, identified to be a hexagonal-structured Al5Co2 phase, was not detected in the same powder when annealed in a differential scanning calorimeter (DSC). Nor was it detected in other Al-based metallic glasses of similar compositions which were annealed around their Tg temperatures by conventional heating. SPS is effective to introduce a unique nanometric intermetallic phase in the amorphous matrix. The abnormal crystallization is attributed to the high applied pressure and non-thermal effects of SPS.

Introduction

Bulk metallic glasses (BMGs) continue to attract significant attention due to their unique properties compared to their crystalline counterparts [1]. However, after decades of research, two major drawbacks still remain largely unresolved. One is their size limitation and the other is their brittle character [1], [2], [3]. Both problems have impeded the potential application of BMGs.

As for the size limitation, powder metallurgy (PM), particularly the spark plasma sintering (SPS) technique, offers an effective solution to the fabrication of large, fully dense BMG preforms [4], [5], [6]. To overcome the intrinsic brittleness of BMGs, an important tactic is to introduce some second phases into the metallic glass matrix by either an in situ or ex situ route [7]. For instance, it can be achieved by partial crystallization during isothermal sintering or subsequent annealing or by blending the metallic glass alloy powder with a selected second phase powder prior to SPS. These second phases can prevent shear bands from localizing and propagating thereby avoiding premature failure [7], [8].

It is thus possible to address the two intrinsic problems that have long faced BMGs by adopting a PM approach such as SPS. This constitutes an important reason why SPS of BMGs has received increasing attention in recent years. Previous studies, however, have mostly focused on achieving fully dense and amorphous BMGs through optimising the SPS parameters (i.e. temperature, holding time and pressure) [9], [10]. Less attention has been dedicated to understanding the crystallization behaviour of BMGs during SPS.

Apart from offering a fast heating rate and a high applied pressure, some non-thermal effects caused by the direct current during SPS could also be significant [11]. These include an increase in the point defect concentration and a decrease in the activation energy for their migration [11], [12]. This is expected to enhance diffusion, facilitating crystallization. Hence, together with the high applied pressure, SPS has the potential to introduce some unique nanometric second phases in the amorphous matrix compared to conventional heating under atmospheric pressure.

The Al86Ni6Y4.5Co2La1.5 metallic glass alloy is a recently developed Al-based BMG [13] and can be readily made into powder by gas atomization. Our recent work has shown that SPS can be used to fabricate 10 mm diameter fully dense amorphous samples of this alloy composition from metallic glass powder [6]. Larger diameter samples also look promising. The fabricated BMG samples show record fracture strength under compression for Al alloys but the ductility remains a major concern. This work reports an abnormal crystallization phenomenon observed in Al86Ni6Y4.5Co2La1.5 metallic glass powder during SPS. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the crystallization product. The results show that SPS offers the unique potential to create a nanometric second phase in the amorphous matrix.

Section snippets

Experimental

Nitrogen gas-atomized Al86Ni6Y4.5Co2La1.5 metallic glass powder (<25 μm [14]) was used. The Tg and onset crystallization temperatures of the metallic glass powder are 271.5 °C and 281.8 °C, respectively [14].

The SPS experiments were conducted on an SPS-1030 model (SPS SYNTEX INC, Japan). A tungsten carbide (WC) die was employed. The isothermal sintering temperature was selected to be 248.5 °C according to a previous study [6], which can ensure full densification without detectable

Results and discussion

The XRD patterns obtained from the SPS-processed Al86Ni6Y4.5Co2La1.5 samples at 248.5 °C for 2–20 min under 200 MPa are shown in Fig. 1. No crystallization was detected by XRD when the isothermal hold was ≤10 min. Crystallization occurred after 20 min at 248.5 °C but the alloy remained largely amorphous according to the residual broad diffraction peak. Some of the diffraction peaks belong to fcc-Al but the rest, indexed to be Al5Co2, were not observed when the same powder was annealed at

Conclusions

TEM analysis confirmed the formation of a nanocrystalline Al5Co2 phase in Al86Ni6Y4.5Co2La1.5 metallic glass powder sintered at 248.5 °C for 20 min under 200 MPa by SPS, where the SPS temperature is about 20 °C below the Tg temperature of the glass powder. This crystalline phase was not observed when the same glass powder was annealed in a DSC furnace. It shows the unique potential of SPS to introduce or develop a nanocrystalline intermetallic phase in the amorphous matrix. The abnormal

Acknowledgements

This work was funded by the Australian Research Council (ARC). The authors would like to thank Prof. Jianqiang Wang of the Institute of Metal Research, Chinese Academy of Sciences, for provision of the metallic glass powder. We also acknowledge assistance from the Centre for Microscopy and Microanalysis (CMM) of The University of Queensland (UQ) and the Australian Microscopy & Microanalysis Research Facility (AMMRF). M.Yan acknowledges supports from the Queensland Smart Future Fellowship (Early

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