Effect of Mechanical Milling and Cold Pressing on Co Powder

A S Bolokang,S Bhero,M J Phasha,D E Motaung

doi:10.1155/2012/290873

A S Bolokang, S Bhero + Show 2 more

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https://doi.org/10.1155/2012/290873

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Abstract

Cold pressing (CP) of the amorphous-like Co powder suppressed most of the XRD peaks, in particular the peak along (100) plane. The DSC curve of unmilled CP Co powder has shown a distinct sharp exothermic peak at 615C°. Upon annealing at 700C°, only the FCC phase with lattice parameter of 3.51 Å was detected by XRD. Our results implied that the exotherm at 615C° corresponds to compaction-pressure-assisted HCP to FCC first-order phase transition. The XRD analysis of 30 h milled powder revealed for the first time the FCC phase with a=3.80 Å. However, due to presence of (100) and (210) peaks, this phase is thought to be FCT with lattice parameters a=b=3.80 and c=3.07 Å. Consequently, the high-energy milling carried out in the current work induced for the first time HCP to FCT transition in Co. Upon CP of milled powder, the lattice parameter a shrunk from 3.80 to 3.75 Å. However, during annealing of the CP milled Co powder at 750C°, the FCT to FCC transition occurred, yielding the FCC phase with a=3.51 Å.

Highlights

Cobalt (Co) is a transition metal used in electronics, magnetic recording [1], and hard materials [2,3,4]
Since the axial c/a ratio decreased from 1.6258 to 1.6222, it follows that the HCP crystal lattice was compressed more along the c-axis via prismatic slip as evidenced by suppressed reflection along the (10-10) plane
Allotropic HCP to FCC phase transformation via mechanical milling (MM) has been reported before [6,7,8,9], the current study finds different results

Summary

Introduction

Cobalt (Co) is a transition metal used in electronics, magnetic recording [1], and hard materials [2,3,4]. Through thermal [5] and mechanical treatment [6,7,8,9], Co metal undergoes the allotropic HCP to FCC phase transformation. The current literature shows that metastable FCC phase is induced by ball milling (BM) [6], cold pressing [10], and thermal treatments [11]. It has been shown in the previous investigations that Co milled with W, V, and C powders forms the complex FCC Co-rich carbide with large lattice parameters [12]. The object of the current study to investigate the effects of MM and cold pressing (CP) on the Co powder, and how both processes affect the thermal behaviour

Experimental Work

Results and Discussions

Conclusions