Elsevier

Advanced Powder Technology

Volume 25, Issue 6, November 2014, Pages 1793-1799
Advanced Powder Technology

Original Research Paper
Effect of ball milling on the physical and mechanical properties of the nanostructured Co–Cr–Mo powders

https://doi.org/10.1016/j.apt.2014.07.009Get rights and content

Highlights

  • The effect of phase transformation on some physical properties such as dislocation density was investigated.

  • The role of transformed FCC percent and initial HCP according to as-received materials on some properties was evaluated.

  • Finally, hardness, morphological changes and particle size distribution of the powders were investigated.

Abstract

In the present study, Co-based machining chips (P1) and Co-based atomized alloy (P2) has been processed through planetary ball mill in order to obtain nanostructured materials and also to comprise some their physical and mechanical properties. The processed powders were investigated by X-ray diffraction technique in order to determine several microstructure parameters including phase fractions, the crystallite size and dislocation density. In addition, hardness and morphological changes of the powders were investigated by scanning electron microscopy and microhardness measurements. The results revealed that with increasing milling time, the FCC phase peaks gradually disappeared indicating the FCC to HCP phase transformation. The P1 powder has a lower value of the crystallite size and higher degree of dislocation density and microhardness than that of the P2 powder. The morphological and particle size investigation showed the role of initial HCP phase and chemical composition on the final processed powders. In addition results showed that in the first step of milling the crystallite size for two powders reach to a nanometer size and after 12 h of milling the crystallite size decreases to approximately 27 and 33 nm for P1 and P2 powders, respectively.

Introduction

Metallic biomaterials are widely used for load-bearing implants because they have higher mechanical properties compared with polymer materials and ceramics [1], [2], [3]. The most important metals used for orthopedic implants include: commercially pure titanium and its alloys, cobalt-based alloys and stainless steel [4]. Among these metallic alloys, Co–Cr–Mo alloys, namely vitallium, have been widely used both as surface replacement such as hip and knee joint replacement and for total hip replacements [5]. The application of Co–Cr–Mo for surgical implants has been due to their excellent mechanical properties, biocompatibility and also their corrosion and wear resistance [6].

The microstructure and above-mentioned properties, in particular the mechanical properties, depend not only on the chemical composition but also on the manufacturing process, include: precise casting (lost wax), forging and powder metallurgy [7]. The presence of inherent defects such as shrinkage, chemical heterogeneity and large grain size, in the precise casting method, leads to low mechanical properties compared to other manufacturing process [8]. On the other hand, powder metallurgy exhibit superior mechanical and chemical properties compared with the two other methods, due to chemical homogeneity, finer grain size, fabrication of complex shaped parts, and control of percent and geometry of porosity in order to improve bone tissue in-growth [5], [9].

Preparing of the powder and knowing the changes in microstructure, morphology, and grain size during the milling of powders prior to sintering step, is one of the key stages of powder metallurgy. Although there are a few researches on the manufacturing composite through powder metallurgy [7], [10], [11], [12], there is a need for further information about the effect of milling on the properties of prepared powders.

Milling is one of the most effective and economical methods to produce powders with nano sized structure [11], [13]. In nanostructure materials with extra fine grains, a high fraction of atoms are in grain boundaries which result in interesting physical and mechanical properties such as high strength and good fracture toughness [4]. During milling, powder particles are undergone high-energy impacts by balls [11]. The high energy impacts result in a high amount of defects such as vacancies, dislocations, grain boundaries and stacking faults in particles which in turn give rise to nanometer crystallite size and phase transformations [4].

The aim of this work was to produce nanostructured Co-based alloy by means of milling from machining chips and gas atomized powders. The current work, focused mainly on the characterization and comparison of some physical and mechanical properties between two kinds of powders before and during milling.

Section snippets

Materials and milling conditions

Machining chips (P1) of Co-based alloy bars and gas atomized powders were supplied by Nobilium (USA) and Carpenter Co. (Sweden), respectively. Fig. 1(a) and (b) shows images of as-received chips and gas atomized powder respectively. As can be seen, the machining chips have a spring-like morphology and alloy powder particles have a spherical shape with about 150 μm in diameter. The chemical compositions of these as-received materials According to the measured by aforementioned companies, are

Phase evaluation during milling

Fig. 2, Fig. 3 indicate the XRD patterns of P1 and P2 after different milling times, respectively. As it can be seen in Fig. 2(a), the pattern of the powder at time 0 (as-received) shows sharp peaks corresponding to the HCP phase (PDF # 05-0727, P63/mmc, ε) and FCC phase (PDF # 15-0806, Fm-3m, γ). This pattern shows that the γ(1 1 1), γ(220), and γ(3 1 1) peaks overlapped with ε(0 0 0 2), ε(112¯0), ε(101¯2) peaks, respectively. According to the XRD pattern of P2 at time 0 (Fig. 3(a)), it is evident

Conclusion

  • 1.

    For machining chips (P1) and gas atomized powder (P2), at the first stage of milling (3 h) the FCC  HCP phase transformation start to happen and the complete phase transformation happens after 6 and 9 h for P1 and P2 powders, respectively.

  • 2.

    The broadening of powders diffraction peaks with increasing milling time implies the decreasing trend of crystallite size of powders. The P1 and P2 powders crystallite size after 6 h of milling time is about 69 and 91 nm, respectively, which decreased to 27 and 34 

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