Grain refinement of biomedical Co–27Cr–5Mo–0.16N alloy by reverse transformation

doi:10.1016/j.matlet.2009.10.001

Materials Letters

Volume 64, Issue 1, 15 January 2010, Pages 49-52

https://doi.org/10.1016/j.matlet.2009.10.001 Get rights and content

Abstract

Advanced grain refinement of a biomedical Ni-free Co–27Cr–5Mo–0.16N alloy without hot or cold plastic deformation was successfully achieved by a reverse transformation from a lamellar (hcp + Cr₂N) phase to an fcc phase. The technique consisted of a two-step heat treatment. First, the solution-treated specimen was subjected to isothermal aging at 1073 K for 90 ks, forming a lamellar structure of hcp and Cr₂N phases. Then, the aged specimen having a completely lamellar microstructure was reverse-treated at temperatures from 1273 to 1473 K, where the fcc phase is stable. The resultant grains were approximately 1/10 of their initial size. Moreover, tensile testing after reverse transformation showed excellent strength with good ductility compared to samples examined before the reverse transformation. Our results will contribute to the development of biomedical Ni-free Co–Cr–Mo–N alloys with refined grain size and good mechanical properties, without requiring any hot workings.

Introduction

Cobalt–chromium–molybdenum (Co–Cr–Mo) based alloys have been used in various medical applications, such as hip and knee joint replacements, metallic coronary stents, and denture bases, because of their good biocompatibility and mechanical properties, and their excellent resistance to wear and corrosion. Improved reliability and safety are required for hip and knee joint replacements, since these replacements are being performed in younger patients, greatly lengthening the implants' period of service.

One of the most effective methods of achieving higher mechanical reliability is grain refinement, because it increases the toughness of the material. So far, conventional grain refinement techniques for medical Co–Cr–Mo based alloys have involved thermomechanical treatment with recrystallization, such as hot forging, swaging, and rolling, and do result in greatly improved mechanical properties [1], [2], [3]. However, when inhomogeneous strains are introduced during the hot working process, uniform fine grains are not produced. Thus, the mechanical properties vary from area to area, owing to the non-uniform grain size distribution. Another grain refinement technique is heat treatment using the solid-phase transformation without any hot or cold plastic deformation process. In carbon steels and high-nitrogen stainless steel, grain refinement was successfully achieved without any deformation process by reverse transformation from a lamellar structure to austenite [4], [5]. It may be possible to apply a similar technique to the Co–27Cr–5Mo–0.16N alloy, because it also undergoes a eutectoid transformation (γ → ε + Cr₂N), analogous to that observed in high-nitrogen stainless steels.

This work examines the grain refinement of Co–27Cr–5Mo–0.16N alloy by reverse transformation from a lamellar structure consisting of ε-hcp and Cr₂N to the γ-fcc phase, without any hot or cold deformation processes.

Section snippets

Material and methods

A commercially purchased Co–27Cr–5Mo–0.16N alloy, in the form of a cylindrical bar 30 mm in diameter, was used in this study. The composition by mass% was: 27.2Cr, 5.5Mo, 0.12Ni, 0.04C, 0.16N, with Co making up the balance. A cubic specimen (10 mm on a side) for microstructural observation, and a rectangular bar specimen (7 × 7 × 30 mm) for tensile testing were cut using an electric discharge machine. The microstructures were observed at the center of the cross-sectional area of the cubic specimen.

Microstructure prior to reverse transformation treatment

Fig. 1(a) and (b) shows an image quality map and an XRD pattern, respectively, of the Co–27Cr–5Mo–0.16N alloy after solution treatment at 1473 K for 3.6 ks. The microstructure consisted of equiaxed, uniform γ grains with no precipitates. The average grain size with standard deviation of the specimen after solution treatment was 201 ± 8.1 μm.

Fig. 2 shows (a) an SEM micrograph, (b) an XRD pattern, and (c) and (d) inverse pole figure orientation maps with a boundary map of the Co–27Cr–5Mo–0.16N alloy

Conclusions

An advanced grain refinement technique, based on heat treatment utilizing a reverse transformation, and variation in the tensile properties with various heat treatments of biomedical Co–27Cr–5Mo–0.16N alloy were investigated. The results obtained are summarized as follows:

1.
The γ phase transformed due to a eutectoid reaction during aging at 1073 K for 90 ks, forming a lamellar structure of ε-hcp and Cr₂N phases in the alloy, which was quite similar to the “pearlite” formed in carbon steels and

Acknowledgements

This research was supported by a grant for Cooperation of Innovative Technology and Advanced Research in Evolutional Area from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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Grain refinement of biomedical Co–27Cr–5Mo–0.16N alloy by reverse transformation

Abstract

Introduction

Section snippets

Material and methods

Microstructure prior to reverse transformation treatment

Conclusions

Acknowledgements

Scr Mater

Mat Sci Forum

Metall Trans A

Cobalt-based alloys for biomedical applications