Nanoarchitectured Co–Cr–Mo orthopedic implant alloys: Nitrogen-enhanced nanostructural evolution and its effect on phase stability
Introduction
Co–Cr–Mo alloys with exceptional corrosion and wear resistances have been used in orthopedic implants such as artificial hip and knee joints [1], [2], [3], [4]. Although as-cast or hot-forged Co–Cr–Mo alloys are practically used for these applications, their strength and elongation to failure are sometimes low due to a coarse microstructure, solidification defects, precipitations, and so on. Thus, there has been a strong demand to improve the mechanical reliability of biomedical Co–Cr–Mo alloys to obtain orthopedic implants with long lifetimes. The strengthening by carbide precipitation has been commonly employed in the present alloys. Recent research, however, suggests that the hard precipitation phases (e.g. M23C6-type carbides) with sizes from 100 nm to several tens of micrometers are detrimental to wear performance, corrosion resistance and biocompatibility [3], [5], [6]. Therefore, much effort has gone into improving the mechanical properties of the Co–Cr–Mo alloys by alternative approaches such as the alloy design and optimization of the hot deformation process [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].
Biomedical-grade Co–Cr–Mo alloys generally exhibit a phase transformation from a face-centered cubic (fcc) γ matrix to a hexagonal close-packed (hcp) ε martensite during quenching, plastic deformation and isothermal heat treatment [3], [10], [11], [12], [13], [19], [20]. In previous studies, we found that ε martensite has conflicting effects on the mechanical properties. It produces hardening as a result of the strain-induced martensitic transformation that enhances the wear properties of Co–Cr–Mo alloys [3], but ε martensite formation reduces the tensile elongation to failure and the cold workability [10], [11], [12]. These findings suggest that optimization of the martensitic transformation behavior is important for enhancing mechanical performance of the Co–Cr–Mo alloys. Nickel is known to stabilize γ phase by suppressing the martensitic transformation of cobalt and cobalt-based alloys; the Ni-containing alloys exhibit excellent elongation to failure [21]. Large amounts of Ni, which may cause allergies and cancer in living organisms [22], have been incorporated in the biomedical Co–Cr-based alloys when a high deformability is required (designated ASTM F90 and F562) [23], [24].
The addition of nitrogen, which can substitute for Ni, to the Co–Cr–Mo alloys is attractive from both practical and scientific viewpoints [11], [12], [13], [14], [15], [16], [17]. The authors have found that just a small amount of nitrogen can stabilize the γ phase by suppressing the athermal ε martensitic transformation on quenching [11], [12], [13], [14]. We have recently demonstrated that an alloy design that combines hot deformation with nitrogen addition dramatically improves mechanical properties [11], [13]. Fig. 1 shows the relationship between the 0.2% proof stress and tensile elongation of the newly developed N-doped Co–29Cr–6Mo (wt.%)1 alloys processed by hot deformation, which is reproduced from Ref. [11]. Here, the results of conventionally prepared alloy specimens are also plotted in this figure. As shown in Fig. 1, the developed alloys (shown by green symbols) exhibit a superior strength–elongation to failure balance compared with other specimens. Since the hot-forged N-doped alloy has an ultrafine-grained structure [11], the strength of this specimen is extremely high. It should be noted that the alloys without nitrogen addition show a severe loss of elongation to failure with increasing the strength. Thus, nitrogen addition is a promising method for developing the high performance components in artificial joint replacement.
The improvement of mechanical properties of N-doped alloys is due to the N addition eliminating athermal ε martensite plates from the microstructure, which induce premature fracture [11], [12]. It is therefore important to clarify the underlying mechanisms for the enhanced γ phase stability on nitrogen doping. Li et al. [25] have recently performed three-dimensional atom probe tomography (3D-APT), which indicated the presence of short-range ordering (SRO) between Cr and N atoms in the γ matrix of a nitrogen-doped Co–29Cr–6Mo alloy. They speculated that this SRO increases the energy barrier for the γ → ε transition. However, the details have not been clearly elucidated.
It is well accepted that nitrogen in steels forms SRO [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. The SRO due to chemical affinity between alloying elements in steels has been intensively studied by various approaches such as electron structure calculations [26], [27], X-ray absorption fine structure spectroscopy [28], small-angle neutron scattering [29] and 3D-APT [30]. In addition, transmission electron microscopy (TEM) has provided indirect information about SRO by revealing a planar array of dislocations [31], [32]. There are only a few TEM studies that have directly imaged SRO domains in aged microstructures (e.g. Ref. [33]).
One of the most important aspects of the N-containing steels can be characterized by the plastic flow behavior [34], [35], [36], [37]. Flow stress is generally categorized into two components: thermal and athermal stresses (i.e., effective and internal stresses, respectively). The former depends on the temperature and the strain rate, whereas the latter is independent of both these parameters. Previous studies of high-N austenitic steels [34], [35], [36], [37] suggested that SRO enhances both the thermal and athermal components of flow stress.
In an fcc alloy with a low stacking fault energy (SFE), perfect dislocations with Burgers vectors a/2[] tend to dissociate into Shockley partial dislocations bounding stacking faults (SFs) in the {1 1 1}γ plane, in accordance with the following equation:where a is the lattice constant of the fcc phase. In addition, ε martensite develops due to regular overlapping of SFs in every second {1 1 1}γ plane [38]. In other words, glide of Shockley partial dislocations can be considered an elementary step in this martensitic transformation. Thus, an in-depth understanding of SRO is necessary to elucidate not only strength/deformation mechanisms, but also the origin of γ phase stabilization on nitrogen addition. However, no correlations have been found among SRO, dislocation glide, and ultimately the γ → ε martensitic transformation for Co–Cr–Mo alloys.
In the present study, we investigated the nanostructures of hot-forged and subsequently heat-treated Co–29Cr–6Mo–N alloys by TEM for interpreting the origin of nitrogen-associated stabilization of γ phase. Compression tests at room temperature were performed at various strain rates and the relationship between the thermal activation of dislocation glide and nanostructure evolution is discussed.
Section snippets
Specimen preparation
A nitrogen-doped alloy Co–29Cr–6Mo–0.20N (wt.%) that conforms with the ASTM F75 standard was prepared by high-frequency induction melting in an Ar atmosphere. Cr2N powder was used as the nitrogen source. The Co–29Cr–6Mo alloys without N doping and with 0.24 wt.% N were also prepared by casting for comparison. The chemical compositions of the alloys were determined by conventional chemical and gas analyses. The metallic elements were determined using an inductively coupled plasma optical emission
Comparison of N-free and N-doped alloys
We first examined the microstructures of the 0.20N alloy by comparing it with that of its N-free counterpart.
Fig. 2 shows a vertical section of the calculated phase diagram for the Co–29Cr–6Mo–xN (0 ⩽ x ⩽ 0.3) system obtained using Thermo-Calc software. The thermodynamic data sets needed for this calculation were acquired from TCS Steels/Fe-alloys Database Version 6. According to the calculated phase diagram, the fcc γ phase is stable at high temperatures, whereas the hcp ε phase is the equilibrium
Origin of γ phase stabilization by nitrogen addition
As mentioned above, adding nitrogen stabilizes the γ phase of the Co–Cr–Mo alloys. The stability of multiphase systems generally depends on thermodynamic considerations, especially the Gibbs energy differences among the phases. However, in the present case, the calculated phase diagram (Fig. 2) suggests that nitrogen hardly affects the γ–ε equilibrium. In addition, Co–Cr–Mo alloys usually have very low nitrogen contents of less than 1 at.%, which corresponds to about 0.25 wt.% for Co–29Cr–6Mo
Conclusions
Nitrogen addition is a promising strategy for improving the mechanical properties of biomedical Co–Cr–Mo alloys. In this work, we have studied the nitrogen-induced structural evolution in the γ (fcc) matrix phase of biomedical Co–29Cr–6Mo alloys by TEM. The interactions between nanostructures and partial dislocations were examined to determine the origin of the enhanced γ phase stability of N-doped alloys. The major findings of the present study are as follows:
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TEM observations revealed the
Acknowledgements
The authors would like to thank Isamu Yoshii, Kimio Wako and Fumiya Sato for sample preparation, and Shun Ito for TEM observations. This research was financially supported by the Global COE Program “Materials Integration (International Center of Education and Research), Tohoku University” and the Regional Innovation Cluster Program from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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