nan Alfirano

Precipitates in Biomedical Co-28Cr-6Mo-(0–0.41)C Alloys Heat-Treated at 1473 K to 1623 K (1200 °C to 1350 °C)

The phase and formation/dissolution of biomedical Co-28Cr-6Mo-xC (x = 0, 0.12, 0.16, 0.24, 0.33, and 0.41, in mass pct) alloys were investigated before and after heat treatment at high temperatures. The heat-treatment temperatures and holding periods were varied from 1473xa0K to 1623xa0K (1200xa0°C to 1350xa0°C) and from 0xa0ks to 43.2xa0ks, respectively. The intermetallic σ phase was observed in the as-cast and heat-treated alloys without added carbon. In the carbon-added alloys, the M23C6-type, π-phase (M2T3X-type carbide with a β-Mn structure), η-phase (M6C-M12C-type), and M7C3-type carbides were observed depending on the carbon content and heat-treatment conditions. A complete precipitate dissolution was achieved in the alloys with a carbon content of 0 to 0.24xa0massxa0pct. The holding time required for complete precipitate dissolution in the carbon-added alloys increased with increasing heat-treatment temperature above 1548xa0K (1275xa0°C). The formation of π-phase and M7C3-type carbides was observed in the carbon-added alloys during heat treatment at high temperatures of 1548xa0K to 1623xa0K (1275xa0°C to 1350xa0°C). Two types of π phase with different lattice constants and chemical compositions were confirmed in the alloys with carbon contents of 0.24, 0.33, and 0.41xa0massxa0pct after heat treatment at 1573xa0K to 1623xa0K (1300xa0°C to 1350xa0°C). The M7C3-type carbide observed at high temperatures occurred in a starlike precipitate with a complicated microstructure.

Precipitates in Biomedical Co-Cr-Mo-C-Si-Mn Alloys

The phase and dissolution behavior of precipitates in biomedical ASTM F75 Co-Cr-Mo-C-Si-Mn alloys were investigated. Alloys of five different compositions, Co-28Cr-6Mo-0.25C-1Si, Co-28Cr-6Mo-0.25C-1Mn, Co-28Cr-6Mo-0.25C-1Si-1Mn, Co-28Cr-6Mo-0.15C-1Si, and Co-28Cr-6Mo-0.35C-1Si, were heat-treated from 1448 to 1548 K. The precipitates observed in the as-cast and heat-treated alloys were carbides (M23C6 type, h-phase, and p-phase) and an intermetallic compound (c-phase). The main precipitates observed after heat treatment at high temperatures such as 1548 K were p-phase and M23C6 type carbide. At these high temperatures, two types of starlike precipitates—dense and stripe-patterned—were observed. The starlike-dense precipitate was the p-phase, and the starlike precipitate with a stripe pattern was identified as the M23C6 type carbide and metallic fcc g-phase. In the alloys heat-treated at 1448 to 1498 K, blocky-dense M23C6 type carbide was primarily observed. c-phase was detected in the Co-28Cr-6Mo-0.15C-1Si alloy under as-cast condition and after heat treatment at 1448–1523 K for a short holding time. The addition of Si seemed to increase the holding time for complete precipitate dissolution because of the effects of Si on the promotion of p-phase formation at high temperatures and the increased carbon activity in the metallic matrix.

Phase and Formation/Dissolution of Precipitates in Biomedical Co-Cr-Mo Alloys with Nitrogen Addition

The precipitates in as-cast and heat-treated biomedical Co-28Cr-6Mo-(0 to 0.35)C-(0.15 to 0.25)N alloys (mass pct) were investigated. Heat treatment was carried out at temperatures of 1473xa0K to 1573xa0K (1200xa0°C to 1300xa0°C) for holding periods of 0 to 43.2xa0ks. In the as-cast and heat-treated Co-Cr-Mo-N alloys, no precipitates were detected; nitrogen effectively inhibited the formation of the σ-phase and stabilized the face-centered cubic (fcc) metallic γ-phase. The precipitates observed in the as-cast and heat-treated Co-Cr-Mo-C-N alloys were of the M23X6 type, M2X type, π-phase (M2T3X type with a β-Mn structure), and η-phase (M6X-M12X type). Complete precipitate dissolution was detected in the alloys with carbon contents of less than 0.3 mass pct regardless of the nitrogen content. The main precipitates were of the M2X and M23X6 types after heat treatment for 1.8 to 43.2xa0ks. The π-phase precipitate was detected in the early stage of heat treatment at high temperatures. The formation of the M2X-type precipitate was enhanced by the addition of nitrogen, although the constitution of the precipitates depended on the balance between the nitrogen and carbon contents and the heat-treatment conditions.

Precipitates in Biomedical Co-Cr-Mo-C-N-Si-Mn Alloys

The microstructures of biomedical ASTM F 75/F 799 Co-28Cr-6Mo-0.25C-0.175N-(0 to 1)Si-(0 to 1)Mo alloys (mass pct) were investigated before and after heat treatment, with special attention paid to the effect of nitrogen on the phases and the dissolution of precipitates. The heat treatment temperatures and holding periods employed ranged from 1448 to 1548xa0K (1175 to 1275xa0°C) and 0 to 43.2xa0ks, respectively. A blocky-dense π-phase precipitate and a lamellar cellular colony, which consisted of an M2X type precipitate and a γ phase, were mainly detected in the as-cast alloys with and without added Si, respectively. The addition of nitrogen caused cellular precipitation, while the addition of Si suppressed it and enhanced the formation of the π phase. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses suggested that a discontinuous reaction, i.e., γ1xa0→xa0γ2xa0+xa0M2X, might be a possible formation mechanism for the lamellar cellular colony. Nitrogen was enriched in the M2X type, η-phase, and π-phase precipitates, but was excluded from the M23X6 type precipitate. Complete precipitate dissolution was observed in all of the alloys under varied heat treatment conditions depending on the alloy composition. The addition of nitrogen decreased the time required for complete precipitate dissolution at low heat-treatment temperatures. At high temperatures, i.e., 1548xa0K (1275xa0°C), complete precipitate dissolution was delayed by the partial melting that accompanied the formation of the precipitates such as the π phase resulting in the boundary between the complete and incomplete precipitate dissolution regions in having a C-curved shape.

Co-Cr Alloys as Effective Metallic Biomaterials

Because of their excellent mechanical properties, high corrosion resistance, and high wear resistance, Co-Cr alloys have been recognized as effective metallic biomaterials and have been used as materials for dental and medical devices since a cast Co-Cr-Mo alloy, Vitallium, was developed in the 1930s. Further increases in the usage of Co-Cr alloys are still expected as well. In this chapter, first, the history and current status of biomedical Co-Cr alloys such as Co-28Cr-6Mo and Co-20Cr-15W-10Ni alloys are reviewed. Their microstructure, processing, and properties are then discussed. Control of the microstructure by optimization of chemical composition of the alloys and thermomechanical treatments is described, and newly developed processing techniques for grain refinement and newly found precipitates such as the π-phase and χ-phase are discussed. As a novel process for implant fabrication, an additive manufacturing technique using an electron beam and a laser beam is mentioned. Finally, the mechanical properties and corrosion and wear resistances of the alloys are presented, and the relationships between the microstructure and properties of the Co-Cr alloys are discussed.

Formation of the χ -Phase Precipitate in Co-28Cr-6Mo Alloys with Additional Si and C

The precipitates of biomedical Co-28Cr-6Mo-(0.5 to 2)Si-(0.05 to 0.35)C alloys (massxa0pct) have been investigated before and after heat treatment, focusing on the formation of the χ-phase precipitate. The precipitates were precisely and directly analyzed after using electrolytic extraction to separate the precipitates from the alloys. Heat treatment was performed at 1523xa0K (1250xa0°C) for a holding time of 0.6xa0ks. The χ-phase precipitate was detected with Si content of 1.3 to 2xa0massxa0pct and C content of 0.05 to 0.15xa0massxa0pct in both the as-cast and heat-treated alloys. The higher Si content and the mid-level C content of about 0.15xa0massxa0pct favored the formation of the χ-phase precipitate. Moreover, the χ-phase precipitate was not observed in the compositional range of the ASTM F 75xa0standard: Si content ≤1.0xa0massxa0pct and C content ≤0.35xa0massxa0pct. In the as-cast Co-28Cr-6Mo-1.3Si-0.15C alloy, which is outside of the ASTM F 75xa0standard, the content of the χ-phase precipitate was around 6xa0pct in area percent; the decrease in the ductility was detected in the tensile test of this alloy. The amount of precipitates decreased due to heat treatment at 1523xa0K (1250xa0°C), where the dissolution of precipitates occurred. After the heat treatment, a single χ-phase precipitate region was detected.

π-Phase and χ-Phase: New Precipitates in Biomedical Co–Cr–Mo Alloys

The phases of precipitates in ASTM F 75 and F 799 Co–Cr–Mo alloys are reviewed with a focus on new precipitates of the π- and χ-phases. The π-phase carbide was observed at temperatures of 1,548–1,623 K where partial melting in Co–Cr–Mo–C system alloys occurred. Si was slightly enriched in the π-phase precipitate and nitrogen substituted carbon in the octahedral sites of the π-phase. It is suggested that the formation and stability in the partial melting region and the transformation of the π-phase take part in the final constitution of the precipitates in ASTM F 75 and F 799 alloys. A χ-phase has been detected in the as-cast and heat-treated Co–Cr–Mo alloy with a midlevel carbon content and the addition of Si. The χ-phase precipitate is an intermetallic compound. The conditions under which the χ-phase is formed are limited by the heat-treatment temperature and chemical composition.

Phase and Morphology of Carbides in ASTM F75 Co-Cr-Mo-C Alloys Formed at 1473 to 1623 K

The phase and morphology of precipitates in heat-treated Co-28Cr-6Mo-xC (x = 0.12, 0.15, 0.25, and 0.35mass%) alloys were investigated. The as-cast alloys were solution-treated in the temperature range of 1473 to 1623 K for 0 to 43.2 ks. Complete precipitate dissolution was observed in all four alloys, each of which had different carbon contents. The holding time for complete dissolution was greater for alloys with greater carbon content. The curve representing the boundary between the complete- and incomplete-dissolution conditions for each alloy is C shaped. Under the incomplete precipitate dissolution conditions of the Co-28Cr-6Mo-0.25C alloy, an M23C6 type carbide and a π-phase (M2T3X type carbide with β-Mn structure) were observed at 1548 to 1623 K, and starlike precipitates with a stripe pattern and with a dense appearance were both observed; the former comprised the M23C6 type carbide + γ-phase, and the latter was the π-phase. In contrast, only a blocky-dense M23C6 type carbide was observed at 1473 to 1523 K.

Heat Treatment of ASTM F75 Co-Cr-Mo-C-Si-Mn Alloys

In this study, the carbide dissolution and formation of biomedical Co-28Cr-6Mo-0.25C-1Si (1Si) and Co-28Cr-6Mo-0.25C-1Mn (1Mn) alloys occurring during solution treatment and aging were investigated. The addition of Si or Mn markedly affected the behavior of precipitates during heat treatment. The alloy with added Si required a longer solution treatment time for complete precipitate dissolution as compared to the alloy with added Mn. Blocky M23C6-type carbide was observed during solution treatment at 1448–1523 K. Starlike precipitates were observed in the 1Si and 1Mn alloys at 1523–1548 K and 1523 K, respectively. The starlike precipitates exhibited two types of morphologies: dense and stripe patterned. The starlike precipitates with dense and stripe-patterned appearances were a uf070-phase and M23C6¬-type carbide + metallic uf067-phase, respectively. An M23C6-type carbide and an uf068-phase were formed during the aging of both the 1Si and the 1Mn alloys. In addition, an intermetallic uf073-phase was detected during the aging of the 1Si alloy at 1373 K.