May 25, 2007 - 1523 K for 7.2 ks was carried out for cast Co-29Cr-6Mo alloy, followed by various aging treatments at 1023 K for up to 21.6 ks. The area fraction.
Materials Transactions, Vol. 48, No. 6 (2007) pp. 1517 to 1522 #2007 The Japan Institute of Metals
Microstructure and Mechanical Properties of Co-29Cr-6Mo Alloy Aged at 1023 K Shingo Kurosu1; * , Naoyuki Nomura2 and Akihiko Chiba3 1
Department of Material Science, Graduate School of Engineering, Iwate University, Morioka 020-8551, Japan Department of Welfare Engineering, Faculty of Engineering, Iwate University, Morioka 020-8551, Japan 3 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 2
Mechanical properties of Co-29Cr-6Mo alloys consisting of " phase and phase were examined at room temperature. Solution treatment at 1523 K for 7.2 ks was carried out for cast Co-29Cr-6Mo alloy, followed by various aging treatments at 1023 K for up to 21.6 ks. The area fraction of the phase in the aged alloys increases with the aging time at 1023 K and reaches 0.6% after aging at 1023 K for 21.6 ks. However, mechanical properties such as 0.2% proof strength, ultimate tensile strength and plastic elongation of the aged alloys do not depend on the aging time. It is found that the phase less than 0.6% does not cause the deterioration of mechanical properties of the aged Co-29Cr-6Mo alloy. [doi:10.2320/matertrans.MRA2007606] (Received March 15, 2007; Accepted March 19, 2007; Published May 25, 2007) Keywords: cobalt-chromium-molybdenum alloy, biomaterial, mechanical property, sigma phase (), aging treatment
1.
Introduction
Cobalt chromium molybdenum (Co-Cr-Mo) alloys have been widely used for surgical prostheses such as hip and knee joint replacement due to their high mechanical properties, corrosion resistance and wear resistance. In as-cast condition, the alloys consist of phase (F.C.C.) and " phase (H.C.P.) formed by martensitic transformation from to " during cooling. As far as Co-Cr-Mo alloys are concerned, properties of the alloys significantly depend on the ratio of these phases. Elongation of Co-29Cr-6Mo alloy increases with increasing the phase fraction.1–3) On the other hand, the quantity of released metal ion tends to decrease with increasing the " phase fraction.4) In addition, wear resistance of Co-Cr-Mo alloys consisting of the " phase are excellent under metal-onmetal condition.5) From the above properties, it can be summarized that the alloy consisting of the phase is favorable for plastic forming and the alloy consisting of the " phase is promising for the application used in corrosion or wear circumstance. Although the effect of heat treatment and carbon addition on the stability of the phase have been reported so far,3) the stability of the " phase has not been investigated in detail yet. At the composition of ASTM-F75(Co-(2730)Cr(57)Mo(mass%)), only the phase is stable at 1473 K, although the " phase and the phase coexist at 1200 K.6) Therefore, the phase inevitably precipitates when the " phase is required as the matrix at the composition. The phase in Co-Cr-Mo alloys has the same crystal structure (P42/mnm) as that appeared in Fe-Cr alloy system. In the ferritic stainless steels (Fe-29Cr-4Mo),7) the phase precipitates in the grain boundaries during aging treatment at 1123 K in the early stage. The Fe-29Cr-4Mo aged for 3.6 ks with the phase fraction of only 0.1% exhibited degradation of resistance to localized corrosion and significant ductility loss of 66%. Therefore the phase is supposed to affect corrosion and mechanical properties of Co-Cr-Mo alloys as well. *Graduate
Student, Iwate University
In previous study, we have investigated corrosion behavior of a Co-29Cr-6Mo alloy consisting of the " phase and the phase in saline.6) The phase mainly precipitates in grain boundaries. In addition, the size is less than 1 mm and the shape is globosity.6) The area fraction of the phase increases with increasing aging time at 1023 K and reaches 0.6% after the aging time of 21.6 ks. Corrosion resistance in saline such as breakdown potential, passive current density and the quantity of released Co ion of the aged Co-29Cr-6Mo alloys does not depend on the area fraction of the phase. Therefore, the effect of phase on the mechanical properties of Co-29Cr-6Mo should be investigated. The objective of this study is to investigate microstructure and mechanical properties of Co-29Cr-6Mo alloy after aging treatment at 1023 K for various times. 2.
Experimental Procedure
2.1 Alloys The vacuum induction melting technique was employed to prepare Co-29Cr-6Mo (mass%) alloy. The chemical composition of investigated Co-29Cr-6Mo alloy was listed in Table 1. Each specimen was cut out by using electrical discharge cutting machine (EDM). The solution heat treatment was carried out at 1523 K for 7.2 ks. In our preliminary experiments, microstructure of the Co-Cr-Mo alloys was affected by cooling rate from 1523 to 1023 K. Thus the cooling rate from 1523 to 1023 K was settled at 0.166 K/s in the present study. The aging treatments were continuously conducted at 1023 K for 0, 1.8, 3.6, 7.2, 14.4 and 21.6 ks, followed by water quenching. Hereafter, these aged specimens are designated as 0, 1.8, 3.6, 7.2, 14.4 and 21.6 ks, respectively.
Table 1
Chemical composition of investigated Co-Cr-Mo alloy.
[mass%]
[mass ppm]
Co
Cr
Mo
Ni
C
N
0
Bal.
28.68
6.01
1.06
