The microstructures of Zr-added Co-29Cr-6Mo alloy compacts fabricated with a metal injection molding (MIM) process and their metal release from the compacts immersed in 1% lactic acid were investigated for medical applications. The relationship between the microstructure and amount of Co released from the compacts is discussed phenomenologically. The relative density of the Co-29Cr-6Mo compacts increased when Zr was added to the powder with amounts of 0.03 and 0.1 mass% and sintered in Ar or N 2 . The amounts of Co released from the compacts containing 0.03 and 0.1 mass% Zr sintered in Ar or N 2 were smaller than those from the other compacts. Therefore, the addition of Zr to the Co29Cr-6Mo powders enhanced the sintering of the compacts and decreased the porosity in the resultant products, leading to the suppression of the Co release from the compacts. When the Zr-added Co-29Cr-6Mo alloy powders were sintered in N 2 , the relative density of the compacts was smaller than that of those sintered in Ar. The powders were nitrided during sintering in N 2 , and the nitrides disturbed the densification during sintering. In addition, a lamellar structure was formed in the Co-29Cr-6Mo and Co-29Cr-6Mo-0.5Zr compacts. The amount of Co released from these compacts was larger than that released from the other compacts because local corrosion occurred at the interface between the different phases in these compacts during immersion in 1% lactic acid. In the MIM process, a small addition of Zr (less than 0.1 mass%) to the Co-29Cr-6Mo alloy is effective for densification during sintering and suppression of the Co release from the compacts.

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