Effect of Cu on Martensite Transformation of CoCrMo alloy for biomedical application

The martensite transformation of Co-29Cr-6Mo-xCu(Co-xCu,x=0,1.5,2,4,wt%)alloys was investigated in order to explore ductile Co-based alloy without Ni for biomedical application.The effect of Cu on martensite transformation of Co-29 Cr-6 Mo alloy was investigated using scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-Ray diffraction(XRD)and electron backscatter diffraction(EBSD)analysis.EBSD results revealed that the fraction ofγ-phase was increasing with increasing of copper content.In detail,the fraction ofγ-phase of aged Co-1.5 Cu alloy was 4.72%and noγ-phase was found in aged Co-0Cu alloy.Furthermore,the fraction ofγ-phase of aged Co-2Cu alloy and Co-4Cu alloy was 51.68%and 46.17%,respectively.This demonstrated that the minimum Cu content which could inhibit martensite transformation in this alloy system was 2 wt%.Calculated phase diagram revealed that increasing the copper content of this alloy system could enlargeγ-phase zone,which means Cu could inhibit the formation ofγ-phase and consequently stabilizeγ-phase.Randomly distributed Cu-rich phase was identified in aged Co-4Cu alloy.The possible reason for the inhibition of martensite transformation by Cu addition is that alloying with Cu atom might decrease charge accumulations and increase atomic bonds,which increase the stacking fault energy ultimately.