Effects of grain size on development of athermal and strain induced ɛ martensite in Co–Cr–Mo implant alloy

Abstract

In the present work, the development of athermal ɛ martensite during quenching of a low carbon Co–Cr–Mo alloy was investigated as a function of the grain size. In addition, a strain induced transformation (SIT) from fcc to hcp was exhibited during compressive plastic straining. It was found that grain size exerts a strong influence on the resultant volume fractions of athermal and strain induced ɛ martensite. In particular, fine grain sizes inhibit the formation of athermal martensite while promoting appreciable volume fractions of ɛ martensite through the SIT mechanism. Moreover, X-ray diffraction analyses indicated that in 10 μm grained structures the volume fraction of strain induced ɛ martensite reaches a saturation level of approximately 0·65 just before compressive fracture. In contrast, increasing grain sizes result in the formation of up to 0·9 volume fraction of SIT martensite. Moreover, the alloy yield strength was found to decrease down to 592 MPa (approximately half the yield strength of the as received alloy). Annealing gave rise to appreciable improvements in the compressive strength (242 MPa) and ductility (0·41) when compared with the as received alloy (2141 MPa and 0·295 respectively). The alloy hardness initially drops from 42 to 29 HRC as the grain sizes increase from 10 to 90 μm. A further reduction in alloy hardness did not occur for grain sizes between 90 and 324 μm. Compression straining did not have a significant effect on the exhibited hardness of the as received alloy and only a moderate effect was found in coarse grained alloys. Alloys with grain sizes of 117 μm exhibited an increase in hardness from roughly 29 to 46 HRC through compression straining up to 0·407. Probable mechanisms are considered to account for the role of grain size on the development of athermal and SIT ɛ martensite.