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

In the present work, the development of athermal epsilon martensite during quenching of a low carbon Ca-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 tha t grain size exerts a strong influence on the resultant volume fractions of athermal and strain induced epsilon martensite. In particular fine grain s izes inhibit the formation of athermal martensite while promoting appreciab le volume fractions of epsilon martensite through the SIT mechanism. Moreov er, X-ray diffraction analyses indicated that in 10 mu m grained structures the volume fraction of strain induced epsilon martensite reaches a saturat ion level of approximately 0.65 just before compressive fracture. In contra st. increasing grain sizes result in the formation of up to 0.9 volume frac tion of SIT martensite Moreover the alloy yield strength was found to decre ase down to 592 MPa (approximately half the yield strength of the as receiv ed alloy). Annealing gave rise to appreciable improvements in the compressi ve strength (242 MPa) and ductility (0.41) when compared with the as receiv ed alloy (2141 MPa and 0.295 respectively). The alloy hardness initially dr ops from 42 to 29 HRC as the grain sizes increase from 10 to 90 mu m. A fur ther reduction in allay hardness did not occur for grain sizes between 90 a nd 324 mu m. Compression straining did not have a significant effect on the exhibited hardness of the as received alloy and only a model ate effect wa s found in coarse grained alloys. Alloys with grain sizes of 117 mu m exhib ited an increase in hardness from roughly 29 to 46 HRC through compression straining up to 0.407. Probable mechanisms are considered to account for th e role of grain size an the development of athermal and SIT epsilon martens ite.