Effects of heat treatment on microstructure and mechanical properties of a cobalt based implant material
Examensarbete för masterexamen
Sandvik Material Technology (SMT) has for a number of years had the medical segment as one of the prioritized areas which has been further emphasized by recent acquisitions of several companies operating in the medical field. The Cobalt based material F75 (Co-28Cr-6Mo) is one of the most materials used for orthopedic implants. The influence on the microstructure and mechanical properties of the material after aging was not fully understood and has been investigated in this study. Co-28Cr-6Mo alloy of ASTM 75 type was cast at Sandvik ABs facility in Sheffield. Three different states were then investigated; as cast, solution heat treaded (SHT) or Hot Isostatic Pressed and solution heat treated (HIP). The solution heat treatment is carried out at 1200°C for 4h. The samples were then aged in a conventional furnace between 600°C and 1300°C up to 4 weeks. The samples were investigated by LOM, SEM, TEM, XRD, DSC and dilatometry. Their mechanical properties were examined by Vickers hardness test, tensile test and Charpy impact toughness test. It was found that the samples consist of a Co-rich matrix with large (10-100μm) carbides spread out in the interdendritic areas. The carbides in the as cast samples have a lamellar structure while the SHTed and HIPed samples have homogenous carbides; the carbides are M23C6 in all samples. After aging small Laves-phase precipitates and small blocky M23C6 carbides are seen in the matrix. The development of the Laves-phase and small M23C6 carbides is strongly time and temperature dependant. At temperatures above 1200°C the carbides start to dissolve. The mechanical properties are all depending on the aging. The hardness is increased for all aging temperature but the behavior differs. The toughness and ductility decrease strongly when aging at 900°C.
Konstruktionsmaterial , Construction materials