Effects of Hot-Forging Reduction Ratio on Microstructure, Mechanical Properties and Dry Friction Wear Behavior of Co-29Cr-6Mo Alloy Without Ni and C Addition for Biomedical Applications

摘要

In order to examine the influence of variation in grain size and microstructure on mechanical properties of Co-29Cr-6Mo alloy for biomedical implant materials, the tensile properties and the dry friction wear characteristics of the forged Co-29Cr-6Mo alloy without Ni and C addition were investigated. The microstructure of as-forged alloys, consisting of fee phase mixed with athermal hcp martensite, is found to become finer as the increasing of forging ratio. The hot-forging process for fabricating the fine microstructure with different grain sizes of 43, 11 and 3 um has been successfully established under the condition that forging temperature of higher than 1273 K is strictly kept during forging. The tensile properties, such as the yield stress, the tensile stress and the elongation, are improved with decreasing the grain size and thereby increasing the volume fraction of the fee phase in the microstructure. Worn surfaces are hardened during the wear tests, forming oxide films. This results from significantly high work hardening rate of the forged alloy, caused by the strain-induced martensitic transformation from an fcc-gamma phase to an hcp-epsilon phase, which contributes to the improvement in the dry wear resistance. Wear mechanisms of the forged alloy are discussed on observations of the wear scars formed on the alloy disc and the alumina ball surfaces. It is suggested that a delamination wear resulting from the fatigue fracture likely occurs under the present dry wear condition.