Microstructural Changes during Isothermal Forging of a Co-Cr-Mo Alloy
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I.
INTRODUCTION
COBALT base (Co-Cr-Mo) alloys are widely used as surgical implant materials, l They are used normally in the as-cast condition and as such suffer from wide variability in mechanical properties and generally low mechanical integrity. This is because of random casting defects such as shrinkage cavities and a coarse nonuniform grain size which tend to reduce their tensile and fatigue properties. It is well recognized that these shortcomings often lead to the premature failure of implants manufactured from cast cobalt alloys. 2 The elimination of casting defects by thermo-mechanical treatments provides a means of improving the mechanical properties of these materials. 3 For instance, major improvements are obtained by hot isostatic pressing cast alloys. 4 This closes and seals the shrinkage cavities. Even greater improvements are obtained by mechanical working of ingots, which breaks down the as-cast structure and refines the grain size while also eliminating the shrinkage cavities. 5 Also, recently it was shown that the corrosion fatigue characteristics of Co-Cr-Mo alloys can be substantially improved by powder metallurgy processing, using hot isostatic pressing to compact prealloyed powders. ~.5.6 In any serious attempt to develop further these improvements, one should develop a clear understanding of the changes in microstructure which occur in these alloys during thermomechanical processing. Additional improvements may be obtained, for instance, by using controlled strain rate isothermal forging, a new technique developed recently for the processing of hard-to-work materials. 8 In this type of forging the dies are kept at the forging temperature. This eliminates die chill and allows microstructures in forgings to be controlled more effectively. It might be used to advantage for implant processing, and the work reported here was initiated to examine this possibility more closely. The purpose of the paper is to outline the basic microstructural changes occurring during controlled strain rate J-P. IMMARIGEON, KRISHNA RAJAN, and W. WALLACE are all Research Officers, National Aeronautical Establishment, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada. Manuscript submitted May 2, 1983. METALLURGICALTRANSACTIONSA
isothermal forging. With this information, the limitations imposed by the material and the processing route on the practical feasibility of fabricating wrought Co-Cr-Mo alloy implants from as-cast ingots are discussed. Finally, suggestions are made for further developments in the processing of surgical implant alloys.
II.
EXPERIMENTAL DETAILS
The forging behavior of the cast alloy was examined by means of uniaxial compression testing, using a specially modified MTS hydraulic testing machine. 9 The compression samples were in the form of small cylindrical billets, 10 mm high and 7 mm in diameter. The tests were carried out in an inert argon atmosphere between flat TZM dies heated by induction. Both the temperature and strain rate were kept constant during compression. A h
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