The bulk of metallic bone repair or replacement parts is currently constructed of either wrought Type 316L stainless steel or cast cobalt-chromium-molybdenum-carbon alloys.1 The latter are typified by Haynes Stellite(1) alloy No. 21 (28 Cr, 5.5 Mo, 2.5 Ni, Bal Co) but more commonly referred to as Vitallium(2) Zimalloy(3) etc. The stainless components are known to suffer from localized corrosion problems such as pitting and crevice attack.2 These problems are, for the most part, eliminated by using the cast cobalt-base alloys, however some mechanical properties are sacrificed. Recently, it has been shown that if the cobalt-base composition corresponding to Haynes Stellite alloy No. 21 is made in a wrought form, the corrosion resistance is significantly improved and the mechanical properties enhanced.3
The more limited mechanical properties of the cast parts are a result of microporosity and segregation patterns that form during solidification. These same...
