RELATIONSHIP BETWEEN MICROSTRUCTURE AND DUCTILITY OF INVESTMENT CAST ASTM F-75 IMPLANT ALLOY

Hip replacement implants fabricated using the ASTM F-75 alloy sometime s fail in a sudden catastrophic way. In general, fractures start at mi crostructural defects subjected to stress-corrosion under chemical att ack by body fluids. Ln this paper the results of a study on the effect of casting parameters on the microstructure of ASTM F-75 are presente d. The preheating mold temperature and the liquid temperature were var ied between 900 and 1000 degrees C, and 1410 and 1470 degrees C, respe ctively. Optimum static strength and ductility were obtained when shri nkage microporosity and the volume fraction of M(23)C(6) ''eutectic'' carbides precipitated at grain boundaries were minimized by increasing the preheating mold temperature to 1000 degrees C and by using interm ediate pouring temperatures of 1455 degrees C. Under these casting con ditions, however, the solidification rates are low, leading to large g rain sizes, which, in turn, reduce the strength of the material under dynamic loading conditions. The volume fraction of the M(23)C(6) ''blo cky'' carbides appears to be independent of the casting conditions; ho wever, their size and spatial distributions determine the strength of the as-cast alloys. (C) 1997 John Wiley & Sons, Inc.