Austempered Ductile Iron (ADI) has been established as an advanced material because of its excellent mechanical properties based on a good combination of wear resistance, toughness and very high strength combined with a relatively high ductility. It has a unique acicular matrix structure that consists of high-carbon austenite and ferrite with graphite nodules dispersed in it [
]. Some previous studies [
] have been carried out on the mechanical properties of alloyed ADI. However, very little information is available regarding the influence of microstructure on the tensile properties and fracture toughness of alloyed ADI. The present investigation was undertaken to examine the influence of microstructure on the tensile properties and the plane strain fracture toughness (K
) of copper and copper nickel alloyed austempered ductile iron (ADI). Two ductile irons with chemical compositions (in wt.%): (a) 3.6C; 2.5Si; 0.28Mn; 0.04Cr; 0.45Cu; 0.01P; 0.014S; 0.066Mg, and (b) 3.07C; 2.15.Si; 0.26Mn; 0.04Cr; 1.6Cu; 1.5Ni; in both alloys balance was Fe, were produced in a commercial electro-induction foundry furnace. Specimens for mechanical testing were machined from the test section of the Y-block. Machined specimens austenitized in a protective argon atmosphere at 900°C for 2h were rapidly transferred to a salt bath at austempering temperature 350°C, held between 1 and 6h and then air-cooled to room temperature. The reported results represent the average values of three measurements. Fracture surfaces of specimens after tensile and fracture toughness tests were examined by a JOEL JSM-6460LV scanning electron microscope (SEM) operated at 25kV. Metallographic investigations were carried out by light microscope. Change in the volume fraction of retained austenite during austempering was determined by the X-ray diffraction technique on diffractometer “Siemens D-500” with Ni filtered CuK radiation.