In particular, die ridges formed on the discharge craters determine the MRR of the material. Figure 6 lists the results concerning the rapidly resolidified layer following EDM under a specified EDM condition, pulse duration: 350. Figures 6(a)-(d) display the EDMed surfaces of specimens with the hypoeutectic composition in which die microstructure contains acicular eutectic silicon particles. Figures 6(e) - (h) display the EDMed surfaces of specimens in which the microstructure contains rod-like and massive primary silicon particles.