An analysis of cascade and subcascade formation in fcc metals by D-T fusion neutron irradiation is made by fitting the experimentally observed distribution of cascade zone size and number of subcascades to the calculated primary recoil energy spectrum. The analysis is made by categorizing subcascades into closely space (Ag, Au) and widely separated (Cu, Ni). The energy subdivided in subcascades is estimated. The estimated energy density in a subcascade is as high as several tens of eV/atom, which is high enough to raise the local temperature far beyond the melting temperature. A discussion is made on the reactions during cascade cooling, suggesting the inadequacy of the conventional radiation damage parameter of DPA. The existence of impact effects from other cascades to reveal the cascade collision induced invisible vacancies as visible vacancy clusters is concluded. A kinetics type of analysis is made on the microstructure evolution by the impact effect, and the sphere of influence of the impact is estimated for each material.