In an elastic collision between two billiard balls. The variation of elastic potential energy is represented by the graph (here r is the distance between centres of the balls and R is radius of the ball)

During collision balls are deformed and elastic potential energy stored in the balls also gradually increases when deformation is maximum, elastic potential energy also becomes maximum. After that, the balls start regaining their original shape and size and potential energy stored in the balls gradually decreases and finally becomes zero.