A new model is developed to describe the accumulation of amorphousness with increasing ion fluence in heavy ion implanted semiconductors. This is based upon simultaneous direct impact amorphisation and thermal relaxation to simpler disorder which can then be reamorphised. Although a defining equation for the amorphousfraction can be formulated and solved to a tri-exponential decay form neither the coefficients nor fluence exponents of this function can be determined exactly analytically but steady state conditions can be predicted analytically. Approximations show that the form of the amorphous fraction-fluence function and steady state amorphous fractions agree, qualitatively with respect to variations of ion flux and temperature, with experimental results.