Heavy ion collisions are one of tools for studying nuclear system far away from itsequilibrium state. This work concerns the most violent collisions in the Ni + Au system for incidentenergies ranging from 32 up to 90 AMeV. These events were detected with the multidetector INDRAand selected by the Principal Component Analysis (multidimensional analysis). This method classifiesthe events according their detection features and their degree of dissipation. We observed twodesexcitation mechanisms: a fusion/fission - evaporation process and a multifragmentation process.Those two coexist from 32 to 52 AMeV whereas only one subsists at 90 AMeV. For those twomechanisms, an component was observed which seems to be linked to the initial phase of the reaction.The energy fluctuations of this component leads to variations in the energy deposit which determinesthe desexcitation of the system.The experimental multifragmentation data of the Ni + Au system (52 and 90 AMeV) were compared tothe predictions of a statistical model and to the experimental data of the system Xe +Sn at 50 AMeV(also detected with INDRA). These comparisons show the lack of collective radial energy forfragments (Z>10) in the Ni + Au system, and show that the degree of multifragmentation depends ofthe thermal excitation energy. Mean kinetic energies of particles and lights fragments (Z10) pour le système Ni + Au, etont montré que le degré de fragmentation dépend principalement de l'énergie d'excitation thermique.Les énergies cinétiques moyennes des particules et des fragments légers (Z<10) sont plus élevées pourle système Ni + Au que pour le système Xe + Sn. Cette observation montre que ces espèces sont plussensibles à la voie d'entrée dans un système asymétrique que dans un système symétrique (pour lemême nombre total de nucléons)
