THE INFLUENCE OF LATTICE-DEFECTS ON THE SOLIDIFICATION PROCESS OF AL-CU ALLOYS

An experimental study of the Al-Cu and Al-Sn systems has been performe d in order to determine the fraction of solid formed after quenching i n a DTA-furnace. The fraction of solid formed after quenching has been calculated using the presently accepted theory for the solidification process and is compared with the experimental results. It is conclude d that the latent heat is not constant and the value found in literatu re is only true for very slow solidification rates. This deviation is assumed to be due to lattice defects formed during the solidification process. The defects formed are assumed to be mono-vacancies. The quan tity of mono-vacancies required to give the fraction of solid found ex perimentally is calculated. It is shown that the free energy of the so lid increased due to the increase of the vacancy concentration during solidification. The change in the free energy decreases the melting po int and increases the partition coefficient between solid and liquid. The homogenisation time required to anneal the supersaturated vacancie s formed, and the critical cooling rate to form this amount are calcul ated. It is concluded that the presently accepted solidification law f or prediction of the fraction formed solid must be modified by introdu cing the formation of lattice defects during the solidification proces s.