Experimental evidence for thermal generation of interstitials in a metallic crystal near the melting temperature

The only intrinsic point defects of simple crystalline metals known from solid state physics are vacancies and interstitials. It is widely believed that while vacancies play a major role in crystal properties and their concentration reaches relatively big values near the melting temperature T-m, interstitials essentially do not occur in thermodynamic equilibrium and their influence on properties is minor. Here, taking aluminum single crystals as an example, we present compelling experimental evidence for rapid thermoactivated growth of interstitial concentration upon approaching T-m. Using high precision measurements of the shear modulus we found a diaelastic effect of up to -1.5% near T-m. It is argued that this effect is mostly due to the generation of dumbbell (split) interstitials. The interstitial concentration c(i) rapidly increases upon approaching T-m and becomes only 2-3 times smaller than that of vacancies just below T-m. The reason for this c(i)-increase is conditioned by a decrease of the Gibbs free energy with temperature, which in turn originates from the high formation entropy of dumbbell interstitials and a decrease of their formation enthalpy at high c(i). Special molecular dynamic simulation confirmed all basic aspects of the proposed interpretation. The results obtained (i) demonstrate the significance of interstitial concentration near T-m that could lead to the revaluation of vacancy concentration at high temperatures, (ii) suggest that dumbbell interstitials play a major role in the melting mechanism of monatomic metallic crystals and (iii) support a new avenue for in-depth understanding of glassy metals.