Abstract
Precursor effects of indium melting have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). MS tests evidenced a sharp drop of dynamic modulus in the temperature range between 418 K and 429 K (melting point). At 429 K, HT-XRD showed partial grain re-orientation, peak profile broadening, in particular in the lower part, and peak shift towards lower angles. Experimental results are consistent with density increase of self-interstitials and vacancies in the crystal lattice before melting. Self-interstitials and vacancies play a synergic role in the solid–liquid (S-L) transformation. The increase of self-interstitials over a temperature range of about 10 K before melting has the effect of weakening interatomic bonds (modulus drop) that favors the successive vacancy formation. Finally, the huge increase of vacancy concentration above 428 K leads to the collapse of crystal lattice (melting).
Highlights
Melting of pure metals has been extensively investigated in the past [1]
A second order polynomial function was chosen because the modulus behavior as temperature increases is certainly affected by two contributions: one due to anharmonicity, which leads to a linear decrease with T, and the other one to free electrons giving a contribution proportional to T2
Precursor effects of indium melting have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD)
Summary
Melting of pure metals has been extensively investigated in the past [1]. Since the discovery of the empirical Lindemann’s criterion [2] stating that melting occurs if the average atomic vibration. Metals 2015, 5 amplitude exceeds a certain value (about 10% of the distance between nearest neighbors), many investigators have tried to find out the microscopic precursor mechanisms leading to crystal melting. Granato [9] derived an interstitial-concentration-dependent free energy, appropriate for calculation of all the thermodynamic properties of crystalline and liquid states of metals. The theory predicts that the shear elastic modulus decreases when the concentration of self-interstitials increases. Gomez et al [11,12] evaluated the percentage of defects near TM and showed that defects form clusters, which are not isolated, but rather linked over long distances near the phase transition Their calculations indicate that the linked clusters are dislocations and melting is consistent with the saturation of the crystal by dislocation loops. HT-XRD permitted to correlate MS data with the structural features of solid and liquid phases as temperature increases up to TM and above
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