Initiation of melting at tilt grain boundaries in austenite depending on the misorientation angle

Using molecular dynamics simulation, the authors studied the influence of misorientation angle and energy of tilt grain boundaries with the misorientation axes \(\left\langle {100} \right\rangle\), \(\left\langle {110} \right\rangle\) and \(\left\langle {111} \right\rangle\) on the melting temperature and nature of early initiation of melting at grain boundaries in austenite. It is shown that with gradual heating, melting begins from the grain boundaries, where there is a violation of the crystal structure and, accordingly, the atoms are located in less deep potential wells. In the case of large‒angle boundaries, melting begins simultaneously along the entire boundary, in the case of small-angle boundaries – in the cores of grain-boundary dislocations. Dependences of the melting temperature of the simulated calculation cells on the angle of grain misorientation and excess energy were obtained. For the misorientation axes \(\left\langle {100} \right\rangle\), \(\left\langle {110} \right\rangle\) and \(\left\langle {111} \right\rangle\), the results were similar. In the region of small misorientation angles (less than 15°), the melting point decreases almost linearly with increasing angle, then, for large-angle boundaries, the decrease becomes less intense. These dependences correlate with the energy of grain boundary formation or with the associated excess energy of the calculation cell. The main quantitative criterion determining the effect of defects on a decrease in melting temperature is excess energy, that is, the energy difference between the considered structure and the ideal crystal, which can also be interpreted as the energy of the consi­dered structure formation. The melting point decreases linearly with increasing excess energy. Obviously, the effect of grain boundaries on the melting point becomes significant only for materials with a very high content of grain boundaries, for example, for materials with a nanocrystalline structure.