Abstract
In order to study the mechanism of liquation cracking in cast alloy 718, a mathematical model was developed to calculate grain boundary liquation in weld thermal cycle. According to microscopic observation of specimen heated at high tempera-tures, the laves cluster has started to melt at about 1383 K. Thus, it was considered on modeling that the grain boundaries were liquated by the melting of laves cluster which were located at the center of the grain boundaries. In this model, two factors were considered for solute diffusion in solid and liquid phase, namely, (1) high diffusivity at the grain boundary region, (2) the balance between the grain boundary energy and solid/liquid interfacial energy. Calculation has shown that the experimental values for the amount of grain boundary liquation fairly agree with the calculated results where the grain boundary diffusivity is to be the ten times value of the bulk solid diffusivity. The calculation has also indicated that the liquid strongly infiltrates along grain boundary with increasing the heating temperature, and with decreasing the heating rate. In addition, the amount of grain boundary liquation increases, as the grain size and laves cluster increases. On the basis of these calculations, it has been elucidated that the liquation cracking in cast alloy 718 strongly depended upon the factors, such as weld thermal cycle, grain size and the amount of laves cluster, etc..
