Mechanism of grain boundary angle on solidification cracking in directed energy deposition Hastelloy X superalloys

Abstract

Solidification cracking occurs only when the grain boundary (GB) angle ($\theta$) exceeds a critical value. This value, known as the critical cracked GB angle (${\theta }^{*}$), can be predicted from the grain coalescence theory based on GB-angle-dependent GB energy. However, the calculated value (${\theta }_c^{*}$) is always less than the measured value in experiments (${\theta }_e^{*}$), which is also confirmed in our directed energy deposition Hastelloy X superalloys. In addition to GB energy, there are evidences showing that GB angle can affect cracking by changing dendrite spacings. We show by experiments and phase field simulations that, same as GB energy, the dendrite spacings at GBs increase with GB angle, but its effect on solidification cracking sensitivity (SCS) is opposite to GB energy. Depending on their relative contributions, three ranges can be identified. In the first range of $\theta <{\theta }_c^{*}$, both dendrite spacings and GB energy have negligible effects on dendrite coalescence, compared to the case inside a grain. In the second range of ${\theta }_c^{*}<\theta <{\theta }_e^{*}$, dendrite spacings counteract the effect of high GB energy on SCS. It is exactly this effect that induces the gap in ${\theta }^{*}$ between theory and experiments. In the third range of $\theta >{\theta }_e^{*}$, GB energy plays a dominant role and leads to severe solidification cracking. After including the effect of dendrite spacings on SCS, we predict ${\theta }^{*}$=15° in directed energy deposition Hastelloy X superalloys, close to the experimental value of 18°. These new findings provide new insights for suppressing cracking by controlling the dendrite spacings near GBs.