Solidification behavior of Sn crystals - Under different temperature gradients

Lead-free solder alloys typically comprise beta-Sn, known for its anisotropy. Our experiments revealed that under standard solidification conditions (Room temperature cooling), characterized by an angular distribution spanning 20-60 degrees, beta-Sn crystals exhibited minimal nucleation and lacked a distinct optimal orientation. However, when a temperature gradient of 295 K/cm was applied during the solidification of beta-Sn crystals, a conspicuous selective orientation emerged. This orientation favored the growth along the [110] grain axis. However, with the further increase in temperature gradient, a significant transition occurs in the growth mechanism of Sn crystals: multiple nucleation centers emerge, leading to dendritic crystal growth. With the further increase in temperature gradient, a significant transformation occurs in the growth mechanism of Sn crystals: multiple-nucleation centers emerge, leading to dendritic crystal growth, and subsequently transitioning towards equiaxed growth. Consequently, the expansion of the temperature gradient results in an increase in undercooling, triggering a series of changes in crystal growth patterns: the solidification front transitions from planar to dendritic growth and further to equiaxed growth. This is because, with the increasing prominence of undercooling, the transport of solutes from the tin solution to the tin melt diminishes. The atomic saturation in the tin melt decreases, causing a decline in the grain growth rate and promoting the formation of equiaxed crystals.