In the present study, single nucleation events have been recorded in the solidification of all the samples. In contrary to the case of multiple nucleation events, this allows to determine crystal growth velocity as a function of undercooling. The crystal growth velocity has been calculated by measuring the time between the start and the end of solidification and dividing it by the diameter of the sample. High speed camera recording the solidification event at 10000 frames per second has been used to determine the start and completion of the solidification. Figure
2 shows that the increase in crystal growth velocity w.r.t increasing undercooling is small at lower undercooling values. However, at undercooling larger than 100–120 K, the increase in crystal growth velocity becomes rapid. The reason for the sudden increase in crystal growth velocity in Ni–Sn is usually attributed to the transition from coupled eutectic growth mode to the uncoupled growth of
α-Ni phase and
β-Ni
3Sn phases.
[7,9,12,13,17,18] This seems to be the case in the present study as well where coupled growth took place at lower undercoolings resulting in a lamellar eutectic microstructure in the inter-dendritic region. At larger undercoolings, different microstructures have been observed which will be discussed below.
Two different microstructures have been observed at larger undercoolings, shown in Figures
4(b) and (c). The microstructure shown in Figure
4(b) is obtained at intermediate undercoolings ranging between 100 and 150K. It consists of a divorced eutectic structure of
β-Ni
3Sn dendrites with
α-Ni phase segregated at the inter-dendritic region. This clearly shows that the solidification velocity has been too fast to allow the formation of lamellar eutectic in this region. As a result, uncoupled growth of
β-Ni
3Sn and
α-Ni phases results in the microstructure shown in Figure
4(b). In addition to the uncoupled growth, divorced eutectics rarely incorporate the interactions between the two eutectic phases, as reported in a recent study.
[29] In case of hypoeutectic and eutectic compositions of Ni–Sn, anomalous eutectic has been found to develop at larger undercooling values.
[7,9,12,13,17,18] In the present study, the microstructure obtained for the largest undercoolings (
i.e. above 165 K) is shown in Figure
4(c) where
α-Ni precipitates could be seen within the
β-Ni
3Sn dendrites. It is possible that due to large undercooling, the solidification begins at a temperature where eutectic dendrite forms in the beginning instead of the
β-Ni
3Sn, which could then undergo re- melting. Similar re-melting of eutectic dendrites has also been speculated during the solidification of CoCrFeNiMnPd
x eutectic high entropy alloy with higher Pd-content.
[16] For the Ni–Sn system, it was proposed by Kattamis and Flemings that the first solid formed at very large undercooling was a highly supersaturated
α-Ni solid solution of eutectic composition (Ni-18.7 at.pct Sn), which grew dendritically and later decomposed to the equilibrium
α-Ni and
β-Ni
3Sn during the post-recalescence.
[9] In another study, three different eutectic systems have been studied and it has been concluded that partial re-melting takes place because of the supersaturation of solute.
[20] In some recent studies involving different Ni–Sn compositions, remelting of eutectic dendrites has been reported resulting in non- equilibrium microstructures.
[14,15] This seems to be the case in the present work as well where due to large undercooling; nucleation could start below the eutectic temperature resulting in the formation of eutectic dendrite instead of
β-Ni
3Sn dendrite. However, due to the rapid increase in temperature during recalescence phase, partial re-melting takes place resulting in excess Ni precipitating out within the
β-Ni
3Sn dendrites. In a recent study, non-equilibrium solidification of hyper- eutectic Nb–Si alloy has been performed using electrostatic levitation. A transition from the primary faceted Nb
3Si phase to the non-faceted Nb phase, and then to the complete eutectic has been observed with the increase of undercooling, however, no re-melting has been reported.
[30]
Micro- and- nano-indentation has been carried out to investigate the mechanical behaviour changes as a function of undercooling. Nano-indents give the information about the mechanical properties while the micro-indents indicate the hardness due to microstructure as well. It can be seen in Figure
6, that the hardness of
β-Ni
3Sn phase is higher than that of the
α-Ni phase. This is mainly because of stronger bonding between the constituents of the intermetallic phase. Intermetallic compounds typically show properties in between those of ceramic and metals, namely having a high hardness, increased brittleness, and a higher melting point.
[3] In Figure
6, the lowest hardness value is observed when the nano-indent lies completely on the
α-Ni phase. The hardness values are slightly increased when the nano-indent lies on a phase boundary or close to it. The hardening effect of the grain or phase boundary is attributed to the well-known dislocation pile-up which takes place at the boundary. It is because the dislocations, which play the most important role in plastic deformation, experience more hindrance in movement due to grain boundaries. Consequently, fine grained structures tend to show higher strength due to more grain boundaries.
[31,32] The effect of phase boundaries together with individual phase hardness is summed up in micro-hardness where the indent covers a larger area. It has been observed that the lamellar microstructure shows greater hardness as compared to the non-lamellar one which means that the micro-hardness decreases with increasing undercooling in this special alloy. This is in accordance with another study where eutectic high entropy alloy samples with lamellar microstructure showed better mechanical properties than the samples where spheroidization of lamellar structure had taken place.
[33] For the Nb–Si alloy, a similar trend has been observed where the Vickers hardness is the highest for the master alloy while lower hardness has been observed for the samples solidified at larger undercoolings.
[30] In another study involving Fe-based multinary alloy, the effect of undercooling on mechanical properties has been investigated.
[21] It has been reported that rapid dendritic growth of Fe-based multinary alloy leads to grain refinement resulting in the formation of equi-axed grains which have higher hardness. The hardness change has been attributed to the combined effect of grain refinement, 2nd phase formation and solute entrapment. In the present study, no new phase has been formed even at the largest undercooling and solute entrapment has not been observed. The decrease in micro-hardness at larger undercoolings is attributed to the formation of non-lamellar microstructure.