Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei

Abstract

The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

Efficacy of Borides in Grain Refining Al-Si Alloys

“…This study further shows that inoculation of Al-Si alloys with Nb-based substrates entirely suppresses the formation of columnar grains and thus the formation of the columnar-to-equiaxed transformation zone. [25] The results shown in Figure 4 indicates that Al-Nb-B master alloys are the most effective. As their intrinsic potency is expected to be lower, due to the less effective growth restriction of Nb, their higher efficacy is due to the higher chemical stability of Nb-based substrates with respect to Ti-based substrates and Al borides.…”

“…The lab made Al-2Nb-2B master alloy we developed contains Nb-based substrates (i.e., Al 3 Nb and NbB 2 ) [25] which are isomorphous to the Ti-based substrates (i.e., Al 3 Ti and TiB 2 ) present in commercial Al-Ti-B master alloys. The combined addition of Nb and B permits to refine the cast structure of Al-Si alloys to a greater extent as after 15 min the grain size is < 300 lm (Figure 4(a)).…”

“…SEM analysis indicates that two types of Nb-based compounds are present, NbB 2 and Al 3 Nb as confirmed by EDS semi-quantitative analysis, and the compounds are actually agglomeration of various particles as outcome of the liquid engineering manufacturing method used to produce the Al-2Nb-2B master alloy. [25] Although found in the concentrated Al-2Nb-2B master alloy, the particles disperse once added to the molten metal. SEM morphological analysis carried out in deep-etched Al-2Nb-2B master alloy samples and TEM investigation confirm the expected hexagonal (NbB 2 ) and tetragonal (Al 3 Nb) crystal lattices as shown in Figure 5(g) where Al 3 Nb particles nucleated onto the surface of a NbB 2 particles.…”

“…The lattice constants for NbB 2 and for Al 3 Nb are a = 3.102 Å and c = 3.285 Å and a = 3.8485 Å and c = 8.615 Å , respectively. [25] The heterogeneous substrates added into the A354 alloy via the Ti-based, B-based, and Al-2Nb-2B master alloys have comparable size and distribution. Assuming that the liquid-solid interfacial energy and the entropy of fusion do not significantly change, from Eq.…”

“…This is mainly due to the highly heterogeneous non-equiaxed dendritic grains which lead to a high variance of the average values. Furthermore, the different average values of the slope b highlight that the master alloys, and therefore the inoculants present (i.e., [25] and thus they could potentially have the same potency. The difference found is then most likely related to the poisoning of the inoculants once in contact with the molten Al-Si alloy.…”