Grain refinement by titanium in the unidirectionally solidified aluminum alloys

Abstract

Grain refinement by Ti in unidirectionally solidified aluminum alloys were studied under the condition to suppress crystal multiplication. Alloys with different Cu and Ti contents were unidirectionally grown from melts at a constant rate by using seeds of pure aluminum single crystals, and the number of grains was examined. The results obtained are as follows(1) When the alloys were Ti-free, single crystals were grown from the seeds with the <100> direction, the prefered orientation in dendritic growth. When Ti was added to the alloys, growth of the crystals formed from the seeds was hindered by many newly crystallized grains at the growing interface.(2) As the Ti content and the solidifying rate increased, the number of the grains increased.(3) Grain refinement effect of Ti was suppressed when the melt was superheated. However, when both Ti and B were present in the melt, the grain refinement was still observed.(4) Grain refinement action of Ti was not affected by types of solid-liquid interface morphology which depended on the Cu contents.(5) Smaller grains were formed near the surfaces, when the alloy contained Ti.(6) When 0.05%Ti was added to the Al-CuAl2 and Al-NiAl3 eutectic alloys, the solidifying interface took the cellullar structure. However, grain refinement effect of Ti was not observed in these eutectic alloys.These results were considered to clarify heterogeneous nucleation due to a Ti compound when the Ti containing alloys were solidified in the absence of crystal multiplication. The mechanism to explain the grain refinement effect by such a Ti compound was also discussed.