The Interface of TiB 2 and Al 3 Ti in Molten Aluminum
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TITANIUM and boron, in the forms of TiB2 and Al3Ti particles in Al-Ti-B master alloys, are introduced into aluminum melt for grain refinement purposes.[1–10] These particles exist separately or as compounded particles in the master alloys.[5,9,11] Upon addition into aluminum melt in a total concentration of about 0.01 wt. pct Ti and 0.002 wt pct B, the particles react with the aluminum melt differently. TiB2 dissolves into aluminum melt very slowly and reacts with aluminum to form a solid solution of (Ti,Al)B2, releasing small percentage of titanium into the aluminum melt.[10] The solubility of TiB2 in aluminum melt is so low that no obvious dissolution can be noticed in industrial grain refinement practice. Analysis also confirmed that TiB2 in Al-Ti-B master alloys in fact is a solid solution of (Ti,Al)B2 with a variable Ti/Al ratio.[9,11] The (Ti,Al)B2 solid solution may not be in a thermodynamic stable state instead of a metastable state by considering the rapid formation of the particles in the production of Al-Ti-B master alloys through salt reaction processes. The effects of the formation of (Ti,Al)B2 solid solutions on grain refinement had been ignored until a dynamic nucleation theory for aluminum grain refinement was proposed recently.[10] The theory proposes that TiB2
XIAOMING WANG, Assistant Professor, WEI VIAN, Ph.D. Student, HAIBIN MA, Master Student, and QINGYOU HAN, Professor, are with the School of Engineering Technology, Purdue University, West Lafayette, IN 47907. Contact e-mail: wang1747@ purdue.edu JIE SONG, formerly Ph.D. Student with the School of Engineering Technology, Purdue University, is now Postdoctoral Researcher with the Colorado School of Mines, Golden, CO 80401. Manuscript submitted September 3, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
reacts with aluminum upon addition into an aluminum melt to form a solid solution of (Ti,Al)B2 and releases titanium, which in the presence of titanium in solution from the dissolution of Al3Ti particles forms a dynamic layer of Al3Ti on the surface of the (Ti,Al)B2 particles. The layer of a couple of atom thickness then nucleates a-Al grains through a peritectic reaction and disappears in the final castings. However, the formation of a two-dimensional single-atom layer of Al3Ti at the TiB2/ Al interface was reported the first time by another group in the same time though contradictory to the peritectic reaction nature of aluminum and Al3Ti.[4,12] In general, it is believed that Al3Ti dissolves into aluminum melt in just a few seconds and the diboride particles remain solid, though no boride particles were observed in castings after grain refinement.[7,8,12–16] A popular grain refinement mechanism has been proposed as TiB2 acts as a substrate for the nucleation of Al3Ti, which then nucleates a-Al grains, i.e., via two steps of heterogeneous nucleation. Meanwhile, titanium in solution offers a constitutional undercooling to restrict the growth of a-Al grains leading to the formation of equiaxed a-Al grains in castings.[5,8,12,17] There are no questions ab
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