Mechanism for Si Poisoning of Al-Ti-B Grain Refiners in Al Alloys
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TRODUCTION
A grain-refined microstructure is usually desirable for Al-alloy castings, since it not only facilitates the casting processes, but also improves mechanical performance of the alloys through reduced cast defects, such as macro-segregation, hot tearing, and porosity. In the Al industry, a common foundry practice for grain refinement is addition of grain refiner prior to casting processes, with Al-5Ti-1B (all the alloy compositions are in wt pct unless stated otherwise) being the most widely used commercial grain refiner. Since the introduction of Al-Ti-B-based grain refiners in early 1950s,[1] extensive experimental investigations and theoretical studies have been carried out to understand the underpinning mechanisms for grain refinement.[2–7] It is now generally accepted that promoting heterogeneous nucleation of numerous aAl grains on TiB2 particles is responsible for grain refinement. Various hypotheses have been proposed to explain the mechanisms of heterogeneous nucleation in the Al-TiB2 system.[3–7] It
YUN WANG, ZHONGPING QUE, ZHONGYUN FAN are with the BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH. Contact e-mail: [email protected] TERUO HASHIMOTO and XIAORONG ZHOU are with the School of Materials, University of Manchester, Manchester M13 9PL. Manuscript submitted April 24, 2020. Article published online September 20, 2020 METALLURGICAL AND MATERIALS TRANSACTIONS A
is realized only until very recently that the formation of an atomic layer of Al3Ti 2-dimensional compound (2DC) on the (0 0 0 1) TiB2 surface is responsible for the high potency of the TiB2 particles for nucleation of aAl grains.[8] The presence of alloying elements has long been recognized as one of the important factors for grain refinement by providing growth restriction. However, this may not be true for all the elements. For instance, Al-5Ti-1B becomes less effective for grain refinement when certain elements are present in the alloy melt, such as Zr,[9–14] Li,[15,16] Cr,[17,18] and high levels of Si.[19–34] The negative effect of solute elements on grain refinement is referred to as ‘‘poisoning’’ in the literature. One of the main explanations to the poisoning effect is that the interaction between solute elements and TiB2 particles leads to the decrease in nucleation potency of the TiB2 particles.[35] Our recent research work on Zr poisoning disclosed that the presence of Zr in the melt results in the dissolution of the pre-existing Al3Ti 2DC layer (formed during the grain refiner production process) and the formation of a Ti2Zr 2DC atomic layer on TiB2 surface, which is responsible for the reduced potency of TiB2 for heterogeneous nucleation of aAl.[36] The phenomenon of Si poisoning has been observed in Al-Si alloys containing more than 3 pct Si inoculated with Al-5Ti-1B grain refiner.[19–23,30–32] Sigworth and Guzowaski[20] and Johnsson et al.[21,22] showed that,
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with inoculation by Al-5Ti-1B, the grain size of Al-Si alloys first decreased slightly and then increased with increasing Si
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