In Situ Observation of the Zr Poisoning Effect in Al Alloys Inoculated by Al-Ti-B
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THE efficiency of the grain refinement of aluminum alloys containing Zr inoculated by Al-5Ti-1B is dramatically decreased[1–5] and is unrecoverable. This phenomenon is generally termed ‘‘Zr poisoning.’’ For example, Ahmady et al.[6] found that the average grain size of Zr-free 7050 inoculated by 0.1 wt pct Al-5Ti-1B was approximately half that of 7050 containing 0.1 wt
YIWANG JIA, SHUBIN WANG, HAIJUN HUANG, DONGHONG WANG, GUOLIANG ZHU, and DAFAN DU are with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. YANAN FU is with Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, CAS, Shanghai 201204, China. ANPING DONG and DA SHU are with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China and also with the Materials Genome Initiative Center, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] BAODE SUN is with the Shanghai Key Lab of Advanced High-temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, the State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China, and also with the Shanghai Innovation Institute for Materials, Shanghai 200444, China. Manuscript submitted February 8, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
pct Zr. Ahmed et al.[7] reported that the grain size of commercial-purity Al inoculated by Al-Ti-B was gradually coarsened with the addition of Zr from 0.05-0.3 wt pct. Bunn et al.[1] found that the poisoning was enhanced by increased holding temperatures and holding times. While the ‘‘Zr-poisoning’’ phenomenon has been widely observed, its mechanism remains controversial. In general, it is believed that the addition of Zr deteriorates the nucleation potency of the heterogeneous nucleation sites in grain refiners. Jones and Pearson[8] deemed through a thermodynamic analysis that TiB2 would be converted to ZrB2 when the Zr concentration in the melt was greater than four times that of Ti. Furthermore, Abdel-Hamid[7] pointed out that a solid solution phase of (Ti, Zr)B2 and/or Al3(Ti, Zr) might more likely form by the substitution of Zr for Ti in TiB2 and/or Al3Ti. This substitution would cause an increase in the boride lattice parameters and would thus increase the disregistry between TiB2{0001} and Al{111}, hindering grain initiation. However, the formation of ZrB2 or (Ti,Zr)B2 has not been confirmed experimentally. An alternative explanation is that the Zr atoms may displace Ti at the TiB2 particle/liquid interface. Since the development of duplex nucleation theory,[9,10] it has been proposed that TiB2 particles are covered by a layer of Al3Ti nucleate a-Al. An extensive high-resolution electron microscopy investigation by Fan et al.[11] further confirmed the existen
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