Achieving grain refinement of α-Al and Si modification simultaneously by La‒B‒Sr addition in Al‒10Si alloys. (1st February 2023)

• Record Type: Journal Article
• Authors: Luo, Qun; Li, Xingrui; Li, Qian; Yuan, Lingyang; Peng, Liming; Pan, Fusheng; Ding, Wenjiang
• Is Part Of: Journal of materials science & technology. Volume 135(2023)
• Page Start: 97
• Page End: 110
• Publication Date: 2023-02-01
• Subjects: Al‒Si casting alloys -- Si modification -- Twinning -- Grain refinement -- Semicoherent interface; Metals -- Periodicals; Materials science -- Periodicals; Materials science; Metals; Periodicals; 620.1105
• DOI: 10.1016/j.jmst.2022.07.009
• Languages: English
• ISSNs: 1005-0302

Abstract

Highlights: Sr–B poisoning effect on Si modification is solved by La addition in Al–10Si alloy. Ductility of refined Al–10Si–0.05La–0.02B–0.015Sr alloy is twice as La-free alloy. La reacts with B to form LaB6 acting as an effective nucleation site of α-Al. Multiply twinning of Si is caused by Sr atoms doping at twin boundaries. Poisoning mechanism of Sr–B are revealed by thermodynamic calculations. Abstract: As-cast Al‒Si alloys always face low ductility because of coarse α-Al grains and large lamellar eutectic Si. Although B-containing refiners and Sr modifiers could refine the α-Al grains and eutectic Si respectively, simultaneous addition leads to an invalid effect of Sr because B-containing refiner has a poisoning effect (PE) on Sr modifier in the Al‒10Si‒B‒Sr alloys. The present work achieves both refinement of α-Al grains and modification of Si merely by the addition of 500 ppm La in Al‒10Si‒0.02B‒0.015Sr alloy, which greatly enhances the ductility and tensile strength. Sr‒B interaction in the Al‒10Si‒0.02B‒0.015Sr melt results in a consumption of Sr by forming Sr‒B intermetallic compound. CALPHAD-type calculations reveal that Sr concentration in liquid before eutectic reaction becomes a key parameter to estimate the alloying modification effect. The addition of La forms LaB6 and thereby releases Sr to protect the Sr modification effect. First-principles calculations illustrate that both Sr and La are beneficial to the formation of twin boundaries in Si particles due to their negative formation energy of twin boundaries. The formed LaB6 has a semicoherent interface with α-Al with the orientation relationship of [1 1 ¯ 0]LaB6 //[1 1 ¯ 0]Al and (1 1 ¯ 1)LaB6 //(1 1 ¯ 1)Al, demonstrating that LaB6 is an efficient nucleation site for α-Al, which contributes to the refinement of α-Al in the Al‒10Si‒0.05La‒0.02B‒0.015Sr alloy. Our findings reveal the micro-mechanism of refinement during the solidification process and develop an effective and simple way to obtain high-performance Al alloys. Graphical abstract: Image, graphical abstract