Design of novel aluminum alloys strengthened by T-Al6Mg11Zn11 phase for high temperature applications

B2. Light Weight Metals
Naoki Takata¹ , Masato Ishihara¹, Satoshi Nakatsuka¹, Asuka Suzuki¹, Makoto Kobashi^1
1 Department of Materials Process Engineering, Nagoya University

Introduction/Purpose: Wrought aluminum alloys (e.g. alloy 2618) with relatively high specific strength are widely used for radial compressor impellers in vehicle turbochargers, whereas their poor high-temperature strength limits the service temperature of compressor impellers for turbochargers. A key requirement for improving strength at elevated temperatures is the formation of a multi-phase microstructure consisting of high volume fractions of stable intermetallic phases. In present study, we focused on two commonly used metals (Mg and Zn) as alloying elements and designed a wrought Al-based alloy strengthened by the T-Al₆Mg₁₁Zn₁₁ (cubic) intermetallic phase in the Al–Mg–Zn ternary system.
Methods: Thermodynamic calculations using reported database of  Al–Mg–Zn ternary system assessed a composition of Al–5Mg–3.5Zn (at.%) with the alpha-Al (fcc) phase reinforced with high fractions (approximately 10%) of T phase. The desigined alloy ingot was prepared by an induction melting furnace in an Ar atmosphere. The prepared ingots were solution-treated and subsequently aged at various temperatures ranging from 450^oC to 200^oC.
Results: We observed that the T phase preferentially precipitated at grain boundaries in the alpha-Al matrix, increasing the area fraction of the T phase at grain boundaries during aging. The granular precipitates of the T phase were dispersed rather homogenously in the alpha-Al matrix with a particular orientation relationship of (1-11)_alpha // (1-21)_T and [011]_alpha // [111]_T at temperatures above 300°C. After aging at 200°C, numerous fine precipitates with a mean size of ~20 nm in the grain interior were observed, which were likely the metastable phase associated with the T phase. The present alloy (pre-aged at 200°C for 1 h) exhibited a high yield strength of approximately 260 MPa at 200°C, much higher than those of the conventional Al alloys at elevated temperatures corresponding to service temperatures for compressor impellers in turbochargers.
Conclusions: Our desigined Al–5Mg–3.5Zn alloy exhibits superior strength at elevated temperatures to the conventional Al alloys.
Selected references
[1] N. Takata, M. Ishihara, A. Suzuki, M. Kobashi, Mater. Sci. Eng. A 739 (2019), 62-70.
[2] N. Takata, T. Okano, A. Suzuki, M. Kobashi, Intermetallics 95 (2018), 48-58.