Improving the Elevated-Temperature Properties by Two-Step Heat Treatments in Al-Mn-Mg 3004 Alloys
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Improving the Elevated-Temperature Properties by Two-Step Heat Treatments in Al-Mn-Mg 3004 Alloys K. LIU, H. MA, and X. GRANT CHEN In the present work, two-step heat treatments with preheating at different temperatures (175 C, 250 C, and 330 C) as the first step followed by the peak precipitation treatment (375 C/48 h) as the second step were performed in Al-Mn-Mg 3004 alloys to study their effects on the formation of dispersoids and the evolution of the elevated-temperature strength and creep resistance. During the two-step heat treatments, the microhardness is gradually increased with increasing time to a plateau after 24 hours when first treated at 250 C and 330 C, while there is a minor decrease with time when first treated at 175 C. Results show that both the yield strength (YS) and creep resistance at 300 C reach the peak values after the two-step treatment of 250 C/24 h + 375 C/48 h. The formation of dispersoids is greatly related to the type and size of pre-existing Mg2Si precipitated during the preheating treatments. It was found that coarse rodlike b¢-Mg2Si strongly promotes the nucleation of dispersoids, while fine needle like b¢¢-Mg2Si has less influence. Under optimized two-step heat treatment and modified alloying elements, the YS at 300 C can reach as high as 97 MPa with the minimum creep rate of 2.2 9 109 s1 at 300 C in Al-Mn-Mg 3004 alloys, enabling them as one of the most promising candidates in lightweight aluminum alloys for elevated-temperature applications. https://doi.org/10.1007/s11663-018-1268-x The Minerals, Metals & Materials Society and ASM International 2018
I.
INTRODUCTION
DUE to the rapid demand from weight-sensitive automotive and aerospace industries for lightweight materials, such as aluminum alloys, on the elevated-temperature applications, Al-Mn 3xxx alloys have been developed to obtain good properties at both room temperature (RT) and elevated temperature, in which the dispersoid-strengthening mechanism plays a significant role.[1–4] Typical industrial applications of Al-Mn 3xxx alloys can be found in the fabrication of the can body used at RT and the heat exchanger applied at elevated temperature.[5,6] In our previous works,[2,3,7–9] the YS and creep resistance at 300 C of Al-Mn-Mg 3004 alloy were improved by modifying the alloying elements, such as Mn, Fe, and Mo, and the addition of TiB2 nanoparticles, to optimize the characters of dispersoids, including the size, volume fraction, and distribution. For instance, Mo was introduced in 3004 alloy to increase the volume fraction of dispersoids and
K. LIU, H. MA, and X.-GRANT CHEN are with the Department of Applied Science, University of Quebec at Chicoutimi, Saguenay QC, G7H 2B1, Canada. Contact email: [email protected] Manuscript submitted August 16, 2017
METALLURGICAL AND MATERIALS TRANSACTIONS B
reduce the area of the dispersoid-free zone (DFZ), leading to a significant increase in both the strength and creep resistance at 300 C.[7] On the other hand, the mechanical properties became worse at high Fe content due t
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