Enhancement of High Temperature Strength of 2219 Alloys Through Small Additions of Nb and Zr and a Novel Heat Treatment
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T-TREATABLE AL alloys have drawn a great deal of attention due to their advantages for structural applications in the automobile and aerospace industries.[1] The 2XXX series Al alloys, which are strengthened by the formation of precipitates, are some of the most studied age-hardenable alloy systems. The precipitation sequence of this alloy is accepted to occur as follows: Supersaturated solid solution (SSSS) fi Guinier–Preston (GP) zones fi h¢¢ (GPII zones) fi h¢ fi equilibrium h.[2] The strength of the alloy is controlled by the metastable h¢¢ and h¢ precipitates. Therefore, it is necessary to introduce new precipitates in the Al matrix or modify the precipitation kinetics of the existing precipitates for further
S. MONDOL, S.K. MAKINENI, S. KUMAR and K. CHATTOPADHYAY are with the Department of Materials Engineering, Indian Institute of Science, Bengaluru 560012, India. Contact e-mail: [email protected] Manuscript submitted April 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
improvement of strength of the alloy at room temperature as well as at elevated temperature. The addition of small amounts of transition metals, such as Sc, Zr, Nb, Hf, V, Ti etc., as micro alloying elements to age-hardenable Al alloys can form stable or metastable L12 type trialuminides, which makes a distinct positive effect on room temperature as well as high temperature strength of the alloy.[3–8] Moreover, the addition of these elements in Al-Cu- and Al-Mgbased alloys can refine the grain size and morphology, and improve recrystallization resistance.[9–16] In the present investigation, Zr and Nb are added to the commercial 2219 alloy to improve its high temperature mechanical properties. Zr has solid solubility of about 0.29 wt pct in Al at the peritectic temperature of 933.8 K (660.8 C).[17] In Al-Zr binary alloy, nanometre-sized Al3Zr precipitates with metastable L12 structure form during aging in the temperature range of 648 K to 698 K (375 C to 425 C).[18,19] These L12 precipitates transform to the equilibrium DO23 phase after prolonged aging at a temperature above 698 K (425 C).[19–22] Although these precipitates have a larger size, they are stable and remain coherent due to the small lattice mismatch with Al (+ 0.75 pct).[23–25] Stability of these L12 precipitates arises from slower
diffusivity of Zr in Al and coherency due to small lattice parameter mismatch of Al3Zr with Al.[3,18,26] Moreover, the addition of Zr improves creep resistance and thermal stability of Al alloys at elevated temperatures.[3,4,27] The solid solubility of Nb in Al is 0.22 wt pct at the peritectic temperature of 913 K (640 C), decreasing to less than 0.1 wt pct at 573 K (300 C). The diffusivity of Nb in Al is also very low.[3,28] Addition of Nb to Al or Al alloy forms equilibrium DO22 type Al3Nb intermetallic. This DO22 crystal structure is stable at high temperature but is very brittle in nature. A few studies have reported the formation of metastable Ll2 structure in Al-Nb alloy.[3,29] L12 type Al3Nb dispersoids can be obtained by app
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