Enhancement of High Temperature Strength of 2219
Alloys Through Small Additions of Nb and Zr
and a Novel Heat Treatment
S. MONDOL, S.K. MAKINENI, S. KUMAR, and K. CHATTOPADHYAY
This paper presents a detailed investigation on the eﬀect of small amount of Nb and Zr
additions to 2219 Al alloy coupled with a novel three-stage heat treatment process. The main
aim of the work is to increase the high temperature strength of 2219 alloy by introducing
thermally stable L1
type ordered precipitates in the matrix as well as by reducing the coarsening
of metastable strengthening h¢¢ and h¢ precipitates. To achieve this, small amounts of Nb and Zr
are added to 2219 alloy melt and retained in solid solution by suction casting in a water-cooled
copper mould having a cooling rate of 10
K/s. The suction cast alloy is directly aged at
673 K (400 °C) to form L1
type ordered coherent Al
Zr precipitates. Subsequently, the alloy is
solution treated at 808 K (535 °C) for 30 minutes to get supersaturation of Cu in the matrix
without signiﬁcantly aﬀecting the Al
Zr precipitates. Finally, the alloy is aged at 473 K
(200 °C), which results in the precipitation of h¢¢ and h¢. Microstructural characterization reveals
that h¢¢ and h¢ are heterogeneously precipitated on pre-existing uniformly distributed Al
precipitates, which leads to a higher number density of these precipitates. This results in a
signiﬁcant increase in strength at room temperature as well as at 473 K (200 °C) as compared to
the 2219 alloy. Furthermore, the alloy remains thermally stable after prolonged exposure at
473 K (200 °C), which is attributed to the elastic strain energy minimization by the conjoint
Zr/h¢ or Al
Zr/h¢¢ precipitates, and the high Zr and Nb solute-vacancy binding energy,
retarding the growth and coarsening of h¢¢ and h¢ precipitates.
Ó The Minerals, Metals & Materials Society and ASM International 2018
alloys have drawn a great
deal of attention due to their advantages for structural
applications in the automobile and aerospace indus-
The 2XXX series Al alloys, which are strength-
ened by the formation of precipitates, are some of the
most studied age-hardenable alloy systems. The precip-
itation 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¢ﬁequilibrium h.
The strength of the
alloy is controlled by the metastable h¢¢ and h¢ precip-
itates. Therefore, it is necessary to introduce new
precipitates in the Al matrix or modify the precipitation
kinetics of the existing precipitates for further
improvement of strength of the alloy at room temper-
ature 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
type trialuminides, which makes a
distinct positive eﬀect on room temperature as well as
high temperature strength of the alloy.
the addition of these elements in Al-Cu- and Al-Mg-
based alloys can reﬁne the grain size and morphology,
and improve recrystallization resistance.
In the present investigation, Zr and Nb are added to
the commercial 2219 alloy to improve its high temper-
ature 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).
In Al-Zr binary alloy, nanome-
Zr precipitates with metastable L1
ture form during aging in the temperature range of
648 K to 698 K (375 °C to 425 °C).
precipitates transform to the equilibrium DO
after prolonged aging at a temperature above 698 K
Although these precipitates have a larger
size, they are stable and remain coherent due to the
small lattice mismatch with Al (+ 0.75 pct).
bility of these L1
precipitates arises from slower
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@example.com
Manuscript submitted April 18, 2017.
Article published online May 1, 2018
METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—3047