Corrosion and Mechanical Properties
of Al-5 At. Pct Cr Produced by Cryomilling
and Subsequent Consolidation at Various
J. ESQUIVEL, K.A. DARLING, H.A. MURDOCH, and R.K. GUPTA
An Al-5 at. pct Cr alloy was produced by high-energy ball milling at liquid nitrogen
temperature followed by consolidation using equal-channel axial extrusion at 200 °C, 300 °C
and 450 °C. The microstructure and corrosion response were compared with a cast alloy of the
same composition. Rather than the intermetallics expected by the phase diagram and seen in the
cast alloy, consolidated HEBM alloys exhibited extended solid solubility of Cr in the aluminum
matrix in addition to a ﬁnely dispersed Cr-rich phase. This led to improvement in the corrosion
behavior as investigated via potentiodynamic polarization and constant immersion tests in NaCl
solution. Hardness and tensile tests were performed to evaluate the mechanical properties. The
highest consolidation temperature (450 °C) contributed to signiﬁcant grain growth and Cr
diﬀusion, lessening the beneﬁcial eﬀects of processing with HEBM.
Ó The Minerals, Metals & Materials Society and ASM International 2018
aluminum possesses excellent corrosion resis-
tance but poor strength, which is generally increased by
additions of alloying elements that form hardening
precipitates; this in turn leads to the formation of
electrochemical heterogeneities and consequently a
decrease in the corrosion performance.
of the common trade-oﬀ between strength and corrosion
properties is in commercial Al-Zn-Mg alloys (7xxx
series) where strength is achieved by the precipitation
of the MgZn
phase, which is anodic in nature and
causes pitting corrosion.
Increasing the precipitation
results in higher strength but poor corrosion
resistance. Commercial alloy design has conventionally
been focused on improving mechanical properties, while
the inevitable corrosion is usually addressed by coatings
for service conditions. This work aims to strengthen
aluminum without the use of corrosion-enhancing
Several non-conventional alloy production techniques
are reported to result in homogeneous and reﬁned
microstructures and therefore superior corrosion prop-
erties owing to their far from equilibrium processes such
as super-fast cooling or severe mechanical deforma-
For example, sputtered Al binary alloys such as
Al-Mo, Al-W, Al-Cr, Al-Nb and Al-Ta have been
reported to exhibit signiﬁcantly high pitting potentials
compared with commercial Al alloys.
ment was attributed to the extended solid solubility of
alloying elements caused by the sputtering process.
Moshier et al. studied the composition of the passive
ﬁlm formed on sputtered Al-Cr alloys and suggested
that this extended solid solubility of Cr in the aluminum
matrix promoted formation of Cr-containing species
(i.e., CrOOH) in the passive layer, which impeded the
ingress of chlorides through the passive ﬁlm, enhancing
the pitting corrosion resistance.
It has also been
suggested that Cr decreases the ability of the dissolving
metal to maintain a critical pit environment, ennobling
the potential needed for stable anodic dissolution.
contents in the range of 5 to 10 at. pct have been
reported to be optimal in retarding the dissolution
kinetics in the pit.
For Cr contents of > 10 at. pct,
hydrolysis of excess Cr ions facilitates the formation of
the critical pit environment and decreases repassivation
capabilities, leading to a decrease in corrosion
For reference, equilibrium solubility of
Cr in Al at 298 K was determined to be
1.07 9 10
Most studies on the corrosion
behavior of sputter-deposited alloys have been focused
J. ESQUIVEL and R.K. GUPTA are with the Department of
Chemical and Biomolecular Engineering, Corrosion Engineering
Program, The University of Akron, Akron, OH 44325-3906. Contact
e-mail: email@example.com K.A. DARLING and H.A. MURDOCH
are with the Weapons and Materials Research Directorate, U.S. Army
Research Laboratory, RDRL-WMM-F, Aberdeen Proving Ground,
Adelphi, MD 21005.
Manuscript submitted August 11, 2017.
Article published online April 24, 2018
3058—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A