M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
831–836
DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING
THE MECHANICAL PROPERTIES AND STRESS CORROSION
RESISTANCE OF 7000 Al ALLOYS
RAZVOJ TOPLOTNE OBDELAVE ZA IZBOLJ[ANJE MEHANSKIH
LASTNOSTI IN NAPETOSTNO KOROZIJSKO ODPORNOST 7000
Al ZLITIN
Mehdi Shakouri1, Mohammad Esmailian1, Saeed Shabestari2
1Advanced Material and Renewable Energy Department, Iranian Research Organization for Science and Technology (IROST),
P. O. Box 3353-5111, Tehran, Iran
2Center of Excellence for High Strength Alloys Technology (CEHSAT), School of Metallurgy and Materials Engineering,
Iran University of Science and Technology (IUST), Narmak, 16846, Tehran, Iran
mshakoory@yahoo.com
Prejem rokopisa – received: 2016-10-10; sprejem za objavo – accepted for publication: 2017-04-19
doi:10.17222/mit.2016.297
A retrogression and re-ageing (RRA) treatment is a three-step heat treatment that can improve both the mechanical strength and
the corrosion resistance in aluminum alloys. In this work, the mechanical and stress corrosion properties under various ageing
treatment conditions were investigated in an Al–8.5Zn–2.1Mg–2Cu–0.2Ag (w/%) alloy. The treatments were the T6
conventional method followed by a retrogression and re-ageing (RRA) treatment. Tensile test, scanning electron microscopy
(SEM), energy-dispersive X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC) were used to investigate the
mechanical and stress corrosion cracking (SCC) properties. The results showed that both, the strength and the corrosion
resistance criteria (SCR), which is defined as the ratio between the remaining strength percent in stressed and un-stressed
conditions, improve after the RRA treatment. The tensile strength and SCR criteria for the T6 heat treatment were 547 MPa and
71 % initially and then increased to 612 MPa and 95 % for the RRA treatment, respectively. Moreover, the EDS results showed
that in grain-boundary precipitates the Cu concentration is much higher for the RRA in comparison with the T6 treatment, but it
is lower for the matrix precipitates in the RRA treatment.
Keywords: aluminium alloys, stress corrosion resistance, precipitation, retrogression and re-ageing
Obdelava z retrogresijo in ponovno o`ivitvijo (angl. RRA) je tristopenjska toplotna obdelava, s katero lahko izbolj{amo tako
mehansko trdnost kot odpornosti proti koroziji pri aluminijevih zlitinah. V pri~ujo~em delu so bile raziskovane mehanske
lastnosti in lastnosti odpornosti proti koroziji pri zlitini Al–8.5Zn–2.1Mg–2Cu–0.2Ag (w/%) pod razli~nimi pogoji obdelave
staranja. Izvedeni so bili testi po konvencionalni T6 metodi, z upo{tevanjem RRA obdelave. Natezni preiskus, vrsti~na
elektronska mikroskopija (SEM), energijsko disperzijska rentgenska spektroskopija (EDS) in DSC-testiranja so bili izvedeni, da
bi raziskali mehanske in napetostno-korozijske lomne lastnosti (angl. SCC). Rezultati so pokazali, da se oba kriterija tako
trdnost kot odpornost proti koroziji, ki definirata razmerje med procentom zaostale trdnosti pri napetostnih in nenapetostnih
pogojih, po RRA-obdelavi izbolj{ata.
Klju~ne besede: aluminijeve zlitine, odpornost proti koroziji, oborine, retrogresija in ponovno staranje
1 INTRODUCTION
The Al–Zn–Mg–Cu series aluminum alloys are preci-
pitation-hardening alloys that are used extensively for
light-weight structural applications, in particular in the
aircraft industry. They have a combination of good
strength and good stress corrosion resistance.1
The most popular heat treatment to gain the peak-
aged strength (T6X temper) is a solution treatment,
quenching, stretching (for stress relief purposes)
followed by artificial aging at 120 °C for 24 h, but this
temper is highly susceptible to SCC. In order to reduce
this susceptibility, an over-aging treatment (T7X temper)
is needed.2 This requirement becomes increasingly de-
manding as the solute contents are increased in commer-
cial alloys in order to improve the mechanical properties
even further.
To improve both the mechanical strength and corro-
sion resistance, it has been proposed to utilize an RRA
treatment. This three-step heat treatment has been shown
to offer a stress corrosion resistance as good as that of a
T7X heat treatment, while keeping a strength compar-
able to that of a T6X temper.3,4 This type of heat treat-
ment comprises three steps. First, an ageing step that
leads to a T6 state. A second step (called retrogression or
reversion) of short duration at high temperature dissolves
part of the initially formed precipitates. Third heat
treatment step at lower temperature leads to the desired
microstructure.5
Stress corrosion cracking occurs under loading in a
corrosive environment. Several investigations have re-
ported that the SCC mechanism involves anodic dissolu-
tion, hydrogen-induced cracking, passive film rupture,
hydrogen embrittlement, magnesium segregation to the
Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836 831
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.78:67.017:620.193:669.715 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)831(2017)
grain boundaries and a precipitate-free zone (PFZ) along
the grain boundary.6–10 However, the microstructural
characteristics of Al-Zn-Mg-Cu high strength aluminum
alloys are well known to have a strong influence not only
on the mechanical properties but also on the SCC sus-
ceptibility. Larger grain-boundary precipitates can trap
more atomic hydrogen to nucleate hydrogen bubbles,
thereby decreasing the hydrogen concentration at grain
boundaries below a critical value is considered to prevent
intergranular SCC fracture.11 Furthermore, the cathodic
grain-boundary precipitates grow by depleting solute
atoms. Studies showed that this leads to the broadening
of the anodic PFZ, which contained no strengthen pre-
cipitation phase and as a result was soft and weak.12 The
combination of tensile stress and anodic dissolution
caused SCC.
SCC resistance is of practical importance for the
industrial applications of the Al–Zn–Mg–Cu series
aluminum alloys. Various heat treatments in these alloys
offer very different SCC properties. The SCR criteria are
proposed to compare the SCC resistance.
The aim of this paper is to investigate the mechanical
strength and SCC of a 7000 series aluminum alloy after
conventional T6 and RRA treatments. The SCR criteria
have been defined to easily compare the SCC resistance
of this alloy after various treatments. SEM investigations
were used to determine the effect of the heat treatments
on the alloy’s microstructure with the aim of studying its
effect on SCC susceptibility. For comparison, the stress
corrosion resistance of the alloy with different heat
treatments was studied by the breaking load method
according to ASTM G139 standard.13
2 EXPERIMENTAL PART
The samples used in this study were received as
8-mm-thick sheets that were homogenized, hot rolled
and heat treated after being alloyed and casted. The
chemical composition of the alloy is shown in Table 1.
Table 1: Chemical composition (in mass fractions, w/%) of fabricated
alloy
Alloy
No. Zn Cu Mg Fe Si Zr Ag
1 8.5 2 2.1 0.18 0.16 0.20 0.19
An induction melting furnace used for melting and
the melt was poured in a water-cooled copper mold. The
as-cast specimens were homogenized at 460 °C for 24 h
and hot rolled to about 33 % reduction. Hot rolling was
performed with 40 min–1 rolling speed at 430 °C. The
specimens were milled to tensile samples according to
the ASTM E8M standard. A schematic showing the pre-
paration of tensile samples from the specimens is given
in Figure 1.
A solution heat treatment was done at 471 °C for 6 h
followed by a water quenching. T6 ageing was per-
formed for 24 h at 120 °C (T6 temper). The scheme of
the retrogression and re-ageing treatment is shown in
Figure 2.
Mechanical properties measurements were made at
ambient temperature on the specimens machined
according to the ASTM E8M-04 small size standard.
The average of three tests was used for each result. The
test’s strain rate was 10–3 /s.
The SEM analysis was performed on a TESCAN
scanning electron microscope. Thermal analysis was per-
formed in a DSC 1 Mettler Toledo differential scanning
calorimeter. Polished alloy disks with a diameter of
5 mm and 0.6 mm thick were sealed in aluminum pans
and heated in a flowing argon atmosphere at a constant
heating rate of 10 °C/min. The flowing rate of the argon
was 100 mL/min.
The stress corrosion tests were performed according
to the ASTM G139 standard. The Neutral 3.5 % Sodium
Chloride Solution was prepared in accordance with the
requirements of the ASTM G44 standard. The ASTM
G49 standard was used for preparation of direct tension
stress corrosion test specimens. A 207 MPa stress for the
cycle of 4 d was applied as per section 8-2 of the ASTM
G139 standard. In order to eliminate the corrosion other
than SCC (such as pitting), some specimens were tested
unstressed. The ratio of remaining strength of stressed
and unstressed samples represented in percent, were
calculated as SCR criteria for different treatment.
3 RESULTS AND DISCUSSION
3.1 Mechanical strength
The mechanical strength of the alloy was 547 MPa
after the T6 treatment and was 612 MPa after the RRA
treatment. The RRA treatment leads to an about 12 %
increase in mechanical strength. While T6 treatment is
carried out to ensure maximum strength; the RRA
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
832 Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Scheme of the RRA treatment
Figure 1: Schematic showing the preparation of tensile samples from
the specimens
treated samples show higher strength. It seems that the
performed T6 treatment does not lead to precipitation of
all the alloying elements and yet there are elements in
the solution. To investigate this further, DSC analysis
was conducted on the samples.
3.2 DSC results
Figure 3 shows the DSC analysis for the T6 and
RRA treatment. Unlike the RRA treatment, the T6 curve
shows two exothermic peaks at temperatures between
200 °C and 245 °C. The RRA curve in this region has
only one small peak at about 240 °C. These peaks are
results of  and  precipitation from solid solution.14–16
In the RRA curve, there exists only one peak related to 
precipitation. This means that there are still a lot of ele-
ments in solid solution after T6 treatment suggesting that
either time or temperature or both are not enough for
diffusion and precipitation of the elements. As a result,
the maximum strength has not achieved.
3.3 Stress corrosion cracking
Stress corrosion tests were performed according to
the ASTM G139 standard for direct tension stress corro-
sion test method. Figure 4 shows the strength of T6 and
RRA samples after exposure to corrosive 3.5 % sodium
chloride solution without stressing and with a 207 MPa
stress after 4 d. As expected, strength is dropped in both
the T6 and RRA treatment after the SCC test. After the
SCC test with 207 MPa constant stresses for 4 d in corro-
sive solution, the strength of RRA treated samples
decreased from 612 MPa to 545 MPa. Accordingly, the
strength of the T6 treated samples dropped from 547
MPa to 367 MPa. Calculating the results in percentages,
gives us a better understanding of the strength reduction.
The test was conducted in corrosive solution in two
stress conditions: un-stressed and with constant 207 MPa
tension stress. The mechanical strength decreased for
both conditions. In order to better demonstrate the SCC
results, it is most useful to calculate the remaining
strength after SCC test and subtract the effect of corro-
sion methods other than SCC (such as pitting). To
achieve this, the percentage of SCR ratio of remaining
strength of stressed and unstressed samples is defined as
stress corrosion resistance SCR. The SCR results of the
alloy after the T6 and RRA heat treatments are shown in
Figure 5. The remaining strengths of the unstressed
samples after RRA and T6 treatments were 94.7 % and
93.3 %, respectively, which are not much different. It
means that unstressed corrosion results for both treat-
ments were almost identical. However, the remaining
strength percentages in the constant stress condition
were very different. The remaining strength percentage
was 89 % for the RRA treatment and 68 % for the T6
treatment. The SCR for the RRA treatment was 95 %
and for the T6 treatment it was 71 %. A significant im-
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836 833
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: The SCR results of the alloy after T6 and RRA heat
treatments
Figure 3: DSC curves for T6 and RRA treatment Figure 4: Direct tension stress corrosion cracking test after T6 and
RRA treatment
grain boundaries and a precipitate-free zone (PFZ) along
the grain boundary.6–10 However, the microstructural
characteristics of Al-Zn-Mg-Cu high strength aluminum
alloys are well known to have a strong influence not only
on the mechanical properties but also on the SCC sus-
ceptibility. Larger grain-boundary precipitates can trap
more atomic hydrogen to nucleate hydrogen bubbles,
thereby decreasing the hydrogen concentration at grain
boundaries below a critical value is considered to prevent
intergranular SCC fracture.11 Furthermore, the cathodic
grain-boundary precipitates grow by depleting solute
atoms. Studies showed that this leads to the broadening
of the anodic PFZ, which contained no strengthen pre-
cipitation phase and as a result was soft and weak.12 The
combination of tensile stress and anodic dissolution
caused SCC.
SCC resistance is of practical importance for the
industrial applications of the Al–Zn–Mg–Cu series
aluminum alloys. Various heat treatments in these alloys
offer very different SCC properties. The SCR criteria are
proposed to compare the SCC resistance.
The aim of this paper is to investigate the mechanical
strength and SCC of a 7000 series aluminum alloy after
conventional T6 and RRA treatments. The SCR criteria
have been defined to easily compare the SCC resistance
of this alloy after various treatments. SEM investigations
were used to determine the effect of the heat treatments
on the alloy’s microstructure with the aim of studying its
effect on SCC susceptibility. For comparison, the stress
corrosion resistance of the alloy with different heat
treatments was studied by the breaking load method
according to ASTM G139 standard.13
2 EXPERIMENTAL PART
The samples used in this study were received as
8-mm-thick sheets that were homogenized, hot rolled
and heat treated after being alloyed and casted. The
chemical composition of the alloy is shown in Table 1.
Table 1: Chemical composition (in mass fractions, w/%) of fabricated
alloy
Alloy
No. Zn Cu Mg Fe Si Zr Ag
1 8.5 2 2.1 0.18 0.16 0.20 0.19
An induction melting furnace used for melting and
the melt was poured in a water-cooled copper mold. The
as-cast specimens were homogenized at 460 °C for 24 h
and hot rolled to about 33 % reduction. Hot rolling was
performed with 40 min–1 rolling speed at 430 °C. The
specimens were milled to tensile samples according to
the ASTM E8M standard. A schematic showing the pre-
paration of tensile samples from the specimens is given
in Figure 1.
A solution heat treatment was done at 471 °C for 6 h
followed by a water quenching. T6 ageing was per-
formed for 24 h at 120 °C (T6 temper). The scheme of
the retrogression and re-ageing treatment is shown in
Figure 2.
Mechanical properties measurements were made at
ambient temperature on the specimens machined
according to the ASTM E8M-04 small size standard.
The average of three tests was used for each result. The
test’s strain rate was 10–3 /s.
The SEM analysis was performed on a TESCAN
scanning electron microscope. Thermal analysis was per-
formed in a DSC 1 Mettler Toledo differential scanning
calorimeter. Polished alloy disks with a diameter of
5 mm and 0.6 mm thick were sealed in aluminum pans
and heated in a flowing argon atmosphere at a constant
heating rate of 10 °C/min. The flowing rate of the argon
was 100 mL/min.
The stress corrosion tests were performed according
to the ASTM G139 standard. The Neutral 3.5 % Sodium
Chloride Solution was prepared in accordance with the
requirements of the ASTM G44 standard. The ASTM
G49 standard was used for preparation of direct tension
stress corrosion test specimens. A 207 MPa stress for the
cycle of 4 d was applied as per section 8-2 of the ASTM
G139 standard. In order to eliminate the corrosion other
than SCC (such as pitting), some specimens were tested
unstressed. The ratio of remaining strength of stressed
and unstressed samples represented in percent, were
calculated as SCR criteria for different treatment.
3 RESULTS AND DISCUSSION
3.1 Mechanical strength
The mechanical strength of the alloy was 547 MPa
after the T6 treatment and was 612 MPa after the RRA
treatment. The RRA treatment leads to an about 12 %
increase in mechanical strength. While T6 treatment is
carried out to ensure maximum strength; the RRA
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
832 Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Scheme of the RRA treatment
Figure 1: Schematic showing the preparation of tensile samples from
the specimens
treated samples show higher strength. It seems that the
performed T6 treatment does not lead to precipitation of
all the alloying elements and yet there are elements in
the solution. To investigate this further, DSC analysis
was conducted on the samples.
3.2 DSC results
Figure 3 shows the DSC analysis for the T6 and
RRA treatment. Unlike the RRA treatment, the T6 curve
shows two exothermic peaks at temperatures between
200 °C and 245 °C. The RRA curve in this region has
only one small peak at about 240 °C. These peaks are
results of  and  precipitation from solid solution.14–16
In the RRA curve, there exists only one peak related to 
precipitation. This means that there are still a lot of ele-
ments in solid solution after T6 treatment suggesting that
either time or temperature or both are not enough for
diffusion and precipitation of the elements. As a result,
the maximum strength has not achieved.
3.3 Stress corrosion cracking
Stress corrosion tests were performed according to
the ASTM G139 standard for direct tension stress corro-
sion test method. Figure 4 shows the strength of T6 and
RRA samples after exposure to corrosive 3.5 % sodium
chloride solution without stressing and with a 207 MPa
stress after 4 d. As expected, strength is dropped in both
the T6 and RRA treatment after the SCC test. After the
SCC test with 207 MPa constant stresses for 4 d in corro-
sive solution, the strength of RRA treated samples
decreased from 612 MPa to 545 MPa. Accordingly, the
strength of the T6 treated samples dropped from 547
MPa to 367 MPa. Calculating the results in percentages,
gives us a better understanding of the strength reduction.
The test was conducted in corrosive solution in two
stress conditions: un-stressed and with constant 207 MPa
tension stress. The mechanical strength decreased for
both conditions. In order to better demonstrate the SCC
results, it is most useful to calculate the remaining
strength after SCC test and subtract the effect of corro-
sion methods other than SCC (such as pitting). To
achieve this, the percentage of SCR ratio of remaining
strength of stressed and unstressed samples is defined as
stress corrosion resistance SCR. The SCR results of the
alloy after the T6 and RRA heat treatments are shown in
Figure 5. The remaining strengths of the unstressed
samples after RRA and T6 treatments were 94.7 % and
93.3 %, respectively, which are not much different. It
means that unstressed corrosion results for both treat-
ments were almost identical. However, the remaining
strength percentages in the constant stress condition
were very different. The remaining strength percentage
was 89 % for the RRA treatment and 68 % for the T6
treatment. The SCR for the RRA treatment was 95 %
and for the T6 treatment it was 71 %. A significant im-
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836 833
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: The SCR results of the alloy after T6 and RRA heat
treatments
Figure 3: DSC curves for T6 and RRA treatment Figure 4: Direct tension stress corrosion cracking test after T6 and
RRA treatment
provement in the stress corrosion cracking has been
achieved after the RRA treatment.
3.4 SEM and EDS analysis
The SEM micrographs of the T6 and RRA samples
are shown in Figures 6 and 7.
Comparison of these two micrographs shows that,
after the RRA treatment the grain-boundary precipitates
have lost their continuity and divided to relatively larger
particles. This is an essential factor for reducing the
intergranular corrosion and consequently reducing the
stress corrosion cracking. Two basic mechanisms of the
SCC in 7000 series aluminum alloys have been pro-
posed: anodic dissolution and cathodic dissolution
(hydrogen embrittlement).6 The precipitates in the grain
boundaries have the electrode potential different from
the Al matrix. This would result in the anodic dissolution
and form defects in a chloride solution. The hydrogen
atoms produced in the crack tip also lead to the hydrogen
embrittlement in the grain boundaries. The large size and
distance of the grain-boundary particles could decrease
the anodic dissolution speed. These particles can also act
as the trapping sites for atomic hydrogen and transform
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
834 Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 9: EDS analysis of RRA sample: a) matrix; b) grain boundary
precipitates
Figure 7: SEM micrograph of RRA sample showing less continuous
precipitates along the grain boundary in comparison to the T6 sample
Figure 6: SEM micrograph of T6 sample showing continuous
precipitates along the grain boundary
Figure 8: EDS analysis of T6 sample: a) matrix, b) grain boundary
precipitates
them to molecular hydrogen bubbles to reduce the con-
centration of the atomic hydrogen at the grain boun-
daries. Both the size and distance of the grain-boundary
precipitates in RRA treated alloy were larger than that in
T6 treated alloy, therefore, the SCR criteria after RRA
treatment is higher than the T6 treatment.
Another factor showing SCC resistance improvement
after RRA treatment is the concentration changes of pre-
cipitates and matrix. The Zn and Cu atoms have opposite
effects on the electrochemical properties of alumi-
num.17,18 In a corrosive medium, the  phase in grain
boundary is very active and anodic with respect to the
matrix.19 Cu enrichment of precipitates after RRA
increases the electrode potential of the grain boundaries
and decreases the activity of grain-boundary phases.
Figures 9 and 10 show the EDS analysis of the grain-
boundary precipitates and matrix after RRA and T6
treatment. Figure 8a shows the Cu-rich matrix after the
T6 treatment while according to Figure 8b, the preci-
pitates are Zn-reach. After RRA treatment, the matrix
depleted from Cu atoms and the grain boundary preci-
pitates were Cu-enrichment (Figures 9a to 9b). Large
variations in concentration of solute atoms happen
during the RRA treatment. Zn atoms have a higher diffu-
sivity than Cu atoms and leave the precipitates first after
the reversion stage.20 As a result, the Zn content of the
matrix increases and a corresponding increase in the Cu
precipitate content happens. This happens reversely in
the T6 treatment, where Zn incorporates first the preci-
pitates, resulting in a high Zn content of the precipitates
and high Cu matrix content. The Zn and Cu atoms have
opposite effects on the electrochemical properties of
aluminum.21 A change in the corrosion behavior of the
7000 series aluminum alloy is related to changes in pre-
cipitate composition and particularly to a Cu enrichment
of the precipitates.22 During the RRA treatment, the
precipitates enrich by Cu atoms (Figure 9b) and this
factor as well as other factors improves the SCC
resistance.
4 CONCLUSIONS
The mechanical strength of an Al–8.5Zn–2.1Mg–
2Cu–0.2Ag (w/%) alloy after conventional T6 treatment
was lower than the mechanical strength after RRA
treatment.
SCR is a useful criteria to compare the SCC in
Al-Zn-Mg-Cu alloys after various heat treatments. The
SCR of the Al–8.5Zn–2.1Mg–2Cu–0.2Ag (w/%) alloy
after conventional T6 treatment was 71 % and was 95 %
after RRA treatment. The SCC improved significantly
after the RRA treatment.
The SCC improvement after the RRA treatment
related to Cu enrichment of the precipitates along with
the discontinuities of the grain-boundary precipitates.
5 REFERENCES
1 T. Dursun, C. Soutis, Recent developments in advanced aircraft
aluminium alloys, Materials & Design, 56 (2014), 862–871,
doi:10.1016/j.matdes.2013.12.002
2 D. Liu, B. Xiong, F. Bian, Z. Li, X. Li, Y. Zhang, F. Wang, H. Liu, In
situ studies of microstructure evolution and properties of an
Al–7.5Zn–1.7Mg–1.4Cu–0.12Zr alloy during retrogression and
reaging, Materials & Design, 56 (2014), 1020–1024, doi:10.1016/
j.matdes.2013.12.006
3 J. Li, N. Birbilis, C. Li, Z. Jia, B. Cai, Z. Zheng, Influence of
retrogression temperature and time on the mechanical properties and
exfoliation corrosion behavior of aluminium alloy AA7150,
Materials Characterization, 60 (2009) 11, 1334–1341, doi:10.1016/
j.matchar.2009.06.007
4 R. Bucci, C. Warren, E. Starke, The Need for New Materials in
Aging Aircraft Structures, Journal of Aircraft, 37 (2000) 1,
122–129, doi:10.2514/2.2571
5 D. Feng, X. M. Zhang, S. D. Liu, and Y. L. Deng, Non-isothermal
retrogression and re-ageing treatment schedule for AA7055 thick
plate, Materials & Design, 60 (2014), 208–217, doi:10.1016/
j.matdes.2014.03.064
6 D. Najjar, T. Magnin, T. J. Warner, Influence of critical surface
defects and localized competition between anodic dissolution and
hydrogen effects during stress corrosion cracking of a 7050 alumi-
nium alloy, Materials Science and Engineering: A, 238 (1997) 2,
293–302, doi:10.1016/S0921-5093(97)00369-9
7 T. Burleigh, The postulated mechanisms for stress corrosion cracking
of aluminum alloys: a review of the literature 1980-1989, Corrosion,
47 (1991) 2, 89–98, doi:10.5006/1.3585235
8 R. Jones, D. Baer, M. Danielson, J. Vetrano, Role of Mg in the stress
corrosion cracking of an Al-Mg alloy, Metallurgical and Materials
Transactions A, 32 (2001) 7, 1699–1711, doi:10.1007/s11661-
001-0148-0
9 E. Pugh, Progress toward understanding the stress corrosion
problem, Corrosion, 41 (1985) 9, 517–526, doi:10.5006/1.3583022
10 A. Sedriks, J. Green, D. Novak, Corrosion processes and solution
chemistry within stress corrosion cracks in aluminum alloys,
Proceedings of NACE, 1971, 569–575
11 T. Tsai, T. Chuang, Role of grain size on the stress corrosion crack-
ing of 7475 aluminum alloys, Materials Science and Engineering: A,
225 (1997) 1–2, 135–144, doi:10.1016/S0921-5093(96)10840-6
12 T. Pardoen, D. Dumont, A. Deschamps, Y. Brechet, Grain boundary
versus transgranular ductile failure, Journal of the Mechanics and
Physics of Solids, 51 (2003) 4, 637–665, doi:10.1016/S0022-
5096(02)00102-3
13 ASTM 139:2005(G)- Standard test method for determining stress-
corrosion cracking resistance of heat-treatable aluminum alloy
products using breaking load method, ASTM International, West
Conshohocken
14 K. Ghosh, N. Gao, Determination of kinetic parameters from
calorimetric study of solid state reactions in 7150 Al-Zn-Mg alloy,
Transactions of Nonferrous Metals Society of China, 21 (2011) 6,
1199–1209, doi:10.1016/S1003-6326(11)60843-1
15 T. Marlaud, A. Deschamps, F. Bley, W. Lefebvre, B. Baroux,
Influence of alloy composition and heat treatment on precipitate
composition in Al–Zn–Mg–Cu alloys, Acta Materialia, 58 (2010) 1,
248–260, doi:10.1016/j.actamat.2009.09.003
16 J. G. Tang, H. Chen, X. M. Zhang, S. D. Liu, W. J. Liu, H. Ouyang,
H. P. Li, Influence of quench-induced precipitation on aging behavior
of Al-Zn-Mg-Cu alloy, Transactions of Nonferrous Metals Society of
China, 22 (2012) 6, 1255–1263, doi:10.1016/S1003-6326(11)
61313-7
17 R. W. Revie, H. H. Uhlig, Uhlig’s corrosion handbook: vol. 51, John
Wiley & Sons, Hoboken 2011
18 G. Silva, B. Rivolta, R. Gerosa, U. Derudi, Study of the SCC
Behavior of 7075 Aluminum Alloy After One-Step Aging at 163° C,
Journal of Materials Engineering and Performance, 22 (2013) 1,
210–214, doi:10.1007/s11665-012-0221-4
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836 835
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
provement in the stress corrosion cracking has been
achieved after the RRA treatment.
3.4 SEM and EDS analysis
The SEM micrographs of the T6 and RRA samples
are shown in Figures 6 and 7.
Comparison of these two micrographs shows that,
after the RRA treatment the grain-boundary precipitates
have lost their continuity and divided to relatively larger
particles. This is an essential factor for reducing the
intergranular corrosion and consequently reducing the
stress corrosion cracking. Two basic mechanisms of the
SCC in 7000 series aluminum alloys have been pro-
posed: anodic dissolution and cathodic dissolution
(hydrogen embrittlement).6 The precipitates in the grain
boundaries have the electrode potential different from
the Al matrix. This would result in the anodic dissolution
and form defects in a chloride solution. The hydrogen
atoms produced in the crack tip also lead to the hydrogen
embrittlement in the grain boundaries. The large size and
distance of the grain-boundary particles could decrease
the anodic dissolution speed. These particles can also act
as the trapping sites for atomic hydrogen and transform
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
834 Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 9: EDS analysis of RRA sample: a) matrix; b) grain boundary
precipitates
Figure 7: SEM micrograph of RRA sample showing less continuous
precipitates along the grain boundary in comparison to the T6 sample
Figure 6: SEM micrograph of T6 sample showing continuous
precipitates along the grain boundary
Figure 8: EDS analysis of T6 sample: a) matrix, b) grain boundary
precipitates
them to molecular hydrogen bubbles to reduce the con-
centration of the atomic hydrogen at the grain boun-
daries. Both the size and distance of the grain-boundary
precipitates in RRA treated alloy were larger than that in
T6 treated alloy, therefore, the SCR criteria after RRA
treatment is higher than the T6 treatment.
Another factor showing SCC resistance improvement
after RRA treatment is the concentration changes of pre-
cipitates and matrix. The Zn and Cu atoms have opposite
effects on the electrochemical properties of alumi-
num.17,18 In a corrosive medium, the  phase in grain
boundary is very active and anodic with respect to the
matrix.19 Cu enrichment of precipitates after RRA
increases the electrode potential of the grain boundaries
and decreases the activity of grain-boundary phases.
Figures 9 and 10 show the EDS analysis of the grain-
boundary precipitates and matrix after RRA and T6
treatment. Figure 8a shows the Cu-rich matrix after the
T6 treatment while according to Figure 8b, the preci-
pitates are Zn-reach. After RRA treatment, the matrix
depleted from Cu atoms and the grain boundary preci-
pitates were Cu-enrichment (Figures 9a to 9b). Large
variations in concentration of solute atoms happen
during the RRA treatment. Zn atoms have a higher diffu-
sivity than Cu atoms and leave the precipitates first after
the reversion stage.20 As a result, the Zn content of the
matrix increases and a corresponding increase in the Cu
precipitate content happens. This happens reversely in
the T6 treatment, where Zn incorporates first the preci-
pitates, resulting in a high Zn content of the precipitates
and high Cu matrix content. The Zn and Cu atoms have
opposite effects on the electrochemical properties of
aluminum.21 A change in the corrosion behavior of the
7000 series aluminum alloy is related to changes in pre-
cipitate composition and particularly to a Cu enrichment
of the precipitates.22 During the RRA treatment, the
precipitates enrich by Cu atoms (Figure 9b) and this
factor as well as other factors improves the SCC
resistance.
4 CONCLUSIONS
The mechanical strength of an Al–8.5Zn–2.1Mg–
2Cu–0.2Ag (w/%) alloy after conventional T6 treatment
was lower than the mechanical strength after RRA
treatment.
SCR is a useful criteria to compare the SCC in
Al-Zn-Mg-Cu alloys after various heat treatments. The
SCR of the Al–8.5Zn–2.1Mg–2Cu–0.2Ag (w/%) alloy
after conventional T6 treatment was 71 % and was 95 %
after RRA treatment. The SCC improved significantly
after the RRA treatment.
The SCC improvement after the RRA treatment
related to Cu enrichment of the precipitates along with
the discontinuities of the grain-boundary precipitates.
5 REFERENCES
1 T. Dursun, C. Soutis, Recent developments in advanced aircraft
aluminium alloys, Materials & Design, 56 (2014), 862–871,
doi:10.1016/j.matdes.2013.12.002
2 D. Liu, B. Xiong, F. Bian, Z. Li, X. Li, Y. Zhang, F. Wang, H. Liu, In
situ studies of microstructure evolution and properties of an
Al–7.5Zn–1.7Mg–1.4Cu–0.12Zr alloy during retrogression and
reaging, Materials & Design, 56 (2014), 1020–1024, doi:10.1016/
j.matdes.2013.12.006
3 J. Li, N. Birbilis, C. Li, Z. Jia, B. Cai, Z. Zheng, Influence of
retrogression temperature and time on the mechanical properties and
exfoliation corrosion behavior of aluminium alloy AA7150,
Materials Characterization, 60 (2009) 11, 1334–1341, doi:10.1016/
j.matchar.2009.06.007
4 R. Bucci, C. Warren, E. Starke, The Need for New Materials in
Aging Aircraft Structures, Journal of Aircraft, 37 (2000) 1,
122–129, doi:10.2514/2.2571
5 D. Feng, X. M. Zhang, S. D. Liu, and Y. L. Deng, Non-isothermal
retrogression and re-ageing treatment schedule for AA7055 thick
plate, Materials & Design, 60 (2014), 208–217, doi:10.1016/
j.matdes.2014.03.064
6 D. Najjar, T. Magnin, T. J. Warner, Influence of critical surface
defects and localized competition between anodic dissolution and
hydrogen effects during stress corrosion cracking of a 7050 alumi-
nium alloy, Materials Science and Engineering: A, 238 (1997) 2,
293–302, doi:10.1016/S0921-5093(97)00369-9
7 T. Burleigh, The postulated mechanisms for stress corrosion cracking
of aluminum alloys: a review of the literature 1980-1989, Corrosion,
47 (1991) 2, 89–98, doi:10.5006/1.3585235
8 R. Jones, D. Baer, M. Danielson, J. Vetrano, Role of Mg in the stress
corrosion cracking of an Al-Mg alloy, Metallurgical and Materials
Transactions A, 32 (2001) 7, 1699–1711, doi:10.1007/s11661-
001-0148-0
9 E. Pugh, Progress toward understanding the stress corrosion
problem, Corrosion, 41 (1985) 9, 517–526, doi:10.5006/1.3583022
10 A. Sedriks, J. Green, D. Novak, Corrosion processes and solution
chemistry within stress corrosion cracks in aluminum alloys,
Proceedings of NACE, 1971, 569–575
11 T. Tsai, T. Chuang, Role of grain size on the stress corrosion crack-
ing of 7475 aluminum alloys, Materials Science and Engineering: A,
225 (1997) 1–2, 135–144, doi:10.1016/S0921-5093(96)10840-6
12 T. Pardoen, D. Dumont, A. Deschamps, Y. Brechet, Grain boundary
versus transgranular ductile failure, Journal of the Mechanics and
Physics of Solids, 51 (2003) 4, 637–665, doi:10.1016/S0022-
5096(02)00102-3
13 ASTM 139:2005(G)- Standard test method for determining stress-
corrosion cracking resistance of heat-treatable aluminum alloy
products using breaking load method, ASTM International, West
Conshohocken
14 K. Ghosh, N. Gao, Determination of kinetic parameters from
calorimetric study of solid state reactions in 7150 Al-Zn-Mg alloy,
Transactions of Nonferrous Metals Society of China, 21 (2011) 6,
1199–1209, doi:10.1016/S1003-6326(11)60843-1
15 T. Marlaud, A. Deschamps, F. Bley, W. Lefebvre, B. Baroux,
Influence of alloy composition and heat treatment on precipitate
composition in Al–Zn–Mg–Cu alloys, Acta Materialia, 58 (2010) 1,
248–260, doi:10.1016/j.actamat.2009.09.003
16 J. G. Tang, H. Chen, X. M. Zhang, S. D. Liu, W. J. Liu, H. Ouyang,
H. P. Li, Influence of quench-induced precipitation on aging behavior
of Al-Zn-Mg-Cu alloy, Transactions of Nonferrous Metals Society of
China, 22 (2012) 6, 1255–1263, doi:10.1016/S1003-6326(11)
61313-7
17 R. W. Revie, H. H. Uhlig, Uhlig’s corrosion handbook: vol. 51, John
Wiley & Sons, Hoboken 2011
18 G. Silva, B. Rivolta, R. Gerosa, U. Derudi, Study of the SCC
Behavior of 7075 Aluminum Alloy After One-Step Aging at 163° C,
Journal of Materials Engineering and Performance, 22 (2013) 1,
210–214, doi:10.1007/s11665-012-0221-4
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836 835
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
19 S. Maitra, G. English, Mechanism of localized corrosion of 7075
alloy plate, Metallurgical and Materials Transactions A, 12 (1981) 3,
535–541, doi:10.1007/BF02648553
20 H. Bakker, H. Bonzel, C. Bruff, M. Dayananda, W. Gust, J. Horvath,
I. Kaur, G. Kidson, A. LeClaire, H. Mehrer, Diffusion in solid metals
and alloys/Diffusion in festen metallen und legierungen: vol. 26,
Springer, Berlin 1990
21 E. Hollingsworth, H. Hunsicker, Corrosion of aluminum and alumi-
num alloys, ASM Handbook, 13 (1987), 583–609
22 J. T. Staley, S. Byrne, E. Colvin, K. Kinnear, Corrosion and stress-
corrosion of 7xxx-w products, Materials Science Forum, 217 (1996),
1587–1592, doi:10.4028/MSF.217-222.1587
M. SHAKOURI et al.: DEVELOPMENT OF A HEAT TREATMENT FOR INCREASING THE MECHANICAL PROPERTIES ...
836 Materiali in tehnologije / Materials and technology 51 (2017) 5, 831–836
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
B. YÜKSEL et al.: CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED ELECTROLESS ...
837–842
CORROSION RESISTANCE OF AS-PLATED AND HEAT-TREATED
ELECTROLESS DUBLEX Ni-P/Ni-B-W COATINGS
KOROZIJSKA ODPORNOST PLATIRANIH IN NEELEKTRI^NO
TOPOLOTNO OBDELANIH DUPLEKS Ni-P/Ni-B-W PREVLEK
Behiye Yüksel1, Garip Erdogan2, Fatih Erdem Bastan2, Rasid Ahmed Yýldýz3
1Istanbul Aydin University, Mechanical Engineering Department, 34668 Istanbul, Turkey
2Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, 54187 Sakarya, Turkey
3TU Istanbul, Mechanical Engineering Faculty, 34437 Istanbul, Turkey
behiyeyuksel@aydin.edu.tr
Prejem rokopisa – received: 2016-10-20; sprejem za objavo – accepted for publication: 2017-03-10
doi:10.17222/mit.2016.304
In this study, Ni-P/Ni-B-W dublex coatings were deposited on carbon steel substrates (AISI 1020) using the electroless plating
process and their microstructure and corrosion properties were systematically evaluated based on different heat-treatment
temperatures. Both, the surface morphology and cross-sectional morphology of the Ni-P/Ni-B-W coatings were studied using a
scanning electron microscope (SEM), while X-ray diffraction (XRD) was applied for examining the structural modifications.
The amorphous coating began to crystallize at a heat-treatment temperature of 350 °C. Potentiodynamic polarization
measurements were carried out in an aqueous medium containing 3.5 % NaCl for evaluating the corrosion resistance of
as-plated and heat-treated dublex coatings. The corrosion potentials of dublex coatings were observed to shift toward more
positive values with increased heat-treatment temperatures. Depending on the heat-treatment temperature, it was identified that
the crystallized dublex coatings generally had better corrosion resistance than the amorphous coating.
Keywords: Ni-P/Ni-B-W coating, corrosion, heat treatment
V {tudiji so bile dvojne Ni-P/Ni-B-W prevleke nane{ene na substrate iz ogljikovega jekla (AISI 1020) s postopkom neelek-
tri~nega platiranja. Sistemati~no so bile ocenjene mikrostruktura ter korozijske lastnosti izdelanih prevlek, glede na razli~ne
temperature toplotne obdelave. Morfologije povr{in in profili izdelanih Ni-P/Ni-B-W prevlek so bili pregledani z vrsti~nim
elektronskim mikroskopom (SEM), medtem ko so bile z rentgensko difrakcijsko analizo (XRD) ugotovljene strukturne
spremembe. Amorfna prevleka je za~ela kristalizirati pri temperaturi toplotne obdelave 350 °C. Korozijska odpornost platiranih
in toplotno obdelanih dupleks prevlek je bila ocenjena s pomo~jo meritev potenciodinami~ne polarizacije v 3,5 % vodni razto-
pini NaCl. Opazovali so korozijske potenciale dupleks prevlek, da bi dobili bolj pozitivne vrednosti z zvi{animi temperaturami
topolotne obdelave. Ugotovljeno je bilo, da imajo kristalizirane dupleks prevleke na splo{no bolj{o korozijsko odpornost kot
amorfne prevleke.
Klju~ne besede: Ni-P/Ni-B-W dvojna prevleka, korozija, toplotna obdelava
1 INTRODUCTION
Studied for the first time in 1946 by Brenner and
Riddell, the electroless plating process has been used in
many industrial applications because of its superior
characteristics over electroplating, such as the ability to
plate insulation materials and having a homogeneous
coating-thickness distribution.1 Among electroless coat-
ing types, electroless nickel coating is the most popular
for having good hardness, wear and corrosion resistance
properties.2 If the electroless Ni coating family is re-
viewed, besides pure Ni coatings, Ni-P and Ni-B coat-
ings deposited using reducing agents such as hypo-
phosphite, borohydride or dimethylamine borane stand
out. The major advantages of Ni-P coatings are their low
cost, high corrosion resistance and easy process control.3
Nonetheless, a borohydride reduced nickel coating is
harder and has higher wear resistance than tool steel and
hard chrome coatings.4,5 On the other hand, some studies
– although they are limited in number – have highlighted
the addition of tungsten to this coating system for in-
creasing its corrosion resistance, which is lower than that
of a Ni-P coating.6–9 As a result of the co-deposition of
this refractory material (which cannot be reduced in an
aqueous solution as metallic tungsten) together with iron
group metals, developing coatings with attractive corro-
sion and tribological properties is possible.10–12 Recently,
research for obtaining multilayer coatings to increase
existing coating corrosion resistance to higher levels has
also been conducted.13,14
The aim of the present study was to evaluate the
effect of different heat-treatment temperatures on the
microstructure and corrosion properties of electroless
Ni-P/Ni-B-W coatings. To the best of our knowledge, no
others studies are available on this topic.
2 EXPERIMENTAL PART
For this experiment, 10 mm × 10 mm × 60 mm plain
carbon steel (AISI 1020) plates were used as a substrate
material for developing Ni-P/Ni-B-W dublex coatings.
Prior to the plating process, the surfaces of all the
Materiali in tehnologije / Materials and technology 51 (2017) 5, 837–842 837
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.193:621.78:669.058 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)837(2017)