J. Cent. South Univ. (2017) 24: 1027-1033

DOI: 10.1007/s11771-017-3505-x

Effects of pre-deformation on microstructure and properties of Al–Cu–Mg–Ag heat-resistant alloy

LIU Xiao-yan(刘晓艳), WANG Zhao-peng(王召朋), LI Qing-shuai(李庆帅), ZHANG Xi-liang(张喜亮),

CUI Hao-xuan(崔好选), ZHANG Xiao-liang(张晓亮)

College of Materials Science and Engineering, Hebei University of Engineering, Handan 056038, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: The effects of the pre-deformation on the microstructure, mechanical properties and corrosion resistance of Al–Cu–Mg–Ag alloys were investigated by means of hardness tests, tensile tests, intergranullar corrosion (IGC) tests and transmission electron microscopy (TEM), respectively. The results show that with the increase of deformation amount, the aging hardening rate increases while the strength of the alloy decreases and then increases. The sample with a pre-deformation of 6% possesses the highest tensile strength due to the refinedly and homogeneously distributed precipitations. The pre-deformation aging accelerates the heterogeneous nucleation of Ω and θ′ phases at dislocations, and also refines the precipitations both in the grains and along the grain boundaries. The precipitation of Ω phase is restrained while that of θ′ phase is accelerated in pre-deformed Al–Cu–Mg–Ag alloy compared with the sample without pre-deformation. In addition, the width of the precipitate free zone decreases with increasing the pre-deformation amount, leading to a narrower IGC passageway. This results in an enhanced IGC resistance of Al–Cu–Mg–Ag alloy treated by pre-deformation aging.

Key words: Al–Cu–Mg–Ag alloy; pre-deformation; dislocation; intergranullar corrosion

1 Introduction

Al–Cu–Mg alloy with trace amounts of Ag is a new aviation aluminum alloy due to its good heat resistant property [1-4]. The main strengthening phases in Al–Cu–Mg–Ag alloys are Ω phases precipitating along {111} planes of the matrix, they remain stable at high temperature compared to the strengthening phases θ′ in the traditional heat resistant Al–Cu–Mg alloys. As a result, Al–Cu–Mg–Ag alloys possess excellent mechanical properties at elevated temperatures [5-8].

The main strengthening mechanism of Al–Cu–Mg– Ag alloy as an aging strengthening alloy is precipitation strengthening. The types of the precipitations and their quantity, distribution, etc confirmedly affect the mechanical properties of the alloy. To optimize the alloy properties, several aging processes of Al–Cu–Mg–Ag alloy have been reported, such as duplex aging [9], interrupted aging [10] and pre-deformation aging [11]. Pre-deformation aging is a common method to decrease the internal stress of the as-quenched aluminium alloys, especially for the thick plate, avoiding cracking during the following manufacturing process. It has been reported that pre-deformation aging could enhance the mechanical properties of aluminium alloys [12-14]. The formation of dislocations before aging could provide nucleation sites for the strengthening phases and accelerate the precipitation of these phases in Al–Cu–Mg–Ag alloys, whereas the relations between pre-deformation aging and mechanical properties are complicated and unclear. RINGER et al [11] have studied the effects of pre-deformation on the precipitation of Ω phase in an Al–4.0Cu–0.3Mg–0.4Ag alloy with a 6% deformation amount. They found that pre-deformation assisted precipitation of θ′ phase at the expense of Ω phase and decreased hardening response were reported during artificial aging. But prior cold work was found to increase the hardening response during natural aging.

Dislocations play an important part in varying the phases. As a result, the factors affecting the dislocation morphology might have great effect on the precipitations as well as the properties of the alloy. On the other hand, no research has been reported about the effect of pre-deformation on the corrosion resistance of Al–Cu–Mg–Ag alloy, even though the corrosion resistance is an important index for the property of Al

alloys. LI et al [15] reported that a 10% pre-deformation could lead to a decreased number of the phases precipitated along the grain boundaries and a narrower precipate free zone (PFZ), resulting in a reduced susceptibility to stress corrosion of 2519 Al alloy. ZHANG et al [16] found a larger pre-deformation amount of rolling combined with stretching could bring in the nore densely and more homogeneously distributed dislocations for Al–5.80Cu–0.20Mg–0.29Mn–0.20Zr alloy, and correspondingly finer and denser precipitates within the grains, discontinuous grain boundary precipitates, as well as narrower precipitate-free zone width after aging. The pre-deformed plate with a 7% rolling plus 3% perpendicular stretching showed better stress corrosion resistance.

In this work, the microstructure and mechanical properties of the aged Al–Cu–Mg–Ag alloy with various deformation amounts were studied. The effect of pre-deformation on the corrosion resistance was also investigated. The mechanism was discussed in details. This can give indispensible information for the investigation and application of Al–Cu–Mg–Ag alloy.

2 Experimental

The experimental material Al–5.3Cu–0.8Mg– 0.5Ag–0.3Mn–0.15Zr (mass fraction, %) was prepared by ingot metallurgy in a crucible furnace. The ingot was homogenized at 500 °C for 24 h [17], and then hot rolled to 6 mm. After being annealed at 400 °C for 2 h, the ingot was cold rolled to a 3 mm thin plate. The tensile specimens were got parallel to the rolling direction. These specimens were solid solution treated at 515 °C for 1.5 h [18], and subsequently water quenched. The pre-deformation was carried out in less than 0.5 h. All of the samples were aged at 185°C. The deformation amount was 0 (T6 sample), 2.5% and 6.0% (T8 sample).

Hardness tests were carried out on 401MVDTM digital Vickers microhardness tester with a 1.96 N load for 10 s according to ISO6507-1 specification. Room temperature mechanical properties were performed on CSS-44100 electronic universal testing machine following ISO 6892-1. The IGC resistance was tested according to ASTM G110. Transmission electron microscopy (TEM) observations were performed on TECNAI G220 transmission electron microscopy.

3 Results

3.1 Mechanical properties

Figure 1 shows the aging hardness curves of the pre-deformated Al–Cu–Mg–Ag alloys aged at 185 °C with various deformation amounts. It is obvious that the initial hardness of the alloy as well as the aging hardening rate increases with increasing the deformation amount. The hardness of the as-quenched alloy without deformation is HV 86 while that of the alloy with a deformation amount 6.0% is HV 133. The time to the peak value of the T6 sample is 4 h while it decreases to 2 h with the deformation amount increasing to 2.5% and 6.0%. The peak values of the two T8 samples are about the same as that of the T6 sample.

Fig. 1 Aging hardening curves of Al–Cu–Mg–Ag alloys with different deformation amounts

The mechanical properties of Al–Cu–Mg–Ag alloys with different deformation amounts are listed in Table 1. The tensile strength and the yield strength change with the same variation tendency. The strength of the sample with a 2.5% deformation is the lowest, even lower than that of the T6 sample. The strength increases with increasing the deformation amount while the elongation of the sample decreases. The sample with a 6.0% deformation amount possesses the highest tensile strength of 514 MPa whereas its elongation decreases to 10.6%.

Table 1 Mechanical properties of peak-aged Al–Cu–Mg–Ag alloys with different deformation amounts

3.2 Corrosion resistance

Figure 2 shows the cross profile pictures of the peak-aged Al–Cu–Mg–Ag alloy with different pre- deformation amounts after immersion in the IGC solution for 6 h. It is obvious that IGC attackes all the samples to a different extent. The maximun corrosion depth of the T6 sample is the largest and many particles fall off from the matrix (Figs. 2(a) and (b)). The maximum corrosion depth decreases and the surface is slightly attacked for the sample with a pre-deformation

amount of 2.5% (Figs. 2 (c) and (d)). Further increasing the pre-deformation amount to 6.0%, the IGC is alleviated and only a few particles fall off from the matrix (Figs. 2(e) and (f)).

Fig. 2 Cross profile pictures of peak-aged Al–Cu–Mg–Ag alloy with different pre-deformation amounts:

In order to clarify the effect of pre-deformation on the IGC property of Al–Cu–Mg–Ag alloy, the maximum depth is measured quantitatively and the corrosion level is evaluated according to ASTM standard. The results are listed in Table 2. It is obvious that the maximum depth decreases with increasing the pre-deformation amount.

Table 2 Intergranular corrosion results of peak-aged Al–Cu– Mg–Ag alloys with different pre-deformation amounts

The corrosion level of the T6 sample is 4 while those of the samples with a pre-deformation amount of 2.5% and 6.0% are both 3. The IGC test results indicate that pre-deformation can enhance the IGC resistance of Al–Cu–Mg–Ag alloy.

3.3 Microstructure

Figure 3 gives the TEM images of the as-quenched Al–Cu–Mg–Ag alloys with different deformation amounts. Some Al₃Zr particles (arrowed) as well as some dislocation lines are observed in the sample with a 2.5% deformation (Fig. 3(a)). With increasing the deformation amount up to 6.0%, a great amount of dislocation lines form (Fig. 3(b)), which might lead to the higher hardness of the as-quenched sample.

Figure 4 demonstrates the TEM images of the peak- aged Al–Cu–Mg–Ag alloys with different deformation amounts. Figures 4(a), (c) and (g) are recorded close to <110>_α and Figs. 4(e) and (f) are obtained close to <100>_α. A large quantity of Ω phases precipitate uniformly in the sample without pre-deformation. Meanwhile a few θ′ phases are also observed and the faint spots belonging to θ′ phase appear in the selective area electron diffraction picture (Fig. 4(a)). Ω phases with a fine size precipitate inhomogeneously (Fig. 4(c)). The precipitations in the sample with a 2.5% deformation amount are less than those in Fig. 4(a). Many θ′ phases are observed in Fig. 4(e) closed to <100>_α and their spots are more significant in the selective area electron diffraction picture (Fig. 4(c)) compared to that in Fig. 4(a). The Ω and θ′ phases both precipitate increasingly and distribute relative homogeneously in the matrix of the sample with a 6.0% deformation amount (Figs. 4(f) and (g)). The quantity of Ω phase decreases whereas that of θ′ phase increases, while the Ω phase is refined and their sizes decrease from 80 nm to 60 nm compared to the T6 sample (Fig. 4(a)).

The pre-deformation aging also affects the morphology of the grain boundaries. Coarse phases precipitate along the grain boundaries of the T6 sample (Fig. 4(b)) and a wider PFZ also forms in the sample. With increasing the pre-deformation amount, the precipitations on the grains are refined and the width of the PFZ decreases. A narrow PFZ is observed in the sample with a 6.0% deformation (Fig. 4(h)).

Fig. 3 TEM images of as-quenched Al–Cu–Mg–Ag alloys with different deformation amounts:

4 Discussion

The above results indicate that pre-deformation before aging plays an important part in the microstructure, mechanical properties and the corrosion resistance of Al–Cu–Mg–Ag alloy.

4.1 Effects of pre-deformation on microstructure

The precipitation processes in Al–Cu–Mg–Ag alloys could be shown as [17]

SSS (supersaturated solid solution)→Mg cluster/

Mg-Ag co-cluster→Ω (Al₂Cu)→θ (Al₂Cu)

SSS (supersaturated solid solution)→Cu cluster

(GP I)→GP II/θ″→θ′→θ (Al₂Cu)

The phase Ω has the same chemical composition with θ′ Al₂Cu. Although no Mg or Ag is included in the Ω phase, it is believed that Mg is a critical component for nucleation of Ω phases rather than Ag [17, 18]. As reported by REICH et al [19], Mg–Ag co-clusters could provide the nucleation sites of Ω phases. Mg and Ag

atoms segregated to a monoatomic layer of the Ω/α interface as Ω phase formed.

Fig. 4 TEM images of peak-aged Al–Cu–Mg–Ag alloys with different deformation amounts:

Because of their same chemical compositions, the phases Ω and θ′ competitively precipitate in the Al–Cu– Mg–Ag alloys. The results of the present research indicate that with increasing the deformation amount, the density of the dislocation increases, and correspondingly, the quantity of Ω phase decreases while that of θ′ phase increases. This implies that dislocations play an important role in the precipitation of the strengthening phases. Many literatures reported that pre-deformation aging could accelerate the precipitation of θ′ phase in Al–Cu–Mg alloys due to the formation of dislocations [12-14]. Dislocations could also accelerate the precipitation of T₁ in Al–Cu–Li alloys [11]. All of these results indicate that dislocations are advantageous for the precipitation of the strengthening phases in these alloys. This might be explained as: first, the interfacial energy could be decreased by dislocations and correspondingly, the resistance of the nucleation decreases; second, the solution atoms enrich around the dislocations and the driving force is enhanced; third, the activation energy of diffusion is decreased because of the short path diffusion channel provided by the dislocations. As a result, the nucleation ratio of the precipitations is enhanced.

However, the comparison of Figs. 4(a), (c) and (g) indicates that the existence of dislocations is adverse for the precipitation of Ω phase. It might be due to its complex nucleation process. The diffusion mechanism is vacancy-depended in the non pre-deformation Al–Cu– Mg–Ag alloy. A great number of Ag/Mg/vacancy co-clusters formed at the initial aging process because of the higher binding energy between Ag and the vacancies compared to that between Cu and vacancies [18] and correspondingly, numerous Ω phases precipitate during the following aging process. A few θ′ phases with the same chemical composition with Ω phase precipitate in the T6 sample. Treated by pre-deformation, the diffusion mechanism in the sample is dislocation-depended. The nucleation of Ω phase is decreased, and correspondingly, the nucleation and precipitation of θ′ phase are accelerated.

4.2 Effects of pre-deformation on mechanical property

The deformation amount also affects the mechanical property of Al–Cu–Mg–Ag alloy. It can be seen from Table 1 that the strength of the sample with a deformation amount of 2.5% is the lowest in the present samples. It might be due to the small quantity of dislocation. Though the dislocations could accelerate the precipitations of the strengthening phases, their distribution is inhomogeneous (Fig. 4(c)). So, the strength effect is decreased. The quantity of dislocation increases with increasing the deformation amount, providing more nucleation sites for the strengthening phases. Meanwhile, the dislocations distribute uniformly in the matrix because of their high density. The nucleation ratio increases and correspondingly, the precipitation is refined, leading to the higher strengthening level.

Though the precipitation of Ω phase is restrained in the present work, they are refined in the dimension. Together with θ′ phase accelerated by dislocations, the strength of the sample with a pre-deformation amount of 6.0% keeps at a relative high level.

4.3 Effects of pre-deformation on corrosion resistance

The results of IGC tests indicate that pre- deformation could enhance the IGC resistance of Al–Cu–Mg–Ag alloy. The IGC generally starts at the grain boundaries and goes on along these areas. As discussed in our early work, the self-corrosion potential of the PFZ, the equilibrium phase (θ) on the grain boundaries and the matrix of Al–Cu–Mg–Ag alloy satisfy E_PFZθmatrix [20]. The IGC property of Al–Cu–Mg–Ag alloy is determined by PFZ with the lowest corrosion potential. Large amounts of solution atoms are consumed to form the coarse equilibrium phase along the grain boundaries for the T6 sample (Fig. 4(a)), resulting in a wider PFZ. This could provide the broad IGC corrosion passage for the T6 sample. Treated by pre-deformation, the dislocations are distributed in the grains as well as closed to the grain boundaries. Some phases also precipitate near the grain boundaries. This leads to the narrower PFZ of the T8 sample and correspondingly, the IGC passageway becomes narrower. With increasing pre-deformation amount, the width of the PFZ decreases, resulting in a higher IGC resistance of Al–Cu–Mg–Ag alloy.

5 Conclusions

1) Pre-deformation aging could accelerate the inhomogeneous nucleation of Ω and θ′ phases, and refine the precipitations both in the grains and along the grain boundaries. The precipitation of Ω phase is restrained while that of θ′ phase is accelerated.

2) The tensile strength of Al–Cu–Mg–Ag alloy decreases and then increases with increasing the deformation amount. The suitable pre-deformation aging process is at 185 °C for 2 h with a 6.0% deformation amount, treated by which, the tensile strength of the alloy is 514 MPa.

3) With increasing the pre-deformation amount, the width of PFZ decreases, leading to a narrower corrosion passageway. The sample with a 6.0% pre-deformation amount possesses an excellent IGC resistance.

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(Edited by FANG Jing-hua)

Cite this article as: LIU Xiao-yan, WANG Zhao-peng, LI Qing-shuai, ZHANG Xi-liang, CUI Hao-xuan, ZHANG Xiao-liang. Effects of pre-deformation on microstructure and properties of Al–Cu–Mg–Ag heat-resistant alloy [J]. Journal of Central South University, 2017, 24(5): 1027-1033. DOI: 10.1007/s11771-017-3505-x.

Foundation item: Project(E2013402056) supported by the Natural Science Foundation of Hebei Province, China; Project(QN2014002) supported by the Science and Technology Research Foundation of Hebei Education Department for Young Teachers in University, China; Project(51601053) supported by the National Natural Science Foundation of China

Received date: 2016-01-21; Accepted date: 2016-07-25

Corresponding author: LIU Xiao-yan, Associate Professor, PhD; Tel: +86-310-8577971; E-mail: x918y@126.com, xiaoyanliu@hebeu.edu.cn