1 Introduction

In the last two decades, various new composite materials and alloys have been fabricated for using in high technological industries [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. One of them is Al–Li alloy which was used for the first time in aerospace structures with a view to reduce their weight. Recently, these alloys have been employed in cryogenic applications such as hydrogen and oxygen fuel tanks in aerospace vehicles [16]. Among various grades of Al–Li alloys, only in the Al–Li 8090 alloy, the second element after aluminum is lithium, while in other, the second element is copper. This issue leads to decrease the mass density of Al–Li 8090 alloy.

In recent years, several studies have been carried out to investigate different applications and characterizations of Al–Li 8090 alloy. Engler and Lucke [17] studied the texture development in the Al–Li 8090 alloy during cold rolling with the aid of X-ray pole figures and orientation distribution function analysis. Watkinson and Martin [18] correlated the changes in hardness of the duplex aged Al–Li 8090 alloy using transmission electron microscopy and differential scanning calorimetry. Kornisarov et al. [19] investigated the microstructure of the precipitates in an Al–Li 8090 alloy qualitatively and quantitatively by techniques of transmission electron microscopy through the stages of retrogression and reaging. Perez-Landazabal [20] employed the relative method to X-ray spectra in order to examine the precipitated mass fraction of δ and δ′ in Al–Li alloys. Eddahbi et al. [21] analyzed the dynamic recrystallization of an Al–Li 8090 alloy at high temperatures and a specific value of the strain rate. Gaber and Afify [22] utilized the variation of thermophysical properties of Al–Li 8090 quenched from the solid solution state to indicate the temperatures at which the phase transformations occur. Bairwa and Date [23] determined tensile properties of Al–Li alloy sheets for three different solutionising temperatures, with three ageing times at each of the three ageing temperatures. Kumaran et al. [24] identified the local maximum transition temperatures at the selected heating rate, precipitation of coherent δ′, dissolution of δ′ and precipitation of stable S′ and δ. They also studied the first differential of temperature dependent ultrasonic parameters as an effective tool to identify precipitation reactions in a slow-treated Al–Li 8090 alloy [25]. Katsikis et al. [26] investigated the microstructural stability during low temperature exposure of Al–Li 8090 alloys. Rajendran et al. [27] performed ultrasonic velocity and attenuation measurements on the as-received and thermally treated Al–Li 8090 alloys. Zhang et al. [28] explored the microstructure, electrical conductivity and mechanical properties of cast Al–2Li–2Cu–0.5 Mg–0.2Zr alloy during heat treatment. El-Aty et al. [29] reported the influence of sample orientation and strain rates on the tensile properties and anisotropy behavior of Al–Li alloys sheet through quasi-static and dynamic uniaxial tensile tests. Zhang et al. [30] anticipated the effect of creep aging on strength and toughness and microstructure evolution of Al–Li alloy using tensile test and Kahn tear test at room temperature, in conjunction with fractograph and transmission microstructure analysis. Liu et al. [31] studied the influences of surface abrasion on microstructure and pitting corrosion of Al–Li alloys.

During the aging heat treatment, the formation of the metastable phase δ′(Al3Li) plays an essential role in the strengthening of the Al–Li 8090 alloy. In other words, the δ′ precipitation has a long-range ordering structure which causes a significant effect on the slip procedure. Consequently, it yields to control the mechanism of dislocation breakdown. Various thermal treatments have been utilized to improve mechanical properties of different alloys such as Al–Li 8090 alloy. Gable et al. [32] predicted the quench sensitivity of Al–Li 8090 alloy via employing different coating rates from the solution heat treatment temperature. Kim et al. [33] enhanced the microhardness of a heat treated Al–Li 8090 alloy by increasing Mg content due to increased solid solution strengthening and reduction of the grain size. Nayan et al. [34] explored the hot deformation behavior of Al–Li alloys in homogenized condition via the hot isothermal compression testing. Rodak et al. [35] produced an ultrafine grained structure based on Al–Li alloys in the solution treated condition and additionally in aging condition.

The main problem in most aging heat treatments is the reduction of the ductility together with the enhancement of the strength. For instance, the thermomechanical aging heat treatments decrease significantly the ductility and make anisotropic properties. In the current study, a new heat treatment method namely as directional quenching is proposed which has the capability to increase simultaneously both of the ductility and strength of Al–Li 8090 alloy.

2 Experimental Procedure

2.1 Specimen

In the present study, a sheet of 2 mm thickness made of Al–Li 8090 alloy with chemical composition given in Table 1 is considered as the starting material. The main impurities are 0.05% Fe, 0.03% Si, 0.02% Ti, and 0.02% Zn (wt%). Two groups of specimen are manufactured. The first group includes small specimen with dimension 10 mm × 10 mm × 2 mm for using in the hardness test with standard number of ASTM E45. The second group includes tensile specimen with standard number of ASTM E8 to utilize in the tensile properties test.

Table 1 Chemical composition of used Al–Li 8090 alloy
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2.2 Applied Aging Heat Treatments

Three different aging heat treatments are employed in this work to improve the mechanical properties of Al–Li 8090 specimen as below

  1. (a)

    T6 → Solution treatment at 535 °C for 1 h + quenching within 0 °C water + aging at 170 °C for various times

  2. (b)

    T8 (thermomechanical heat treatment) → Solution treatment at 535 °C for 1 h + quenching within 0 °C water + 14% cold rolling + aging at 170 °C for various times

  3. (c)

    Directional quenching (the proposed method) → Solutionizing treatment along a specific direction at 535 °C for 1 h + quenching along a specific direction within 0 °C water + aging at 170 °C for various times

The proposed directional quenching method is similar to T6 heat treatment process. However, in this new method, heating and cooling process are conducted in a specific one direction. For this purpose, the specimen inserted in a castable alumina as the insulator in such a way that only one of six faces of the specimen is exposed to quenching. This procedure is depicted schematically in Fig. 1.

Fig. 1
figure 1

Schematically representation of an inserted sample in a castable alumina as the insulator

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2.3 Microscopic Examinations

2.3.1 TEM

In order to perform the associated microstructural examinations, a Jeol, JEM microscope 2000FX operated at 200 kV is put to use. Thin discs of 2 mm diameter are punched from the prepared foils and then they are electro-polished via a twin jet device using a solution comprising 25% nitric acid and 75% methanol cooled to − 25 °C.

2.3.2 SEM

PHILIPS XL-30 scanning electron microscope (SEM) is utilized for fractography analyses of specimen.

2.4 Mechanical Properties

The Brinell hardness test of 31.35 kg loading in accordance with ASTM E45 standard number, and tensile property test with ASTM E8 standard number are performed on the related specimen.

2.5 Thermal Stress Analysis

In order to study the thermal gradients and thermoelastic stress distribution in specimen due to the quenching process, the finite element method via ANSYS software is employed.

The flowchart including all of the processes is depicted in Fig. 2.

Fig. 2
figure 2

Flowchart of the experimental procedure

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3 Results and Discussion

3.1 Thermal Stress Analysis

The results of the thermal stress analyses using finite element method which are demonstrated the distribution of thermoelastic stress are presented in Figs. 3 and 4 corresponding to T6 and directional quenching methods, respectively. It is observed that the value of thermoelastic stress associated with the directional quenching is so higher and is distributed more uniformly compared to the T6 counterpart.

Fig. 3
figure 3

Thermal stress analysis for a heat treated sample via T6 method after various times at = 0.25 s, bt = 5 s, ct = 10 s, dt = 15 s

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Fig. 4
figure 4

Thermal stress analysis for a heat treated sample via directional quenching method after various times at = 0.25 s, bt = 5 s, ct = 10 s, dt = 15 s

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The high thermoelastic stress with uniform distribution obtained by the directional quenching method leads to nucleation and growth of δ′ fine phase which results into improve simultaneously both of the strength and ductility of specimen. In Fig. 5a, the TEM microstructure of a heat treated sample via the directional quenching method is shown. The uniform distribution of the δ′ fine phase can be seen clearly. It is indicated that the diameter of these δ′ spherical precipitates is within the range of 4–24 nm. Also, In Fig. 5b, the selected area diffraction pattern (SADP) associated with the δ′ spherical precipitates is illustrated. According to the SADP along [1 1 0] direction, it is obvious that the structure of these precipitates is an ordered L12.

Fig. 5
figure 5

a TEM image of a heat treated sample via the directional quenching method, b SADP associated with the δ′ spherical precipitates along [1 1 0] direction

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3.2 Hardness Property

In Fig. 6, the variation of hardness with time corresponding to different aging heat treatments is illustrated. It can be seen that all of the plots represent the same pattern. At first, an increment in the value of hardness occurs up to a maximum point. Thereafter, the hardness of specimen reduces with time. The reason of the initial increment in the hardness is due to the creation of GP zones and intermediate precipitations which nucleate and grow coherently with the matrix. It should be noted that a more homogenous distribution of these precipitations leads to enhance the hardness more efficiently. Through transformation of the intermediate precipitations to δ′ (Al3Li) coherent phase, the maximum hardness achieves. After this maximum point, by passing the aging time, the precipitations grow to a high level which causes to decrease the hardness.

Fig. 6
figure 6

Variation of hardness with time corresponding to different heat treatment techniques

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Through comparison of the directional quenching and T6 heat treatment method, it is revealed that the value of the hardness at the maximum point associated with the direction quenching method is higher than that of the T6 one. Moreover, the needed time to obtain the maximum hardness decreases from 48 h in T6 method to 10 h in the directional quenching method. The reason of such improvements is the creation of high value of the thermos-elastic stress in the directional quenching method. This high stress acts as a driving force to create strengthening δ′ (Al3Li) precipitates in a more short time with higher volume fraction.

By comparing the results associated with the directional quenching and T8 heat treatment methods, it is found that the values of the hardness in the maximum points are approximately the same, but the needed time to acquire the maximum hardness reduces from 34 h in T8 method to 10 h in the directional quenching method. It means that the effect of thermos-elastic stress due to the directional quenching process on the atomic diffusion, nucleation and growth of δ′ phase is more than that of the stress due to the cold work in T8 process.

3.3 Tensile Properties

In Tables 2, 3, 4 and 5, the yield strength, ultimate tensile strength, and elongation of a non-heat treated sample, T6, T8 and directional quenching heat treated samples are given. The results associated with the peak of hardness are presented in the highlighted column of the tables. In accordance with the presented results, in T6 and T8 heat treatment methods, the strength enhances, but the elongation decreases significantly compared to the non-heat treated sample. However, in the directional quenching method, the strength of the heat treated sample increases as well as the elongation is higher than that of T6 and T8 methods. The reason of this observation is uniform distribution of δ′ phase as mentioned in the thermal stress analysis.

Table 2 Tensile properties of non-heat treated sample
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Table 3 Tensile properties of heat treated specimen via T6 method with aging temperature of 170 °C
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Table 4 Tensile properties of heat treated specimen via T8 method with aging temperature of 170 °C
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Table 5 Tensile properties of heat treated specimen via directional quenching technique with aging temperature of 170 °C
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3.4 Fractography

In Figs. 7, 8 and 9, the SEM micrographs of cross-sectional fracture pattern at the time associated with the peak of hardness as well as before and after it are represented for samples which are heat treated with different heat treatment methods. For the heat treated sample with T6 method, an ununiformed distribution of ductility can be seen in the cross-sectional fracture pattern. For example, in Fig. 7b, a complete brittle fracture is observed at point A, but at point B, the type of fracture tends to a ductile one. This pattern is probably due to the ununiformed distribution of δ′ phase during the T6 aging heat treatment.

Fig. 7
figure 7

SEM micrographs of cross-sectional fracture patterns corresponding to T6 heat treatment technique with aging temperature of 170 °C: at = 34 h, bt = 48 h, ct = 72 h

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Fig. 8
figure 8

SEM micrographs of cross-sectional fracture patterns corresponding to T8 heat treatment technique with aging temperature of 170 °C: at = 6 h, bt = 34 h, ct = 72 h

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Fig. 9
figure 9

SEM micrographs of cross-sectional fracture patterns corresponding to directional quenching heat treatment technique with aging temperature of 170 °C: at = 8 h, bt = 10 h, ct = 12 h

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For the heat treated sample with T8 method, a smooth cross-sectional fracture pattern is found which indicates a complete brittle fracture. However, for the heat treated sample with the directional quenching method, the presence of cavities with appropriate depth in the cross-sectional fracture pattern indicates a significant deformation of the sample during the fracture procedure. These cavities are distributed uniformly within the cross-sectional fracture pattern.

4 Conclusion

The main objective of this work was to introduce a new aging heat treatment method namely as the directional quenching technique to improve the mechanical properties of Al–Li 8090 alloy. To this end, the solutionizing treatment as well as quenching were conducted along a specific direction. In order to confirm the advantage of this method, the results obtained from it were compared with those of T6 and T8 conventional heat treatment methods. It was observed that in opposite to the two other methods, the directional quenching technique has the capability to enhance simultaneously both of the strength and ductility of Al–Li 8090 alloy. Moreover, based upon the conducted finite element based thermal stress analysis, it was demonstrated that there is a uniform distribution with high volume fraction of δ′ (Al3Li) precipitates within the improved heat treated sample via the directional quenching method.