Introduction

Al–Zn–Mg–Cu (7xxx series) alloys possess a usage period for almost a century due to the extensive applications in aerospace and military industry to make various fuselage, wing beam, rib, wing wall, and so on [1, 2]. Satisfactory combinations of high strength, yield-strength ratio, preferable fracture toughness, fine corrosion resistance and practicable fatigue property have increasingly made it crucial in aforementioned fields [3]. Due to accelerating development of industrial application, high level demands of service performances for 7xxx series aluminum alloys are proposed, which means the alloy design turns to higher content designing of major alloying elements (mainly the zinc element) [4]. In the past twenty years, several new 7xxx alloys are registered, namely 7136 alloy, 7056 alloy and 7095 alloy, which have almost one percent enhancement on zinc content compared with typical 7055 alloy registered in the 1990s [5,6,7].

Compared with traditional 7xxx series aluminum alloys (5.0–8.0 wt% zinc content) [8,9,10,11,12], high-zinc alloys (no less than 9.0 wt% zinc content) have relatively faster ageing response degree and bigger number density of precipitates [13]. Furthermore, the variation of ageing precipitation kinetics and property performance has some differences from the traditional ones [14]. Accordingly, the microstructure variation of the high-zinc ones has its characteristics. Early researches have proved that traditional alloys endure obvious periods of one-step ageing treatments [15]. That means a certain time to peak-ageing state and then access over-ageing state with a typical ageing temperature of 120 °C. That is also the reason of choosing 120 °C/24 h as a commonly used peak-ageing treatment regime for traditional ones [13, 16]. Accordingly, precipitation characteristics have fine and dispersive matrix precipitates and continuously distributed grain boundary precipitates in common [17]. However, equivalent researches on high-zinc alloys have found the variations of microstructure of high-zinc alloys compared with the traditional ones. TEM technique can observe precipitates in hundred within an image, which makes it possible to be used to represent geometric features of precipitates with general image analyses. Thus, statistics obtained from several images can successively quantize the precipitates [18].

In the present study, a high-zinc Al–Zn–Mg–Cu alloy treated at several ageing temperatures is fully studied on the aspects of hardness, electrical conductivity and tensile properties. The microstructures with typical under-ageing, peak-ageing and over-ageing state are observed by TEM observations and the characteristics of matrix precipitates are quantitatively investigated.

Experimental Procedure

The researches were launched on samples extracted from an Al–9.2Zn–2.0 Mg–1.9Cu alloy extrusion with infinitesimal Zr, Fe and Si elements. The samples were solution treated with an optimized regime of 460 °C/1.5 h, followed by water-quenching and ageing treatment at 100, 120 and 140 °C. A 430 VSD Vickers hardness survey meter was used to monitor ageing hardening processes and each hardness datum was extracted from the average value of nine effective values. The electrical conductivity was measured by a current electrical conductivity meter performed under indoor temperature and the value is also gained by nine effective ones. Tensile specimens were machined to a gauge of Φ10 × 60 mm (M10) and the tensile direction was parallel to extrusion orientation. The tensile speed was two millimeters per minute and the result of three effective duplicate samples were averaged to obtain a tensile value. Disk samples for TEM observations were extracted from metal sheets with a line cutting thickness of about three millimeters. Then they were manually rubbed to a thickness of 40–60 mm. Every five disks are used to create minute holes by contrary jetted methanol solution with a quarter of nitric acid. The sample preparation temperature is no higher than −20 °C under a voltage pressure of 10–30 V. TEM examinations were conducted on a F2010 transmission electron microscope machine. The matrix precipitate size distribution was measured from the TEM images with the aid of an Image Plus Pro software.

Results and Discussion

Hardness, Electrical Conductivity and Tensile Properties

The hardness values of the alloy aged at (100, 120, 140) °C are shown in Fig. 1a. First the alloy aged at 100 and 120 °C is under discussion. The hardness exhibits a rapid increase during the initial few hours, a more placid increase up to 24 h and then maintain a platform till 128 h with little fluctuations. Here the hardness values of 120 °C are larger than that of 100 °C during ageing processes and the platform periods for them are 210–216 and 212–221 HV, respectively. As for the alloy aged at 140 °C, the hardness takes almost 12 h to reach a peak value of 218 HV, maintains a relatively short period till 28 h and then decreases continuously.

Fig. 1
figure 1

The alloy aged at 100, 120 and 140 °C: a hardness, b electrical conductivity

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Correspondingly, the electrical conductivity, as shown in Fig. 1b, also presents quits rules. Here we can see that the electrical conductivity increases with the enhancing of ageing temperature or the prolonging of ageing time. From the initial ageing point to the last point, the increase rates of electrical conductivity at 100, 120 and 140 °C enlarge successively. Among them, the enhancement from 120 to 140 °C is more remarkable than that from 100 to 120 °C.

Based on aforementioned hardness and electrical conductivity results, tensile properties of the alloy aged at 120 °C are proposed, as shown in Fig. 2. From 6 to 72 h, the ultimate tensile strength (UTS) has small fluctuations and almost maintains a terrace with a strength value range of 692–697 MPa. The yield strength (YS) shows an obvious increase within the initial 24 h and then maintain a platform with a strength value range of 655–659 MPa. The elongation has no conspicuous increase or decrease trend and is larger than 10%. So it is reasonable to consider that the alloy aged by 120 °C/24 h has the peak-ageing (PA) state with UTS, YS, elongation and electrical conductivity values of 697 MPa, 655 MPa, 12.9% and 17.3 MS m−1, respectively.

Fig. 2
figure 2

Tensile properties of the alloy with an ageing temperature of 120 °C

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Hence, the alloy aged at 120 °C for 4 h and 64 h are chosen as under-ageing (UA) state and over-ageing (OA) state, respectively, to conduct further microstructure observations combined with the PA state alloy.

TEM Observations

The selected area diffraction patterns (SADPs) for the alloy treated by UA, PA and OA states, which are projected along <100> and <112> orientations, are shown in Fig. 3. The matched diffraction spots for <100> and <112> orientations from the Al matrix have been labelled. As for the alloy with UA state, the corresponding spots at {1,1/4,0} and {1,7/4,0} in <100> projection come from GPI zone which is coherent with the matrix. In these two positions, the diffraction spots are quite obvious. The spots and rays at 1/3 and 2/3 of {220} position along <100> and <112> orientations, respectively, represent semi-coherent η′ phase. Some legible coherent GPII spots are found near a half of {311} position in <112> orientation. Hence, the man precipitates in the UA samples are GPI zone, GPII zone and η′ phase. In the case of the alloy with the PA state, it is obvious that the corresponding spots from GPII zone and η′ phase are still quite clear, which reveals the main precipitates under PA state are GPII zone and η′ phase. In the PA samples, no distinct GPI zone or GPII zone are found in the SADPs while the spots and rays representing η′ phase are plain, which uncover the dominating precipitates of the alloy under OA state. This demonstrates that the primary precipitates evolve from “GPI zone, GPII zone and η′ phase” to “GPII zone and η′ phase” to “η′ phase” with the alloy aged from the UA state to the PA state to the OA state.

Fig. 3
figure 3

Selected-area diffraction patterns (SADPs) in a–c <100> and d–f <112> for the alloy treated by a, d 120 °C/4 h (under-ageing state), b, e 120 °C/24 h (peak-ageing state) and c, f 120 °C/64 h (over-ageing state)

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Based on the SADPs analysis, matrix precipitates (MPs) and grain boundary precipitates (GBPs) of the UA, PA and OA alloy are observed, as shown in Fig. 4.

Fig. 4
figure 4

TEM images of a–c matrix precipitates and d–f grain boundary precipitates of the alloy treated by a, d 120 °C/4 h (under-ageing state), b, e 120 °C/24 h (peak-ageing state) and c, f 120 °C/64 h (over-ageing state)

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The matrix precipitates show small dimension and dispersive distribution. With ageing degree deepening, the size of matrix precipitates enlarges and a significant increase in the amount of big size precipitates happens. In the UA alloy, most matrix precipitate has an upper size limit of about 6–7 nm and distribute densely. In the OA alloy, the size of matrix precipitates enlarges obviously with an upper size limit of about 10–12 nm. The precipitate distribution is sparser than that of the UA alloy. As the ageing degree deepens to the OA state, the size of matrix precipitates increases furtherly and the upper size limit is about 14–16 nm. Among them, the amount of big size precipitates for the OA alloy is obviously larger than that of the PA alloy. The precipitate distribution is more scattered compared with the PA alloy.

The grain boundary precipitates show a similar trend. In the UA alloy, the grain boundary precipitates exhibit approximately successive distribution and the size is quite small. In the PA alloy, the size of grain boundary precipitates enlarges and an obvious discontinuous distribution of them appears. In the OA alloy, the size of them furtherly enlarges and the degree of discontinuous distribution deepens.

Here we can see that the size of precipitates has an obvious enhancement from the UA state to the PA state to the OA state. The matrix precipitate distribution becomes increasingly broader while the average matrix precipitate size becomes larger and larger. The discontinuous degree of grain boundary becomes more severe.

Conclusions

In this work, the microstructure characteristics combined with precipitate geometrical features, electrical conductivity, hardness and tensile properties in a high-zinc Al–9.2Zn–2.0Mg–1.9Cu alloy treated by single stage ageing treatments are investigated. The conductivity increases continuously at the ageing temperature of 100, 120 and 140 °C while the hardness increases severely in the initial period and then maintain a platform of 100 and 120 °C. The hardness increases severely in the initial period and then decrease continuously at the ageing temperature of 140 °C. The tensile strength keeps a platform while the yield strength increases in the initial 24 h and then maintain a platform. Based on property measurements, the samples with typical under-ageing state, peak-ageing state and over-ageing state are selected to observe microstructure. The results of SADPs observations indicate that the main precipitates in the alloy evolve from “GPI zone, GPII zone and η′ phase” to “GPII zone and η′ phase” to “η′ phase” when the ageing state transforms from the UA state to the PA state to the OA state. The Bright field TEM observations reveal that the precipitate characteristics has big variations from the UA state to PA state to OA state. As the ageing degree deepens, the average matrix precipitate size become larger, the matrix precipitate size distribution become broader and the grain boundary precipitate size become larger.