High-Temperature Mechanical Behavior of an As-Extruded Al-5Zn-2Mg-0.3Cu (in wt.%) Alloy

Abstract

To ensure that Al alloys are being served as the high-temperature structural components for applications in aerospace and transportation, it is necessary to investigate their high-temperature mechanical behavior and failure mechanism. In this paper, the mechanical behavior of the as-extruded Al-5Zn-2Mg-0.3Cu (in wt.%) alloy was studied and compared under different high-temperature tensile-testing conditions. It was found that the yield strength and the ultimate tensile strength of the alloy gradually decreased with the increase in temperature, but its elongation ratio showed a slightly increasing trend. Failure analysis demonstrated that there were a lot of ductile dimples on the fracture surfaces and that obvious necking occurred for the samples being tensile-tested at different temperatures. Surface observation revealed that the initiation of micro-cracks was mainly attributed to the self-cracking of the brittle phase particles. Moreover, when the testing temperature was between 450 °C and 550 °C, micro-cracks could also occur at the interface between phase particles and the Al matrix.

1. Introduction

Since aluminum (Al) alloys have many advantages, such as low density, good strong plasticity, corrosion resistance, and good electrical and thermal conductivity, they have a wide application prospect in the fields of rail transit, ship, and aerospace [1,2,3,4,5]. At room temperature, their mechanical properties can well meet the standard of material selection for light-weight and high-strength structural materials [6]. However, with the rapid development of aerospace technology and transportation, the service temperature of Al alloys being the high-temperature structural components in engine blocks, cylinder heads, and engine room components is becoming harsher and harsher [7,8]. For example, the temperature of the Al alloy for its application as a missile shell can reach 400 °C (0.6 T_m) or above during the operation in question [9]. In high-temperature conditions, the mechanical degradation of Al alloys has become a key constraint to their application [10]. Therefore, it is of great significance to develop high-strength and heat-resistant Al alloys for ensuring their service reliability at elevated temperatures.

Compared with other Al alloys, the Al-Zn-Mg-Cu (7xxx) series alloys have a relatively high specific strength, good workability and corrosion resistance, and are often used as part of an aircraft’s skin and wing frame [11,12,13]. In addition, the high-temperature mechanical properties of the 7xxx series Al alloys are good [14,15,16]. In the research regarding the mechanical behavior of 7015 and 6061 Al alloys at temperatures ranging from room temperature to 500 °C, Oñoro et al. reported that the tensile strength of the 7015 Al alloy was significantly higher than that of the 6061 Al alloy, but that the tensile strength of the 7015 Al alloy at 400 °C was still lower than 100 MPa [14]. Yang et al. reported that the main softening mechanisms of the 7050 Al alloy at 350 °C/0.01 s⁻¹ were dynamic recovery, dynamic precipitation, and coarsening of precipitates, while the main softening mechanisms at 450 °C/10 s⁻¹ were dynamic recovery, dynamic recrystallization, and deformation heating [15]. Due to these reasons, the tensile strength of the as-extruded 7075 Al alloy at 550 °C was only 20 MPa [16]. So far, although the mechanical behavior of the 7xxx series Al alloys at elevated temperatures above 400 °C has been investigated, few studies about the effect of temperature on the micro-damage mechanisms at elevated temperatures can be referred.

In this work, through investigating and comparing the mechanical behavior of an as-extruded Al-5Zn-2Mg-0.3Cu (in wt.%) (7N01) Al alloy at the elevated temperature of above 400 °C, the effects of temperature on the mechanical properties and failure mechanisms were disclosed. The study of this topic would significantly deepen the understanding of the relationship between the microstructure and cracking mechanism of Al alloys at elevated temperatures. Moreover, the obtained high-temperature mechanical data can be used for guiding whether the investigated alloy is suitable for application in engine components.

2. Experimental Procedures

2.1. Material and Sample Preparation

The material used in this experiment is a commercial as-extruded 7N01 Al alloy sheet with a thickness of 10 mm and an extrusion ratio of 10:1, which was provided by the CSR Qingdao Sifang Co. Ltd. of Qingdao, China. The concrete chemical composition of the alloy was determined using an inductively coupled plasma spectrometer (ICP-AES) (Agilent, Beijing, China), as listed in

Table 1. Chemical composition of the 7N01 Al alloy (in wt.%).

Chemical Composition	Zn	Mg	Cu	Mn	Cr	Ti	Zr	Si	Fe	Al
Content	4.96	1.95	0.28	0.45	0.25	0.08	0.22	0.28	0.35	91.18

.

The bulk metallographic specimens with a dimension of 10 mm × 10 mm × 3.5 mm were cut from the as-extruded 7N01 Al alloy sheet using a wire, electrical discharge cutting machine. Here, samples with their exposed large surface being perpendicular to the extrusion direction (ED), transverse direction (TD) and normal direction (ND) were defined as ED sample, ND sample and TD sample, respectively. Similarly, the plate-shaped tensile specimens were cut from the sheet along the extrusion direction and their surfaces were parallel to the sheet surface. The shape and dimensions of the tensile specimen are shown in Figure 1. Then, the surfaces of the bulk metallographic specimens and tensile specimens were ground by sequentially using 400, 800, 2000 and 5000 grit papers, followed by mechanical polishing with 2.5 μm and 1.0 μm of diamond grinding paste until no small scratches could be observed on the polished surfaces using an optical microscope (OM, VHX-900F) (Keyence, Osaka, Japan) with a magnification of 500×. After that, the polished surfaces of the metallographic specimens were etched with 16 mL HNO₃ + 1 mL HF + 3 g Cr₂O₃ + 83 mL H₂O.

2.2. Microstructural Observations

X-ray diffraction (XRD; D/Max 2400) (Rigaku, Tokyo, Japan) with a radiation source of monochromatic Cu-Ka (wavelength of 0.154056 nm), a step length of 0.02°, a scanning range of 10° to 90°, a scanning rate of 10°/min and an accelerated voltage of 40 kv was employed for analyzing the phase component of the alloy. Based on the MDI Jade 9 software, the diffraction peak was calibrated and the existing phases in the alloy were determined. The etched surfaces of differently orientated samples were observed using an optical microscope (OM, VHX-900F) (Keyence, Osaka, Japan). The size, morphology and distribution of phase particles were observed using a scanning electron microscope (SEM, EM Crafts Cube II) (EmCrafts, Hanam-si, Republic of Korea). Based on the energy dispersive spectrum (EDS, Oxford Xplore Compact 30) (Oxford instruments, Oxford, UK) analysis and elemental scanning, the main chemical components and their distribution of phase particles were determined.

2.3. High-Temperature Tensile Testing

The tensile testing of the as-extruded 7N01 Al alloy at 400 °C, 450 °C, 500 °C and 550 °C were carried out using a tensile testing machine (MTS, 370.10) (Mechanical Testing & Simulation, Eden Prairie, MN, USA) equipped with a high-temperature environmental furnace. The tensile samples were loaded on the test machine with their gauge sections being located in the middle of the environmental furnace. After the temperature had reached the set values, the samples were held at the temperature for 2 h to ensure that they would reach the set temperature. During the heat preservation process, the temperature fluctuation in the high-temperature environmental furnace was ±1 °C. Then, the samples were subjected to high-temperature tensile tests at the tensile rate of 1 × 10⁻³ s⁻¹. In order to reflect the error range of the high-temperature mechanical properties of samples, three parallel samples were used for each condition.

2.4. Failure Analysis

After the high-temperature tensile testing, the tensile fracture surfaces were washed with alcohol and air-dried. Then, the fracture morphologies of the samples which had failed at 400 °C, 450 °C, 500 °C and 550 °C were observed using SEM. A three-dimensional (3D) metallographic microscope (OM, VHX-900F) (Keyence, Osaka, Japan) was used to observe the 3D morphologies of the tensile fracture surfaces. In order to characterize the plastic deformation and failure mechanisms of the samples that had been tensile-tested at different elevated temperatures the side surfaces with a distance of 2 mm and 5 mm from the fracture surfaces on the basis of their corresponding cross-sectional shrinkage ratios were observed using SEM.

3. Results

3.1. Microstructural Characterization

Figure 2 shows the microstructure of ED, TD and ND oriented samples. It can be seen that the grain structures of the samples with different orientations are equiaxed and the grain sizes are quite uniform. The average grain sizes of the ED, ND and TD samples were determined to be 6.9 μm, 7.5 μm and 7.9 μm, respectively. In comparison, the grain size of the ED sample was slightly smaller than that of the other two samples. Figure 3 is the result of the SEM observations and EDS analysis of the as-extruded 7N01 Al alloy. It can be seen that there are some bright, white, irregular particles with a size of less than 5 μm, in the matrix (Figure 3a). Moreover, these particles are aggregated and preferentially distributed along the extrusion direction. The EDS spectrum and elemental scanning reveal that the main components of these particles are Al, Fe, Mn and Si (Figure 3b,c). The XRD phase analysis of the as-extruded 7N01 Al alloy is shown in Figure 4. It can be seen that the alloy is mainly composed of an α-Al₁₅(MnFe)₃Si₂ phase and the Al matrix, because the low solubility of the Fe and Mn elements can easily cause the formation of the α-Al₁₅(MnFe)₃Si₂ phase [17].

3.2. Mechanical Properties at Elevated Temperatures

Figure 5 shows the high-temperature tensile-stress–strain curves of the as-extruded 7N01 Al alloy at temperatures of 400 °C, 450 °C, 500 °C and 550 °C. In order to describe and compare their mechanical properties at different elevated temperatures, the yield strength (σ_0.2), ultimate tensile strength (UTS) and elongation ratio (ε_f) are determined, as listed in

Table 2. Yield strength, tensile strength and elongation ratio of the as-extruded 7N01 Al alloy.

Testing Temperature	σ0.2/MPa	UTS/MPa	εf/%
400 °C	269 ± 6	308 ± 2	18 ± 1
450 °C	260 ± 1	286 ± 1	21 ± 1
500 °C	256 ± 1	276 ± 1	20 ± 1
550 °C	255 ± 2	273 ± 2	23 ± 1

. It can be seen that the σ_0.2 and UTS values of the alloy decrease with the increase in testing temperature, but the alloy’s plasticity changes slightly at different temperatures. At the temperatures of 400 °C, 450 °C, 500 °C and 550 °C, the σ_0.2 values of the alloy are, respectively, 269 MPa, 260 MPa, 256 MPa and 255 MPa; the UTS values are, respectively, 308 MPa, 286 MPa, 276 MPa and 273 MPa and the ε_f values are, respectively 18%, 21%, 20% and 23%. It is demonstrated that the alloy has good high-temperature mechanical strength because its yield strength can be higher than 250 MPa at an elevated temperature ranging from 400 °C to 550 °C.

3.3. Fracture Mechanism

Figure 6 shows the tensile fracture morphologies of the as-extruded 7N01 Al alloy tested at 400 °C, 450 °C, 500 °C and 550 °C. It can be seen that the tensile samples at different temperatures show the obvious necking phenomenon and that their fracture surfaces are distributed with a large number of plastic dimples, indicating their typical ductile fracture characteristics. Meanwhile, the size of plastic dimples on the fracture surfaces increases with the testing temperature. In addition, phase particles can be observed at the bottom of the dimples. Figure 7 shows the optical observations of the fracture surfaces and the three-dimensional morphologies of the alloy at different temperatures. It can be seen that the maximum height-difference values caused by the plastic dimples on the fracture surfaces of samples tensile-tested at 400 °C, 450 °C, 500 °C and 550 °C are 214.76 μm, 281.83 μm, 256.78 μm and 347.68 μm, respectively. Generally, the size and depth of plastic dimples can reflect the plastic-deformation capability of metallic materials, in which larger and deeper plastic dimples correspond to a better plasticity [15]. In contrast, for the sample being tensile-tested at 400 °C, the plastic dimples on its fracture surface have the smallest size and the shallowest depth, resulting in the lowest elongation ratio. For the sample being tensile-tested at 550 °C, the largest size and the deepest plastic dimples on the fracture surface ensure the highest elongation ratio.

In order to disclose the micro-damage mechanism of the alloy at different temperatures, the side surfaces 2 mm and 5 mm away from the fractures were observed using SEM, as shown in Figure 8. It can be seen that, for the tensile samples tested at different temperatures, slip traces are not obvious and the plastic deformation is weak in the area 5 mm away from the fractures and their corresponding cross-sectional shrinkage ratios of 7%~8%. However, micro-cracks can be observed in this area and their initiation is mainly due to the self-cracking of coarse phase particles. For the area 2 mm away from the fracture surfaces, the cross-sectional shrinkage ratios of the corresponding sections of tensile samples are 36%~38%. In this area, a large number of slip traces can be observed, indicating that severe plastic deformation occurs. In addition, the initiated micro-cracks are mainly located in the interior of the phase particles. However, when the testing temperature is between 450 °C and 550 °C, micro-cracks can also occur at the interface between the phase particles and the Al matrix.

4. Discussion

4.1. Effect of Temperature on the High-Temperature Mechanical Properties

Based on the measured mechanical properties at high temperatures, it can be seen that the mechanical strength of the 7N01 Al alloy decreases with the increase in temperature. It has been reported that the main strengthening mechanisms of the 7xxx series Al alloys were the dislocation strengthening and the second-phase strengthening [11,18]. Among these, the second-phase strengthening mechanism includes the cut-through mechanism and the Orowan mechanism [19]. When the size of the second-phase particles exceeds 3 nm, the main strengthening mechanism of the alloy will change from the cut-through mechanism to the Orowan bypass mechanism [11,19]. In this work, the size of the phase particles in the 7N01 Al alloy is about 5 μm, much larger than 3 nm. Therefore, the strengthening mechanism should be mainly ascribed to the Orowan bypassing mechanism. However, with the temperature increasing, the enhancement degree due to the Orowan bypassing mechanism gradually weakens [11]. In addition, although the phase particles can effectively pin the dislocation movement and play an important role in strengthening the alloy [18,19], their pinning effect on the dislocation movement and grain boundaries sliding is remarkably degraded at elevated temperatures [18]. Therefore, the high-temperature yield strength and ultimate tensile strength of the as-extruded 7N01 Al alloy exhibit a decreasing trend with the increase in temperature.

4.2. Effect of Temperature on the Failure Mechanisms

In general, casting defects and brittle phases in Al alloys can easily cause stress concentration and subsequently induce crack initiation and propagation at elevated temperatures, which could remarkably reduce their high-temperature mechanical properties [20,21,22]. In the investigation about the mechanical behavior of the die-cast Al–10Si–Mn–Mg alloy at high temperatures, Ahna et al. reported that the initiation of micro-cracks was mainly attributed to the self-cracking of brittle Si-containing phase particles during the tensile-testing process [23]. Similarly, Han et al. studied the mechanical behavior of the TiB₂/Al-Si composite at high temperatures and found that the fracture mechanism was mainly ascribed to the breaking of the silicon particles and the debonding of the TiB₂ particle–matrix [24]. For the as-extruded 7N01 Al alloy investigated in this study, the phase particles formed are mainly α-Al₁₅(MnFe)₃Si₂, also existing in other Al–Zn–Mg alloys [25]. Qian et al. reported that, when the amount of Mn was higher than 0.4 wt.% in the Al–Si alloys, the formed α-Al₁₅(MnFe)₃Si₂ phase could effectively pin the grain boundary sliding and then induce the enhancement of the high-temperature tensile strength of the alloys [26]. It has been reported that the addition of Mn in Al–Si alloys with lower Fe content can cause the transformation of the brittle β-Al₅SiFe phase into the plastic α-Al₁₅(MnFe)₃Si₂ phase [26,27]. However, compared with the Al matrix, the α-Al₁₅(MnFe)₃Si₂ phase is still brittle and highly susceptible to fragmentation during the extrusion, resulting in the zonal aggregation and preferential distribution of broken α-Al₁₅(MnFe)₃Si₂ phase particles along the extrusion direction (Figure 3a). Therefore, the stress concentration in the α-Al₁₅(MnFe)₃Si₂ phase particles leads to their preferential cracking in order to coordinate the plastic deformation of the Al matrix during the high-temperature tensile process. In addition, in the investigation of the mechanical behavior of the Ti-2Al-Nb-based alloys at elevated temperatures, Wu et al. reported that the interfacial bonding strength of the interfaces between the high-strength TiB-reinforced phase and the matrix decreases with increasing temperature [28]. Therefore, for the tensile samples tested at temperatures above 400 °C, besides the self-cracking of the α-Al₁₅(MnFe)₃Si₂ phase particles, the initiation of microcracks can be observed at the interface between the α-Al₁₅(MnFe)₃Si₂ phase particles and the Al matrix in the area 2 mm away from the fracture surfaces. However, for the area 5 mm away from the fracture surfaces, the cross-sectional shrinkage ratio is only 7%~8% and the occurred plastic deformation is quite small. Thus, the incompatible plastic deformation at the interface between the α-Al₁₅(MnFe)₃Si₂ phase particles and the Al matrix is weak and can hardly cause the interfacial cracking. Then, only the self-cracking of the α-Al₁₅(MnFe)₃Si₂ phase particles can be observed far from the fracture surfaces. On the basis of the results of this work, it seems that controlling the size and quantity of phase particles should effectively enhance the high-temperature mechanical properties of Al alloys.

5. Conclusions

Through investigating and comparing the mechanical behavior and micro failure mechanisms of the as-extruded 7N01 Al alloy at high temperatures, the main conclusions are as follows:

(1)

The as-extruded 7N01 Al alloy is mainly composed of α-Al₁₅(MnFe)₃Si₂ particles and Al matrix; its yield strength and ultimate tensile strength decrease with the increase in temperature, but its elongation ratio changes slightly.

(2)

For the samples being tensile-tested at different temperatures, a lot of ductile dimples on the fracture surfaces and obvious necking can be observed.

(3)

The initiation of micro-cracks is mainly attributed to the self-cracking of the α-Al₁₅(MnFe)₃Si₂ particles and the interfacial cracking at the interface between the phase particles and the Al matrix.