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Creep behavior of solid solutioned and annealed Al-7075 alloy processed by equal channel angular pressing
Materials Science & Engineering A 765 (2019) 138225
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Creep behavior of solid solutioned and annealed Al-7075 alloy processed by equal channel angular pressing
T
Fatemeh Shahriyari, Mohammad Hossein Shaeri*, Fatemeh Bavarsiha, Mohammad Talafi Noghani Department of Materials Science and Engineering, Imam Khomeini International University (IKIU), Qazvin, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Al-7075 alloy Equal channel angular pressing Creep properties Creep mechanism Microstructure
Effect of equal channel angular pressing ECAP process on creep properties of solid solutioned and annealed Al7075 alloy was investigated at different temperatures and applied stresses. The microstructures of treated alloys before ECAP process were analyzed via transmission electron microscopy (TEM) and electron back scatter diffraction (EBSD). Mechanical properties of ECAPed alloy were also characterized. The fracture surface of samples was also observed by scanning electron microscopy (SEM). Subsequently, the creep mechanisms at different temperatures and stresses were discussed. The results showed that applying the initial pass of ECAP can effectively improve the mechanical properties and creep resistance of Al-7075 alloys due to impeding the dislocations movement as a result of dislocation density growth. However, applying multi passes of ECAP on annealed Al7075 alloy did not have a positive effect on creep strength. Increasing the fraction of HAGBs and grain refinement of multi-pass annealed Al-7075 ECAPed alloy deteriorate the creep characteristic. The maximum creep rate was achieved for annealed sample after 1 pass of ECAP at 503 K under 70 MPa. Furthermore, according to different values of stress exponent, n, creep mechanism of treated samples altered. Dislocation creep as a dominant creep mechanism of annealed Al-7075 ECAPed alloy at both temperatures (503,543 K) was not altered after initial pass of ECAP; however it was replaced by grain boundary sliding after multi passes of process. Core creep mechanism of solid solutioned sample was also changed from Coble diffusion creep to viscos glide dislocation after one pass of ECAP.
1. Introduction The study of severe plastic deformation (SPD) methods over the last decades has been much more extensive [1–4]. Numerous experimental studies have shown that SPD techniques enhance the mechanical properties of alloys due to ultra-fine grained (UFG) or nano-grained (NG) formation at room or high temperatures [54][5]. As UFG and NG is not stable, grain growth and recrystallization can happen when working temperature exceeds about 0.3 Tm. It may deteriorate the creep behavior of alloys [5–8]. On the other hand, it is also rational that the nano-sized grains and second phases or precipitates in some alloys can prevent dislocations motion due to their pinning roles. Consequently, it keeps grain boundary from sliding and rotation and may improve the creep resistance of alloys [9]. Altogether, the effect of SPD methods such as equal channel angular pressing (ECAP) on the creep behavior of alloys is still remained a challenge. Al and its alloys have been widely used in some industries due to their lightweight, excellent strength to weight ratio, good corrosion
*
resistance and high thermal and electrical conductivity [10]. Among aluminum alloys, Al-7075 which is based on the Al–Zn–Mg–Cu alloy system (the 7xxx series aluminum alloys) have been applied in the aerospace, marine, automobile and aviation industries [11]. Until now, many researches have been conducted to investigate the effect of SPD methods on the mechanical, tribological, corrosion, fatigue and superplastic behavior of Al and its alloys [12–20]. However, the researches about creep behavior of SPD processed Al alloys are very limited. It was found in previous researches that the creep resistance of pure Al considerably increases after one pass of ECAP, however at subsequent passes, it starts to decrease due to increasing the amount of high-angle grain boundaries (HAGBs) [21–26]. Studies about creep behavior of ECAPed binary Al–Sc alloy and ternary Al–Mg-Sc alloy also indicated the detrimental effect of ECAP on the creep resistance after multi-passes of ECAP [27]. It was reported that the creep rate of these alloys increased up to 100–200% after initial ECAP passes. However, subsequent ECAP passes significantly reduced the creep life of the alloys [28,29]. Since the creep properties of Al-7075 is of great importance in
Corresponding author. Tel.: +98 28 33901190; fax: +98 28 3378 6579. E-mail addresses: [email protected] (F. Shahriyari), [email protected] (M.H. Shaeri).
https://doi.org/10.1016/j.msea.2019.138225 Received 12 April 2019; Received in revised form 26 July 2019; Accepted 27 July 2019 Available online 28 July 2019 0921-5093/ © 2019 Published by Elsevier B.V.
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Table 1 Chemical composition of Al-7075 alloy used in current research.
Table 2 The Al-7075 samples codes and conditions.
Ti
Zn
Cr
Mg
Mn
Cu
Fe
Si
Al
Sample code
Sample condition
0.02
5.70
0.21
2.65
0.04
1.50
0.09
0.07
base
O O1 O4 SS SS1
Annealing at 415 °C for 2 h followed by cooling in furnace for 24 h O + 1 pass ECAP at Tamb O + 4 passes ECAP at Tamb Solution treatment at 470 °C for 2 h followed by water quenching SS+ 1 pass ECAP at Tamb
many applications and the effect of ECAP process on the creep behavior of Al-7075 has not been investigated previously; evaluating the effect of ECAP process on the creep behavior of this alloy seems to be worthy. Hence, in this study, firstly two different heat treatments: annealing and solid solution treatment were implemented on Al-7075 alloy to obtain different microstructures. Then, the effect of ECAP process on creep strength as well as creep mechanism of pre-treated alloy was investigated.
Tamb: room temperature.
nitric acid and 70% methanol. Furthermore, the electropolishing with 30% nitric acid and 70% methanol solution was employed for EBSD sample preparation. The detailed information of the examination procedure and sample preparation for microstructure characterization has been reported in our previous papers [30–32]. Tensile specimens were prepared according to ASTM-E8 and tensile testing was done using a Zwick-Roell machine at a rate of 1 mm min−1 with the load direction parallel to the specimen axis. The yield strength, ductility (elongation to failure) and ultimate tensile strength of the samples were then evaluated. Each test was repeated at least 3 times to ensure the repeatability of the results. The Vickers microhardness test was performed on the cross section of the specimens by a VMHTAUTO Leica Wechsler microhardness tester. Each test was repeated at least 5 times near the center of samples. In order to evaluate the creep properties of Al-7075 alloy before and after ECAP process, the tension-creep test was used. The creep samples were machined with a gage diameter and length of 4.5 and 25 mm, respectively. The creep tests were performed with the applied stress of 70, 75, 80, 85 and 90 MPa at 503 and 543 K for O samples and 140, 145, 150, 155 and 160 MPa at 543 and 583 K for SS samples using a SANTAM SCT-30 creep tester. It should be mentioned that due to the higher strength of the Al-7075 in solid solution condition compared with that of annealed one, higher temperatures and stress were considered for creep test of solution treated samples. The surface microstructure of samples after creep test was also observed via AIS2300C scanning electron microscopy (SEM).
2. Material and method 2.1. Samples fabrication The material used in this study was extruded rods of Al-7075 alloy with diameter of 20 mm. The chemical composition of the alloy is presented in Table 1. At first, the extruded rods were machined into the cylinders with 11 mm diameter and 120 mm length. Prior to ECAP, annealing and solid solution treatment were carried out as pre-treatments. In order to prove the microstructure of Al-7075 annealed alloy (Al 7075-O), the Xray diffraction analysis (XRD) was performed using an X'pert PRO MPD Panalytical (Cu-Kα radiation) diffractometer. Then, ECAP was conducted at the ambient temperature using a solid die with channel angle of Φ = 90° and outer curvature angle of Ψ = 20°. The schematic of the ECAP die is presented in Fig. 1. Table 2 summarizes the samples condition and the code of each one. The deformation route of ECAP was BC in which the sample is rotated 90° between the ECAP passes in one direction and the pressing speed of ECAP was 0.5 mm/min. It should be noted that SS samples were processed just for one pass due to the inducement of catastrophic microcracks after multi passes of ECAP, while, the annealed samples were ECAPed up to 4 passes without cracking.
3. Results
2.2. Sample characterizations
3.1. Microstructure analysis
Microstructural aspects of ECAPed specimens were characterized by transmission electron microscopy (TEM) (JEOL JEM 3010 operating at 300 keV), and field emission scanning electron microscopy (FESEM) equipped with an electron back-scattered diffraction (EBSD) camera (Quanta 3D FEI operating at 15 kV and 10 nA). TD cross section of samples was chosen for evaluation of microstructure. TEM samples were prepared by twin-jet electro-polishing using an electrolyte of 30%
Precipitates characteristics of both annealed and solid solutioned Al7075 alloy before and after ECAP process have been studied by TEM. Fig. 2 presents the high magnification TEM images of the pre-treated samples before and after ECAP process. As it can be seen, the annealed sample (Al 7075-O) consists of mainly coarse and minor quantities of fine MgZn2 precipitates in Al matrix. Fig. 3 shows the XRD pattern of the initial Al 7075 alloy SEM microstructure and chemical analysis of precipitates in EDS analysis are shown in Fig. 4. The presence of MgZn2 peaks in XRD pattern and the results of EDS microanalysis prove that the microstructure consists of mainly α-Al as matrix and MgZn2 as major precipitates [18,19]. By increasing the passes of ECAP process, Al 7075-O4 sample merely presents the finer MgZn2 phase and the coarse MgZn2 precipitates disappear. TEM micrographs of solid-solutioned alloys before and after 1 pass of ECAP process are also depicted in Fig. 2 (d) and (e). As it expected, the microstructure of solid solutioned sample before and after ECAP process does not show any second phases. The absence of second phase in the microstructure of 1-pass ECAPed solid solutioned sample revealed that no precipitate was formed during 1 pass of ECAP at RT. Fig. 5 shows the inverse pole figures (IPF) maps as well as grain boundary analysis of unECAPed and ECAPed annealed samples. Obviously, initial microstructure of annealed sample includes large grains with sizes larger than 90 μm. While annealed sample exhibits the bimodal microstructure after 1 pass of ECAP composing both large and
Fig. 1. Schematic of ECAP die used in this work. 2
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Fig. 2. TEM micrographs of pretreated Al- 7075 alloys; annealed alloy, a) before ECAP, b) 1 pass ECAPed and c) 4 passes ECAPed as well as solid-solutioned alloys, d) before ECAP and e) 1 pass ECAPed.
3.2. Mechanical analysis The experimental results of the microhardness as well as tension tests are presented at Table 3. It is seen that the annealed sample has the lowest mechanical properties. Irrespective of pre-treatment, applying ECAP enhanced the hardness, yield strength and tensile strength. After 4 passes of ECAP, hardness and tensile strength of the annealed sample increases over 100%. Whereas, this growth can be seen only after 1 pass of ECAP for SS sample. It should be mentioned that ECAP processing decreases the total elongation. Strength and hardness improvement of ECAPed samples may be attributed to grain refinement and considerable growth of dislocation density after ECAP process [33–35]. The more significant efficacy of ECAP process in mechanical properties improvement of solid solutioned samples in comparison with those of annealed samples could be originated from dynamic strain aging as a result of formation of GP zones during ECAP process [31,36,37]. 3.3. Creep Fig. 3. XRD pattern of Al-7075 annealed sample.
Creep behaviors of annealed Al-7075 (Al 7075-O) alloy before ECAP process at 503 K and 543 K are shown in Fig. 6. The experimental results are recorded in the plots of strain-time (ε-t) and strain rate-time (ε°-t). It should be noted that strain-time and strain rate-time curves of all samples are represented in only three applied stress to facilitate the analysis of the graphs. The results indicates that the creep rupture life of Al 7075-O alloy at 503 K is about 335 min at applied stress of 70 MPa; however, it is reduced to 228 and 142 min by increasing applied stress to 80 and 90 MPa, respectively (Fig. 6b). As can be seen in Fig. 6d, the creep rupture times of Al 7075-O sample at 543 K are 26, 112 and 134 min at applied loads of 70, 80 and 90 MPa, respectively. Fig. 7 shows the creep behavior of Al 7075-O alloy after 1pass ECAPed process at different stress and temperatures of creep test. As seen, applying single pass of ECAP at 503 K notably increases the creep rupture life of Al 7075-O alloy to 434, 376 and 147 min during increasing applied stresses from 70 to 80 and 90 MPa, respectively
small equiaxed grains, applying 4 pass of ECAP process demonstrates only small equiaxed grains. Quantitative analysis of grain boundary maps performed by TSL software is summarized in Fig. 5g. As indicated, 46% of grain boundaries at Al 7075-O sample depict low angle boundaries with the misorientation angles from 2° to 5°. Following applying 1 pass of ECAP process, although the fraction of grain boundaries with the average misorientation between 2° and 5° exhibits a slight reduction; the fraction of grain boundaries with 5°–10° misorientation considerably decreases and finally great amounts of high-angle grain boundaries (> 15°) are detected. By increasing the passes number of ECAP process to 4 passes most of the low angle boundaries are replaced with highangle grain boundaries. It is worth-mentioning that EBSD microstructural evaluation of solid solutioned sample after 1 pass of ECAP does not show any remarkable differences with Al 7075-O1 sample. 3
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Fig. 4. a) SEM microstructure of Al 7075-O alloy and b) EDS micro-analysis of the selected area.
(Fig. 7b). On the other hand, increasing the passes of ECAP to 4 passes considerably declines the creep rupture life to 51, 63 and 81 min in applied stresses of 70, 80 and 90 MPa, respectively (Fig. 8b). Creep behaviors of Al 7075-SS alloy before and after ECAP process at 543 and 583 K are illustrated in Fig. 7, when tested under the 140, 150 and 160 MP. As can be seen the failure of Al 7075-SS sample at 543 K occurs after 126, 104 and 79 min when tested at applied loads of 140, 150 and 160 MPa, respectively (Fig. 9b). As can be observed, the creep rupture life of Al 7075-SS sample at 583 K significantly declines to 72, 57, 44 min under applied stresses of 140, 150 and 160 MPa (Fig. 9d). Fig. 10 shows the creep behavior of ECAPed solid solution sample at both temperatures of 543 K and 583 K under the applied stress of 140, 150 and 160 MPa. It is clear that after 1 pass of ECAP, the creep rupture life of SS samples at 543 K considerably grew up to 146, 115 and 93 min by increasing the applied stresses from 140 up to 160 MPa. In order to evaluate the effects of temperature and stress on the creep rate and identifying the creep mechanisms, the logarithmic plots
Table 3 Average mechanical properties of Al-7075 alloy before and after ECAP process (YS: yield stress, UTS: ultimate tensile stress, Δ: ductility, H: hardness). Sample
YS (MPa)
UTS (MPa)
Δ (%)
O O1 O4 SS SS1
169± 3 288± 3 384± 4 235± 7 598± 9
230±5 300± 2 450± 6 368± 6 624± 8
15.5± 5.29± 9± 3 17.2± 8.67±
H (Hv)
2 1 7 5
60± 3 111± 4 140± 3 95± 5 189± 4
of the steady-state creep rates versus the applied stresses for all samples are illustrated in Fig. 11. Meanwhile, the stress exponents of samples, obtained from the slop of ε° - σ plots, are summarized in Table 4. The stress dependence of the steady-state creep rate can be ascribed by power-law equation:
ε o = Aσ n
(1)
Where A is independent of σ, and stress exponent of creep rate (n) can
Fig. 5. EBSD inverse pole figure and grain boundary maps of annealed Al-7075, (a, d) unECAPed sample, (b, e) 1 pass ECAPed sample and (c, f) 4 passes ECAPed sample. (g) Quantitative analysis regarding the faction of LAGBs and HAGBs in annealed samples at different conditions. (The unit triangle depicted the orientation of each grain). 4
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Fig. 6. Creep behavior of Al 7075-O sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively.
4. Discussions
be described by following equation:
n=(
∂lnε o )T ∂lnσ
4.1. Microstructure (2) According to microstructural observations, although annealed Al7075 alloy contained coarse MgZn2 phases in Al matrix (Figs. 3–4), applying one pass of ECAP results in fragmentation of coarse MgZn2 precipitates in which coarse phases could hardly been observed in Al 7075-O1 sample (Fig. 2b). It is also obvious that after ECAP process, the microstructure became wavy and ill-defined indicating the presence of high dislocation density as well as non-equilibrium state of the alloy. Increasing the passes of ECAP process demonstrated the finer MgZn2 phase at Al 7075-O4 sample (Fig. 2c). The microstructure of Al-7075 solid solution sample with and without ECAP process, however, was free from any clear second phases (Fig. 2c and d). Based on the inverse pole figures (IPF) maps of samples, initial microstructure of annealed sample included large grains with sizes larger than 90 μm (Fig. 5a and d). The microstructure of annealed sample after 1 pass of ECAP comprised some large elongated grains stemmed from the shear strain during ECAP (with a mean size of 3 μm) as well as some small equiaxed ultra-fined grains (with size of about 700 nm) (Fig. 5b and e) as a result of dislocation tangles formation in deformation bands [38,39]. The homogenous equiaxed grains along the transverse direction (TD) were utterly obvious with the size of about 600 nm after 4 passes of ECAP (Fig. 5c and f). Quantitative analysis of IPF and grain boundary analysis manifested that applying ECAP process could have a profound effect on the misorientation of grains. As it is shown in Fig. 5g, the microstructural analysis effectively confirmed the greater fraction of high angle boundaries (HAGBs) by applying ECAP and also increasing the number of passes [32]. As expected ECAP process did not the microstructure of solid solution samples.
As can be seen in Table 4, the value of n is about 4.67 for Al 7075-O sample at 503 K, which indicates the dominance of dislocation creep as the main creep mechanism. Increasing creep temperature to 543 K leads to decreasing the n value to 4.01. Applying single pass of ECAP for annealed sample presents the higher values of n at both temperatures (about 5.24 and 4.56 at 503 and 543 K, respectively). The stress exponent of Al 7075-O4 sample at both temperatures are significantly lower than both Al 7075-O and –O1 samples (n = 3.15 and 2.18 for 503 and 543 K, respectively). It is also clear that stress exponent decreases with increasing the creep temperature from 503 to 543 K. When it comes to solid solution samples, while the stress exponent of Al 7075-SS sample is remarkably low (n = 1.56), it then notably grows to about 3.36 after one pass of ECAP. Increasing temperature from 543 to 583 K presents lower stress exponent of 1.18 and 2.87 for both Al 7075-SS and Al 7075-SS1 samples, respectively. Fig. 10 depicts the SEM images of fracture surfaces of annealed and solid solutioned samples after creep at temperature of 543 K and applied stresses of 80 and 150 MPa in different processing condition, respectively. It seems that the large precipitates of annealed sample are incoherent with the matrix and can easily detach from matrix; therefore they encourage the microvoids nucleation as well as large dimple formation, which are easily detected in Al 7075-O sample (Fig. 12a). It can be seen that the surface of Al 7075-O1 sample presents two kinds of dimples including large and small size (Fig. 12b). In the case of Al 7075O4 sample, the number of submicron-dimples becomes greater and consequently the dimples density enhances (Fig. 12c). Comparison of fracture surfaces of solid solutioned samples before and after ECAP process in Fig. 12 (d) and (e) reveal that, similar to annealed samples, the dimples size reduced considerably after 1 pass of ECAP. Meanwhile the surface of Al 7075-SS1 sample contains both large and small size dimples due to the bimodal microstructure.
4.2. Creep rupture life Based on the experimental results of strain-time and strain rate-time plots, the creep behavior of Al-7075 alloy was studied before and after ECAP process. As expected, a creep curve of Al 7075-O generally included three stages: the short primary, well-defined secondary (steady state) and tertiary stages. With increasing the applied stress, the creep strain rate increased and correspondingly the primary and secondary
Fig. 7. Creep behavior of Al 7075-O1 sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively. 5
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Fig. 8. Creep behavior of Al 7075-O4 sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively.
formation of LAGBs in first pass of ECAP, occurrence of creep aging during ECAP process of solid solutioned has also contributed in improvement of creep life within the first pass of ECAP. During the SPD process, the remained tiny precipitates can be dissolved, and accordingly the amount of solute elements responsible for subsequent static or dynamic strain aging is increased. Hence, the creep aging of the sample imposed during creep deformation can influence the creep rate of the sample [44].
stages were shortened while the tertiary stage was prolonged (Fig. 6). The earlier beginning of tertiary stage and the higher creep rate suggested much shorter creep rupture life of the alloy exposed to the higher stress [40]. The effect of temperature on the creep behavior of the Al 7075-O can be realized by comparison of curves in Fig. 6. It is clear that increasing temperature rapidly increased the creep strain rate and decreased the creep rupture life [40]. It is interesting to note that the secondary creep stage of Al 7075-O alloy at 543 K under 90 MPa, was considerably shortened in such a way that the strain rate curve almost included primary and tertiary stages. Effect of ECAP process on the creep behavior of annealed Al-7075 alloy at 503 and 543 K are depicted in Figs. 7 and 8. Single pass of ECAP notably increased the creep strength of Al 7075-O alloy, whereas applying more passes (4 passes ECAP) led to a significant decrease. Regarding to microstructural analysis (Fig. 5b and e), low-angle grain boundaries (LAGBs) combined with elongated grains in Al7075–O1 sample retarded the dislocation motion; therefore, creep strength significantly increased up to 40% after 1 pass of ECAP [41,42]. Significant transformation of LAGBs to HAGBs and grain refinement of Al 7075-O4 sample (Fig. 3c and d) could facilitate the grain boundary sliding and ultimately deteriorated the creep strength by approximately 50% compared with that of Al 7075-O1. This result is in agreement with Blum et al. [8,43]. They suggested that when grains become smaller than sub-grains, significant softening causes the creep behavior deterioration. It is worth to note that these results are significantly consistent with mechanical properties of samples. Evaluating the effect of temperature on the creep behavior of ECAPed Al 7075-O reveals that by increasing the temperature of creep test from 503 K to 543 K, the creep rupture life of single passes ECAPed sample (Al 7075-O1) as well as multi-passes ECAPed sample (Al 7075O4) significantly drop. Decreasing the creep rupture life of ECAPed samples by increasing the creep temperature could be explained by the higher diffusion and greater activation of new sliding systems [40]. Applying ECAP process enhanced the creep resistance of solid solutioned sample (Fig. 9). As indicated in Fig. 10, the strain rate of steady state region of Al 7075-SS1 was greatly developed after one pass of ECAP process. Similar to annealed samples, increasing the temperature of creep test resulted in remarkable reduction in creep strength of solid solutioned samples. Creep rupture life of SS1 decreased up to 50% by increasing testing temperature from 543 to 583 K. Alongside the
4.3. Creep mechanisms Creep mechanisms were assessed based on the stress exponent's valve of samples (Fig. 11 and Table 4). The n value of Al 7075-O sample was calculated 4.67 which is almost similar to the n value reported for coarse grained pure Al subjected to creep testing (n = 4.5) [23,45,46]. Increasing the n value of this sample via increasing the creep test temperature indicated that dislocation creep could be a dominate creep mechanism [23]. The reduction of n value by increasing the creep test temperature may originate from the increment of intergranular dislocation glide and climb as dominant rate-controlling flow processes [47,48]. The n value of annealed sample increased to 5.24 and 4.56 after applying 1pass ECAP at 503 K and 543 K; respectively. This may verify that the improvement of creep properties can be explained by increment of the dislocation density and LAGBs amounts after single pass of ECAP (Fig. 5g). The lower value of stress exponent for Al 7075-O4 at 503 K (n = 3.15) compared to Al7075–O (n = 4.67) and –O1 samples (n = 5.24) represented the influence of more intensive grain boundary sliding on the creep deformation of UFG material [21,23]. It seems that the creep of 4-passes ECAPed sample at lower temperature deteriorated due to the generation and movement of dislocations inside the grains as well as grain boundary sliding [49]. Regarding to microstructural analysis, replacing LAGBs with HAGBs and formation of UFG microstructure at 7075-O4 sample could verify this idea very well [50]. Similar to Al 7075-O1 sample, the stress exponent of the Al 7075-O4 alloy also decreased with increasing the creep temperature. This behavior can be interpreted in terms of easier dislocation movement and also grain boundary sliding in creep deformation at higher temperatures [51]. When it comes to solid solutioned samples, while the n value was greatly low (n = 1.56) before ECAPed, it increased up to 3.36 after
Fig. 9. Creep behavior of Al 7075-SS sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively. 6
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Fig. 10. Creep behavior of Al 7075-SS1 sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively.
Fig. 11. Logarithmic plot of minimum creep rate versus applied stress for Al-7075 samples before and after ECAP process at both creep temperatures of 503 and 543 K, (a) O sample, (b) O1 sample, (c) O4 sample, (d) SS sample and (e) SS1 sample. Table 4 Stress exponent of Al-7075 samples at both creep temperatures of 503 and 543 K in different conditions. Sample
O
Temperature (K) N
503 4.67
O1 543 4.01
503 5.24
O4 543 4.56
503 3.15
SS1
SS 543 2.18
543 1.50
583 1.18
543 3.36
583 2.87
4.4. Creep fracture surface
single pass of ECAP. As it was previously mentioned, higher creep temperature was applied for solid solutioned samples. This higher temperature creep test could activate coble mechanism in Al 7075-SS sample [40]. However after applying 1 pass of ECAP, the dominant creep mechanism altered from coble creep to viscos glide dislocation creep. Reducing the n value of solid solutioned samples by increasing the temperature demonstrated that the coble diffusion creep and viscose glide process of dislocation could be a dominant mechanism for Al 7075-SS1 samples, respectively [52]. The change of dislocation mechanism form coble creep to dislocation glide creep by applying one pass of ECAP is in consistent with higher creep properties of 1-pass ECAPed Al 7075-SS in comparison with that of unECAPed Al 7075-SS.
According to the SEM observations of the fracture surfaces of creepruptured samples, the fracture surface of one pass ECAPed annealed samples was greatly covered with the finer submicron-dimples in comparison with that of initial annealed specimen, however, some large dimples could be found (Fig. 12a and b). It is worth noting that the difference in dimples sizes deflected bimodal microstructure of Al 7075O1 sample [17]. The formation of UFG microstructure after 4 passes of ECAP produced small-sized dimples in fracture surface of Al 7075-O4 sample (Fig. 12c). These small-sized dimples could easily coalescence together and ultimately evolve into large ones. The coalescence of microvoids and microcracks proves the creep properties deterioration of Al 7075-O4 sample [15]. Applying 1pass of ECAP process on Al 7075-SS alloy also 7
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Fig. 12. SEM micrographs of the fracture surfaces of creep-ruptured Al-7075 alloy at different conditions, (a) O, (b) O1, (c) O4, (d) SS and (e) SS1 samples.
demonstrated the finder dimples than that of Al 7075-SS. It should be noted that the fracture mechanism of all samples was predominantly ductile fracture.
Processed by ECA Pressing, (2005). [8] W. Blum, Y. Li, Y. Zhang, J.T. Wang, Deformation Resistance in the Transition from Coarse-Grained to Ultrafine-Grained Cu by Severe Plastic Deformation up to 24 Passes of ECAP, (2011). [9] L. Wang, F. Qiu, Q. Zhao, M. Zha, Q. Jiang, Superior high creep resistance of in situ nano-sized TiC(x)/Al-Cu-Mg composite, Sci. Rep. 7 (2017) 4540. [10] R. Abrahams, J. Mikhail, P. Fasihi, Effect of friction stir process parameters on the mechanical properties of 5005-H34 and 7075-T651 aluminium alloys, Mater. Sci. Eng. A 751 (2019) 363–373. [11] P.V. Kumar, G.M. Reddy, K.S. Rao, Microstructure, mechanical and corrosion behavior of high strength AA7075 aluminium alloy friction stir welds – effect of post weld heat treatment, Defence Technology 11 (4) (2015) 362–369. [12] S. Gholami, E. Emadoddin, M. Tajally, E. Borhani, Friction stir processing of 7075 Al alloy and subsequent aging treatment, Trans. Nonferrous Metals Soc. China 25 (9) (2015) 2847–2855. [13] M. Ilieva, R. Radev, Effect of Severe Plastic Deformation by ECAP on Corrosion Behaviour of Aluminium Alloy AA 7075, (2016). [14] S. Kuramoto, K. Horibuchi, I. Aoi, K. Oh-ishi, Effect of severe plastic deformation on mechanical properties and microstructure in 7075 aluminum alloy, J. Jpn. Inst. Light Metals 64 (6) (2014) 241–248. [15] Y.C. Lin, Z.-W. Wang, D.-G. He, Y. Zhou, M.-S. Chen, M.-H. Huang, J.-L. Zhang, Effects of pre-treatments on precipitate microstructures and creep-rupture behavior of an Al–Zn–Mg–Cu alloy, J. Mater. Res. 31 (9) (2016) 1286–1295. [16] J. Robinson, R.L. Cudd, J. Evans, Creep Resistant Aluminium Alloys and Their Applications, (2003). [17] A. Esmaeili, M.H. Shaeri, M.T. Noghani, A. Razaghian, Fatigue behavior of AA7075 aluminium alloy severely deformed by equal channel angular pressing, J. Alloy. Comp. 757 (2018) 324–332. [18] M. Chegini, M.H. Shaeri, Effect of equal channel angular pressing on the mechanical and tribological behavior of Al-Zn-Mg-Cu alloy, Mater. Char. 140 (2018) 147–161. [19] M. Shaeri, M. Salehi, S. Seyyedein, M. Abutalebi, J.K. Park, Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment, Mater. Des. 57 (2014) 250–257. [20] A. Dashti, M. Shaeri, R. Taghiabadi, F. Djavanroodi, F. Vali Ghazvini, H. Javadi, Microstructure, texture, electrical and mechanical properties of AA-6063 processed by multi directional forging, Materials 11 (12) (2018) 2419. [21] V. Sklenička, J. Dvořák, M. Svoboda, Creep in ultrafine grained aluminium, Mater. Sci. Eng. A 387 (2004) 696–701. [22] V. Sklenička, J. Dvořák, M. Svoboda, P. Král, B. Vlach, Effect of Processing Route on Microstructure and Mechanical Behaviour of Ultrafine Grained Metals Processed by Severe Plastic Deformation, Materials Science Forum, Trans Tech Publ, 2005, pp. 83–88. [23] V. Sklenicka, J. Dvorak, P. Kral, Z. Stonawska, M. Svoboda, Creep processes in pure aluminium processed by equal-channel angular pressing, Mater. Sci. Eng. A 410 (2005) 408–412. [24] V. Sklenička, P. Král, J. Dvořák, M. Kvapilová, M. Kawasaki, T.G. Langdon, Effect of Equal-Channel Angular Pressing on the Creep Resistance of PrecipitationStrengthened Alloys, Materials Science Forum, Trans Tech Publ, 2011, pp. 897–902. [25] J. Dvořák, P. Král, M. Kvapilová, M. Svoboda, V. Sklenička, Microstructure Stability and Creep Behaviour of a Cu-0.2 wt.% Zr Alloy Processed by Equal-Channel Angular Pressing, Materials Science Forum, Trans Tech Publ, 2011, pp. 821–826. [26] P. Král, J. Dvořák, A. Jäger, M. Kvapilová, Z. Horita, V. Sklenička, Creep properties of aluminium processed by ECAP, Kovove Mater 54 (2016) 441–451. [27] V. Sklenička, J. Dvořák, M. Kvapilová, M. Svoboda, P. Král, I. Saxl, Z. Horita, Effect of Equal-Channel Angular Pressing (ECAP) on Creep in Aluminium Alloys, Materials
5. Conclusion In this research, the creep behaviors and creep mechanisms of annealed and solid-solutioned Al-7075 alloy were investigated before and after ECAP process. The following conclusions can be derived from the results: The creep response of Al-7075 alloy was related to temperature and primary microstructure. The main creep mechanism of annealed Al7075 alloy was dislocation creep at 503 K. With an increase in creep temperature to 543 K, a greater creep deformation was observed due to creep mechanism alteration to rate-controlling flow processes such as dislocation glide and climb. Increasing the n value of ECAPed annealed Al-7075 alloy after first pass indicated the creep strength enhancement. This can be attributed to higher amounts of LAGBs and dislocation jungles after single pass of ECAP. After applying multi passes of ECAP, the creep resistance was markedly deteriorated with respect to replacing LAGBs with HAGBs as well as activation of grain boundary sliding at 503 K. Increasing the creep temperature to 543 K presented the lower n for both ECAPed Al 7075-O alloys as a result of higher dislocation movements and grain boundary sliding. Regarding to solid solutioned Al 7075-SS alloy, coble mechanism which was the dominant creep mechanism changed to viscos glide dislocation creep after applying 1 pass of ECAP at 543 and 583 K. References [1] L. Zuev, Macroskopicheskaya fizika plasticheckoy deformatsii, Uspehi fiziki metallov 16 (2015) 35–60. [2] L.B. Zuev, On the waves of plastic flow localization in pure metals and alloys, Ann. Phys. 16 (4) (2007) 286–310. [3] A. Berezina, T. Monastyrska, O. Davydenko, O. Molebny, S. Polishchuk, Effect of severe plastic deformation on structure and properties of Al-Sc-Ta and Al-Sc-Ti alloys, Nanoscale research letters 12 (1) (2017) 220. [4] V. Segal, modes and processes of severe plastic deformation (SPD), Materials 11 (7) (2018) 1175. [5] U. Bansal, M. Sharma, Development of mechanical properties by severe plastic deformation methods, Development 2 (5) (2015). [6] P. Kral, J. Dvorak, W. Blum, E. Kudryavtsev, S. Zherebtsov, G. Salishchev, M. Kvapilova, V. Sklenicka, Creep study of mechanisms involved in low-temperature superplasticity of UFG Ti-6Al-4V processed by SPD, Mater. Char. 116 (2016) 84–90. [7] C. xu, T.G. Langdon, Creep and Superplasticity in a Spray-Cast Aluminum Alloy
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[41] W. Blum, J. Dvořák, P. Král, P. Eisenlohr, V. Sklenička, Effect of grain refinement by ECAP on creep of pure Cu, Mater. Sci. Eng. A 590 (2014) 423–432. [42] R. Marlin, F. Cosandey, J. Tien, The effect of predeformation on the creep and stress rupture of an oxide dispersion strengthened mechanical alloy, Metallurgical Transactions A 11 (11) (1980) 1771–1775. [43] W. Blum, Y. Li, K. Durst, Stability of ultrafine-grained Cu to subgrain coarsening and recrystallization in annealing and deformation at elevated temperatures, Acta Mater. 57 (17) (2009) 5207–5217. [44] V. Dudko, R. Kaibyshev, E. Salakhova, Effect of subgrain structure on the creep strength of alloy 1207, Phys. Met. Metallogr. 107 (1) (2009) 90–95. [45] D.G. Brandon, Quantitative Relation between Properties and Microstructure: Proceedings of an International Conference, Haifa, Israel, July 27-Aug. 4, 1969, Israel Univ. Pr, 1969. [46] A. Yousefiani, F. Mohamed, J. Earthman, Creep Rupture Mechanisms in Annealed and Overheated 7075 Al under Multiaxial Stress States, (2012). [47] M. Kawasaki, I.J. Beyerlein, S.C. Vogel, T.G. Langdon, Characterization of creep properties and creep textures in pure aluminum processed by equal-channel angular pressing, Acta Mater. 56 (10) (2008) 2307–2317. [48] E.A. Marquis, D.N. Seidman, D.C. Dunand, Effect of Mg addition on the creep and yield behavior of an Al–Sc alloy, Acta Mater. 51 (16) (2003) 4751–4760. [49] Z. Wang, Y. Zhao, D.L. Kohlstedt, Dislocation creep accommodated by grain boundary sliding in dunite, J. Earth Sci. 21 (5) (2010) 541–554. [50] P. Kral, J. Dvorak, V. Sklenicka, T. Masuda, Z. Horita, K. Kucharova, M. Kvapilova, M. Svobodova, The effect of ultrafine-grained microstructure on creep behaviour of 9% Cr steel, Materials 11 (5) (2018) 787. [51] P. Shahbeigi, A. Sarkar, K. Murty, H. Brody, R. Scattergood, Grain Boundary Sliding Mechanism during High Temperature Deformation of AZ31 Magnesium Alloy, (2016). [52] Z. Ma, L. Zhan, C. Liu, L. Xu, Y. Xu, P. Ma, J. Li, Stress-level-dependency and bimodal precipitation behaviors during creep ageing of Al-Cu alloy: experiments and modeling, Int. J. Plast. 110 (2018) 183–201. [54] F. Shahriyari, et al., Effect of friction hardening on the surface mechanical properties and tribological behavior of biocompatible Ti-6Al-4V alloy, Journal of Manufacturing Processes 31 (2018) 776–786, https://doi.org/10.1016/j.jmapro. 2017.12.016 In press.
Science Forum, Trans Tech Publ, 2007, pp. 2904–2909. [28] R. He, M. Wang, X. Zhang, B. Yaping Zong, Influence of Second-phase Particles on Grain Growth in AZ31 Magnesium Alloy during Equal Channel Angular Pressing by Phase Field Simulation, (2016). [29] J. Dvorak, P. Král, M. Svoboda, M. Kvapilová, V. Sklenička, Enhanced Creep Properties of Copper and its Alloys Processed by ECAP, (2014). [30] M. Shaeri, M. Shaeri, M. Ebrahimi, M. Salehi, S. Seyyedein, Effect of ECAP temperature on microstructure and mechanical properties of Al–Zn–Mg–Cu alloy, Prog. Nat. Sci.: Materials International 26 (2) (2016) 182–191. [31] M. Shaeri, M. Shaeri, M. Salehi, S. Seyyedein, M. Abutalebi, Effect of equal channel angular pressing on aging treatment of Al-7075 alloy, Prog. Nat. Sci.: Materials International 25 (2) (2015) 159–168. [32] M. Shaeri, M. Shaeri, M. Salehi, S. Seyyedein, F. Djavanroodi, Microstructure and texture evolution of Al-7075 alloy processed by equal channel angular pressing, Trans. Nonferrous Metals Soc. China 25 (5) (2015) 1367–1375. [33] E. Khafizova, R. Islamgaliev, Effect of severe plastic deformation on the structure and mechanical properties of Al-Cu-Mg alloy, IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2014012081. [34] S. Hashimoto, T. Suzuki, A. Vinogradov, Hardening Mechanisms of Metals and Alloys Produced by SPD, Materials Science Forum, Trans Tech Publ, 2006, pp. 967–970. [35] P. Snopiński, T. Tański, M. Król, Effect of SPD processing on the structure and properties of Al–Mg alloy, Inżynieria Materiałowa 39 (1) (2018). [36] M. Moradpour, F. Khodabakhshi, H. Eskandari, Dynamic strain aging behavior of an ultra-fine grained Al-Mg alloy (AA5052) processed via classical constrained groove pressing, Journal of Materials Research and Technology 8 (1) (2019) 630–643. [37] S. Zhao, C. Meng, F. Mao, W. Hu, G. Gottstein, Influence of severe plastic deformation on dynamic strain aging of ultrafine grained Al–Mg alloys, Acta Mater. 76 (2014) 54–67. [38] C. Xu, Z. Horita, T.G. Langdon, Microstructural evolution in an aluminum solid solution alloy processed by ECAP, Mater. Sci. Eng. A 528 (18) (2011) 6059–6065. [39] L.W. Meyer, K. Sommer, T. Halle, M. Hockauf, Crack growth in ultrafine-grained AA6063 produced by equal-channel angular pressing, J. Mater. Sci. 43 (23–24) (2008) 7426–7431. [40] M.E. Kassner, Fundamentals of Creep in Metals and Alloys, ButterworthHeinemann, 2015.
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Creep behavior of solid solutioned and annealed Al-7075 alloy processed by equal channel angular pressing
T
Fatemeh Shahriyari, Mohammad Hossein Shaeri*, Fatemeh Bavarsiha, Mohammad Talafi Noghani Department of Materials Science and Engineering, Imam Khomeini International University (IKIU), Qazvin, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Al-7075 alloy Equal channel angular pressing Creep properties Creep mechanism Microstructure
Effect of equal channel angular pressing ECAP process on creep properties of solid solutioned and annealed Al7075 alloy was investigated at different temperatures and applied stresses. The microstructures of treated alloys before ECAP process were analyzed via transmission electron microscopy (TEM) and electron back scatter diffraction (EBSD). Mechanical properties of ECAPed alloy were also characterized. The fracture surface of samples was also observed by scanning electron microscopy (SEM). Subsequently, the creep mechanisms at different temperatures and stresses were discussed. The results showed that applying the initial pass of ECAP can effectively improve the mechanical properties and creep resistance of Al-7075 alloys due to impeding the dislocations movement as a result of dislocation density growth. However, applying multi passes of ECAP on annealed Al7075 alloy did not have a positive effect on creep strength. Increasing the fraction of HAGBs and grain refinement of multi-pass annealed Al-7075 ECAPed alloy deteriorate the creep characteristic. The maximum creep rate was achieved for annealed sample after 1 pass of ECAP at 503 K under 70 MPa. Furthermore, according to different values of stress exponent, n, creep mechanism of treated samples altered. Dislocation creep as a dominant creep mechanism of annealed Al-7075 ECAPed alloy at both temperatures (503,543 K) was not altered after initial pass of ECAP; however it was replaced by grain boundary sliding after multi passes of process. Core creep mechanism of solid solutioned sample was also changed from Coble diffusion creep to viscos glide dislocation after one pass of ECAP.
1. Introduction The study of severe plastic deformation (SPD) methods over the last decades has been much more extensive [1–4]. Numerous experimental studies have shown that SPD techniques enhance the mechanical properties of alloys due to ultra-fine grained (UFG) or nano-grained (NG) formation at room or high temperatures [54][5]. As UFG and NG is not stable, grain growth and recrystallization can happen when working temperature exceeds about 0.3 Tm. It may deteriorate the creep behavior of alloys [5–8]. On the other hand, it is also rational that the nano-sized grains and second phases or precipitates in some alloys can prevent dislocations motion due to their pinning roles. Consequently, it keeps grain boundary from sliding and rotation and may improve the creep resistance of alloys [9]. Altogether, the effect of SPD methods such as equal channel angular pressing (ECAP) on the creep behavior of alloys is still remained a challenge. Al and its alloys have been widely used in some industries due to their lightweight, excellent strength to weight ratio, good corrosion
*
resistance and high thermal and electrical conductivity [10]. Among aluminum alloys, Al-7075 which is based on the Al–Zn–Mg–Cu alloy system (the 7xxx series aluminum alloys) have been applied in the aerospace, marine, automobile and aviation industries [11]. Until now, many researches have been conducted to investigate the effect of SPD methods on the mechanical, tribological, corrosion, fatigue and superplastic behavior of Al and its alloys [12–20]. However, the researches about creep behavior of SPD processed Al alloys are very limited. It was found in previous researches that the creep resistance of pure Al considerably increases after one pass of ECAP, however at subsequent passes, it starts to decrease due to increasing the amount of high-angle grain boundaries (HAGBs) [21–26]. Studies about creep behavior of ECAPed binary Al–Sc alloy and ternary Al–Mg-Sc alloy also indicated the detrimental effect of ECAP on the creep resistance after multi-passes of ECAP [27]. It was reported that the creep rate of these alloys increased up to 100–200% after initial ECAP passes. However, subsequent ECAP passes significantly reduced the creep life of the alloys [28,29]. Since the creep properties of Al-7075 is of great importance in
Corresponding author. Tel.: +98 28 33901190; fax: +98 28 3378 6579. E-mail addresses: [email protected] (F. Shahriyari), [email protected] (M.H. Shaeri).
https://doi.org/10.1016/j.msea.2019.138225 Received 12 April 2019; Received in revised form 26 July 2019; Accepted 27 July 2019 Available online 28 July 2019 0921-5093/ © 2019 Published by Elsevier B.V.
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Table 1 Chemical composition of Al-7075 alloy used in current research.
Table 2 The Al-7075 samples codes and conditions.
Ti
Zn
Cr
Mg
Mn
Cu
Fe
Si
Al
Sample code
Sample condition
0.02
5.70
0.21
2.65
0.04
1.50
0.09
0.07
base
O O1 O4 SS SS1
Annealing at 415 °C for 2 h followed by cooling in furnace for 24 h O + 1 pass ECAP at Tamb O + 4 passes ECAP at Tamb Solution treatment at 470 °C for 2 h followed by water quenching SS+ 1 pass ECAP at Tamb
many applications and the effect of ECAP process on the creep behavior of Al-7075 has not been investigated previously; evaluating the effect of ECAP process on the creep behavior of this alloy seems to be worthy. Hence, in this study, firstly two different heat treatments: annealing and solid solution treatment were implemented on Al-7075 alloy to obtain different microstructures. Then, the effect of ECAP process on creep strength as well as creep mechanism of pre-treated alloy was investigated.
Tamb: room temperature.
nitric acid and 70% methanol. Furthermore, the electropolishing with 30% nitric acid and 70% methanol solution was employed for EBSD sample preparation. The detailed information of the examination procedure and sample preparation for microstructure characterization has been reported in our previous papers [30–32]. Tensile specimens were prepared according to ASTM-E8 and tensile testing was done using a Zwick-Roell machine at a rate of 1 mm min−1 with the load direction parallel to the specimen axis. The yield strength, ductility (elongation to failure) and ultimate tensile strength of the samples were then evaluated. Each test was repeated at least 3 times to ensure the repeatability of the results. The Vickers microhardness test was performed on the cross section of the specimens by a VMHTAUTO Leica Wechsler microhardness tester. Each test was repeated at least 5 times near the center of samples. In order to evaluate the creep properties of Al-7075 alloy before and after ECAP process, the tension-creep test was used. The creep samples were machined with a gage diameter and length of 4.5 and 25 mm, respectively. The creep tests were performed with the applied stress of 70, 75, 80, 85 and 90 MPa at 503 and 543 K for O samples and 140, 145, 150, 155 and 160 MPa at 543 and 583 K for SS samples using a SANTAM SCT-30 creep tester. It should be mentioned that due to the higher strength of the Al-7075 in solid solution condition compared with that of annealed one, higher temperatures and stress were considered for creep test of solution treated samples. The surface microstructure of samples after creep test was also observed via AIS2300C scanning electron microscopy (SEM).
2. Material and method 2.1. Samples fabrication The material used in this study was extruded rods of Al-7075 alloy with diameter of 20 mm. The chemical composition of the alloy is presented in Table 1. At first, the extruded rods were machined into the cylinders with 11 mm diameter and 120 mm length. Prior to ECAP, annealing and solid solution treatment were carried out as pre-treatments. In order to prove the microstructure of Al-7075 annealed alloy (Al 7075-O), the Xray diffraction analysis (XRD) was performed using an X'pert PRO MPD Panalytical (Cu-Kα radiation) diffractometer. Then, ECAP was conducted at the ambient temperature using a solid die with channel angle of Φ = 90° and outer curvature angle of Ψ = 20°. The schematic of the ECAP die is presented in Fig. 1. Table 2 summarizes the samples condition and the code of each one. The deformation route of ECAP was BC in which the sample is rotated 90° between the ECAP passes in one direction and the pressing speed of ECAP was 0.5 mm/min. It should be noted that SS samples were processed just for one pass due to the inducement of catastrophic microcracks after multi passes of ECAP, while, the annealed samples were ECAPed up to 4 passes without cracking.
3. Results
2.2. Sample characterizations
3.1. Microstructure analysis
Microstructural aspects of ECAPed specimens were characterized by transmission electron microscopy (TEM) (JEOL JEM 3010 operating at 300 keV), and field emission scanning electron microscopy (FESEM) equipped with an electron back-scattered diffraction (EBSD) camera (Quanta 3D FEI operating at 15 kV and 10 nA). TD cross section of samples was chosen for evaluation of microstructure. TEM samples were prepared by twin-jet electro-polishing using an electrolyte of 30%
Precipitates characteristics of both annealed and solid solutioned Al7075 alloy before and after ECAP process have been studied by TEM. Fig. 2 presents the high magnification TEM images of the pre-treated samples before and after ECAP process. As it can be seen, the annealed sample (Al 7075-O) consists of mainly coarse and minor quantities of fine MgZn2 precipitates in Al matrix. Fig. 3 shows the XRD pattern of the initial Al 7075 alloy SEM microstructure and chemical analysis of precipitates in EDS analysis are shown in Fig. 4. The presence of MgZn2 peaks in XRD pattern and the results of EDS microanalysis prove that the microstructure consists of mainly α-Al as matrix and MgZn2 as major precipitates [18,19]. By increasing the passes of ECAP process, Al 7075-O4 sample merely presents the finer MgZn2 phase and the coarse MgZn2 precipitates disappear. TEM micrographs of solid-solutioned alloys before and after 1 pass of ECAP process are also depicted in Fig. 2 (d) and (e). As it expected, the microstructure of solid solutioned sample before and after ECAP process does not show any second phases. The absence of second phase in the microstructure of 1-pass ECAPed solid solutioned sample revealed that no precipitate was formed during 1 pass of ECAP at RT. Fig. 5 shows the inverse pole figures (IPF) maps as well as grain boundary analysis of unECAPed and ECAPed annealed samples. Obviously, initial microstructure of annealed sample includes large grains with sizes larger than 90 μm. While annealed sample exhibits the bimodal microstructure after 1 pass of ECAP composing both large and
Fig. 1. Schematic of ECAP die used in this work. 2
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Fig. 2. TEM micrographs of pretreated Al- 7075 alloys; annealed alloy, a) before ECAP, b) 1 pass ECAPed and c) 4 passes ECAPed as well as solid-solutioned alloys, d) before ECAP and e) 1 pass ECAPed.
3.2. Mechanical analysis The experimental results of the microhardness as well as tension tests are presented at Table 3. It is seen that the annealed sample has the lowest mechanical properties. Irrespective of pre-treatment, applying ECAP enhanced the hardness, yield strength and tensile strength. After 4 passes of ECAP, hardness and tensile strength of the annealed sample increases over 100%. Whereas, this growth can be seen only after 1 pass of ECAP for SS sample. It should be mentioned that ECAP processing decreases the total elongation. Strength and hardness improvement of ECAPed samples may be attributed to grain refinement and considerable growth of dislocation density after ECAP process [33–35]. The more significant efficacy of ECAP process in mechanical properties improvement of solid solutioned samples in comparison with those of annealed samples could be originated from dynamic strain aging as a result of formation of GP zones during ECAP process [31,36,37]. 3.3. Creep Fig. 3. XRD pattern of Al-7075 annealed sample.
Creep behaviors of annealed Al-7075 (Al 7075-O) alloy before ECAP process at 503 K and 543 K are shown in Fig. 6. The experimental results are recorded in the plots of strain-time (ε-t) and strain rate-time (ε°-t). It should be noted that strain-time and strain rate-time curves of all samples are represented in only three applied stress to facilitate the analysis of the graphs. The results indicates that the creep rupture life of Al 7075-O alloy at 503 K is about 335 min at applied stress of 70 MPa; however, it is reduced to 228 and 142 min by increasing applied stress to 80 and 90 MPa, respectively (Fig. 6b). As can be seen in Fig. 6d, the creep rupture times of Al 7075-O sample at 543 K are 26, 112 and 134 min at applied loads of 70, 80 and 90 MPa, respectively. Fig. 7 shows the creep behavior of Al 7075-O alloy after 1pass ECAPed process at different stress and temperatures of creep test. As seen, applying single pass of ECAP at 503 K notably increases the creep rupture life of Al 7075-O alloy to 434, 376 and 147 min during increasing applied stresses from 70 to 80 and 90 MPa, respectively
small equiaxed grains, applying 4 pass of ECAP process demonstrates only small equiaxed grains. Quantitative analysis of grain boundary maps performed by TSL software is summarized in Fig. 5g. As indicated, 46% of grain boundaries at Al 7075-O sample depict low angle boundaries with the misorientation angles from 2° to 5°. Following applying 1 pass of ECAP process, although the fraction of grain boundaries with the average misorientation between 2° and 5° exhibits a slight reduction; the fraction of grain boundaries with 5°–10° misorientation considerably decreases and finally great amounts of high-angle grain boundaries (> 15°) are detected. By increasing the passes number of ECAP process to 4 passes most of the low angle boundaries are replaced with highangle grain boundaries. It is worth-mentioning that EBSD microstructural evaluation of solid solutioned sample after 1 pass of ECAP does not show any remarkable differences with Al 7075-O1 sample. 3
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Fig. 4. a) SEM microstructure of Al 7075-O alloy and b) EDS micro-analysis of the selected area.
(Fig. 7b). On the other hand, increasing the passes of ECAP to 4 passes considerably declines the creep rupture life to 51, 63 and 81 min in applied stresses of 70, 80 and 90 MPa, respectively (Fig. 8b). Creep behaviors of Al 7075-SS alloy before and after ECAP process at 543 and 583 K are illustrated in Fig. 7, when tested under the 140, 150 and 160 MP. As can be seen the failure of Al 7075-SS sample at 543 K occurs after 126, 104 and 79 min when tested at applied loads of 140, 150 and 160 MPa, respectively (Fig. 9b). As can be observed, the creep rupture life of Al 7075-SS sample at 583 K significantly declines to 72, 57, 44 min under applied stresses of 140, 150 and 160 MPa (Fig. 9d). Fig. 10 shows the creep behavior of ECAPed solid solution sample at both temperatures of 543 K and 583 K under the applied stress of 140, 150 and 160 MPa. It is clear that after 1 pass of ECAP, the creep rupture life of SS samples at 543 K considerably grew up to 146, 115 and 93 min by increasing the applied stresses from 140 up to 160 MPa. In order to evaluate the effects of temperature and stress on the creep rate and identifying the creep mechanisms, the logarithmic plots
Table 3 Average mechanical properties of Al-7075 alloy before and after ECAP process (YS: yield stress, UTS: ultimate tensile stress, Δ: ductility, H: hardness). Sample
YS (MPa)
UTS (MPa)
Δ (%)
O O1 O4 SS SS1
169± 3 288± 3 384± 4 235± 7 598± 9
230±5 300± 2 450± 6 368± 6 624± 8
15.5± 5.29± 9± 3 17.2± 8.67±
H (Hv)
2 1 7 5
60± 3 111± 4 140± 3 95± 5 189± 4
of the steady-state creep rates versus the applied stresses for all samples are illustrated in Fig. 11. Meanwhile, the stress exponents of samples, obtained from the slop of ε° - σ plots, are summarized in Table 4. The stress dependence of the steady-state creep rate can be ascribed by power-law equation:
ε o = Aσ n
(1)
Where A is independent of σ, and stress exponent of creep rate (n) can
Fig. 5. EBSD inverse pole figure and grain boundary maps of annealed Al-7075, (a, d) unECAPed sample, (b, e) 1 pass ECAPed sample and (c, f) 4 passes ECAPed sample. (g) Quantitative analysis regarding the faction of LAGBs and HAGBs in annealed samples at different conditions. (The unit triangle depicted the orientation of each grain). 4
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Fig. 6. Creep behavior of Al 7075-O sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively.
4. Discussions
be described by following equation:
n=(
∂lnε o )T ∂lnσ
4.1. Microstructure (2) According to microstructural observations, although annealed Al7075 alloy contained coarse MgZn2 phases in Al matrix (Figs. 3–4), applying one pass of ECAP results in fragmentation of coarse MgZn2 precipitates in which coarse phases could hardly been observed in Al 7075-O1 sample (Fig. 2b). It is also obvious that after ECAP process, the microstructure became wavy and ill-defined indicating the presence of high dislocation density as well as non-equilibrium state of the alloy. Increasing the passes of ECAP process demonstrated the finer MgZn2 phase at Al 7075-O4 sample (Fig. 2c). The microstructure of Al-7075 solid solution sample with and without ECAP process, however, was free from any clear second phases (Fig. 2c and d). Based on the inverse pole figures (IPF) maps of samples, initial microstructure of annealed sample included large grains with sizes larger than 90 μm (Fig. 5a and d). The microstructure of annealed sample after 1 pass of ECAP comprised some large elongated grains stemmed from the shear strain during ECAP (with a mean size of 3 μm) as well as some small equiaxed ultra-fined grains (with size of about 700 nm) (Fig. 5b and e) as a result of dislocation tangles formation in deformation bands [38,39]. The homogenous equiaxed grains along the transverse direction (TD) were utterly obvious with the size of about 600 nm after 4 passes of ECAP (Fig. 5c and f). Quantitative analysis of IPF and grain boundary analysis manifested that applying ECAP process could have a profound effect on the misorientation of grains. As it is shown in Fig. 5g, the microstructural analysis effectively confirmed the greater fraction of high angle boundaries (HAGBs) by applying ECAP and also increasing the number of passes [32]. As expected ECAP process did not the microstructure of solid solution samples.
As can be seen in Table 4, the value of n is about 4.67 for Al 7075-O sample at 503 K, which indicates the dominance of dislocation creep as the main creep mechanism. Increasing creep temperature to 543 K leads to decreasing the n value to 4.01. Applying single pass of ECAP for annealed sample presents the higher values of n at both temperatures (about 5.24 and 4.56 at 503 and 543 K, respectively). The stress exponent of Al 7075-O4 sample at both temperatures are significantly lower than both Al 7075-O and –O1 samples (n = 3.15 and 2.18 for 503 and 543 K, respectively). It is also clear that stress exponent decreases with increasing the creep temperature from 503 to 543 K. When it comes to solid solution samples, while the stress exponent of Al 7075-SS sample is remarkably low (n = 1.56), it then notably grows to about 3.36 after one pass of ECAP. Increasing temperature from 543 to 583 K presents lower stress exponent of 1.18 and 2.87 for both Al 7075-SS and Al 7075-SS1 samples, respectively. Fig. 10 depicts the SEM images of fracture surfaces of annealed and solid solutioned samples after creep at temperature of 543 K and applied stresses of 80 and 150 MPa in different processing condition, respectively. It seems that the large precipitates of annealed sample are incoherent with the matrix and can easily detach from matrix; therefore they encourage the microvoids nucleation as well as large dimple formation, which are easily detected in Al 7075-O sample (Fig. 12a). It can be seen that the surface of Al 7075-O1 sample presents two kinds of dimples including large and small size (Fig. 12b). In the case of Al 7075O4 sample, the number of submicron-dimples becomes greater and consequently the dimples density enhances (Fig. 12c). Comparison of fracture surfaces of solid solutioned samples before and after ECAP process in Fig. 12 (d) and (e) reveal that, similar to annealed samples, the dimples size reduced considerably after 1 pass of ECAP. Meanwhile the surface of Al 7075-SS1 sample contains both large and small size dimples due to the bimodal microstructure.
4.2. Creep rupture life Based on the experimental results of strain-time and strain rate-time plots, the creep behavior of Al-7075 alloy was studied before and after ECAP process. As expected, a creep curve of Al 7075-O generally included three stages: the short primary, well-defined secondary (steady state) and tertiary stages. With increasing the applied stress, the creep strain rate increased and correspondingly the primary and secondary
Fig. 7. Creep behavior of Al 7075-O1 sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively. 5
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Fig. 8. Creep behavior of Al 7075-O4 sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively.
formation of LAGBs in first pass of ECAP, occurrence of creep aging during ECAP process of solid solutioned has also contributed in improvement of creep life within the first pass of ECAP. During the SPD process, the remained tiny precipitates can be dissolved, and accordingly the amount of solute elements responsible for subsequent static or dynamic strain aging is increased. Hence, the creep aging of the sample imposed during creep deformation can influence the creep rate of the sample [44].
stages were shortened while the tertiary stage was prolonged (Fig. 6). The earlier beginning of tertiary stage and the higher creep rate suggested much shorter creep rupture life of the alloy exposed to the higher stress [40]. The effect of temperature on the creep behavior of the Al 7075-O can be realized by comparison of curves in Fig. 6. It is clear that increasing temperature rapidly increased the creep strain rate and decreased the creep rupture life [40]. It is interesting to note that the secondary creep stage of Al 7075-O alloy at 543 K under 90 MPa, was considerably shortened in such a way that the strain rate curve almost included primary and tertiary stages. Effect of ECAP process on the creep behavior of annealed Al-7075 alloy at 503 and 543 K are depicted in Figs. 7 and 8. Single pass of ECAP notably increased the creep strength of Al 7075-O alloy, whereas applying more passes (4 passes ECAP) led to a significant decrease. Regarding to microstructural analysis (Fig. 5b and e), low-angle grain boundaries (LAGBs) combined with elongated grains in Al7075–O1 sample retarded the dislocation motion; therefore, creep strength significantly increased up to 40% after 1 pass of ECAP [41,42]. Significant transformation of LAGBs to HAGBs and grain refinement of Al 7075-O4 sample (Fig. 3c and d) could facilitate the grain boundary sliding and ultimately deteriorated the creep strength by approximately 50% compared with that of Al 7075-O1. This result is in agreement with Blum et al. [8,43]. They suggested that when grains become smaller than sub-grains, significant softening causes the creep behavior deterioration. It is worth to note that these results are significantly consistent with mechanical properties of samples. Evaluating the effect of temperature on the creep behavior of ECAPed Al 7075-O reveals that by increasing the temperature of creep test from 503 K to 543 K, the creep rupture life of single passes ECAPed sample (Al 7075-O1) as well as multi-passes ECAPed sample (Al 7075O4) significantly drop. Decreasing the creep rupture life of ECAPed samples by increasing the creep temperature could be explained by the higher diffusion and greater activation of new sliding systems [40]. Applying ECAP process enhanced the creep resistance of solid solutioned sample (Fig. 9). As indicated in Fig. 10, the strain rate of steady state region of Al 7075-SS1 was greatly developed after one pass of ECAP process. Similar to annealed samples, increasing the temperature of creep test resulted in remarkable reduction in creep strength of solid solutioned samples. Creep rupture life of SS1 decreased up to 50% by increasing testing temperature from 543 to 583 K. Alongside the
4.3. Creep mechanisms Creep mechanisms were assessed based on the stress exponent's valve of samples (Fig. 11 and Table 4). The n value of Al 7075-O sample was calculated 4.67 which is almost similar to the n value reported for coarse grained pure Al subjected to creep testing (n = 4.5) [23,45,46]. Increasing the n value of this sample via increasing the creep test temperature indicated that dislocation creep could be a dominate creep mechanism [23]. The reduction of n value by increasing the creep test temperature may originate from the increment of intergranular dislocation glide and climb as dominant rate-controlling flow processes [47,48]. The n value of annealed sample increased to 5.24 and 4.56 after applying 1pass ECAP at 503 K and 543 K; respectively. This may verify that the improvement of creep properties can be explained by increment of the dislocation density and LAGBs amounts after single pass of ECAP (Fig. 5g). The lower value of stress exponent for Al 7075-O4 at 503 K (n = 3.15) compared to Al7075–O (n = 4.67) and –O1 samples (n = 5.24) represented the influence of more intensive grain boundary sliding on the creep deformation of UFG material [21,23]. It seems that the creep of 4-passes ECAPed sample at lower temperature deteriorated due to the generation and movement of dislocations inside the grains as well as grain boundary sliding [49]. Regarding to microstructural analysis, replacing LAGBs with HAGBs and formation of UFG microstructure at 7075-O4 sample could verify this idea very well [50]. Similar to Al 7075-O1 sample, the stress exponent of the Al 7075-O4 alloy also decreased with increasing the creep temperature. This behavior can be interpreted in terms of easier dislocation movement and also grain boundary sliding in creep deformation at higher temperatures [51]. When it comes to solid solutioned samples, while the n value was greatly low (n = 1.56) before ECAPed, it increased up to 3.36 after
Fig. 9. Creep behavior of Al 7075-SS sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively. 6
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Fig. 10. Creep behavior of Al 7075-SS1 sample, (a, c) strain–time and (b, d) strain rate–time curves at creep temperatures of 503 and 543 K, respectively.
Fig. 11. Logarithmic plot of minimum creep rate versus applied stress for Al-7075 samples before and after ECAP process at both creep temperatures of 503 and 543 K, (a) O sample, (b) O1 sample, (c) O4 sample, (d) SS sample and (e) SS1 sample. Table 4 Stress exponent of Al-7075 samples at both creep temperatures of 503 and 543 K in different conditions. Sample
O
Temperature (K) N
503 4.67
O1 543 4.01
503 5.24
O4 543 4.56
503 3.15
SS1
SS 543 2.18
543 1.50
583 1.18
543 3.36
583 2.87
4.4. Creep fracture surface
single pass of ECAP. As it was previously mentioned, higher creep temperature was applied for solid solutioned samples. This higher temperature creep test could activate coble mechanism in Al 7075-SS sample [40]. However after applying 1 pass of ECAP, the dominant creep mechanism altered from coble creep to viscos glide dislocation creep. Reducing the n value of solid solutioned samples by increasing the temperature demonstrated that the coble diffusion creep and viscose glide process of dislocation could be a dominant mechanism for Al 7075-SS1 samples, respectively [52]. The change of dislocation mechanism form coble creep to dislocation glide creep by applying one pass of ECAP is in consistent with higher creep properties of 1-pass ECAPed Al 7075-SS in comparison with that of unECAPed Al 7075-SS.
According to the SEM observations of the fracture surfaces of creepruptured samples, the fracture surface of one pass ECAPed annealed samples was greatly covered with the finer submicron-dimples in comparison with that of initial annealed specimen, however, some large dimples could be found (Fig. 12a and b). It is worth noting that the difference in dimples sizes deflected bimodal microstructure of Al 7075O1 sample [17]. The formation of UFG microstructure after 4 passes of ECAP produced small-sized dimples in fracture surface of Al 7075-O4 sample (Fig. 12c). These small-sized dimples could easily coalescence together and ultimately evolve into large ones. The coalescence of microvoids and microcracks proves the creep properties deterioration of Al 7075-O4 sample [15]. Applying 1pass of ECAP process on Al 7075-SS alloy also 7
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Fig. 12. SEM micrographs of the fracture surfaces of creep-ruptured Al-7075 alloy at different conditions, (a) O, (b) O1, (c) O4, (d) SS and (e) SS1 samples.
demonstrated the finder dimples than that of Al 7075-SS. It should be noted that the fracture mechanism of all samples was predominantly ductile fracture.
Processed by ECA Pressing, (2005). [8] W. Blum, Y. Li, Y. Zhang, J.T. Wang, Deformation Resistance in the Transition from Coarse-Grained to Ultrafine-Grained Cu by Severe Plastic Deformation up to 24 Passes of ECAP, (2011). [9] L. Wang, F. Qiu, Q. Zhao, M. Zha, Q. Jiang, Superior high creep resistance of in situ nano-sized TiC(x)/Al-Cu-Mg composite, Sci. Rep. 7 (2017) 4540. [10] R. Abrahams, J. Mikhail, P. Fasihi, Effect of friction stir process parameters on the mechanical properties of 5005-H34 and 7075-T651 aluminium alloys, Mater. Sci. Eng. A 751 (2019) 363–373. [11] P.V. Kumar, G.M. Reddy, K.S. Rao, Microstructure, mechanical and corrosion behavior of high strength AA7075 aluminium alloy friction stir welds – effect of post weld heat treatment, Defence Technology 11 (4) (2015) 362–369. [12] S. Gholami, E. Emadoddin, M. Tajally, E. Borhani, Friction stir processing of 7075 Al alloy and subsequent aging treatment, Trans. Nonferrous Metals Soc. China 25 (9) (2015) 2847–2855. [13] M. Ilieva, R. Radev, Effect of Severe Plastic Deformation by ECAP on Corrosion Behaviour of Aluminium Alloy AA 7075, (2016). [14] S. Kuramoto, K. Horibuchi, I. Aoi, K. Oh-ishi, Effect of severe plastic deformation on mechanical properties and microstructure in 7075 aluminum alloy, J. Jpn. Inst. Light Metals 64 (6) (2014) 241–248. [15] Y.C. Lin, Z.-W. Wang, D.-G. He, Y. Zhou, M.-S. Chen, M.-H. Huang, J.-L. Zhang, Effects of pre-treatments on precipitate microstructures and creep-rupture behavior of an Al–Zn–Mg–Cu alloy, J. Mater. Res. 31 (9) (2016) 1286–1295. [16] J. Robinson, R.L. Cudd, J. Evans, Creep Resistant Aluminium Alloys and Their Applications, (2003). [17] A. Esmaeili, M.H. Shaeri, M.T. Noghani, A. Razaghian, Fatigue behavior of AA7075 aluminium alloy severely deformed by equal channel angular pressing, J. Alloy. Comp. 757 (2018) 324–332. [18] M. Chegini, M.H. Shaeri, Effect of equal channel angular pressing on the mechanical and tribological behavior of Al-Zn-Mg-Cu alloy, Mater. Char. 140 (2018) 147–161. [19] M. Shaeri, M. Salehi, S. Seyyedein, M. Abutalebi, J.K. Park, Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment, Mater. Des. 57 (2014) 250–257. [20] A. Dashti, M. Shaeri, R. Taghiabadi, F. Djavanroodi, F. Vali Ghazvini, H. Javadi, Microstructure, texture, electrical and mechanical properties of AA-6063 processed by multi directional forging, Materials 11 (12) (2018) 2419. [21] V. Sklenička, J. Dvořák, M. Svoboda, Creep in ultrafine grained aluminium, Mater. Sci. Eng. A 387 (2004) 696–701. [22] V. Sklenička, J. Dvořák, M. Svoboda, P. Král, B. Vlach, Effect of Processing Route on Microstructure and Mechanical Behaviour of Ultrafine Grained Metals Processed by Severe Plastic Deformation, Materials Science Forum, Trans Tech Publ, 2005, pp. 83–88. [23] V. Sklenicka, J. Dvorak, P. Kral, Z. Stonawska, M. Svoboda, Creep processes in pure aluminium processed by equal-channel angular pressing, Mater. Sci. Eng. A 410 (2005) 408–412. [24] V. Sklenička, P. Král, J. Dvořák, M. Kvapilová, M. Kawasaki, T.G. Langdon, Effect of Equal-Channel Angular Pressing on the Creep Resistance of PrecipitationStrengthened Alloys, Materials Science Forum, Trans Tech Publ, 2011, pp. 897–902. [25] J. Dvořák, P. Král, M. Kvapilová, M. Svoboda, V. Sklenička, Microstructure Stability and Creep Behaviour of a Cu-0.2 wt.% Zr Alloy Processed by Equal-Channel Angular Pressing, Materials Science Forum, Trans Tech Publ, 2011, pp. 821–826. [26] P. Král, J. Dvořák, A. Jäger, M. Kvapilová, Z. Horita, V. Sklenička, Creep properties of aluminium processed by ECAP, Kovove Mater 54 (2016) 441–451. [27] V. Sklenička, J. Dvořák, M. Kvapilová, M. Svoboda, P. Král, I. Saxl, Z. Horita, Effect of Equal-Channel Angular Pressing (ECAP) on Creep in Aluminium Alloys, Materials
5. Conclusion In this research, the creep behaviors and creep mechanisms of annealed and solid-solutioned Al-7075 alloy were investigated before and after ECAP process. The following conclusions can be derived from the results: The creep response of Al-7075 alloy was related to temperature and primary microstructure. The main creep mechanism of annealed Al7075 alloy was dislocation creep at 503 K. With an increase in creep temperature to 543 K, a greater creep deformation was observed due to creep mechanism alteration to rate-controlling flow processes such as dislocation glide and climb. Increasing the n value of ECAPed annealed Al-7075 alloy after first pass indicated the creep strength enhancement. This can be attributed to higher amounts of LAGBs and dislocation jungles after single pass of ECAP. After applying multi passes of ECAP, the creep resistance was markedly deteriorated with respect to replacing LAGBs with HAGBs as well as activation of grain boundary sliding at 503 K. Increasing the creep temperature to 543 K presented the lower n for both ECAPed Al 7075-O alloys as a result of higher dislocation movements and grain boundary sliding. Regarding to solid solutioned Al 7075-SS alloy, coble mechanism which was the dominant creep mechanism changed to viscos glide dislocation creep after applying 1 pass of ECAP at 543 and 583 K. References [1] L. Zuev, Macroskopicheskaya fizika plasticheckoy deformatsii, Uspehi fiziki metallov 16 (2015) 35–60. [2] L.B. Zuev, On the waves of plastic flow localization in pure metals and alloys, Ann. Phys. 16 (4) (2007) 286–310. [3] A. Berezina, T. Monastyrska, O. Davydenko, O. Molebny, S. Polishchuk, Effect of severe plastic deformation on structure and properties of Al-Sc-Ta and Al-Sc-Ti alloys, Nanoscale research letters 12 (1) (2017) 220. [4] V. Segal, modes and processes of severe plastic deformation (SPD), Materials 11 (7) (2018) 1175. [5] U. Bansal, M. Sharma, Development of mechanical properties by severe plastic deformation methods, Development 2 (5) (2015). [6] P. Kral, J. Dvorak, W. Blum, E. Kudryavtsev, S. Zherebtsov, G. Salishchev, M. Kvapilova, V. Sklenicka, Creep study of mechanisms involved in low-temperature superplasticity of UFG Ti-6Al-4V processed by SPD, Mater. Char. 116 (2016) 84–90. [7] C. xu, T.G. Langdon, Creep and Superplasticity in a Spray-Cast Aluminum Alloy
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