- Email: [email protected]
Anisotropy of mechanical properties and crystallographic texture in hot rolled AZ31+XSr sheets
Available online at www.sciencedirect.com
Journal of Magnesium and Alloys 7 (2019) 466–473 www.elsevier.com/locate/jma
Full Length Article
Anisotropy of mechanical properties and crystallographic texture in hot rolled AZ31+XSr sheets Alireza Sadeghi a,∗, Hossein Mortezapour a, Javad Samei b, Mihriban Pekguleryuz c, David Wilkinson b a School
of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada c Department of Mining and Materials Science, McGill University, Montreal, QC, Canada
b Department
Received 26 November 2018; received in revised form 17 April 2019; accepted 17 April 2019 Available online 31 May 2019
Abstract The influences of Sr on the microstructure, texture and mechanical properties including flow and anisotropy behavior of AZ31 alloys are investigated. Slabs containing no, 0.4 and 0.8 wt% of strontium were cast and subjected to hot rolling. Results indicate that Sr reduces the basal texture intensity (23% in the AZ31+0.8Sr alloy) and homogenizes the distribution of strain in uniaxial tension. Furthermore, Sr increases both the strength coefficient and the strain hardening exponent in all directions. In the transverse direction, enhancements are more significant. Moreover, Sr enhances the combination of tensile strength and total elongation; i.e., toughness, whether tests are performed parallel to the rolling, diagonal or transverse directions, to significant extents. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: AZ31; Hot rolling; Mechanical properties; Microstructure; Strontium; Texture.
1. Introduction Strontium additions are initially introduced to Mg alloys for grain refinement [1–5]. Different mechanisms are suggested for this effect range from growth restriction [1,2,4] to the introduction of nucleating particles [1,3]. The grain refining effect of Sr is most effective at low concentrations (<0.1 wt%) and when Al is not present in the alloy [4]. However, other researchers have reported that Sr increases the growth restriction factors of Al and Zn in AZ31 [5]. Pekguleryuz et al. first introduced alloys containing higher Sr concentrations up to 1.5wt% and named the new alloys as the AJ series [6,7]. In these alloys, Al and Sr form temperature resistant intermetallics which lock the grain boundaries and enhance creep performance. Such alloys have high volume
∗
Corresponding author. E-mail address: [email protected] (A. Sadeghi).
fractions of precipitates which reduce necessary formability for wrought applications. The first attempts to use Sr in wrought Mg alloys was completed by Sadeghi et al. [8,9] and Borkar et al. [10,11] for AZ31 and M1 alloys respectively. Results indicate that Sr changes the mechanical properties of wrought alloys by: (1) Reducing the concentration of solute atoms in the Mg matrix and reducing the solute hardening effect. Previous reports show that Sr reduces the solid solubility of Al and Zn in the Mg matrix [5,12]. (2) Forming new precipitates such as Al4 Sr and Mg17 Sr2 which contribute to hardening and reduce ductility by crack initiation and propagation [9,10,13]. (3) Reducing the basal texture intensity by particle stimulated nucleation (PSN) during hot deformation [8,11]. In general, median concentrations of Sr (0.2–0.8 wt%) in Mg alloys provide enough formability to form wrought alloys.
https://doi.org/10.1016/j.jma.2019.04.005 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Later, other researchers have investigated the effect of higher concentrations of Sr in precipitation of wrought Mg alloys. For instance, Wu et al. [14] added high concentrations of Sr in AZ31 and have reported that the formed precipitates are different at low and high concentrations of Sr. High concentrations of Sr procedure a continuous web of precipitates at the interdendritic regions. This web of precipitates might be useful for creep resistant applications, but it limits formability which is necessary for wrought applications. The effect of Sr in hot extrusion of AZ31+XSr alloys is previously studied, and the effect of Sr on yield anisotropy of extruded sheets in tension and compression are previously reported [15]. It has been shown that the tensile and compressive strength of these alloys is more dependent on the extrusion temperature compared to the level of Sr. With increasing extrusion temperature, yield asymmetry increases. Nevertheless, the amount of yield asymmetry increase reduces when Sr is present in the alloy. In the present paper, we report on the cast and hot rolled AZ31+0.4% and 0.8 wt% Sr plates. The two compositions are selected based on the promising results obtained in previous research published by the authors. Microstructures and mechanical properties of the sheets are studied, and the texture evolution is compared. Based on the obtained results, the influence of strontium on the mentioned parameters are discussed.
2. Experimental procedure To investigate the effect of Sr concentration on the planar anisotropy of hot-rolled AZ31+XSr sheets, 1 × 10 × 10 cm AZ31 plates were cast with 0, 0.4 and 0.8 wt% Sr. Alloy compositions were synthesized in graphite crucibles using an induction furnace under CO2 –SF6 gas mixture cover. Measured quantities of 90Sr–10Al master alloys were used to introduce Sr in the molten AZ31. Samples were cast into BN covered preheated steel dies at 720 °C. Cast plates were rolled to 2 mm thick sheets by hot rolling at 350 °C. Reduction from 10 mm to 2 mm involved 10 consecutive rolling passes with approximately 15% thickness reduction for each pass. Dog-bone tensile specimens (ASTM-E8) were wire-cut from the rolled sheet, oriented in the rolling (RD), 45° from rolling (DD) and transverse (TD) directions. An MTS universal testing machine equipped with an Aramis digital image correlation (DIC) system was used. The stress response was recorded as a function of longitudinal and width strain. The microstructure of the prepared sheets was examined by optical and scanning electron microscopy (FEG-SEM, Hitachi S-4700). To reveal the microstructure features, the RD-ND cross-section of the sheets were first ground, then polished by diamond paste, following with a picric-acetic solution for etching. In order to measure the twin fraction, optical images of fractured samples were taken. Microstructure was chosen up to approximately 200 μm below the fracture surface. The chosen area was measured by ImageJ software. At higher magnifications the number of twins were counted manually
467
and by dividing it by the total area, twin density was calculated. Energy dispersive spectroscopy (EDS) was used in SEM for phase identification. Volume fraction and morphology of Al4 Sr precipitates were determined by Thermo-Calc simulation and quantitative metallography, respectively. Deformation texture in the rolled sheets was examined by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer using Co Kα radiation on the sheet surface (RDTD). Incomplete pole figures of {0001} and {101¯ 0} were recorded and reconstructed using a texture analysis software (TexTools). 3. Results and discussion 3.1. Microstructures Microstructures of the studied samples after hot rolling are presented in Fig. 1. All alloys showed signs of dynamic recrystallization (DRX). Moreover, shear bands containing fine DRX grains and twins could be easily distinguished. Based on the AZ31-Sr Pseudo-binary phase diagram [12], at 350 °C the Mg17 Al12 precipitate is dissolved in the matrix. The only stable precipitates at this temperature are various Al–Mn precipitates and Al4 Sr. By increasing the Sr content, the stable Al–Mn precipitate at 350 °C changes from Mn4 Al11 to Al8 Mn5 . At room temperature, for all the samples the thermodynamically stable phases are Al99 Mn23 , Mg17 Al12 , MgZn and Al4 Sr. However, thermodynamic equilibrium may not be reached during the actual hot rolling experiments, and other unstable phases might be present [16]. It can be concluded that in the AZ31 sample (Fig. 1(a) and (b)), most of the precipitates are sharped edge Al–Mn precipitates. A few globular and elongated Mg17 Al12 precipitates are also observed. In the Sr-containing samples, the major precipitate is Al4 Sr. With increasing Sr concentration, the area fraction of this precipitate is increased (compare with Fig. 1(e)–(c)). The precipitates form long stringers in the rolling direction. Based on the Thermo-Calc simulation, at equilibrium, there is approximately 0.009 and 0.018 wt% of Al4 Sr in AZ31+0.4 and 0.8Sr, respectively. A higher magnification image of the precipitate stringers in the AZ31+0.8Sr sample is shown in Fig. 2. The image shows how newly recrystallized grains are initiated from the precipitate interface. Such grains are presumed to be formed by particle stimulated nucleation (PSN). Detailed EBSD analysis on the same set of alloys are performed previously by the authors which show that crystallographic orientation of the new grains are different from their parents [8]. Beyond serving as nucleation sites for new grains, precipitates actively block grain boundary migration and prevent grain growth. EDS mapping has been performed on a region covering two stringers with different compositions to identify the precipitate type causing PSN (Fig. 2(b)). Based on the individual element maps, it could be concluded that the upper stringer is Al–Mn and the lower stringer is Al–Sr. The Al–Mn precipitates could also be roughly distinguished by their large and
468
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Fig. 1. Microstructures of (a) and (b) AZ31, (c) and (d) AZ31+0.4Sr and (e) and (f) AZ31+0.8Sr, as-rolled at 350 °C. Rolling direction is horizontal and normal direction is vertical.
Fig. 2. (a) SEM microstructure (b) EDS result of two stringer lines with different compositions in of AZ31+0.8Sr hot rolled at 350 °C.
sharp morphology. In contrast, the eutectic Al–Sr precipitates form smaller round particles. The SEM image shows that recrystallized grains are associated with the Al–Sr precipitates and not the Al–Mn ones. Micrographs of Fig. 2 also show the formation of voids and small cracks between individual particles along the stringer line. Properly aligned discontinuities may act as stress concentrators in tension and cause premature failure with lower than expected elongation.
and AZ31+0.8Sr samples are 6.0 and 4.9, respectively (23% decrease by adding 0.8%Sr). The shape of the basal component in the AZ31+0.8Sr sample shows spreading in the TD direction. Spreading of basal poles has been previously observed in Mg alloys containing RE elements [17]. Prismatic pole figures show that no special alignment has formed in the plane directions. 3.3. Mechanical properties
3.2. Texture evolution Basal and prismatic pole figures after hot rolling at 350 °C are presented in Fig. 3. All the three samples have a strong basal texture. Formation of basal texture by extrusion of AZ31+Sr samples has been reported previously [8]. Prismatic pole figures show a random distribution of poles around the circle in each alloy. Although the addition of Sr has not changed the texture components, the texture intensity is reduced as presented in Fig. 4. The maximum intensity of the basal poles figure for AZ31 is 6.4 while that of AZ31+0.4Sr
Engineering stress–strain curves for the hot-rolled AZ31XSr sheets are presented in Fig. 5. In AZ31, the diagonal direction shows the highest tensile elongation (Fig. 5(a)). Although adding 0.4 and 0.8 wt% Sr did not notably affect elongation in DD it significantly enhanced elongations in RD and TD (Fig. 5(b) and (c)). In Fig. 5(d)–(f), the mechanical properties of the three alloys are compared in different directions. As presented, Sr improved both strength and elongation in RD and TD and it remarkably increased strength in DD without any notable influence on elongation.
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
469
Fig. 3. Basal and prismatic pole figures of AZ31, AZ31+0.4Sr and AZ31+0.8Sr after hot rolling at 350 °C.
Fig. 4. Influence of Sr on pole figure maximum intensity of basal and prismatic planes.
Quantitative analyses of flow curves are presented in Fig. 6. The power-law (Holloman) constitutive equation, σ = Kεn , was fitted to the curves using MATLAB Curve Fitting Toolbox. The strength coefficient, K, and hardening exponent, n, were determined with R2 >0.99 for all curves. As demonstrated in Fig. 6 (a) and (b), Sr increases the strength coefficient and hardening exponent for all sample directions. Addition of Sr improved texture (Fig. 3), and increased the number and size of obstacles, i.e. Al4 Sr, against easy glide of dislocations which along with solid solution hardening increased the strength and work hardening. The increase of work-hardening rate is in line with previous reports that have
shown that increasing Sr concentration in AZ31 decreases the Al concentration in solid solution and thereby enhances dislocation motion [9]. The impact of Sr on tensile elongation is shown in Fig. 6 (c). The most significant effect is seen in the RD and TD directions. Fig. 6(d) indicates that toughness, i.e., the area blew the flow curves, is increased by adding Sr in all directions. This is due to enhancement of combination of strength-ductility by adding Sr which is more pronounced (approximately 88%) in RD. This could be interesting when crashworthiness is important. As previously discussed elongation in DD is less affected by Sr concentration. Adding 0.4 wt% Sr first decreased the elongation perhaps due to solid solution hardening, and adding further Sr to 0.8wt% increased the elongation which can be attributed to softening the matrix as described before; however, toughness in DD was continuously increased by adding Sr. Twin density is measured in the fractured samples close to the fracture surface (Fig. 7(a)). Results indicate that by increasing Sr concentration, twin density is increased for all sample directions. Moreover, the 3D plot of Fig. 7(b) shows that the samples fractured in the TD and DD present the lowest and highest twinning density, respectively. This observation is in line with the stress–strain results of Fig. 5 which indicate that the TD and DD samples deform with the lowest and highest average elongations, respectively. Therefore, it can be concluded that the grains in the DD samples are well oriented for twin activation, and twinning plays an essential role in work-softening and elongation in this direction. In
470
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Fig. 5. Engineering stress-strain curves in RD, TD and ND for (a) AZ31, (b) AZ31+0.4Sr, (c) Az31+0.8Sr and comparing the three compositions in (d) RD, (e) TD and (f) DD.
Fig. 6. Influence of Sr on (a) strength coefficient, (b) work-hardening exponent, and (c) elongation to fracture.
Fig. 7. (a) Twin density at fracture of samples containing various levels of Sr in different sample directions. (b) twin density vs. sample orientation and Sr content and (c) normalized twin density vs. normalized fracture strain.
Fig. 7(c) normalized twin density is plotted versus normalized fracture strain. While results show that strain and twin density are proportional, the effect of Sr and sample direction on fracture strain is much less in the DD samples compared to the others. Effect of Sr on twinning activity at the regions next to fracture surfaces are further discussed later.
It is well-known that inhomogeneous deformation causes necking and eventually fracture in ductile materials. Inhomogeneous deformation occurs due to heterogeneities in microstructure, and due to mechanical instability at higher strain levels. To investigate influence of Sr on distribution of strain along specimens during tensile tests, strain maps were ob-
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
471
Fig. 8. Longitudinal strain distribution of the studied alloys in different directions (a) AZ31, (b) AZ31+0.4Sr, (c) AZ31+0.8Sr.
tained using DIC method during tensile tests. Fig. 8 presents evolutions of longitudinal strain distribution for AZ31+XSr samples in all at different during plastic deformation. To investigate evolutions of homogeneity of strain distribution during defomration, five locations were chosen in the hot zones in Fig. 8, i.e., those with higher strains shown in red, and the average strain in the hot zones was calculated. Strain Ratio was calculated at different steps during deformation according to the following equation: Str ain Ratio = δ =
average local strain in hot zones t rue global st rain
(1)
Obviously, the strains at hot zones are larger than the global strain; therefore, always 1< δ. When there is significant strain localization along the specimen, the strains at hot zones are large and δ is larger. Evolutions of δ is determined for the samples and results are presented in Fig. 9. Fig. 9 show that distribution of strain in the specimens along different directions containing various amounts of Sr. Comparing Fig. 5(a)–(c) with Fig. 9(a)–(c), it can be concluded that elongation reached to higher strains when strain distribution was more homogeneous. Fig. 9(f) clarifies that rapid increase of δ in TD led to earlier fracture compared to RD and DD. As presented in Fig. 9(d), adding Sr constantly enhanced uniform distribution of strain in RD in such a way that elongation reach to its maximum amongst all the specimens in RD at 0.8 wt% of Sr. Fig. 9(e) indicates that Sr improved uniform distribution of strain in DD. Although this was not reflected in improvement of elongation in DD
it led to enhancement of toughness; i.e., increase of strength without notable trade-off of elongation. As shown by the circles, progressive formation of 45° shear bands occurred in DD direction. As can be seen in Fig. 9(f), adding 0.8 wt% Sr reduced the rate of δ development in TD to an approximately linear condition and consequently elongation was improved for about 40%. In Mg alloys, plastic deformation occurs by a combination of twinning and dislocation motion. As mentioned above, the high affinity of Sr to Al dilutes the Al concentration in the Mg matrix through the formation of Al4 Sr [16]. This reduces the critical resolved shear stress for dislocation movement and twinning enables greater post-uniform elongation. This can explain the improved homogenization of strain in the Sr-added specimens. Hazeli et al. [18] have studied twinning activity in AZ31Sr samples by EBSD and acoustic emission. They have shown that the twinning activity is strongly attributed to plastic strain at fracture. As shown in Fig. 9(d)–(f), Sr is increasing true plastic strain by reducing strain ratio. This observation supports the findings of Fig. 7 where reported that Sr is increasing twinning activity at the regions next to the fracture surface in all sample directions. The R-value at different directions of samples containing different levels of Sr is plotted in Fig. 10(a)–(c) up to the necking strain. Results indicate that in AZ31 and AZ31+0.4Sr the R-value in the DD and TD are at the lowest and highest, respectively. However, in AZ31+0.8 (Fig. 10(c)) the R-value at RD is the lowest. Fig. 10(d)–(f) shows that Sr decreases
472
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Fig. 9. Evolutions of Strain Ratio, δ, during deformation.
Fig. 10. R-values of studied alloys in different directions and strains.
R-values of AZ31+XSr samples at different directions. The R-values are initially between 0.5 and 1 but increases rapidly to around 2 in RD and DD and close to 3 in TD. Based on the R-value diagrams, it can be suggested that the AZ31 alloy generally resists thinning and shear localization. This
can also be explained by the strong basal texture where the crystal orientation with its c-axis parallel to ND is unable to provide easy slip direction for the dislocations. As noted previously, by adding Sr, the basal texture is weakened, and work-hardening is increased. Therefore, further sheet thinning
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
occurs in the Sr-added specimens which is consistent with reducing the R-values. 4. Conclusions 0.4 and 0.8 wt% of Sr were added to an AZ31 alloy. The influences of Sr are investigated on microstructure, mechanical properties, and crystallographic texture of the hot-rolled AZ31 and Sr-added alloys. The following conclusions are drawn: (1) Al4 Sr particles were identified at the grain boundaries in the microstructures of Sr-added alloys. As expected, the number and size of the particles are larger in the 0.8Sradded alloy. Evidences of particle stimulated nucleation is observed across the microstructure of Sr-added alloys leads to reducing basal texture intensity. (2) The strength coefficient and work-hardening exponent in the Holloman’s constitutive equation was continuously increased by adding Sr in all direction. Elongations to fracture are increased in all cases expect for the AZ31+0.4Sr in DD perhaps due to significant of solid solution hardening; however, the combination of strength-ductility (i.e., toughness) was improved in all cases. Toughness improved approximately 88% in RD by adding 0.8 wt% Sr which is interesting when crashworthiness in a critical parameter. (3) According to the DIC maps, distribution of strain become more homogenous by adding Sr in all directions. This is in good agreement by improvement of toughness in the Sr-added alloys. (4) Elongation in TD is significantly smaller compared to RD and DD in AZ31 alloy. This is attributed to rapid evolutions of inhomogeneous distribution of strain along with the highest development of R-value in AZ31 in TD. The elongation and toughness in TD improved for approximately 42% and 58% by adding 0.8 wt% of Sr, respectively. Conflict of interest None.
473
Acknowledgment A. Sadeghi and H. Mortezapour acknowledge the support of the Iran National Science Foundation (INSF) Grant number 95832506. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. References [1] M. Yang, F. Pan, R. Cheng, A. Tang, Mater. Sci. Eng. A 491 (1–2) (2008) 440–445. [2] M. Yang, F. Pan, R. Cheng, A. Tang, J. Mater. Sci. 42 (24) (2007) 10074–10079. [3] Y.C. Lee, A.K. Dahle, D.H. StJohn, Metall. Mater. Trans. A 31 (11) (2000) 2895–2906. [4] D.H. StJohn, M. Qian, M.A. Easton, P. Cao, Z. Hildebrand, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 36 (7) (2005) 1669–1679. [5] R. Cheng, F. Pan, S. Jiang, C. Li, B. Jiang, X. Jiang, Prog. Nat. Sci. Mater. Int. 23 (1) (2013) 7–12. [6] E. Baril, P. Labelle, M. Pekguleryuz, Jom 55 (11) (2003) 34–39. [7] M. Pekguleryuz, E. Baril, Mater. Trans. 42 (7) (2001) 1258–1267. [8] A. Sadeghi, M. Hoseini, M. Pekguleryuz, Mater. Sci. Eng. A 528 (7–8) (2011) 3096–3104. [9] A. Sadeghi, S. Shook, M. Pekguleryuz, Mater. Sci. Eng. A 528 (25) (2011) 7529–7536. [10] H. Borkar, M. Hoseini, M. Pekguleryuz, Mater. Sci. Eng. A 537 (2012) 49–57. [11] H. Borkar, R. Gauvin, M. Pekguleryuz, J. Alloys Compd. 555 (2013) 219–224. [12] A. Sadeghi, M. Pekguleryuz, J. Mater. Res. 26 (7) (2011). [13] R.J. Cheng, F.S. Pan, S. Jiang, X.Q. Jiang, C. Li, Mater. Sci. Technol. (U.K.) 29 (2) (2013). [14] L. Wu, F. Pan, M. Yang, R. Cheng, J. Mater. Sci. 48 (16) (2013) 5456–5469. [15] A. Sadeghi, S. Shook, M. Pekguleryuz, Mater. Sci. Eng. A 528 (25–26) (2011) 7529–7536. [16] A. Sadeghi, M. Pekguleryuz, J. Mater. Sci. 47 (14) (2012) 5374–5384. [17] D. Griffiths, B. Davis, J.D. Robson, Metall. Mater. Trans. A 49 (1) (2018) 321–332. [18] K. Hazeli, A. Sadeghi, M.O. Pekguleryuz, A. Kontsos, Mater. Sci. Eng. A 578 (2013).
Journal of Magnesium and Alloys 7 (2019) 466–473 www.elsevier.com/locate/jma
Full Length Article
Anisotropy of mechanical properties and crystallographic texture in hot rolled AZ31+XSr sheets Alireza Sadeghi a,∗, Hossein Mortezapour a, Javad Samei b, Mihriban Pekguleryuz c, David Wilkinson b a School
of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada c Department of Mining and Materials Science, McGill University, Montreal, QC, Canada
b Department
Received 26 November 2018; received in revised form 17 April 2019; accepted 17 April 2019 Available online 31 May 2019
Abstract The influences of Sr on the microstructure, texture and mechanical properties including flow and anisotropy behavior of AZ31 alloys are investigated. Slabs containing no, 0.4 and 0.8 wt% of strontium were cast and subjected to hot rolling. Results indicate that Sr reduces the basal texture intensity (23% in the AZ31+0.8Sr alloy) and homogenizes the distribution of strain in uniaxial tension. Furthermore, Sr increases both the strength coefficient and the strain hardening exponent in all directions. In the transverse direction, enhancements are more significant. Moreover, Sr enhances the combination of tensile strength and total elongation; i.e., toughness, whether tests are performed parallel to the rolling, diagonal or transverse directions, to significant extents. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: AZ31; Hot rolling; Mechanical properties; Microstructure; Strontium; Texture.
1. Introduction Strontium additions are initially introduced to Mg alloys for grain refinement [1–5]. Different mechanisms are suggested for this effect range from growth restriction [1,2,4] to the introduction of nucleating particles [1,3]. The grain refining effect of Sr is most effective at low concentrations (<0.1 wt%) and when Al is not present in the alloy [4]. However, other researchers have reported that Sr increases the growth restriction factors of Al and Zn in AZ31 [5]. Pekguleryuz et al. first introduced alloys containing higher Sr concentrations up to 1.5wt% and named the new alloys as the AJ series [6,7]. In these alloys, Al and Sr form temperature resistant intermetallics which lock the grain boundaries and enhance creep performance. Such alloys have high volume
∗
Corresponding author. E-mail address: [email protected] (A. Sadeghi).
fractions of precipitates which reduce necessary formability for wrought applications. The first attempts to use Sr in wrought Mg alloys was completed by Sadeghi et al. [8,9] and Borkar et al. [10,11] for AZ31 and M1 alloys respectively. Results indicate that Sr changes the mechanical properties of wrought alloys by: (1) Reducing the concentration of solute atoms in the Mg matrix and reducing the solute hardening effect. Previous reports show that Sr reduces the solid solubility of Al and Zn in the Mg matrix [5,12]. (2) Forming new precipitates such as Al4 Sr and Mg17 Sr2 which contribute to hardening and reduce ductility by crack initiation and propagation [9,10,13]. (3) Reducing the basal texture intensity by particle stimulated nucleation (PSN) during hot deformation [8,11]. In general, median concentrations of Sr (0.2–0.8 wt%) in Mg alloys provide enough formability to form wrought alloys.
https://doi.org/10.1016/j.jma.2019.04.005 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Later, other researchers have investigated the effect of higher concentrations of Sr in precipitation of wrought Mg alloys. For instance, Wu et al. [14] added high concentrations of Sr in AZ31 and have reported that the formed precipitates are different at low and high concentrations of Sr. High concentrations of Sr procedure a continuous web of precipitates at the interdendritic regions. This web of precipitates might be useful for creep resistant applications, but it limits formability which is necessary for wrought applications. The effect of Sr in hot extrusion of AZ31+XSr alloys is previously studied, and the effect of Sr on yield anisotropy of extruded sheets in tension and compression are previously reported [15]. It has been shown that the tensile and compressive strength of these alloys is more dependent on the extrusion temperature compared to the level of Sr. With increasing extrusion temperature, yield asymmetry increases. Nevertheless, the amount of yield asymmetry increase reduces when Sr is present in the alloy. In the present paper, we report on the cast and hot rolled AZ31+0.4% and 0.8 wt% Sr plates. The two compositions are selected based on the promising results obtained in previous research published by the authors. Microstructures and mechanical properties of the sheets are studied, and the texture evolution is compared. Based on the obtained results, the influence of strontium on the mentioned parameters are discussed.
2. Experimental procedure To investigate the effect of Sr concentration on the planar anisotropy of hot-rolled AZ31+XSr sheets, 1 × 10 × 10 cm AZ31 plates were cast with 0, 0.4 and 0.8 wt% Sr. Alloy compositions were synthesized in graphite crucibles using an induction furnace under CO2 –SF6 gas mixture cover. Measured quantities of 90Sr–10Al master alloys were used to introduce Sr in the molten AZ31. Samples were cast into BN covered preheated steel dies at 720 °C. Cast plates were rolled to 2 mm thick sheets by hot rolling at 350 °C. Reduction from 10 mm to 2 mm involved 10 consecutive rolling passes with approximately 15% thickness reduction for each pass. Dog-bone tensile specimens (ASTM-E8) were wire-cut from the rolled sheet, oriented in the rolling (RD), 45° from rolling (DD) and transverse (TD) directions. An MTS universal testing machine equipped with an Aramis digital image correlation (DIC) system was used. The stress response was recorded as a function of longitudinal and width strain. The microstructure of the prepared sheets was examined by optical and scanning electron microscopy (FEG-SEM, Hitachi S-4700). To reveal the microstructure features, the RD-ND cross-section of the sheets were first ground, then polished by diamond paste, following with a picric-acetic solution for etching. In order to measure the twin fraction, optical images of fractured samples were taken. Microstructure was chosen up to approximately 200 μm below the fracture surface. The chosen area was measured by ImageJ software. At higher magnifications the number of twins were counted manually
467
and by dividing it by the total area, twin density was calculated. Energy dispersive spectroscopy (EDS) was used in SEM for phase identification. Volume fraction and morphology of Al4 Sr precipitates were determined by Thermo-Calc simulation and quantitative metallography, respectively. Deformation texture in the rolled sheets was examined by X-ray diffraction (XRD) using a Bruker D8 X-ray diffractometer using Co Kα radiation on the sheet surface (RDTD). Incomplete pole figures of {0001} and {101¯ 0} were recorded and reconstructed using a texture analysis software (TexTools). 3. Results and discussion 3.1. Microstructures Microstructures of the studied samples after hot rolling are presented in Fig. 1. All alloys showed signs of dynamic recrystallization (DRX). Moreover, shear bands containing fine DRX grains and twins could be easily distinguished. Based on the AZ31-Sr Pseudo-binary phase diagram [12], at 350 °C the Mg17 Al12 precipitate is dissolved in the matrix. The only stable precipitates at this temperature are various Al–Mn precipitates and Al4 Sr. By increasing the Sr content, the stable Al–Mn precipitate at 350 °C changes from Mn4 Al11 to Al8 Mn5 . At room temperature, for all the samples the thermodynamically stable phases are Al99 Mn23 , Mg17 Al12 , MgZn and Al4 Sr. However, thermodynamic equilibrium may not be reached during the actual hot rolling experiments, and other unstable phases might be present [16]. It can be concluded that in the AZ31 sample (Fig. 1(a) and (b)), most of the precipitates are sharped edge Al–Mn precipitates. A few globular and elongated Mg17 Al12 precipitates are also observed. In the Sr-containing samples, the major precipitate is Al4 Sr. With increasing Sr concentration, the area fraction of this precipitate is increased (compare with Fig. 1(e)–(c)). The precipitates form long stringers in the rolling direction. Based on the Thermo-Calc simulation, at equilibrium, there is approximately 0.009 and 0.018 wt% of Al4 Sr in AZ31+0.4 and 0.8Sr, respectively. A higher magnification image of the precipitate stringers in the AZ31+0.8Sr sample is shown in Fig. 2. The image shows how newly recrystallized grains are initiated from the precipitate interface. Such grains are presumed to be formed by particle stimulated nucleation (PSN). Detailed EBSD analysis on the same set of alloys are performed previously by the authors which show that crystallographic orientation of the new grains are different from their parents [8]. Beyond serving as nucleation sites for new grains, precipitates actively block grain boundary migration and prevent grain growth. EDS mapping has been performed on a region covering two stringers with different compositions to identify the precipitate type causing PSN (Fig. 2(b)). Based on the individual element maps, it could be concluded that the upper stringer is Al–Mn and the lower stringer is Al–Sr. The Al–Mn precipitates could also be roughly distinguished by their large and
468
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Fig. 1. Microstructures of (a) and (b) AZ31, (c) and (d) AZ31+0.4Sr and (e) and (f) AZ31+0.8Sr, as-rolled at 350 °C. Rolling direction is horizontal and normal direction is vertical.
Fig. 2. (a) SEM microstructure (b) EDS result of two stringer lines with different compositions in of AZ31+0.8Sr hot rolled at 350 °C.
sharp morphology. In contrast, the eutectic Al–Sr precipitates form smaller round particles. The SEM image shows that recrystallized grains are associated with the Al–Sr precipitates and not the Al–Mn ones. Micrographs of Fig. 2 also show the formation of voids and small cracks between individual particles along the stringer line. Properly aligned discontinuities may act as stress concentrators in tension and cause premature failure with lower than expected elongation.
and AZ31+0.8Sr samples are 6.0 and 4.9, respectively (23% decrease by adding 0.8%Sr). The shape of the basal component in the AZ31+0.8Sr sample shows spreading in the TD direction. Spreading of basal poles has been previously observed in Mg alloys containing RE elements [17]. Prismatic pole figures show that no special alignment has formed in the plane directions. 3.3. Mechanical properties
3.2. Texture evolution Basal and prismatic pole figures after hot rolling at 350 °C are presented in Fig. 3. All the three samples have a strong basal texture. Formation of basal texture by extrusion of AZ31+Sr samples has been reported previously [8]. Prismatic pole figures show a random distribution of poles around the circle in each alloy. Although the addition of Sr has not changed the texture components, the texture intensity is reduced as presented in Fig. 4. The maximum intensity of the basal poles figure for AZ31 is 6.4 while that of AZ31+0.4Sr
Engineering stress–strain curves for the hot-rolled AZ31XSr sheets are presented in Fig. 5. In AZ31, the diagonal direction shows the highest tensile elongation (Fig. 5(a)). Although adding 0.4 and 0.8 wt% Sr did not notably affect elongation in DD it significantly enhanced elongations in RD and TD (Fig. 5(b) and (c)). In Fig. 5(d)–(f), the mechanical properties of the three alloys are compared in different directions. As presented, Sr improved both strength and elongation in RD and TD and it remarkably increased strength in DD without any notable influence on elongation.
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
469
Fig. 3. Basal and prismatic pole figures of AZ31, AZ31+0.4Sr and AZ31+0.8Sr after hot rolling at 350 °C.
Fig. 4. Influence of Sr on pole figure maximum intensity of basal and prismatic planes.
Quantitative analyses of flow curves are presented in Fig. 6. The power-law (Holloman) constitutive equation, σ = Kεn , was fitted to the curves using MATLAB Curve Fitting Toolbox. The strength coefficient, K, and hardening exponent, n, were determined with R2 >0.99 for all curves. As demonstrated in Fig. 6 (a) and (b), Sr increases the strength coefficient and hardening exponent for all sample directions. Addition of Sr improved texture (Fig. 3), and increased the number and size of obstacles, i.e. Al4 Sr, against easy glide of dislocations which along with solid solution hardening increased the strength and work hardening. The increase of work-hardening rate is in line with previous reports that have
shown that increasing Sr concentration in AZ31 decreases the Al concentration in solid solution and thereby enhances dislocation motion [9]. The impact of Sr on tensile elongation is shown in Fig. 6 (c). The most significant effect is seen in the RD and TD directions. Fig. 6(d) indicates that toughness, i.e., the area blew the flow curves, is increased by adding Sr in all directions. This is due to enhancement of combination of strength-ductility by adding Sr which is more pronounced (approximately 88%) in RD. This could be interesting when crashworthiness is important. As previously discussed elongation in DD is less affected by Sr concentration. Adding 0.4 wt% Sr first decreased the elongation perhaps due to solid solution hardening, and adding further Sr to 0.8wt% increased the elongation which can be attributed to softening the matrix as described before; however, toughness in DD was continuously increased by adding Sr. Twin density is measured in the fractured samples close to the fracture surface (Fig. 7(a)). Results indicate that by increasing Sr concentration, twin density is increased for all sample directions. Moreover, the 3D plot of Fig. 7(b) shows that the samples fractured in the TD and DD present the lowest and highest twinning density, respectively. This observation is in line with the stress–strain results of Fig. 5 which indicate that the TD and DD samples deform with the lowest and highest average elongations, respectively. Therefore, it can be concluded that the grains in the DD samples are well oriented for twin activation, and twinning plays an essential role in work-softening and elongation in this direction. In
470
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Fig. 5. Engineering stress-strain curves in RD, TD and ND for (a) AZ31, (b) AZ31+0.4Sr, (c) Az31+0.8Sr and comparing the three compositions in (d) RD, (e) TD and (f) DD.
Fig. 6. Influence of Sr on (a) strength coefficient, (b) work-hardening exponent, and (c) elongation to fracture.
Fig. 7. (a) Twin density at fracture of samples containing various levels of Sr in different sample directions. (b) twin density vs. sample orientation and Sr content and (c) normalized twin density vs. normalized fracture strain.
Fig. 7(c) normalized twin density is plotted versus normalized fracture strain. While results show that strain and twin density are proportional, the effect of Sr and sample direction on fracture strain is much less in the DD samples compared to the others. Effect of Sr on twinning activity at the regions next to fracture surfaces are further discussed later.
It is well-known that inhomogeneous deformation causes necking and eventually fracture in ductile materials. Inhomogeneous deformation occurs due to heterogeneities in microstructure, and due to mechanical instability at higher strain levels. To investigate influence of Sr on distribution of strain along specimens during tensile tests, strain maps were ob-
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
471
Fig. 8. Longitudinal strain distribution of the studied alloys in different directions (a) AZ31, (b) AZ31+0.4Sr, (c) AZ31+0.8Sr.
tained using DIC method during tensile tests. Fig. 8 presents evolutions of longitudinal strain distribution for AZ31+XSr samples in all at different during plastic deformation. To investigate evolutions of homogeneity of strain distribution during defomration, five locations were chosen in the hot zones in Fig. 8, i.e., those with higher strains shown in red, and the average strain in the hot zones was calculated. Strain Ratio was calculated at different steps during deformation according to the following equation: Str ain Ratio = δ =
average local strain in hot zones t rue global st rain
(1)
Obviously, the strains at hot zones are larger than the global strain; therefore, always 1< δ. When there is significant strain localization along the specimen, the strains at hot zones are large and δ is larger. Evolutions of δ is determined for the samples and results are presented in Fig. 9. Fig. 9 show that distribution of strain in the specimens along different directions containing various amounts of Sr. Comparing Fig. 5(a)–(c) with Fig. 9(a)–(c), it can be concluded that elongation reached to higher strains when strain distribution was more homogeneous. Fig. 9(f) clarifies that rapid increase of δ in TD led to earlier fracture compared to RD and DD. As presented in Fig. 9(d), adding Sr constantly enhanced uniform distribution of strain in RD in such a way that elongation reach to its maximum amongst all the specimens in RD at 0.8 wt% of Sr. Fig. 9(e) indicates that Sr improved uniform distribution of strain in DD. Although this was not reflected in improvement of elongation in DD
it led to enhancement of toughness; i.e., increase of strength without notable trade-off of elongation. As shown by the circles, progressive formation of 45° shear bands occurred in DD direction. As can be seen in Fig. 9(f), adding 0.8 wt% Sr reduced the rate of δ development in TD to an approximately linear condition and consequently elongation was improved for about 40%. In Mg alloys, plastic deformation occurs by a combination of twinning and dislocation motion. As mentioned above, the high affinity of Sr to Al dilutes the Al concentration in the Mg matrix through the formation of Al4 Sr [16]. This reduces the critical resolved shear stress for dislocation movement and twinning enables greater post-uniform elongation. This can explain the improved homogenization of strain in the Sr-added specimens. Hazeli et al. [18] have studied twinning activity in AZ31Sr samples by EBSD and acoustic emission. They have shown that the twinning activity is strongly attributed to plastic strain at fracture. As shown in Fig. 9(d)–(f), Sr is increasing true plastic strain by reducing strain ratio. This observation supports the findings of Fig. 7 where reported that Sr is increasing twinning activity at the regions next to the fracture surface in all sample directions. The R-value at different directions of samples containing different levels of Sr is plotted in Fig. 10(a)–(c) up to the necking strain. Results indicate that in AZ31 and AZ31+0.4Sr the R-value in the DD and TD are at the lowest and highest, respectively. However, in AZ31+0.8 (Fig. 10(c)) the R-value at RD is the lowest. Fig. 10(d)–(f) shows that Sr decreases
472
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
Fig. 9. Evolutions of Strain Ratio, δ, during deformation.
Fig. 10. R-values of studied alloys in different directions and strains.
R-values of AZ31+XSr samples at different directions. The R-values are initially between 0.5 and 1 but increases rapidly to around 2 in RD and DD and close to 3 in TD. Based on the R-value diagrams, it can be suggested that the AZ31 alloy generally resists thinning and shear localization. This
can also be explained by the strong basal texture where the crystal orientation with its c-axis parallel to ND is unable to provide easy slip direction for the dislocations. As noted previously, by adding Sr, the basal texture is weakened, and work-hardening is increased. Therefore, further sheet thinning
A. Sadeghi, H. Mortezapour and J. Samei et al. / Journal of Magnesium and Alloys 7 (2019) 466–473
occurs in the Sr-added specimens which is consistent with reducing the R-values. 4. Conclusions 0.4 and 0.8 wt% of Sr were added to an AZ31 alloy. The influences of Sr are investigated on microstructure, mechanical properties, and crystallographic texture of the hot-rolled AZ31 and Sr-added alloys. The following conclusions are drawn: (1) Al4 Sr particles were identified at the grain boundaries in the microstructures of Sr-added alloys. As expected, the number and size of the particles are larger in the 0.8Sradded alloy. Evidences of particle stimulated nucleation is observed across the microstructure of Sr-added alloys leads to reducing basal texture intensity. (2) The strength coefficient and work-hardening exponent in the Holloman’s constitutive equation was continuously increased by adding Sr in all direction. Elongations to fracture are increased in all cases expect for the AZ31+0.4Sr in DD perhaps due to significant of solid solution hardening; however, the combination of strength-ductility (i.e., toughness) was improved in all cases. Toughness improved approximately 88% in RD by adding 0.8 wt% Sr which is interesting when crashworthiness in a critical parameter. (3) According to the DIC maps, distribution of strain become more homogenous by adding Sr in all directions. This is in good agreement by improvement of toughness in the Sr-added alloys. (4) Elongation in TD is significantly smaller compared to RD and DD in AZ31 alloy. This is attributed to rapid evolutions of inhomogeneous distribution of strain along with the highest development of R-value in AZ31 in TD. The elongation and toughness in TD improved for approximately 42% and 58% by adding 0.8 wt% of Sr, respectively. Conflict of interest None.
473
Acknowledgment A. Sadeghi and H. Mortezapour acknowledge the support of the Iran National Science Foundation (INSF) Grant number 95832506. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. References [1] M. Yang, F. Pan, R. Cheng, A. Tang, Mater. Sci. Eng. A 491 (1–2) (2008) 440–445. [2] M. Yang, F. Pan, R. Cheng, A. Tang, J. Mater. Sci. 42 (24) (2007) 10074–10079. [3] Y.C. Lee, A.K. Dahle, D.H. StJohn, Metall. Mater. Trans. A 31 (11) (2000) 2895–2906. [4] D.H. StJohn, M. Qian, M.A. Easton, P. Cao, Z. Hildebrand, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 36 (7) (2005) 1669–1679. [5] R. Cheng, F. Pan, S. Jiang, C. Li, B. Jiang, X. Jiang, Prog. Nat. Sci. Mater. Int. 23 (1) (2013) 7–12. [6] E. Baril, P. Labelle, M. Pekguleryuz, Jom 55 (11) (2003) 34–39. [7] M. Pekguleryuz, E. Baril, Mater. Trans. 42 (7) (2001) 1258–1267. [8] A. Sadeghi, M. Hoseini, M. Pekguleryuz, Mater. Sci. Eng. A 528 (7–8) (2011) 3096–3104. [9] A. Sadeghi, S. Shook, M. Pekguleryuz, Mater. Sci. Eng. A 528 (25) (2011) 7529–7536. [10] H. Borkar, M. Hoseini, M. Pekguleryuz, Mater. Sci. Eng. A 537 (2012) 49–57. [11] H. Borkar, R. Gauvin, M. Pekguleryuz, J. Alloys Compd. 555 (2013) 219–224. [12] A. Sadeghi, M. Pekguleryuz, J. Mater. Res. 26 (7) (2011). [13] R.J. Cheng, F.S. Pan, S. Jiang, X.Q. Jiang, C. Li, Mater. Sci. Technol. (U.K.) 29 (2) (2013). [14] L. Wu, F. Pan, M. Yang, R. Cheng, J. Mater. Sci. 48 (16) (2013) 5456–5469. [15] A. Sadeghi, S. Shook, M. Pekguleryuz, Mater. Sci. Eng. A 528 (25–26) (2011) 7529–7536. [16] A. Sadeghi, M. Pekguleryuz, J. Mater. Sci. 47 (14) (2012) 5374–5384. [17] D. Griffiths, B. Davis, J.D. Robson, Metall. Mater. Trans. A 49 (1) (2018) 321–332. [18] K. Hazeli, A. Sadeghi, M.O. Pekguleryuz, A. Kontsos, Mater. Sci. Eng. A 578 (2013).