Strength and Ductility with Dual Grain-Size and Texture Gradients in AZ31 Mg Alloy
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Using surface rotation rolling (SRR) treatment with various vertical loads (120 to 280 N) while maintaining other processing parameters (rotation speed and horizontal velocity), the SRR-treated AZ31 Mg alloy sheets exhibit through-thickness gradients of both grain size and basal texture, as revealed by cross-sectional hardness tests and microstructure analysis. An optimal strength–ductility combination is achieved as the vertical load is around 200 N. The corresponding microstructure exhibits two characters: (1) The affected depth reaches the very center of the sheet and (2) the gradients approach the allowable maximum. Texture gradient is found to be the major contributor to the significant ductility enhancement.
Magnesium (Mg) alloy, the lightest among structural materials, is an attractive material for reducing vehicle weight thus increasing the efficiency in terms of fuel consumption and reducing the emission of greenhouse gas.[1,2] However, their applications have been limited due to their low strength and poor deformability. According to the fundamental principle of tailoring the relative activities of basal slips, non-basal slips, and twins, several approaches to improving the deformability and strength of Mg alloys have been proposed, such as increasing processing temperature,[3,4] alloying with appropriate solutes,[5, 6, 7, 8] refining grain sizes,[9, 10, 11] weakening textures,[12, 13, 14] and forming metallic laminates.[15, 16, 17]
Refining grain sizes of magnesium alloys can be accomplished by severe plastic deformation (SPD) processes, such as equal channel angular pressing (ECAP)[10,18] and high-pressure torsion (HPT).[19,20] Refining grain sizes in the surface layer has been demonstrated to enhance mechanical properties of the bulk materials, and corresponding techniques are easy to scale up. For instance, surface mechanical attrition treatment (SMAT) can refine grains in the surface layer of different materials[21, 22, 23, 24] and develop the grain-size gradient from the surface. Corresponding to the gradient of grain size, the incompatibility of plastic deformation will develop in different depths, and results in an extra strain hardening and a long-range back stress. Compared with homogeneous grain refining,[22,24] refining grain sizes in the surface layer could result in a superior ductility–strength combination of surface-treated materials. In addition to refining grain sizes with a gradient, texture can also influence the tensile strength and ductility because of the strong plasticity anisotropy in Mg alloys. Recently, Chen et al. has found that, in addition to refining grain sizes, SMAT can also modify textures with a gradient distribution along depth. Such texture gradient significantly improves the ductility–strength combination compared with the grain-size gradient, e.g., in steel,[22,26] aluminum, and magnesium alloys without strong basal textures.
In our experiment, we modified grain size and texture in the surface layer of AZ31 sheets on both sides by surface rotation rolling (SRR) technique. Compared with SMAT[25,29] and other similar surface treatments,[23,30,31] SRR treatment can be utilized for large plates and result in fine surface finishing. By changing vertical loads, we tailored the microstructure with gradients in grain size and basal texture contents, and conducted tensile testing of SRR-treated samples. An optimal strength–ductility combination is achieved as the vertical load is around 200 N. The roles of the grain-size gradient and the texture gradient are discussed in terms of their influences on strength and ductility. Texture gradient is found to be the major contributor to the significantly enhanced ductility.
Tensile specimens with a gauge dimension of 15 mm × 5 mm were machined parallel to the rolling direction of the sheet. For each SRR-treated sample, we prepared two tensile specimens. Tensile tests were carried out using a Zwick testing machine at a strain rate of 1 × 10−3 s−1 at room temperature. All the cross sections mentioned in this study are perpendicular to the transversal direction (TD) of the sample. The cross-sectional microhardness of the SRR-treated samples was measured using a Buehler 402SXV Microhardness Tester with a load of 10 g and a dwell time of 10 seconds. At least five indents were conducted for each depth from the surface. Cross-sectional microstructure was characterized using a scanning electron microscope (SEM) with an electron backscatter diffraction (EBSD) detector, with the step sizes varying from 0.2 to 2 μm. HKL Channel 5.0 software was used to analyze and process the EBSD data. EBSD samples were prepared by electrochemical polishing using perchloric acid solution (25 mL perchloric acid + 475 mL ethanol) at − 20 °C and applying 17 V for about 7 minutes. Cross-sectional microstructure in the surface layer was also characterized by transmission electron microscope (TEM, JEOL 2100) at the operating voltage of 200 kV. TEM samples were prepared by gluing two cross-sectional thin foils in a face-to-face manner and then inserting it into a copper ring for thinning by Ar ion milling.
To understand the hump feature in Figure 2(a), we further characterized the variation of the cross-sectional hardness, the depth of the hardened layer, and the microstructures along the depth for different vertical loads. The cross-sectional hardness of SRR-treated samples exhibits a gradient variation, as plotted in Figure 2(b) as a function of the depth from the surface, according to which, we determined the depth of the hardened layer. The depth of the hardened layer increases with the increasing P, and is increased throughout the whole sample when P exceeds 200 N. The cross-sectional hardness increases with the vertical load. The hardness in the near surface increases to 112 HV compared with 65 HV in the as-received material when P = 120 N, and finally reaches a peak value of 130 HV when P exceeds 200 N. With further increasing of P, up to 280 N, the hardness at different depths continuously increases except at the top surface, where the hardness reaches a peak of 130 HV and the saturation depth increases up to 60 μm. The larger scatters and fluctuation in the mean value of hardness within the top surface layer are presumed to indicate the occurrence of over hardening that induces flaws and hardness reduction.
The Thickness and Volume Fractions of Hardened/Grain-Refined/Deformed/Texture-Weakened Layers with Different Loads
250 (44.4 pct)
60 (13.3 pct)
260 (57.8 pct)
240 (53.3 pct)
450 (100 pct)
120 (26.7 pct)
450 (100 pct)
450 (100 pct)
450 (100 pct)
160 (35.6 pct)
450 (100 pct)
450 (100 pct)
Accompanying with grain-size gradient, texture also exhibits gradient along the depth, i.e. the content of basal texture is gradually weakened from the center area outward to the surface layer, as shown in Figure 4. Weakening basal texture, evidenced by the expansion of (0001) orientation cluster, occurs until the maximum depth of 200 μm when P = 120 N, and it extends to the center of the sample when P increases to 200 N. The basal texture is further weakened along the depth with P = 280 N. Weakening of basal textures at different loads is also quantified and compared as shown in Figure 4(d). The reduction of basal texture contents in SRR-treated samples is calculated as the relative reduction compared with the as-received sample, where the basal texture contents are the summation of intensity of (0001) orientations that are within a critical degree around ND. Error bars in Figure 4(d) indicate the variations in measurement when the critical angle varies within the range of 8 to 12 deg. It is noted that for each sample, the depth of texture-weakened layer is nearly the same as that of hardened layer or deformed layer, as listed in Table I.
The tensile strength increases monotonically with the vertical load P. This is attributed to the increase in the hardness and the thickness of the hardened layer. However, the ductility exhibits an abnormal fluctuating trend. At P = 120 N, grain refinement and hardening happen within a limited depth on the surface, and the degree of texture weakening is similarly limited. Therefore, in terms of back stress, the positive effects of grain-size and texture gradients on hardening and ductility are limited. Correspondingly, the ductility follows the usual “stronger–less ductile” trend. This is consistent with the results of those SMAT materials with no obvious (or enough) texture gradient, which exhibit the monotonous decrease in ductility with monotonous increase of strength.[28,34,38] At the optimal load of 200 N, the gradients in hardness/strength expand to the whole thickness and result in the spreading of back stress in the whole volume. Accompanied with grain-size gradient from the surface ~ 150 μm layer, the gradient weakening of basal textures gives extra contribution to back stress because it can create gradients in plastic strains, which, in turn, generates back stress. This auxiliary effect originates from the large plastic anisotropy in hexagonal metals. In addition to back stress hardening, weakened basal texture itself is proposed to improve ductility by facilitating more basal/non-basal slips and/or activating tensile twinning,[12,14] although comprehensive mechanisms are lacking. At P = 280 N, ductility decreases again after the hardening had already reached the very center. This can be explained by the fact that all gradients including hardness, grain size, and basal texture have decreased in terms of the degree. Meanwhile, the grain refinement on the surface reaches the maximum and dislocations stored in the grains of the middle area increase extensively. As a consequence, such over hardening leads to lack of further strain hardening during deformation as well as the reduction of ductility. An additional reason is that surface damage is likely to occur upon over hardening.
Thus, it could be inferred that gradient, in terms of textures, is more important than grain-size gradient and defect gradients, including dislocation and deformation twinning, for the improvement in ductility. This is supported by three arguments. First, the depth of texture gradient is much larger than that of grain-size gradient and twinning gradient, but it is in coincidence with the depth of hardness gradients that controls the back stress distribution. Second, the gradient of weakening basal texture in hcp crystals contributes much more than the back stress generated by grain-size gradient, as mentioned above, because of the strong anisotropy of plastic deformation. Third, one system, with mainly grain-size gradient and dislocation-density variations across the thickness but with a little amount of texture gradients, usually exhibits monotonic “harder–less ductile” trend.[28,34,38]
In summary, the surface rotation rolling method is applied to produce AZ31 Mg alloy sheet with dual gradients in terms of grain size and weakened basal textures. This is promising for large-scale applications in metal plate forming or processing. The optimal strength–ductility happens when the volume exhibiting gradient in strength reaches the maximum allowable limit without over hardening to reduce the gradient in the surface or the center. It is suggested that the texture gradient is more important in improving ductility than the grain-size gradient, especially for hexagonal close-packed structures exhibiting significant plastic anisotropy.
This study is supported by the National Key Research and Development Plan (Grant No. 2016YFB0701201), the National Science Foundation of China (Nos. 51471107 and 51671132), and the Materials Genome Initiative Center, Shanghai Jiao Tong University.