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Jianyue Zhang, Xuzhe Zhao, Deʼan Meng, and Qingyou Han, Utilization of surface nanocrystalline to improve the bendability of AZ31 Mg alloy sheet, Int. J. Miner. Metall. Mater., 29(2022), No. 7, pp.1413-1424. https://doi.org/10.1007/s12613-022-2414-7
Cite this article as: Jianyue Zhang, Xuzhe Zhao, Deʼan Meng, and Qingyou Han, Utilization of surface nanocrystalline to improve the bendability of AZ31 Mg alloy sheet, Int. J. Miner. Metall. Mater., 29(2022), No. 7, pp.1413-1424. https://doi.org/10.1007/s12613-022-2414-7
Research Article

Utilization of surface nanocrystalline to improve the bendability of AZ31 Mg alloy sheet

Author Affilications
  • Corresponding author:

    Jianyue Zhang      E-mail: zhang.12278@osu.edu

    Qingyou Han      E-mail: hanq@purdue.edu

  • A surface nanocrystalline was fabricated by ultrasonic shot peening (USSP) treatment at AZ31 Mg alloy. The effect of nanocrystalline thickness and its placed side (external or internal) on the bendability was studied by a V-bending test. Three durations, 5, 10, and 15 min, were applied to form the surface nanocrystalline with thicknesses of 51, 79, and 145 μm, respectively. Two-side treatment led to a similar bendability as that of as-received. One-side internal treatment for 5 min resulted in an improved bendability while the improvement was limited and degenerated for longer treatment. The improvement was related to the drawing back of the neutral axis. The one-side external treatment also improved the bendability, and the improvement was due to the redistribution of strain and stress during bending. With nanocrystalline at external side, it resulted in a larger stress but a smaller strain at the convex, which prevented the happening of crack during bending.

  • Severe plastic deformation (SPD) is a method developed to fabricate the grains with a size of submicrometer (average grain size between 100–1000 nm) or nanometer (average grain size < 100 nm) [14]. Among these SPD grain refining methods, ultrasonic shot peening (USSP) is currently of great concern to achieve a surface modification by nanocrystallization [57]. During ultrasonic shot peening, a large number of flying balls impact at the surface of the material at a short time. The repeated impacting of the flying balls provides a high strain rate during the ultrasonic shot peening process. By applying USSP, a nanocrystalline will be obtained on the surface of the treated materials [810]. The surface related properties, such as fatigue [1113], wear resistance [1416], and corrosion behavior [9,1618], can be greatly modified after the surface nanocrystallization. Meanwhile, the mechanical strength can be improved after USSP treatment [1921].

    Magnesium alloys are regarded as the new structural alloys with the low density, promising mechanical properties, and outstanding energy storage capacity [2228]. In Mg alloys, it is previously reported that the formed surface nanocrystalline by USSP treatment can enhance the surface properties, such as wear resistance and corrosion resistance, with applying suitable process parameters during USSP treatment [2932]. At the same time, similar to other alloys, Mg alloys after USSP treatment also result in a reduction of ductility [3335]. The nanocrystalline processed by SPD methods is always more brittle than the coarse grain counter. The nanocrystalline with high densities of grain boundary and dislocation has a higher strain rate sensitivity than its coarse grain counter, which leads to an increased sensitivity of crack nucleation and growth [3638]. Compared with Al alloys and steel, the ductility of Mg alloys, especially at room temperature, is relatively lower, which is due to the strong basal texture and its limited slip systems [3839]. Therefore, the reduced ductility of Mg alloys after USSP treatment increases greater concern of applicability of USSP in Mg alloys. However, the bending performance of Mg alloys after surface nanocrystalline has not been investigated so far.

    In this study, the effect of surface nanocrystalline on the bendability of Mg alloys was investigated by a V-bending test at room temperature. Three treatment durations were conducted to obtain different thicknesses of nanocrystalline. Both two sides and just one single side USSP treatment were applied. The roles of surface nanocrystalline at both the external and internal sides were investigated. The basic rules about how the surface nanocrystalline affects the bendability in Mg alloys were provided.

    In this study, as-rolled Mg–3Al–1Zn–0.3Mn (all in wt%) alloy sheet with a thickness of 3 mm was selected for USSP treatment. The detailed experimental set up of USSP process can be found in previous works [5,29]. Both two sides and just one side were USSP treated with a duration of 5, 10, or 15 min.

    The USSP treated specimens were cut along the center of the treated area and the cross section was polished by SiC abrasive paper to 2000 grit and etched in a solution of 50 mL ethanol, 5 mL water, 5 mL acetic, and 5 g picric acid for optical microscope (OM) test. For X-ray diffraction (XRD) test, the treated surface was performed on the Riguku D/max 2500PC X-ray diffractometer with Cu-K radiation, and the test was conducted by a scan step of 0.02° per step in the 2θ ranging from 10° to 90°. Electron-back scattering diffraction (EBSD) method was applied to study the microstructure after bending test. The EBSD sample preparation consisted of mechanical polishing by SiC paper to 2000 grit and electro-polishing in a commercial AC2 electrolyte with a voltage of 20 V and duration of 60 s at −20°C. EBSD was conducted on a JEOL 7800F field emission gun scanning electron microscope (SEM) and the result was analyzed by HKL Channel 5.0 software.

    The V-bending was applied to test the bending performance. The USSP treated AZ31 Mg alloy sheets were cut by CNC machine from the center of the treated area, into the specimens with a length of 50 mm and a width of 12 mm, along the rolled direction. The V-bending test was conducted at room temperature on a CMT6305-300 kN test machine with a V-punch radius of 4 mm and a die with consistent bending angle of 120°. The punch speed was set to 3 mm/min. For single side treatment, the treated side was placed at the internal or external side. In total, there were four states of specimens tested in the study, as shown in Fig. 1: (a) as-received, (b) nanostructure at both sides, (c) nanostructure at the internal side, and (d) nanostructure at the external side. The both sides treated was named as USSP-both-x min (x = 5, 10, and 15), the one side treated was named as USSP-external-x min or USSP-internal-x min (x = 5, 10, and 15). To ensure the replicability of V-bending, each sample condition was tested at least three times.

    A finite element method (FEM) with simple two-dimension model was applied to simulate the stress and strain distribution during V-bending process using ABAQUS commercial software. The plasticity properties of the as-received and USSP treated materials were obtained from previous report [40]. For the USSP treated AZ31 Mg alloy, a gradient nanostructure was set by a virtual temperature technique. A continuously variable temperature field was assigned and a one-to-one relationship between temperature and mechanical properties was created to obtain the gradient mechanical properties at the treated surface along the depth. Both the simulated stress field and the strain field after bending were calculated.

    Fig. 1.  Schematic diagrams of the position of nanostructure during bending process: (a) as-received, (b) nanostructure at both sides, (c) nanostructure at the internal side, and (d) nanostructure at the external side.

    The XRD was conducted to analyze the crystallographic orientations and the grain size after ultrasonic shot peening process. Fig. 2 shows the XRD patterns of as-received and USSP treatment for 5, 10, and 15 min. From the XRD results, only α-Mg peak is detected in as-received. Before USSP treatment, as-received AZ31 Mg alloy shows a typical strong basal texture. After USSP treatment, the peak of (0001) is greatly reduced and the peaks of (10ˉ10) and (10ˉ11) become relatively strong, indicating a random grain orientation. An enlarged full width half maximum of the peaks after USSP is also exhibited. Such a larger full width half maximum is related to a smaller grain size, and the grain size can be evaluated according to Williamson-Hall formula [41]:

    Fig. 2.  (a) XRD patterns of AZ31 alloy before and after USSP treatment and (b) the calculated grain sizes after USSP treatment.
    d=Kλβcosθ
    (1)

    where d is the grain size, K is the Scherrer constant (0.9), λ is the X-ray wavelength, β is the full-width-half-max (FWHM), and θ is the diffraction angle. The calculated average grain sizes at the top surface after USSP treatment for 5, 10, and 15 min are 45, 42, and 37 nm, respectively, seen in Fig. 2(b).

    Fig. 3(a) shows the as-received OM images of AZ31 Mg alloy sheet from the transverse direction (TD). The grains are homogenous and equiaxed in as-received, and the average grain size is about 9 μm. After USSP treatment for 5 min, it shows that the top surface of the sample has been deformed heavily. A boundary-invisible surface layer with a dark appearance is formed. This boundary-invisible surface layer is the formed nanocrystalline according to our previous reports [2930]. Due to the grain refinement, the grain boundary of the new formed nanocrystalline at the top surface cannot be observed in OM as the grain size is too small. The detailed morphologies of the formed nanostructure can be also found in our previous study [2930]. For the thickness of boundary-invisible nanocrystalline layer, it increases from 51 μm after 5 min treatment to 79 and 145 μm after 10 and 15 min.

    Fig. 3.  OM images taken from the TD direction after ultrasonic shot peening: (a) as-received, (b) USSP-5 min, (c) USSP-10 min, and (d) USSP-15 min.

    The microhardness distribution along the depth after USSP treatment is shown in Fig. 4. For as-received, the hardness is uniformly distributed of HV ~61. For USSP-5 min treated sample, the microhardness at the surface is improved to HV 123 and decreases gradually to HV 65 at the depth of 300 µm. After 10-min treatment, the microhardness at the top surface is enhanced to HV 127. For 15-min treatment, the microhardness is further improved to HV 145, and returns to HV 65 at the depth of 400 µm. It is apparent that the microhardness improved area is larger than the thickness of nanocrystalline, which is because the USSP induces compressive residual stress with a larger depth, and then also improves the hardness [2930].

    Fig. 4.  Distribution of the Vickers microhardness as a function of distance to the top surface after different USSP treated times.

    The load–stroke curves of USSP-both AZ31 magnesium alloy sheets are present and compared with as-received in Fig. 5. From the load–stroke curves, the as-received is with a poor bendability, and the sheet fractures at a punch distance of around 4 mm. The bendability of the USSP treated sample is closed to the as-received, presenting a similar punch distance of about 4.5 mm before the happening of crack and fracture. Meanwhile, it needs a larger bending load after USSP treatment. The increment of USSP time results in a higher bending load value as the nanocrystalline thickening. Our previous study [40] shows the USSP treatment will increase the mechanical stress with sacrificing the ductility. The ductility is always regarded to be a key factor that will affect the bendability of the material. However, the change of the bending performance of AZ31 alloy after USSP treatment is negligible. It may be due to the poor bendability of as-rolled AZ31 sheet. With a strong basal texture, as-rolled AZ31 sheet is difficult to corporate the deformation in c-axis and its bendability is limited. As the bendability of as-rolled AZ31 is already poor, the further reduction of bendability after USSP is limited even though the ductility is reduced after USSP treatment.

    Fig. 5.  Load–stroke curves of as-received and USSP-both sheets with different USSP treated times.

    For one side treatment, the load–stroke curves of USSP-internal with different durations are present in Fig. 6. The bending loads of the USSP-internal are also obviously higher than the as-received while the loading distances are improved. The USSP-internal-5 min sample presents a better bendability than the as-received, reaching ~6.5 mm before fracture. After 10-min USSP treatment, the bendability is reduced to ~6 mm. The bending performance of 15-min treated sample is further reduced, becoming slight better than the as-received. The result shows this one-side treated USSP-internal samples with asymmetric structure can improve the bending performance, and the improvement is related to the thickness of nanocrystalline. The result also indicates the nanocrystalline at the internal side provides a higher resistance to bend.

    Fig. 6.  Load–stroke curves of USSP-internal sheets with different treated times.

    For USSP-external samples, the load–stroke curves with different USSP treated durations are present in Fig. 7. From the load–stroke curves, the loading distances of the USSP-external are improved with the bending loads similar to that of as-received. For USSP-internal-5 min sample, the bending curve is similar to that of as-received with a load distance of 4.5 mm. For USSP-internal-10 min sample, the bending load is a bit smaller and the loading distance before fracture is also just ~4.5 mm. For the USSP-internal-15 min sample, the punch distance is greatly increased and the rising value of the load at the tail of the curve indicates the sample has touched the die mold and the sample is successfully shaped. The result shows the external side with a nanocrystalline exhibit a greatly improved bendability. Meanwhile. It indicates the nanocrystalline at external side provides a decrement of applied load during bending process.

    Fig. 7.  Load–stroke curves of USSP-external sheets with different treated times.

    In this study, a nanocrystalline was formed on the surface of AZ31 Mg alloy sheet by USSP treatment, which greatly changed the bending behaviors. An improved bendability was obtained by adjusting both the thickness of surface nanocrystalline and the placed position (internal or external). The possible reasons for this improvement and related mechanism are discussed as below.

    A simple schematic illustration of bending process is present in Fig. 8. During the bending process, the external side, zone a, is under tension, while zone c, the internal side, is under compression. In theory, zone b is stress free and is located at the centric line, which is called the neutral axis. However, the neutral axis will be shifted during the bending process as the deformation at external and internal sides are not symmetrical. For steel and Al alloys, it will be shifted towards the inner side as the external side is with a larger strain during the bending process [4247]. Differently, Mg alloys present a shift towards outer region according to previous reports [4850] because of its high tension–compression yield asymmetry. The shift of neutral axis leads to extra shear stress during bending process, and it is accepted that the reduction of neutral axis shift is beneficial for the bendability. It is previously reported the neutral axis shift can be achieved by introducing asymmetry structure [5154]. Yilamu et al. [51] reported the neural axis of stainless steel clad aluminum sheet was shifted towards the clad steel layer as it was a strong layer. This shift towards the strong layer was also reported by Kagzi et al. [52] in aluminum–steel bimetallic sheet. In Mg alloys, this shift was also reported in Mg–Al bimetallic sheet [53] and Mg/Al/steel clad composites [54]. After USSP treatment, as the formed surface nanocrystalline is with higher strength, it can easily imply the shift of neutral axis should be activated after USSP treatment, especially for one side treatment.

    Fig. 8.  Schematic illustration of the bending process with the observation direction from normal direction (ND)–rolling direction (RD) plane.

    The coefficient of the neutral axis, k-value, is a standard to measure the offset of the neutral axis after bending, and the k-value can be expressed as the followings [55]:

    α=t1/t
    (2)
    k=0.5α2+α1tR
    (3)

    where α, R, t, and t1 are the coefficient of incrassation, the inside bending radius, the original thickness, and the thickness after bending, respectively. When k-value is less than 0.5, it means the neutral axis has been shifted to the internal zone, and a k-value more than 0.5 means the shift to the external zone. In this study, the R values can’t be measured correctly because the majority of the samples are fractured during the bending. From the equations, it is clearly found that the k-value is strongly dependent on the α value. Therefore, the α value is calculated to represent the shift of neutral axis. A α value of >1 means the shift towards the external zone, and a value of <1 means to the internal zone. The thickness before and after bending and the α values of all the test samples are summarized in Table 1. The α value of the as-received is 1.0226, which means the neutral axis of the as-received AZ31 sheet shifts to the external zone during bending, which is common in Mg alloys. For the USSP-both samples, the α values of USSP durations of 5, 10, and 15 min are 1.0145, 1.0195, and 1.0108, respectively, which is similar to that of as-received. So, the shift of neutral axis is also towards to external zone for the USSP-both. For the USSP-internal-5 min, the α value reduces to just 1.0036, which means a very limited shift of neutral axis. For the USSP-internal-10 min and USSP-internal-15 min samples, the α values are less than 1, which means the neutral axis shifts towards the internal zone. For the USSP-external samples, the α values for 5, 10, and 15 min are 1.0413, 1.0337, and 1.0403, respectively. The values of USSP-external are bigger than that of the as-received, which means the shift of the neutral towards external side is more obvious. The result is consistent with previous finding that the neutral axis is shifting towards the strong side in inhomogeneous materials [5154]. During the bending process, the deformation of the neutral axis is pure bending while the external or internal region undergoes a nonuniform bending deformation to corporate the thinning or thickening of the sheet. By introducing nanocrystalline at internal side, a α value close to 1 is obtained in USSP-internal-5 min as well as an improved bendability. By applying surface nanocrystalline at internal side, the bendability is improved by reducing the shift of neutral layer while this improvement is limited. A thicker nanocrystalline in USSP-internal-10/15 min sample results in shift of neutral layer towards internal side, which leads to a decreased bendability.

    Table  1.  Plate thicknesses before and after bending and the related coefficients of incrassation
    SampleThickness before bending / mmThickness after bending / mmα
    As-received2.652.711.0226
    USSP-both-5 min2.742.781.0145
    USSP-both-10 min2.562.611.0195
    USSP-both-15 min2.762.791.0108
    USSP-internal-5 min2.732.741.0036
    USSP-internal-10 min2.902.730.9413
    USSP-internal-15 min2.762.130.7717
    USSP-external-5 min2.662.771.0413
    USSP-external-10 min2.672.761.0337
    USSP-external-15 min2.482.581.0403
     | Show Table
    DownLoad: CSV

    It is clear that the shift of neutral layer can’t explain the improved bendability in USSP-external-15 min as its shift of neutral axis is more obvious than that of as-received. To clarify the effect of surface nanocrystalline on the deformation behavior during bending process as well as the reason of improved bendability in USSP-external sample, the microstructural evolution after the bending was measured by an EBSD from the TD direction, and the twinning structure, low-angle grain boundary (LAGB), and high-angle grain boundary (HAGB) were all characterized. As displayed in Fig. 8, three feature areas are selected: (a) the external area close to the convex, (b) the middle area close to the neutral axis, and (c) the internal area close to the concave. Three typical samples are selected: (1) as-received, (2) USSP-internal-15 min, and (3) USSP-external-15 min.

    For as-received in Fig. 9, the external surface zone is under tension during the bending process and few twin structures are observed. Close to the surface, a lot of white area is present, which is the residual stress concentrated area. Due to the strong basal texture and the limited sliding system, the residual stress is more concentrated during bending, and cracks are generated at the convex of the external side, which leads to the failure and fracture. For the middle zone b, the upper part is similar to that in the outer surface layer, which is under tension during the bending. The lower part is full of the (10ˉ12) tension twin structure. From the inverse pole figure (IPF) of RD direction, a lot of grain rotation is present with the shift of c-axis from the ND to the RD direction. For the inner surface layer, less (10ˉ12) tension twinning is found in the internal side as the shift of c-axis is almost completed at the internal region. From the Kernel Average Misorientation (KAM) map, it is seen the external side is with a much smaller strain. As the internal side is under compression, it is easy to be deformed by twining. As a result, there is no severe stress concentration at the internal side. The KAM map shows that the stress is concentrated at the external side in the as-received AZ31 during bending.

    Fig. 9.  EBSD maps of the as-received at various positions: (a) external surface zone, (b) middle zone, and (c) internal surface zone.

    For USSP-internal-15 min, the EBSD maps of the external surface zone, middle zone, and internal surface zone are shown in Fig. 10. For the external surface zone of USSP-internal-15 min sample, the edge of the outer surface is clearly detected. From the KAM map, there are lots of strain left, which results in white regions as well. The twins in the outer zone of the USSP-internal are also barely found. For the middle zone, the USSP-internal-15 min sample also presents the shift of c-axis. What should be mentioned is that, for the internal surface zone in Fig. 10(c), the small cracks with multi-directions are also detected and there are lots of white areas among them. From the KAM map, it is seen that the internal side is with a much larger residual stress, compared with that of as-received. The result indicates the formed strong nanocrystalline at the internal side is more resistant to compress, compared with its coarse counter during bending process. That explains the increased applied loads in USSP-both and USSP-internal samples.

    Fig. 10.  EBSD maps of the USSP-internal-15 min at various positions: (a) external surface zone, (b) middle zone, and (c) internal surface zone.

    The EBSD of USSP-external-15 min is illustrated in Fig. 11. For the external surface zone, a layer of white area is observed on the surface, which is not found in the as-received and USSP-internal-15 min. The white area is the formed nanocrystalline from the USSP process, according our previous studies [2930]. Similar to the as-received, in the external zone, the slip still dominates the main contribution of the plastic deformation as it is with a strong basal texture, and few twins are found. Compared with the as-received and USSP-internal-15 min, it is found a region with higher residual strain beneath the nanocrystalline from the KAM map. In zone b of the USSP-external-15 min, the shift of c-axis is also observed from the IPF map of RD direction. In the internal zone, unlike the USSP-internal-15 min sample, there are no multi-directional cracks and less white areas. However, compared with the as-received, the internal side is with a high strain, seeing from the KAM map. The internal side of USSP-external-15 min is the coarse grains, same as that in the as-received. From Fig. 9, it is seen that the strain is concentrated at the external side in the as-received, which leads to the cracks and failure. By forming a nanocrystalline at the external side, the strain is redistributed. The strain at the external side is concentrated not at the surface layer but a sublayer beneath the nanocrystalline, which reduces the sensitivity of cracking. Moreover, the internal side of USSP-external-15 min is more severely compressed during bending. For Mg alloys with a strong texture, it is easy to coordinate the compression process by twinning [5657]. As a result, the formed nanocrystalline in USSP-external-15 min leads to a redistribution of strain and stress during bending process.

    Fig. 11.  EBSD maps of the USSP-external-15 min at various positions: (a) external surface zone, (b) middle zone, and (c) internal surface zone.

    To better understand the redistribution of stress and strain during bending process, a two-dimensional model was developed to simulate the stress and strain distribution during the bending process of both as-received and USSP treated samples. A surface nanocrystalline with gradient properties was set. The thickness of the gradient nanostructure was set as 100 µm and the yield stress of the top surface was set as 500 MPa, according to the previous report [35]. As the hardness linearly decreased along the depth in Fig. 4, the yield stress was also set as a linearly decrease along the depth and from 500 MPa at the surface to 128 MPa at the depth of 100 μm. For the bending process, the simulated load distance was set as 4 mm, which was before the fracture happening during bending process, according to the load–stroke curves.

    The simulation results of the equivalent plastic strain (PEEQ) and the Mises stress (S. Mises) with average crit (Avg) of 75% are present in Fig. 12. For the as-received, the maximum Mises stress is 2.32 × 102 MPa and the maximum PEEQ is 7.96 × 10−2. The maximum compression stress is located at the concave of the bending center, and the maximum tension stress is located at the convex. For USSP-both, the maximum Mises stress increases to 5.027 × 102 MPa, which is double of that of the as-received. The maximum compress is still at the concave and the maximum tension is at the convex. However, the distribution of the stress is more concentrated near the surface zone. For PEEQ, it shows a similar distribution, and the maximum value is 8.947 × 10−2, just slightly bigger than that of the as-received. The result implies that the USSP-both treatment enhances the required stress for bending while a high stress doesn’t affect the strain distribution. For the USSP-internal, the maximum Mises stress is 4.71 × 102 MPa and located at the internal surface zone. The result shows the formed nanocrystalline at the internal zone endures a high stress during the bending process, which is consistent with the EBSD result in Fig. 10. For PEEQ, it has the highest value of 9.06 × 10−2. For USSP-external, the maximum Mises stress is 4.71 × 102 MPa and it is located at the external surface zone while the PEEQ value here is just ~5.00 × 10−2. This means the nanocrystalline at the external zone leads to a higher stress but a reduced strain at the external zone. This redistribution from USSP-external treatment is benefit for the bending process. For external zone, the nanocrystalline is with a high yield stress but a low ductility. The redistribution is beneficial for the nanocrystalline to coordinate deformation during bending. Meanwhile, the internal side is with a higher strain, which is also beneficial for bending deformation. It is because the compression at internal zone is more easily to coordinate in Mg alloys by the activation of (10ˉ12) tension twin. The above results show the USSP-external leads to a redistribution of stress and strain and this redistribution can improve the bendability in USSP-external samples and a thicker nanocrystalline provides a greater improvement.

    Fig. 12.  Finite element analysis of stress and strain of the as-received, USSP-both, USSP-external, and USSP-internal samples after a load distance of 4 mm during the bending process.

    In this study, a three-point V-bending experimental investigation was conducted to study the bendability of AZ31 Mg alloy sheets after surface nanocrystalline by USSP treatment. The effect of nanocrystalline’s thickness and placed positions on the bendability was investigated. The bending behavior after surface nanocrystallization was investigated by the EBSD analysis and simulated by FEM method. The major conclusions from the experimental and simulation results are summarized as follows:

    (1) A surface nanocrystalline was successfully fabricated on the surface of AZ31 Mg alloy sheet by ultrasonic shot peening method. By increasing the duration of treatment from 5 to 10 and 15 min, the thickness of the formed surface nanocrystalline was increased from 51 to 79 and 145 μm, respectively.

    (2) The bendability after surface nanocrystallization at both sides (USSP-both) was similar to that of as-received AZ31 Mg alloy, while a higher applied load was needed. The fracture happened at a load distance of 4.5 mm. The increased applied load was originated from the nanocrystalline at internal side.

    (3) The bendability was improved by applying single side treatment at internal side for 5 min. This improvement was from the drawing back of the neutral axis during bending process but the improvement is limited. Longer USSP treatments (10 and 15 min) degenerated the bendability to that of the as-received.

    (4) The single external side treatment also resulted in an improved bendability and a thicker nanocrystalline exhibited a greater improvement. The improved bendability in USSP-external treatment was originated from the redistribution of stress and strain at the external and internal sides during bending process. The formed nanocrystalline at external surface leaded to a larger stress and a smaller strain at convex, which is beneficial for the bending in Mg alloys.

    This work was financially supported by the Natural Science Basic Research Program of Shaanxi, China (No. 2021JQ-250) and the Fundamental Research Funds for the Central Universities (No. 300102220301). J.Y. Zhang also thanks Prof. Milan Rakita (Purdue University) for his assistance on the ultrasonic shot peening equipment setup.

    The authors declare no conflict of interest.

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