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The effect of large thickness-reduction on microstructure evolution and tensile properties of Mg-9Al-1Zn alloy (AZ91) processed by hard-plate rolling (HPR) was investigated. Increasing rolling reduction from 55 % to 85 % increases the volume fraction and refines average size of fine grains (

In this study, we successfully prepared a Mg-6Zn-0.2Ca alloy by utilizing sub-rapid solidification (SRS) combined with hard-plate rolling (HPR), whose elongation-to-failure increases from ∼17 % to ∼23 % without sacrificing tensile strength (∼290 MPa) compared with its counterpart processed via conventional solidification (CS) followed by HPR. Notably, both samples feature a similar refined grain structure with an average grain size of ∼2.1 and ∼2.5 μm, respectively. However, the high cooling rate of ∼150 K/s introduced by SRS modified both the size and morphology of Ca2Mg6Zn3 eutectic phase in comparison to those coarse ones under CS condition. By subsequent HPR, the Ca2Mg6Zn3 phase was further refined and dispersed uniformly by severe fragmentation. Specially, the achieved supersaturation containing excessive Ca solute atoms due to high cooling rate was maintained in the SRS-HPR condition. The mechanisms that govern the high ductility of the SRS-HPR sample could be ascribed to following reasons. First, refined Ca2Mg6Zn3 eutectic phase could effectively alleviate or avoid the crack initiation. Furthermore, excessive Ca solute atoms in α-Mg matrix result in the yield point phenomenon and enhanced strain-hardening ability during tension. The findings proposed a short-processed strategy towards superior performance of Mg-6Zn-0.2Ca alloy for industrial applications.

An annealed polypyrrole (PPy)/poly(vinyl alcohol) PVA film was successfully applied to the ZnCo2O4 nanorod surface via a liquid-phase synthesis approach and a subsequent annealing process. PVA play...

A novel Mg-8Al-2Sn-1Zn alloy with a bimodal structure prepared by hard-pate-rolling (HPR) exhibits both higher tensile strength and ductility than its fine-grained counterparts prepared by conventional rolling. By delicate electron back-scatter diffraction (EBSD) analysis, we found fine grains with weak basal texture is beneficial for basal slip and favors initial deformation. Meanwhile, coarse grains with a strong basal texture could accommodate abundant newly generated dislocations, promoting work hardening after fine grains are saturated with dislocations. For the first time, individual roles of fine and coarse grains and their synergy effect on enhancing ductility in bimodal structured Mg alloys is clarified.

Pulse current assisted treatment has gained more attention in metallic materials, which is efficient and energy-saving, as compared with traditional heat treatment. However, the microstructural evolution of magnesium alloys with varied pre-deformation under pulsed current is still unclear. In this work, AZ91 alloy is subjected to gradient rolling and pulse current treatment (∼40 s). After gradient rolling, a transition zone from the initial to deformed microstructure was obtained in the sample, and the evolution of phases and twins can be analyzed precisely and continuously. Due to the difference in the Joule heat distribution of pulse current, a large number of fine Mg17Al12 phase are precipitated and dispersed, the precipitation and dissolution process of Mg17Al12 phase occur simultaneously. The region with moderate deformation has the largest recrystallization fraction, and the { 10 1 ¯ 1 } − { 10 1 ¯ 2 } double twins and { 10 1 ¯ 1 } compression twins are the preferred nucleation sites in the early stage of pulse current treatment. The weaken of texture is due to the newly formed grains with non-basal orientations. This work provides a new insight into the microstructural control of pre-deformed magnesium alloys under pulse current assisted treatment.

The control of precipitation density has always been a difficult issue in the application of precipitation strengthening mechanism. Here we reported the precipitation of Mg2Ca in a novel Mg-2Al-0.8Sn-0.5Ca (wt. %, ATX2105) alloy promoted by multidirectional rolling. Microstructure characterization revealed that the dislocation density of the multidirectional rolling ATX2105 sample was 3 . 6 × 1 0 13 m − 2 higher than that of unidirectional rolling sample after the second pass of rolling, which can effectively promote the precipitation of Mg2Ca during the heat preservation process between rolls. Benefiting from the combination of more precipitates and finer grains, the yield strength of the multi-directionally rolled ATX2105 was ∼41 MPa higher than its unidirectionally rolled counterpart. Eventually, the multidirectional rolling ATX2105 sample showed a good combination of strength and ductility, exhibiting an ultimate tensile strength of ∼302 MPa, a yield strength of ∼251 MPa, and an elongation of ∼23.4%. In this work, the role of multidirectional rolling in promoting precipitation is clarified for the first time, which may provide new ideas for improving the effectiveness of precipitation enhancement.

Serious centerline segregations limited the application of twin-roll casting (TRC) to produce Al–Mg–Si alloys. The formation of centerline segregations was closely associated with solidification behaviors especially the flow of residual liquid. In the present study, we chose 0.07 wt% Cr as grain refiner of Al–1.2Mg–0.7Si–0.1Fe (AA6061) alloys, and we found the secondary dendrite arm spacing around the segregations sharply decreased from ~25 to ~9 μm. Grain refinement was probably attributed to the Cr addition induced formation of Al45Cr7 phases acting as heterogeneous nucleation sites for fcc-Al, which decreased the flowing tendency of residual liquid by providing extra flowing resistance, and heterogeneous nucleation for fcc-Al depleted the residual liquid. As a result, the quantity of residual liquid at the centerline was significantly reduced, and then the centerline segregations of TRC were significantly restrained by alloying with 0.07 wt% Cr. Tensile properties of solution treated (T4) and peak aged (T6) alloys were synchronously optimized. In particular, the uniform elongation of peak aged alloys increased from ~9% to ~12%, and the fracture elongation increased from ~15% to ~18%. Hence, this study may provide a simple and economic method to restrain the centerline segregations of TRC, and also improve the tensile properties.

The combined addition of 0.2 wt% Ca-Sb in Al-11Mg2Si alloy led to significant refinement of the eutectic Mg2Si from a coarse, needle-like morphology to spherical particles. Specifically, a further increase of Ca-Sb addition to 1.0 wt% resulted in the microstructure transition from hypoeutectic to hypereutectic characteristics. The refinement of eutectic Mg2Si could be attributed to the selective adsorption of Ca and Sb atoms on Mg2Si surfaces, which inhibited the growth of the eutectic phase in the preferential growth direction. Importantly, Ca and Sb elements were involved in the formation of heterogeneous nuclei for primary Mg2Si when Ca-Sb content increased to 1.0 wt%, which introduced the transition of composition from hypoeutectic to hypereutectic structures. Comparing to the unmodified alloy, the Al-11Mg2Si alloy modified with 0.2 wt% Ca-Sb exhibited significantly enhanced elongation to failure of ~7.9% (from ~1.3% of the unmodified alloy) and ultimate tensile strength of ~222 MPa (from ~180 MPa of unmodified alloy). The spheroidization of eutectic Mg2Si after complex modification is believed to be the underlying reason for the greatly enhanced tensile properties.

Achieving high strength-ductility synergy is a great challenge in low-alloyed Mg alloys. In this work, a new Mg–1.0Zn–0.45Ca–0.35Sn–0.2Mn (wt.%, ZXTM1000) alloy was designed and fabricated by sub-rapid solidification (SRS) to overcome the dilemma. After hot rolling and annealing, the new alloy sheet exhibited an excellent tensile yield strength (YS, ∼270 MPa) and elongation (∼21%). Microstructure characterization revealed that the high YS was mainly attributed to the fine grains (∼3 μm) and high density of spherical Ca2Mg6Zn3, Mg2Ca, and α-Mn precipitates. Moreover, the homogeneous grain size distribution, weakened TD-split texture and the activation of multiple types of slips contributed to the enhanced ductility. The findings demonstrate an effective way to fabricate low-alloyed Mg alloys with high strength and ductility by combining the SRS and hot rolling.

The present study investigated influences of the Al3Sc particle content on the evolution of bimodal grain structure and mechanical properties at both room and elevated temperatures of Al–7Mg–Sc alloys processed by hard-plate rolling (HPR). Bimodal grain structures were obtained in HPRed Al–7Mg–Sc alloys where ultrafine/fine grains decreased in size and increased in amount with increasing volume fraction of Al3Sc precipitates. The formation and evolution of bimodal grain structure are associated with (i) the different stability of various-orientated grains, and (ii) the volume fraction of Al3Sc precipitates. Ultrafine/fine (sub) grains form preferentially along grain boundaries (GBs) of //normal direction (ND)- and //ND-oriented coarse grains during HPR. In contrast, the //ND-oriented coarse grains show relatively high stability nearly without subdivision. Furthermore, increased volume fraction of Al3Sc precipitates in Al–7Mg–0.4Sc promotes dislocation accumulation, favoring recrystallization nucleation, and meanwhile restricts grain growth, which results in more and refined ultrafine/fine grains. Thereby, the refined grain structure and increased volume fraction of Al3Sc precipitates in the Al–7Mg–0.4Sc alloy contribute to improved room-temperature yield strength (YS) and elongation, i.e. ~482 MPa and ~8%, which is much higher than that of ~420 MPa and ~4% in the binary Al–7Mg alloy. Meanwhile, Al–7Mg–0.4Sc exhibited higher hardness when annealed at 150–300 °C and also higher YS when subjected to a tensile test at ≤ 200 °C due to the more stabilized grain structure pinned by dispersed Al3Sc and additional strengthening effect from Al3Sc precipitates. Nevertheless, when subjected to a tensile test at ≥ 250 °C, the Al–7Mg–0.4Sc alloy exhibited much lower YS than binary Al–7Mg. The transition could be explained by more favored grain boundary sliding (GBS) and great dislocation loss, due to the easier occurrence of dynamic recrystallization (DRX) in Al–7Mg–0.4Sc during tensile test at elevated temperatures.

In this study, we are pleased to report a low-alloyed Mg–1.0Zn–0.45Ca–0.35Sn–0.2Mn (wt. %) alloy sheet fabricated by hot rolling, showing superplastic deformation ability with tensile elongation of ~410%±30% at 573 K and 1 × 10−3 s−1. The superplasticity is co-dominated by grain boundaries sliding (GBS) and solute drag creep. Unlike high-alloyed Mg alloys with dispersed precipitates, this low-alloyed system keeps its fine-grained microstructure by the co-segregation of Zn and Ca atoms at grain boundaries (GBs) coupled with a few precipitates. Although the co-segregation of Zn and Ca atoms at GBs are not favored for GBS, the resulting superior thermostability is of great importance to achieve superplastic deformation. Thus, we put forward a new path in achieving superplasticity in low alloyed Mg alloys by using a complementary thermodynamic and kinetic stabilization approach.

The Hall-Petch (H–P) relation for bimodal-grained Mg–Al–Zn alloys consisting of both ultra-fine grains (UFGs)/fine grains (FGs) and coarse grains (CGs) is discussed separately for yield stress and flow stress according to different deformation behaviors of UFGs/FGs and CGs. The yield behavior is controlled by basal dislocation slip of UFGs/FGs and twinning of CGs collectively. Thereafter, non-basal slip is activated in basal-oriented CGs as deformation proceeds. Considering the synergy effect of UFGs/FGs and CGs, we present modified H–P relations for bimodal-grained Mg–3Al–1Zn (AZ31) and Mg–9Al–1Zn (AZ91) alloys containing either few or numerous dispersed particles, respectively, by which the calculated strengths depict experimental values well.

Effects of Sb modification on the eutectic microstructure and tensile properties of hypoeutectic Al–11Mg2Si alloy were systematically investigated in this work. Eutectic Mg2Si phase in hypoeutectic Al–11Mg2Si alloy was successfully refined from the long rod-like structure into fine spherical morphology when introducing 0.2 wt% Sb, whereas 1.0 wt% Sb reversely led to the formation of coarse eutectic Mg2Si phase. Specifically, the elongation to failure in the 0.2 wt% Sb modified alloy was significantly improved to 9.7% as compared to the unmodified counterpart (1.5%), and the ultimate tensile strength increased from 183 MPa to 208 MPa. This result can be attributed to the spheroidization of eutectic Mg2Si phase with Sb modification, which greatly relieved the stress concentration and restrained the crack source from initiating and propagating. The achieved results could shed new light on a more efficient and economical way to obtain high-performance hypoeutectic Al–Mg2Si alloys.

In conventional analysis of instability, a rough prediction of uniform deformation was obtained due to taking material parameters as constants. In this study, the constitutive equation with varying parameters for Zn-5%Al alloy at 340 °C is employed to predict the critical values of uniform strain in tension based on Considere criterion and Hart criterion, respectively. It should address the factor of strain rate in the characterization of the capability of uniform deformation on superplastic alloys, or for that matter, on any rate-dependent material. Comparison and analysis indicated that the results on Hart criterion have the better predictability of uniform deformation than Considere criterion. The Considere criterion is dependent on strain path, while Hart crtierion is merely dependent on the values of strain and strain rate in tension, and is independent on the strain path or the deformation condition or the deformation history. Therefore, the uniform strain vs. strain rate relation can be taken as a quantitative reference for designing a reasonable strain path during superplastic forming with increase of formability and reduction of forming time.

With the development of numerical calculation and precision forming, constitutive equations are required to possess high accuracy and good reliability, rather than simplicity of mathematical form. Due to simple algorithm and constant parameters, the conventional constitutive models can not be suited to describing superplastic flow behavior which represents complex responses with a large strain. In this study, through surface fitting on experimental data from tension tests performed over a wide range of strain rates, tensile velocities and loads, an empirical approach was proposed to establish constitutive equation for complex superplastic behavior of Zn-5%Al alloy at 340 °C. The empirical constitutive equation not only represents the strain dependence and the strain rate dependence of stress, but also reflects the coupling effects of strain and strain rate on stress, which can not be achieved by traditional models. A comparison between the predicted flow stresses and the experimental data verified that the empirical constitutive equation has high accuracy and good reliability on modeling superplastic flow behavior of Zn-5%Al alloy at 340 °C in a wide range of strains 0~2.5 and strain rates 7.0×10-5~8.0×10-2s-1.

In order to precisely describe superplastic flow behavior of Al–Zn–Mg–Zr alloy, a phenomenological constitutive equation and an empirical constitutive equation were established based on the experimental data, which were obtained from the constant strain rate tensions (1.0 × 10 −4 to 8.0 × 10 −2 s −1 ) at 530 °C. Through verification of the two constitutive equations with the experimental data in constant strain rate tensions, constant velocity tensions and constant load tensions, it was indicated that the phenomenological constitutive equation has very high accuracy in the local range of some strain rates and strains. The empirical constitutive equation has higher accuracy and more comprehensive reliability in a wide range of strain rates (1.0 × 10 −4 to 8.0 × 10 −2 s −1 ) and strains (0–2.0) due to considering the coupling effects of strain and strain rate on stress, compared with the phenomenological constitutive equation. In addition, both the phenomenological constitutive equation and the empirical constitutive equation have a good ability to model the superplastic flow behavior of Al–Zn–Mg–Zr alloy at 530 °C under other deformation conditions besides constant strain rate tension.

In this study, constitutive equations for superplastic materials with an extra large elongation were investigated through mechanical analysis. From the view of phenomenology, firstly, some traditional empirical constitutive relations were standardized by restricting some strain paths and parameter conditions, and the coefficients in these relations were strictly given new mechanical definitions. Subsequently, a new, general constitutive equation with varying parameters was theoretically deduced based on the general mechanical equation of state. The superplastic tension test data of Zn-5%Al alloy at 340 °C under strain rates, velocities, and loads were employed for building a new constitutive equation and examining its validity. Analysis results indicated that the constitutive equation with varying parameters could characterize superplastic flow behavior in practical superplastic forming with high prediction accuracy and without any restriction of strain path or deformation condition, showing good industrial or scientific interest. On the contrary, those empirical equations have low prediction capabilities due to constant parameters and poor applicability because of the limit of special strain path or parameter conditions based on strict phenomenology.