Acta Metall Sin  2018, Vol. 54 Issue (12): 1745-1755    DOI: 10.11900/0412.1961.2018.00174
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Effects of Tool Rotation Rates on Superplastic Deformation Behavior of Friction Stir Processed Mg-Zn-Y-Zr Alloy
Guangming XIE1(), Zongyi MA2, Peng XUE2, Zongan LUO1, Guodong WANG1
1 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Abstract

Compared to conventional Mg-Al and Mg-Zn system magnesium alloys, the Mg-Zn-Y-Zr heat-resistant alloy exhibits high thermal stability due to the addition of Y earth element, which is an ideal candidate for producing high strain rate superplasticity (HSRS, strain rate≥1×10-2 s-1). Recently, the HSRS of Mg-Zn-Y-Zr alloy was achieved by friction stir processing (FSP), because the FSP resulted in the generation of fine and equiaxed recrystallized grains and fine and homogeneous second phase particles. However, the study on superplastic deformation mechanism of FSP Mg-Zn-Y-Zr alloy at various parameters is limited relatively. Therefore, at the present work, six millimeters thick as-extruded Mg-Zn-Y-Zr plates were subjected to FSP at relatively wide heat input range of rotation rates of 800 r/min to 1600 r/min with a constant traverse speed of 100 mm/min, obtaining FSP samples consisting of homogeneous, fine and equiaxed dynamically recrystallized grains and fine and uniform Mg-Zn-Y ternary phase (W-phase) particles. With increasing rotation rate, within the FSP samples the W-phase particles were broken up and dispersed significantly and the recrystallized grains were refined slightly, while the fraction ratio of the high angle grain boundaries (grain boundaries misorientation angle≥15°) was increased obviously. Increasing rotation rate resulted in an increase in both optimum strain rate and superplastic elongation. For the FSP sample obtained at 1600 r/min, a maximum elongation of 1200% was achieved at a high-strain rate of 1×10-2 s-1 and 450 ℃. Grain boundary sliding was identified to be the primary deformation mechanism in the FSP samples at various rotation rates by superplastic data analyses and surfacial morphology observations. Furthermore, the increase in rotation rate accelerated superplastic deformation kinetics remarkably. For the FSP sample at 1600 r/min, superplastic deformation kinetics is in good agreement with the prediction by the superplastic constitutive equation for fine-grained magnesium alloys governed by grain boundary sliding mechanism.

 ZTFLH: TG456.9
Fund: Supported by National Natural Science Foundation of China (Nos.51774085 and 51671190) and Fundamental Research for the Central Universities (No.N170704013)
 Fig.1  OM images of parent Mg-Zn-Y-Zr alloy (a) and FSP samples at rotation rates of 800 r/min (b), 1200 r/min (c) and 1600 r/min (d) (FSP—friction stir processing, the black particles are the second phases) Fig.2  SEM images of parent Mg-Zn-Y-Zr alloy (a) and FSP samples at rotation rates of 800 r/min (b), 1200 r/min (c) and 1600 r/min (d) (The white particles are the second phases) Fig.3  EPMA element maps of Mg (a, d), Zn (b, e) and Y (c, f) in parent Mg-Zn-Y-Zr alloy (a~c) and FSP sample at rotation rate of 1600 r/min (d~f) Fig.4  XRD spectra of parent Mg-Zn-Y-Zr alloy (a) and FSP sample at rotation rate of 1600 r/min (b) Fig.5  EBSD orientation maps of parent Mg-Zn-Y-Zr alloy (a) and FSP samples at rotation rates of 800 r/min (b), 1200 r/min (c) and 1600 r/min (d) (The black and white lines represent the high angle grain boundaries (HAGBs, grain boundaries misorientation angle≥15°) and low angle grain boundaries (LAGBs, 2°≤grain boundaries misorientation angle<15°), respectively) Fig.6  Grain boundary misorientation angle distributions of parent Mg-Zn-Y-Zr alloy (a) and FSP samples at rotation rates of 800 r/min (b), 1200 r/min (c) and 1600 r/min (d) (The black curves show the random misorientation angle distribution for hcp structural metal) Fig.7  Relationships of elongation with initial strain rate at 400 ℃ (a) and 450 ℃ (b) for FSP samples at rotation rates of 800, 1200 and 1600 r/min with traverse speed of 100 mm/min Fig.8  Relationships of flow stress with initial strain rate at 400 ℃ (a) and 450 ℃ (b) for FSP samples at rotation rates of 800, 1200 and 1600 r/min (m—strain rate sensitive exponent) Fig.9  Macrographs of untested and failed tensile FSP specimens at rotation rates of 800 r/min (a), 1200 r/min (b) and 1600 r/min (c) at test temperatures of 400 ℃ (a) and 450 ℃ (b, c) Fig.10  SEM images of fracture surface of FSP samples at 800 r/min deformed under 400 ℃ and 1×10-3 s-1 (a), at 1200 r/min deformed under 450 ℃ and 3×10-3 s-1 (b) and at 1600 r/min deformed under 450 ℃ and 1×10-2 s-1 (c) Fig.11  Relationships between $ε˙kTd3D0-1G-1b-4$ and normalized effective stress $σG-1$ for FSP samples at rotation rates of 800, 1200 and 1600 r/min ($ε˙$—strain rate, σ—flow stress, D0—pre-exponential constant for diffusivity, G—shear modulus, b—Burgers vector, k—Boltzmann's constant, T—absolute temperature, R—gas constant, d—grain size)