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CHINESE ABSTRACT ENGLISH ABSTRACT TABLE OF CONTENT LIST OF TABLES LIST OF FIGURES CHAPTER I. INTRODUCTION II. LETERATURE SURVEY A. Introduction of Superplasticity 1. Superplasticity in Zn—Al alloy 2. Mechanical aspects of superplasticity 3. Requirements for superplastic a. Fine grain size b. Presence of second phase c. Nature of grain boundary structure d. Shape of grains e. Mobility of grain boundaries f. Grain boundaries and their resistance to tensile separation B. Grain Refinement of Duplex Alloys 1. Phase separation 2. Mechanical working C. Techniques of Mechanical Working For Effective Grain Refinement 1. Rolling 2. Equal channel angular pressing (ECA pressing 3. Torsion D. Microstructural Characteristics and Mechanisms of Superplastic Deformation 1. Microstructural characteristics a. Grain growthth b. Grain boundary curvature c. Grain sliding, rotation and displacement 2. Mechanisms of superplastic deformation a. Dislocation models b. Diffusion models E. High—Strain—Rate Superplasticity (HSRSP) 1. Introduction of high—strain—rate superplasticity 2. Deformation mechanisms of high—strain—rate superplasticity a. HSRSP attained by liquid phase enhanced grain boundary sliding b. HSRSP attained by intense plastic straining 3. Commercial applications for HSRSP III. EXPERIMENTAL PROCEDURES A. Materials Preparation B. Shear Forging C. Macroscopic Observation D. Scanning Electron Microscopy Examination E. Microhardness Test F. The Superplastic Test G. Determination of Strain Rate Sensitivity H. Formability Test IV. RESULTS AND DISCUSSIONS A. Morphology of the Thermal Treated Zn — 22 wt.% Al Alloy B. Deformation Characteristics of the Shear Forging C. Microstructural Evolution of the Zn—Al Alloy during Shear Forging D. The Superplastic Tensile Test 1. The effect of testing temperature on the superplasticity 2. The effect of shear forging route on the superplasticity 3. The effect of shear forging cycle on the superplasticity 4. The effect of initial structure prior to the shear forging on the superplasticity 5. The effect of annealing time on the superplasticity E. Microstructural Evolution during Superplastic Deformation F. Determination of Strain Rate Sensitivity G. The Formability and the Microhardness Tests H. The Effect of Zr addition in the Zn — 22 wt.% Al Alloy V. CONCLUTIONS REFFERENCE List of Tables Table 2.1 Examples of typical superplastic materials [1-12]. Table 3.1 Chemical compositions of ZnAl and Zn—Al—Zr alloys. Table 3.2 Four types of heat treatment applied to the Zn—Al and Zn—Al—Zr alloys. Table 4.1 Percentage of elongation tested at various temperatures and strain rates for the No. 3 thermal treated and shear forged Zn—Al alloy by Route C for 5 cycles. Table 4.2 Percentage of elongation tested at 250℃ under various strain rates for the No. 3 thermal treated and shear forged Zn—Al alloy by three kinds of forging route for 3 cycles. Table 4.3 Percentage of elongation tested at 250℃ under various strain rates for the No. 3 thermal treated and shear forged Zn—Al alloy by Route C for various forging cycles. Table 4.4 Percentage of elongation tested at 250℃ under various strain rates for the Zn—Al alloy specimens subjected to the No. 1—3 thermal treatment and shear forging by Route C for 3 cycles. Table 4.5 Percentage of elongation tested at 250℃ under various strain rates for the Zn—Al alloy specimens subjected to the No. 3 thermal treatment, shear forging by Route C for 5 cycles and an annealing treatment at 240℃ for various time periods. Table 4.6 Formability properties in the No. 3 thermal treated Zn—Al and Zn—Al—Zr alloys. Table 4.7 Formability properties in the shear forged Zn—Al and Zn—Al—Zr alloys by Route C for 3 cycles. Table 4.8 Microhardness (Hv) of the No. 3 thermal treated and shear forged Zn—Al and Zn—Al—Zr alloys by Route C for 1—6 cycles. Table 4.1 Percentage of elongation tested at 250℃ under various strain rates for the No. 3 thermal treated, shear forged Zn — Al and Zn—Al—Zr alloys by Route C for 3 cycles. List of Figures Fig. 2.1 The phase diagram of Zn—Al system. Fig. 2.2 The tensile ductility of natural aged Zn—Al alloys at various temperatures [13]. Fig. 2.3 The tensile elongation (a) and the flow stress (b) of cold rolled Zn—Al alloys at various temperatures. The tensile elongation and flow stress of Zn—Al alloys prior to the cold rolling (dashed line) are also shown for comparison [14]. Fig. 2.4 TTT diagram for Zn—Al alloy [15]. Fig. 2.5 Schematic illustration of the logarithmic plot of superplastic materials, stress versus strain rate [16]. Fig. 2.6 Schematic illustration of the microstructural changes that occur during grain refinement by phase separation from a non-equilibrium microstructure [16]. Fig. 2.7 Schematic illustration of the microstructural changes that occur during grain refinement by mechanical working of duplex alloys [16]. Fig. 2.8 A schematic diagram of ECA Pressing [20]. Fig. 2.9 A schematic diagram of torsion straining [21]. Fig. 2.10 Replica showing evidence for enhanced boundary curvature produced by superplastic deformation of a monotectoid Zn—Al alloy, and (b) Removal of curvature on subsequent annealing [35]. Fig. 2.11 Scanning electron micrographs of a deformed sample of SnPb eutectic alloy at different stages of deformation. (a) (f) the strains involved are 120, 180, 270, 540, 610 and 800 percent, respectively. Significant grain boundary sliding is evident [36]. Fig. 2.12 Scanning electron micrographs at increasing time intervals during the tensile straining of a thin foil in the microscope at 1 MeV, stress 18.5 MNm-2, temperature 373 K (m=0.3); grain switching, ‘pinching off’ of grains by grains of and curving of interphase boundaries present [37]. Fig. 2.13 Schematic illustration of the strain rate dependence of flow stress in a superplastic material [41]. Fig. 2.14 Dislocation pile-up models of superplastic flow. The rate at which the grains slide past each other can be controlled by (i) the removal of a pile-up of lattice dislocations (ii) The emission of lattice dislocations from grain boundary ledges and (iii) by the removal of pile-ups of grain boundary dislocations [43]. Fig. 2.15 Grain switching through diffusional mass transport as postulated by Ashby and Verrall (a1-a2) and modified by Spingarn and Nix (b1-b2-b3) [16]. Fig. 2.16 Schematic illustration of the Ball-Hutchison model modified by contribution of subgrains to grain boundary sliding [50]. Fig. 2.17 The variation in superplastic strain rate as a function of the inverse of grain size [56, 57]. Fig. 3.1 Procedure flow chart. Fig. 3.2 The model of shear forging; (a) the apparatus used for shear forging; (b) the photograph of the model; (c) sample chart of the model and (d) the punchers with angle 0 and 60. Fig. 3.3 A schematic diagram of shear forging. Fig. 3.4 Hung—Ta HT—8150 materials testing system equipped with a 5—zone segment electric furnace. Fig. 3.5 Geometries of specimen for superplastic tensile test. Fig. 3.6 A typical load-elongation plot of the step strain rate test. Fig. 3.7 Specimen geometry specified by ASTM E9 for compression test, (b) specimen dimensions for the compression type formability test. Fig. 4.1 Morphologies of various thermal treated Zn — 22 wt.% Al alloys. (a) NA for 1 hour, matrix with a small amount of dispersoids, (b) NA for I hour + AA at 240℃ for 1 hour, coexisted granular and lamellar structure of phases, (c) NA for 2 weeks + AA at 240℃ for 1 hour, granular mixture of , (d) direct quench and aged at 175℃ for 1 hour. Fig. 4.2 EPMA of a No. 3 thermal treated Zn — 22 wt.% Al alloy; (a) BEI showing equiaxed , (b) elementary digital mapping of Al, (c) elementary digital mapping of Zn. Fig. 4.3 Macrographs of the Zn—Al alloy specimens shear forged by Route A for (a) N = 0, (b) N = 1/2 and (c) N = 1. The contour of markers embedded on the sideface of the specimens are re-drawn for reference. Fig. 4.4 Macrographs of the Zn—Al alloy specimens shear forged by Route B for (a) N = 1/4, (b) N = 3/4 and (c) N = 5/4. The contour of markers embedded on the sideface of the specimens are re-drawn for reference. Fig. 4.5 Macrographs of the Zn—Al alloy specimens shear forged by Route C for (a) N = 1, (b) N = 3/2 and (c) N = 2. The contour of markers embedded on the sideface of the specimens are re-drawn for reference. Fig. 4.6 SEM morphology of the No. 1 thermal treated Zn—Al alloy shear forged for 3 cycles by (a) Route A, (b) Route B, and (c) Route C. Fig. 4.7 SEM morphology of the No. 2 thermal treated Zn—Al alloy shear forged for 3 cycles by (a) Route A, (b) Route B, and (c) Route C. Fig. 4.8 SEM morphology of the No. 3 thermal treated Zn—Al alloy shear forged for 3 cycles by (a) Route A, (b) Route B, and (c) Route C. Fig. 4.9 SEM morphology of the No. 1 thermal treated Zn—Al alloy shear forged by Route C for (a) 3, (b) 5, and (c) 7 cycles. Fig. 4.10 SEM morphology of the No. 2 thermal treated Zn—Al alloy shear forged by Route C for (a) 3, (b) 5, and (c) 7 cycles. Fig. 4.11 SEM morphology of the No. 3 thermal treated Zn—Al alloy shear forged by Route C for (a) 3, (b) 5, and (c) 7 cycles. Fig. 4.1 Microstructures of the Zn—Al alloy subjected to a No. 1 thermal treatment, shear forging by Route C for 7 cycles and annealing treatment at 240℃ for (a) 1 min. (b) 10 min. and (c) 60 min. Fig. 4.13 Microstructures of the Zn—Al alloy subjected to a No. 2 thermal treatment, shear forging by Route C for 7 cycles and annealing treatment at 240℃ for (a) 1 min. (b) 10 min. and (c) 60 min. Fig. 4.14 Microstructures of the Zn—Al alloy subjected to a No. 3 thermal treatment, shear forging by Route C for 7 cycles and annealing treatment at 240℃ for (a) 1 min. (b) 10 min. and (c) 60 min. Fig. 4.15 Appearance of the pull-to-failure Zn—Al alloy specimens after the superplastic tensile testing at (a) 250, (b) 200 and (c) 150℃ in a No. 3 thermal treated and shear forged by Route C for 5 cycles condition. The numerical numbers 1—4 indicates the specimens in the prior to test condition, and tested conditions under of 1.5 × 10-3, 1.5 × 10-2 and 4 × 10-1 s-1, respectively. Fig. 4.1 The tensile elongation of No. 3 thermal treated and shear forged Zn—Al alloy specimens by Route C for 5 cycles tested at 150, 200 and 250℃ as a function of initial strain rate. Fig. 4.17 Appearance of the pull-to-failure Zn—Al alloy specimens after the superplastic tensile testing at 250℃ in No. 3 thermal treated and shear forged for 3 cycles by (a) Route A, (b) Route B and (c) Route C conditions. The numerical numbers 1—4 indicates the specimens in the prior to test condition, and tested conditions under of 1.5 × 10-3, 1.5 × 10-2 and 4 × 10-1 s-1, respectively. Fig. 4.1 The tensile elongation of No. 3 thermal treated and shear forged Zn—Al alloy specimens by Route A—C for 3 cycles tested at 250℃ as a function of initial strain rate. Fig. 4.19 Appearance of the pull-to-failure Zn—Al alloy specimens after the superplastic tensile testing at 250℃ in No. 3 thermal treated and shear forged by Route C for (a) 3, (b) 5 and (c) 7 cycles conditions. The numerical numbers 1—4 indicates the specimens in the prior to test condition, and tested conditions under of 1.5 × 10-3, 1.5 × 10-2 and 4 × 10-1 s-1, respectively. Fig. 4.1 The tensile elongation of No. 3 thermal treated and shear forged Zn—Al alloy specimens by Route C for 3, 5 and 7 cycles tested at 250℃ as a function of initial strain rate. Fig. 4.21 Appearance of the pull-to-failure Zn—Al alloy specimens after the superplastic tensile testing at 250℃ in (a) No. 1, (b) No. 2 and (c) No. 3 thermal treated and shear forged by Route C for 3 cycles conditions. The numerical numbers 1—4 indicates the specimens in the prior to test condition, and tested conditions under of 1.5 × 10-3, 1.5 × 10-2 and 4 × 10-1 s-1, respectively. Fig. 4.1 The tensile elongation of No. 1—3 thermal treated and shear forged Zn—Al alloy specimens by Route C for 3 cycles tested at 250℃ as a function of initial strain rate. Fig. 4.23 Appearance of the pull-to-failure Zn—Al alloy specimens after the superplastic tensile testing at 250℃ in No. 3 thermal treated, shear forged by Route C for 5 cycles and annealed at 240℃ for (a) 0 min. (b) 10 min. and (c) 60 min. conditions. The numerical numbers 1—4 indicates the specimens in the prior to test condition, and tested conditions under of 1.5 × 10-3, 1.5 × 10-2 and 4 × 10-1 s-1, respectively. Fig. 4.24 The tensile elongation of Zn—Al alloy specimens subjected to a No. 3 thermal treatment, shear forging by Route C for 5 cycles and an annealing treatment at 240℃ for 0, 10 and 60 minutes tested at 150, 200 and 250℃ as a function of initial strain rate. Fig. 4.25 Morphologies of the Zn—Al alloy specimen which was thermo-mechanical processed (No. 3 thermal treated and shear forged by Route C for 3 cycles) and tensile tested at 250℃ under an initial strain rate of 1.5 × 10-3 s-1, (a) grip area, (b) gage section, SEI, and (c) gage section, BEI. Fig. 4.1 Morphologies of the Zn—Al alloy specimen which was thermo-mechanical processed (No. 3 thermal treated and shear forged by Route C for 3 cycles) and tensile tested at 250℃ under an initial strain rate of 1.5 × 10-2 s-1, (a) grip area, (b) gage section, SEI, and (c) gage section, BEI. Fig. 4.27 Morphologies of the Zn—Al alloy specimen which was thermo-mechanical processed (No. 3 thermal treated and shear forged by Route C for 3 cycles) and tensile tested at 250℃ under an initial strain rate of 4 × 10-1 s-1, (a) grip area, (b) gage section, SEI, and (c) gage section, BEI. Fig. 4.28 The flow stress versus strain rate for a No. 3 thermal treated and shear forged Zn—Al alloy by Route C for 3 cycles. (b) The resulted strain rate sensitivity as a function of strain rate. Fig. 4.29 side-view and (b) top-view macrographs of No. 3 thermal treated (N = 0) and thermo-mechanical processed (by Route C for N = 3) Zn—Al alloy specimens after the formability test, and the load versus displacement curves of the compressive test for (c) N = 0 and (d) N = 3 conditions. Fig. 4.30 side-view and (b) top-view macrographs of No. 3 thermal treated (N = 0) and thermo-mechanical processed (by Route C for N = 3) Zn—Al—Zr alloy specimens after the formability test, and the load versus displacement curves of the compressive test for (c) N = 0 and (d) N = 3 conditions. Fig. 4.31 Microstructural evolution of a No. 3 thermal treated Zn — 22 wt.% Al — 0.1 wt.% Zr alloy in the course of shear forging by Route C. (a) N = 1, (b) N = 2, (c) N = 3, (d) N = 4, (e) N = 5 and (f) N = 6. Fig. 4.32 Appearance of the pull-to-failure (a) Zn—Al and (b) Zn—Al—Zr alloy specimens after the superplastic tensile testing at 250℃ in a No. 3 thermal treated and shear forged by Route C for 3 cycles conditions. The numerical numbers 1—4 indicates the specimens in the prior to test condition, and tested conditions under of 1.5 × 10-3, 1.5 × 10-2 and 4 × 10-1 s-1, respectively. Fig. 4.1 A comparison of the superplastic tensile elongation data for Zn—Al and Zn—Al—Zr alloys tested at 250℃ under various initial strain rate in a No. 3 thermal treated and shear forged by Route C for 3 cycles conditions. Fig. 4.34 A comparison of the superplastic characteristics at 250℃ in No. 3 thermal treated and shear forged Zn—Al and Zn—Al—Zr alloys by Route C for 3 cycles. (a) The flow stress versus strain rate. (b) The strain rate sensitivity versus strain rate. Fig. 4.1 Morphologies of the Zn—Al—Zr alloy specimen which was thermo-mechanical processed (No. 3 thermal treated and shear forged by Route C for 3 cycles) and tensile tested at 250℃ under an initial strain rate of 1.5 × 10-3 s-1, (a) grip area, (b) gage section, SEI, and (c) gage section, BEI. Fig. 4.36 Morphologies of the Zn—Al—Zr alloy specimen which was thermo-mechanical processed (No. 3 thermal treated and shear forged by Route C for 3 cycles) and tensile tested at 250℃ under an initial strain rate of 1.5 × 10-2 s-1, (a) grip area, (b) gage section, SEI, and (c) gage section, BEI. Fig. 4.37 Morphologies of the Zn—Al—Zr alloy specimen which was thermo-mechanical processed (No. 3 thermal treated and shear forged by Route C for 3 cycles) and tensile tested at 250℃ under an initial strain rate of 4 × 10-1 s-1, (a) grip area, (b) gage section, SEI, and (c) gage section, BEI.
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