Severe Plastic Deformation of Ferritic and Austenitic Steels

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$dissertation global and local fracture properties of metal matrix ...$

Chapter II Fig. 2.1: Steady-state microstructures (BSE images) of pure iron and Fe-17wt% Co after HPT at 20°C and 450°C. The significant effect of deformation temperature on the structural size is reflected in the torque curves. Fig. 2.2 depicts the torque measured during deformation and the Vickers microhardness in the saturated states. The curves show all the same behaviour: a steep rise of the torque at low shear strains that decreases slightly with preceding deformation until a specific saturation strain εsat is reached. From the beginning of saturation the torque remains constant for all samples. It is clearly evident that the processing temperature essentially dominates both the saturation torque Msat and the saturation strain εsat. Fig. 2.2: In-situ torque curves and Vickers microhardness in the saturated states given for pure iron and Fe-17wt% Co. HPT was performed at 20°C and 450°C, respectively. 10
Chapter II An increase in the deformation temperature leads to a decrease in these two parameters. It is obvious that the onset of steady state is shifted towards lower torques and smaller shear strains, respectively. The effect of deformation the temperature is also reflected in the microhardness measured near the edge of the discs and is shown in Fig 2.2. By increasing the HPT temperature lower microhardness values were determined. Due to the Hall-Petch relation high hardness values are measured in microstructures with smaller grain sizes. The interaction of dislocations at grain boundaries (pile-up) is considered to be the mechanistic process responsible for the resistance to plastic deformation from grain refinement. A further characteristic of the microhardness evolution is seen in the different materials. Alloying results in both an increase in hardness and an increase in the thermal stability of the HPT deformed microstructure. The above described features are based on results of bcc metals. Fig. 2.3 displays the TEM microstructures of fcc metals such as pure Ni and Ni- 24wt% Fe. The samples were deformed at 20°C to a strain of 45. By comparing both micrographs one can clearly see that alloying affects the microstructural sizes in the steady state. In both materials the grain boundaries become clearly visible and in the case of pure nickel the shape of the structural element is more polygonal. The influence of the processing temperature in pure nickel is illustrated in Fig. 2.4. The curves show the same behaviour as described above: a steep rise of the torque at low shear strains that decreases slightly with preceding deformation. At large strains the saturation strain εsat is reached again. It is clearly evident that independent on the crystal structure severe plastic deformation results in a significant grain refinement. The steady state microstructures are essentially dominates by the HPT temperature. Fig. 2.3: TEM micrographs of a) pure Ni and b) Ni- 24wt% Fe after HPT at 20°C to a strain of ε = 45 at a radius of 3 mm. 11
Page 1: Severe Plastic Deformation of Ferri
Page 4 and 5: Affidavit IV I declare in lieu of o
Page 6 and 7: Danksagung Mein Dank gilt natürlic
Page 8 and 9: Abstract thermal treatments for sho
Page 10 and 11: Kurzfassung Das großtechnisch am w
Page 12 and 13: Contents IV.4 Evolution of the Micr
Page 14 and 15: Chapter I SPD provide several benef
Page 16 and 17: Chapter I I.3 Principle of High Pre
Page 18 and 19: Chapter I Fig. 1.3: The big HPT too
Page 20 and 21: Chapter I behaviour. Slip dissoluti
Page 24 and 25: Chapter II Fig. 2.4: In-situ torque
Page 26 and 27: Chapter II 14
Page 28 and 29: Chapter III 16 Fe C Cr Cu Ni Co Si
Page 30 and 31: Chapter III III.2 Magnetic Properti
Page 32 and 33: Chapter III cooling-down process to
Page 34 and 35: Chapter III Ring Method The SQUID a
Page 36 and 37: Chapter III independent on the appl
Page 38 and 39: Chapter III 26
Page 40 and 41: Chapter IV The effect of Mn on aust
Page 42 and 43: Chapter IV the designations denote
Page 44 and 45: Chapter IV Fig. 4.4: In-situ torque
Page 46 and 47: Chapter IV Fig. 4.5: a) Microstruct
Page 48 and 49: Chapter IV TEM studies. Transmissio
Page 50 and 51: Chapter IV conditions such as the a
Page 52 and 53: Chapter IV the average grain size i
Page 54 and 55: Chapter IV 42 For each processing t
Page 56 and 57: Chapter V substructure-free, fully
Page 58 and 59: Chapter V (I) High local stresses o
Page 60 and 61: Chapter V Fig. 5.4: Stress-strain c
Page 62 and 63: Chapter V Fig. 5.6: SSRT fracture s
Page 64 and 65: Chapter V (III) A high hydrogen abs
Page 66 and 67: Conclusions discontinuous dynamic r
Page 68 and 69: Conclusions From the technical poin
Page 70 and 71: Appendix 58
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Appendix VII.2 HR-TEM microstructur
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Appendix VII.4 SSRT fracture surfac
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Appendix Fig A.9: Higher magnificat
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Appendix VII.5 Microstructures foll
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List of Appended Papers Paper E S.
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Paper A 1. Introduction Over the la
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Paper A transmission electron micro
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Paper A Fig. 4: BSE images of HPT d
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Paper A Fig. 7: SEM images of HPT d
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Paper A temperatures and it seems t
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Abstract Paper B iIIiB Tailoring th
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Fig. 1: Dimensions and orientation
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Paper B evaluated to quantify the s
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Paper B grained metallic materials
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Paper C iIIiC Using Ring Samples to
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Paper C All sets of samples were de
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Paper C Therefore the rings for the
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Paper C (iv) The stress distributio
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Paper D 1. Introduction Metals and
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Paper D Fig. 2: Sequence of microgr
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Paper D approximately 0.4 µm; a se
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Paper D behaviour was studied in-si
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Paper E 1. Introduction Recrystalli
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Paper E consist of a substructure w
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Paper E the centre to the edge afte
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Paper E suspended. This indicates t
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Paper F 1. Introduction Over the pa
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Paper F F - 4 Fig. 1: Schematic ill
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Paper F Fig. 2 demonstrates the rad
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Paper F strains the initial coarse
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Paper F Fig. 6: a) TEM micrograph o
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Paper F Fig. 7: Sequence of TEM mic
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Paper F Fig. 7g represents the micr
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Paper F 4. Discussion The present s
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Paper F Ad iii) It is well known th
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Paper F 5. Conclusions A deformatio
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Severe Plastic Deformation of Ferritic and Austenitic Steels

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