Effect of Large Shear Deformation on Rail Steels and Pure Metals

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3 3 Results and Discussion 3.3 Mechanical Properties In order to characterize the changes in the mechanical properties due to the large shear deformation, various methods were applied either after or during the deformation. In the following section, the results <strong>of</strong> these measurements are shortly described. 3.3.1 In-situ Measurement <strong>of</strong> the Torque The development <strong>of</strong> a reliable and accurate possibility to measure the torque during deformation 42, 69 allowed it to quickly determine changes in the mechanical strength without applying other methods afterwards. The torque that is measured contains next to the torque necessary to deform the sample MD solely a contribution from the region <strong>of</strong> the burr, MB. From the torque MD, the shear stress can be calculated according to Equation 3.1: Md = 2πτ(r)r 2 dr (3.1) However, the contribution <strong>of</strong> the burr is not easily evaluated, the in-situ measurement <strong>of</strong> the torque shall only be used to measure relative changes <strong>of</strong> the strength for different strains, different materials or different regimes <strong>of</strong> deformation (HPT and CHPT). When the experimental setup is kept constant (especially the hydrostatic pressure and the tool geometry), also MB will be quite constant and the results <strong>of</strong> the measurement are very accurate and reproducible. Figure 3.15a shows typical in-situ measured torque curves <strong>of</strong> the investigated pure metals. At the beginning <strong>of</strong> the deformation, a region <strong>of</strong> intense strain hardening is present until the increase <strong>of</strong> the strength reachs zero and a kind <strong>of</strong> steady state deformation occures. 36 The onset <strong>of</strong> this steady state deformation coincidences with the saturation in the decrease <strong>of</strong> the structure size and can also be seen in the microhardness and tensile tests. The strain necessary to reach this steady state deformation depends on the material and seems to be influenced by the crystalline structure (fcc vs. bcc). 26 Figure 3.15: Comparison <strong>of</strong> the in-situ measured torque curves for (a) pure metals and (b) rail steels.
3.3 Mechanical Properties In contradiction to pure metals, in the rail steels no steady state deformation could be reached. Figure 3.15b depicts the measured torque curves for the rail steels. It can be seen that the increase in the strength is highest for Dobain 430, and lowest for 900A. When the measured torques for the steels are compared to the measured torque for the pure metals, the difference in tensile strength can be estimated (almost 100 percent between Armco iron in saturation and 900A after ɛeq = 16). The torque measured in CHPT experiments shows the same characteristic as in monotonic HPT experiments. For pure metals, the torque saturates after high strains, while in the steels there is still an increase, although this is much smaller than in the monotonic HPT. The differences between CHPT and HPT are the levels, at which the torque saturates and the necessary values <strong>of</strong> strain to reach this steady state deformation (pure metal). The smaller the ∆ɛ was, the lower is the torque and the earlyer (in terms <strong>of</strong> total equivalent strain) the saturation is reached. A detailed investigation <strong>of</strong> these observations is given in Papers D, H and I. Figure 3.16 depicts what happens when HPT and CHPT experiments are combined. It can be seen the the different strength levels (and therefore necessarily the microstructures) can at least be partial converted into one another. This is especially surprising when a HPT experiment is performed first, by applying <strong>of</strong> cyclic loading conditions, the torque decreases almost down to the level that is reached when only a cyclic loading (with the same ∆ɛ) is applied. This process is even repeatable for some times, although it can be seen that it is not totally reversible. This area <strong>of</strong> SPD is at the moment completely unexplored. The first results <strong>of</strong> our experiments will be published in a forthcoming paper 70 Figure 3.16: in-situ measured torque curves for combined HPT and CHPT experiments (a) Armco iron, ∆ɛ = 0.5, one block → ɛeq,total = 32 and (b) nickel, ∆ɛ = 2, one block → ɛeq,total = 32. 3.3.2 Tensile Tests To obtain absolute values <strong>of</strong> the mechanical strength, specimens for tensile tests were machined out <strong>of</strong> HPT-samples. 42, 69 From each sample, two specimens are obtained. With this method, the validity <strong>of</strong> the in-situ torque measurement as well as the influence <strong>of</strong> the hydrostatic pressure was investigated, see Paper C. Figure 3.17 show the tensile strength for the 27 3
Page 1 and 2: University of Leob
Page 3: ASTRID, LÄTITIA
Page 7 and 8: Danksagung Die hier vorliegende Arb
Page 9 and 10: Summary The investigation o
Page 11 and 12: Kurzfassung Die Untersuchung der Au
Page 13 and 14: Sobald jemand in einer Sache Meiste
Page 15 and 16: Contents 1 Introduction 1 1.1 Motiv
Page 17 and 18: Contents E.3.2 Mechanical strengths
Page 19 and 20: 1.1 Motivation 1 Introduction Today
Page 21 and 22: 1.2 Objectives The aim of</
Page 23 and 24: 2 Methods of Sever
Page 25 and 26: 2.2 Equal Channel Angular Pressing
Page 27 and 28: 2.2 Equal Channel Angular Pressing
Page 29 and 30: 3 Results and Discussion In the fol
Page 31 and 32: 3.1 Structural Evolution of
Page 43: 3.2 Structural Evolution of
Page 47 and 48: 3.3 Mechanical Properties Figure 3.
Page 49 and 50: 3.3 Mechanical Properties Figure 3.
Page 51 and 52: 3.3 Mechanical Properties can be se
Page 53 and 54: 4 Conclusions The methods o
Page 55 and 56: [1] G. Baumann, H.-J. Fecht, and S.
Page 57 and 58: Bibliography [36] T. Hebesberger, H
Page 59 and 60: 5 List of appended
Page 61 and 62: A Structural Refinement of<
Page 63 and 64: A.1 Introduction A.1 Introduction K
Page 65 and 66: A.3 Results Figure A.2: Micrograph
Page 67 and 68: A.4 Discussion Figure A.3: Microgra
Page 69 and 70: Bibliography to paper A [1] W. Lojk
Page 71 and 72: B Structural Changes of</st
Page 73 and 74: B.1 Introduction B.1 Introduction D
Page 75 and 76: B.3 Results and Discussion after a
Page 77 and 78: Figure B.5: 2Θ scans of</s
Page 79 and 80: B.4 Conclusion B.4 Conclusion • T
Page 81 and 82: Bibliography to paper B [1] G. Baum
Page 83 and 84: C Strain Hardening during High Pres
Page 85 and 86: C.1 Introduction C.1 Introduction R
Page 87 and 88: C.3 Results With this equation it i
Page 89 and 90: C.4 Discussion Figure C.4: SEM micr
Page 91 and 92: Bibliography to paper C [1] T. Hebe
Page 93 and 94: D Formation of sur
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D.1 Introduction D.1 Introduction T
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D.3 Results D.3.1 Monotonic <strong
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D.3 Results Figure D.4: SEM microgr
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D.4 Discussion Figure D.6: SEM micr
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occurring on the surface of
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Bibliography to paper D [1] Y. K. C
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E High Pressure Torsion of<
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E.1 Introduction E.1 Introduction T
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E.3 Results Figure E.2: SEM microgr
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E.3.2 Mechanical strengths E.4 Disc
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Bibliography to paper E [1] L. Wang
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F TEM Investigation of</str
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F.1 Introduction F.1 Introduction C
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F.4 Discussion see Figure F.2b. The
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Figure F.3: In-situ measured torque
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F.4 Discussion Figure F.6: Comparis
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F.6 Appendix • Electron energy-lo
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[1] M. Zelin. Acta Mater., 50(17):4
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G Fracture Processes in Severe Plas
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G.2 Introduction G.2 Introduction T
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G.4 Results toughness test. It can
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G.5 Discussion Figure G.3: (a) Load
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G.5.2 Fracture G.5 Discussion Fract
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Bibliography to paper G [1] J. D. E
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H Cyclic High Pressure Torsion <str
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H.1 Introduction H.1 Introduction I
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Figure H.2: Sample preparation for
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H.3 Results With increasing ∆ɛ a
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H.3 Results Figure H.7: SEM microgr
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H.3 Results between 2°and 15°, th
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H.4 Discussion comparable to ECAP R
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H.5 Conclusions bon steels as a fun
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H.5 Conclusions • The steady stat
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Bibliography to paper H [1] Y. T. Z
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Bibliography to paper H [33] C. Buq
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I Structural Evolution during Cycli
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I.1 Introduction I.1 Introduction T
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I.3 Results Figure I.2: SEM microgr
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I.4 Discussion be seen that saturat
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I.5 Conclusions structure size is a
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[1] H. Mughrabi. Mater. Sci. Eng.,
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Effect of Large Shear Deformation on Rail Steels and Pure Metals

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