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

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Section 7 the matrix is one of the problems associated with the production of cast MMCs, as well [85, 86]. In cast MMCs, the distribution of the reinforcement particles in the matrix alloy is influenced by several factors, such as the rheological behavior of the matrix melt, the particle incorporation method, interactions of the particles and the matrix before, during and after mixing, the changing particle distribution of during solidification [85, 86]. As was shown in [87], the post solidification processing of the cast MMCs extrusion or rolling can modify the particle distribution, but complete declustering can not be achieved even at high extrusion ratios. Therefore, an application of ECAP could be useful in this case, as well. But additional investigations are necessary to be performed in order to create a model analogously to the case of the PM-MMCs. 97
8. Summary Section 8 This work deals with the fracture properties of cast and powder metallurgy (PM) metal matrix composites (MMCs) with various particle volume fractions of Al2O3 and SiC - particles. The age hardening conditions of the matrix are also varied. The average particle size is 10 µm for the Al2O3 and 100 µm for the SiC particles. The global tensile and fracture properties are measured by standard tests, but the work is focused on the determination of local fracture properties, i.e. the fracture properties at different positions along the crack front. A new procedure was developed to investigate the local conditions for void initiation near the crack tip. It consists of: 1) the determination of the crack tip opening displacement at the moment of void initiation, CODvi, for considered particles near the crack tip; 2) the estimation of the stress tensor at the position of a particle by the HRR-theory through the determined CODvi-values; 3) the estimation of the stress tensors in both the particle and in the matrix by the Mori- Tanaka approach. An extremely high scatter of the critical crack tip opening displacement, CODi, which is a measure of the local fracture initiation toughness, is found for all MMCs. Neither the particle coordinates, r and θ, nor the particle dimensions have a decisive influence on the value of CODi. The crack tip opening displacement at the moment of void initiation, CODvi, which is considered to be the measure of the local void initiation toughness, shows also a very high scatter in all investigated specimens. The CODvi-values slightly decrease with increasing angle to the crack plane, θ. Near-zero CODvi-values are found for alumina particles in cast MMCs. It is shown that it might be related with the shape of the crack tip plastic zone which has its maximum extension at θ ≈ 70° comparable with the distance between the particle and the crack tip. No marked influence of the distance, r, or the particle size on the values of CODi and CODvi is revealed for the MMCs. In the cast MMCs, a shifting of the main mechanism of the void initiation from particle fracture to particle/matrix decohesion with increasing aging time is revealed. In the PM MMC, SiC particles are only fractured, independently on the aging condition. The analysis shows that the maximum principal stresses in particles at the moment of void initiation, σ p max, have a rather small scatter. The σ p max-values, e.g., 974±58 MPa for a Al2O3- 98
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MONTANUNIVERSITÄT MONTANUNIVERSIT
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TABLE OF CONTENTS 1. Introduction 5
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7.4.4. The effect of ECAP on the lo
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Section 1 The main results of this
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Section 2 2.2. The influence of the
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Section 2 high strength, fine reinf
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Section 2 etched to mark the inclus
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Section 2 (4.) The specimen prepara
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Section 3 Fig. 3.1. The digital ele
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height [µm] height [µm] 10 0 -10
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Section 3 Fig. 3.5. The observation
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Section 3 3.3.2. The determination
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Section 3 The Eshelby tensor does n
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equivalent stress Section 3 Fig. 3.
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Assume σ m eq Calculate matrix sec
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Section 4 Table 4.1. Chemical compo
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4.1.1.3. Fracture mechanics tests S
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Section 4 Fig. 4.4. A view of a com
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Section 4 Table 4.3. Mechanical pro
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εfr [%] εfr [%] N 16 12 8 4 0 150
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Section 4 4.2. The effect of the he
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Section 4 It should be noted that n
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CODi [µm] 40 30 20 10 0 Section 4
Page 47 and 48: Section 4 Table 4.4. Results of the
Page 49 and 50: CODi [µm] COD i [µm] CODi [µm] 2
Page 51 and 52: Cast MMCs 10%-RT 10%-8h 10%-24h 10%
Page 53 and 54: COD vi [µm] COD vi [µm] CODvi [µ
Page 55 and 56: CODvi [µm] CODvi [µm] 80 60 40 20
Page 57 and 58: Section 4 Table 4.6. The values of
Page 59 and 60: σ p max [MPa] σ p max [MPa] σ p
Page 61 and 62: σ p max [MPa] σ p max [MPa] 1200
Page 63 and 64: σ interf max [MPa] σ interf max [
Page 65 and 66: Section 4 4.5. The relation between
Page 67 and 68: Section 4 Table 4.9. Conditions for
Page 69 and 70: Ln(Ln(1/(1-F σ))) Ln(Ln(1/(1-F σ)
Page 71 and 72: Section 5 diameter of 2…8 µm and
Page 73 and 74: MnS-inclusion r θ MnS-inclusion Se
Page 75 and 76: σ interf [MPa] σ interf [MPa] σ
Page 77 and 78: Section 6 where n is the number of
Page 79 and 80: Ji [kN/m] J i [kN/m] 25 20 15 10 5
Page 81 and 82: Section 7 7.2. A model by Tan and Z
Page 83 and 84: Section 7 its initial length, li. O
Page 85 and 86: 1 µm Section 7 5 µm Fig. 7.3. A s
Page 87 and 88: Section 7 a) b) c) Fig. 7.4. Micros
Page 89 and 90: Section 7 It is important that no p
Page 91 and 92: Section 7 between the Rtot and the
Page 93 and 94: Section 7 Fig. 7.8. The fracture su
Page 95 and 96: 10 z [mm] 5 -10 -15 Section 7 0 -20
Page 97: Section 7 Table 7.3. Data on measur
Page 101 and 102: Section 8 The effect of the homogen
Page 103 and 104: Table 9.1. Data on stress for Speci
Page 113 and 114: Table 9.11. Data on stress for Spec
Page 119 and 120: Table 9.17. Results of the stereoph
Page 121 and 122: 1 2 1 2 3 3 4 5 6 5 9 119 7 7 8 9 6
Page 123 and 124: 8 9 10 6 10 122 11 12 13 14 11 12 1
Page 125 and 126: 9 10 11 12 13 124 50 µm Fig. 9.6.
Page 127 and 128: 1 2 1 2 3 3 4 4 126 5 5 6 9 7 50 µ
Page 129 and 130: 1 1 2 2 3 4 3 4 5 5 128 6 7 6 7 50
Page 131 and 132: Fig. 9.12. Fracture surface with ma
Page 133 and 134: 1 2 3 1 2 3 4 5 6 7 8 9 10 4 5 132
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Fig. 9.30. Fracture surface with ma
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Fig. 12.36. Fracture surface of the
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[14] Lewandowski JJ, Liu C, Hunt WH
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[42] Rice JR, Rosengren GF. Plane s
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[70] ASTM E1820-01, “Standard Tes
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