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PREDICTIVE SCIENCE AND TECHNOLOGY IN MECHANICS AND MATERIALS

Modeling of Lath Martensitic Microstructures and Failure Evolution in Steel Alloys

[+] Author and Article Information
T. M. Hatem

Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695-7910

M. A. Zikry

Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695-7910zikry@ncsu.edu

J. Eng. Mater. Technol 131(4), 041207 (Sep 01, 2009) (10 pages) doi:10.1115/1.3183780 History: Received February 20, 2009; Revised June 23, 2009; Published September 01, 2009

A multiple-slip dislocation-density-based crystalline formulation, specialized finite-element formulations, and Voronoi tessellations adapted to martensitic orientations were used to investigate dislocation-density activities and crack tip blunting in high strength martensitic steels. The formulation is based on accounting for variant morphologies and orientations, retained austenite, and initial dislocations densities that are uniquely inherent to martensitic microstructures. The effects of variant distributions and arrangements are investigated for different crack and void interaction distributions and arrangements. The analysis indicates that for certain orientations related to specific variant block arrangements, which correspond to random low angle orientations, cracks can be blunted by dislocation-density activities along transgranular planes. For other variant block arrangements, which correspond to random high angle orientations, sharp crack growth can occur due to dislocation activities along intergranular planes.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

The alignment of variants in space, relative to each other and others planes

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Figure 2

Lath martensite hierarchical microstructure, defined in four scales architecture: parent austenite grains, packets, blocks, and laths

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Figure 3

Microstructural model used in the current study; (a) the distribution of variants in blocks and packets for low angle GBs specimen (Model 1), variants numbered as in Table 1; (b) the distribution of variants in blocks and packets for high angle GBs model (Model 2); (c) loading, geometry, and boundary conditions

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Figure 4

Normalized mobile dislocation densities for most active slip systems for low angle GBs; (a) slip system (112)/[111¯] and (b) slip system (1¯12)/[11¯1] at a nominal strain of 7.5%. Values normalized by the mobile dislocation density saturation value ρm,s(α)=6.86×1014 m−2

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Figure 5

Normalized immobile dislocation densities for most active slip systems in low angle GBs specimen; (a) slip system (112)/[111¯] and (b) slip system (1¯12)/[11¯1] at a nominal strain of 15%. Values are normalized by immobile dislocation-densities saturation values ρim,s(α)=1.16×1016 m−2

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Figure 6

Behavior for low angle GBs, at a nominal strain of 7.5%; (a) shear slip contours γ, (b) lattice rotation in degrees, (c) normalized normal stresses, (d) normalized shear stresses; stress values are normalized by the static yield stress

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Figure 7

[010]γ upper hemisphere stereographic projection for variant no. 3 slip system relative to the loading direction, the locus of maximum resolved shear stress (the inner circle) and the long direction of the variants, denoted as  ∗ in (a) the slip directions and (b) the slip planes

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Figure 8

Normalized mobile dislocation densities for most active slip systems for high angle GBs for (a) slip system (112)/[111¯] and (b) slip system (1¯12)/[11¯1] at a nominal strain of 15%

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Figure 9

Normalized immobile dislocation densities for most active slip systems for high angle GBs for (a) slip system (112)/[111¯] and (b) slip system (1¯12)/[11¯1] at a nominal strain of 15.

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Figure 10

Results obtained for the high angle GBs specimen, at a nominal strain of 7.5% (a) shear slip contours γ, (b) lattice rotation in degrees, (c) normalized normal stresses, and (d) normalized shear stresses

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Figure 11

Microstructural model and void distribution

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Figure 12

Shear slip around the crack tip indicating crack tip microvoid interactions for low angle GBs (a) and (b), and high angle GBs (c) and (d)

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Figure 13

Normal stresses around the crack tip indicating crack tip microvoid interactions for low angle GBs (a) and (b), and high angle GBs (c) and (d)

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