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RESEARCH PAPERS

# Crystal Plasticity Based Fe Model for Understanding Microstructural Effects on Creep and Dwell Fatigue in Ti-6242

[+] Author and Article Information
Gayathri Venkataramani, Dhyanjyoti Deka, Somnath Ghosh

Computational Mechanics Research Laboratory, Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43202

J. Eng. Mater. Technol 128(3), 356-365 (Feb 28, 2006) (10 pages) doi:10.1115/1.2204942 History: Received October 17, 2005; Revised February 28, 2006

## Abstract

This paper is aimed at identifying key microstructural parameters that play important roles in the failure initiation of polycrystalline $Ti-6242$ subjected to creep and dwell loading. A finite element model, incorporating rate dependent elastocrystal plasticity, is developed for analyzing evolving variables in material microstructure. The crystal plasticity parameters are characterized by a combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process involving Genetic algorithms (Ga). Accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscope scans are incorporated in the computational model of polycrystalline $Ti-6242$ for constant strain rate, creep, and dwell tests. The computational model is used for the identification of possible microstructural variables that may result in local crack initiation. Basal normal stress, equivalent plastic strain, and stress in loading direction are considered as candidate parameters, of which the former is chosen as most probable from results of creep and dwell experiments and simulations. Creep induced load shedding phenomena is observed to lead to high value stresses that cause failure. The role of grain orientation with respect to the loading axis and misorientation with its neighbors, in causing load shedding and stress localizations is explored.

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## Figures

Figure 7

Validation of the Ti-6242 computational model with experimental results for constant strain rate test

Figure 8

Validation of the Ti-6242 computational model with experimental results for tension creep

Figure 9

Evolution of basal normal stress with time for four grains with high σnb in tension creep

Figure 2

Schematic diagrams showing (a) the nonorthogonal basis and slip systems in a hcp crystal and (b) the orthogonal basis and slip systems in bcc crystals

Figure 3

Orientation assignment to the finite element mesh: (a) experimentally observed (0001) pole figure with 14,799 points; (b) OPAM simulated pole figure with 2744 points; (c) FE model showing element orientations; and (d) OIM image produced by EBSD scan

Figure 4

Comparison of crystallographic texture using Rodrigue’s vector representation (a) from orientation imaging microscopy, and (b) from OPAM based simulations. A is the density of Rodrigue’s vector points.

Figure 5

Histograms of the number fraction of experimental and simulation misorientation distribution: (a) experimental misorientation distribution; (b) simulated MOD with OPAM+MPAM based orientation

Figure 6

Histograms of the number fraction of grains as a function of the fraction of their neighbors having a low misorientation (<15deg): (a) experimental microtexture from OIM; (b) simulated final microtexture with OPAM+MPAM+MTPAM

Figure 10

Evolution of basal normal stress in the neighbors of grains with high basal normal stresses: (a) grain 1 and (b) grain 2 in Fig. 9

Figure 1

(a) Microstructure of a forged α+βTi-6242 alloy consisting of transformed β (dark phase) colonies in a matrix of equiaxed primary α grains (light phase) and (b) schematic of a constituent transformed β colony

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