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Research Papers

# Defect Occurrence and Modeling for the Thermomechanical Processing of Aerospace Alloys

[+] Author and Article Information
S. L. Semiatin, T. J. Turner

Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson AFB, OH 45433-7817

P. D. Nicolaou

UES, Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432

J. P. Thomas

Universal Technology Corp., 1270 North Fairfield Road, Dayton, OH 45432

In backscattered-electron (BSE) images taken in a scanning-electron microscope (SEM), the alpha phase is dark and the beta (or martensitic alpha) phase is white (or gray).

Taylor factors were calculated assuming axisymmetric deformation and plastic flow controlled by the alpha phase for which the ratio of the critical resolved shear stresses for basal $⟨a⟩$, prism $⟨a⟩$, and pyramidal $⟨c+a⟩$ slip was 1:0.7:3 (33).

J. Eng. Mater. Technol 130(2), 021001 (Mar 12, 2008) (8 pages) doi:10.1115/1.2840958 History: Received June 28, 2007; Revised November 06, 2007; Published March 12, 2008

## Abstract

Mechanism-based models for the evolution of defects during the thermomechanical processing of aerospace titanium- and nickel-based alloys are reviewed. These defects include those comprising microstructural/metal-flow irregularities and those that are damage related (i.e., cracks and cavities). The development of undesirable/nonuniform microstructures and cavities during the mill processing of alpha/beta titanium alloys is addressed first. Relatively simple, diffusion-based models of spheroidization and coarsening are applied to quantify the propensity for microstructure nonuniformities. Similarly, first-order micromechanical models have been formulated to estimate the effect of local crystallographic texture on nonuniform flow, the generation of triaxial stresses, and cavity growth/closure in alpha/beta titanium alloys with a colony-alpha microstructure. The occurrence of nonuniform grain structures (and so-called ALA, or “as large as,” grains) in cast, wrought, and powder-metallurgy superalloys is also discussed. A physics-based model to treat the topology of recrystallization and the evolution of ALA grains in such materials is proposed.

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## Figures

Figure 1

Microstructures developed in Ti–6Al–4V during primary processing: (a) optical and (inset) SEM BSE micrographs of the colony-alpha microstructure and (b) SEM BSE micrograph of the fine equiaxed-alpha microstructure

Figure 2

Diffusion-controlled processes during the breakdown of the colony-alpha microstructure: (a) static coarsening of alpha plates, (b) static spheroidization of remnant alpha plates following hot working, and (c) classical static coarsening of equiaxed-alpha particles

Figure 3

Cavitation developed during hot tension testing of Ti–6Al–4V with a colony-alpha microstructure: (a) optical micrograph, (b) high-magnification SEM BSE image, and (c) electron-backscatter-diffraction (EBSD) inverse-pole-figure map indicating the presence of hard (basal-oriented) and soft colonies around the cavity. The tension axis is vertical in all micrographs.

Figure 4

Ratio of the cavity-growth parameter under nonuniaxial stress states to that for uniaxial tension (ηts∕η) as a function of the stress triaxiality (ratio of the mean stress σM to the effective stress σ¯). The data points (determined using hot torsion and notched tension tests on Ti–6Al–4V) (27-29) are compared to predictions from models of Rice and Tracey (30) and Pilling and Ridley (31).

Figure 5

Model results for cavitation in a Ti–6Al–4V pancake forging: (a) continuum FEM model predictions of the local stress state at the midheight as a function of effective strain and distance from the free surface and (b) a comparison of measurements and predictions of the average cavity size after a height reduction of 50% at 815°C. In (b), the sensitivity of the model predictions to input data is illustrated by varying ηts by ±25% relative to its nominal value (i.e., Q=0.75, 1, or 1.25).

Figure 6

Micromechanical-model predictions for cavitation in Ti–6Al–4V with a colony-alpha microstructure, which was hot pancake forged at 815°C: (a) stress triaxiality in adjacent hard and soft colonies at the equatorial free surface as a function of effective strain and the Taylor-factor ratio (Mh∕Ms) and (b) cavity size after a 50% reduction as a function of the Taylor-factor ratio and distance from the equatorial free surface. The cavity-size predictions in (b) are compared to measurements (data points).

Figure 7

Micrographs illustrating the effect of strain path on cavitation in samples of Ti–6Al–4V with a colony-alpha microstructure subjected to hot deformation at 815°C and an effective strain rate of 0.04s−1: (a) forward torsion to a surface effective strain of 0.99, or forward torsion to a surface effective strain of 0.99 followed by (b) reversed torsion to a surface effective strain of 0.25, or (c) uniaxial compression along the prior torsion axis to a height strain of −0.29. The torsion/compression axis is vertical, and the radial direction is horizontal in the micrographs.

Figure 8

Model predictions of the reduction in cavity volume fraction as a function of macroscopic effective strain during compression of Ti–6Al–4V with a colony-alpha microstructure following torsion to the indicated surface effective strains. Torsional prestraining and subsequent compression were both conducted at 815°C and an effective strain rate of 0.04s−1. The predictions are compared to experimental results (data points).

Figure 9

Mesoscale model for simulating the evolution of recrystallization nucleated at grain boundaries and intragranular particles in superalloy ingots with coarse columnar grains: (a) schematic illustration of the topology of recrystallization and (b) predicted size of the remnant, unrecrystallized (ALA) grains for the cases in which (i) the original topology was retained throughout the simulation (solid line) and (ii) the original topology was converted to a necklace-only one once the intragranular recrystallized regions had percolated throughout the initial ingot grains (broken line). The nucleation rate was one nucleus per 100μm2 of boundary and per unit strain, the grain-boundary velocity was 10μm per unit strain, and the volume of each nucleus was 1000μm3.

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