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Process Modeling of Microtextured Region Breakdown in Ti-6242 to Improve Dwell Fatigue Properties

Goal: The titanium alloy Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) has been the structural material of choice for use in high-pressure compressors for gas turbine engines of aircraft due to its high strength-to-weight ratio and excellent high temperature mechanical properties. However, Ti-6242 is susceptible to dwell fatigue at low temperature due to crack growth on low-angle boundaries along the primary α grains, in particular small faceted cracks that link up to form a “quasi-cleavage” surface. These low-angle boundaries occur within so-called microtextured regions (MTR) that consist of many neighboring primary α grains with similarly oriented c-axes of the hexagonal close-packed atomic lattice. In-flight failures of turbine blades made from these and other titanium alloys have led to investigation of the prevalence of MTR in forged parts. Post-mortem observations of specimens have revealed facet clusters along the fracture surface within the MTR. In this work, the crystal plasticity finite element method in WARP3D is used to model the processing of MTR within Ti-6242. Several configurations of MTR within representative microstructures have been simulated under single and multiple loading axes to determine the critical strain to break down the MTR and introduce more disorientation into the microstructure. These and ongoing computational studies provide a complement to experimental data from AFRL and mill processors that demonstrate a load direction dependence on the texture evolution of this material.

Date: 1 May 2016 - 30 August 2019

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Timothy J. Truster
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The titanium alloy Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) has been the structural material of choice for use in high-pressure compressors for gas turbine engines of aircraft due to its high strength-to-weight ratio and excellent high temperature mechanical properties. However, Ti-6242 is susceptible to dwell fatigue at low temperature due to crack growth on low-angle boundaries along the primary α grains, in particular small faceted cracks that link up to form a “quasi-cleavage” surface. These low-angle boundaries occur within so-called microtextured regions (MTR) that consist of many neighboring primary α grains with similarly oriented c-axes of the hexagonal close-packed atomic lattice. In-flight failures of turbine blades made from these and other titanium alloys have led to investigation of the prevalence of MTR in forged parts. Post-mortem observations of specimens have revealed facet clusters along the fracture surface within the MTR. In this work, the crystal plasticity finite element method in WARP3D is used to model the processing of MTR within Ti-6242. Several configurations of MTR within representative microstructures have been simulated under single and multiple loading axes to determine the critical strain to break down the MTR and introduce more disorientation into the microstructure. These and ongoing computational studies provide a complement to experimental data from AFRL and mill processors that demonstrate a load direction dependence on the texture evolution of this material.