High Performance Steels: Damage and Fracture in Steels
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Ana Araujo, Vesuvius USA; C. Tasan, Massachusetts Institute of Technology; Jonah Kleem-Toole, Colorado School of Mines; Louis Hector, General Motors Global Technical Center; Tilmann Hickel, Bam Federal Institute For Materials Research And Testing; Benjamin Adam, Oregon State University
Wednesday 2:00 PM
March 2, 2022
Room: 252C
Location: Anaheim Convention Center
Session Chair: Kayla Molnar, Los Alamos National Laboratory; Louis Hector, General Motors
2:00 PM
In-situ Investigation of Strain Partitioning and Localization in a Dual Phase Steel Up to and Beyond Necking: Hyunseok Oh1; Krista Biggs1; Onur Güvenç1; Hassan Ghassemi-Armaki1; Narayan Pottore1; Cemal Cem Tasan1; 1Massachusetts Institute of Technology
Strain partitioning and localization processes are triggered during the deformation of multi-phase metallic materials. However, microscopic digital image correlation is typically limited to low strain levels (< ~15%) due to surface topography evolution, which hinders the understanding of strain partitioning and localization processes at higher strain levels. Here, we address these challenges by developing a method that relies on serial in-situ scanning electron microscope mechanical tests of samples pre-strained to different levels. Applying this method to a dual-phase steel, we observe that ferrite and martensite on average exhibit a linear strain partitioning trend throughout the deformation even beyond necking. Strain localization bands with different microscopic strain paths develop in the microstructure, and the frequency of these bands differs at different stages of deformation. Martensitic constituents play an important role in the strain localization processes and the resulting microscopic strain paths.
2:20 PM
Strain Path Dependence of Microstructural Strain Path Development: An Experimental-numerical Study on Dual-phase Steels: Julian Rackwitz1; Onur Güvenç1; Cem Tasan1; 1Massachusetts Institute of Technology
In-situ SEM experiments revealed that microstructural strain paths can differ significantly from macroscopically imposed paths. For example, micro-level local strain paths ranging from uniaxial tension to plane-strain tension can be seen in a single material when the macroscopically imposed strain path is the former. Our previous work revealed the importance of local martensitic configurations on this behavior in dual-phase (DP) steels deformed in uniaxial tension. For a better understanding of the damage behavior in this material, this phenomenon should be investigated under multiaxial loading conditions as well. To fill this gap, we present here experiments using a custom-designed bulge test setup, and supporting crystal plasticity simulations, which enable us to discuss the influence of multiaxial loading conditions on the microstructural strain path development and, in turn, in the ductile damage process.
2:40 PM
Microstructure and Mechanism Based Lifetime Predictions in Various Weldment Failures Under Complex Thermomechanical Conditions: Yi Yang1; Yanfei Gao1; Jorge Penso2; Zhili Feng3; 1University of Tennessee Knoxville; 2Shell; 3Oak Ridge National Laboratory
Stress relief cracking and other high-temperature failures on/near weldments are a major issue dictating structural integrity and performance. Complex synergetic factors affect the generation/evolution of welding residual stress, which in turn controls the lifetime. Various mechanisms have been proposed in the past, but lacking a direct and quantitative connection between laboratory tests and actual lifetime prediction. Two critical issues are identified and addressed by our research team. 1)a quantitative prediction of welding residual stress is very sensitive to the dynamic strain-hardening behavior in the material constitutive law. Characterization of such property and also non-destructive residual stress measurements by advanced diffraction techniques are needed. 2)microstructure-based lifetime prediction framework has been developed that simulates the entire processes of intergranular cavity nucleation and growth, together their coalescence into grain boundary cracks. The roles of precipitate kinetics on/near grain boundaries and stress relaxation due to precipitate-dependent viscoplastic deformation can also be incorporated and investigated.
3:00 PM
Strain Partitioning Analysis during Tensile Tests in Intercritically Deformed Steels: Unai Mayo1; Nerea Isasti1; Jose Rodriguez-Ibabe1; Pello Uranga1; 1CEIT and TECNUN (University of Navarra)
Different characterization techniques in the macro, meso and microscale were applied to analyze the strain partitioning during a tensile test in an intercritically deformed Nb-V microalloyed steel. A grain identification and correlation technique based on EBSD was developed together with a discretization methodology, enabling the distinction between different ferrite populations (non-deformed ferrite, NDF, and deformed-ferrite, DF, grains). Nanoindentation technique was applied in the initial microstructure to measure the nanohardness for both ferrite families and the microstructural evolution during the interrupted tensile tests was monitored using EBSD scans. The grain boundary and Kernel Misorientation maps show a different evolution of low- and high-angle grain boundary during tensile tests for different initial NDF/DF balances. NDF population accommodates most of the deformation at initial strain intervals. For higher strains, NDF and DF grains behave similarly to the strain applied.
3:20 PM Break
3:35 PM
Tensile Behaviour and Martensitic Transformations in a Cold Rolled Medium Mn Steel: Thomas Kwok1; 1Imperial College London
Medium Mn steels (4-12 wt% Mn) have shown to exhibit a combined Twinning Induced Plasticity and Transformation Induced Plasticity (TWIP+TRIP) effect, allowing for large elongations (>30%) and high strain hardening rates (2-3 GPa). However, the tensile behaviour of cold rolled medium Mn steels has not been investigated in detail. In this study, tensile tests were conducted on a novel 8 wt% medium Mn steel in the as-annealed and cold rolled conditions. Compared to the as-annealed steel, the cold rolled steels (10-20% reduction) showed a large increase in strength and strain hardening rate without a significant loss in ductility. Microstructural changes during plastic deformation were investigated using Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). It was found that at a similar equivalent strain, cold rolling introduced a larger density of defects compared to uniaxial tensile strain, resulting in increased transformation from austenite to martensite.