Influence of the Strain History on TWIP Steel Deformation Mechanisms in the Deep-Drawing Process
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TWIP steels attract interest globally due to their unique combination of strength (up to 1800 MPa UTS) and ductility (60–100 pct elongation).[1,2] The microstructure of TWIP steel is austenite (fcc) with low stacking fault energy (SFE) between 15 and 40 mJ/ m2[1] favorable for activation of twinning deformation mode and particular strain hardening behavior. Twin boundaries are effective in blocking dislocation motion, and at the same time they can act as slip planes to accommodate the dislocations.[2] It is widely understood that the balance between slip and twinning activity depends on the stacking fault energy[1] but the orientation dependence of preferable deformation modes is less investigated.
R. LAPOVOK, I. TIMOKHINA, A-K. MESTER, and M. WEISS are with the Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia. Contact e-mail: r.lapovok@ deakin.edu.au A. SHEKHTER is with the Aerospace Division, Defence Science and Technology, Fisherman’s Bend, VIC 3207, Australia. Manuscript submitted December 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
A study on the orientation dependence of deformation mechanisms in TWIP steel has been reported by Karaman et al.[3] using single crystals of Hadfield manganese steel. Tensile samples were deformed in three different crystallographic orientations, h001i, h111i, and h123i. Microscopic observation revealed that in crystals oriented along the [111] direction, twinning is the primary macroscopic deformation mode, while slip governs the macroscopic deformation in the crystals oriented along the [001] direction. Furthermore, a Hadfield steel single crystal was tested in two deformation modes, namely tension in [001] and 1510 directions and compression in [001], ½ 123; ½ 111 directions to study the competing mechanisms of slip and twinning.[4] A model for the twin nucleation stress as a function of the crystallographic orientation and stress direction was developed. Plastic deformation of a polycrystalline material will start either with slip or twinning in each crystal depending on the orientation of the crystal and the level of shear stress. If the shear stress on a slip system reaches the critical value ssrss (critical resolved shear stress), slip is initiated. According to Schmid’s law, uniaxial tension starts slip when the resolved shear stress on a closed packed plane reaches critical slip stress.[5] The slip directions for fcc crystals are h110i within {111} planes.
Twinning in fcc crystals occurs by shear on the (111) plane in the 112 direction with mirror shifted planes ð111Þ and reflected direction h112i.[6] Twinning nucleates by Shockley partial dislocations with a Burger vector [7] a Therefore, the 6 112 on {111} close-packed planes. resolved shear stress on the slip plane needs to exceed a critical value, stw rss , in order to move the twinning dislocation. As the resolved shear stress can be calculated by the Schmid’s law, often the criteria for the dominant deformation mechanism are based on the comparison of the Sc
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