The Role of Grain Orientation and Grain Boundary Characteristics in the Mechanical Twinning Formation in a High Manganes
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THERE is an ongoing search for materials with enhanced performance that can meet industrial needs and requirements. This has led to the development of new materials that provide outstanding performance. Twinning-induced plasticity (TWIP) steel is an example, possessing ~ 1 GPa ultimate tensile strength with ~ 50 pct elongation.[1–3] This exceptional combination of mechanical properties is due to the mechanical twinning on straining, contributing to the plastic strain through continuous introduction of obstacles against dislocation glide and refinement of the microstructure. As a result, the mechanical twinning progressively reduces the dislocation mean free path via grain
VADIM SHTERNER, ILANA B. TIMOKHINA, and HOSSEIN BELADI are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3217, Australia. Contact email: [email protected] ANTHONY D. ROLLETT is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted August 15, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
refinement, enhancing the dislocation multiplication phenomenon known as the dynamic Hall–Petch effect.[4–8] Mechanical twinning nucleation is described, in most cases, as a heterogeneous process,[9–12] which presumably occurs at the places with imperfections in the lattice arrangement or at grain boundaries.[13] The most popular theory describes a reaction of dissociation of one full dislocation into two Shockley and Frank partial dislocations.[9,10] This reaction takes place only in materials with a certain range of stacking fault energy (SFE) (i.e., 20 to 50 mJ/m2).[9,10] It requires the operation of multiple slip systems[14] and pileup of the dislocations,[15–17] which occur to accommodate the imposed strain. Therefore, the extent of mechanical twinning is influenced by various parameters, namely, grain size,[15,17] strain rate,[18] grain orientation,[14,19,20] steel composition,[21,22] and deformation temperature,[23,24] the latter of which affects both SFE and, thus, the deformation mechanism. As mentioned previously, grain boundaries appear to be the most favorable nucleation sites for mechanical twinning, as they consist of dislocation or defect aggregates, and the abrupt orientation change across them leads to the stress localization on straining. Grain boundaries are highly anisotropic, depending on the
divergence in the atomic arrangement between two adjacent grains and the grain boundary plane character.[25–27] As demonstrated experimentally, the propensity for mechanical twinning strongly depends on the grain boundary misorientation angle for hexagonal-close-packed metals (e.g., Mg[28] and Zr[29]). The mechanical twins appear to nucleate more frequently on boundaries with low misorientation angles (i.e., in a range of 5 to 10 deg) in Mg. However, the twinning propensity progressively reduces with the misorientation angle to some extent that almost no twinning is observed in boundaries with a misorientation angle greater tha
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