Twin formation from a twin boundary in Mg during in-situ nanomechanical testing

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 759; pp. 142 - 153
• Main Authors: Jiang, Lin, Kumar, M. Arul, Beyerlein, Irene J., Wang, Xin, Zhang, Dalong, Wu, Chuandong, Cooper, Chase, Rupert, Timothy J., Mahajan, Subhash, Lavernia, Enrique J., Schoenung, Julie M.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 24.06.2019; Elsevier BV
• Subjects: Characterization; Coalescing; Deformation; Dislocations; Electrons; Embryos; Grains and interfaces; Magnesium alloys; Modeling/simulations; Nanoindentation; Plasticity; Questions; Single crystals; Stress distribution; Three dimensional models; Twin boundaries; Twinning; Characterization; Modeling/simulations; Grains and interfaces; Magnesium alloys; Plasticity
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2019.04.117

An important fundamental question regarding deformation twinning is whether it is possible for twins to nucleate at boundaries or interfaces when specific stress fields are present. A corollary that follows from this question is: if this is indeed possible, what determines the proper stress field and how does it occur at the nanoscale? Here, we demonstrate the application of an in-situ nanoindentation approach to confine and dynamically capture the stages in the formation of a deformation twin at an internal twin boundary in single crystal Mg. We observe the formation of contraction twin embryos at the pre-existing extension twin boundary, and the subsequent propagation of the twin embryos into the crystal. We reveal an intermediate step, involving the coalescence of tiny embryos into a larger embryo before the nucleus emanates into the crystal. De-twinnning of the twin embryos is captured during unloading and shown to leave a remnant nanosized twin (<10 nm) after complete unloading. A 3D full-field, crystal plasticity model identifies that the twin type and variant selection of the stable embryo are governed by the internal local stress state prevailing at the pre-existing twin boundary. The possible role played by <c+a> dislocations and boundary structure (incoherent vs. coherent) in embryo formation, as suggested by the TEM and modeling analyses, are discussed.