Deformation of lamellar FCC-B2 nanostructures containing Kurdjumov-Sachs interfaces: Relation between interfacial structure and plasticity

摘要

Dual-phase lamellar microstructures containing alternating regions of plastically soft and hard phases are known to produce alloys with exceptional combination of strength and ductility. Here, by coupling high-resolution transmission electron microscopy and molecular dynamics (MD) simulations, we have investigated the deformation mechanisms prevalent in lamellar microstructures with soft fcc and harder bcc-ordered intermetallic B2 whose interfaces follow the Kurdjumov-Sachs (KS) orientation relationship. We have identified two key structural features at such an fcc/B2 KS interface. The mating KS-fcc (111) interfacial plane contains periodically arranged 1/6 < 112 >(fcc) predominantly screw partial dislocations that are separated by extended dislocation "core-overlap" regions. The KS interface also contained steps and ledges with several steps exhibiting fcc-B2 lattice continuity between the {111}(fcc) and {011}(B2). The effects of such interfaces on the uniaxial deformation of fcc-B2 multicrystal nanostructures, as a function of lamellae thickness, were studied using MD simulations. We observed that the screw-like interfacial partials facilitated the KS interfacial sliding and strain accumulation at the interphase interfaces, and reduced the yield strength of the composite material compared to a pure-fcc reference material. Deformation character depends on lamellae thickness. Thin B2 lamellae (similar to 4 angstrom) sheared via twinning to drastically lowered flow stress such that the flow-strength was comparable to the pure fcc constituent phase. In contrast, thicker B2 lamellae (similar to 12 angstrom) sheared via a slip-transfer mechanism, which allowed the fcc-B2 composite to maintain its flow-strength. Thus, the atomic structure of fcc/B2 KS interfaces was directly linked to dominant operative plastic deformation mechanisms.