Deformation Twinning and Transformations in Concentrated Solid-Solution fcc Fe- and Ni-Based Alloys Under Irradiation

The objective of this talk is to reveal the influence of irradiation defect microstructures on deformation twinning and deformation-induced phase transformations in face centered cubic (fcc) alloys. Concentrated solid-solution fcc alloys form the basis for the most ubiquitous structural materials used throughout society (e.g. stainless steels), and for novel alloy concepts such as high entropy alloys. When these alloys are exposed to far-from-equilibrium irradiation conditions, they evolve microstructures containing a complex distribution of point defects (vacancies and interstitials) and extended defects such as dislocation loops and voids. These irradiated microstructures are well known to cause hardening and embrittlement, which are generally ascribed to localized deformation via dislocation channeling. Meanwhile, the effects of the irradiated microstructure on lower-temperature deformation mechanisms – specifically twinning and martensitic phase transformations – have typically been overlooked. But recent studies have suggested that irradiation defects can activate twinning and phase transformations over a wider range of loading conditions (i.e. higher temperatures, lower strain rates). The present study uses simulations to reveal how vacancies and voids control the deformation mechanisms, and complementary experiments will illuminate irradiation + deformation microstructures.<br/><br/>We begin with molecular dynamics (MD) simulations of three fcc concentrated solid solutions: Fe-50%Ni, Ni-50%Ti, and Ni-20%Cr (all in at%). Four different simulation boxes are constructed for each composition: pristine, containing a distribution of vacancies, containing a void with radius 5 atoms, and containing a void with radius 8 atoms. In Fe-50%Ni, strain-induced martensitic transformations occur most readily in the voided simulations. Strain concentrates through dislocation pileup at voids, enabling martensite needle nucleation. In Ni-50%Ti, voids also enhance the tendency for martensitic transformations to occur, though to a lesser degree than in Fe-50%Ni. More critically, however, the transformation in Ni-50%Ti is stress-induced and is not associated with any dislocation plasticity. This allows for a reversible transformation in Ni-50%Ti, representative of the shape-memory effect observed in this class of alloys. Finally, in Ni-20%Cr, deformation is highly dependent upon the loading orientation. Phase transformations occur readily when loaded along the [001] direction, but are preceded by deformation twins when loaded along the [101] direction.<br/><br/>MD results will be used to explain experimental observations from nanoindentation and microcompression pillars conducted on irradiated 304L stainless steel and Ni-base Alloy 625. The 304L stainless steel is neutron irradiated to doses up to 28 displacements per atom (dpa) at 375^○C, and exhibits strain-induced martensite associated with cavities. The Alloy 625 is neutron irradiated to 1 dpa at 300-400^○C. Grain orientation-specific nanoindentation reveals deformation microstructures consistent with MD predictions.