245 research outputs found
Cohesive and magnetic properties of grain boundaries in bcc Fe with Cr additions
Structural, cohesive, and magnetic properties of two symmetric
and tilt grain boundaries (GBs) in pure bcc Fe and in dilute
FeCr alloys are studied from first principles. Different concentration and
position of Cr solute atoms are considered. We found that Cr atoms placed in
the GB interstice enhance the cohesion by 0.5-1.2 J/m. Substitutional Cr in
the layers adjacent to the boundary shows anisotropic effect on the GB
cohesion: it is neutral when placed in the (111) oriented Fe grains, and
enhances cohesion (by 0.5 J/m) when substituted in the boundary layer of
the (210) grains. The strengthening effect of the Cr solute is dominated by the
chemical component of the adhesive binding energy. Our calculations show that
unlike the free iron surfaces, Cr impurities segregate to the boundaries of the
Fe grains. The magnetic moments on GB atoms are substantially changed and their
variation correlates with the corresponding relaxation pattern of the GB
planes. The moments on Cr additions are 2-4 times enhanced in comparison with
that in a Cr crystal and are antiparallel to the moments on the Fe atoms
Modulated Martensite: Why it forms and why it deforms easily
Diffusionless phase transitions are at the core of the multifunctionality of
(magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain
effects under external fields are obtained in low symmetric modulated
martensitic phases. We outline the origin of modulated phases, their connection
with tetragonal martensite and consequences for their functional properties by
analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the
atomic to macroscale. Geometrical constraints at an austenite-martensite phase
boundary act down to the atomic scale. Hence a martensitic microstructure of
nanotwinned tetragonal martensite can form. Coarsening of twin variants can
reduce twin boundary energy, a process we could follow from the atomic to the
millimetre scale. Coarsening is a fractal process, proceeding in discrete steps
by doubling twin periodicity. The collective defect energy results in a
substantial hysteresis, which allows retaining modulated martensite as a
metastable phase at room temperature. In this metastable state elastic energy
is released by the formation of a 'twins within twins' microstructure which can
be observed from the nanometre to millimetre scale. This hierarchical twinning
results in mesoscopic twin boundaries which are diffuse, in contrast to the
common atomically sharp twin boundaries of tetragonal martensite. We suggest
that observed extraordinarily high mobility of such mesoscopic twin boundaries
originates from their diffuse nature which renders pinning by atomistic point
defects ineffective.Comment: 34 pages, 8 figure
- …