Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575°C and 675°C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening ''flat'' curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.

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We study microstructure selection during during directional solidification of a thin metallic sample. We combine in situ X-ray radiography of a dilute Al-Cu alloy solidification experiments with three-dimensional phase-field simulations. We explore a range of temperature gradient G and growth velocity V and build a microstructure selection map for this alloy. We investigate the selection of the primary dendritic spacing ⇤ and tip radius ⇢. While ⇢ shows a good agreement between experimental measurements and dendrite growth theory, with ⇢ ⇠ V 1/2 , ⇤ is observed to increase with V (@⇤/@V > 0), in apparent disagreement with classical scaling laws for primary dendritic spacing, which predict that @⇤/@V < 0. We show through simulations that this trend inversion for ⇤(V ) is due to liquid convection in our experiments, despite the thin sample configuration. We use a classical di↵usion boundary-layer approximation to semi-quantitatively incorporate the e↵ect of liquid convection into phase-field simulations. This approximation is implemented by assuming complete solute mixing outside a purely di↵usive zone of constant thickness that surrounds the solid-liquid interface. This simple method enables us to quantitatively match experimental measurements of the planar morphological instability threshold and primary spacings over an order of magnitude in V . We explain the observed inversion of @⇤/@V by a combination of slow transient dynamics of microstructural homogenization and the influence of the sample thickness.

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