In the present work, Co-Cr-Mo alloy compacts with a unique bimodal microstructural design, harmonic structure design, were successfully prepared via a powder metallurgy route consisting of controlled mechanical milling of pre-alloyed powders followed by spark plasma sintering. The harmonic structured Co-Cr-Mo alloy with bimodal grain size distribution exhibited relatively higher strength together with higher ductility as compared to the coarse-grained specimens. The harmonic Co-Cr-Mo alloy exhibited a very complex deformation behavior wherein it was found that the higher strength and the high retained ductility are derived from fine-grained shell and coarse-grained core regions, respectively. Finally, it was observed that the peculiar spatial/topological arrangement of stronger fine-grained and ductile coarse-grained regions in the harmonic structure promotes uniformity of strain distribution, leading to improved mechanical properties by suppressing the localized plastic deformation during straining.

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In the pursuit of creating new structures with improved overall mechanical properties, we produced a duplex stainless steel with both high strength and good plasticity, which combines in a harmonized way a refined microduplex structure with a coarse structure. This is accomplished by applying a mixed process of mechanical milling followed by spark-plasma sintering for a duplex steel powder, leading to a microstructure with a gradual and continuous transition between coarse and ultra-fine-grained regions at the microscale. This type of structure is referred to as Harmonic. The grain refinement at the surface of the milled powder particles occurs through the recrystallization of the severely-deformed surface layers during sintering, and results in a microduplex structure which gradually transitions to a coarse duplex structure away from the particles surface. Both the recrystallization mechanism and the effect of this refined structure on the sinterability and final mechanical properties of the compacts are investigated.

The process of grain refinement in Ni 3 Al by high-pressure torsion (HPT) was investigated up to 100 turns. The entire diametric sections of the deformed samples were analyzed by optical microscopy and image processing methods in order to evaluate both the spatial distribution and the volume fractions of the nanocrystalline and coarse grains. A thick band of nanocrystalline phase was formed in the middle section of the samples, and a structure containing mainly coarser ordered fragments was present in the vicinity of the top and bottom surfaces. Pseudotwinning along {111} was observed at the boundaries of coarse-fragmented grains as well as the inside of the fragments and is put forward as a possible mechanism for disordering and nanocrystalline structure formation in Ni 3 Al.

Samples of Ni 3 Al intermetallic compound were subjected to deformation by high-pressure torsion (HPT). The plastically-deformed structure revealed a bimodal character: coarse grains, retaining a degree of long-range order, surrounded by regions of nanocrystalline, disordered grains. It was inferred that the grain refinement proceeds in an inhomogeneous manner throughout the sample. Grains as large as 100 nm in size were shown to contain only a low density of perfect dislocations, but a large density of nanotwins and stacking faults. These planar defects appeared to originate from the grain boundaries, suggesting that grain boundaries are active sources for Shockley-partial dislocations. Their formation is accompanied by a deviation of the microhardness dependence on grain size from the Hall-Petch behavior, potentially suggesting the activation of a deformation mechanism different from the one acting in coarse structures. The hardness saturates at a significantly larger grain size than in the case of nanostructured pure Ni.

Polycrystalline nickel‐based superalloys for turbine disc applications typically employ complex alloy chemistry in order to produce a high volume fraction of gamma‐prime (γ′) precipitates for the optimisation of mechanical properties [1]. The precipitate coarsening causes a gradual loss of coherency between γ′ precipitates and γ matrix when materials serving elevated temperatures, therefore resulting in the degradation of its mechanical performance [2]. In this work, we report new experimental observations for diffusion‐mediated secondary γ′ precipitate coarsening (See Fig. 1) within a near‐zero misfit alloy RR1000 in a cyclic manner that these precipitates coarsen and split periodically [3]. Using absorption‐corrected energy‐dispersive X‐ray (EDX) spectroscopy within the scanning transmission electron microscope (STEM) [4], compositional variations for secondary γ′ precipitates as a function of coarsening behaviour under have been investigated. We have observed clear cyclic variations in the elemental concentrations of Co, Ti and Al within the secondary γ′ as a function of ageing time. STEM/EDX spectrum imaging and electron tomography on individual secondary γ′ have revealed local enrichment of Co within the core of secondary γ′ (See Fig. 2). STEM‐EDX analysis of the γ‐γ′ interface revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix region near the γ‐γ′ interface (See Fig. 3). Our experimental results, coupled with complementary modelling and synchrotron X‐ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, we show that elemental inhomogeneities, produced within both matrix and precipitates, are induced by complex interactions between thermodynamics and diffusion kinetics. These elemental inhomogeneities will likely affect the kinetics of coarsening and therefore must be taken into account when predicting the microstructure likely to be produced when the material is exposed to different heat treatment regimes. More generally, our findings suggest the importance of considering diffusion kinetics when attempting to understand the microstructural evolution of advanced superalloys. Our discovery renders the potential to retain the overall γ‐γ′ coherence in nickel‐based superalloys when exposed to elevated temperatures, and therefore to improve its creep properties.