Heat treatment of additively manufactured Al-Ce based multicomponent alloys leads to complex microstructure evolution. In this research, the ability to extend the phase transformation theories ...involving nucleation of a product phase from a heterogeneous multi-phase microstructure typical to that of additively manufactured samples is explored. The Al-10Ce-8Mn (wt%) was used as a model alloy system. Under additive manufacturing conditions different solidification microstructures were obtained due to spatial and temporal variations of thermal gradients (G) and liquid-solid interface velocities (R) within a given melt pool. Near the melt pool boundary (high G and low R, referred as MPB region), initially, Al20Mn2Ce forms from the liquid followed by a eutectic of FCC Al and Al11Ce3. In the melt pool interiors (low G and high R referred as ES region) a eutectic structure between FCC Al and Al20Mn2Ce is observed. During subsequent heat treatments, the MPB and ES regions transform into different sets of microstructures. In the MPB region, a fine globular microstructure containing FCC Al, Al11Ce3, Al6Mn, and Al12Mn results from the decomposition of Al20Mn2Ce. In the ES region a faceted Al51Mn7Ce4 plate phase results from the decomposition of Al20Mn2Ce. The formation of the Al51Mn7Ce4 phase within the eutectic microstructure at the boundaries of FCC Al and Al20Mn2Ce has not been reported in the literature. These two distinct phase transformation pathways are rationalized based on the role of driving force on the nucleation of (Al6Mn) and/or metastable intermetallic (Al51Mn7Ce4) phases at the interface of aluminum (FCC) and the non-equilibrium intermetallic (Al20Mn2Ce) phases.
•Phase decomposition in an additively manufactured Al-Ce-Mn alloy.•Al20Mn2Ce measured as non-stoichiometric compound.•Local Equilibrium controlling decomposition pathway.
While many perovskites remain crystalline during the oxygen evolution reaction (OER) in alkaline media, some highly active perovskites become amorphous. We studied the local structure changes of ...perovskites LaCoO3, Ba0.5Sr0.5Co0.8Fe0.2O3‑δ, and SrCo0.8Fe0.2O3‑δ before and after OER by X-ray absorption spectroscopy. No change in either local structure or OER activity was observed for LaCoO3, while considerably enhanced OER activities and the conversion of the local structure from corner-sharing octahedra to edge-sharing octahedra were noted for Ba0.5Sr0.5Co0.8Fe0.2O3‑δ and SrCo0.8Fe0.2O3‑δ as a result of the OER. Possible processes responsible for the structural change and enhanced OER activities are discussed.
Fe–C–Mn–Al steels have the potential to substitute for commercial Ni–Cr stainless steels. For the development of Fe–C–Mn–Al stainless steels, phase transformations play an important role. Our methods ...of studying the phase transformations of the steel include heating, cooling, and/or annealing. The results of our study show that spinodal decomposition, an atomic ordering reaction and the transformation of the L12 phase to kappa-carbide occur in the Fe–C–Mn–Al steel. After cooling, the austenite decomposes by the spinodal mechanism into solute-lean and solute-rich austenite phases. The solute-rich austenite phase also transforms into the L12 phase via the ordering reaction upon cooling to lower temperatures. After quenching and prolonged annealing, the L12 phase grows in the austenite and finally transforms into kappa-carbide. This L12 phase to kappa-carbide transformation has not been observed previously.
Pressure‐induced phase transformations (PIPTs) occur in a wide range of materials. In general, the bonding characteristics, before and after the PIPT, remain invariant in most materials, and the bond ...rearrangement is usually irreversible due to the strain induced under pressure. A reversible PIPT associated with a substantial bond rearrangement has been found in a metal–organic framework material, namely tmenH2Er(HCOO)42 (tmenH22+=N,N,N′,N′‐tetramethylethylenediammonium). The transition is first‐order and is accompanied by a unit cell volume change of about 10 %. High‐pressure single‐crystal X‐ray diffraction studies reveal the complex bond rearrangement through the transition. The reversible nature of the transition is confirmed by means of independent nanoindentation measurements on single crystals.
A reversible pressure‐induced phase transformation associated with a substantial bond rearrangement is discovered in the metal–organic framework tmenH2Er(HCOO)42 (tmenH22+=N,N,N′,N′‐tetramethylethylenediammonium). The transition is first‐order and is accompanied by a unit cell volume change of about 10 %. X‐ray diffraction studies reveal the complex bond rearrangement process.
Electron-electron interactions can render an otherwise conducting material insulating, with the insulator-metal phase transition in correlated-electron materials being the canonical macroscopic ...manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator-metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator-metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.
•Short review on phase field modeling of martensitic phase transformation.•Three different phase field approaches were identified for modeling martensitic transformation.•No phase field simulation ...was identified for tetragonal to monoclinic transformation.
In the last few decades, the phase field method has shown tremendous capabilities of predicting microstructure evolutions at the mesoscale scale. This method was widely used for modeling martensitic phase transformation, where the displacive character was a challenging problem for the counterpart sharp interface approach. Martensitic phase transformation, which is an invariant plane stress twinning, drives a myriad of phase transition phenomena of paramount importance to many structural applications. This article provides a literature review of the past phase field modeling studies used to capture the formation and growth of martensite.
Mg-Li-Al alloys with a body-centred cubic (BCC) structure can exhibit exceptional specific strengths in combination with excellent ductility and corrosion resistance. In general, the strength of ...these alloys is very sensitive to the processing temperature due to the occurrence of various phase transformations. Although different phases have been identified in these alloys, their corresponding transformation mechanisms and unique role played in controlling the mechanical properties have never been studied in depth. In this work, we identified the phase transformation sequence by in-situ synchrotron X-ray diffraction. Moreover, we investigated the evolution of precipitation and their morphology using transmission and scanning electron microscopy, together with simulations based on the phase field modelling and first-principles calculations. Phase transformation sequence of Al-rich zone → θ (D03−Mg3Al) → AlLi was confirmed during anisothermal ageing. A braided structure resulting from spinodal decomposition was found to be the optimized microstructure for achieving the peak strength. Nanocrystalline α-Mg phase at the interface between θ and the matrix was identified as the main reason for softening in the alloy. The core-shell model for θ → AlLi transformation is observed and verified. Our findings deepen the understanding of BCC Mg-Li-Al alloys and pave a pathway to develop new generation of ultralight alloys with stronger strength and better stability.
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Strain-induced phase transformation studies have been conducted in a Ti–6Al–2Sn–4Zr–6Mo alloy. This alloy was subjected to β sub-transus deformation upon slow cooling from the β-phase field and the ...effect of strain on the extent and morphology of the phase transformation during hot-deformation was determined. In addition, the transformation kinetics and the morphology of the newly formed α-phase were studied following deformation under different cooling conditions. By applying strain, the rate of the β-to-α phase transformation increased significantly during deformation as well as during slow cooling following deformation. This increase in the kinetics of the β-to-α transformation can be ascribed to strain-induced phase transformation. Also, the extent of deformation of the β-phase had a marked effect on the resulting morphology and size of the newly formed α-phase. Transformation of un-deformed β-phase rendered α-phase of acicular morphology only. However, following deformation of the β-phase, acicular as well as globular α-phase morphologies have been observed upon cooling.
Molecular dynamics simulations are performed to investigate temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium shape-memory alloys. Our results provide detailed ...insights into the origins of the experimentally reported characteristics of phase transformations at the nanoscale, such as the decrease of the transformation temperature with grain size and the disappearance of the plateau in the stress-strain response. The relevant atomic scale processes, such as nucleation, growth, and twinning are analyzed and explained. We suggest that a single, unified mechanism—dominated by the contribution of a local transformation strain—explains the characteristics of both temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium.
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