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In a narrow temperature window in going from the isotropic to highly chiral orders, cholesteric liquid crystals exhibit so-called blue phases, consisting of different morphologies of long, space-filling double twisted cylinders. Those of cubic spatial symmetry have attracted considerable attention in recent years as templates for soft photonic materials. The latter often requires the creation of monodomains of predefined orientation and size, but their engineering is complicated by a lack of comprehensive understanding of how blue phases nucleate and transform into each other at a submicrometer length scale. In this work, we accomplish this by intercepting nucleation processes at intermediate stages with fast cross-linking of a stabilizing polymer matrix. We reveal using transmission electron microscopy, synchrotron small-angle X-ray diffraction, and angle-resolved microspectroscopy that the grid of double-twisted cylinders undergoes highly coordinated, diffusionless transformations. In light of our findings, the implementation of several applications is discussed, such as temperature-switchable QR codes, micro-area lasing, and fabrication of blue phase liquid crystals with large domain sizes., The arrangements of defect lines in liquid crystal blue phases provide promising templates for photonic crystals. Here Liu et al. investigate in detail how inter-phase transitions proceed, by intercepting nucleation with on-demand cross linking of a supporting polymer matrix.

The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

This paper outlines a single crystal plasticity theory for the mesoscale resolution of the diffusionless transformation process of deformation twinning. Unlike prevalent crystal plasticity models of lattice transformation that characterize volume fraction evolution of coexisting parent and product lattices at a material point, our model alternates between two deformation gradient decompositions, depending on whether or not the lattice is transforming, so that only a single lattice exists at a material point at all times. This approach permits a proper spatiotemporal resolution of the transformation, and further captures its associated residual lattice distortions. Our formulation was implemented as a VUMAT subroutine on the finite element solver ABAQUS\Explicit and used to model the twinning of a magnesium single crystal that was loaded under compression at different angles. As verified against multiple experiments from the literature, our model successfully characterizes for moderate strain: (a) the anisotropic stress-strain behavior of the Mg crystal, (b) the resulting spatial cross-hatch pattern of spindle shaped twins, and (c) the natural completion times of the twinning transformation. These results indicate that our mesoscale theory is capable of an in-depth spatiotemporal investigation of twinning and can be potentially extended to similar diffusionless transformation processes in single crystals.

Phase transformation in hydrogenated iron during depressurization from several gigapascals was investigated through in-situ synchrotron radiation X-ray diffraction and post-mortem electron backscatter diffraction measurements. The hydrogenated iron under 8.6 GPa at 293 K showed a double hexagonal close-packed (dhcp) structure, and it gradually transformed into a body-centered cubic (bcc) structure with decreasing pressure. The final crystal structure consisted entirely of a bcc phase. The structural change from dhcp to bcc structure was diffusionless-type phase transformation. The bcc phase showed lath morphology and could grow during aging under a constant pressure of 1.9 GPa, which indicated that it was bainitic-type transformation that required hydrogen diffusion or desorption.

Since the discovery of bainite, research over many decades has revealed a substantial amount of information about the mechanism of the bainite transformation in steels. Elements of the theory are now routinely being used in many parts of the world in the design of novel alloys and in the interpretation of a variety of experimental data. However, current experimental and theoretical understanding is limiting technological progress. The purpose of this atom probe tomography study was to track atom distributions during the bainite reaction in a nanocrystalline steel. The results are providing new experimental evidence on subjects critically relevant to the understanding of the atomic mechanisms controlling bainitic ferrite formation, such as the incomplete transformation phenomenon, the carbon supersaturation of ferrite, and the plastic accommodation of the surrounding austenite.

The transformation behaviour of shape memory alloys is influenced by lattice defects. Due to these defects the characteristic temperatures and mode of transformation can be varied. To examine the effect of dislocations on the transformation behaviour, CuZnAl and CuAlNi alloys were deformed plastically by different amounts in the stable austenitic and in the martensitic state. The induced dislocations can be arranged as random dislocation forests, dislocation groupings, subgrain boundaries, tangles and cell walls. LM and TEM investigations connected with DSC measurements were conducted to characterize changes in microstructure and functional properties. A critical amount of plastic deformation promotes a premartensitic solid-solid state reaction even in Cu-based SMA and retards or inhibits the martensitic transformation.

Study of a phase transformation of the kind BCC-to-18 R (51) 3 during water quenching of duplex Fe-Mn-Al-C alloys by means of the light microscopy (LM), scanning electron microscopy (SEM) and TEM

Differential scanning calorimetry and transmission electron microscopy have been used to study thermal fatigue due to diffusionless phase transformation cycling in pure cobalt. Thermal cycling through the allotropic (hcp ↔ fcc) transformation results in a temperature shift of the calorimetric peaks, which means a delay of the transformation. In addition, the transformation enthalpy, which is greater on heating than on cooling, diminishes when the number of transformation cycles increases. This is interpreted as being due to an evolution of the microstructure. Transmission electron microscopy shows the appearance of transformation-induced defects, which are mainly sessile dislocations. We can interpret the calorimetry results (enthalpy evolution and transformation delay) as due to the interactions between interface dislocations and these sessile dislocations.

The phase transformation of graphitelike BN (h-BN) to wurtzite-type high-pressure BN (w-BN) was investigated through shock-recovery techniques under quasihydrodynamic and nonhydrodynamic shock compressions and under various strain conditions. The experimental results support a diffusionless mechanism, by which the hydrodynamic c-axis compression of h-BN is preferred. This mechanism is topologically considered based on the relationship of crystal structures between h-BN and w-BN. The presence of deviatoric stress and strain depresses the yield of w-BN and the development of w-BN (100) relative to (002).

The kinetics of single‐interface motion in the diffusionless transformation of Au‐Cd single crystals containing 47.5 atomic percent Cd were studied. A diffusion‐controlled relaxation phenomenon was found with an activation energy of the order of 24,000 calories per mole. The two observed characteristics of diffusionless transformation of this alloy, that the transformation does not take place isothermally, and that the velocity of transformation on cooling is considerably and consistently larger than that on heating, can be successfully interpreted from the relaxation hypothesis.

The gamma to alpha transformation in iron-nickel (0–15 at.% nickel), iron-chromium (0–10 at.% chromium) and iron-silicon (0–2.7 at.% silicon) alloys has been measured as a function of the cooling rate. The rate required to produce maximum undercooling in the different alloys is compared. Even at the fastest rate (5500°C/sec) none of the alloys transformed martensitically, all forming massive alpha of the same composition as the gamma. By applying the Kaufman regular solution analysis, it is shown that the free-energy change accompanying the massive transformation (10–80 cal mole −1 ) is about the same in all three binary alloy systems. This is significantly smaller than the free-energy change of iron—nickel alloys at the martensite start temperature (MST), 260–320 cal mole −1 , whether the MST be that measured in alloys with more than 18 at. % nickel or that deduced by extrapolation of iron-nickel—carbon data to zero carbon. There is a similar difference in the free-energy change for massive and martensite (by extrapolation) transformations in iron-chromium alloys.

Martensite is a needle-shaped microstructure formed by a rapid, diffusionless transformation and significantly affects the mechanical properties of materials. Here, in two-dimensional ReS2 we show that martensite-like domain structures can form via a diffusionless transformation, involving small lattice deformations. By analyzing the strain distribution and topology of the as-grown chemical vapor deposition samples, we find that cooling-induced strain at the ReS2/substrate interface is responsible for the mechanical loading and is essential for martensite-like domain formation. Meanwhile, the effect of cooling rate, flake size and substrate on the microstructures revealed the mechanical origin of the transformation. The strain-induced lattice reconstructions are rationalized and possibly lead to ferroelastic effects. In view of the strong anisotropy in electronic and optical properties in two dimensional materials like ReS2, opportunities exist for strain-correlated micro/nanostructure engineering, which has potential use in next-generation strain-tunable devices. Two-dimensional materials have well-defined atomic-scale structure, which has the potential to be tuned by processing. Here, substrate-induced straining during the growth of ReS2 causes the formation of martensite-like domain structures.

The fundamental understanding of the effects of rare earth (RE) on the phase transformation of Fe-based alloys is essential in the development of advanced RE-containing high strength steels. In this work, we investigated the effects of La and Ce on the martensitic phase transformation of Fe in terms of the driving force and minimum energy path (MEP) using the first-principles calculations. The calculations of formation energy show that the RE substitution increases the stabilization of fcc-Fe and decreases that of bcc-Fe, thus decrease the driving force of phase transformation. Meanwhile, the analysis based on the generalized solid-state nudged elastic band (G-SSNEB) method indicates that in the RE-containing systems, an additional energy barrier appears when the phase transformation proceeds along the Bain transition path to c/a ratio of 1.14 in the FM state, which is associated with a sudden decrease in magnetization due to the hybridization between the orbitals of the RE atoms and those of the nearest neighbor Fe atoms. The results of potential-energy surfaces (PES) confirm that magnetic ordering and lattice volume change simultaneously with the Bain transition, and in comparison with pure Fe, the variation between the onset and the end structural volumes of the phase transformation is slight for the RE-containing Fe systems.

It is desirable to develop high temperature shape memory alloys with a martensitic transformation start temperature (Ms) above 100°C and a recoverable strain of about 4‐6% in the shape memory effect. The latter property is achieved with low variant reorientation stress due to easy detwinning of martensite and high plastic deformation stress due to precipitation strengthening. We have previously demonstrated the facilitation of detwinning for Ti–Zr–Ni–Pd quaternary alloy systems through controlling the crystal structure of martensite, and proposed that Ti–(15‐20)Zr–49.7Pd (at.%) and surrounding Ni‐containing compositions are candidates. In this study, aging temperature was optimized for a Ti–20Zr–49.7Pd alloy, and an excellent shape memory effect with Ms above 130°C, low reorientation stress around 200 MPa, high plastic deformation stress around 1800 MPa, and large recovery strain of 4.5% was achieved. On the other hand, short‐range ordering of solute atoms occurs just above the reverse transformation temperature and decreases Ms.

Abstract Zirconia-based shape memory ceramics (SMCs) exhibit anisotropic mechanical response when undergoing elastic deformations as well as during austenite–martensite phase transformation. This behavior results in different types of strain incompatibility at grain boundaries, which we study here using a micromechanical model. A single-crystal model is implemented to provide a full mechanistic three-dimensional description of the anisotropic elastic as well as martensitic transformation stress–strain response, including non-Schmid behavior caused by the significant volume change during martensitic transformation. This model was calibrated to and validated against compression tests of single-crystal zirconia micro-pillars conducted previously, and then used to model bi-crystals. Upon the introduction of a grain boundary, the simulation provides detailed information on the nucleation and evolution of martensite variants and stress distribution at grain boundaries. We identify bi-crystal configurations which result in very large stress concentrations at very low deformations due to elastic incompatibility, as well as others where the elastic incompatibility is relatively low and stress concentrations only occur at large transformation strains. We also show how this approach can be used to explore the misorientation space for quantifying the level of elastic and transformation incompatibility at SMCs grain boundaries. Graphical abstract Micromechanics models provide insights on grain boundary elastic and phase transformation strain incompatibility in shape memory zirconia

Martensitic crystal structures are usually obtained by rapid thermal quenching of certain alloys, which induces stress and subsequent shear deformation. Here, we demonstrate that it is also possible to intentionally excite a suitable transverse acoustic phonon mode to induce a local shear deformation. We irradiate the surface of a partially stabilized zirconia plate with intense terahertz pulses and verify martensitic transformation from the tetragonal to the monoclinic phases by Raman spectroscopy and the observed destructive spallation of the zirconia microcrystals. We calculate the phonon modes in tetragonal zirconia and determine the effective channel that triggers the transformation. These modes can be excited via the Klemens process. Since terahertz pulses can induce a specific local shear deformation beyond thermal equilibrium, they can be used to elucidate phase transformation mechanisms with dynamical approaches. Terahertz-induced martensitic transformation is considered to be useful for material strengthening and shape memory ceramics.

We systematically compared the mechanical properties of CrCoNi, a recently emerged prototypical medium-entropy alloy (MEA) with face-centered-cubic (FCC) structure, with hallmark FCC alloys, in particular, the well-known austenitic 316L and 316LN stainless steels, which are also concentrated single-phase FCC solid solutions and arguably next-of-kin to the MEAs. The tensile and impact properties, across the temperatures range from 373 K to 4.2 K, as well as fracture toughness at 298 K and 77 K, were documented. From room temperature to cryogenic temperature, all three alloys exhibited similarly good mechanical properties; CrCoNi increased its tensile uniform elongation and fracture toughness, which was different from the decreasing trend of the 316L and 316LN. On the other hand, the stainless steels showed higher fracture toughness than CrCoNi at all temperatures. To explain the differences in macroscopic mechanical properties of the three alloys, microstructural hardening mechanisms were surveyed. CrCoNi MEA relied on abundant mechanical twinning on the nanoscale, while martensitic transformation was dominant in 316L at low temperatures. The deformation mechanisms in the plastic zone ahead of the propagating crack in impact and fracture toughness tests were also analyzed and compared for the three alloys.

The singular change of the order parameter at the first order martensitic transformation (MT) temperature restricts the caloric response to a narrow temperature range. Here the MT is tuned into a sluggish strain glass transition by defect doping and a large elastocaloric effect appears in a wide temperature range. Moreover, an inverse elastocaloric effect is observed in the strain glass alloy with history of zero-field cooling and is attributed to the slow dynamics of the nanodomains in response to the external stress. This study offers a design recipe to expand the temperature range for good elastocaloric effect.

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ​K or 773 ​K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ​K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.

Hydrogen embrittlement can occur in steels with metastable phases, due to activation of the hydrogen-enhanced decohesion mechanism upon transformation. Meanwhile, recent investigations suggest that alloys undergoing e-martensite transformation may exhibit resistance to hydrogen embrittlement. To better understand hydrogen effects in these alloys, we investigate the hydrogen-induced microstructural transformations in a metastable Fe45Mn35Co10Cr10 alloy. To this end, we electrochemically charge unstrained samples, quantify the hydrogen evolution by thermal desorption spectroscopy, and observe microstructural transformations by scanning electron microscopy techniques. Through these analyses, we find that the hydrogen-induced e-martensite formation is dependent on the crystallographic orientation of the austenite grains, and takes place preferentially along Σ3 boundaries. Further charging of hydrogen induces extension twinning within the martensite. We examine the microstructural factors influencing these transformations to better understand the hydrogen-microstructure interactions.

The effect of Cu addition on crystal structure, compressive properties and shape-memory effect of Ni50Mn25Ga25−x Cu x alloys was studied. With increasing Cu content, the type of crystal structure evolves following a sequence: L21 → 10M → 2M → 2M+γ. Addition of Cu significantly improves room temperature ductility. In polycrystalline Ni50Mn25Ga17Cu8 alloy, a full recoverable strain equal to 7 pct was achieved. High martensitic transformation temperature and large shape-memory effect makes this material potential candidate in high-temperature shape-memory applications.

The Ti–35Nb–7Zr–5Ta (wt%, TNZT) alloy was reinforced with TiB2 and synthesized by L-PBF. The relatively small TiB2 particles change the solidification structure from cellular to columnar-dendritic and lead to submicron TiB precipitation in the β matrix. This results in pronounced grain refinement and reduction of texture. However, the microstructure of the additively manufactured TNZT-TiB2 is still different from the as-cast, unreinforced TNZT, which contains equiaxed and randomly oriented grains. The β phase is less stable in the as-cast samples, leading to stress-induced martensitic transformation and recoverable strain of 1.5%. The TNZT with 1 wt% of TiB2 presents significantly higher compressive strength (σYS = 495 MPa) compared to unreinforced samples (σYS = 430 MPa), without sacrificing ductility or altering Young’s modulus (E ≈ 46 GPa). The addition of a small fraction of TiB2 to the TNZT alloy synthesized by L-PBF is a promising alternative for manufacturing sophisticated components for biomedical applications. Graphical abstract