Search

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

This study proposes to characterize the effect of long-term thermal cycling on transformation temperatures and thermodynamic parameters of polycrystalline CuAlBeCr shape memory alloy. Structural, morphological and thermal characterizations were performed. The samples were submitted up to 1000 cycles using a dual-bath thermal cycler apparatus. The results show that the thermal cycling has a considerable impact on the transformation behavior of the alloy. Up to 500 cycles, the alloy experienced a one-stage reverse martensitic transformation; after this, it was split into two reactions. From 100 cycles, an increase in temperature ranges ( $${A}_{\text {f}}-{A}_{\text s}$$ ) values of the reverse martensitic transformation was observed, associated with the increase in elastic energy. Our study expands the spectrum of knowledge regarding thermal and structural properties, for CuAlBe system alloys, indicating the thermal cycling conditions to which the alloy must be subjected in order to preserve the characteristics required in a given practical application.

We examined the microstructure, phase stability, mechanical properties and deformation behaviors of cast (Zr0.58Cu0.42)100-xAlx (x = 0, 3, 5, 7, 10) bulk metallic glass composites (BMGCs). With increasing Al content, the glass-forming ability of the new Zr-rich Zr–Cu–Al alloys gradually increases, enabling the fabrication of BMGCs for the alloys containing more than 3 at.% Al. The as-cast structure changes from Cu10Zr7 + CuZr2 for the Al-free base alloy to glass + crystal for the Al-added alloys. The new Zr-rich Zr–Cu–Al BMGCs exhibit a large fracture strain of ∼3.4–7.8% and a high fracture strength of ∼1731–1984 MPa under compression. The compressive fracture strain of Zr-rich Zr–Cu–Al alloys can be explained by the percolation theory. The (Zr0.58Cu0.42)95Al5 composite containing ∼70 vol.% crystalline phase possesses the largest plastic strain of ∼6%, and fracture strength of over 1900 MPa under compressive condition. The superior plastic deformation capability under compression is related to the following factors: (1) The formation of three types of shear bands with distinct morphological characteristics, (2) the plastic deformation of B2–CuZr phase itself, together with stress-induced martensitic transformation from B2–CuZr phase to B19’ phase, and (3) the interaction between crystals and shear bands. The present results have implications for better understanding the deformation mechanisms of the Zr-rich Zr–Cu–Al BMGCs and for designing high-performance BMGCs with enhanced plasticity.

Phase transformation changes numerous properties of materials. Ti–Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications, especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-to-ductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.

We report the mechanical and microstructural characteristics of a medium-entropy alloy, Co17.5Cr12.5Fe55Ni10Mo5 (atomic percent, at%) at cryogenic temperatures, down to a record low temperature of 0.5 K. The alloy exhibits excellent strength and ductility combined with a high strain-hardening rate in the entire temperature range investigated. Its property profile, including the yield strength, ultimate tensile strength, strain hardening capability, and absorbed mechanical energy, is better than those of most alloys and HEAs used in cryogenics. Within the interval of extremely low temperatures considered (0.5–4.2 K), the alloy exhibits several unusual features, including anomalies of the temperature dependence of the yield strength and tensile ductility, discontinuous plastic deformation (DPF), and a change in the propensity for the deformation-induced martensitic transformation. While the occurrence of these effects in the same temperature interval may be fortuitous, we hypothesize that they are interrelated and provide a tentative explanation of the observed phenomena on this basis.

Shape memory alloys (SMAs) are being increasingly applied as thermally driven actuators. The commercially available SMA elements, however, frequently contain stress risers, either due to internal or surface defects or due to the required component shape. Stress risers represent a potential danger for the preliminary fatigue failure of such actuators but its actual mechanism is not very clear. This paper presents a combined experimental (2D DIC analysis of surface strains) and numerical analysis (SMA model with plastic deformation) of the stress, strain and phase fraction fields evolving in a thin NiTi shape memory ribbon with an artificial notch subjected to cyclic cooling-heating through transformation range under constant external force. It appeared that, even if only very low tensile stress is externally applied upon thermal cycling, local tensile stress at the notch tip sharply increases during the first cooling due to the forward martensitic transformation (MT) proceeding heterogeneously in space. This heterogeneity gives rise to plastic deformation at the notch-tip, which gradually accumulates upon thermal cycling and shields the notch tip from tensile overloading. Hence, fatigue performance of thermal NiTi actuator with stress riser depends very much on the plastic deformability of the alloy.

The effects of hot rolling and subsequent annealing on microstructure and mechanical behavior of an equiaxed-structure TC21 titanium alloy were investigated. The microstructure observation showed that no phase transformation and dynamic recrystallization occurred during rolling due to the low rolling temperature (750 °C). During annealing, the annealing temperature influenced the microstructure and mechanical properties more remarkably than the annealing time. At the annealing temperature of 820 °C, compared with the rolled samples, the value of strength decreased and the value of plasticity increased, which was largely related to the reduction of strain hardening effect. At the annealing temperature of 880 °C, the increase of β phase content induced the simultaneous occurrence of stress-induced martensitic transformation (SIMT) and dislocation slip during tensile and then resulted in the double yield phenomenon. At the annealing temperature of 920 °C, the value of strength increased, and the value of plasticity decreased obviously, which was related to the precipitation of secondary α phase (αs) and martensitic (α″). However, prolonging annealing time from 1 h to 6 h had a little effect on mechanical properties, because the phase content and grain morphology had a little change with the variation of annealing time. In addition, the fracture morphologies of all annealed samples were composed of equiaxed dimple, indicating a typical ductile fracture.

This paper describes the development and characterisation of bainitic steel for rail applications based on carbide-free, low-alloy steel. The results show that after rolling and subsequently cooling, the designed carbide-free bainitic steel exhibits better mechanical performance than standard pearlitic steel. This is because of its fine, carbide-free bainitic microstructure, which consists of bainitic ferrite and retained austenite laths. Microstructural and mechanical property analysis was carried out using scanning and transmission electron microscopy, X-ray diffraction, hardness measurements, tensile and low-cycle fatigue tests. The obtained results demonstrate that during low cyclic deformation, a partial transformation of the retained austenite into deformed martensite α′ takes place, and strain-induced martensitic transformation occurs. The initial strengthening of the material during low-cycle fatigue was caused by the transformation of austenite into martensite and the increase in the dislocation density of the steel. In addition, an optimal amount of retained austenite in the form of thin layers and islands (dimensions not exceeding 1 µm) made it possible to obtain a high yield while maintaining the high plasticity of the steel. These microstructural features also contributed to the high crack resistance of the tested carbide-free bainitic steel.

This work concurrently investigates the microstructure evolution and stress state during continuous cooling of 1045 steel gear parts. A finite element algorithm was developed by two-way coupling of microstructural and thermal fields. Thermal coefficients were correlated to microstructural evolution, and a phase transformation kinetics model was considered to be both temperature and time dependent. Magee's rule was chosen for martensitic transformation modeling to incorporate the effect of stress state on microstructural field. The dilation curves for the benchmark sample show that martensite starting temperature increases when the stress state is considered in microstructure modeling. To determine the validity of the presented model, a cylindrical specimen was quenched in water and oil media, and their predicted microstructures were compared with experimental results. A reasonable harmony between simulated and experimental results was observed. The simulation was then performed for internal gear parts quenched in oil and water media. The simultaneous monitoring of microstructure and stress evolution during continuous cooling of an internal gear part was conducted. It was shown that simultaneous and separate transformations occurring between the tooth and root region have a determining role in the sign of stress for internal gear parts. Lastly, based on monitoring of different phase transformation scenarios occurring during water and oil quenching, a hybrid quenching was proposed to increase martensite volume fraction at the tooth region of an oil-quenched sample without altering the microstructure at root region.

The electron beam additive manufacturing (EBAM) method was applied in order to fabricate rectangular-shaped NiTi component. The process was performed using an electron beam welding system using wire feeder inside the vacuum chamber. NiTi wire containing 50.97 at.% Ni and showing martensitic transformation near room temperature was used. It allowed to obtain a good quality material consisting of columnar grains elongated into the built direction growing directly from the NiTi substrate, which is related to the epitaxial grain growth mechanism. As manufactured material showed martensitic and reverse transformations diffused over the temperature range from −10 to 44 °C, the applied aging at 500° C moved the transformation to higher temperatures and transformation peaks became sharper. The highest recoverable strain of about 3.5% was obtained in the as-deposited sample deformed along the deposition direction. In the case of deformation of the alloy aged at 500 °C for 2h, the formation of martensite occurs at significantly lower stress; however, at about 2.5% the stress begins to increase gradually and only a small shape recovery was observed due to a higher martensitic transformation temperature. In situ SEM tensile deformation in the direction perpendicular to deposition direction showed that the martensite began to appear at the surface of the sample and at the grain boundaries due to heterogeneous nucleation. In situ studies allowed to determine the following crystallographic relationships between B2 and B19’ martensite: (100)B2||(100)B19’and (100) B2 || (011)B19’; (011)B2|| (001)B19’and$${(011)}_{\mathrm{B}2}||{\left(11\bar{1 }\right)}_{\mathrm{B}1{9}^{\mathrm{^{\prime}}}}$$(011)B2||111¯B19′. Samples aged at 500 °C exhibited fully austenitic microstructure; however, with increasing degree of deformation, the formation of martensite was observed. The majority of needles were tilted about 45° with respect to the tensile direction, and the presence of type I (11$$\bar{1 }$$1¯) invariant twin boundaries was observed at higher degrees of deformation.

To elucidate the dynamic strain aging (DSA) mechanism that causes serrated flow in the tensile curves of an intercritical annealed steel with retained austenite (RA) and intercritical ferrite, the multi-aspect microstructural characterization and micromechanical deformation behavior were systematically investigated in this study. Our results demonstrate that the DSA phenomenon is dependent on the strain rate, deformation temperature, and strain. This is accompanied by deformation-induced martensitic transformation (DIMT) and the generation of crystallographic defects in nucleating and propagating Portevin-Le Chatelier bands. An increase in the deformation temperature and strain rate leads to an increase in the stacking fault energy of RA. This inhibits the growth of martensitic embryos and retards the kinetics of martensitic transformation of RA, consequently decreasing or suppressing the DSA phenomenon. Furthermore, neither the long-range diffusion model of carbon atoms nor the short-range diffusion model on the reorientation of C-Mn complexes could reasonably explain the DSA phenomenon in this study. Based on the in situ synchrotron X-ray diffraction results and established mathematical models stemming from the dislocation multiplication theory, the DSA could be attributed to the periodic instantaneous dislocation multiplication in constituent phases caused by burst DIMT.

This paper aimed to study the influence of the substrate and the arc voltage on the structure and functional properties of the NiTi shape memory alloy produced by wire arc additive manufacturing (WAAM). The gas metal arc based WAAM process was used for the deposition of 3-layered NiTi walls on a titanium or steel substrate at different arc voltages using a Ni50.9Ti49.1 wire. It was found that in the sample deposited on the Ti substrate, the Ti2Ni phase appeared and the Ti concentration in the NiTi phase increased to 50.5 at.%. An increase in the arc voltage influenced the volume fraction of Ti2Ni precipitates in the 1st layer but hardly affected the chemical composition of the NiTi phase in all layers as a result, the martensitic transformation temperatures do not depend on the arc voltage. Therefore, the deposition of the Ni-rich NiTi wire on a Ti substrate allowed for the production of the Ti-rich NiTi walls undergoing the martensitic transformation and demonstrated the shape memory behaviour at high temperatures. An increase in the arc voltage hardly affected the shape memory behaviour but decreased the strain up to failure due to an increase in the volume fraction of the brittle Ti2Ni phase, which in turn facilitated the formation of cracks during deformation. In the sample deposited on the steel substrate, the NiTiFe solid solution and Ti-C, Ni3Ti4, and Ni3Ti2 precipitates formed in the 1st layer. An increase in the arc voltage led to an increase in the Fe concentration in the NiTiFe solid solution from 17 at.% to 42 at.% in the 1st layer. From layer to layer, the Fe concentration decreased; however, it remained larger than 1.5 at.% and completely suppressed the martensitic transformation and the shape memory effects in the NiTi sample deposited on the steel substrate.

The Ti-Ni-Hf shape memory alloy powders were prepared by plasma rotating electrode process. And the powders were featured with the perfect sphericity and smooth surface. Moreover, the grain size was refined in the Ti-Ni-Hf alloy powders due to the rapid cooling rate during plasma rotating electrode process. It was found that the Ti-Ni-Hf alloy powders had the typical self-accommodation configuration including spear-like, mosaic-like and lath-like martensite. Various twins, such as (001) compound twin, (011) type I twin as well as {111} type I twin, were observed in Ti-Ni-Hf alloy powders. Differing from the bulk Ti-Ni-Hf alloy, two-stage transformation corresponding to B2⇌B19’ martensitic transformation was detected in the as-received Ti-Ni-Hf alloy powders. In addition, the martensitic transformation behaviors were closely related to the annealing temperatures. The Ti-Ni-Hf alloy powders with low annealing temperature of 500 ​°C still exhibited two-stage martensitic transformation, while two-stage martensitic transformation changed into single martensitic transformation in the Ti-Ni-Hf alloy powders with the higher annealing temperature of 900 ​°C. And finally the possible mechanisms for the evolution of martensitic transformation behaviors were discussed.

The microstructure, martensitic transformation and mechanical/functional properties of Ti–Ta-Hf alloys with various thermo-mechanical treatments were investigated. The results reveal that the hot-rolling could refine the grain size and introduce a certain number of defects, resulting in the disappearance of martensitic transformation. The as-casted and solution treated Ti–Ta-Hf alloys were composed of α'' martensite phase and smaller volume of β phase. In contrast, the grain size of solution treated Ti–Ta-Hf alloy was slightly less than that of as-casted Ti–Ta-Hf alloy. This should be responsible to the higher yield stress and superior strain recovery characteristics for solution treated Ti–Ta-Hf alloy. The yield stress for the dislocation slip and the maximum recoverable strain of solution treated Ti–Ta-Hf alloy were 723 ​MPa and 5.06%, respectively.

The grain-size-dependent microstructure evolution and overall mechanical behavior of polycrystalline NiTi thin films have not been widely studied under thermomechanical loading. We propose a non-isothermal phase field model with an additional grain boundary energy term introduced into the local energy density to take into account the contribution from the grain boundary in the polycrystalline system. We investigate the grain-size effects in the process of temperature-induced martensitic transformation, detwinning and reorientation of martensitic variants, and superelastic deformation. This multi-variant two-dimensional phase field study showed that, as the average grain size of the thin film drops from 1.5 μm to 10 nm, the temperature-induced martensitic transformation tends to be inhibited. The polytwinned martensitic microstructure tends to be replaced by single-variant martensite microstructure in the grains. The martensitic transformation will not even take place once the grain size is around 10 nm. Meanwhile, the stress-induced martensitic detwinning and reorientation are less active for all types of martensite variants. The shape memory functionality may be weakened with decreasing grain size. Regarding the superelastic behavior, the stress-induced martensitic transformation is delayed with decreasing average grain size. The stress plateau tends to be higher and less noticeable as the grain size drops. The underlying mechanism of this grain-size effect is associated with the inhibiting effect of the grain boundary, the fraction of which will noticeably increase as the average grain size drops, and thus the phase transformation area will significantly reduce in the polycrystalline thin film.

As a modification of quenching and partitioning processing for martensitic steels to obtain mixed microstructures of martensite and austenite aiming at improving strength plasticity, the process of quenching and dynamic partitioning is much simple in practical, especially suitable for components with larger cross-sections. In this paper, a kind of air-quenched low-carbon steel was designed and prepared. The martensitic transformation behaviors at different cooling rates, the structure–property relationship and the strengthening–toughening mechanisms of testing steel under a different processing were explored. The results show that the testing steel exhibits good hardenability that fairly martensitic transformation occurs while it is cooled at the rate of 0.1~1.0°C.s-1. The microstructures of plate samples with 25~40 mm in thickness after air quenching are composed of lath martensite and strip-like retained austenite of about 5 vol.%. Once the air-quenched samples are tempered at 450°C, the volume fraction and thermal stability of retained austenite are further reinforced, and good comprehensive properties are obtained with Rp0.2≈1200 MPa, Rm≈1400 MPa, A≈18% and KV≈1000 kJ.m-2.

The phenomenon of electrochemical phase formation in metals and alloys via a supercooled liquid state stage was discussed. Assuming the electrodeposited metal to be a product of formation and ultrarapid solidification of supercooled metallic liquid, a possibility of metastable phase formation during electrodeposition of polymorphous metals was suggested. It was anticipated that the polymorphic transition of the metal’s metastable form to the stable one occurs by shear, as does the martensitic transformation. To enable revealing an orientation relationship between grains of the two phases, a method for X-ray texture analysis of metals was developed using a combination of direct pole figures. It was established that the phase formation during electrodeposition of polymorphous metals produces metastable modifications typical of entities that crystallized from a liquid state at extremely high rates. In regards polymorphic transitions in metal electrodeposition, certain orientation relationships were observed between grains of the stable and the metastable phase, which is typical of phase transformations proceeding at extremely high rates. The results obtained provided additional arguments in favor of the phenomenon under discussion.

A new Q&P processing route, i.e., applying austempering below Ms in normal two-step Q&P route (termed AQ&P), was proposed and applied to a medium-carbon steel. The effects of isothermal transformation at the quenching temperature on the microstructure evolution and mechanical properties of AQ&P steel were investigated. It was found that both the strength and the elongation of the steel were improved using AQ&P processing route. The yield strength and tensile strength were improved by ~ 100 and ~ 160 MPa, respectively, with similar elongation through austempering for 600 s compared to that of specimens treated by normal two-step Q&P route. Therefore, the product of strength and elongation was increased by 18.8% after AQ&P treatment. In addition, the volume fraction of retained austenite was first decreased and then increased with the extension of the quenching time. Compared to a normal two-step Q&P route, the retained austenite decreased about 6% after austempering for 600 s in AQ&P route. Moreover, the medium-carbon steel experienced a continuous martensitic transformation during isothermal holding at the quenching temperature. The phase transformation during isothermal holding includes two stages: quick martensitic transformation and slow bainitic transformation.

A Fe–3wt pctSi alloy was gas nitrided to study the effect of Si on the Fe nitride formation. Both ε-Fe3N1+x and γ′-Fe4N were observed at nitriding conditions only allowing to form single-phase γ′ layers in pure α-Fe. During short nitriding times, ε and γ′ simultaneously grow in contact with Si-supersaturated α-Fe(Si). Both nitrides almost invariably exhibit crystallographic orientation relationships with α-Fe, which are indicative of a partially displacive transformation of α-Fe being involved in the initial formation of ε and γ′. Due to Si constraining the Fe nitride growth, such transformation mechanism becomes highly important to the nitride layer formation, causing α-Fe-grain-dependent variations in the nitride layer morphology and thickness, as well as microstructure refinement within the nitride layer. After prolonged nitriding, α-Fe is depleted in Si due the pronounced precipitation of Si-rich nitride in α-Fe. The growth mode of the compound layer changes, now advancing by conventional planar-type growth. During nitriding times of 1 to 48 hours, ε exists in contact with the NH3/H2-containing nitriding atmosphere at a nitriding potential of 1 atm−1/2 and 540 °C, only allowing for the formation of γ′ in pure Fe, indicating that Si affects the thermodynamic stability ranges of ε and γ′.

Medium-manganese steel is a third-generation high-strength steel for automobiles, which has the characteristics of high strength and high plasticity. The transformation-induced plasticity (TRIP) phenomenon that occurs during the straining process can improve the work hardening ability of medium-manganese steel and delay the necking phenomenon. The microstructure of 0.1C-5Mn medium-manganese steel was observed by SEM as a two-phase structure of fine-grained ferrite and retained austenite. The law of austenite transformation and strain of medium-manganese steel under uniaxial tension is obtained through magnetic method, and the retained austenite of medium-manganese steel shows a decreasing tendency with increasing strain. The functional relationship between retained austenite and equivalent strain in medium-manganese steels is derived from a dynamic model of martensitic transformation. By using the microstructure characteristics of meso-manganese steel and mesomechanical method, a meso-manganese steel constitutive model including the TRIP effect was established, and the accuracy of the method was verified by comparing it with the curves obtained from experiments and other constitutive models. The model forms the basis to simulate the behavior of medium-manganese steels.

The first part of the paper is devoted to regular features of microstructure formation and evolution of martensitic transformations during post-deformation annealing at 400 – 700°C. The second part presents results of a study of shape recovery characteristics reflecting their evolution as a function of the varying structural state and features of martensitic transformation during post-deformation annealing. Conditions for implementation of anomalously high characteristics of shape recovery under bending with nonisothermal loading are determined and analyzed.

The major challenges hindering the application of high-temperature shape memory alloys are their decreased mechanical strength and thermal stability. Herein, we present high-temperature shape memory properties of the Cu15Ni35Ti25Hf12.5Zr12.5 high-entropy alloy with improved mechanical properties and superior shape memory characteristics. Under an immense stress of 1400 MPa, the alloy exhibited a shape recovery ratio of 88.7%. Using an in-situ high-temperature X-ray diffractometer, the alloy was established to undergo phase transformation from B19′ to B2 phase. Additionally, the alloy exhibited a martensitic transformation temperature higher than 100 °C. The fracture strength and ductility were 1670 MPa and 24.7%, respectively, as measured by compression tests. The deformation mechanism and shape recovery phenomenon of this shape memory alloy are also discussed. This study demonstrates the potential of high-entropy shape memory alloys for high-temperature applications.

The effect of rare earth element Er addition on the microstructure and phase transformation behavior of Ni_{50}Ti_{50-x}Er_{x} (x =0, 1, 5) shape memory alloy was investigated experimentally. The results showed that the microstructure of Ni-Ti-Er ternary alloys consists of the NiEr precipitate and the NiTi matrix. A one-step martensitic transformation was observed in all alloys. The martensitic transformation temperature M_s increased gradually with increasing Er content.

In this research, the effects of annealing temperature (500, 550, and 600 °C) on the microstructure and mechanical properties of a conventionally (1 cycle) and severely (6 cycles) deformed Fe–24Ni martensitic steel, processed by accumulative roll bonding (ARB), were investigated. Electron backscattered diffraction (EBSD) analysis revealed that annealing at 500 and 550 °C for 30 min was insufficient for completing the reverse martensite transformation, and consequently, the microstructure consisted of austenite and martensite. However, complete reverse martensitic transformation has been accomplished after annealing at 600 °C for 30 min for both deformed samples. Both yield strength (YS) and ultimate tensile strength (UTS) were increased after first and sixth cycle of rolling. Subsequent, annealing led to a decrease in YS and UTS with improvement in uniform elongation ( e u ). In addition, the work hardenability was also improved by annealing the deformed samples.

The solvus temperature of face-centered cubic (FCC) phase in Fe–34Mn–xAl–4Cr–7.5Ni (x = 13, 14, and 15) shape memory alloys with different Al contents, and their abnormal grain growth and superelasticity at various temperatures were evaluated. With increasing Al content, the solvus temperature of the FCC phase decreased and the FCC precipitates became finer. Whereas cyclic heat treatment induced abnormal grain growth (AGG) in all samples, large grains were obtained more easily in the alloys with higher Al content. The critical stress for martensitic transformation increased with increasing Al content. The x = 14 alloy is the optimal composition considering grain growth and superelasticity. The newly developed Fe–34Mn–14Al–4Cr–7.5Ni alloy, in which single-crystal can easily be fabricated by AGG, exhibited superelasticity at temperatures ranging from − 263 °C (10 K) to 27 °C (300 K), with a very small temperature-dependence of the critical stress, comparable to that of conventional Fe–34Mn–13.5Al–3Cr–7.5Ni alloy.

High-entropy alloys (HEAs) have attracted great research interest owing to their good combination of high strength and ductility at both room and cryogenic temperatures. However, expensive raw materials are always added to overcome the strength-ductility trade-off at low temperatures, leading to an increased production cost for the cryogenically used alloys. In this work, a series of nitrogen-doped FeMnCoCr HEAs have been processed by homogenization annealing, cold rolling and recrystallization annealing followed by water quenching. The microstructural evolution and mechanical properties of the alloys are studied systematically. The Fe49Mn30Co10Cr10N1 alloy shows excellent mechanical properties at both 293 K and 77 K. Particularly, the yield and ultimate tensile strength of 1078 and 1630 MPa are achieved at the cryogenic temperature, respectively, while a satisfactory uniform elongation of 33.5 % is maintained. The ultrahigh yield strength results from the microstructure refinement caused by the activation of athermal martensitic transformation and mechanical twinning that occur in the elastic regime together with the increased lattice friction due to the cryogenic environment. In the plastic regime, the dynamic Hall-Petch effect caused by twinning, martensitic transformation, and reverse transformation together with the high barrier to dislocation motion jointly contribute to the ultrahigh tensile strength. Simultaneously, the transformation induced plasticity (TRIP) and the twinning induced plasticity (TWIP) effects jointly contribute to the ductility. The design strategy for attaining superior mechanical properties at low temperatures, i.e. by adjusting stacking fault energy in the interstitial metastable HEAs, guides the development of high-performance and low-cost alloys for cryogenic applications.

Friction stir processing (FSP) on weld toes of arc-welded high-strength steel joints was found to significantly increase the fatigue strength. This can be explained by an accompanied increase in the radius of curvature at the weld toes and grain refining, solid solution hardening in the topmost layer. Solid solution hardening seemed to be accompanied with the WC tool wear during FSP. The topmost layer contained the supersaturated W and C, leading to martensitic transformation. To clarify the alloying mechanism in the topmost layer, the stop action technique was employed during FSP, immediately followed by water-cooling the tool and the steel plate. The interface between the WC tool and the topmost steel-plate layer in the obtained keyhole samples was investigated. As tool rotational speed increased, Fe atoms diffused from the steel plate into the WC-tool Ni-based binder regions. Subsequently, some of the WC particles in the tool reacted with the Fe atoms to form Fe4W2C. The Fe4W2C particles agglomerated at the interface, followed by formation of a thin layer. The intermediate layer was fragmented and decomposed by FSP shear stress and friction heat, and alloying the topmost steel-plate layer with the WC-tool constituent elements.

Martensitic transformation, magnetic entropy, and direct adiabatic temperature changes in Ni48Mn39.5Sn12.5−xInx (x = 2, 4, 6) metamagnetic Heusler bulk and grain-constrained ribbon alloys were studied. All alloys showed a typical L21 structure in austenite and the 4O structure in martensite. Their relative volume contributions changed depending on In content. With increasing In concentration, the martensitic transformation temperature increased, whereas the Curie temperature of austenite decreased. The magnetic entropy change under magnetic field of 5 T attained maximum of 20 J/kgK in the bulk and 14.4 J/kgK in the ribbon alloys with the Ni48Mn39.5Sn8.5In4 nominal composition. The corresponding adiabatic temperature change under 1.7 T yielded 1.3 K for the Ni48Mn39.5Sn8.5In4 bulk alloy. Despite grain confinement, melt spinning was found to stabilize martensite phase. Changes observed were discussed with relation to strengthened covalency imposed by In substitution. Graphic abstract

The solid-state magnetic cooling (MC) method based on the magnetocaloric effect (MCE) is recognized as an environmentally friendly and high-energy-efficiency technology. The search or design of suitable magnetic materials with large MCEs is one of the main targets at present. In this work, we apply the chemical and hydrostatic pressures in the Ni35Co15Mn35−xFexTi15 all-d-metal Heusler alloys and systematically investigate their crystal structures, phases, and magnetocaloric performances experimentally and theoretically. All the alloys are found to crystallize in an ordered B2-type structure at room temperature and the atoms of Fe are confirmed to all occupy at sites Mn(B). The total magnetic moments decrease gradually with increasing Fe content and decreasing of volume as well. The martensitic transformation temperature decreases with the increase of Fe content, whereas increases with increasing hydrostatic pressure. Moreover, obviously enhanced magnetocaloric performances can also be obtained by applied pressures. The maximum values of magnetic entropy change and refrigeration capacity are as high as 15.61(24.20) J (kg K)−1 and 109.91(347.26) J kg−1 with ΔH = 20(50) kOe, respectively. These magnetocaloric performances are superior to most of the recently reported famous materials, indicating the potential application for active MC.

The formation energy of martensite nuclei in the austenite matrix is calculated. Nanoclusters with ferromagnetic order, which exist in austenite above the Curie temperature, reduce the formation energy of a critical martensite nucleation center when exposed to an external magnetic field. The data obtained are explained by the magnetic separation of the initial phase under the action of a magnetic field. A fluctuation increase in nanovolumes with a ferromagnetic order in austenite increases the energy in a atoms group of the matrix phase with a parallel spins arrangement. As a result, the nucleation rate of the martensite phase increases and the martensitic transformation proceeds more completely.

Electron backscatter diffraction is a modern experimental method for local structure and texture investigation, which makes it possible to establish the presence and types of the various boundaries between the elements of the mesostructure such as low or high angle, special and interphase boundaries. Moreover, this technique can demonstrate the migration of boundaries during structural and phase transformations. This study estimated the possible spectrum of crystallographic misorientations of intercrystalline boundaries in additively manufactured titanium alloy Ti-6Al-4V using orientation microscopy and crystallographic calculations based on Burgers orientation relationship during β→α-transformation. The study has established that the boundaries between grains of α-phase are characterized by the misorientation angles of 11±2 °, 61±5 °, 89±3 °. The majority of high-angle boundaries are characterized by misorientation angles in the range of 57-65 °. The study also ascertained that the experimental spectrum of intercrystalline boundaries in the α-phase reveals the displacive nature of β→α-transformation in titanium alloys.

The study covers the effect of recrystallization annealing temperature and time on the characteristic temperatures of martensitic transformations and mechanical properties of the Ti–50.7at.%Ni shape memory alloy in the form of wire after cold drawing at room temperature. Six modes of post-deformation annealing with different temperatures and holding times were studied for the alloy to obtain structures with different sizes of recrystallized grains. The recrystallized grain size was determined by electron backscatter diffraction (EBSD). It was shown that the size of recrystallized grains increases from 2.5 to 9 μm, with both an increase in the annealing temperature (600– 700 °С) and an increase in the holding time (0.5–5.0 h). The characteristic temperatures of direct and reverse martensitic transformations were determined using differential scanning calorimetry. It was shown that the threefold growth of the recrystallized grain size reduces the starting temperature of the direct martensitic transformation, and extends the temperature range of the reverse martensitic transformation. The results of mechanical tests (stretching tests) at room temperature showed that an increase in the grain size leads to a decrease in the dislocation yield strength and an increase in the phase yield strength. It was established that the dislocation yield strength obeys the Hall–Petch law, and the phase yield strength is determined by the test temperature position relative to the starting (or peak) temperature of the direct martensitic transformation. Heat treatment modes for specific products should be recommended taking into account these two competing factors, as well as reverse martensitic transformation temperatures determining the alloy strain recovery temperatures.

This study presents the degree of promotion of deformation-induced γ−e martensitic transformation by hydrogen charging and the associated fracture behavior at various strain rates ranging from 10−5 to 10−2 s−1. A decrease in the strain rate from 10−2 to 10−5 s−1 promotes the deformation-induced γ−e martensitic transformation regardless of hydrogen charging (65%→82% in area fraction for uncharged specimens, 68%→84% for hydrogen-charged specimens). Hydrogen charging, which provides 11.7 mass ppm hydrogen concentration, further promotes the γ−e martensitic transformation. However, the degree of promotion of the transformation by hydrogen is insensitive to the strain rate. Corresponding to the promotion of the γ−e martensitic transformation, the hydrogen embrittlement susceptibility increases with a decrease in the strain rate. For instance, the elongation of the hydrogen-charged specimens decreases from 36 to 32% by decreasing strain rate from 10−2 to 10−5 s−1. Hydrogen uptake deteriorates the resistance to crack initiation and propagation. Furthermore, the primary effect of the decrease in strain rate on hydrogen embrittlement is the acceleration of the crack propagation. In addition to the promotion of γ−e martensitic transformation, a decrease in the strain rate in the presence of hydrogen may cause hydrogen localization at the crack tip, which assists brittle-like martensite cracking.