Deposition of various coatings on surface of engineering components with the aim to improve their performance concerning wear, corrosion, friction, and thermal protection is already a standard practice. In special cases, depositing metallic NiTi shape memory alloy coatings may be a viable alternative for hard ceramic coatings. NiTi coatings offer additional benefits originating from their unique functional thermomechanical properties. However, fabrication of thick NiTi coatings turned out to be difficult. Standard electroplating and laser cladding methods are not suitable for NiTi, the most widely used plasma spray methods tend to produce chemically inhomogeneous coatings that do not transform martensitically, cold sprayed NiTi coatings suffer from poor adhesion to the substrates. In this work, we report on first ever successful fabrication of thick NiTi coatings (100–300 μm) that display functional thermomechanical properties and simultaneously show very good adherence to the substrate. We used high velocity air fuel thermal spray method to fabricate NiTi coatings deposited on mild steel using four different sets of fabrication parameters. Chemical composition, porosity, microstructure, phase transformation and functional thermomechanical properties of the NiTi coatings were evaluated. Although the coatings contain inhomogeneous microstructure, voids, oxide particles, high density of dislocation defects and internal stress, they undergo martensitic transformation upon cooling and/or mechanical loading. As sprayed NiTi coatings need to be annealed to display functional thermomechanical properties. Despite their limited tensile strength, the coatings displayed thermal actuation in 3-point bending tests and shape memory effects in nanoindentation and scratch tests.

In order to explore whether the tensile actuation performance of Ni50.13 Ti29.22 Hf20.65 (at.%) high temperature shape memory alloy (HTSMA) fabricated by laser powder bed fusion (LPBF) can be further improved, three post-processing heat treatments commonly used in NiTiHf research were applied (650 degrees C/3 h; 1100 degrees C/3 h; 1100 degrees C/3 h + 650 degrees C/3 h). Microstructures, textures, and secondary phase oxide particles in LPBF-fabricated and heat-treated alloys were characterized, and functional thermomechanical properties were evaluated. It was found that post-processing heat treatments modify: (i) the austenitic grain size, shape and texture, (ii) the size, morphology, surface area fractions of fine homogeneously dispersed oxide nanoparticles created during the LPBF fabrication, (iii) the chemical composition of the matrix and oxide particles, (iv) increase the size of martensite domains, (v) the transformation temperatures, (vi) the stability of transformation response upon thermal cycling, (vii) do not affect significantly the strength but improve ductility of LPBF-fabricated alloy in isothermal tensile and compression tests, and (viii) increase actuation strain and decrease cyclic stability in tensile actuation tests. It is concluded that transformation temperatures and tensile actuation performance of LPBF-fabricated NiTiHf alloy containing finely dispersed oxide nanoparticles can be manipulated by post-processing heat treatments, which promote diffusion across oxide/matrix interfaces and affect the chemical composition of the matrix, oxide nanoparticles, and ultimately the size of domains in martensite variant microstructures. (c) 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & ( http://creativecommons.org/licenses/by/4.0/ )

Severe plastic deformation is an effective process to modify materials’ structures. In this work, its new modification entitled channel angular extrusion was applied to a metastable metal-matrix composite consisting of a Ag matrix and spherical Cu particulates. During this process, the rod sample deforms in an inhomogeneous way and exhibits a gradient microstructure that is characterized by ellipsoidal Cu particulates at the edge of the sample but elongated and fragmented rectangular ones in the center. In addition to the different shapes, the edge and center of the sheet also differ in preferential orientations: the ⟨110⟩ direction predominates in the center of the sheet, while the ⟨111⟩ direction dominates at the sheet edge. The changed angle of the {111} shear plane relative to the extrusion direction explains these differences.

It is well known that NiTi shape memory alloys need to be strengthened to display stable stress-strain-temperature responses in cyclic thermomechanical loads. Strengthening by final cold work/heat treatment at 773 K for 10 min commonly used in industry gives rise to superelastic NiTi wires and sheets with nanocrystalline microstructure that display recoverable stress-strain-temperature responses with acceptable functional fatigue but limited structural fatigue performance (Nf = similar to 5000) and low ductility (similar to 12%). The limited ductility often results in wire fractures due to accidental overloading, which is a serious problem for engineering applications. Plastic deformability of NiTi wires at high stresses thus becomes of concern besides cyclic stability of functional thermomechanical properties. Moreover, strengthening via cold work/heat treatment cannot be applied to bulk NiTi components, thin films and/or additively manufactured NiTi components. Therefore, alternative ways of strengthening NiTi based alloys have been recently explored in the SMA field. In this work, we show that cyclic superelastic performance of coarse grain superelastic NiTi wires can be significantly improved and yield stress for plastic deformation can be increased while assuring high strength (similar to 1 GPa) and large ductility (similar to 80%) by low temperature aging at 523 K for aging times optimized based on the results of Monte Carlo simulation of Ni diffusion. It is claimed that the local chemical inhomogeneity introduced by low temperature aging into the alloy microstructure increases critical stress for [100](001) dislocation slip in martensite which stabilizes cyclic superelasticity, widens superelastic window by increasing yield stress for plastic deformation and improves plastic deformability of NiTi at high stresses. This strategy opens a new pathway for designing coarse grained NiTi alloys with improved functional fatigue performance, high strength, and excellent plastic deformability.

Directed energy deposition enables efficient fabrication of complex Ti-6Al-4V components, yet process-induced microstructural gradients and porosity across the build height remain poorly understood with respect to their effects on mechanical and fatigue performance. In this study, we investigate the relationships between directed energy deposition process parameters, the resulting microstructure, and anisotropic mechanical behavior. The study expands the current understanding of directed energy deposition-processed Ti-6Al-4V by providing a detailed analysis of microstructural evolution in both build directions and presenting new insights into fatigue properties. The microstructural and mechanical characteristics of the alloy are systematically described with an emphasis on their mutual relationship. The material exhibited a predominantly α’ martensitic microstructure, with no significant variation in phase fraction as a function of distance from the build platform, consistent with the measured hardness. Electron backscatter diffraction analysis did not reveal a pronounced crystallographic texture, whereas transmission electron microscopy observations identified deformation twins within the α’ phase. Porosity was mainly oriented perpendicular to the build direction. The combination of a martensitic microstructure and the presence of pores resulted in low ductility. Under cyclic loading, pores acted as crack initiation sites, while their planar orientation limited their effect on crack propagation, indicating that the mechanical response was governed primarily by the martensitic microstructure rather than pore-assisted crack growth. Overall, the results highlight the critical role of microstructure and inherent defects in controlling the mechanical behavior of additively manufactured components and provide a basis for process optimization and post-processing strategies such as heat treatment or hot isostatic pressing.

The Ti-6Al-4V alloy is highly desired for use in the biomedical and aerospace industries, but components produced using laser powder bed fusion (LPBF) often require post-processing to meet the strict surface quality and mechanical requirements of these industries. This study investigates the influence of laser surface treatment under different protective atmospheres (air, argon, and nitrogen) on the microstructure, mechanical, tribological, and biological properties of Ti-6Al-4V alloy previously produced by LPBF. A continuous-wave laser (200 W, 1070 nm) was used to remelt the surfaces of as-printed samples. Comprehensive characterization was performed using XRD, XPS, Raman spectroscopy, SEM/EDS, hardness testing, tribological measurements, and in-vitro cytotoxicity assays. The laser-treated samples exhibited a significant transformation of the surface microstructure from martensitic α′-Ti to a fine α + β phase mixture, along with the formation of hard compounds such as titanium oxides and nitrides. The depth of the remelted layer varied depending on the processing atmosphere, with the deepest and hardest layer observed for samples treated in air. All laser treatments substantially enhanced surface microhardness and dry sliding wear resistance compared to untreated samples. The most favorable combination of low friction, minimal wear, and surface uniformity was achieved with the argon-treated sample. In-vitro tests confirmed that all treated surfaces remained non-cytotoxic, supporting their potential for biomedical applications.

Recently, application of advanced technologies for fabrication of thick NiTi shape memory alloy coatings with functional thermomechanical properties have been explored to protect surfaces of engineering components to be used in harsh environments. Since these metallic coatings are expected to display functional thermomechanical properties, there is a need to evaluate these properties without detaching the coatings from substrates. Nanoindentation methods, although they were not developed to test thermomechanical functional properties, such as superelasticity, shape memory or vibration damping, are obvious candidates for this. Therefore, we prepared two superelastic NiTi alloy samples with slightly different microstructures and functional mechanical properties, characterized by conventional isothermal compression tests and compared the results with the results of nanoindentation experiments. In this work, A novel methodology of nanoindentation mapping was implemented to evaluate spatially resolve superelasticic properties including functional fatigue of NiTi. It is shown that nanoindentation mapping of hardness and reduced modulus using Berkovich indenter can be used to characterize microstructural heterogeneity of NiTi coatings. On the other hand, results of multicycle nanoindentation mapping using spherical indenter (determination of the evolution of recovered depth, accumulated unrecovered depth and dissipated energy with number of cycles), proved to be very useful to evaluate the functional mechanical properties of coatings – for example, this method enabled to distinguish functional fatigue performance of annealed NiTi from that of the aged NiTi alloy containing Ni4Ti3 precipitates.

Prolonged annealing of Ag-Cu metastable metal-matrix composite produced by compaction of Cu@Ag core-shell powder, revealed dramatic transformations in its microstructure. Heat treatment resulted in pronounced changes in particle morphology, extended grain boundary diffusion, and the formation of surface pores at the Cu/Ag interface, as well as extended voids within the bulk material. These phenomena are interpreted as consequences of the reduction of surface, interface and grain boundary energy, and diffusion and wetting processes, including pore migration and coagulation. Based on experimental observations, a model has been developed that explains the growth and migration of the pores leading to the formation of extended voids. This study provides new insight into the mechanisms of microstructural evolution in metal-matrix composites when annealed at relatively high temperatures, and opens pathways for their targeted application in advanced technologies such as electronic contacts.

This study presents a sustainable approach to the synthesis of niobium carbide (NbC) using carbon obtained from thermal plasma valorisation of waste polypropylene (PP), followed by its application as a ceramic reinforcement in CoCrFeNiMn high-entropy alloy (HEA) produced via powder metallurgy. Pure niobium powder and carbon soot were mechanically alloyed as raw materials in Nb: C ratios of 1:1 and 1:2 under high-energy ball milling for up to 8 h, both with and without n-heptane as a process control agent (PCA). Phase formation was monitored using X-ray diffraction (XRD), while scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed to characterize the particle morphology and composition. The absence of n-heptane favoured NbC formation and prevented pyrophoric oxidation, with a Nb: C ratio of 1:1, yielding approximately 19 wt% NbC after only 4 h of milling, while the remaining fraction consisted predominantly of carbonsupersaturated Nb(C), serving as a highly reactive precursor for subsequent NbC formation. The 4 h NbC powders were subsequently incorporated into CoCrFeNiMn HEA at 5, 10, and 15 wt% and consolidated by spark plasma sintering (SPS) at 1000 degrees C under a pressure of 48 MPa. Mechanical testing revealed an increase in compressive yield strength (CYS) with increasing NbC content, starting already at 5 wt% NbC and reaching an approximately 40% improvement at 15 wt% NbC compared to the unreinforced alloy. At the highest investigated loading, the composite achieved a yield strength of 717 MPa and a hardness of 310 HV30, corresponding to a nearly 15% increase in hardness, while retaining excellent compressive plasticity (>40%).

Nitinol technology, besides utilizing the functional thermomechanical properties derived from the B2 cubic to B19' monoclinic martensitic transformation, also exploits the excellent plastic deformability of NiTi in the martensite state. It originates from the unique mechanism of plastic deformation of the B19' martensite by kwinking involving dislocation slip based kinking assisted by deformation twinning. Although the mechanism of plastic deformation of martensite by kwinking was revealed only very recently, various unusual phenomena that can only be rationalized by kwinking, have been reported in literature in the last 50 years. These phenomena include: 1) cold working with a high degree of reduction without introducing cracks, 2) excellent plastic deformability in the martensite state (plastic deformation up to 80

This work presents a pilot study on a strained complex concentrated alloy based on refractory elements: MoNbTaTiZr. Initially, the as-cast and homogenization-annealed conditions were characterized. After casting, the alloy consists of two solid solutions with BCC 1 and BCC 2 crystal structures. Homogenization annealing promotes the growth, ordering, and refinement of the BCC 2 phase. TEM and AES analyses indicate possible Zr segregation at grain boundaries in the as-cast state. In contrast, annealing followed by cooling results in the formation of Ti-Zr-based particles without segregation. Subsequently, the annealed alloy was cold-rolled, and its microstructure was investigated. During rolling, grain fragmentation occurs within the structure. In addition to the two BCC solid solutions, a phase with an FCC crystal structure is identified after rolling. Its composition corresponds to the Zr2Ta phase, which is a Laves phase of the A2B type. Rotational relationships, relatively rare in rolled materials with BCC structures, are identified. The texture components found after 10 % rolling deformation are related to that present after 20 % deformation by a 45 degrees(110) rotation, and this component is related to that appearing after 30 % deformation by a 20 degrees(100) rotation. No strong classical rolling texture develops; however, some specific orientation components and their rotational relationships can be observed. Schmid and Taylor factor maps demonstrate that, despite deformation, the alloy remains capable of further strain accumulation and plastic deformation. Twinning is also observed after rolling, which may be beneficial, as deformation twinning contributes to improved ductility in the alloy.

The non-continuous multi-cycle nanoindentation along with scratch and wear tests were performed on super elastic (SE) and shape memory (SM) NiTi alloys in order to observe the behaviour of super elasticity and shape memory effect. A spherical indenter with radius 10 mu m was used in scratch test under three loads (100, 250 and 500 mN) to evaluate critical limits of scratch resistance and in wear test to evaluate material behavior in repetitive scratch under subcritical loads (100 and 250 mN). The study finds that SE shows more elastic recovery after removing the load from the indenter. However, in case of SM the recovery is less pronounced with remaining plastic deformation, indicating that stress induced martensite phase remains in the alloy. Wear test resulted in the pile of the material during multiple ploughing along the track as is observed from the morphology of track profile. The differences between the materials become smaller when the load in the wear test increases, with even the SE showing marked plastic deformation.

NiTiHf high-temperature shape memory alloys (HTSMAs) fabricated via laser powder bed fusion (LPBF) suffer from poor tensile strength preventing their successful use in engineering applications. We managed to fabricate NiTiHf HTSMA showing tensile actuation under stresses up to 500 MPa in the temperature range 250-350 degrees C. A tensile strength of 821 MPa, an actuation strain 2.34 % under 300 MPa tensile stress with negligible irrecoverable plastic strain, and fracture upon cooling under 600 MPa tensile stress are the best values reported so far for LPBFfabricated NiTiHf HTSMAs in the literature. The enhanced tensile strength reaching half of the strength of the conventional cast and thermomechanically processed NiTiHf HTSMAs was attributed to the lack of micropores (porosity 0.02 %). The tensile actuation performance is claimed to be facilitated by the strengthening effect from homogeneously distributed oxide nanoparticles introduced naturally by the LPBF fabrication. The oxide nano-particles decrease the size of domains of (001) compound twinned martensite created by the forward martensitic transformation upon cooling under stress, which reduces actuation strain but also suppresses dislocation slip in martensite. This dimensional strengthening mechanism lowers actuation strain but increases cyclic stability of the actuation response of the LPBF NiTiHf HTSMA.

The origin of cyclic instability of stress-strain-temperature responses of NiTi shape memory alloy (SMA) in cyclic thermomechanical loads (functional fatigue) represents one of the unsolved problems in the SMA field. In particular, the mechanism by which incremental plastic strains and permanent lattice defects are generated during thermomechanical cycling is not known. To reveal it, we evaluated recoverable strain, plastic strain and permanent lattice defects in austenite created by forward and reverse martensitic transformations (MT) proceeding under wide range of external tensile stress by closed loop thermal cycling under constant stress and transmission electron microscopy (TEM) analysis of permanent lattice defects in austenite consisting of slip dislocations and deformation bands, variation of crystal lattice orientation and elastic strain field in grains. We propose that forward MT proceeding upon cooling under external stress generates plastic strain via slip of [100](001) dislocations that nucleate at habit plane interfaces, glide in martensite across entire grains and disappear within grain boundaries. Reverse MT upon heating under external stress generates plastic strain also via [100](001) dislocation slip but on multiple slip systems within interfacial microstructure layers created by reorientation of martensite that propagate prior habit plane interfaces transforming the oriented martensite into plastically deformed austenite. Both forward and reverse MT proceed via cooperative transformation/twinning/ slipping within large number of grains enabling shape strains in grains to remain compatible at grain boundaries. When NiTi wire is subjected to cyclic thermomechanical loading, incremental plastic strains and permanent lattice defects are generated by the proposed deformation mechanisms anytime the forward and reverse MTs occur under external stress above certain thresholds characteristic for each NiTi wire. The plastic strains and permanent lattice defects accumulate upon thermomechanical cycling which brings about functional fatigue. Assuming dislocation slip in martensite as the origin of functional fatigue, it is discussed how NiTi-based alloys can be strengthened against it.

The instability of cyclic thermomechanical responses of NiTi (functional fatigue) represents one of the unsolved problems of NiTi technology. It has been intuitively understood that it originates from plastic deformation accompanying Martensitic Transformation (MT), but it is not known why and how it occurs. The mechanism by which thermomechanically loaded NiTi generates plastic strains has remained blurred for several decades despite its importance and research effort aimed at revealing the origin of functional fatigue. Recently, we investigated incremental plastic strains, martensite variant microstructures, martensite textures, and permanent lattice defects generated by forward and reverse MTs proceeding under tensile stress in experiments on superelastic (SE) and shape memory (SME) NiTi wires having recrystallized nanograin microstructure. In this work, based on the results of these earlier works, we propose the mechanism by which forward and reverse MTs proceeding under stresses above certain thresholds generate incremental plastic strains, the magnitudes of which are characteristic for stress–temperature conditions at which the MTs occurred. We claim that plastic strains are generated by [100](001) dislocation slip in (001) compound twinned martensite filling whole grains of nanocrystalline NiTi wires cooled and/or deformed at constant temperature under stress above certain stress thresholds. Dislocation slip in martensite is proposed to occur as a part of the cooperative transformation/twinning/slipping proceeding simultaneously within large number of grains allowing thus for strain compatibility to be achieved at grain boundaries of the nanocrystalline NiTi wire. The incremental plastic strains generated whenever the forward and/or reverse MTs occur above stress thresholds in cyclic thermomechanical loadings give rise to functional fatigue. It is discussed (i) how incremental plastic strains accumulating during cyclic thermomechanical loading cause functional fatigue of nanocrystalline NiTi wires, (ii) how stress–temperature diagrams updated with information on magnitudes of incremental plastic strains generated by forward and reverse MT under stress characterize functional fatigue performance of NiTi, and (iii) why SE wires show better functional fatigue performance than the SME wires.

NiTi-20Hf high-temperature shape memory alloys (HTSMAs) were fabricated using laser powder bed fusion (LPBF) additive manufacturing with low laser power and medium scanning speeds. Two alloys S1 (60 W, 120 mm/s) and S2 (70 W, 100 mm/s) were prepared using slightly different processing parameters. Both alloys displayed high strength 2 GPa, partial shape memory effect 1

Metastable metal-matrix composites are structures in which the constituent phases-the matrix and the particulate reinforcements-exist in a state of chemical non-equilibrium. These materials are most effectively produced through rapid powder consolidation techniques such as the spark plasma sintering. This study investigates and characterizes the multiscale microstructure of a Cu-Ag metastable metal-matrix composite, focusing on the morphology of the particulates, chemical composition, and dislocation density in both the as-produced and mechanically deformed states. Furthermore, the study elucidates the individual stress contributions, providing a comprehensive explanation of the mechanical factors underlying the observed differences in tensile yield strength between the metastable composite and the pure Ag matrix.

Metal matrix composites represent an interesting class of materials with an exclusive combination of properties. In this study, a unique Ag–W metastable metal matrix composite was produced from W@Ag core–shell powders using a spark plasma sintering technique at a temperature of 700 °C and a pressure of 80 MPa. The microstructures of a default powder and as-produced composite were observed by scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. As expected, the composite is characterized by a dual microstructure: a soft matrix of pure Ag with a submicrometer grain size reinforced by W particulates with a grain size of up to 30 μm. In addition, tensile and compression tests were performed with a deformation rate of 10–3 s−1 at ambient temperature. The value of the compression yield stress of Ag–W MMC is higher than the compression yield stress of pure Ag by approximately 467