The temperature dependence of the mechanical properties of the CoCrFeNi medium-entropy alloy (MEA) manufactured by laser-directed energy deposition (L-DED) and additionally annealed at 1200 °C for 24 h was studied. The microstructure of the as-deposited alloy was represented by a single-phase face-centered cubic structure with coarse columnar grains and a high density of dislocation. Annealing resulted in the development of recrystallization and a reduction in dislocation density. The CoCrFeNi alloy produced by L-DED demonstrated mechanical properties comparable with those of the fine-grained equiatomic CoCrFeMnNi alloy, produced by casting followed by thermomechanical processing. Namely, as-deposited CoCrFeNi had a yield strength (YS) and ultimate tensile strength (UTS) of YS = 370 MPa and UTS = 610 MPa at room temperature, and YS = 565 MPa and UTS = 965 MPa at cryogenic temperature, along with a ductility of ~60%. Annealing resulted in a decrease in strength to YS = 180/350 MPa at 293/77 K. A quantitative analysis of various strengthening mechanisms showed that some strength increment of the as-deposited alloy was ensured by the high dislocation density formed during L-DED.

Herein, structure, mechanical properties, and oxidation behaviour of a refractory (CrHfMoNbTaTiVZr)(84)Si-16 complex concentrated alloy (RCCA) prepared by vacuum arc melting were studied. Consisting of C11(b)-MeSi2, C14 Laves, bcc, and Me5Si3 phases, the alloy exhibited better 1200 degrees C-specific yield strength (45 MPa cm(3) g(-1)), fracture toughness (5.77 +/- 1.58 MPa m(1/2)), and 1 h-mass gain at 1200 degrees C (similar to 60 mg cm(-2)) than most of current RCCAs.

The paper presents a novel gradient-structuring strategy to enhance mechanical properties of metastable austenitic stainless steels (MASSs) by bulk-dominated cold deformation and following low-temperature phase reversion annealing. To obtain a large-scale gradient structure, the rod of an AISI 321 MASS was subjected to cold rotary swaging (CRS) to a maximum area reduction of 91 %. The gradient distribution of the alpha '-martensite volume fraction was attained in the as-swaged rod. Meanwhile, cellular austenite and lamellar austenite/ alpha '-martensite were found in the rod center, whereas the ultrafine-grained alpha '-martensitic microstructure was observed at the rod edge. During further low-temperature annealing (at 550-650 degrees C), the martensitic alpha'->gamma reversion occurred, which was accompanied by inheritance of the gradient structure morphology from the as- swaged material. Medium- (at 700 degrees C) and high-temperature annealing (at 800 degrees C) caused microstructure coarsening due to the diffusional alpha'->gamma reversion, static recrystallization, and grain growth. After CRS, the duplex < 111 >- and < 100 >-fiber texture of austenite and < 110 >-fiber texture of the alpha '-martensite were detected in the rod center, while shear textures B/B of austenite and D1/D2 of the alpha '-martensite were derived at the rod edge. Low- temperature annealing developed the < 111 >-fiber texture of austenite in the rod center. Through the improved strain-hardening behavior, increased strength and ductility of the AISI 321 MASS were achieved after low- temperature annealing. Thus, the strength-ductility trade-off was overcome.

High-entropy alloys (HEAs) with a face-centered cubic (FCC) structure exhibit remarkable strength at cryogenic temperatures; their fatigue properties remain relatively unexplored, however. The majority of the existing research focuses on the fatigue behavior of CoCrFeMnNi alloy systems at room temperature. The present study reports the fatigue behavior of a C-doped CoCrFeMnNi alloy at 77 K. The program alloy was obtained by vacuum induction melting and subsequent thermomechanical processing, due to which a recrystallized fine-grained microstructure reinforced by Me23C6 carbides was formed. Reducing the test temperature from 293 to 77 K was found to led to an increase in both yield strength and ultimate tensile strength by similar to 25-30 %. Besides at cryogenic temperature the fatigue strength limit after 106 cycles increased from 460 MPa to 620 MPa. The exceptional fatigue strength achieved at 77 K is due to the cumulative effect of hardening mechanisms, specifically dislocation and dispersion hardening, as well as due to the development of deformation twinning. The data obtained provided valuable insights into the potential use of C-doped CoCrFeMnNi alloys under cryogenic conditions, enhancing our understanding of their performance capabilities in extreme temperature environments.

The effect of a heterogeneous structure obtained via cold rotary swaging (CRS) and post-deformation annealing (PDA) on the dynamic mechanical properties of a non-equiatomic 49.5Fe-30Mn-10Co-10Cr-0.5C (at.%) medium-entropy alloy at room and cryogenic temperatures was studied. CRS to a reduction of 92% and subsequent PDA at 500–600 °C developed a heterogeneous structure consisting of a twinned γ-matrix and dislocation-free γ-grains in the rod core and an ultrafine-grained microstructure of γ-phase at the rod edge. Therefore, the maximum stress (σm) value increased. Charpy V-notch impact toughness (KCV) decreased after CRS to a reduction of 18% and stabilized after further straining. However, the contribution of the crack initiation energy consumption (KCVi) increased, while the crack propagation energy consumption (KCVP) decreased. PDA resulted in increases in KCVi and KCVP. A ductile-to-brittle transition occurred from −90 °C to −190 °C. Cryogenic Charpy impact testing of the heterostructured material revealed inflections on impact load–deflection curves. The phenomenon contributed to an increase in KCVP, providing a longer crack propagation path. The heterostructured material possessed an excellent σm-KCV combination in the temperature range between −90 °C and +20 °C.

Ti-6.5Al-2Zr-1Mo-1V/TiB metal matrix composites with 3 wt.% of TiB2 were obtained using vacuum arc melting and spark plasma sintering methods and compared with an unreinforced Ti-6.5Al-2Zr-1Mo-1V alloy. The microstructures of the unreinforced Ti6.5Al-2Zr-1Mo-1V alloy in the as-cast and as-sintered conditions were quite typical and consisted of colonies of α-lamellae embedded in the β matrix. The microstructure of the as-cast Ti-6.5Al-2Zr-1Mo-1V/TiB composite composed of TiB fibers randomly distributed within the two-phase α/β matrix, while the as-sintered composite had a network-like microstructure, in which areas of the two-phase α/β matrix were delineated by walls of TiB fibers. At room temperature, the yield strength of the as-cast and as-sintered Ti-6.5Al-2Zr-1Mo-1V alloy were 800 and 915 MPa, respectively, with a plasticity of 18% in both conditions. The addition of TiB fibers contributed to a ~40 and 50% strength increment, with values of 1100 and 1370 MPa for the as-cast and as-sintered composites, respectively. In the as-sintered composite, the strengthening effect reduced at 400 °C and almost disappeared at elevated temperatures of 800–950 °C. The as-cast composite showed much higher strength during warm and hot deformation—at 800–950 °C, the yield strength of the as-cast composite was 50% higher compared to the Ti-6.5Al-2Zr-1Mo-1V unreinforced alloy. A higher rate and degree of globularization were established for the as-cast composite compared to the unreinforced alloy. For the as-sintered composite, a noticeably lower rate and degree of globularization was shown. During hot compression of the as-cast composite, TiB fibers reoriented towards the metal flow direction, while the network microstructure formed in the as-sintered composite transformed into clusters of borides unevenly distributed within the matrix. Based on the obtained results, the apparent activation energy of plastic deformation was calculated, and the operating deformation mechanisms were discussed both for the as-cast and as-sintered composites. The Arrhenius flow stress model and the dynamic material model were used to evaluate the deformation behavior of composites beyond the experimentally studied temperatures and strain rates.

Microstructure and mechanical properties of the Fe65Co12.5Ni12.5Cr9.5C0.5 (in at.%) medium-entropy alloy obtained by the laser-based powder bed fusion (PBF-LB) was investigated in the as-produced condition and after (i) annealing in the interval 800-1100 degrees C or (ii) cold rolling followed by annealing at 800 degrees C. The as-produced microstructure was found to be surprisingly stable; noticeable recrystallization occurred only after annealing at 1100 degrees C. Cold rolling resulted in the formation of typical deformed microstructure, while further annealing led to the formation of a fine-grained recrystallized microstructure. A considerable improvement in cryogenic mechanical properties was obtained after some thermo-mechanical treatments due to the transformation-induced plasticity (TRIP) effect. The cold-rolled alloy had the best cryogenic strength characteristics; yield strength and ultimate tensile strength were found to be 1740 MPa and 2235 MPa, respectively. High strength was combined with a pronounced value of elongation to fracture of 24 %. Detailed analysis of microstructure and corresponding mechanical behavior changes caused by various processing of the as-produced alloy are presented.

In this study, low-density (< 6 g/cm(3)) AlCrFeTiX (x = Co, Ni, Cu) high-entropy alloys were produced by mechanical alloying and spark plasma sintering (SPS), and their structures and mechanical properties after SPS and annealing at 1000 degrees C for 24 h were reported. Both after SPS and annealing, the AlCrFeTiX (x = Co, Ni, Cu) alloys consisted of an L2(1) matrix phase with embedded bcc and C14 Laves phase particles. All the alloys had a nano-sized structure after SPS that retained in the AlCrFeTiCo and AlCrFeTiNi alloys after annealing. In the AlCrFeTiCu alloy, the annealing led to a coarsening, with an increase in the size of structural constituents above 1 mu m. Compression tests showed that the AlCrFeTiX (x = Co, Ni, Cu) alloys after SPS were brittle at 25 degrees C, but the AlCrFeTiCo alloy exhibited the highest peak strength of 3792 MPa. At 600 degrees C, the AlCrFeTiCo alloy after SPS also demonstrated the best performance, with the yield strength, peak strength, and plastic strain of 1960 MPa, 2121 MPa, and 1.8 %, respectively. The annealing did not eliminate the room-temperature brittleness, but improved the mechanical properties at 600 degrees C. The AlCrFeTiCo alloy showed a 15 %-increase in yield strength (2264 MPa) and more than 2.5 times higher compressive plasticity (4.7 %). The AlCrFeTiNi alloy became more ductile (1 %) and stronger (2200 MPa). For the AlCrFeTiCu alloy, the annealing had a negative effect that resulted in a halved plasticity at 600 degrees C. The AlCrFeTiCo and AlCrFeTiNi alloys after annealing showed record-high specific yield strength values at 600 degrees C, which were 383 and 371 MPa*cm(3)/g, respectively. With these values, the AlCrFeTiCo and AlCrFeTiNi alloys outperformed all the low-density medium-/high-entropy alloys available in literature to date obtained both by SPS or conventional casting. The structure formation and mechanical properties, as well as the response of these features to annealing, were extensively discussed.

The effect of cold rotary swaging and subsequent annealing on the microstructure, texture and mechanical properties of a Fe 49.5 Mn 30 Co 10 Cr 10 C 0.5 middle-entropy alloy was studied. Finite element modeling predicted inhomogeneous stress distribution and temperature gradient during deformation. Microstructure analysis indicated the development of deformation-induced gamma -> 8 8 martensitic transformation during earlier steps of cold rotary swaging (20-40 % reduction). However, a single-phase face-centered cubic structure was attained after 60-90 % reduction due to reverse 8 ->gamma transformation. Meanwhile, a twin-matrix microstructure was observed in the bar center, whereas an ultrafine microstructure was formed at the bar edge. Post-deformation annealing at 600 degrees C caused the onset of static recrystallization and thereby the formation of ultrafine dislocation-free grains. Apparently, after 60-90 % reduction, a < 100 >- and < 111 >- fiber texture gradient along the bar cross-section was developed. Besides, at the bar edge, a pronounced B/B / B shear texture was obtained that transformed into a Cube texture after annealing at 700 degrees C. Compared to the as-swaged material (yield strength (YS) =1250 MPa; ultimate tensile strength (UTS) = 1520 MPa; elongation to failure (EF) = 6.5 %), annealing at 500 degrees C resulted in additional strengthening (YS = 1655 MPa; UTS = 1660 MPa), but EF did not change noticeably. Yet, annealing at 600 degrees & Scy; degrees & Scy; was accompanied by an essential increase in both EF (to 11 %) and YS (to 1500 MPa). The applied approach can be used for obtaining a material with an attractive strength-ductility combination due to overcoming the strength-ductility trade-off.

Laser shock peening (LSP) is a relatively novel and promising surface hardening method. An absorbing layer, which is needed to protect the specimen surface from undesirable thermal effects caused by laser irradiation, should be considered as one of many varying parameters. The physical characteristics of the coating and its adhesion to the specimen surface can significantly influence the result of LSP. In this study, three commonly used absorbing coatings, namely black polyvinylchloride tape with a sticky layer, aluminum foil, and black alkyd paint were used to cover three-millimeter-thick plates of the Ti-6Al-4V titanium alloy with globular or lamellar microstructures. LSP of one side of the plates was carried out with a power density of 10 GW/cm2. The hole drilling method was used to evaluate residual stresses. The aluminum foil was found to be the optimal option for LSP of the Ti-6Al-4V titanium alloy. Microstructural investigations carried out using EBSD analysis suggested that no significant reduction in grain size, twinning, or dislocation density growth occurred as a result of LSP irrespective of the initial structure.

For this work, the behavior of the ZhS6K alloy (Russian grade) in the process of direct laser deposition was investigated. Two samples, a “small” one (40 × 10 × 10 mm3) and “large” one (80 × 16 × 16 mm3), were fabricated with direct laser deposition. In both samples, the typical dual-phase γ/γ’ microstructure with cuboidal shape of the γ’ precipitates was observed. Both specimens revealed a similar tendency to continuous increasing in hardness from the bottom to the top associated with the refinement of γ’ precipitates. The “small” sample was essentially crack-free, while the “large” one underwent extensive cracking. The possible effects of various factors, including thermal history, size, and shape of the gamma grains, on cracking behavior were discussed.

In this study, we analysed the effect of Fe, Mn, or Fe and Mn additions (5 or 10 at%) on the structure, mechanical properties, deformation behaviour, including microstructure evolution, and oxidation resistance of a lightweight (density of 3.92 g/cm3) intermetallic Al55Cr23Ti22 complex concentrated alloy (CCA) with a L12 + C11b structure. The additions of Fe, Mn, or Fe and Mn (at the expense of Cr) retained the density below 4 g/cm3 and resulted in forming a D8a phase (Th6Mn23-prototype; cF120; Fm-3m) in all the alloys, except for the Al55Cr18Ti22Mn5 alloy. The D8a phase nucleated with a different morphology within or adjacent to the C11b phase, and adopted an orientation relationship of (110)C11b || (44 0)D8a, [3 3 1]C11b || [1 11]D8a, which provided coherent C11b/D8a interfaces. Alloying with Mn or Fe and Mn had either neutral or negative effect on the mechanical properties, while the Fe additions boosted the strength along with some decrease in the compressive plasticity at room temperature. Specifically, the Al55Cr13Ti22Fe10 alloy showed a yield strength of 250 MPa at 1000 degrees & Scy;, which was 60 % higher compared to the Al55Cr23Ti22 alloy. During plastic deformation, all the alloys demonstrated pronounced strain hardening at T <= 800 degrees C and steady state flow at 1000 degrees C. Post-deformation observation of the microstructure of the Al55Cr13Ti22Fe10 alloy showed the inhomogeneous distribution of the strain-induced defects between the L12 matrix and (C11b + D8a) regions at 800 degrees C and partial recrystallisation of these phases at 1000 degrees C. All the alloys exhibited complex oxidation behaviour with multistage oxidation kinetics. The addition of 5 at% of Fe or Mn was beneficial for the oxidation resistance, while the further increase in their contents intensified the mass gain after a certain time. The latter effect was more pronounced in the Al55Cr13Ti22Mn10 alloy than in the Al55Cr13Ti22Fe10 alloy, because of forming a loose Al2O3 oxide layer. All the alloys investigated exhibited a superior synergy of 1000 degrees C-specific yield strength and compressive plasticity at room temperature, as well as a lower mass gain after 100 h at 1000 degrees C, compared to both lightweight and the most oxidation-resistant refractory CCAs.

Additive manufacturing enables the efficient production of intricate parts and the creation of multi-material mixtures of functionally graded materials. Such materials can demonstrate spatially variable mechanical and functional properties and thus can be attractive for various applications. In this work, we have produced gradient materials from a mixture of 316L stainless steel and pure Al by direct energy deposition. It was revealed that a relatively small (5–10 wt. ∼ 6×10^-4 to ∼ 1.5-3.0×10^-5 mm^2/N/m . The changes in properties were associated with the transformation of the FCC structure of steel to dual-phase BCC + B2, which occurred in reasonable agreement with the CALPHAD calculations. The obtained gradient materials can be possibly used for cost-effective shielding applications.

Features of microstructure, mechanical properties, and cracking behaviour of the René 80-type superalloy manufactured by laser-based directed energy deposition (DED-LB) have been investigated. Different cracking behaviours were observed in two thin walls fabricated by DED-LB under different conditions. The defect-free thin wall was produced at low power and scanning speed, while increasing energy input during the process led to intensive cracking. Two types of cracks that developed in the René 80-type superalloy were identified: solidification hot cracking and ductility-dip cracking (DDC), caused by σ-phase particles. It was found that transition from an equiaxed structure to a columnar one increased the susceptibility to crack propagation.

The TiNbZr/(Ti, Nb)B metal matrix composite with 2.5 vol.% of borides was produced by vacuum arc melting. The composite was then cold-rolled to thickness strains of 10, 20, 50, or 80%. In the initial condition, the composite had a network-like microstructure consisting of the soft TiNbZr matrix (dendrites) and the rigid (Ti, Nb)B shell (interdendritic space). In comparison with the as-cast condition, cold rolling increased strength by 17–35%, depending on the thickness strain. After the maximum thickness strain of 80%, yield strength and ultimate tensile strength of the composite achieved 865 and 1080 MPa, respectively, while total elongation was found to be 5%. Microstructural analysis revealed that cold rolling to 50% resulted in the formation of crossing shear bands caused by the considerable difference in deformation behavior of the matrix and reinforcements. Cold rolling to 80% led to the formation of a lamellar-like microstructure comprising the interlayers of the (Ti, Nb)B phase between the TiNbZr laths. The maximum strain (80% cold rolling) shortened the (Ti, Nb)B fibers into nearly equiaxed particles, with a length to diameter ratio of ~2.

Recently, Al-containing refractory high/medium-entropy alloys (RH/MEAs) have attracted special attention from materials scientists worldwide. Some of these alloys demonstrated an excellent combination of high-temperature strength and room-temperature tensile ductility provided by the formation of a structure consisting of body-centred cubic (bcc) matrix and B2 nanodomains/particles. However, oxidation resistance and phase stability, and, more importantly, the resulting mechanical properties of the Al-containing RH/MEAs, which are critical characteristics for high-temperature structural materials, remain unexplored. Herein, these aspects were extensively investigated for the Al7.5(NbTiZr)92.5 (in at%) alloy having an initial bcc+B2 structure. The alloy was subjected to long-term (500 h) annealing or oxidation tests at 600, 700, and 800°C followed by the evaluation of mechanical properties during uniaxial tension at 22 and 600°C. Microstructural studies of the specimens exposed to long-term annealing showed the formation of Zr5Al3 particles that precipitated along the grain boundaries and within grain interiors. The volume fraction of these particles was the highest (∼16%) after annealing at 600°C and the lowest (∼0.5%) after annealing at 800°C. Oxidation tests revealed that the alloy was prone to pesting phenomena, which intensified with the temperature. The alloy could sustain pesting only up to 10 h at 600°C, while it experienced a complete disintegration into powder after 5 h at 800°C. The poor oxidation resistance originated from the formation of volumetric, non-protective (Al,Nb,Ti,Zr)O4 oxide. While insignificantly affecting the strength, long-term annealing or oxidation deteriorated the ductility of the Al7.5(NbTiZr)92.5 alloy both at 22 and 600°C. The Zr5Al3 phase embrittled the alloy after long-term annealing at 600 or 700°C, but marginally reduced the ductility in the 800°C-annealed specimens due to the low volume fraction and relatively homogeneous distribution of this phase. The oxidation for 1 h decreased the room-temperature ductility owing to the premature macroscopic localisation of plastic deformation induced by synergistic effects from the volumetric (Al,Nb,Ti,Zr)O4 oxide. This study highlights the necessity for the examination of a set of properties of promising RH/MEAs to critically assess perspectives of these alloys to find applications as new high-temperature materials in aircraft and aerospace sectors.

For a refractory Nb-Ti-Zr-Cr alloy system, we show that the effect of increasing chemical complexity, which is accepted as the main source for unique properties of high-entropy alloys, on the strength and oxidation resistance is sensitive to the particular elements added. A transition from pure Nb to NbTi and then to NbTiZr alloys that retained a single-phase body-centred cubic (bcc) structure improved the ambient-to-high-temperature strength and oxidation resistance at 1000 C. However, the further addition of Cr that resulted in the Laves phase formation broke the trend. In the case of dual-phase bcc + Laves phase alloys, binary Nb100-xCrx alloys were much stronger at T > 600 C than microstructurally similar multicomponent refractory complex concentrated alloys (RCCAs). Meanwhile, ternary (NbTi)(100-x)Cr-x alloys had the best oxidation resistance due to an exclusive formation of a protective Cr2O3 layer. This study emphasises opposite strategies for designing high-performance single-phase bcc and Laves phase-containing Nb-Ti-Zr-Cr RCCAs.

Ti–6.5Al–2Zr–1Mo–1V/TiB metal-matrix composites were produced by vacuum arc or selective laser melting (SLM) methods using 0.7 and 2 wt.

The search for new high-entropy alloys (HEAs) with desired properties is an urgent problem that is hardly solvable experimentally due to the extremely large number of possible alloy compositions. Thus, methods for theoretical prediction of HEA's properties play a key role. Currently, effective predictive models are based on machine learning methods and modern data analysis algorithms. Here we address developing data-driven machine learning models (DDML) to predict the ductility of HEAs. We have built several DDMLs and found that the best approach is based on the Support Vector Classifier, which significantly outperforms phenomenological models (balanced accuracy of 0.784 and F-score of 0.824). By combining this model with a previously developed yield strength prediction model, we have predicted and fabricated novel HEAs of the Al-Cr-Nb-Ti-V-Zr system with good mechanical properties. An obtained Al1Cr9Nb35Ti5V40Zr10 alloy demonstrates a combination of high strength at room and elevated temperature, combined with good ductility at room temperature.