Precipitation Of Intermetallic Phases Research Articles

Grain Structure and Tensile Properties of As-Cast and Aged Pb–1.1%Sn–0.05%Ca

Abstract The effects of temperature parameters of casting procedure and aging time at room temperature on the grain structure and tensile properties of Pb–1.1%Sn–0.05%Ca alloy for positive grids of lead-acid batteries were studied by means of optical metallography, scanning electron microscopy, and tensile properties measurements. It was established that with melt or mold temperatures rising, grains in the alloy structure become larger, which causes an increase in the elongation and a decrease in the ultimate tensile strength of the alloy. Natural aging for 32 days does not noticeably change grain size but affects tensile properties of the alloy. Final ultimate tensile strength, yield strength, and Young’s modulus are found to be 2.7, 4.9, and 1.3 times higher, respectively, but the elongation is 2.4 times lower because of the precipitation of intermetallic phases.

Electrical property enhancement of non-heat-treatable wrought aluminum alloys using graphene additives

With growing efforts of electrification, aluminum’s role as a light-weight conductor material has become increasingly prominent. There is a critical need to improve the electrical performance of aluminum at room temperature and high operating temperatures. In this study, the effect of graphene nanoparticle additives on the electrical performance of a non-heat treatable alloy were (AA3003) explored. Graphene’s unusual structure and electronic properties were used to improve AA3003 properties. Here, the effects of graphene on the evolution of electrical properties and microstructural features have been explored on lab scale hot extruded AA3003-graphene composites. Hot pressing schedules and extrusion temperatures were varied to investigate changes in intermetallic dispersion characteristics in the presence of dispersed graphene. We measured a reduction of 10.3 % in the temperature coefficient of resistance in the AA3003 sample with 0.05 wt% graphene extruded at 400 °C, along with a maximum increase of 1.1 % in electrical conductivity at 20 °C. Increasing the hot-pressing times up to 8 hours was also found to consistently increase the electrical conductivity, due to increased precipitation of intermetallic phases. Despite being a non-heat treatable alloy, AA3003 displays interesting precipitation dynamics and grain recrystallization trends that can be modulated with varying levels of heat treatment, graphene concentrations, and hot extrusion process parameters.

Microstructural evolution and phase analysis of SS410–Al2O3–SiC multilayered functionally graded composite fabricated through laser cladding

In the present study, a functionally graded metal-ceramic composite structure comprising SS410, Al2O3, and SiC particles with varying composition, structure and properties in a single deposit was successfully fabricated using direct energy deposition (DED). The composition varied from 90 % SS410 with 10 % (xAl2O3 + ySiC) to 10 % SS410 with 90 % (xAl2O3 + ySiC), where x = 70 % and y = 30 % in a five-layered FGM design. The microstructure, phase evolution and mechanical properties of the components with different composition gradients were characterized by microscopy, energy dispersive spectroscopy, wavelength dispersive spectroscopy, X-ray diffraction and micro hardness. The investigation revealed that complex phases such as C0.12Fe0.79Si0.09, M7C3, Fe3C, and Fe3Si formed as a result of the interaction between SiC and SS410 during the cladding process. Microstructural analysis showed a transition from columnar dendrites at the bottom zone of the cladding to a typical cellular structure and carbide particle agglomerates in the middle zone, with finer grains at the top zone. This gradation in microstructure correlated with an increase in microhardness ranging from 299 HV at the bottom to 966 HV at the top. This enhancement in hardness is primarily attributed to solid solution strengthening from C and Si originating from the decomposition of SiC and the precipitation of intermetallic phases.

Effect of Artificial Aging in the Microstructure and Corrosion Performance of Superduplex UNS S32750 Stainless Steel

Superduplex stainless steels have great mechanical and corrosion properties. However, its chemical composition makes it prone to intermetallic phase precipitation during thermal processing. Sigma (σ), chi (χ), and chromium nitrides (Cr2N) remove Cr and Mo from the matrix, reducing the corrosion and mechanical resistance. Understanding the effects of thermal processing on the secondary phase’s precipitation and depletion of the material’s performance is crucial to its applications. Thus, this work aims to analyze the behavior of the corrosion performance of the UNS S32750 after thermal treatment at 800°C, for 60, 180, 300, and 420 minutes, in comparison to the as-received material. Optical emission spectrometry, X-ray diffraction, and SEM with backscattered electrons (BSE) were used to evaluate the material. The corrosion performance was evaluated with the cyclic potentiodynamic polarization technique. The main results and conclusions obtained in the study were a decomposition of the ferrite phase into the χ and σ phases, with the formation of the χ phase being predominant in shorter times, while for longer aging times σ formed in greater quantities. It was also possible to verify a more aggressive corrosion trend for aged samples in the regions adjacent to the formation of the χ and σ phases. It was also possible to observe that the losses generated in corrosion resistance were greater for aging times longer than 60 minutes. The aging treatment significantly reduced the material’s corrosion resistance in conjunction with the formation of precipitates.

Mechanical Properties of Thermally Annealed Cu/Ni and Cu/Al Multilayer Thin Films: Solid Solution vs. Intermetallic Strengthening

In this study, Cu/Ni and Cu/Al multilayers, with individual layer thickness varying from 25 nm to 200 nm, and co-sputtered Cu-Ni and Cu-Al single layer films were deposited at room temperature via magnetron sputtering and further annealed from 100 °C to 300 °C. The mechanical and microstructural properties of the as-deposited and annealed samples were characterized by nanoindentation, x-ray diffraction, and scanning electron microscopy. Both multilayer systems exhibit an increase in hardness with increasing annealing temperature. However, the Cu/Ni system shows a gradual and moderate hardness increase (up to 30%) from room temperature to 300 °C, while the Cu/Al system displays a sharp hardness surge (~150%) between 125 °C and 200 °C. The co-sputtered Cu-Ni and Cu-Al samples consistently demonstrate higher hardness than their multilayered counterparts, albeit with distinctly different temperature dependence—the hardness of Cu-Ni increases with annealing temperature while Cu-Al maintains a constant high hardness throughout the entire temperature range. The distinct thermal strengthening mechanisms observed in the two metallic multilayer systems can be ascribed to the formation of solid solutions in Cu/Ni and the precipitation of intermetallic phases in Cu/Al. This study highlights the unique advantage of intermetallic strengthening in metallic multilayer systems.

Heat-Affected Zone Microstructural Study via Coupled Numerical/Physical Simulation in Welded Superduplex Stainless Steels

Superduplex stainless steels (SDSS) are known for their combination of good mechanical properties and excellent corrosion resistance, enabled by the microstructural balance between austenite and ferrite and an amount of alloying elements. Their application in welded components is, however, limited by the possibility of the precipitation of intermetallic phases and microstructural misbalance, which might hinder their properties, especially in the heat-affected zone (HAZ). This work introduces a methodology that relies simultaneously on physical and numerical simulations to study the HAZ in a UNS S32750 SDSS. Dimensions of the fusion zone and thermal cycles were calibrated for a numerical model using preliminary welding trials. Numerically simulated cycles for each heat input (HI) were physically reproduced in a Gleeble® simulator, and the heat-treated samples were characterized and compared with real specimens welded using the same parameters. Thermal curves resulting from the numerical simulations were successfully replicated by the Gleeble®, indicating adequate application of the desired HI. The hardness and microstructural results from simulated and welded specimens were also found to be quite similar. Therefore, the proposed methodology showed itself adequate not only for the study of duplex stainless steels, but also of materials with similar thermal and mechanical properties, including the extrapolation of welding parameters.

Kinetics of Intermetallic Phase Precipitation in Manual Metal Arc Welded Duplex Stainless Steels.

The article presents the influence of heat treatment on the kinetics of transformations in lean duplex LDX2101 steel and a weld made of standard duplex 2209 material, which was welded by manual metal arc welding. Changes in the microstructure, hardness, and magnetic phase content were analyzed after heat treatment was conducted at a temperature of 800 °C for a period ranging from 15 to 1440 min. Light and scanning microscopy, Vickers hardness measurements, and magnetic phase content measurements using a ferritoscope were used for the research. In the LDX2101 steel, the presence of δ-ferrite and γ austenite was identified and additional Cr2N nitrides were observed in the heat-affected zone. After heat treatment, the decomposition of δ ferrite into γ2 austenite and Cr2N nitrides was observed in both areas. In the case of weld made by the coated electrode in 2209 grade, a ferritic-austenitic microstructure with allotriomorphic austenite (γA), Widmanstätten austenite (γW), and idiomorphic austenite (γI) and δ-ferrite area with "bee swarms" of fine precipitations of chromium nitrides Cr2N and non-metallic inclusions (NMIs) of slag, formed during the welding process, are observed in the as-welded state. After heat treatment, the presence of the χ phase (after 15 min of annealing) and the σ phase (after 120 min of annealing) was additionally identified. The kinetics of intermetallic phase evolution in welds made from 2209 material were presented. The obtained results of hardness measurements and metallographic tests were correlated, which allowed for a quick check of the precipitation processes on the used element.

Optimization of magnetic domain structure and soft magnetic properties of FeCoNiGd medium-entropy alloys by gadolinium addition

Soft magnetic high-entropy alloys (HEAs) have attracted much attention due to their excellent mechanical properties and high electrical resistivity. However, the magnetic dilution caused by non-magnetic elements in HEAs greatly reduces their saturation magnetization (Ms). In this work, FeCoNiGdx medium-entropy alloys (MEAs) were prepared by adding gadolinium, a rare earth element with ferromagnetism near room temperature, into FeCoNi alloy to study its influence on the microstructure and magnetic properties. The results demonstrate that the addition of Gd can promote the precipitation of intermetallic compound phases in FeCoNi alloy. When x decreases from 1 to 0.5, the intermetallic compound phases change from GdM2 (M=Fe, Co, Ni) phase to GdM5 phase, and it becomes Gd2M17 phase if x ≤ 0.07. Moreover, the addition of a minor amount of Gd significantly optimizes the remanence (Br), while slightly worsens the saturation magnetization (Ms) and coercivity (Hc). Finally, the FeCoNiGd0.03 MEA achieves the best comprehensive magnetic properties with high Ms= 149.58 emu/g, low Hc= 434.3 A/m and Br= 0.231 mT owing to the high content of ferromagnetic elements and reasonable distribution of the magnetic domain structure. This work can provide insights into the role of rare earth elements on microstructure and magnetic properties of MEAs and HEAs.

Study Of the Effect of Titanium Additions on The Mechanical and Corrosion Properties of AISI 316 Powder Metallurgical Steel

Context: Powder metallurgy uses metallic and/or non-metallic powders that, through mixing, compacting, and sintering operations, allow obtaining large series of products. In austenitic stainless steels, Cr23C6-type carbides can precipitate at temperatures between 450 and 950 °C. When this occurs, the steel is susceptible to being attacked at its grain boundaries by a phenomenon called sensitization. Titanium is added as a ‘stabilizer’ because it has a greater affinity with carbon for the formation of species at a temperature of approximately 900 ºC, and, during cooling, it consumes the carbon forming MC-type carbides, inhibiting the precipitation of Cr23C6. Method: The composition and morphology of the powders were characterized, leading to the formulation of an alloy matrix consisting of a mixture of AISI 316 steel powders of two different particle-size distributions in a proportion that produced the highest density and the lowest porosity in the sintered material. Titanium was added at two levels (0,4 and 1,0 wt%), and sintering was carried out with nitrogen. The corrosion rate was determined by potentiodynamic polarization. Vickers hardness and pin-on-disk wear tests were performed. The stages were complemented with a microstructural analysis. Results: The addition of 0,4 wt% of titanium decreased the steel’s rate of corrosion, albeit in the absence of passivation. The microstructure consists of austenite, ferrite, and TiC precipitates. The addition of 1,0 wt%Ti showed an increase in the corrosion rate, with a microstructure containing austenite, ferrite, TiC carbides, and the Laves ɳ-Fe2Ti phase. Conclusions: The results were compared against thermodynamic simulations in the Thermo-Calc software, which were consistent with the microstructural analysis, showing the phenomena of stabilization as well as the precipitation of intermetallic phases and highlighting the importance of establishing strict controls in the formulation of powder metallurgical alloys due to the transformations that can take place due to the effect of the thermal cycles of the process.

Effect of layer thickness and laser emission mode on the microstructure of an additive manufactured maraging steel

In this work, a high-performance maraging steel M300 was processed by laser powder bed fusion, where the layer thickness and the laser emission mode were modified. As-built microstructures were studied by considering the metallurgical phenomena taking place during printing: melting of the material deposited over previous layers; rapid solidification of the melt pool; and martensite-to-austenite reversion and precipitation of intermetallic phases induced by the combined effects of repeated heating and cooling cycles in the process. After printed, parts were subjected to ageing treatments at temperatures ranging from 480 to 540 °C. Microstructural characterization results evidenced the importance of layer thickness as a key parameter to modify the solidification cell size of the as-built samples. Two different phases were identified from the X-ray diffraction patterns in the microstructure: BCT martensite and FCC austenite. It was found that the tetragonality of martensite remained constant across different layers for all as-built conditions regardless of the successive reheating cycles, which are experienced during subsequent melting passes. No significant effect of the studied printing parameters was observed on the evolution of the microstructure during ageing. Finally, it was observed that the hardness of aged microstructures is comparable to that obtained by conventional manufacturing methods.

Predicting the early stages of solid-state precipitation in Al-rich Al-Pt alloys

The high strength of structural aluminium alloys depends strongly on the controlled precipitation of specific intermetallic phases, whose identity and crystal structure can be difficult to predict. Here, we investigate the Al-Pt system, which shows some similarity to the Al-Cu system as one of the main intermetallic phases, Al2Pt, is nearly isostructural with θ′ (Al2Cu), the metastable phase responsible for the high-strength of Al-Cu alloys. Using a combination of density-functional theory (DFT) calculations and classical nucleation theory (CNT) applied to the Al-Pt system, we design a workflow to predict the thermodynamics of solid solution, intermediate phases such as GP zones, stable and metastable precipitates, and their precipitation sequence. This workflow can be applied to an arbitrary binary alloying system. We confirm the known stable phases Al4Pt, Al21Pt8, Al2Pt, Al3Pt2, AlPt (α&β), Al3Pt5, AlPt2 (α&β) and AlPt3 (α&β). We also reveal the possible existence of two phases of chemical formulae Al5Pt and Al3Pt. This large number of intermetallic phases is due to the strong bonding between Al and Pt, which also leads to a significantly favourable Pt solute formation energy in the Al matrix. Our findings are compared with the known precipitation characteristics of the binary Al-Cu and Al-Au systems. We find that the θ′-like Al2Pt precipitate phase has a lower coherent interfacial energy than θ′. Our calculations strongly suggest that Al2Pt will precipitate first in Al-rich Al-Pt alloys and will form bulk-like interfaces similar to η (Al2Au) rather than like θ′ (Al2Cu).

Influence of Friction Stir Weld Parameters on the Corrosion Susceptibility of EN AW‐7075 Weld Seam and Heat‐Affected Zone

Friction stir welding enables joining of high‐strength, lightweight aluminum alloys, e.g., EN‐AW‐7075, below the melting point by induced plastic deformation. Therefore, heat transfer into the adjacent regions beneath the weld seam is significantly reduced as compared to fusion welding processes such as laser beam welding. However, specific zones along the weld seam area are susceptible to localized corrosion due to grain growth and the precipitation of intermetallic phases. Thus, several approaches toward lowering the corrosion susceptibility of the heat‐affected zone are presented. Special interest is given to increasing the plastic deformation by the use of novel multipin welding tools that eventually facilitate reduced heat input during welding as a result of substantially lower tool revolutions. The corrosion behavior is tested by means of full material immersion tests and electrochemical measurements which provide insight into the corrosion kinetics. Using pre‐ and postmortem microstructural analysis, the mechanisms influencing the initiation of corrosion can be identified. Supported by in‐operando temperature measurements, the varied welding parameters and their interrelationships to corrosion resistance can be derived. Furthermore, recommendations on optimal welding parameters to obtain enhanced corrosion resistance can be deduced.

Influence of build orientation and aging on corrosion behaviour of 18Ni300 maraging steel produced via Powder Bed Fusion using Laser Beam (PBF-LB)

Conventional maraging steel corrodes uniformly but additive manufactured (AM) maraging steel can corrode differently, depending upon the angle of exposure of build layers to the corrosive media. The primary objective of this investigation is to examine the effect of build orientation and solution aging treatment (HT) on the corrosion behaviour of the AM maraging steel and to compare with that produced through the conventional route of casting and hot rolling. In this investigation, corrosion behaviour of maraging steel samples produced by Powder Bed Fusion using Laser Beam (PBF-LB), with different build orientations of 0°, 45°, and 90°, was investigated in a 3.5% NaCl solution. Corrosion resistance of the as-built (AB) samples was found to increase with decrease in the angle of inclination and related to structural heterogeneity and grain boundary density. However, corrosion resistance of the AB samples was decreased following heat treatment (solutioning and aging) and was controlled by precipitation of intermetallic phases and reversed austenite generation. While there was dominance of protective oxide layer of TiO2 in the AB and conventional sample, this oxide layer was not observed on the heat treated (HT) samples. Further, a marked difference was observed in pitting corrosion of the AB and HT samples.

Laser In Situ Synthesis of Gradient Fe-Ti Composite during Direct Energy Deposition Process

The suitability of the direct energy deposition process of exothermic powders Fe-Ti in joining dissimilar metals to produce small parts of a complete shape for various applications is considered. The procedure of the direct energy deposition of commercial pure iron and titanium in various proportions and the modes of the process are described. Optical microscopy and SEM with EDX analysis, X-ray analysis, and microhardness measurements of laser-fabricated intermetallics are applied. Intermetallic compounds of FeTi, Fe2Ti, eutectoids, complex titanium oxides and nitrides, and iron carbides are found. Interlayer and trans-layer cracks and pores are observed. A microhardness growth from 150 HV to 900 HV was obtained for all samples due to the precipitation of brittle intermetallic phases in the gradient Fe-Ti system during the DED. The dispersion of microhardness values becomes significant in Ti-rich areas; there, pores and cracks are found. The revealed structure features are considered in relation to published results and explained. Increased concentrations of Ti to Ti + Fe = 3:1 on the Fe- and Fe + Ti -substrate with concentrations of Ti + Fe = 1:1 and Ti + Fe = 1:3 lead to increasing hardness and its distribution, but also increases in residual microstress. Recommendations are given to reduce the power during the direct energy deposition of titanium layers and to apply Fe-substrate, which can reduce residual stress, pores, and cracks.

Design of Fe36.29Cr28.9Ni26.15Cu4.17Ti1.67V2.48C0.46 HEA using a new criterion based on VEC: Microstructural study and multiscale mechanical response

The present work updated empirical parameter ranges with a high adjustment to the experimental results as obtained by the means of the exploratory data analysis (EDA) method. A new high entropy alloy with FCC structure reinforced by precipitation of intermetallic phases fabricated by vacuum-argon induction melting was designed and modeled. The main results showed that the valence electron concentration (VEC) is the predominant empirical phase prediction parameter in high entropy alloys and that the lattice packing factor is necessary to determine the presence of intermetallic phases effectively. Phase stability ranges based on VEC associated with the base crystal structures and the presence of intermetallics corroborated computationally by CALPHAD and experimentally were reported. The interdendritic zone proved to be a preferential precipitation zone with precipitates of σ, TiC, and γ' phases. The γ' phase showed a size difference between the dendritic and interdendritic zone associated with diffusive Ti mechanisms. The nanoscale mechanical response determined that dislocation creep and reinforcement in the interdendritic zone are predominant creep mechanisms that obtained an effective entanglement of the dislocations increasing the strain hardening coefficient. The mechanical response of the alloy obtained is superior to the average of the alloys with FCC structure. It maintains a high ductility that allows reaching an energy absorption and damage tolerance of 43.56 GPa% showing severe plastic slip lines being in the range of use for aerospace applications and hydrogen tanks.

Orderly nucleation and competitive growth behaviors of Ti-Al intermetallic compounds in Ti/TiAl3 diffusion couple under high temperature

Precipitation and growth of intermetallic phases during thermal processing play an important role in the formation and evolution of heterogeneous microstructure of Ti-Al intermetallic based alloys, which have not been adequately understood. Herein, we study the nucleation and growth behaviors of Ti-Al intermetallic compounds in Ti/TiAl3 diffusion couple based on high temperature annealing. We combine scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) to analyze the microstructure and crystallization behavior of newly-formed intermetallic phases. The experimental results showed that phase transformations of α-Ti → Ti3Al → TiAl → TiAl2 and TiAl3 → Ti2Al5 → TiAl2 occurred orderly during annealing at temperatures from 950 ℃ to 1150 ℃. Intermetallic compounds containing Ti3Al, TiAl, TiAl2 and Ti2Al5 were formed at Ti/TiAl3 interface and comprised four distinct single-phase layers. The newly-precipitated intermetallic grains usually presented certain crystallographic orientation relationships with the adjacent phases. Growth kinetics analysis indicated that the growth of intermetallic layers was governed by volume diffusion and grain boundary diffusion, respectively, during annealing at 950 ℃ and 1050 ℃. Both interfacial reaction and volume diffusion contributed to the growth of intermetallic layers at 1150 ℃. Besides, the competitive growth between different intermetallic phases was revealed. To provide more guidance for the design of high-performance Ti-Al intermetallic based alloys, the hardness and elastic modulus of the above intermetallic compounds were determined preliminarily in the current work.

Intermetallic phase precipitation and oxidation behavior of Fe–20Cr–0.5Nb–2Mo (at%) high-Cr ferritic alloy at high temperatures

The precipitation behavior of intermetallic compounds, such as C14 Laves phase and Chi (χ) phase, and oxidation behavior of an Fe–20Cr–0.5Nb–2Mo (at%) high-Cr ferritic alloy were investigated at high temperatures between 1023 K and 1123 K. During aging/oxidation heat treatment at 1073 K, the protective Cr2O3 is rapidly formed on the surface, consecutively, a high density of fine Laves precipitates, (Fe,Cr)2(Mo,Nb), are formed throughout the α-Fe matrix and preferably at the α/Cr2O3 interfaces as well as α-Fe grain boundaries. The Laves precipitation causes local changes of solute concentrations in the surrounding α-matrix, i.e., decrease in Nb and increase in Mo. These local changes of phase equilibria in the vicinity of the α-grain boundaries and α/Cr2O3 interfaces result in the formation of the χ-phase which is not supposed to be precipitated. Moreover, χ precipitation at the α/Cr2O3 interfaces is most favored in case the alloy is oxidized at 1073 K. In the short-term oxidation at 1073 K, the Cr-rich χ interfacial precipitates, Fe36Cr12(Mo8,Nb2), suppress the growth of Cr2O3. However, as the interfacial χ precipitates coarsen in the long-term oxidation at 1073 K, localized stress concentrations are generated at intersections between the Cr2O3 layer and the α/χ interfaces. Consequently, these facilitate cracking of the adjacent surface oxide, then oxide ‘rehealing’, and formation of the abnormal-Cr2O3 process. Eventually, the precipitation of interfacial χ phase indirectly causes an acceleration growth of the surface Cr2O3 on the alloy.

Austenite Reversion Behavior of Maraging Steel Additive-manufactured by Laser Powder Bed Fusion

This study was set to fundamentally investigate the characteristics of austenite reversion occurring in maraging steels additive-manufactured by laser powder bed fusion (L-PBF). The maraging steel samples manufactured under different L-PBF process conditions (laser power P and scan speed v) were subjected to heat treatments at 550°C for various durations, compared with the results of the austenitized and water-quenched sample with fully martensite structure. The L-PBF manufactured samples exhibited the martensite structure (including localized austenite (γ) phases) containing submicron-sized cellular structures. Enriched alloy elements were detected along the cell boundaries, whereas such cellar structure was not found in the water-quenched sample. The localized alloy elements can be rationalized by the continuous variations in the γ-phase composition in solidification during the L-PBF process. The precipitation of nanoscale intermetallic phases and the following austenitic reversion occurred in all of the experimental samples. The L-PBF manufactured samples exhibited faster kinetics of the precipitation and austenite reversion than the water-quenched sample at elevated temperatures. The kinetics changed depending on the L-PBF process condition. The enriched Ni element (for stabilizing γ phase) localized at cell boundaries would play a role in the nucleation site for the formation of γ phase at 550°C, resulting in the enhanced austenite reversion in the L-PBF manufactured samples. The variation in the reaction kinetics depending on the L-PBF condition would be due to the varied thermal profiles of the manufactured samples by consecutive scanning laser irradiation operated under different P and v values.

Mechanical properties of Al-Nb in situ metal-matrix composites fabricated by constrained high pressure torsion at 10 GPa and subsequent annealing

Mechanical alloying of dissimilar metals by means of severe plastic deformation techniques is known to be a perspective method of obtaining in situ metal matrix composites with advanced properties. Recent investigations found out that besides severe mixing of elements, intensive formation of non-equilibrium intermetallic phases takes place. Presence of intermetallic particles in the composite affects both its physical and mechanical properties. In this work we have realized the fabrication of Al-Nb composites in one step by means of high-pressure torsion technique. A non-trivial phenomenon of growth of tensile strength with increasing number of revolutions during HPT was revealed. It was explained by more homogeneous mixing of components and precipitation of intermetallic phase in the vicinity of Al-Nb interphases. The microstructural observations and X-ray diffraction analysis allowed to reveal Al3Nb intermetallic phase in the as deformed composites, while the equilibrium temperature of formation of this phase is above 600°C. Varying the number of revolutions and post deformation annealing allows obtaining composites with different fractions of intermetallic particles. This approach can be considered as a way for the development of composites with a tailorable complex of physical and mechanical properties.

Precipitation of metastable intermetallic phases and nanoindentation behaviors of a Ti-20Zr-9 Nb-4Sn (at%) alloy during aging treatment

In this study, the microstructure evolution and nanoindentation behavior of a Ti-20Zr-9 Nb-4Sn (at%) alloy aged at 600 ºC for 1–60 h were investigated by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, Vickers microhardness test and nanoindentation technique to show the potential for precipitation strengthening effect of (Zr,Sn)-rich precipitates. The Vickers hardness in the 600 ºC aged specimens increased from 218 HV to the maximum of 242 HV with increasing the aging time from 1 to 13 h. With further prolonging the aging time to 60 h, the hardness decreased to 219 HV at 45 h aging and then almost keep constant at 45–60 h aging. The metastable intermetallic (Zr,Sn)-rich precipitates were formed in all aged specimens and their composition is highly dependent on the aging time as a result of elemental partitioning during aging. In the specimen aged for 24 h, the nanoindentation hardness (18.6 GPa) of the (Zr,Sn)-rich precipitates (Zr3Sn-type) was much higher than those of the α precipitate (9.3 GPa) and matrix (7.6 GPa), which suggests that the hard (Zr,Sn)-rich precipitate should be responsible for the high Vickers hardness in the specimen. These findings show that the precipitation hardening effect in Ti-Zr-Nb-Sn alloys is not just limited to the well-known isothermal ω and/or α precipitates and the (Zr,Sn)-rich precipitates also exhibit great potential for precipitation strengthening effect.