Supersaturated Solid Solution Research Articles

Wire-based friction stir additive manufacturing towards isotropic high-strength-ductility Al-Mg alloys

ABSTRACT The formation of eutectic Al-Mg2Al3 phases along the grain boundaries makes it difficult to improve the mechanical properties of high-magnesium-content aluminum alloys. Here, wire-based friction stir additive manufacturing(W-FSAM) was proposed as a novel solid-state additive manufacturing technology. The shearing, transport, and thermo-plasticisation processes of the deposited materials were achieved by a rotational tool and a stationary barrel. The original Mg2Al3 phases were refined and dissolved into the matrix and formed supersaturated solid solution structures. The refined grains with a diameter of 1.27 ± 0.64 μm were achieved through the dynamic recrystallization process. The components achieved isotropic mechanical properties due to the homogeneous equiaxed fine-grain structure. The ultimate tensile strength reached 415 ± 11 MPa with an elongation of 31.0 ± 2.0%, superior to the commercial 5xxx products. The enhanced strain hardening capacity was achieved by the suppressed emission of dislocations from grain boundaries and the interaction between the supersaturated solid solute atoms and mobile dislocation.

Grain boundary sensitization kinetics of cold-rolled Al–Mg alloys

As super-saturated solid solutions of Al-Mg, 5XXX series aluminum alloys are susceptible to sensitization via intergranular precipitation of the anodic β-phase, which promotes intergranular corrosion, exfoliation and stress corrosion cracking under environmental conditions. This study presents important updates to a Johnson–Mehl–Avarami–Kolmogorov (JMAK) type model for low-temperature sensitization, correlating the intergranular corrosion response to impingement of locally sensitized regions surrounding discrete β-phase grain boundary precipitates. It is demonstrated that the sensitization response of these alloys can be approached as a combination of two independent contributions: the geometric configuration of grain boundaries passing through the microstructure that are most prone to sensitization, and the rate that these boundaries sensitize due to the formation of the β-phase. This allows for the large sensitization response variations found between nominally identical materials produced by different suppliers, which originate due to a lack of constraints within current cold-rolled plate tempers, to be removed as a sample-dependent linear scaling factor that is separate of the rate kinetics. The JMAK model describes the kinetics of 5xxx series sensitization with excellent accuracy across all data available in the literature. The results of the model imply that sensitization at environmental temperatures proceeds via a site-saturated process, with the β-phase forming on a set density of preferential nucleation sites. It is shown that site-saturation allows for extension of the JMAK model to non-isothermal aging profiles and supports a diffusion pathway dominated by pipe diffusion to the interface followed by precipitate growth via the collector plate mechanism.

Enhancing mechanical and corrosion properties of Al-Zn-Mg-Cu alloy through electropulsing and aging alternate treatment

The contradiction between mechanical and corrosion properties restricts the further research and application of Al-Zn-Mg-Cu alloy. Studies report a new technique, electropulsing and aging alternate treatment (EPAAT), including twice electropulsing and aging treatment, to solve this contradiction by precisely controlling the intragranular precipitates and grain boundary precipitates (GBPs). The results show that the grain boundary precipitates (GBPs, η) are discontinuous and the intragranular precipitates (η’) are not coarsened after EPAAT. The precise regulation of precipitated phase by EPAAT can be attributed the differing effect of electropulsing current on the GBPs and the intragranular precipitates. Utilize electropulsing primary treatment to obtain the supersaturated solid solution. Following natural aging, the small continuous phases at the grain boundary and the GP zones within the grain were precipitated again. The small continuous GPBs of the sample diffuse along the grain boundaries and become discontinuously distributed, while the GP zones within the grain dissolve with under the electropulsing secondary treatment. After artificial aging, the intragranular phase is fully precipitated, yet the GBPs remain unchanged, thereby achieving phase regulation. Consequently, the icorr of Al-Zn-Mg-Cu alloy decreases from 28 to 5.87 μA/cm2 and the tensile strength increases from 511 MPa to 524 MPa. The EPAAT technology realizes the accurate treatment of Al-Zn-Mg-Cu alloys and provides a new solution for improving strength and corrosion properties.

Influence of the Constant Magnetic Field on the Density of an Aging Alloy of Beryllium Bronze BrB-2

This paper presents the main experimental results of measuring the density of beryllium bronze BrB-2, aged in the constant magnetic field and in its absence, using the hydrostatic weighing method. The temperature and time dependences of the density of beryllium bronze BrB-2 during the decomposition of a supersaturated solid solution in a constant magnetic field of strength 557.2 kA/m were studied, at aging temperatures of 250, 300, 350 and 400°C and aging duration from 0 to 1 hour, and a comparative comparison analysis of their behavior with the corresponding microhardness dependencies was carried out. The correlation was discovered between the temperature and time dependences of the density and microhardness of beryllium bronze BrB-2, so at an aging temperature of 350°C a sharp maximum in the density of the alloy is observed, and similarly, at the same point the absolute maximum microhardness is achieved. In addition, it was found that the application of a constant magnetic field to the aging process under all heat treatment modes always leads to an increase in the density and microhardness of beryllium bronze BrB-2 compared to the density of the alloy aged in its absence. A physical interpretation is given to the observed facts and patterns. In the future, it is planned to find a correlating function between two independent measured characteristics density and microhardness, which is important for solid state physics and materials science from a scientific and practical point of view.

Research on the microstructure, mechanical and fatigue performance of 7075/6061 dissimilar aluminum alloy fusion welding joint treated by nanoparticle and post-weld heat treatment

The microstructure and mechanical properties of 7075/6061 high-strength dissimilar aluminum alloy fusion welds, after TiC nanoparticle-assisted welding and heat treatment, were discussed, and their fatigue performance was analyzed. The results indicate that the significant increase in hardness at the weld zone with T6 treatment compared to T5 is due to the solution treatment providing supersaturated solid solution for subsequent aging precipitation. T5 treatment causes the precipitation in the heat affected zones, thereby increasing the hardness of these regions. The joints exhibit excellent yield strength and tensile strength after heat treatment, with the elongation performance being optimal in T6 state. The fatigue performance of dissimilar aluminum alloy joints treated with nanoparticle and heat treatment is superior to the joints with single riveting. Porosity defects and microcracks generated during welding are prone to stress concentration, with interconnected pores and easily propagating cracks forming fatigue sources for pores and cracks. The crack propagation behavior is influenced by the pinning effect of TiC nanoparticles at the grain boundaries, and the second phase particles hinder crack propagation along the grain boundaries, forcing cracks to extend towards the 6061 side or the HAZ of the lower strength 6061 matrix. It demonstrates that the method of combining nanoparticle-assisted melt inert-gas welding and T6 heat treatment improves the fatigue life of 7075/6061 dissimilar aluminum alloy joints.

Enhancing corrosion fatigue performance of 17-4 PH stainless steel by simultaneous aging and plasma nitriding

This study investigated the effects of simultaneous aging and low-temperature plasma nitriding on the corrosion fatigue performance of 17-4 PH stainless steel. Plasma nitriding was conducted under two temperatures of 400 and 500 °C and, for comparison, some other specimens were subjected to aging treatment at the same temperatures and times. Microstructural characterization by scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD) confirmed similar structures at the core in the nitrided and aged specimens, thereby, the influence of nitrogen at the surface of 17-4 alloy could be carefully investigated. X-ray diffraction and grazing incidence X-ray diffraction (GIXRD) identified expanded martensite as the matrix within the nitrided layer, accompanied by CrN, Fe3N, and Fe4N phases. X-ray photoelectron spectroscopy (XPS) analysis revealed high nitrogen concentration, in regions very close to the surface, after nitriding at 500 °C. The N1s spectra shifted towards lower binding energies, indicating interaction between nitrogen and the matrix atoms, and the formation of nitrides in the nitrided layer. The nitrided thicknesses experienced close values at 400 °C (10 h) and 500 °C (5 h), provided at 400 °C the layer was mostly a supersaturated solid solution but at 500 °C it was composed of iron nitrides and chromium nitride. High hardness values of above 1600 HV0.1 were obtained after optimum plasma nitriding as the result of combined effects of solution strengthening induced by nitrogen atoms and the presence of dispersed nitride phases. EBSD results indicated residual stress within the nitrided layer, which would affect the corrosion fatigue resistance. Experimental findings demonstrated that both aging and plasma nitriding substantially enhanced the corrosion fatigue resistance of 17-4 PH stainless steel in 3.5 wt% NaCl. Moreover, simultaneous aging and plasma nitriding proved superior behavior in corrosion fatigue, primarily due to delayed crack initiation and improved fatigue resistance, without the risk of overaging which is detrimental to mechanical properties.

Effect of dispersoid formation on age hardening and precipitation behavior of an Al-Si-Mg(-Cu) alloy with Mn and Cr addition

In the present study, Mn and Cr elements were added to the alloy to induce the formation of the α-AlFeMnCrSi dispersoids during solution treatment. The effect of the dispersoids on subsequent age-hardening response and precipitation behavior was investigated. The results indicate that there is a partially coherent structure between the dispersoid and the α-Al matrix, but the coherence is weak. The precipitation of dispersoids resulted in accelerated age-hardening response and precipitation kinetics, but weakened peak-aged hardening effect. With the aging time increasing from 0 to 16 h, a new precipitation sequence was proposed in Mn+Cr modified Al-Si-Mg(-Cu) alloy: Supersaturated solid solution (SSSS) → GP zones (under-aged) → β'' + β' (peak-aged) → Si + β'' + β' + Mg2Si + Q + α (over-aged). Among them, the extra precipitates were heterogeneous nucleated on the dispersoids, which depended on the orientation relationship between the dispersoid and the precipitation, the interfacial energy of the dispersoid/matrix/precipitation, and the local solute element concentration. Heterogeneous precipitation of coarse particles on dispersoids resulted in the formation of precipitation-free zones (PFZ), which was mainly responsible for the weakened peak-age hardening effect. Moreover, compared to the base alloy, finer and denser precipitates were obtained in the matrix of the modified alloy, due to the increased density of dislocations induced by the dispersoids and the higher Mg supersaturation.

Study of minor Sc and Zr additions effect on silicon rich Al–Mg–Si aluminum alloy microstructure during Sc multistage thermal treatment

The study addresses the effect of multistage thermal treatment on microstructure formation of Al–Mg–Si series alloy with scandium and zirconium additions in Mg/Si = 0.3 proportion. Basic alloy AlM–Si without Sc and Zr and them modification AlMg1SiZrSc were cast. AlMgSiScZr alloy multi-stage thermal treatment included the following 4 annealing stages –550 °C 8 h + 440 °C 8 h + 500 °C 0.5 h + 180 °C 5 h. For AlMg1Si alloy it was performed following 550 °C 8 h + 180 °C 5 h practice. AlMgSiScZr microstructure was examined using scanning (SEM) and transmission (TEM) microscopy both in as-cast state and after each stage of thermal treatment, for AlMg1Si – both in as-cast state and after final thermal treatment. Coarse intermetallic particles were examined using SEM, while fine particles were examined using TEM. Besides, microhardness values were measured after various thermal treatment stages. It was found out, that coarse intermetallic compounds of Fe2Mg7Si10Al18 type are formed in both alloys during as-cast structure cooling down with such compounds partial dissolution during further thermal treatment. At the same time particles, that can be attributed either to (AlSi)3ScZr or to AlSc2Si2τ-phase, occur in inter-grain boundary of alloys with scandium-zirconium additions. No traces of scandium solid solution discontinuous decomposition have been identified during ingot cooling down. Thermal treatment during 8 h at 550 °C 8 and 8 h at 440 °C results in formation of the phase, which can represent both AlSc2Si2 and Al5SiZr2. At the same time nanoparticles (AlSi)3ScZr do not form during this thermal treatment stage. Heating to 500 °C during 30 min interval allows complete dissolution of magnesium in supersaturated aluminum solid solution. During the final stage β’’ particles (Mg5Si6), representing the major type, form in both alloys; in these conditions scandium does not produce significant effect on their formation.

Metastable phases of Ag–Si: amorphous Si and Ag-nodule mediated bonding

Metastable phases such as supersaturated solid solutions, supercooling, and amorphous phases are well-known in metallurgy. They are often composed in non-equilibrium states and can be transformed into a stable phase by overcoming an energy barrier with driving forces. Particularly, it has been widely used for material strengthening and heterogeneous nucleation of precipitates in solids is mainly induced by heat treatments for supersaturated solid solutions. However, little is known about the metastable phases of the Ag–Si alloy, although it is a well-known simple binary eutectic alloy. Here, we show that the metastable phases composed of amorphous Si and supersaturated Ag solid solution are induced by the eutectic reaction under rapid cooling of Ag–Si. Furthermore, the solute Si in the Ag matrix reacts with oxygen to precipitate Ag by-products, which grow as nodules. The Ag nodules have high crystallinity and robust interfacial structures, and the nodule growth leads to the formation of cross-links between the Ag–Si particles. We also demonstrate the Ag nodule-mediated bonding where the rapidly cooled Ag–Si ribbon is directly used as a bonding medium, indicating the possibility of using it as a high-temperature bonding material with low-temperature processes.

Anomalous temperature-dependent strength of copper alloy manufactured by laser-beam powder bed fusion

This study reports an anomalous temperature-dependent tensile behavior of laser-beam powder bed fusion (PBF-LB) processed Cu–Cr–Zr alloy. The yield strength of the alloy initially decreases as the temperature increases to 200±5 MPa and then increases to 350±11 MPa at 500°C before reducing to 234±6 MPa at 600°C. The microstructure consists of elongated Cu grains with a high concentration of Cr solute (∼1 mass%), resulting from rapid solidification during the PBF-LB process. Transmission electron microscopy for the specimens deformed at 500°C revealed the presence of numerous nanoscale Cr-rich particles embedded inside the supersaturated solid solution of the Cu matrix. Nanoscale particles can act as barriers to dislocation motion, leading to an increase in internal stress during plastic deformation at elevated temperatures. This work provides the high potential of post heat treatments for achieving superior mechanical performance using high solute supersaturation formed by the PBF-LB process.

Microstructure and properties of Cu–Ni–Si–Cr alloy fabricated by vacuum-assisted die casting via direct aging

In this study, the Cu−3.18Ni−0.75Si−0.28Cr (wt.%) alloy was fabricated using vacuum-assisted die casting (VADC), and the microstructure and properties of the alloy before and after direct aging treatment were investigated. The synergistic effects of pressure and high cooling rate during the VADC process yield excellent composition uniformity within the alloy, characterized by fine-grained microstructures and the formation of a high-energy metastable supersaturated solid solution. The subsequent direct artificial aging (723 K/6 h) leads to the precipitation of nano-sized δ−Ni2Si phases, thereby enhancing both the mechanical properties and electrical conductivity of the VADC alloy. The alloy exhibited an impressive synergy of the ultimate tensile strength (659 ± 14 MPa), hardness (250.7 ± 3.8 HV), and elongation (21.5 ± 3.6 %), with an excellent conductivity of 32.3 ± 0.4 % IACS. With the considerable potential for industrial applications, the novel fabrication method enables high-efficiency preparation and near-net shape forming of high-performance Cu–Ni–Si–Cr alloy.

The impact of small Zr addition to Al–Ni cast alloy for elevated temperature applications

Most aluminium casting alloys are based on the eutectic system Al–Si, which has weak thermal properties. Alloys from the eutectic Al–Ni system on the other hand have higher thermal stability and good casting properties and are therefore promising for casting applications. Using casting simulations, a copper mold was designed to achieve cooling rates above 100 K/s with the aim of producing a Zr-rich supersaturated solid solution. Microstructural analysis revealed that the addition of Zr leads to notable changes in the microstructure characterised by the formation of primary Al3Zr and αAl dendrites. Subsequent heat treatment revealed that Zr-enriched alloys undergo significant age hardening, which showed an improvement in microhardness due to effective Al3Zr precipitation.

Electropulsing rapidly refined the microstructure and enhanced the strength and plasticity of Cu-3.5Ti alloy

Recrystallization annealing is a necessary process for refining the grain of processed metallic materials and eliminating work hardening. However, it can cause a large amount of precipitation and coarsening of the second phase in supersaturated solid solution alloys, which is detrimental to the subsequent processing and aging processes aimed at controlling the precipitation phase. This article compared the effects of electropulsing (EPT) and conventional heat treatment (CHT) processes on the microstructure and mechanical properties of cold-rolled Cu-3.5Ti alloy. The results indicated that compared with CHT, EPT achieved full recrystallization of the alloy at a lower temperature (650 °C, 100 °C lower than CHT) and in a shorter time (10 s, 1/360 of CHT), and effectively inhibited the precipitation of β-Cu4Ti phase, resulting in a fine and uniform supersaturated solid solution. This process significantly improved both the strength and plasticity of the alloy. The yield strength, tensile strength and elongation to failure of the EPT-650 °C/10 s sample were 570 MPa, 598 MPa and 21.2 %, respectively, which were 27.8 %, 25.9 % and 19.1 % higher than those of the CHT-750 °C/1 h sample. EPT significantly increased the recrystallization nucleation rate of the alloy, while its rapid heating method inhibited the precipitation of β-Cu4Ti phase, ensuring the recrystallization process was not affected by the precipitated phase, which was the main reason for rapid recrystallization and grain refinement of Cu-3.5Ti alloy by EPT at lower temperatures and shorter time.

One step synthesis network supersaturation α phase through electron beam bombardment

Solid solution strengthening is a highly effective approach for enhancing the surface strength of Mg alloys. However, the realization of solid solution of Al in Mg remains a significant obstacle. Thus, a novel environmentally friendly technique for continuous electron beam bombardment is proposed to break through this bottleneck. The results revealed that a significant quantity of Al dissolved into the α-Mg phase, forming a reticulated supersaturated solid solution. Furthermore, the supersaturated α-Mg serves as an effective barrier and promotes notable microstructural refinement benefited from its enriched grain boundaries, contributing to a remarkable enhancement in their corrosion performance, an order of magnitude decrease. Additionally, AZ91D wear was reduced from 1.0 mg to 0.5 mg. Modified Mg alloy exhibited a noteworthy decrease of 50 % in wear when compared to its initial condition. This study provided insight into the development of efficient modification of Mg alloys by electron beam scanning.

Metastable defect phase diagrams as roadmap to tailor chemically driven defect formation

Thermodynamic bulk phase diagrams have become the roadmap used by researchers to identify alloy compositions and process conditions that result in novel materials with tailored properties. Recent experimental studies show that changes in the alloy composition can drive not only transitions in the bulk phases present in a material, but also in the concentration and type of defects they contain. Defect phase diagrams in combination with density functional theory provide a natural route to study these chemically driven defects. Our results reveal, however, that direct application of equilibrium bulk thermodynamics can fail to reproduce experimentally observed defect formation. Therefore, we extend the concept to metastable defect phase diagrams to account for kinetic limitations that prevent the system from reaching equilibrium. We apply this concept to successfully explain the formation of large concentrations of planar defects in supersaturated Fe-Nb solid solutions. We then utilize it to design suitable conditions for synthesis, which we subsequently realized experimentally, successfully validating the formation of the predicted defects in Mg-Al-Ca alloys. The concept offers new avenues for the design of materials performance by tailoring defect structures.

A STUDY OF PROPERTIES OF AUSTENITIC STEEL IN THE INITIAL STATE

High-manganese steel is used in the rocket, space, and defense industries due to its low specific gravity, stable austenitic structure over a wide temperature range, and ability to strengthen under mechanical deformation. However, the wider application of this steel is hindered by such factors as: instability of mechanical properties in the initial state, tendency to thermal embrittlement and very poor machinability. Therefore, the work is aimed at solving the problems of eliminating the above-mentioned negative factors. Conducted studies of the effect of temperature-time parameters on the structure and properties of steel made it possible to establish the mechanism of thermal embrittlement of 9Г28Ю9МВБ steel. During slow cooling from hot deformation temperatures with speeds of the order of 0.03 º/s in steel, a large number of gray-blue coarse phase particles are released, which are located on the grain boundaries, which causes a sharp decrease in impact viscosity. It was established that in the temperature range of 500-800 ºС, the solid solution disintegrates with the release of particles of the strengthening K-phase - (Fe, Mn)3 AL, Cx. It was determined that the simultaneous decrease in hardness and impact strength in the temperature range of 750-950ºС provides a significant improvement in machinability. The lowest values of mechanical properties are achieved during aging at 700 ºС, the improvement of machinability in this case is maximal compared to exposure to other temperatures. So, for example, with an increase in the temperature of isothermal holding at 650ºС for 35 hours. machinability improves by almost 1.7 times, and for the same time at 700 ºС – by 2.4 times. When the holding temperature is increased to 950ºС for 35 hours. the machinability of steel increases only 1.6 times, which is due to the less intense decomposition of the saturated solid solution. Significant anisotropy of mechanical properties is formed as a result of the development of liquid chemical heterogeneity and its imitation during further thermomechanical processing. The value of the coefficient of anisotropy in sample bars of different grades of initial steel reaches values from 1.5 for a bar with a cross section of 105x105 mm to 3.9 units for a bar with a diameter of 120 mm. The study of the influence of the annealing temperature on the homogeneity of austenitic steel made it possible to develop a mode of heat treatment of austenitic steel 9Г28Ю9МВБ, which consists of heating to 1250°С, holding for 2 hours. at this temperature and subsequent cooling in water, in order to fix a homogeneous supersaturated solid solution. After such heat treatment, the striated microchemical heterogeneity is significantly reduced, the anisotropy coefficient does not exceed 1.1 units for all types of steel assortment.

Crystallization of FeC Iron Monocarbide During Peritectoidal Transformation of the Lamellar Eutectoid of Ledeburite White Eutectic Cast Iron

The previously unknown process of homogeneous and heterogeneous crystallization of FeC iron monocarbide and its co-crystallizations with ε-carbide Fe2C from a supersaturated solid solution based on ε-carbide Fe2C or polycarbide quasi-eutectic formed in the process of peritectoid decomposition during prolonged heating (isothermal annealing) of the lamellar eutectoid ledeburite in cast eutectic white iron has been investigated. Crystallization of 2D monolayers of FeC monocarbide allotropes in the form of translucent extended and elastic crystalline nanofilms has been experimentally proved. The carbide phases in white cast iron can be characterized as a single isomorphic and isostructural quasi-carbide solid solution, which structurally crystallizes as a mixture of carbide phases as a quasi-eutectic, in which the carbon content is free to vary widely without indentification of the carbide phases proper. The decomposition product of the lamellar eutectoid as a result of peritectoid transformation during isothermal annealing is polycarbide with a gradient crystal lattice of solid solutions corresponding in carbon concentration to this or that carbide.

Formation of metastable phases during spark plasma sintering of Fe@Au core–shell particles

ABSTRACT The present work reports the formation of Fe-Au metastable alloys by galvanic replacement, pressureless spark plasma sintering (SPS) and acid treatment. The reaction of partial galvanic replacement between micrometer-sized iron particles and HAuCl4 solution allowed synthesising core–shell Fe@Au particles. When the Fe@Au particles were treated in HCl solution, iron was selectively dissolved and hollow Au particles were formed. Pressureless SPS of the Fe@Au core–shell particles led to the formation of a metastable intermetallic phase, FeAu3, and a supersaturated solid solution, Au(Fe). The latter was resistant to HCl solution; at the same time, the FeAu3 phase decomposed during HCl treatment to form a porous gold structure. This work shows that a combination of galvanic replacement, SPS and acid treatment can be used for the preparation of metastable Fe-Au alloys with different phase compositions and particle morphologies.

Rapid-thermal-process pre-treatment promoted precipitation towards strengthening hard magnetism of Sm2Co17-type magnets

Dislocations can act as heterogeneous nucleation sites but their annihilations upon heating sacrifice point defects nearby, weakening the driving force for precipitate nucleation. In order to keep a high density of dislocations and point defects together, here rapid-thermal-process (RTP) pre-treatment was carried out to promote the precipitate nucleation in Sm2Co17-type permanent magnets that evolve gradual formation and dissociation of dislocations during concurrent precipitation and recrystallization. As exhibited in a model magnet Sm25Co49.3Fe17.1Cu5.6Zr3.0 (wt.%) with high-density dislocations at solution-treated state, the RTP pre-treatment can induce excess point defects owing to the migration of solute atoms towards equilibrium state and the quenching of point defects, and suppress the early-stage diffusion-controlled dissociation of dislocations. After whole-process isothermal aging and slow cooling, the RTP pre-treated magnet possesses much higher fraction of hexagonal SmCo5 (1:5H) nanoprecipitates than the non-pre-treated one, giving rise to effective enhancements in coercivity Hcj from 26.20 to 30.34 kOe and in knee-point field Hk from 14.43 to 19.20 kOe. These findings add insight into controlling precipitation in dislocation-bearing supersaturated solid solutions, which are feasible for strengthening the Sm-Co-based magnets.

Slow precipitate coarsening kinetics in extremely fine nanograined Ni-Cr alloys

Grain boundaries significantly influence the nucleation and growth of precipitates in supersaturated solid solutions. However, due to the inherent instability of grain boundaries in nanograined metals, the precipitation process usually occurs concurrently with grain growth during thermal annealing, making it difficult to reveal the grain size effect on the precipitate coarsening behavior. In this study, nanograined Ni-48 Cr (at.%) supersaturated alloys with grain sizes ranging from about 85 nm to 10 nm were fabricated by plastic deformation, in which the relaxed grain boundaries led to the extremely fine nanograined structure having good thermal stability. During the high-temperature annealing process, nanoscale Cr particles were precipitated from the nanograined matrix, forming a nanograined dual-phase structure. The growth kinetics of the Cr precipitates in the extremely fine nanograined structures are different from the larger grain size samples, showing a trend that the smaller the as-deformed grain size, the slower the precipitate coarsening rate. Detailed microstructural characterization and kinetic analysis indicate that the extremely fine nanograined structure may change the rate-limiting process for precipitate coarsening from lattice diffusion to grain boundary diffusion as grain size is refined to about a few tens nanometers, which leads to a slow coarsening rate. The nanograined dual-phase Ni-Cr alloy of the annealed 10 nm sample shows the highest strength among these samples as well as excellent thermal stability, which provides guidance for the design of high-performance alloys.