Precipitation-strengthened Alloys Research Articles

Effect of adaptive control of cooling rate on microstructure and mechanical properties of laser additively manufactured Inconel 718 thin walls

Ni-alloy Inconel 718 (IN718) is finding ever-increasing applications in various industries like aviation and aerospace. IN718 is a precipitation-strengthened alloy which derives its strength mainly from the intermetallic phases, γ” (Ni3Nb) along with the γ’ (Ni3(Al, Ti)) phase which can be precipitated using standard heat treatment schedules. However, the consummation of Nb in Laves phase formation during solidification tends to deteriorate the strength of the material. During laser additive manufacturing (LAM) by directed energy deposition (DED), the cooling rate variation along the build direction causes variations in the microstructure, properties and Laves phase distribution. The study aims to minimize the heterogeneities along the build direction by monitoring and controlling the cooling rate during DED of IN718. The feedback control system involves the use of two pyrometers in tandem with a leading pyrometer measuring the molten pool temperature and a trailing pyrometer measuring the temperature at a distance behind the molten pool. Two sets of thin walls of 120 layers were fabricated with one set being of constant process parameters without feedback control and the other set being of adaptive-controlled process parameters. The adaptive control was aimed at maintaining the cooling rates above a desired limit by changing the process parameters, namely the laser scan speed and the powder feed rate. The Laves phase morphology, which affects the strength of the deposited part deleteriously was finer and more discrete in the adaptive-controlled walls which led to their easier dissolution during heat treatment. This in turn helped in the precipitation of more amount of strengthening phases during heat treatment which led to better mechanical properties. The characterizations of the two sets of walls reflected the superior part quality of the adaptive-controlled wall in terms of homogeneity of microstructure, enhanced microhardness and tensile strength, thereby validating the importance of adaptive control of cooling rate in ensuring better quality in LAM parts.

Synergistic effects of chemical composition on precipitates, dislocations, and mechanical properties of precipitation-strengthened alloys

The properties of materials are intricately dependent on their chemical composition, and microstructures. Achieving a precise modulation in the composition and microstructure of materials to enhance properties significantly is a huge challenge. In this work, we uncovered the effects of nickel and silicon composition on the precipitates, dislocations, and properties of Cu–Ni–Si alloys. The results show that the volume fraction of precipitates increased from 0.84 % to 4.80 % with the increase of Ni and Si contents. The average diameter of the precipitates increased from 6.20 nm to 20.21 nm, and the dislocation density increased from 2.16 × 1013 m−2 to 4.08 × 1015 m−2 concerning the Ni and Si contents. In addition, the dislocation type changes from half-edge dislocation and half-screw dislocation to screw dislocation. The modified WHWA (Williamson Hall-Warren Averbach) method was proved to be the most accurate way of calculating dislocation density. As anisotropy was present in the alloy, adding a dislocation contrast factor and removing some lower-intensity diffraction peaks could further correct the obtained results and reduce the calculation error. Furthermore, the above analysis and findings were employed to design a Cu-3.21Ni-0.66Si alloy with a high yield strength (814 MPa), high ultimate tensile strength (970 MPa), and comparable electrical conductivity of 42.4 % IACS. This work offers a meaningful way to develop precipitation-strengthening alloys.

Precipitation behavior of supersaturated solid-solubility CuCrZr alloy by additive manufacturing

As a typical precipitation-strengthened alloy, the improvement of CuCrZr alloy performance can be achieved by promoting the precipitation of alloying elements. However, the low solubility of Cr limits further enhancement of the performance of conventionally processed CuCrZr alloys. In this study, a supersaturated solid solution CuCrZr alloy was prepared using laser powder bed fusion (LPBF), and the precipitation of solute atoms was promoted through heat treatment, resulting in an alloy with excellent comprehensive performance. The influence of different heat treatment processes on the precipitation behavior and performance of the alloy was investigated. Microstructural analysis revealed the presence of micrometer-scale Cr particles in the specimens after solution treatment (ST) and solution-aging treatment (SAT), while the direct-age-hardening (DAH) specimen exhibited dispersed nanoscale Cr precipitations. Notably, a Nishiyama-Wassermann (N-W) relationship was observed between the Cr precipitations and the Cu matrix, significantly enhancing the mechanical properties of the alloy. The DAH specimen exhibited an ultimate tensile strength (UTS) of 661.54 MPa, a hardness of 231.03 HV and an electrical conductivity of 65.05 % IACS, demonstrating the best match between mechanical properties and electrical conductivity. The outstanding properties can be credited to the uniformly distributed Cr nano-precipitates and slim lattice distortion. Additionally, the main strengthening mechanism is precipitation strengthening for CuCrZr alloy.

Strengthening from dislocation restructuring and local climb at platelet linear complexions in Al-Cu alloys

Stress-driven segregation at dislocations can lead to structural transitions between different linear complexion states. In this work, we examine how platelet array linear complexions affect dislocation motion and quantify the associated strengthening effect in Al-Cu alloys using atomistic simulations. The presence of platelet complexions leads to the faceting of the dislocations, with nanoscale segments climbing upwards along the platelet growth direction, resulting in a complex configuration that restricts subsequent dislocation motion. Upon deformation, the leading partial dislocation must climb down from the platelet complexions first, followed by a similar sequence at the trailing partial dislocation, in order to overcome the precipitates and commence plastic slip. The dislocation depinning mechanism of linear complexions is strikingly different from traditional precipitation-strengthened alloys, where dislocations overcome obstacles by either shearing through or looping around obstacles. The critical shear stress required to unpin dislocations from platelet complexions is found to be inversely proportional to precipitate spacing, which includes not just the open space (as observed in Orowan bowing) but also the region along the platelet particle where climb occurs. Thus, linear complexions provide a new way to modify dislocation structure directly and improve the mechanical properties of metal alloys.

A novel under-aging design to improve the creep rupture life of a precipitation-strengthened Fe–Ni-based superalloy

The under-aging, peak-aging, and over-aging microstructures of a precipitation-strengthened Fe–Ni-based alloy were designed by applying different heat treatment procedures. Tensile creep tests were conducted at 700 °C and 250 MPa on samples exhibiting different strengthening γ′ precipitate volume fractions and diameters. The creep life of the alloy in the under-aged state was increased by more than 10% compared to the peak-aged condition, which provided a new approach to designing the heat treatment regime that improve the creep rupture life and plasticity of precipitation-strengthened alloys. The improvement in creep properties of the under-aged state was attributed to the transition from shearing to Orowan looping of γ′ particles by matrix dislocations during the dynamic precipitation and coarsening of γ′ phase, increasing intragranular deformation resistance. The under-aged microstructure design allowed reducing the grain boundary sliding contribution to overall strain during creep. As a result, the steady-state creep stage of the alloy in the under-aged state was extended. Additionally, relationships between applied stress and nucleation, growth, and coarsening of the γ′ phase have been evidenced.

Anisotropic growth of nano‐precipitates governed by preferred orientation and residual stress in an Al‐Zn‐Mg‐Cu alloy

Through an understanding of diffusion, precise control of the size distribution of nano-precipitates can be essential to developing superior properties in precipitation-strengthened alloys. Although a significant influence of crystallographic orientation on the diffusion process is known to exist in low-symmetry hexagonal close-packed alloys, such anisotropic diffusion is still unidentified in high-symmetry cubic alloys. In this work, we reveal the diffusion-controlled coarsening induced anisotropic growth process of nano-precipitates in an Al-Zn-Mg-Cu alloy. Our experimental and theoretical studies demonstrate that with an increase in the residual stress, the diffusion-controlled coarsening rate is slow along the 〈112〉 fiber texture in the alloy matrix with smaller grain sizes. As such, we find that the diffusion activation energy will be increased along the preferred orientation with largest residual stress, which leads to a reduced diffusion-controlled coarsening rate. Specifically, we demonstrate that the increase in the volume fraction of nano-precipitates originates from the rapid grain-boundary controlled coarsening of the grain-boundary precipitates. Based on these results, an underlying microstructural design strategy is proposed, involving the crystallographic orientation, the residual stress and the grain boundaries to manipulate the precipitate size distribution in this class of alloys.

Ultrasonic provoked network-like intragrain carbide precipitations strengthen and ductilize a C-doped CoCrNi medium entropy alloy

A strategy bypassing “rolling+annealing” while involving merely ultrasonic vibration-modulated static compressive loading was established to enhance the strength-ductility synergy in a C-doped CoCrNi medium entropy alloy. Specifically, by superimposing ultrasonic vibration of 20 KHz in frequency and 20 μm in amplitude onto a compressive load of ∼180 MPa (∼60% of yield strength) applied on a coarse grain CoCrNiC0.1 alloy for 25 min, yield strength, ultimate tensile strength, and total elongation of the alloy were largely improved to 530 MPa, 950 MPa, and 36.5%, showing respectively an increment of 77%, 28%, and 53% from the original state (300 MPa, 740 MPa, and 23.8%). Structure analysis on the ultrasonically processed CoCrNiC0.1 alloy indicates that intragrain precipitation of Cr7C3 networks of a characteristic size ∼20 μm provoked by ultrasonic vibration was the main cause for the strength-ductility synergy. Our work provides a promising path to engineer the strength-ductility synergy in precipitation-strengthened alloys.

Ultra-high work hardening mediated by three-level precipitation in a Ni2CoCr0.5Si0.3Al0.2Ti0.2 medium entropy alloy

The three levels of precipitation with the primary Ni16Ti6Si7 intermetallic, the secondary large-sized L12 phase and the tertiary diffused distributed nanoscale L12 particles is achieved in an aged Ni2CoCr0.5Si0.3Al0.2Ti0.2 medium entropy alloy (MEA). The yield strength of 1222 MPa and ultimate strength of 1626 MPa as well as the uniform elongation of 16% and excellent work hardening rate exceeding 4.5 GPa are obtained. The precipitation strengthening model is employed to understand the comprehensive effect of the shearing and Orowan strengthening on yield strength. The synergistic strengthening mechanisms, including dislocation Orowan mechanisms for the Ni16Ti6Si7 intermetallic, the shearing and Orowan mechanisms of large-sized L12 precipitation, the shearing mechanism and pining effect from the small-sized L12 precipitation, together with Lomer-Cottrell (LC) locks and nano-spacing stacking fault (SF) networks, result in the ultra-high work hardening during plastic deformation, which provides a novel strategy for the design of excellent precipitation-strengthened alloys.

A performance comparison of additive manufactured creep-resistant superalloys

Creep-resistant nickel, cobalt based superalloys, selected for a high-speed flight application, deposited using Wire + Arc Additive Manufacturing (WAAM), was reported. Three different alloys, Haynes 188, Inconel 718, and Rene 41, were deposited, and tested for their high-temperature tensile properties, and the results compared with wrought data. The alloys were tested from ambient temperature to 1000°C in their as-deposited condition and after undergoing industry standard age-hardening and solutionising heat-treatments, to down select the best performing alloy under two different processing conditions. The mechanical strength of the alloys fell short of the maximum achievable in wrought condition. Precipitation-strengthened alloys, Inconel 718 and Rene 41 were found to have underperformed the most significantly, whereas solid-solution-strengthened Haynes 188 suffered the least due to WAAM.

Abnormally high work hardening ability and excellent comprehensive properties of copper alloys due to multiple twins and precipitates

In the present work, the Cu-1.35Ni-1Co-0.55Si alloy with multiple twins and ultra-low interfacial mismatch precipitates was obtained by liquid nitrogen dynamic plastic deformation (DPD) and aging treatment. It has abnormally high work hardening ability, which is more than twice that of the previous DPD treatment without aging. In addition, it has excellent comprehensive properties, and its strength, electrical conductivity, and elongation are 816.4 MPa, 40.2% IACS and 11.3% respectively. The fine multiple twins do not coarsen significantly during peak aging, and the dispersed nano precipitates (∼3 nm) further enhance the strength of the Cu-Ni-Co-Si alloy. In addition, the low interfacial mismatch strain between the precipitates and the matrix also ensures adequate elongation. The Cu-Ni-Co-Si alloy has high strength, electrical conductivity, and ductility under the simultaneous action of various mechanisms (grain boundary strengthening, precipitation strengthening, dislocation strengthening and stacking faults). This strategy paves a new avenue for developing precipitation-strengthened alloys with excellent comprehensive properties.

Eutectic, precipitation-strengthened alloy via laser fusion of blends of Al-7Ce-10Mg (wt.%), Zr, and Sc powders

Laser powder-bed fusion was used to create an Al-7Ce-10Mg-0.71Zr-0.23Sc (wt.%) alloy, using hypoeutectic Al-7Ce-10Mg (wt.%) pre-alloyed powder blended with either elemental Zr and Sc powders (∼20 µm in size), or pre-alloyed, concentrated Al-10Zr and Al-10Sc (wt.%) powders (comprised of 1-5 µm Al3Zr or Al3Sc particles embedded in a <45 μm Al-rich matrix). The effects of process parameters and powder selection on the homogeneity of the as-fabricated microstructure were investigated via metallography and compared to predictions from a simple numerical model describing the dissolution of Zr powders in the melt pool during printing. Both experiments and model show that decreasing scan speed leads to greater dissolution of Zr and Sc, but remelting has no significant effect. The samples produced with Al-10Zr and Al-10Sc powder additions show greater amounts of Zr and Sc dissolution than those produced with elemental Zr and Sc additions, because of the smaller size (5 µm for prealloyed vs. 20 µm for elemental) and lower melting point (1580 and 1320°C for Al3Zr, Al3Sc vs. 1850 and 1541°C for Zr, Sc) of the dissolving species. The alloys fabricated with prealloyed powders display larger amounts of very fine grains (1-2 µm) nucleated by primary L12 Al3(Zr,Sc) precipitates, and a clear hardening response during aging, consistent with supersaturated Zr and Sc forming a high number density of secondary L12 Al3(Zr,Sc) nanoprecipitates.

The as-cast precipitation-strengthened N-doped Fe40Mn40Co10Cr10 high-entropy alloys

The as-cast Fe40Mn40Co10Cr10-based (in at.%) high-entropy alloys were doped with different amounts of V and N. It was shown that a certain percentage of V and N cause significant strengthening due to the V2N-type nitrides precipitation. The precipitation-strengthened alloy with 2 at.% V and 1 at.% N had yield strength of 440/940 MPa and uniform elongation of 36/18 % at 293/77 K. Experimental data on the phase composition of the alloys agree reasonably with the CALPHAD predictions.

Customized development of promising Cu-Cr-Ni-Co-Si alloys enabled by integrated machine learning and characterization

Two types of alloys, Cu-Ni-Co-Si and Cu-Cr-Zr, are considered candidate materials for next-generation integrated circuits due to their superior comprehensive performance. However, the rapid development of these two types of alloys remains difficult using conventional simulation techniques. Machine learning offers a new tool for accelerating the design and discovery of new materials with required property profiles. Herein, composition-process-property database of the six-element Cu-Cr-Ni-Co-Si-Zr alloys were established, and a novel strategy of customized performance design for different application environments was proposed. Then, four alloys with different performance characteristics were rapidly screened from 850,500 candidates using a multi-property segmented screening method, and the predicted results agreed well with the experimental results. Importantly, the developed Cu-1.0Cr-1.0Ni-2.5Co-0.8Si alloy was used as a bridge alloy to link the Cu-Ni-Co-Si and Cu-Cr-Zr alloys together, filling the gap in the mid-segment performance (220–240 HV, 45–65% IACS) of Cu-based alloys. Interestingly, the studied alloy was a dual-phase precipitation-strengthened alloy. It was found that the small spherical (Co, Ni)2Si phase was the main influence on the micro-hardness and strength, while the large rod-shaped Cr3Co5Si2 phase was the main reinforcing phase that affected ductility and electrical conductivity. The design method proposed in this paper accelerates the development of the Cu-Cr-Ni-Co-Si alloy system, which has great potential for application in integrated circuits and heat sinks.

Manipulation of precipitation and mechanical properties of precipitation-strengthened medium-entropy alloy

Medium-entropy alloys (MEAs) strengthened by coherent L12 precipitates were recently developed. However, incoherent precipitates are often unexpectedly formed owing to the composition complexity. Their regulation needs to be elucidated. In this study, we clarify the influence of prestrain-induced dislocations on the formation of both L12 and L21 precipitates in the most representative precipitation-strengthened (FeCoNi)86Ti7Al7 MEA. The formation of incoherent L21 precipitates was promoted by the prestrain. Their amount was increased, while their size was refined by the prestrain before isothermal aging. The formation of the coherent L12 precipitates was indistinctively affected by the prestrain. Consequently, the room-temperature tensile properties were improved. A tensile yield strength of 1127 MPa, ultimate tensile strength of 1369 MPa, and satisfactory tensile elongation of 11.2% were obtained. This study provides insights into the regulation of the formation of incoherent precipitates and optimization of the mechanical performances of precipitation-strengthened alloys.

Effects of Recrystallization on Tensile Anisotropic Properties for IN738LC Fabricated by Laser Powder Bed Fusion

This study demonstrates the effects of recrystallization on tensile properties and the anisotropy of IN738LC, a typical γ’ precipitation-strengthened alloy, at both room and high temperatures via the laser powder bed fusion process. The nonrecrystallized columnar microstructure, subjected to standard IN738LC heat treatment up to 1120 °C, and the almost fully recrystallized microstructure, heat-treated at 1204 °C, were compared. The tensile properties strongly depend on whether recrystallization was completed as well as the tensile direction. This can be explained by microstructure characterization, featuring the Taylor factor in the tensile direction, average grain size estimated by ellipse approximation, and the relationship between the grain shape and tensile direction. The shape of the recrystallized grains and the distribution of coarse MC carbides inside the recrystallized grains were determined by the microstructure in an as-built state. In high-temperature tensile tests conducted in the horizontal direction, the separation of the columnar grains caused a brittle fracture. In contrast, dimples were observed at the fracture surface after recrystallization, indicating scope for further improvement in ductility.

A strategy for enhancing the mechanical property of the precipitation-strengthened medium-entropy alloy

Medium-entropy alloys strengthened by the L12-type coherent precipitates were recently proposed, whereas the mechanical property still needs to be improved. Here, we demonstrated a strategy for notably improving the strength of the representative (CoCrNi)94Ti3Al3 alloy, in which the refinement of grain structures and the promotion of precipitation were concurrently achieved by pre-strain followed by aging. A refined grain structure containing incoherent σ-precipitates and a large number of coherent L12-precipitates was formed, which enhances the yield strength and ultimate tensile strength up to 1431 MPa and 1683 MPa respectively. Furthermore, decreasing pre-strain temperature or increasing strain further aids the enhancement. The present study opens an avenue for fabricating high-performance precipitation-strengthened alloys.

Cuboidal γ' phase coherent precipitation-strengthened Cu–Ni–Al alloys with high softening temperature

Coherent precipitation of cuboidal γ' phase is an efficient strategy to improve high temperature properties in nickel-based alloys. The present work aims to introduce this strengthening mechanism into copper alloys to solve the issue of deficient heat resistance. The results shown that a series of cuboidal γ' coherent precipitation-strengthened Cu100-y(Ni3/4Al1/4)y alloys were obtained in the (Ni+Al) content range of 20 at. % to 50 at. %. These alloys are shown to exhibit excellent comprehensive mechanical properties, high melting point, and a softening temperature of 1273K or more than 1273K. Their hardness and yield strength increased with the rise of temperature, ascribing to splitting of γ′ phase. AFM surface current mode and three-dimensional reconstruction of microstructure was performed to determine that γ phases remain conductive body in Cu100-y(Ni3/4Al1/4)y alloys, and meanwhile temperature-dependent resistivity was characterized and discussed, which showed the improvement of resistivity thermal stability attributing to coherent precipitation of γ' phase. Comparing to other Cu commercial alloys, cuboidal γ' phase coherent precipitation-strengthened alloys possess a great potential under high-temperature loads and service environments. In further researches, continued efforts will be required in the multi-component design of Cu100-y(Ni3/4Al1/4)y alloys to improve the comprehensive performance.

Effects of Filler Metals on Heat-Affected Zone Cracking in IN-939 Superalloy Gas-Tungsten-Arc Welds

Filler metals play an important role in reducing the weld cracking in nickel-based superalloys. In order to improve the welding conditions of IN939 superalloy, a wide range of solid solutions and age-hardenable filler metals were investigated in this study. Five solid solutions (HAYNES 230, IN625, IN617, HASTELLOY X, HAYNES 25) and two precipitation-strengthened alloys (IN718, HAYNES C-263) were used as filler metal to weld IN939 superalloy via gas-tungsten-arc welding. Microstructural studies were performed using optical microscopy and field-emission scanning electron microscopy. The results revealed that IN939 alloy is susceptible to liquation cracking in the heat-affected zone. In addition, the primary γ′ particles had grown into “ogdoadically diced cubes” of about of 2 μm in side length. The microstructure of the weld pool made with various filler metals was observed to be made up of fine spherical γ′ particles with a diameter of about 0.2 μm. The study of mechanical properties and thermodynamic behavior of the weld showed that the filler metals with lower concentrations of (Al + Ti + Nb + Ta + Mo + W) than the base metal can effectively attenuate the PWHT cracking. We found that optimal IN939 alloy weld performance can be achieved using HAYNES C-263 as filler metal, followed by IN617 and then IN625.

Molecular dynamics simulation of structural transformations in Cu-Al system under pressure

Aluminium-copper (Al–Cu) compounds are one of the most-studied precipitation-strengthened alloy systems. Mechanical properties of Cu-Al systems considerably dependent on the phase composition. Excellent properties primarily depend on the intrinsic microstructures formed during processing stages, particularly the precipitated phases or the so-called intermetallics, along with various defects and impurity segregation, have important influences on the composite strength. Study of fabrication techniques to obtain composites with improved mechanical properties, careful investigation of phase composition, dynamics and kinetics are of high importance. Molecular dynamics simulation is used to study on the atomistic level the process of formation of Al/Cu composite from two initially separated crystals by severe plastic deformation. The proposed model is the simplification of scenario, experimentally observed previously. However, even in such a simple model, understanding of the mechanisms underlying in the process of composite formation can be obtained.

Forming of the Structure and Functional Properties of the Precipitation-Strengthened CuNi2Si1 Alloy

The work presents the results on the structure of CuNi2Si1 copper alloy. The alloy was treated in two variants: supersaturation - aging (variant I) and supersaturation - cold rolling - aging (Variant II).The structure of the CuNi2Si1 alloyed copper were analyzed by high resolution transmission electron microscopy (HRTEM). The TEM investigation showed in the Cu matrix after applying cold rolling after solution heat treatment, during aging at 600°C, causes the Ni2Si phase occurrence immediately after the begin of aging. Cold rolling (50% reduction) of the CuNi2Si1 alloy after supersaturation changes the mechanism and kinetics of precipitation and provides possibilities for production of broader sets of functional properties.