Prime Precipitates Research Articles

Effects of carbon content on precipitate evolution and crack susceptibility in additively manufactured IN738LC

Hot cracking is a major bottleneck preventing the additive manufacturing community from adopting precipitation-strengthened nickel-base superalloys, such as the IN738LC. Prior literature demonstrates the beneficial outcome of increasing the carbon content within IN738LC to alleviate its hot cracking problem. However, the effect of carbon content on the gamma prime precipitation and grain recrystallization was not fully addressed. Here, we fabricated five sample sets of IN738LC with different carbon contents and subjected these samples to two separate heat treatment processes. The precipitate and grain evolution were monitored under the backscattered electron imaging and electron backscattered diffraction studies. While the carbon addition could assist in addressing the hot cracking problem, horizontal delamination cracks were detected during the fabrication of large samples when the overall carbon content was above 0.4 wt.%, highlighting the need for care when introducing carbon for the purpose of resolving hot cracking.

Gamma prime precipitation in as-deposited Ni-based superalloy IN713LC

The as-deposited microstructure of an additively manufactured Ni-based superalloy fabricated using laser powder-bed-fusion has been investigated using atomic resolution transmission electron microscopy. It is shown that γ′ precipitates exist in the as-deposited condition, albeit in a non-equilibrium state. The precipitates are present as nano-structured domains throughout the microstructure. These domains exhibit clear L12 ordering but show highly irregular matrix-precipitate interfaces.

Development of morphology and lattice misfit in modified Ni-base superalloy with Al, Co and Ni additions

Abstract This research work studied the effect of aluminium, cobalt and nickel additions in superalloys grade GTD-111 and IN-738 on morphology of gamma prime particles and their lattice misfits after subjected to heat treatment and long-term cyclic exposure by using SEM and XRD. It was found that the particles became more cuboidal in shape after heat treatment in modified GTD-111 and IN-738 alloys. When adding 1 wt% aluminium and 6 wt% nickel, larger negative values of lattice misfit were found at −0.57 % and −0.28 %, respectively. However, 6 wt% cobalt addition reduced the negative value of the lattice misfit to −0.20 % in modified GTD-111 and small positive lattice misfit (+0.08 %) was found in modified IN-738. Also, modified GTD-111 alloy shows more spherical shape of the gamma prime precipitates than those in modified IN-738. After long-term cyclic exposure at 1173 K for 1440 ks, it was found that the particles become more spherical shape comparing to those of heat treated ones. The shape of particles in both modified alloys with 1 wt% aluminium and 6 wt% nickel additions maintained more cuboidal compared to others. Furthermore, the effect of 6 wt% cobalt addition is more pronounced for modified IN-738 than that of modified GTD-111.

Using θ′ interfaces as templates for planar L12 precipitation in AlCuMnZr alloys

• Al 3 Zr L1 2 / θ' co-precipitates form faster in additively manufactured Al-Cu-Mn-Zr (ACMZ) alloys • Rapid solidification allows for higher matrix solute contents • Interfacial Zr segregation rate is dependent on the Zr matrix content • Theta prime precipitates are perfect templates for planar Al 3 Zr L1 2 formation Controlled Mn and Zr additions to Al-Cu alloys have allowed for the improved retention of mechanical properties after extended 350°C exposures by stabilizing the main strengthening θ' (Al 2 Cu) phase. Ultimately, θ' /L1 2 (Al 3 Zr) co-precipitate formation stabilizes θ' most effectively; however, Zr diffuses sluggishly and has low solubility in aluminum castings. Increasing the Zr segregation rate would allow for faster and more effective θ' /L1 2 co-precipitation. It is demonstrated that the Zr segregation rate is faster when the Zr matrix content is higher. A much higher Zr matrix content facilitated by rapid cooling during additive manufacturing (AM) was achieved that produces faster θ' /L1 2 co-precipitation, which is shown by scanning transmission electron microscopy and atom probe tomography experiments. It was also found that Zr continuously segregates to θ' interfaces up to the most aggressive heat treatment studied such that planar L1 2 precipitates remain after the metastable θ' dissolves. In this manner, we demonstrate that θ' coherent interfaces serve as perfect templates to form stable planar L1 2 precipitates that can provide strength at higher temperatures than traditional θ' strengthened AlCu alloys. This work introduces an alloy design strategy that uses metastable precipitates to quickly nucleate and grow co-precipitates with a desired geometry that contain slow diffusing elements. These ideas can be applied to engineer more heat resistant alloys by taking advantage of high solute matrix contents enabled by rapid cooling during additive manufacturing.

Effects of Re and Co additions on lattice parameters and lattice misfit in cast Ni-based superalloys

Abstract Morphological evolution of gamma prime precipitates was governed by an elemental partitioning ratio between the gamma and gamma prime phases. In the present study, Ni-based MGA 1400 alloy was modified by adding 4 wt% of Re and 6.4 wt% of Co by vacuum arc melting. The effects of Re and Co additions on lattice parameters of gamma and gamma prime phases were studied by TEM and XRD techniques. Calculated lattice misfit values obtained from XRD and TEM techniques were used to identify the shape and morphology of gamma prime precipitated particles after long-term exposure at 1173 K for 1800 ks? After long-term exposure, gamma prime particles became much coarser. The addition of Re retarded the coarsening rate of the gamma prime precipitates, resulting in smaller gamma prime particles at the same annealing times. Even though, both Re and Co atoms are strongly partitioned in the gamma matrix, the effect of Re and Co additions on lattice misfits was contradictory. The addition of Re produced rounded corner cuboidal morphology of gamma prime precipitates due to higher magnitude with negative lattice misfit (−0.3857%), while Co addition provided a more rounded shape of gamma prime particles due to the lattice misfit value approach to zero (+0.0936%).

An insight into a novel Mg2InSm prismatic plate in Mg-In-Sm alloy

• A novel Mg 2 InSm prismatic platelet can form in the aged Mg−In−Sm alloys. • The platelet has an aspect ratio of ∼25 and can keep a coherent relation with matrix. • The platelet is deemed as generalized β ″ with a (Mg 2 In)Sm-type D0 19 structure. Prismatic precipitate platelet is always purposefully designed in the microstructure of magnesium alloys due to its greater contribution to yield stress. In this study, with an introduction of In into Mg-Sm system, a category of novel { 10 1 ¯ 0 } α prismatic platelets has replaced thoroughly the traditional β ′ precipitate formed in magnesium rare earth (Mg-RE) alloys. Herein, the microstructural characteristics of platelet are investigated particularly by atomic scale scanning transmission electron microscopy. It is confirmed that the platelet has a Mg 2 InSm composition and can maintain a coherent relationship with α-Mg matrix. Importantly, on account of the similarities between In and Mg atoms, the Mg 2 InSm prismatic platelet could be structurally categorized as a generalized β ″ precipitate with a (Mg 2 In)Sm-type D0 19 structure when both In and Mg are regarded as an equivalent atom. Thus, the addition of In into Mg-Sm alloy induces the formation of β ″ precipitate. Furthermore, the formed β ″ prismatic platelets generally have a large average aspect ratio. The findings are of great significance to construct the effective precipitation strengthening phases and optimize the microstructure of Mg-based alloys. With an addition of In into Mg-Sm system, a type of novel { 10 1 ¯ 0 } α prismatic platelet has occurred and replaced the traditional β \prime precipitate. The prismatic platelet has a composition of Mg 2 InSm and can be categorized into a generalized β ″ phase with a (Mg 2 In)Sm-type D0 19 structure when Mg and In are deemed as a mixed atom.

Effect of heavy ion irradiation dose rate and temperature on α′ precipitation in high purity Fe-18%Cr alloy

Cr-rich alpha prime precipitates (CrRP) induce hardening and embrittlement of FeCr alloys, but the kinetics of CrRP formation due to particle irradiation are not well understood. In this study, Fe18wt.%Cr alloy in solid solution state and pre-aged to produce relatively coarse CrRP was irradiated with 8 MeV Fe ions. The irradiation conditions involved two midrange doses of 0.37 and 3.7 displacements per atom (dpa), a wide range of dose rates (10−5–10−3 dpa/s) and temperatures (300–450 °C). The distributions of CrRP after irradiation were studied with atom probe tomography (APT). The critical irradiation conditions to suppress CrRP formation were identified as 300 °C and 10−3 dpa/s; CrRP formation occurred readily at lower dose rates or higher temperatures. From 0.37 to 3.7 dpa, CrRP were observed to slightly grow at 350 °C and strongly coarsen at 450 °C. Specimens with pre-existing CrRP evolved into a similar precipitate distribution as detected after ion irradiation on solution annealed specimens at 300–350 °C to 0.37 dpa, indicating that the precipitate microstructure approaches a quasi-equilibrium for doses <1 dpa. Limited shrinking of pre-existing CrRP was observed after irradiation at 450 °C to 0.37 dpa, indicating a higher recovery rate at this temperature. The evolution of CrRP is quantitatively explained by employing corrections to the historic Nelson-Hudson-Mazey precipitate stability model, and a radiation modified precipitation mechanism is proposed to account for the competition between radiation enhanced diffusion and ballistic dissolution which results in the modifications on both size and solute concentration of CrRP.

Phase transformations during continuous cooling in Inconel 718 alloys manufactured by laser powder bed fusion and suction casting

Understanding alloy phase transformations during continuous cooling is important for post-processing design and optimization. In this work, continuous-cooling-transformation (CCT) diagrams of Inconel 718 alloys manufactured by laser powder bed fusion (LPBF) and suction casting are developed under different homogenization conditions. Unlike the available CCT diagrams in the reported studies, no gamma double prime and gamma prime precipitates can be observed. NbC and delta are determined to be the precipitates after cooling from the gamma matrix. Importantly, homogenization time and manufacturing methods are found to affect the Nb homogeneity in the matrix near NbC particles and thus significantly influence the precipitation process of the delta phase, which has a high content in Nb. In the alloys with high Nb homogeneity, the nucleation process mainly contributes to the precipitation, whereas in the alloys with low Nb homogeneity, the precipitation is primarily associated with the growth process. Subgrains are found to form after cooling at 0.1 K/s and can cause the highest hardness in samples. This work provides a new viewpoint on the study of processing-structure-property relationships during cooling in Inconel 718 and is beneficial to the development of alloy post-processing strategies.

On as-built microstructure and necessity of solution treatment in additively manufactured Inconel 939

ABSTRACT Increased adoption of additively manufactured superalloys has led to the consideration of revised heat treatment approaches for these materials. The rapid cooling during additive manufacturing processes has been seen to suppress gamma prime (γ′) precipitation, which has raised the possibilities for omitting the high-temperature solution treatment step that usually precedes ageing heat treatment for these alloys. In this work, the as-built microstructure of a high gamma prime fraction superalloy Inconel 939 is presented, where the absence of any γ′ precipitation is notable. However, transmission electron microscopy shows the presence of nano-sized Eta (η) phase. It is shown that the omission of solution treatment leads to the growth of the deleterious η phase upon ageing, which results in embrittlement in tensile loading. It is concluded that at least for this particular alloy the solution treatment plays a critical role in the establishment of the required microstructure and hence cannot be omitted from the heat treatment.

Microstructure, mechanical properties and fracture behavior of NiCoCrTiAl and FeNiCoCr new alternative binders for WC based hardmetals

Hardness and fracture strength of WC-Ni-Co-Cr-Ti-Al cemented carbides have been measured at room temperature in as-HIPed and solution-aged conditions. These treatments are applied to modify the size of gamma prime precipitates, which are intrinsically formed within the metallic binder of these ceramic-metal composites during the sintering process. Compositions containing approx. 28–29 vol% metal content exhibit hardness values in the range of those reported for similar grades of WC-Co hardmetals. Optimized aluminum additions lead to materials with fracture strength values only 15% lower than those reported for the same WC-Co commercial references. These results suggest gamma prime precipitation hardening as a potential strategy for improving the performance of WC-Co materials at high temperatures.Regarding Fe-Ni-Co-Cr alloys are potential candidates for partial substitution of Co content in WC-based hardmetals. It has been investigated WC coarse grade with 15 wt%(FeNiCoCr). The Cr content has been adjusted in order to avoid the precipitation of M7C3 carbides. Within the corresponding carbon windows, fracture strength values range from 2.8 GPa to 3.0 GPa. These values are within the range of standard WC-Co grades with similar binder contents and WC grain sizes.

Effect of annealing and supercritical CO2 exposure at 750°C on the tensile properties of stainless steel and Ni‐based structural alloys

AbstractSubsize dogbone tensile specimens of alloys 304H, 310HCbN, and 740H were exposed to 30 MPa supercritical CO2 (sCO2) for up to 4000 h at 750°C. Similar exposures in laboratory air were used to assess the specific role of sCO2 exposure on the alloy tensile properties. Type 304H specimens were only embrittled by exposure in sCO2 due to the formation of a thick Fe‐rich oxide and C ingress in the bulk of the alloy. For 310HCbN and 740H, no effect of the sCO2 environment was observed and a decrease of the alloy's ductility was associated with previously observed microstructure evolutions, for example, grain boundary carbide formation. The 740H was tested in the solution annealed condition and self‐aging took place during 750°C exposures in sCO2 and air with the formation of fine gamma prime precipitates leading to significant tensile strength increase. It was found that both 740H and 310HCbN have adequate sCO2 compatibility for high‐temperature commercial applications.

Laser powder bed fusion of a new high gamma prime Ni-based superalloy with improved weldability

This study reports the crack-free fabrication of a new high gamma prime precipitation hardened Ni-based superalloy by Laser Powder Bed Fusion (LPBF) process. In the as-built condition, scanning electron microscopy and electron backscattered diffraction analysis indicated the presence of elongated columnar grains with an orientation along 〈1 0 0〉 direction. Following the sub-solvus heat treatment cycle determined using Thermo-Calc software, crack-free microstructure with columnar grains was observed along 〈1 0 0〉 orientation. High-resolution scanning electron microscope analysis revealed a bimodal distribution of gamma prime precipitates and a uniform distribution of carbide precipitates in the heat-treated samples. As a result of heat treatment, hardness increased from 365 HV to 508 HV. The heat-treated samples also displayed excellent yield strength, ultimate tensile strength, and elongation of 939 MPa, 1368 MPa, and 23% respectively at room temperature. Mechanical testing at elevated temperatures confirmed the yield strength, tensile strength, and elongation reached 80 MPa, 106 MPa, and 34% respectively at 1121 °C. The stress rupture test of the heat-treated samples revealed good creep properties with rupture times of 559 h and 75 h, respectively, at 927 °C and 982 °C, for stresses of 152 MPa and 138 MPa.

Turbine Blade Tip Repair by Laser Directed Energy Deposition Additive Manufacturing Using a Rene 142–MERL 72 Powder Blend

Laser directed energy deposition (LDED) was used with a powder blend comprising 75 wt.% Rene 142 and 25 wt.% of Merl 72 (4275M72) for turbine blade tip repair applications. Sound samples could be deposited at ambient temperature on Haynes 230. The microstructural analyses showed the presence of fine gamma prime precipitates in the as-deposited samples, while after aging, the alloy possessed around 40 vol.% with a bimodal precipitate size distribution. Also, the alloy contained Ta-Hf-W carbides in different sizes and shapes. Tensile testing from room temperature up to 1366 K was performed. The 4275M72 deposits possessed higher tensile properties compared to Rene 80 in this temperature range but lower elongations at the elevated temperatures. The creep properties of 4275M72 samples at 1255 K were superior to Rene 80. Also, the oxidation resistance of deposited 4275M72 was similar to Rene 142. The combination of high mechanical properties, creep behavior, and oxidation resistance of LDEDed 4275M72 makes it a suitable alloy for tip repair of turbine blades.

Precipitate Dissolution During Deformation Induced Twin Thickening in a Coni-Base Superalloy Subject to Creep

The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal gamma prime distribution has been examined at 800C and 300MPa. The rupture life of the alloy is comparable to that of RR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealed the presence of continuous gamma prime precipitates and M23C6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary gamma prime precipitates occurred at the expense of the fine tertiary gamma prime. Long planar deformation bands, free of gamma prime, were also observed to traverse individual grains ending in steps at the grain boundaries. Examination of the deformation bands confirmed that they were microtwins. Long sections of the microtwins examined were depleted of gamma prime stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolution of the precipitates is suggested based on the Kolbe reordering mechanism.

Reactive sintering of WC-Ni-Co-Cr-Ti-Al cemented carbides and precipitation of gamma prime in their metallic binder phases

Cemented carbides processed from WC-Ni-Co-Cr3C2-TiAl3 powder mixtures by HIP after sintering present a homogeneous precipitation of gamma prime in their metallic binder phases. Other reaction products include alumina precipitates and (Tix,W 1-x)C mixed carbides. The formation of these phases is consistent with the highly exothermic reactions detected by DSC during the heating ramp of the sintering cycle. The amount of alumina and gamma prime phases present in the sintered materials depends on the oxygen content of the powder mixtures. Refinement of gamma prime precipitates depends critically on the cooling rate after HIPing. Nanoindentation experiments have confirmed the presence of age hardening effects of samples which were previously solution treated. As expected, gamma prime precipitation is induced at lower temperatures in compositions with higher Al contents in the binder phase.

Effects of helium on irradiation-induced phase instability in a NiCrFeTiNbAl alloy

The Ni–Fe–Cr based superalloy with percentage additions of Nb, Al and Ti, called "Inconel X750 superalloy", is studied with its importance in nuclear application. Under reactor neutron irradiation, the strengthening phase Ni3(Al, Ti) in the form of gamma prime precipitates were somehow disordered, and dissolved eventually, by which the mechanical properties of materials were degraded. It is of high interests to investigate the effects of transmutation production helium on radiation-induced instability of gamma prime in the materials. The transmission electron microscope in situ heavy ion irradiations with and without pre-injected helium are performed purposely. The precipitates were disordered by heavy ion irradiation after 0.06 displacement per atom (dpa). The presence of 2×10−4 atoms helium delayed the disordering of Ni3(Al, Ti) precipitates to 0.68 dpa. With 4×10−4 atoms helium, the precipitates required doses of > 2.7 dpa to become disordered. In addition, the precipitation of M23C6 particles were observed at 600 °C irradiation to 5.4 dpa with 4×10−4 atoms helium pre-injected.

Microstructures at 1250°C of Chromia–Forming Carbides– Reinforced Nickel–based Alloys after Addition of Tantalum; Effects of the Cr and Co Contents

Cast nickel-based alloys can access to good mechanical properties at elevated temperatures even in total absence of gamma prime precipitates. Versions in which TaC carbides are present in significant quantities have recently emerged at laboratory scale but further optimization is required prior to possible industrial use. The main remaining problem is an insufficient stability of the TaC phase due to competition with chromium. This latter element is an essential element for the hot corrosion resistance and it cannot be removed. However, a better rating in Cr and a limited introduction of cobalt, element known to be more favorable for TaC stability, may lead to TaC-reinforced nickel-based superalloys. In this work thermal tests and thermodynamic calculations contributed to a better knowledge of the effect of added cobalt onthe nature of the carbides population in chromium-rich alloys based on nickel. On the one hand three alloys with different amounts in added cobalt were synthesized and exposed to 1250°C isothermally on long time. On the other hand thermodynamic calculations were carried out using a database especially designed for this used but not yet perfect. Adding Co to a {Ni, 25 wt.%Cr}-base containing carbon and tantalum obviously allows favoring predominant TaC at the expense of chromium carbides. The TaC fraction was accurately determined for each Co addition and verified by image analysis. Thermodynamical calculations showed the same trends but with a different origin: chromium carbides exist alone in the alloy when Co starts to be added while TaC are already present in the real alloy in absence of cobalt. Improvements need to be brought to the used database to better predictions and to take benefit of it to explore the possible high temperature microstructures when the contents in Co, Ta, Cr and C vary over more extended intervals than tested in this work.

Natural-mixing guided design of refractory high-entropy alloys with as-cast tensile ductility.

Metallic alloys containing multiple principal alloying elements have created a growing interest in exploring the property limits of metals and understanding the underlying physical mechanisms. Refractory high-entropy alloys have drawn particular attention due to their high melting points and excellent softening resistance, which are the two key requirements for high-temperature applications. Their compositional space is immense even after considering cost and recyclability restrictions, providing abundant design opportunities. However, refractory high-entropy alloys often exhibit apparent brittleness and oxidation susceptibility, which remain important challenges for their processing and application. Here, utilizing natural-mixing characteristics among refractory elements, we designed a Ti38V15Nb23Hf24 refractory high-entropy alloy that exhibits >20% tensile ductility in the as-cast state, and physicochemical stability at high temperatures. Exploring the underlying deformation mechanisms across multiple length scales, we observe that a rare β'-phase plays an intriguing role in the mechanical response of this alloy. These results reveal the effectiveness of natural-mixing tendencies in expediting high-entropy alloy discovery.

Development of Pulsed Current Arc Welding to Preclude Carbide Precipitates in Hastelloy X Weldment Using ERNiCr-3

In this study, gas tungsten arc (GTA) and pulsed current gas tungsten arc (PCGTA) welding on Hastelloy X were performed using ERNiCr-3. The welded plates are free from the porosity, inclusion, liquation cracks, and hot cracks. The weld bead profile shows a lower bead width in PCGTA than GTA weldment. The segregations of Cr, Mo, Nb, and Ti were identified in GTA with the development of Fe3Mo3C (M6C), Fe2MoC (M3C), Cr23C6, Cr2Ti, Ni8Nb phases. While in PCGTA, Nb and Ti segregations alone were observed with the formation of phases like Ni3Ti, Cr2Ti, Ni8Nb, respectively. The presence of γ prime precipitate (Ni3Ti) observed in PCGTA improved the ultimate tensile strength (UTS) by 8.56% compared to GTA. The UTS and ductility were found inferior in both the weldments when compared to the base metal. This is due to the precipitation of Ni8Nb phase and the existence of Cr2Ti phase, respectively. The PCGTA weldment also shown improved microhardness by 12.21% and impact energy by 9.78% compared to GTA, respectively. The refined crystallite size and precluded carbide precipitates in PCGTA are responsible for the enhanced properties.

Microstructural characterization of three different size of gamma prime precipitates in Rene 65

Cast and wrought Ni-based superalloy Rene 65 was developed for turbine engine components applications. In the as-extruded condition the alloy presents trimodal distribution of gamma prime (γ′) precipitates. A detailed microstructural and chemical characterization of the precipitates is presented at a near atomic-scale before and after a softening sub-solvus heat treatment. Scanning and Transmission Electron Microscopy (SEM and TEM) as well as Atom Probe Tomography (APT) were employed to identify the differences in compositions. In the as received condition differences in Cr, Ti and Al content were found between the three precipitate sizes. After a softening sub-solvus heat treatment, the interface between secondary and grown tertiary γ′ with the matrix had become chemically homogenized. Lattice mismatch of secondary and tertiary γ′ were empirically calculated based on the APT compositions.