Standard Heat Treatment Research Articles

Low-Temperature Superplasticity and High Strength in the Al 2024 Alloy with Ultrafine Grains.

This study aims to achieve an ultrafine-grained (UFG) Al 2024 alloy superplasticity at temperatures lower than the traditional ones for commercial Al alloys (400-500 °C). The UFG structure with a mean grain size of 100 nm produced in the alloy by high-pressure torsion at room temperature provided a very high strength-microhardness (HV0.1) of 286 ± 4, offset yield strength (σ0.2) of 828 ± 9 MPa, and ultimate tensile strength (σUTS) of 871 ± 6 MPa at elongation to failure (δ) of 7 ± 0.2%. Complex tensile tests were performed at temperatures from 190 to 270 °C and strain rates from 10-2 to 5 × 10-5 s-1, and the values of flow stress, total elongation and strain rate-sensitivity coefficient were determined. The UFG alloy was shown to exhibit superplastic behavior at test temperatures of 240 and 270 °C. For the first time, 400% elongation was achieved in the alloy at an unusually low temperature of 270 °C (0.56 Tm) and strain rate of 10-3 s-1. The UFG 2024 alloy after superplastic deformation was found to have higher strength (150-160 HV) than that after the standard strengthening heat treatment T6.

Culinary Recipes Development from Camel Meat Treated with the Enzyme Preparation Protosubtilin

Most recently, non-traditional sources of animal protein, including camel meat, are of increasing interest. Due to the coarse-grained fibrous structure of the camel muscle tissue, ready-made dishes from this meat raw material have a hard consistency. In this regard, the study aim is to develop a rational regimen for softening Kalmyk Bactrian camel breed meat using the enzyme preparation protosubtilin G20x, to develop culinary dishes from meat raw materials pre-treated with this preparation, and also to compare with the study results of the protosubtilin G3x impact on softening meat raw materials. In terms of organoleptic quality indicators, camel meat complies with the GOST 7269-2015 and has the following content of the main nutrients: proteins – 21.6 %; fats – 7.2 %; minerals – 0.97 %. The moisture mass fraction is 76.7 %; energy value accounts for 151 kcal/100 g; moisture-binding capacity ranges from 73.3 to 80.7 %. By the microstructure, the camel muscle tissue is the same as in most butcher beast species, but the muscle fibers diameter ranges from 10 to 250 microns, indicating a coarse-grained fibrous structure. Moreover, there is a large amount of connective tissue in different cuts. Therefore, meat raw materials will remain tough after standard heat treatment. According to the listed indicators, a man can identify camel meat as a dietary full-fledged meat raw material. Still due to the meat hardness, pre-treatment is necessary to soften it. The optimal processing mode of meat raw materials treatment with an enzyme preparation protosubtilin G20x is the following: the aqueous solution concentration – 0.05 %; medium temperature – 40 °С; duration – 1 hour. The finished culinary dish from pre-processed raw materials received the highest score – 5 points. A comparative analysis of the protosubtilin G3x and G20x impact on the meat raw materials structure demonstrated that the preparation G20x was optimal for processing Kalmyk Bactrian camel breed meat, since at a lower concentration it enabled to increase the moisture-binding capacity of raw materials and finished dish yield.

In Envelope Additive/Subtractive Manufacturing and Thermal Post-Processing of Inconel 718.

This study investigated the application of an in envelope additive/subtractive (LPBF) manufacturing method (Matsuura LUMEX-Avance-25) to fabricate IN718 benchmarking coupons. The coupons were then examined comprehensively for surface finish both with and without high-speed micro-machining. The microstructure of the manufactured IN718 coupons was investigated thoroughly in the as-fabricated condition and following three different standard and one non-standard post-processing heat treatments. As built coupons revealed columnar grain morphology mainly along the <100> direction with a cellular dendritic sub-grain structure and without any strengthening precipitates. Grain size, aspect ratio, and texture were maintained after each of the applied four heat treatments. Only one of the standard heat treatments resulted in the δ phase formation. The other three heat treatments effectively dissolved the Laves phase preventing the δ formation while promoting the formation of γ′/γ″ precipitates. Despite the observed differences in their microstructures, all of the heat treatments resulted in similar yield and ultimate tensile strength values that ranged between 1103−1205 MPa and 1347−1387 MPa, respectively. These values are above the minimum requirements of 1034 MPa and 1241 MPa for the wrought material. The non-standard heat treatment provided the highest elongation of 24.0 ± 0.1% amongst all the heat-treated specimens without a significant loss in strength, while the standard heat treatment for the wrought parts resulted in the lowest elongation of 18.3 ± 0.7% due to the presence of δ phase.

Effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy

In this work, the effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy for Generation Ⅳ nuclear reactor application was systematically investigated. The results show that different deformed microstructures were observed during cold rolling, including dislocation tangles, slip bands, and microtwins (MTs). The dominant cold rolling deformation mechanism of the alloy gradually changed from planar slipping to deformation twinning, which was a little different from Ni-/Co-based superalloys. A texture transition from Copper {112}< 111 > to Brass {110}< 112 > and Goss {110}< 001 > at 25 % rolling reduction was detected due to the changed deformation mechanism. The standard heat treatment alloy exhibited a yield strength of 213 MPa and good elongation of 54 %. By increasing the rolling reduction up to 55 %, the yield strength was significantly enhanced to 872 MPa, which was attributed to the combined strengthening from both MTs and dislocations. Fine-spaced MTs with coherent interfaces can impede dislocation motion effectively and play a critical role in the strengthening, contributing 75–235 MPa to the yield strength. MTs can also provide ample room for dislocation accumulation, which maintained a reasonable elongation of 12 %. Moreover, due to the texture evolution during cold rolling, the averaged Schmid factor (SF) values for {111}< 110 > slip system exhibited a decreasing trend with the increased rolling reduction, contributing to the progressive increase in yield strength. The present work demonstrates that the formation of MTs at high rolling reductions is an essential reason for the improvement of mechanical properties of the alloy, which also provides a new strategy for designing and optimizing high-performance solid-solution strengthened Ni-Fe-based superalloys.

Laser powder bed fusion of high-strength and corrosion-resistant Inconel alloy 725

The development of additive manufacturing, or three-dimensional (3D) printing, technologies has produced breakthroughs in the design and manufacturing of products by enhancing design freedom and minimising manufacturing steps. In addition, the complex, unique microstructures imparted by the additive processes offer prospects of unprecedented advances to produce high-performance metal alloys for high-temperature and corrosive environments. Here, we present the first additive manufacturing of Inconel alloy 725, an advanced nickel-base superalloy that is the widely accepted gold standard material of choice for oil and gas, chemical, and marine applications. We explore the printability of Inconel alloy 725 and identify a wide processing space to build material with a crack- and near-pore-free microstructure. The conventionally heat-treated Inconel alloy 725 has an equiaxed, near-fully recrystallised microstructure containing copious twin boundaries and nano-precipitates. It also displays promising tensile properties and corrosion resistance compared to its wrought counterpart. Our work opens the door toward additive manufacturing of Inconel alloy 725 components with optimised microstructure and topology geometry for applications in harsh environments. • We additively manufacture Inconel alloy 725, for the first time, by laser powder bed fusion. • We reveal that Inconel alloy 725 is highly printable exhibiting a wide processing window for near defect-free microstructure. • We find that Inconel alloy 725 responses well to industrial standard heat treatment compared to other common additively manufactured alloys. • We demonstrate that Inconel alloy 725 shows promising tensile properties and corrosive resistance compared to conventional wrought material.

Influence of pre- and post-weld heat treats on microstructures of laser welded GTD-111 with IN-718 as filler metal

Abstract The present research work studied and evaluated conditions of pre-weld and post-weld heat treatments for the laser welding process, aiming to avoid weld cracking, which usually occurs after tungsten inert gas and/or laser welding processes in cast nickel-based superalloys with high aluminum and titanium contents. The pre-weld heat treatment conditions used in the experiments were (1) 1473 K for 7.2 ks, (2) 1433 K for 7.2 ks + 1298 K for 56.7 ks, (3) 1413 K for 7.2 ks + 1298 K for 56.7 ks, (4) 1393 K for 7.2 ks + 1298 K for 56.7 ks. A laser welding process on cast nickel-based superalloys, grade GTD-111, with metal filler of Inconel 718, a nickel-iron based alloy was conducted. From all obtained results, it was found that the pre-weld heat treatment conditions significantly influence microstructures before laser welding. After laser welding and post-weld heat treatment at a temperature of 1473 K for 7.2 ks followed by precipitation aging at a temperature of 1118 K for 86.4 ks (standard heat treatment), all welded specimens exhibited similar microstructures and hardness values. Furthermore, no weld cracking was observed in all welded specimens.

Improving high-temperature mechanical properties of laser powder bed-fused Inconel 738 alloy by hot isostatic pressing: Tailoring precipitates and healing defects

Microscopic defects and inferior high-temperature mechanical properties that occur during laser powder bed fusion (LPBF) of Inconel 738 alloys can severely hinder their industrial applications. Hot isostatic pressing (HIP) is a promising method to overcome these issues. In this study, the effects of HIP on the defects, grain structure, precipitated phase behavior, room- and high-temperature mechanical properties of an as-built Inconel 738 alloy were investigated. The results showed that, after HIP treatment, the defect densities in the XY and YZ planes of the as-built Inconel 738 alloy decreased from 0.537% and 0.292% to 0.251% and 0.123%, respectively. Moreover, the grain morphology of the as-built Inconel 738 alloy changed from elongated columnar grains to coarse equiaxed grains after HIP treatment, and the grain size became six times larger than that of the original sample, which was attributed to recrystallization and grain growth. Moreover, HIP treatment eliminated cellular dendritic segregation and significantly reduced dislocation densities in the as-built Inconel 738 alloy. Consequently, fewer M23C6 carbides were formed, but block-like M23C6 carbides were uniformly distributed at the grain boundaries after a combination of HIP and standard heat treatments. The HIP treatment reduced the room-temperature tensile properties of the as-built Inconel 738 alloy because of grain growth, low dislocation density, and coarser precipitates. However, the high-temperature (800 °C) tensile properties of the HIP sample with standard heat treatment (σuts = 827.8 MPa, σy = 744.3 MPa, εf = 2.2%) increased significantly compared to those of the as-built sample with standard heat treatment (σuts = 715.6 MPa, σy = 653.3 MPa, εf = 0.6%) and alleviated the embrittlement fracture owing to the formation of uniform carbides and lower defect densities. These results indicate that HIP treatment can significantly improve the high-temperature mechanical properties of LPBF-built nickel-based superalloys with high Al and Ti contents.

Investigation on the influence of Ta on the microstructure evolution of Ni-based superalloy DZ411 during directional solidification, heat treatment, and long-term aging

The microstructure evolution in directionally solidified Ni-based superalloys DZ411 with different Tantalum (Ta) contents (2.72, 3.10, and 4.00 wt%) was investigated. After directional solidification, three alloys with different Ta contents were subjected to standard heat treatment: solution treatment (1230 °C/2 h/AC)+ primary aging (1120 °C/2 h/AC)+ secondary aging (870 °C/24 h/AC). Then, following isothermal long-term aging treatments were performed at 950 °C for 50, 100, 200, 500 and 1000 h. The addition of Ta can lead to the increase of the volume fractions of MC carbides, eutectics, and γ′ phases during directional solidification. In addition, with the increase of Ta content, the mismatch of γ/γ′ increases, thus, the shape of γ′ becomes more regular. After solution heat treatment, the MC carbides are decomposed, and more M23C6 carbides are precipitated along the grain boundaries in alloys with increasing Ta contents. Besides, the eutectic cannot be completely eliminated. After secondary aging, more secondary γ′ phases are precipitated on the γ matrix channels in alloys with increasing Ta contents. Furthermore, the addition of Ta can delay the coalescence of the γ′ phase during long-term aging, which can be attributed to the reduction of the coarsening rate K according to the precipitation coarsening theory of Lifshitz and Slyozov and Wanger (LSW).

Effect of heat treatments on the microstructure and mechanical properties of IN738LC prepared by electron beam powder bed fusion

The heat treatment of non-weldable nickel-based superalloy prepared by electron beam powder bed fusion (EB-PBF) is vital for the properties of parts. The current study aims to use a heat treatment method to increase the properties of the as-built EB-PBF IN738LC and explore the applicability and necessity of standard heat treatment designed for cast IN738 on the EB-PBF counterpart. Different heat treatment (HT) methods, such as standard heat treatment (SHT), hot isostatic pressing (HIP), and HIP+SHT, have been performed. The microstructures before and after HT are scrutinized focusing on the evolution of the precipitates. Tensile tests were carried out to evaluate the effect of the heat treatment. The grain morphology can be altered by a HIP process but not the SHT process. The HIP process induces more carbides to be precipitated; the carbide changes from MC to M 23 C 6 during the SHT process, and the γ’ phase changes from the cuboidal shape in the as-built state to the spherical shape in SHT processes. The refined grain and disappearance of pores are the reasons for the increased elongation ratios of HIP samples. Combining the HIP and SHT processes helps refine the grain and improve the mechanical properties. • The elongated grains in EBSM IN738LC are different from the equiaxed grains in cast IN738LC. • The different heat-treated process of EBSM IN738LC samples were compared for the first time. • The refined grain and disappearance of pores are the reasons for the increased elongation ratios of HIP samples. • Combining the HIP and SHT processes helps refine the grain and increase the mechanical properties (YS, UTS, and total elongation ratios).

Effect of remelting solution heat treatment on microstructure evolution of nickel-based single crystal superalloy DD5

The microstructure evolutions of nickel-based single crystal superalloy DD5 after solution treatment at different temperatures as well as heat treated with various routes were systematically investigated by differential scanning calorimeter (DSC), optical microscope (OM), scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). The formation and elimination mechanisms of unfavorable incipient melting microstructure emerged during heat treatment were explored in detail. Microstructure observations reveal that a higher solution temperature can speed up the element diffusion, resulting in a more uniform element distribution. A reduced degree of lattice mismatch between γ phase and γ' phase would be obtained, which leads to an increase in the average size and content of γ' phase. However, excessively high solution temperature would cause the pre-melting occurred at the interface of γ matrix and γ' phase, causing the formation of incipient melting microstructure. The incipient melting microstructure can be eliminated by the subsequent homogenization process under the conditions that the solution temperature exceeds the solvus temperature of γ' phase. The formation course of incipient melting microstructure includes the pre-melting at the interface of γ matrix and γ' phase, the expansion of molten liquid, and the formation by the resolidification of molten liquid. The elimination of incipient melting microstructure begins with the dissolution of fine eutectic, followed by the dissolution of coarse γ' phase and the filling of pores, and eventually the dissolution of residual eutectic. Compared with standard heat treatment, remelting heat treatment can provide better homogenization effect of element for DD5 superalloy.

Effect of hot isostatic pressing on microstructural and micromechanical properties of additively manufactured 17–4PH steel

Microstructural and mechanical characterization techniques were utilized to establish the effect of the combined hot isostatic pressing (HIP) and standard H1025 heat treatment schedule on the properties of 17–4PH steel manufactured by means of laser powder bed fusion (LPBF). The as-printed (AP) sample initially consists of a dual-phase microstructure, with 78% martensite and 22% retained austenite. Then, the combined HIP and H1025 applied to the AP sample resulted in lower retained austenite of 7%. The microstructural analysis indicates that the post-processing after printing alleviated the process-induced defects reducing the overall defect content from 0.37% to 0.01%. Moreover, it is observed that the characteristic melt pools are dissolved, resulting in a homogeneous microstructure with reduced grain sizes. A room-temperature compression testing was then applied to the sample before and after post-processing treatment. It has been identified that, in both samples, the retained austenite transforms into martensite, a characteristic of the transformation-induced plasticity effect. The critical stress to initiate the dynamic transformation was then determined to be lower in the HIP sample than that of the AP sample. Based on the results, it can be concluded that the combined HIP+H1025 processing employed in this work effectively increased the mechanical properties of LPBF-fabricated 17–4PH steel. • 17–4Ph steel was fabricated via laser powder bed fusion. • Characterization of the samples was done by the means of EBSD analysis. • Using double differential method, the critical stresses for initiation of dynamic transformation of HIP sample was higher.

Microstructure evolution and hardness enhancement of LMD Inconel 718 multilayer in as-deposited and post heat treated: A comparative study

ABSTRACT This work aims to study the influence of heat treatment on the microstructure of laser metal deposited multi-layered Inconel 718 alloy. As-deposited samples were subjected to standard heat treatment, and the resulting microstructure and mechanical properties are studied. When observed under optical microscope and scanning electron microscope, the microstructure revealed microstructural homogenisation. In this case, the grain morphology shows that Nb-rich Laves phase in as-deposited sample is converted to small irregular granules in post heat treatment. The main finding of this study is that after heat treatment, the continuous bands of Nb-rich Laves phase are dissolved into small patches, as observed in elemental mapping through Energy Dispersive Spectroscopy. Also, Nb and Ti are distributed uniformly in the microstructure forming γ´ (Ni3Ti) and γ´´ (Ni3Nb) strengthening precipitates in Ni matrix. The precipitates developed has resulting in 50–60% higher hardness (419 HV) in the heat-treated sample compared to as-deposited sample.

Long-term structural stability and excellent mechanical properties of CoCrNi system medium entropy alloys

CoCrNi medium entropy alloys (MEAs) are promising candidates for high- temperature application, but their structural stability and mechanical properties during long-term service are still an open question. This work comprehensively investigated the microstructure and mechanical properties at both room and high temperatures of CoCrNi MEAs after being aged for 500 h at 850 °C. It has been shown that the long-term aged CoCrNi MEA has held a single γ phase, a nanoparticulate and discontinuous block γ′ phase, and η-Ni3(Ti,Ta) phase in acicular Widmanstätten, cellular and rod-like shapes with the addition of Al, Ti, and Ta. Long-term aged CoCrNi MEAs hold same levels (or sometimes slightly weaker levels) of room- and high-temperature strengths as those MEAs under the standard heat treatment. Specifically, CoCrNi MEA with 2%Al, 2%Ti, and 1%Ta presents an ultimate strength of 1012 MPa at room temperature and a yield strength of 695 MPa at 700 °C. Surprisingly, all of the relevant long-term aged MEAs exhibit superior plasticity to that under standard heat treatment. The anomalous mechanical properties are firstly attributed to the γ' phase precipitation. Next, η phase plays an important role in strengthening, improving or at least harmless plasticizing effect since the η phase has the preferential orientation relationship of [011]γ//[2110]η, {111}γ//{0001}η with the γ matrix. It has been confirmed that the strengthening mechanisms are ascribed to the dislocation accumulation and proliferation, formation of stacking faults and nanotwinning in γ, γ' and η phase induced by the precipitation of γ′ and η phase.

Вплив комплексного модифікування на структуру й властивості жароміцного нікелевого сплаву

In this work, a study of the influence of complex modification of the ZhS3DK-VІ nickel-based heat-resistant super alloy was conducted. The modification was performed out according to different technological schemes: with highly dispersed titanium carbonitrides Ti[Ti(C,N)] particles, with Ti [Ti(C,N)]+CoAl2O4 and Ti[Ti(C,N)]+CoAl2O4+Ni-Y complex that made it possible to investigate the specifics of the influence of the volume modifier Ti[Ti(C,N)] separately, and its complex effect with surface modifierCoAl2O4 and rare earth metals (with Ni-Y master alloy use) in the macro- and microstructure of a heat-resistant nickel alloy. It was shown that the use of modifiers allows for significant refining of macrograins, and the greatest effect was obtained with Ti[Ti(C,N)]+CoAl2O4 and Ti[Ti(C,N)]+Ni-Y+CoAl2O4complexes. Microstructure study allows to show that the value of interdendritic distances in both the case of modification with Ti[Ti(C,N)] and Ti[Ti(C,N)]+Ni-Y+CoAl2O4 complexes essignificantly decreased, from 100…130 μm to 80...120 μm, which is the result of many nuclei formation during crystallization in the volume of the alloy and with highly intense heat transfer from the melt by CoAl2Omodified ceramic mould after melt pouring. Based on the results of the influence of various types of modification on the microstructure of the alloy under study, it was established that modification according to all schemes allows obtaining carbides of smaller sizes, but grown sizes of carbonitrides was observed. Globular particles of carbide and carbonitrides are evenly located along the cross-section of the samples under study. In all cases of modification, a decrease in the value of the shrinkage microporosity was observed. The mechanical properties of the specimens after standard heat treatment shows, that modification with complexes under study allows provide mechanical properties that meet the specification requirements. The best complex of alloy mechanical properties at room temperature, fracture toughness and stress-rupture strength provides with Ti[Ti(C,N)]+Ni-Y+CoAl2O4 modifiers complex.

Microstructure, Mechanical and Wear Behaviour of Deep Cryogenically Treated EN 52 Silchrome Valve Steel.

This study has compared the performance of cryogenically processed EN 52 Silchrome valve steel with untreated material. After completing the standard heat treatment process, EN 52 steel material specimens are subjected to a deep cryogenic process with varying soaking temperatures. The parameters of the deep cryogenic procedure were changed to find the best wear qualities. The key features of valve steel, such as microstructure, mechanical, and wear behaviour are evaluated by conducting a test study. The evolution of wear mechanisms after enhancing qualities of EN 52 steel is studied using scanning electron microscopy. The mechanical and wear behaviour improved due to factors such as fine carbide precipitation, conversion of residual austenite, and carbide refining formed after cryogenic treatment. With a maximum reduction in wear rate of up to 45%, the deep cryogenic treatment of EN 52 steel with a soaking temperature of −140 °C was the ideal parameter. All three cryo-treated samples had better properties than the untreated EN 52 valve steel.

Phase composition and fatigue crack growth behavior of Inconel 718 under additive manufacturing

AM samples both in as-deposited and heat-treated states have been subjected to microstructural and phase characterization. Mechanical testing has been performed to heat-treated samples with respect to the build directions. AM samples were made by Direct Laser Deposition under the three different levels of efficient energy from Inconel 718 powder. As-deposited samples contained dispersed particles of Fe2Ti Laves phase, NbC and NbNC as well as starting clusters of γ′ phase. Although the standard heat treatment at 980 °C was found to result in partial dissolution of Laves phase precipitation of δ phase occurred around it. Depletion in Nb of Ni-based matrix due to formation of NbC, NbNC and Laves phase in as-deposited states causes precipitation of γ′ phase as the main strengthening phase in heat-treated states. Fatigue crack propagation manners were analyzed with fractography. Fine columnar dendrite structure of samples tested in parallel build direction to the loading is much more resistant to fatigue crack propagation. Fatigue crack bypass dispersed Laves phase which is fixed in matrix by δ phase.

Influence of post-processing heat treatment on the cyclic deformation behaviour of AlSi10Mg aluminium alloy subjected to laser powder bed fusion

The effect of heat treatment on the cyclic deformation behaviour of AlSi10Mg aluminium alloy manufactured by laser powder bed fusion (LPBF) was investigated. Uniaxial fully-reversed strain-controlled fatigue tests were conducted on polished smooth specimens in the low-cycle and high-cycle fatigue regimes for three material states: no heat treatment, stress relief heat treatment and T6 heat treatment. The effect of the heat treatment on microstructure, ductility, hardness and tensile properties was also investigated. It was found that the microstructural transformation associated with the heat treatments increased the ductility, and reduced the hardness, the tensile strength, and the fatigue resistance, particularly in the low-cycle fatigue regime. Thus, these standard heat treatments used in conventional heat-treatable aluminium alloys are not recommended for LPBD aluminium alloys subjected to cyclic strain loading.

Independent effects of laser power and scanning speed on IN718's precipitation and mechanical properties produced by LBPF plus heat treatment

The laser energy density was believed to control microstructure and mechanical properties of additive-manufactured Inconel 718 (IN718) alloy. However, in this study, remarkable differences were found in the microstructures, tensile mechanical properties, and fracture surfaces of industrial IN718 alloy (with densities higher than 99.75%), which were fabricated by Laser Powder Bed Fusion (LPBF) processes with almost identical laser energy densities and the same standard heat treatment. The results show that these deviations are primarily ascribed to the independent effects of laser power and scanning speed during LPBF-processes on the precipitation of IN718 alloy. The reasons for these variations are discussed based on the thermal history of melt pool and pre-solidified layer during the LPBF-process, which were further analyzed and supported by computational fluid dynamics simulation based on the finite volume method.

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.

Effect of phosphorus segregation on carbides and creep damage of Ni–Fe-based GH984G alloy

The effect of phosphorus segregation on carbides and creep damage of Ni–Fe based GH984G alloy, for the application of advanced ultra-super critical (A-USC) power plant, were investigated under the creep condition of 750 °C and 200 MPa. The results indicate that the primary precipitates of GH984G alloy are MC, M23C6 and γ′ after standard heat treatment. P-doping has no effect on the grain size, γ′ size and morphology. Nevertheless, P atoms segregate at the grain boundaries and around the MC carbides, resulting in a change in the morphology of M23C6 carbides at grain boundary from film-like to granular and a decrease in the density and area percentage of MC carbides. The creep behavior of GH984G alloy at 750 °C/200 MPa changes significantly after P doping and the creep properties (creep life and elongation) are optimized with the increasing P content. The damage feature is microcrack at grain boundaries for P-free alloy, while the creep cavities in P-doped alloy. The fracture mode changes from intergranular fracture to transgranular fracture with the increasing P doping for GH984G alloy after creep at 750 °C/200 MPa. The optimizing of creep properties and transformation of fracture mode of GH984G alloys after P doping are mainly attributed to increase of grain boundary cohesion and the change of carbides caused by the P segregation behaviors during creep process.