Cr-rich M23C6 Carbides Research Articles

Substructure boundary's enhancing strain hardening ability in additively manufactured 0.75 at%C doping NiCoCr medium entropy alloy

In this study, the effect of post heat treatment on microstructural evolution and tensile properties of 0.75% of carbon doped NiCoCr MEA alloy manufactured by laser powder bed fusion (LPBF) was investigated. The post heat treatment of LPBF-built 0.75C MEA alloy was selected and carried out at 700 °C for 1hr, respectively. The microstructural observation in heat-treated 0.75C MEA HT sample shows a lower stacking fault energy (SFE) with wider stacking fault width (SFW) compared to as-built condition. In-situ nano-sized precipitates are formed along the sub-boundaries of the as₋built 0.75C MEA and 0.75C MEA HT samples which are estimated as 1.25 % and 2.45 %, respectively. After post heat treatment process, the size and ratio of nano-sized precipitates enhanced and identified as Cr-rich M23C6 carbide. Subsequently, the yield and tensile strengths increase from 823.2 MPa to 872.7 MPa and 1.05 GPa–1.15 GPa for 0.75C MEA and 0.75C MEA HT conditions, respectively which is attributed to the prevailing solid solution strengthening and precipitation strengthening. The deformation twins in as−built specimen are transformed to twin bundle once the local strain increases from 5% to 20%. The 0.75C MEA HT deformed specimen exhibits high dislocation accumulation with retained stable substructure in local strain 20% area due to the pinning effect of nano-sized precipitates which stabilizes and strengthens the substructure boundary i.e., Dynamic Hall-Petch mechanism. These results provide a new perception for achieving better strength performance by post heat treatment process for NiCoCr-based medium entropy alloys (MEA) alloy.

The transient thermal ageing of Eurofer 97 by mitigated plasma disruptions

Plasma disruptions in a commercial-scale tokamak will impose high magnitude, short-duration thermal loads on its plasma-facing first wall. Despite a plethora of mitigation measures, these severe, off-normal events are likely to briefly expose the structural materials of the first wall to significant temperature excursions. Eurofer 97, the reference structural material for the plasma-facing first wall of the EU DEMO tokamak, is a reduced activation 9Cr steel with a normalised and tempered ferritic/martensitic microstructure. Repeated exposure to the thermal effects of disruptions over the operating lifetime of a reactor may promote a cumulative evolution of Eurofer 97′s microstructure, affecting key material properties crucial to first wall performance. This novel transient thermal degradation mechanism has been explored via a laser-based transient heating experiment, supported by time-dependent finite element thermal analysis studies of DEMO’s water-cooled lithium–lead first wall during a mitigated plasma disruption. Transient-affected samples of Eurofer 97 were characterised via scanning and transmission electron microscopy techniques (SEM/TEM), including electron backscatter diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDX), and selective area electron diffraction (SAED). Microhardness and magnetisation testing data are also presented. A single 700 °C thermal transient was found sufficient to coarsen and partially recrystallise Eurofer 97′s tempered martensite sub-grains at the tungsten-Eurofer 97 interface. Further transient exposure at 700 °C resulted in the significant growth of equiaxed grains, the nucleation of intergranular Cr-rich M7C3 and M23C6 carbides, and the coarsening of V-rich and intra-granular Ta-rich MX precipitates. The microstructural effects of 850 °C transients are also reported. Notably, after 1,000 transients at 850 °C the hardness of Eurofer 97 was found to have decreased by 32 %.

Effect of long-term thermal aging on lead-bismuth eutectic corrosion behavior of 9Cr ferritic/martensitic steel

In lead-cooled fast reactor (LFR) systems, the liquid lead-bismuth eutectic (LBE) coolant provides a corrosive environment that damages the steel components during high-temperature operation. This study investigated the microstructural deterioration of 9Cr ferritic/martensitic (F/M) steel under thermal aging at 550 °C for 2,000, 10,000, or 20,000 h and its effect on oxidation corrosion in an LBE environment using multiscale characterization techniques. The results indicated that the thickness of the internal oxidation zone (IOZ) increased significantly with extended thermal aging, whereas that of the spinel layer remained relatively constant. The abundant subgrain boundaries that emerged during extensive thermal aging facilitated Fe diffusion, and the enlarged Cr-rich M23C6 carbides contributed to the formation of preferential oxidation regions, accelerating IOZ layer growth. The spinel layer formed from the IOZ was influenced by microstructural defects within the IOZ. A theoretical model describing the accelerated oxide layer growth due to thermal aging was developed. These findings support the advancement of LFR technology.

Heat-Treated Inconel 625 by Laser Powder Bed Fusion: Microstructure, Tensile Properties, and Residual Stress Evolution

This work investigates the impact of different heat treatments on the evolution of the microstructure, tensile properties, and residual stresses of Inconel 625 (IN625) processed by laser powder bed fusion (LPBF). Applying a heat treatment is an essential step to mitigate the high residual stresses in the components produced by LPBF and, simultaneously, to design the mechanical properties of the components. A high magnitude of residual stress can involve deformation and reduce the fatigue resistance of the components. In the current work, heat treatments performed at 600, 800, and 870 °C provided minimal modification on the dimensions of the grains but involved the formation of new phases, which increased the tensile strength. The results showed mitigation of the residual stresses at 800 and 870 °C correlated with the formation of Cr-rich M23C6 carbides and δ phases, respectively. Finally, the solution annealing at 1150 °C triggered recrystallization with the formation of sub-micrometric carbides, reducing the residual stresses. The solution annealing treatment involved an improvement of the ductility and a reduction in tensile strength. This work provides a guide to understanding the microstructure, residual stress, and mechanical properties evolution of the IN625 alloy under heat treatments.

Oxidation behavior and corrosion properties of 2-GPa grade Co-reduced ultrahigh-strength stainless steel

This study investigated the oxidation behavior and corrosion resistance of 2-GPa grade ultrahigh-strength stainless steel. Results revealed that the addition of V to Co-reduced FerriumS53 (7Co1V) accelerated the precipitation of MC carbide during aging, while inhibiting the precipitation of Cr-rich M23C6 carbide. A significant amount of Cr was predicted in the microstructure of the alloys, and this microstructure increased the partitioning of Cr to the alloy surface and provided enhanced resistance against corrosion pitting. These findings suggest that the addition of V into Co-reduced stainless secondary hardening steels can enhance their oxidation and corrosion resistance without compromising their mechanical properties.

On the normal and inverse Portevin–Le Chatelier behavior in hot rolled Inconel 617 superalloy

In the present study the effect of hot rolling process on the normal and inverse Portevin-Le Chatelier behavior was investigated. The hot rolling process was carried out at 950 °C, 1050 °C, and 1150 °C to capture the effect of thermomechanical process on elimination or postponing of dynamic strain aging phenomenon in Inconel 617 during subsequent deformation process. The tensile test was performed within the temperature range between 350 °C and 750 °C at a constant strain rate. The microstructure evaluations were carried out by optical microscope and scanning electron microscope. The serrated flow was observed in the stress-strain curves at specific temperatures. The type of serrations changed from type B to type C at the strain rate of 5 × 10−4 s−1 as the tensile test temperature was enhanced. The change of normal PLC effect to the inverse PLC effect was intensified when the hot rolling temperature was increased; Hence, the inverse PLC effect was observed at 750 °C in the sample which had rolled at 1150 °C. Whilst the serrated flow was not seen at 750 °C in the samples which had rolled at 950 °C and 1050 °C. The critical strain, for the initiation of the serrated flow was found to reduce with the increasing temperature of hot rolling from 950 °C to 1150 °C. After tensile test, the microstructural evaluations revealed the proven substantial role of precipitation of γ′ and the Cr-rich M23C6 carbides in pinning of the mobile dislocations, which led to the inverse PLC effect.

M3C2 (Cr3C2 Type) carbides formed in steel

As an authigenic alloy phase, M3C2 (Cr3C2 type) carbide, Cr-rich M3C2 carbide with a simple orthorhombic crystal structure, was confirmed to be present in an 11%Cr ferritic/martensitic steel normalized at 1323 K (1050 °C) for 0.5 h and tempered at 873 K (600 °C) for 1.5 h by transmission electron microscopy. The identified Cr-rich M3C2 carbides were found to be the main phase as the Cr-rich M23C6 and M2C carbides according to the present TEM examination, and to have the following characteristics: irregular block, branch crystal-like and needle-like morphologies; larger sizes ranging from approximately 100 to 350 nm in diameter and approximately 200–470 nm for the length of the long axis of the needles; approximate chemical formula of (Cr0.55Fe0.4W0.05)3C2. The formation mechanism of the M3C2 carbides during the tempering is discussed. The M3C2 carbides may be transformed by the reaction of carbon atoms in the matrix with Cr-rich M7C3 carbides formed in situ from Cr-enriched M3C carbides, which were formed from Fe-rich M3C carbides already present in the normalized steel by the diffusion of Cr solute atoms.

Pitting Performance of Cold- and Hot-Rolled Nickel-Saving High-Strength Metastable Austenitic Stainless Steel

Nowadays, nickel-saving metastable austenitic stainless steel (MASS) has become the right solution to meeting the growing requirement of higher strength, better corrosion resistance and more cost saving for the automobile industry. Better understanding of the pitting mechanism of the MASS after either cold- or hot-rolled can offer guidance for the producing of high-performance automobile steel. In the current work, for uncovering the pitting mechanism of the cold- and hot-rolled MASS, the microstructural evolution and pitting performance of nickel-saving metastable austenitic stainless (MASS) steel after cold- (CR) and hot-rolling (HR) were researched via electron microscopy technique and electrochemical methods. Austenite composites the main phase of the MASS. Small amounts of martensite film were proven to form in the MASS. The precipitation of Cr-rich M23C6 carbides was observed in the CR-MASS, while no carbides existed in the HR-MASS. The pitting resistance of the HR-MASS was better than the CR-MASS, which could be attributed to the fact that the stable pits in CR-MASS were initiated near the carbides, whereas the MnS inclusion would serve as the initiation sites for stable pits in HR-MASS. Findings in this work will provide a guidance for developing new generation MASS for automobile industry.

Plastic injection molding dies using hybrid additively manufactured 420/CX stainless steels: electrochemical considerations

This research focused on the corrosion resistance and microstructure of hybrid additively manufactured (HAM) samples of AISI 420/CX (420/CX SS) stainless steels. Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott–Schottky analyses as well as the electrochemical noise (EN) technique were used to evaluate the electrochemical behavior of the as-built and heat-treated HAM parts in NaCl solution. The results showed a more protective passive layer formed on the CX SS side. The distribution of Cr-rich M23C6 carbides in matrix of 420 SS side resulted in a lower corrosion resistance compared to the CX SS side. The noise data analysis confirmed an increase in the galvanic currents of the HAM parts after heat treatment. The stochastic analysis revealed the interface in the heat-treated condition increases pit growth more than the as-built one due to the evolution of nano-sized intermetallic compounds of Al-N/ (Cr, Nb) (N, C) at the heat-treated interface area.

Fatigue fracture mechanism of T92/HR3C dissimilar metal weld joints at elevated temperature

The T92/HR3C dissimilar metal weld joints were fabricated by manual tungsten inert gas (TIG) welding with Inconel 82 as the filler metal. After fatigue tests at 640 °C, it was found that the location of the fatigue fracture shifted from the weld metal (WM) to the heat-affected zone of the T92 steel (T92-HAZ) with a decrease in stress amplitudes (171–189 MPa to 155–166.5 MPa). The fatigue crack propagation behavior of the two fracture modes mentioned above was monitored by an in-situ scanning electron microscopy (SEM) imaging test. In addition, the results of the transmission electron microscopy (TEM) analysis showed that under high-stress amplitudes, the initiation of cracks at the weld metal was caused by the interaction between dislocations with Nb(C,N) in the matrix and M23C6 on the grain boundary. At long-term high temperature, the carbides precipitated in the weld metal played a role in strengthening the grain boundaries, so the matrix structure still maintained high strength and stability. However, the dislocation movement in the T92-HAZ accelerated the transformation of lath martensite into equiaxed subgrains, and the coarsening and aggregation of Cr-rich M23C6 carbides resulted in the initiation of cracks in this region. Therefore, as the stress amplitude decreased, the fatigue fracture location of the DMWJs shifted from the weld metal to T92-HAZ.

Microstructure evolution, phase transformation and mechanical properties of IN738 superalloy fabricated by selective laser melting under different heat treatments

Selective laser melting (SLM) is a new method for manufacturing IN738 superalloy hot-end components. This study evaluated the microstructure evolution, phase transformation, and mechanical properties of IN738, fabricated using SLM before and after different heat treatments. The results showed that the microstructure of the as-SLMed sample was mainly columnar, with cell grains aligned in the building direction (BD) due to the high cooling rate. Many Laves phases and nanoscale Ti-, Nb-, and Ta-rich MC carbides precipitated in the sub-grain boundaries. The grains exhibited full recrystallization and serious coarsening at a solution temperature of 1260 °C. The residual Laves phases completely dissolved when the solution temperature rose above 1140 °C. After solution heat treatment (SHT), the primary MC carbides obviously grew up and were distributed as grains on the grain boundaries, and a large number of fine and homogeneous γ′ phases precipitated from the γ matrix. After aging heat treatment (AHT), hardness, ultimate tensile strength (UTS), and elongation (EL) decreased due to an increase in γ′ size and a reduction in uniformity. With increasing aging temperatures, this situation worsened. However, the yield strength (YS) increased under AHT at 750 °C, which was attributable to the formation of chain-like Cr-rich M23C6 and MC carbides on the grain boundaries. Finally, the overall performance of SLMed IN738 samples improved under solution treatment at 1200 °C for 2 h, followed by aging at 750 °C for 24 h, with increases of UTS and YS from 1205 MPa to 1,351, and from 850 MPa to 1319 MPa, respectively.

Microstructural/mechanical characterizations of electron beam welded IN738LC joint after post-weld heat treatment

The effect of post-weld heat treatment on microstructure and mechanical properties of electron beam welded IN738LC joint has been investigated. The fusion zone consists of γ-Ni matrix, bimodal-size γʹ particles, granular MC carbides (Ti-rich) and Cr-rich M23C6 carbide chain in IN738LC joint subjected to post-weld heat treatment. The secondary γʹ particle in fusion zone precipitates and grows ceaselessly with the increased aging time. The coarsening of γʹ particle follows the Lifshiitz-Slyozov-Wagner coarsening theory with a constant coarsening rate of 1.003 × 10−27m3/s. The secondary γʹ particle in base metal shows a similar size with that of fusion zone, but a lower volume fraction (52%) than that of fusion zone (54%). M23C6 carbide is easier to precipitate along high angle grain boundaries. As aging time increases, the morphology of M23C6 carbide changes from discrete granular shape to continuous chain. The thickness of M23C6 carbide also increases dramatically. Owing to smaller grain size and higher solute concentration in fusion zone, more M23C6 carbides form in FZ than that of base metal. With the increased aging time, the tensile strength of IN738LC joint at 900°C decreases gradually, while the elongation of joint firstly increases and then decreases, which is attributed to the increased channel width of γ-Ni matrix. Due to the formation of continuous M23C6 carbide chain, the rupture life of IN738LC joint firstly increases and then decreases as aging time increases. Combined with the mechanical properties and size stability of γʹ precipitates in IN738LC joint, 1125°C/2 h + 850°C/24 h is an optimized post-weld heat treatment regime.

Effects of process parameters on microstructure and mechanical properties of Al0.5CoCrFeNi high entropy alloy thin sheets using pinless friction stir welding

The present study investigates the effects of welding procedure parameters on microstructure and mechanical properties through pinless friction stir welding (PFSW) for thin sheets of the Al0.5CoCrFeNi high entropy alloy (HEA). Results demonstrate that the joints comprise four different regions, that is, stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM). The heat index (HI) is shown as an important factor affecting the microstructure and mechanical properties of the joints. It influences recrystallization, the development of second phases, and even the weld tool's wear. The HI correlates significantly with welding process parameters, affecting the Al–Ni phase volume fraction (the B2 precipitate). The optimum amount of HI almost occurs at the welding speeds of 10 mm min−1, producing a high degree of B2 precipitates and leading to fine grains in the HAZ. EDS analysis shows that the high level of Cr in the SZ and TMAZ is ascribed to the emerging Cr-rich M23C6 carbides. At the optimum welding conditions and three specific temperatures of 25, 300, 500 °C, the UTS and the ductility of the best joint reaches 423 MPa and 20%, 495 MPa and 21%, and 646 MPa and 21%, respectively, comparable to those of the base metal (605 MPa and 28%, 642 MPa and 19%, and 702 MPa and 30%, respectively). Even at high temperatures, the higher tensile strength might be ascribed to the formation of extremely thermally stable B2 precipitates, resulting in small grains. Hence, the pinless FSW of the Al0.5CoCrFeNi is appropriate for high-temperature applications.

Characterization of the nature and morphology of coarse precipitation in various oxide dispersion strengthened steels

Oxide Dispersion Strengthened (ODS) steels are candidate materials for both fission and fusion nuclear reactors. In this study, the microstructure of ferritic (Fe-14Cr-1W) and ferritic / martensitic (Fe-9Cr-1W) ODS steels has been characterized after two different processing routes: Hot Extrusion (HE) and Hot Isostatic Pressing (HIP). Transmission Kikuchi Diffraction (TKD) revealed the presence of Ti-rich precipitates on every specimen. They are identified by X-Ray Diffraction (XRD) as Ti(C,N) and Ti-O (Ti2O3 or TiO2). Intergranular M7C3 and M23C6 have been observed on most Fe-14Cr ODS steels except one, where no Cr-carbides have been found. In Fe-9Cr ODS, intergranular Cr-rich M23C6 carbides have been found. The lack of M7C3 on Fe-9Cr ODS is possibly linked to the matrix phase transformation, not occurring in the Fe-14Cr ODS. Cr-carbides display highly elongated shapes for the Fe-14Cr HE specimens that could be detrimental to the mechanical behavior of the material.

Effect of grain boundary precipitates on the stress rupture properties of K4750 alloy after long-term aging at 750 °C for 8000 h

In K4750 alloy, the evolution of grain boundary (GB) precipitates, including the degradation of blocky MC carbide particles and the precipitation of granular/needle-like η phase particles, were observed after long-term aging (LA) at 750 °C for 8000 h. During MC degradation, the Ti and C released from the MC carbide combined with Ni and Cr, respectively, in the γ matrix to form η-Ni3Ti phase and Cr-rich M23C6 carbide. Large amounts of granular η phase precipitated at GBs and the needle-like η phase grew gradually from GBs toward the grain interior. Because of the growth of the η phase through absorbing γ′ phase, γ′-depleted zones were formed around the η phase. The evolution of the MC carbide and η phase was primarily responsible for the decrease of the stress rupture life and the increase of elongation. After an LA sample was tested at 750 °C and 360 MPa, the residual strain distribution was investigated by electron backscatter diffraction (EBSD). The results showed that the residual strain mainly distributed at GBs, especially in the region of MC degradation and at the edges of η phases, which was closely related to the appearance of phase interfaces. Microvoids/cracks easily initiated at phase interfaces, then easily extended along the γ′-depleted zones, thus the stress rupture life of LA samples was substantially shorter than that of samples subjected to the standard treatment. In particular, because of large amounts of fine degraded MC, granular M23C6 and granular η phase particles distributed at GBs after 750 °C /8000 h LA and microvoid/crack formation could be hindered by the formation of dimples, which led to an increase of elongation.

Effect of carbon content, deformation and annealing on the structure and properties of interstitial TRIP high-entropy alloys

In this work, the effect of carbon content on the structure and mechanical properties of Fe(50-x)Mn30Co10Cr10Cx (x=0; 0.5; 1.0 at.%) interstitial high entropy alloys was studied. In the as-cast condition, the alloys were composed of the face centred cubic (fcc) and hexagonal close packed (hcp) phases; the amount of the hcp phase decreased from 46% to 20% as the carbon concentration increased from 0 to 1 at.%. The carbon content had a limited effect on the strength of the alloys, yet the ductility was enhanced significantly as the result of interstitial alloying. Furthermore, Fe49Mn30Co10Cr10C1 alloy was cold rolled to a reduction of 56% reduction and annealed at 700-900°C for 1 hour. After cold working, the alloy has attained almost fully the hcp structure. Annealing resulted in (i) transformation of the deformation-induced hcp phase into fcc; (ii) development of recovery and recrystallization; (iii) precipitation of Cr-rich M23C6 carbides. The cold-rolled alloy was very strong but brittle; annealing resulted in softening and increase of the ductility. The relationships between the chemical composition, structure and mechanical properties of the alloys are briefly discussed.

Identification of precipitate phases in CLAM steel

Precipitate phases in a reduced activation ferritic/martensitic steel CLAM (HEAT-0912) were observed and analyzed using transmission electron microscopy and energy dispersive spectroscopy. The steel samples were normalized at 980 °C for 30 min and then tempered at 760 °C for 90 min followed by an air cooling. Through the selected-area electron diffraction pattern analyses and energy dispersive X-ray data, eight kinds of precipitate phases were identified to be Ta-rich MX carbonitride with a f.c.c. crystal structure, Ta-rich M3X2 carbonitride with a base-centered monoclinic crystal structure, Ta-rich MSi2 phase with a hexagonal structure, V-rich MX carbonitride with a f.c.c. crystal structure, V-rich M2C carbide with an orthorhombic-simple crystal structure, Cr-rich M23C6 carbide with a f.c.c. structure, Cr-Ta-rich M7C3 carbide with an orthorhombic-simple crystal structure, Cr-rich M6C carbide with a f.c.c. structure, respectively. There appear to have no previous reports of the Ta-rich MSi2, Ta-rich M3X2, V-rich M2C and Cr-rich M6C phases identified in reduced activation ferritic/martensitic steels.

Effect of carbon on recrystallised microstructures and properties of CoCrFeMnNi-type high-entropy alloys

The effect of carbon content (x = 0, 0.5, and 2.0 at.%) on the phase composition, microstructure, and mechanical properties of thermomechanically processed CoCrFeMnNi system high-entropy alloys was studied. The molar fraction of Cr in the program Co1Cr0.25Fe1Mn1Ni1 alloy was reduced to 0.25 compared with the equiatomic alloy to increase the solubility of carbon in the face-centred cubic solid solution. The as-cast alloys were cold rolled to 80% thickness reduction and then annealed at 600–1000 °C for 1 h. The addition of carbon to the CoCrFeMnNi alloys resulted in an increase in the temperature at which recrystallisation starts and in the precipitation of the Cr-rich M23C6 carbide particles. The volume fraction of the second phase increased with an increase in the carbon content and decreased with increasing annealing temperature. The formation of precipitates in the carbon-doped alloys strongly limited the matrix grain growth owing to the pinning effect. The thermomechanical processing of the alloys resulted in a reasonable work-hardening capacity and, therefore, in rather high ultimate tensile strength and ductility. Increasing the carbon content led to a pronounced increase in strength and a weak decrease in the ductility of the CoCrFeMnNi alloys. After annealing at 800 °C, the yield strength of the alloys increased from 313 MPa to 636 MPa, whereas the total elongation decreased from 56% to 43% with an increase in the carbon percentage from 0 at.% to 2.0 at.%. Decreasing the testing temperature to 77 K led to a simultaneous increase in both strength and ductility of the alloys. The alloy with 2.0 at.% of carbon demonstrated an attractive combination of mechanical properties at cryogenic temperature, presenting a yield and ultimate tensile strength of 786 MPa and 1218 MPa, respectively, along with a ductility of 52%. The effect of the carbon content on the microstructure development and strengthening mechanisms was discussed.

Insight into Type IV cracking in Grade 91 steel weldments

In this work, an insight into the premature Type IV cracking was undertaken to clarify its mechanisms in Grade 91 steel pipe weldments. High-resolution microscopy observations of the as-welded heat-affected zone (HAZ) reveal that the commonly recognized fine-grained region susceptible of cracking on the edge of HAZ belongs to the inter-critical HAZ (ICHAZ), rather than the fine-grained HAZ (FGHAZ). Instrumented indentation tests uncover that the ICHAZ is the weakest region across the weld, exhibiting the largest displacement and the lowest hardness in three thermal stages. Localized deformation of matrix grains and high stress triaxiality in the ICHAZ promoted nucleation of creep cavities along grain boundaries. This localized deformation was induced by the creep strength mismatch of matrix grains with different Cr concentrations. Cavity-free regions exhibit a relatively homogenous Cr distribution, whereas, an inhomogeneous Cr distribution is observed in the cavity-containing regions. It is believed that this local Cr inhomogeneity in the ICHAZ is caused by the partial dissolution of Cr-rich M23C6 carbides and an insufficient homogenization during rapid welding thermal cycles.

Phase Transformation Strengthening of Hot Extruded Inconel 625 under High-temperature Load Environment

Microstructure evolution and properties of hot-extruded Inconel 625 alloy were investigated at different creep temperatures, aging time and strain rates. The experimental results indicate that the Inconel 625 alloy exhibits an excellent creep resistance at 700 °C and below. When the creep temperature rises to 750 °C, the creep resistance falls drastically due to the failure of phase transformation strengthening and the precipitation of a large amount of δ phase and σ phase at the grain boundary. The special temperature-sensitive characteristics of Inconel 625 alloy play a very important role in its fracture. When the strain rate is 8.33×10−3 s−1, the strength of the specimen is higher than that of other parameters attributed to the effect of phase transformation strengthening. With the increase of Ni3 (Al, Ti), the phase transformation strengthening inhibits thickening of the stacking faults into twins and improves the overall mechanical properties of the alloy. With the increase of the aging time, the granular Cr-rich M23C6 carbides continue to precipitate at the grain boundary, which hinders the movement of the dislocations and obviously increases the strength of the samples. Especially, the yield strength increases several times.