Stress Rupture Properties Research Articles

Effect of carbon content on the microstructure and stress rupture properties of a 4th-generation nickel-based single crystal superalloy

The balance between the stress rupture properties and castability of a 4th-generation Ni-based single crystal superalloy can be achieved by addition of appropriate carbon content. In this work, the effects of carbon content on the microstructure and stress rupture properties of a 4th-generation Ni-based single crystal superalloy were investigated in detail. The results showed that minor carbon addition could aggravate the dendrite segregation, while the addition of carbon had no obvious effect on the rafting and distortion of γ′ phase during stress rupture deformation. At 760 °C/810 MPa and 1100 °C/210 MPa, the stress rupture lives of the three experimental alloys initially increased and subsequently decreased with the increasing carbon content. However, under condition of 1140 °C/137 MPa, the increased carbon content resulted in the reduction of the stress rupture lives of alloy. It was discovered that minor carbon elements addition (0.045 wt%) can promote formation of interfacial dislocation networks and improve creep resistance at high temperatures. At intermediate temperature, the introduction of carbon could reduce the stacking fault (SF) energy of γ matrix, thus facilitating the formation of SFs in γ matrix. The M6Cgranular carbides precipitated at high temperature, which was considered to be beneficial to stress rupture properties.

Tailoring microstructural stability and stress rupture properties of a solid-solution strengthened Fe-Ni-based superalloy via niobium addition

The influence of niobium (Nb) addition on the microstructural stability and 750 °C/135 MPa stress rupture properties of a solid-solution strengthened Fe-Ni-based superalloy designed for fourth-generation (Gen IV) nuclear reactors was systematically studied. The results showed that the stress rupture life first increased and then decreased with the increase of Nb content. The alloy with the addition of 0.5 wt% Nb exhibited the best stress rupture properties, with a stress rupture life of 164 h. Compared to Nb-free alloy, the stress rupture life has been increased by 382 %. The enhanced creep performance of this alloy was closely related to its excellent high-temperature microstructural stability, which can be explained as follows: homogeneous and optimal grain size distribution, elemental Nb provided good solid-solution strengthening effects, fine granular secondary carbides relieved stress concentration and strengthened grain boundaries (GBs), dynamic precipitation of nano-sized NbC carbides effectively pined dislocations. Dynamic recrystallization (DRX) behavior can be observed in all alloys. The mechanisms related to DRX behavior were discussed, and it was deemed that the smaller average grain size, higher blocky NbC volume fraction, and increased stacking-fault energy (SFE) may be responsible for the facilitated DRX in the alloy with higher Nb content, which greatly caused local softening and consequently, decreasing the creep resistance of the alloy. Important guidance for the further design and development of γ′-free Fe-Ni-based superalloys for Gen IV nuclear reactors with good high-temperature microstructural stability and creep performance could be summarized from the present study.

Effect of Carbon on the Microstructures and Stress Rupture Properties of a Polycrystalline Ni-Based Superalloy

Effect of Carbon on the Microstructures and Stress Rupture Properties of a Polycrystalline Ni-Based Superalloy

Effect of Overheating Treatment on Microstructure and Stress Rupture Properties of a Nickel-Based Single-Crystal Superalloy

Effect of Overheating Treatment on Microstructure and Stress Rupture Properties of a Nickel-Based Single-Crystal Superalloy

Effect of stress-relief heat treatment on the stress-rupture properties of a nickel-based superalloy manufactured by laser powder bed fusion

Residual stress is a ubiquitous phenomenon in laser powder bed fusion (LPBF) metal parts due to high temperature gradients and uneven heating during building processes. Subsequent stress-relief (SR) heat treatment is usually conducted to reduce the geometrical distortion and microscopic cracks induced by residual stress after the LPBF process. Using electron microscopy, X-ray diffraction, and stress-rupture tests, the experimental results revealed that the stress-rupture life of a nickel-based superalloy, IN625 alloy, subjected to SR treatment at 870°C and 980°C is inferior to that of the as-built (AB) alloy. The fundamental cause for this phenomenon is that more δ phase precipitates in the grains of the AB alloy during the stress-rupture test at 750°C/230 MPa, resulting in an increased dispersion hardening effect and a decreased ductility. Furthermore, the Laves and δ phases along grain boundaries in the SR treated alloys are more likely to coarsen, which is the second reason for the decreased stress-rupture life of the SR treated alloys.

Effects of stabilized heat treatment on stress rupture properties of high Si-bearing austenitic stainless steel weld metal

The 15Cr-9Ni-Nb austenitic stainless steel weld metal with a Si content of 3.5 wt% was prepared via gas tungsten arc welding and then held at 900 °C for 3 h for the stabilized heat treatment (SHT). The stress rupture properties of the as-welded (AW) and SHT weld metals at 550 °C were evaluated via the Larson-Miller parameter. The microstructure evolution was discussed during the 550 °C stress rupture process. The coarse σ-phase and relatively fine G-phase formed on the δ-ferrite during aging at 550 °C. In the AW weld metal, the continuous δ-ferrite with a large amount of coarse σ-phase led to the formation and expansion of cracks during the stress rupture process, which accelerated the eventual rupture and damaged the stress rupture properties. The SHT decreased the δ-ferrite content and formed a large amount of nanoscale NbC precipitated in the matrix. The decreased δ-ferrite content avoided the rapid formation and expansion of cracks and the nanoscale NbC blocked the dislocation movement during the stress rupture process, which improved the stress rupture properties.

The diminishing positive effect of phosphorus on the creep behavior with the decrease of applied stress in a Ni-Fe-based alloy

Phosphorus can increase the creep and stress-rupture properties of the turbine disk materials served in high stress with short life. However, the effect of P on the creep of boiler tube in advanced ultra-supercritical power plants served in low stress with long life is still unknown. Combining the formula derivation and experimental analysis, the varying effects of P on the creep with the decrease of stress were clarified. The positive effect of P on prolonging the creep rupture life reduces with the decrease of creep stress, even vanishes when the creep stress decreases to 93 MPa, 133 MPa and 140 MPa in the alloy doping 100 ppm, 200 ppm and 400 ppm P, respectively. The degeneration of M23C6 and the dynamic segregation of P at grain boundary during long-term creep under low stress can decrease the deformation coordination between the adjacent grains and the grain boundary binding force, which makes the positive effect of phosphorus on the creep diminish.

New insights into the stress rupture life enhancement mechanisms of a nickel-based superalloy

In this paper, the effects of hot isostatic pressing (HIP) and heat treatment (HT) on the stress rupture properties of nickel-based superalloy K417G were investigated. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to analyze the microstructure, fracture morphology, determination of crack source and dislocation evolution after stress rupture fracture. The results indicate that, HIP specimens demonstrate a 19.5% reduction in stress rupture life compared to as-cast specimens, whereas HIP + HT specimens exhibit a 41% increase in stress rupture life, reaching a value of 63.04 h. The fracture mechanism of the samples under different heat treatment conditions is different. There are a large number of pores in the as-cast specimen, which leads to the initiation and propagation of cracks and eventually result in fracture. The pores in the HIP specimen are almost eliminated. However, the coarsening and uneven distribution of γ′ phase led to the weakening of the hindrance to dislocation movement. Additionally, the increase in elongated carbides at grain boundaries accelerates crack initiation and propagation. Under HIP and HT, the cubic γ′ phase is uniformly precipitated, which effectively hinders the movement of dislocation lines. Granular M23C6 carbides precipitate at grain boundaries, acting as pinning points to retard grain boundary sliding and crack propagation, thus improving the stress rupture life.

Synergistic Effects of Boron and Rare Earth Elements on the Microstructure and Stress Rupture Properties in a Ni-Based Superalloy.

The synergistic effects of boron (B) and rare earth (RE) elements on the microstructure and stress rupture properties were investigated in a Ni-based superalloy. The stress rupture lifetime at 650 °C/873 MPa significantly increased with the addition of B as a single element. Furthermore, the stress rupture lifetime reached its peak (303 h), with a certain amount of B and RE added together in test alloys. Although the grain size and morphology of the γ' phase varied a little with the change in B and RE addition, they were not considered to be the main reasons for stress rupture performance. The enhancement in stress rupture lifetime was mostly attributed to the segregation of the B and RE elements, which increased the binding force of the grain boundary and improved its strength and plasticity. In addition, the enrichment of B and RE inhabited the precipitation of carbides along grain boundaries. Furthermore, nano-scale RE precipitates containing sulfur (S) and phosphorus (P) were observed to be distributed along the grain boundaries. The purification of grain boundaries by B and RE elements was favorable to further improve the stress rupture properties.

The influence of γ′ morphology and size on stress rupture properties in Ni-base superalloy IN738LC

This study investigates the influence of γ′ precipitate morphology and size on the creep deformation of the cast superalloy IN738LC under conditions of 760–982 °C and 78–674 MPa. Two microstructures were obtained by different heat treatments. The analysis of the experiments show that, according to the applied stress at 760 °C and 871 °C, the S1A material, with a bimodal distribution with large cuboidal γ′ and small spherical γ′, exhibited the longer creep life compared to those with a monomodal distribution with spherical γ′ precipitates. In the creep test at 760 °C, the S1A material exhibited a stacking fault within the primary cuboidal-shaped γ′ phase. The main deformation mechanism observed was isolated stacking fault (ISF) formation. Conversely, for the S2A material, an extended stacking fault (ESF) was observed, where dislocations continuously traversed several γ′ and γ phases. At 871 °C in low-stress creep tests, the S1A alloy displayed a dislocation network resulting from thermal activation, with localized stacking faults. In the S2A alloy, three-dimensional dislocations generated by climb were primarily observed around spherical γ′ precipitates, accompanied by localized stacking faults. At 982 °C, both S1A and S2A alloys exhibited rafting under lower stress, displaying similar lifetimes. The formation of a regular dislocation network at the interface between γ′ and γ matrix and rafting of γ′ precipitate proved beneficial in improved creep resistance of the alloy.

Effect of Pt on Stress Rupture Properties of Pt-Modified Nickel Aluminide Coatings at 1100 °C.

Platinum plays a crucial role in the superior high-temperature oxidation resistance of Pt-modified nickel aluminide (PtAl) coatings. However, PtAl coatings usually serve in thermo-mechanical coupling environments. To investigate whether Pt contributes to the high-temperature mechanical properties of PtAl coating, stress rupture tests under 1100 °C/100 MPa were performed on PtAl coatings with varying Pt contents. The different coatings were obtained by changing the thickness of the electroplated Pt layer, followed by a diffusion heat treatment and the aluminizing process in the present work. The results of the stress rupture tests indicated that an increasing Pt content resulted in a significant decrease in the stress rupture life of PtAl-coated superalloys under 1100 °C/100 MPa. Theoretical calculations and microstructural analysis suggested that an increased coating thickness due to the Pt content is not the main reason for this decline. It was found that the cracks generated close to the substrate in high-Pt-coated superalloys accelerated the fracture failure.

Microstructure and mechanical properties of a first-generation single crystal superalloy CMSX-6

CMSX-6 alloy is a first generation of single crystal high temperature alloy and has been widely applied for the aeroengine and gas turbine blades. In the present study, the microstructure, tensile properties, stress rupture properties and fracture properties of the as-cast and heat-treated CMSX-6 alloy were investigated. The experimental results show that under the heat-treated condition the γ′ phase was about 0.5 μm, the volume fraction was about 66%, and the residual eutectic phase volume was 2%~5%. The tensile and stress rupture properties was similar that of the first generation of single crystal alloys, DD3 and DD407 developed by Beijing Institute of Aeronautical Materials and General Institute of Iron and Steel Research, China. And the results also indicate that CMSX-6 alloy has good fracture properties, oxidation resistance properties and corrosion resistance properties, and therefore CMSX-6 alloy has very good application prospect for the aeroengine and gas turbine blades materials.

The Effect of Precipitates on the Stress Rupture Properties of Laser Powder Bed Fusion Inconel 718 Alloy

Improving the high-temperature stress rupture properties of Inconel 718 (IN718) alloys is crucial for enhancing aircraft engine performance. By using the laser powder bed fusion (LPBF) technique, IN718 alloys were crafted at varying volumetric energy densities (VED) in this study. The dendrite growth mode, reinforcing phase distribution and high temperature stress rupture properties of various VED samples were investigated. The results showed that the stress rupture life and the uniform elongation of the samples both first increased and then decreased with the increase in VED. When the VED was 60 J/mm3, the maximum rupture life and elongation of the sample were 43 h and 3.8%, respectively. As the VED increased, the angle of dislocation in the dendrite decreased while the spacing between primary dendrite arms increased, resulting in an increase in the size and volume fraction of the Laves phase. Following a heat treatment, the δ phase would nucleate preferentially around the dissolved Laves phase causing an increase in the volume fraction of the δ phase with the increase in VED. The creep voids readily formed around the δ phase are distributed along the grain boundaries, while the inhomogeneous δ phase and fine grains facilitated crack initiation and propagation. Furthermore, a significant quantity of the δ phase consumed the Nb element, thereby hindering adequate precipitation in the γ″ phase and causing cracks.

Microstructure evolution and strengthening mechanism of carbides-reinforced Co–Cr–Nb–W alloy under high-temperature stress-rupture

A novel Co–Cr–Nb–W alloy with a carbide-dominant microstructure was designed in view of the excellent characteristics of NbC and γ-Co matrix to use as wear-resistant components on integral shrouds. The actual service conditions of the alloy were simulated using heat treatment experiments to investigate the influence of high temperature on their microstructure and mechanical properties. The stress-rupture property of the alloy was improved by exposing them to high temperatures. Carbides in the matrix bore the main load under external stress. The fracture mechanism of the Co–Cr–Nb–W alloy is ductile combined with localized brittle failure around large carbides. M6C was formed from the degeneration of MC and decomposition of M23C6 during the heat treatment process along with the two carbides transformation as follows: MC + matrix → M6C and M23C6 + matrix → M6C. The elimination of lamellar M23C6 by carbide transformation and supplemental precipitation of fine M6C around skeleton MC were conducive to longer rupture life. Stacking faults (SFs) played a crucial role in the strengthening mechanism of the Co–Cr–Nb–W alloy. The external stress activated different slip planes in the γ matrix to form “X-shaped” crossing stacking faults (CSFs). Lomer-Cottrell (L–C) locks in CSFs were confirmed by transmission electron microscopy (TEM) under two-beam conditions. SFs can interact with precipitates, dislocations, and grain boundaries to enhance the strengthening mechanism. The SF width was increased by heat treatment, which further increased the dislocation accumulation in the matrix and promoted the formation of CSFs and L–C locks. Therefore, the heat treatments were beneficial to the enhancement of the stress-rupture property of the Co–Cr–Nb–W alloy.

Influence of stabilization heat treatment temperature on microstructure and stress rupture properties of Inconel 706 superalloy

Although the stabilizing heat treatment between solution treatment and aging treatment in Inconel 706 alloy is important to improve its stress rupture life, the influence of stabilization heat treatment temperature on the microstructure and stress rupture properties has not been well understood. In this study, heat treatments consisting of three stabilization temperatures (800 °C, 845 °C, and 880 °C) were performed in Inconel 706 alloy. The results showed that the stress rupture properties were improved with the increase of stabilization temperature. When compared with stabilization at 800 °C, the alloy stabilized at 880 °C exhibited a 96% increase in stress rupture life and a 110% increase in elongation. Furthermore, increasing the stabilization temperature led to the increased amount of η phase at grain boundaries. The η phase appeared in granular, rod-shaped or needle-shaped morphology. The microstructure in the grain interior exhibited compact γ'/γ" coprecipitates in the samples stabilized at 800 °C and 845 °C, while that showed noncompact γ'/γ" coprecipitates in the sample stabilized at 880 °C. The formation of 2.34 vol% η phase was found to be beneficial for the stress rupture properties. The improvement in stress rupture properties was ascribed to the presence of η phase and the associated precipitate-free zones at grain boundaries.

Influence of cooling strategy on microstructure, deformation mechanism and mechanical properties of a nickel-based superalloy

In this study, the influence of cooling strategy after solution treatment on microstructure and mechanical properties of a novel cast and wrought nickel-based superalloy is studied. It was found that continuous oil cooling (CC) from the solution temperature led to formation of large primary γ' and ultrafine tertiary γ'. Interrupted cooling (IC), which involves furnace cooling from the solution treatment temperature (1080 °C) to a lower temperature (such as 1010 °C, 980 °C or 950 °C) first followed by an interrupted oil cooling, shows significant influence on microstructure. Owing to a short-period furnace cooling, secondary γ' started to form in these interrupted cooled samples and their volume fraction and size increased continuously with decreased IC temperature. In contrast, the volume fraction of tertiary γ' decreased with decreased IC temperature. The CC sample was found to deform mainly by dislocation glide/climb and extension of stacking faults at 650 °C/950 MPa. With increased volume fraction of large secondary γ' in the interrupted cooled samples, deformation became increasingly planar, i.e., the formation of stacking faults and nanotwins increasingly became the dominant deformation mechanisms. The cooling strategy shows no obvious influence on tensile properties and impact toughness but shows significant influence on stress rupture properties. With decreased IC temperature and coarsened microstructure, the stress rupture lifetime continues to decrease.

Evolution of microstructure and 800 °C/294 MPa stress rupture property of cast Ni-based superalloys during long-term thermal exposure: Role and behavior of primary MC carbide

The role and behavior of primary MC carbide and their effects on the microstructure and stress rupture property of the cast Ni-based superalloys during long-term thermal exposure at elevated temperature are systemically investigated in this work. It is found that the chemical compositions of primary MC are proportional to the Nb/Ti and (Ti + Nb)/(Mo + W) ratios of alloys. During long-term thermal exposure, the thermal stability of primary MC can be quantitatively evaluated by its decomposition degree defined as Dt=(V0-Vt)/V0 × 100%, which is closely associated with the Nb/Ti and (Mo + W)/Cr ratios of alloys. The decomposition process, degree, form and mechanism of primary MC are revealed in detail. The STEM observations with the EDS analyses show that primary MC decomposition is an inter-diffusion process of elements between the primary MC carbide and γ matrix, which can increase the volume fraction of γ' phase, restrain the precipitation of topologically close-packed (TCP) phases and promote the coarsening of grain boundaries (GBs). The combination of these microstructural evolutions, together with the appropriate Al + Ti + Nb and W + Mo contents in the matrix, eventually results in a remarkable improvement in the stress rupture property of long-term thermally exposed cast Ni-based superalloys.

Effect of P and B compound strengthening on the evolution of phases and stress rupture properties of the IN718 superalloy during long-term aging at 650 °C

The influences of P and B compound strengthening on the precipitated phases, stress-rupture properties and deformation mechanisms of a direct-aged IN718 superalloy during long-term aging at 650 °C are investigated. The results indicate that the combined addition of P and B reduces the coarsening rate of γ″ phases and markedly inhibits the precipitation and growth of δ phases and α-Cr phases. P and B compound strengthening could improve the stress-rupture property of the alloy at 680 °C/700 MPa by increasing the strength of intragranular and grain boundary. In the grain interior, P and B compound strengthening could inhibit the intragranular diffusion and hinder the coarsening of γ″ phases, thus increasing the stability of γ″ phases that could reduce the degree of cut by dislocations. In addition, the combined addition of P and B reduces the stacking fault energy of the alloy and forms the high-density twin, improving the ability of the alloy to impede dislocation movement. At the grain boundary, P and B compound strengthening could inhibit the grain boundary diffusion, hindering the nucleation and growth of the δ phase and the α-Cr phase, and the crack initiation and propagation. After aging at 650 °C for 5000 h, the combined addition of P and B could also inhibit the precipitation of TCP-σ phases at grain boundaries, but the TCP phases have no obvious effect on the stress-rupture property of the alloys due to their lower amounts and sizes. With the increase in aging time, the permanent deformation mechanism of the two alloys changes from twinning to dislocation shear γ" phase and twinning.

Effect of stabilizing treatment on microstructure and stress rupture properties of phosphorus microalloyed Inconel 706 alloy

Effect of stabilizing treatment on microstructure and stress rupture properties of phosphorus microalloyed Inconel 706 alloy

Effects of Si on the stress rupture life and microstructure of a novel austenitic stainless steel

A novel Si-bearing austenitic stainless steel was designed as the cladding material for lead-cooled fast reactors with an operating temperature range of 450 °C–550 °C, and the effects of Si (1.0 wt% - 2.0 wt%) on the creep life and fractured microstructure of material were studied at 550 °C and 650 °C. Results show that the addition of Si increased the stress rupture life of steel at 550 °C whereas brought in an opposite effect at 650 °C, which was closely related to the precipitation behavior of σ phase at these two temperatures during creep. At 550 °C the precipitation of σ phase was quite slow, but it was substantially accelerated at 650 °C. Since the experimental materials used in this work were in a cold worked state, so deformation twins existed and they were discovered to be able to promote the nucleation and growth of σ phase on the grain boundaries. With the increase of Si content at 550 °C, σ phase became larger in quantity and denser in distribution on the grain boundary, which improved the anti-cracking ability of grain boundary and enhanced the creep life of material. However, at 650 °C, σ particles apparently coarsened and congregated on the grain boundary under thermal activation, leading to an evident drop in the stress rupture property of steel.