Intermediate Temperature Embrittlement Research Articles

A novel D022-strengthened medium entropy alloy with outstanding strength-ductility synergies over ambient and intermediate temperatures

Precipitation-strengthened medium/high-entropy alloys (MEAs/HEAs) have great potential for high-temperature applications. In this study, we designed a novel Ni45.9Fe23Cr23V4Nb3Mo1B0.1 (at.%) MEA alloy, hardened by the D022 (Ni, Fe, Cr)3(Nb, V)-type nanoprecipitates, with an excellent strength-ductility combination from room to elevated temperatures. Specifically, the tensile strengths, at 700 and 800 °C, could be maintained as high as 845 and 589 MPa, respectively; meanwhile, elongations at all testing temperatures exceeded 25 % without any intermediate-temperature embrittlement. The temperature-dependent deformation mechanisms were unraveled using multi-scale characterizations, which involved profound slip planarities, such as stacking fault (SF) networks and deformation twins (DTs). Furthermore, the critical resolved shear stress (CRSS) to initiate SFs in both face-centered cubic (FCC) and D022 phases was evaluated, and the possible reasons for the origin of anomalous DTs at 800 °C were discussed in detail. The main findings demonstrate that the shearable D022 nanoparticles can provide the FCC matrix with considerable dislocation storage capacity, reinforcing strain hardening at ambient and intermediate temperatures. This work provides fundamental insights into the controllable design and deformation mechanisms of high-performance D022-strengthened MEAs/HEAs.

Evading the intermediate-temperature embrittlement of L12-strengthened high entropy alloys by alleviating intergranular oxidation

L12-strengthened high entropy alloys (HEAs) usually suffer from severe intermediate-temperature embrittlement. In this study, we revealed that the intermediate-temperature embrittlement of a (Ni2Co2FeCr)92Nb4Al4 L12-strengthened HEA originates from the severe oxidation of intergranular D019 precipitates which weakens the grain-boundary cohesion. With the minor addition of 1.5 at% Si, we successfully achieved a brittle-to-ductile transition in the alloy via alleviating intergranular oxidation. The alloyed Si preferentially partitioned into the intergranular precipitates, significantly improved their oxidation resistance and pinning effects on the grain boundaries, which effectively mitigated environmentally assisted grain-boundary damage and thus suppressed the crack propagation upon deformation.

Comments on the Intermediate-Temperature Embrittlement of Metals and Alloys: The Conditions for Transgranular and Intergranular Failure

The intermediate-temperature embrittlement range was examined for Fe, Al, Cu, and Ni alloys. It was found that this embrittlement occurs in many alloys, although the causes are very diverse. Embrittlement can be due to fine matrix precipitation, precipitate free zones, melting of compounds at the grain boundaries, segregation of elements to the boundaries, and, additionally for steel, the presence of the soft ferrite film surrounding the harder austenite matrix. Grain boundary sliding and segregation to the boundaries seem to dominate the failure mode at the base of the trough when intergranular failure takes place. When cracking is due to the presence of hydrogen or liquid films at the boundary, then the dissociation along the boundaries is so easy, it is often independent of the strain rate and is always intergranular. In the other cases when failure occurs, if the deformation is carried out at a high strain rate, it is normally transgranular (e.g., hot rolling giving rise to edge cracking). However, when the strain rate is reduced to that of creep (e.g., bending during continuous casting of steel), failure can also take place by grain boundary sliding, and intergranular failure then becomes the favoured mode.

Intermediate temperature embrittlement and Portevin-Le Châtelier effect of Inconel 625 alloy caused by carbides

The phenomena of intermediate temperature embrittlement (ITE) and Portevin-Le Châtelier (PLC) effect are common in nickel-based superalloys, which seriously restrict the machinability and serviceability of the alloys. The ITE and PLC effect of Inconel 625 alloy caused by carbides and hot ductility were studied by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscopy (TEM), and Gleeble 3500 thermal simulation equipment. The results show that hot ductility has a strong dependence on temperatures of Inconel 625 alloy. During the heating process, ductility initially decreases at 600 ℃ and reaches a minimum at 800 ℃ as the temperature increases, which was due to the formation of microcracks. Dynamic recrystallization can be observed at 900 ℃, and then ductility begins to drop until 1212 ℃ due to material melting. Meanwhile, the variation trend of the cooling process is consistent with the heating process. ITE exists in both the heating process and the cooling process, occurring at 600–900 ℃ and 600–1000 ℃ respectively of Inconel 625 alloy. The microcracks caused by the precipitation of carbides (NbC, (Nb, Ti)C, and M23C6) at grain boundaries led to the formation of ITE. The size, shape, distribution position, and type of carbides have different degrees of influence on the formation of microcracks. In addition, the serrated flow was observed throughout the test temperature range of Inconel 625 alloy. With the increase of the temperature, the type and amplitude of the serration changed, which is caused by the interaction between carbides (solute atoms) and dislocations.

Trace cerium/ yttrium addition for inhibiting sulfur-induced intermediate temperature embrittlement in ultrahigh-purity copper

Sulfur (S), as the common impurity prevalent in copper (Cu) and Cu-based alloys, often induces the intergranular embrittlement at intermediate temperatures even with a ppm concentration, a phenomenon variously called “intermediate temperature embrittlement” (ITE). However, the inhibition of such an embrittlement has been rarely reported. In this study, we investigate the influence of trace cerium/ yttrium (Ce/Y) addition on the ductility of the ultrahigh-purity Cu containing minute amounts of S impurities at temperatures between room temperature (RT) and 900 °C. The results show that both Ce and Y additions can improve the ductility of the CuS alloy at any test temperature, which is attributed to the inhibition of Ce/Y on the grain boundary (GB) segregation of S by forming the RE-S precipitates. By comparison, the modification of ITE caused by S impurities is not significant after trace Ce is introduced, while the addition of small amounts of Y can completely eliminate the ITE phenomenon at temperatures of 300–750 °C. This results from the size of the RE-S precipitates and the structure of the transition nanolayers between the RE-S precipitates and Cu matrix, which is dependent on the diffusion rates of the dissolved S, Ce and Y atoms in Cu during the tensile deformation. Our findings provide a comprehensive experimental and computational understanding on the fundamental mechanisms of rare earth elements inhibiting the embrittlement induced by impurities in metallic materials.

Metallographic Evaluation of Increased Susceptibility to Intermediate Embrittlement of Engine Valve Forgings Made of NCF 3015 High Nickel and Chromium Steel.

This paper focused on determining the increased tendency of cracking after the die forging process of high nickel and chromium steel. The increase in carbon content in austenitic nickel-chromium steel promoted the tendency of valve forgings to forging intergranular crack on the valve head. Attention was paid to issues related to the chemical composition of the material to be considered when hot forming nickel-chromium steel components. Optical and scanning electron microscopies were used to examine the microstructure and fracture features of the samples removed from a fractured valve head. The embrittlement was due to microcavity formation at grain boundaries. Creep theory at grain boundaries was used to explain crack formation. The tensile behavior was interpreted from the evolution of the microstructure during deformation and referred to intermediate brittleness to explain the effect of carbon. It was found that the increased carbon content of the nickel-chromium steel and the strong undercooling observed at the edges of the valve head are factors that promote a reduction in grain boundary cohesion and enhance intermediate temperature embrittlement. Finally, it was found that the formation of a heterogeneous structure manifested by the presence of grain boundary M23C6-type carbides in the austenitic matrix was most likely related to the occurring brittleness.

Small punch investigation of the dynamic strain aging behaviour and intermediate temperature embrittlement of Inconel 625

In this study, the deformation behaviour and mechanical property evolution of Inconel 625 were studied at different temperatures and deformation rates by using small punch test (SPT). With the increase in the temperature, small punch characteristic parameters decreased, and an approximate platform was found in the temperature range of 300–600 ℃. The dynamic strain aging (DSA) characteristics were observed in the load-displacement curves over a similar temperature range. To analyse the small punch DSA behaviour, the serration types, critical displacement, and maximum load drop were examined. The serration types changed from Type A to Type C when the temperature increased or the deformation rate decreased. The abnormal DSA was proven at around 600 ℃. Then, the influence of the strain rate was analysed and the intermediate temperature embrittlement was found at 600–700 ℃ as the strain rates decreased. Considering the difference between the SPT and the uniaxial tensile test, the influence of stress triaxiality on DSA was discussed. The similar change rule of serration types was presented. SPT was proven to be a reasonable method to evaluate the DSA behaviour which could replace uniaxial tensile test.

The mechanism of intermediate temperature embrittlement of cast irons by magnesium

Metallic alloys such as steels and nickel-based alloys are potentially vulnerable to high-temperature brittleness in the range of 500 °C to 1000 °C that results in a loss of ductility. Ferritic spheroidal graphite cast irons show similar loss of ductility at temperatures between 350 °C and 500 °C, suggesting the term intermediate temperature brittleness. Previous works have shown that sulfur and magnesium are detrimental while phosphorus added at a reasonable level is beneficial in preventing loss of ductility in ferritic spheroidal graphite castings.In order to clarify the role of magnesium, a ferritic spheroidal graphite cast iron with high silicon content and low sulfur and phosphorus contents was subjected to tensile tests between room temperature and 650 °C. It was found to be susceptible to embrittlement at intermediate temperatures around 400 °C. MgO precipitates were observed on the fracture surface, which increased in size with test temperature, confirming the role of the environment that was reported by previous researchers. This finding is cross-checked with the results of the literature and allows us to propose a scheme for intermediate temperature embrittlement of ferritic spheroidal graphite iron.

Microstructures and High Temperature Tensile Behaviours of Laser Powder Bed Fusion Fabricated Al-Mn-Sc Alloy

The introduction of Sc/Zr inoculates to aluminium (Al) alloys for laser powder bed fusion (LPBF) provides numerous benefits, including laser processability improvement, solidification microstructure control and mechanical property enhancement. Though great efforts have been put into tailoring the microstructure and room temperature mechanical properties via process parameter optimisations, the potential roles of Sc/Zr inoculate modified Al alloys for high-temperature applications were still underexplored. In this study, the microstructural stability and the elevated temperature tensile behaviours of LPBF-processed Al-Mn-Sc alloy were systematically evaluated. The alloy demonstrated high microstructural stability after both heat treatment and high-temperature tensile testing for up to 573 K. The applied tensile testing temperature and strain rate played significant influences on the elevated temperature tensile properties and deformation behaviours. Unusual intermediate temperature embrittlement (also known as ductility dip) and yield drop behaviours were observed under certain testing temperature and strain rate regimes, and the underlying deformation mechanisms were elucidated in detail. The present study is expected to shed light on future high-performance, high-temperature Al alloy development for the LPBF process.

A novel single-crystal L12-strengthened Co-rich high-entropy alloy with excellent high-temperature strength and antioxidant property

Severe intermediate-temperature embrittlement as well as rapid softening and oxidation at high temperatures are significant problems for high-entropy alloys (HEAs) in polycrystalline structures, which greatly limit their applications in modern industries. In this study, a novel Co-rich single-crystal high-entropy alloy (HEA), Co41Ni35Al11·5Ta2·5Cr4Ti6 (at.%), was innovatively developed. After isothermally aged at 1250 °C, this alloy exhibits an ultrahigh volume fraction of cuboidal L12-type nanoprecipitates (∼80%) which is higher than those of previous Co-based superalloys. More importantly, the present HEA shows excellent mechanical performance at both ambient and elevated temperatures, and no intermediate temperature embrittlement and rapid softening were observed. Noticeably, the yield strength can still maintain at ∼800 and ∼500 MPa when tested at 900 and 1000 °C, which are superior to other conventional single-crystal alloys. Moreover, extraordinary oxidation resistance can be achieved at the high temperature of 1000 °C due to the formation of a compact sandwich-structured oxide layer.

Eliminating embrittlement of GH4065A superalloy at intermediate temperature by a novel grain boundary strengthening mechanism

Intermediate temperature embrittlement (ITE) exists widely in Ni-based superalloys, which seriously restricts the processing and service properties of alloys. The ITE of a Ni-based wrought superalloy GH4065A was eliminated by long-term aging treatment. With the extension of aging time, the fracture characteristics of the alloy changed from intergranular fracture to dimple. EDS and AES observations showed that the intergranular diffusion and segregation of W and Mo elements increased the grain boundary strength of the long-term aging alloy. Intergranular M6C carbide precipitates have also been shown to be beneficial for plastic deformation.

Yield strength anomaly evaluation of W-free Co-Ni-Al-based superalloys during high temperature tensile tests

High temperature tensile behaviors of two W-free Co-Ni-Al-based γ'-strengthened superalloys at 773 K ~ 1073 K were investigated. The tensile strength and elongation of two superalloys decrease with increasing temperature. The fracture mechanism changes from dimple fracture to brittle fracture. A detailed observation of the fracture shows that the intermediate temperature embrittlement caused by decohesion of slip plane plays a significant role in reducing high temperature performance. Both superalloys reach to peak yield strength at 923 K. Occurance of yield strength anomaly (YSA) is derived from the synergistic effects of dislocation locking strengthening and dynamic strain aging strengthening. Dislocation cross-slipping hinders the dislocation migration. Dislocations' unpinning to the moving solutes increases the yield strength at 723 K ~ 973 K. A rapid weakening of grain boundaries strength decreases the yield strength at 1073 K.

Effect of post fabrication aging treatment on the microstructure, crystallographic texture and elevated temperature mechanical properties of IN718 alloy fabricated by selective laser melting

The effect of building direction and post fabrication aging treatment on the microstructure, crystallographic texture and high temperature mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) method was investigated. After aging, arc-shaped structures seen in as-fabricated samples disappeared and converted into a mixture of columnar and equiaxed grains. Nano-sized γ″ and/or γ′ precipitates were formed upon aging; however, MC type carbides and Laves phase encountered in as-fabricated samples were not dissolved completely after aging. Moreover, aging did not alter the texture ((001)//building direction (BD)) of as-fabricated samples. Mechanical properties of the alloys under tension were influenced by the build direction, aging time and test temperature. As-fabricated samples produced in vertical direction exhibited higher room temperature strengths with lower ductility due to orientation of overlapped prior melt pools. Room temperature tensile test results revealed that peak aging caused a significant improvement in ultimate tensile strength (UTS), from 1066.5 MPa and 998.4 MPa to 1408.5 MPa and 1330.4 MPa whereas elongation values decreased from 27.5% and 32.2% to 19.6% and 23.7% in vertically and horizontally built samples, respectively. Peak-aged samples (aged at 700 °C for 8 h) tested at 600 °C displayed serrated regions in their stress-strain curves due to dynamic strain aging (DSA). Although strength values of the samples displayed an expected decrease by temperature, ductility of the samples reduced to minimum at temperatures around 700–800 °C, which was attributed to intermediate temperature embrittlement.

Intermediate temperature embrittlement in a precipitation-hardened high-entropy alloy: The role of heterogeneous strain distribution and environmentally assisted intergranular damage

Severe intergranular embrittlement has been found in a wide range of polycrystalline metallic materials in the intermediate temperature regime, setting severe limits on their engineering applications. In this study, we have systematically investigated the origin of such temperature-dependent premature tensile failure in a precipitation-hardened high-entropy alloy. We highlight the role of heterogeneous strain distribution and environmental attack in facilitating intergranular crack initiation and propagation at intermediate temperatures. The dislocation accumulation has been found in the vicinity of grain boundaries to accommodate thermally activated grain-boundary sliding. Grain boundaries with extensive dislocation pile-ups served as preferential sites for the initiation of voids and associated cracks. The growth and linkage of microcracks lead to the destructive tensile failure. More importantly, environmentally assisted grain-boundary damage plays a vital role in exacerbating the embrittlement. In contrast to the laboratory air atmosphere, tensile testing in an inert argon atmosphere protected the specimens from the environmental damage and helped to recover the tensile ductility at intermediate temperatures. Electron microscopy analyses have uncovered unique deformation substructures, in which both Orowan looping and particle shearing took place. These findings provide a fundamental understanding of the temperature-dependent deformation behaviors of polycrystalline high-entropy alloys, and this understanding is of a great significance for developing high-performance structural materials for elevated temperature applications.

On the fracture process of intermediate temperature embrittlement of pure copper in electrical-assisted tension

This paper investigated the intermediate temperature embrittlement phenomenon in the electrical-assisted tensile test of copper. Both the reduction of area and elongation decreased significantly in electrical-assisted tension. Many cavities were found on the surface of fractured sample, and most of the cavities are located near the grain boundary and a few of them are in the grain interior. Based on experimental and computational results, it is concluded that the fracture behavior is mainly ruled by the aggregation and coalescence of cavities that is possibly induced by trace concentration of sulfur solutes.

Control of nanoscale precipitation and elimination of intermediate-temperature embrittlement in multicomponent high-entropy alloys

Thermally stable high-entropy alloys (HEAs) consisting of a high density of coherent precipitates show a great potential for high-temperature applications. In this work, we systematically investigated the phase stability and coarsening kinetics of L12-type coherent precipitates in a Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B (at.%) HEA isothermally aged at 800, 900 and 1000 °C. Aged microstructures in the grain interiors under this temperature range were essentially dominated by the uniform precipitation of multicomponent L12 (Ni, Co, Fe, Cr)3(Ti, Al)-type precipitates. The coarsening kinetics of these intragranular L12 precipitates were quantitatively determined, which were adequately characterized by the classical Lifshitz-Slyozov-Wagner model. The activation energy for coarsening was determined to be 378 kJ/mol, which is relatively higher than that of conventional Ni or Co-based superalloys, suggesting a slow elemental diffusion in the HEA matrix. More importantly, the heterogeneous precipitation and the associated metastable phase transformation mechanism along grain boundaries (GBs) were carefully analyzed. Localized chemical heterogeneity was identified within the discontinuous L12 phase at the GBs, which thermodynamically destabilizes the L12 structure and encourages the formation of brittle Heusler phase. Finally, we establish a unique duplex-aging strategy that can be efficiently utilized for GB stabilization, by which these detrimental intergranular heterostructures can be greatly eliminated, leading to an exceptional resistance to intermediate-temperature embrittlement, along with enhanced tensile strengths. These findings will not only shed light on the precipitation mechanisms in compositionally complex HEAs but also generate new opportunities to the interfacial design of HEAs for advanced high-temperature applications with superior properties.

Control of Nanoscale Precipitation and Elimination of Intermediate-Temperature Embrittlement in Multicomponent High-Entropy Alloys

Thermally stable high-entropy alloys (HEAs) consisting of a high density of coherent precipitates show a great potential for high-temperature applications. In this work, we systematically investigated the phase stability and coarsening kinetics of L12-type coherent precipitates in a Ni-Co-Fe-Cr-Al-Ti-based HEA isothermally aged at 800, 900 and 1000 °C. Aged microstructures in the grain interiors under this temperature range were essentially dominated by the uniform precipitation of multicomponent L12 (Ni, Co, Fe, Cr)3(Ti, Al)-type precipitates. The coarsening kinetics of these intragranular L12 precipitates were quantitatively determined, which were adequately characterized by the classical Lifshitz-Slyozov-Wagner model. The activation energy for coarsening was determined to be 378 kJ/mol, which is relatively higher than that of conventional Ni or Co-based superalloys, suggesting a slow elemental diffusion in the HEA matrix. More importantly, the heterogeneous precipitation and the associated metastable phase transformation mechanism along grain boundaries (GBs) were carefully analyzed. Localized chemical heterogeneity was identified within the discontinuous L12 phase at the GBs, which thermodynamically destabilizes the L12 structure and encourages the formation of brittle Heusler phase. Finally, we establish a unique duplex-aging strategy that can be efficiently utilized for GB stabilization, by which these detrimental intergranular heterostructures can be greatly eliminated, leading to an exceptional resistance to intermediate-temperature embrittlement, along with enhanced tensile strengths. These findings will not only shed light on the precipitation mechanisms in compositionally complex HEAs but also generate new opportunities to the interfacial design of HEAs for advanced high-temperature applications with superior properties.

Phase transformations in reduced-activation duplex alloy Fe52Mn30Cr18 under isothermal heat treatment

Reduced-activation Fe-Mn-Cr alloys have been regarded as the candidate structural materials for advanced nuclear systems. The phase stability of one alloy at high temperature is critically required for its application. In this work, a series of phase transformations in a new type reduced-activation duplex alloy Fe52Mn30Cr18 under isothermal heat treatment were investigated. The ferrite phase in the alloy converted to χ phase after the alloy treated at 550°C for 3 h. The cube-on-cube orientation relationship was found between χ phase and the original ferrite phase with the lattice constant ratio of 3:1. The χ phase was eutectoid decomposed into austenite and σ phases with lamellar structures due to the diffusion of Cr atoms at temperatures above 800 °C. According to the evolution of the crystal structure and magnetic properties, the transformation from ferrite to χ phase was considered as atomic rearrangement in the adjacent unit cells, accompanied by the disappearance of ferromagnetism in the alloy. The microhardness results indicated that the χ phase formation could be one of the potential reasons for the intermediate temperature embrittlement in the high Cr Fe-based alloys.

Tensile, creep behavior and microstructure evolution of an as-cast Ni-based K417G polycrystalline superalloy

The Ni-based K417G superalloy is extensively applied as aeroengine components for its low cost and good mid-temperature (600–900°C) properties. Since used in as-cast state, the comprehensive understanding on its mechanical properties and microstructure evolution is necessary. In the present research, the tensile, creep behavior and microstructure evolution of the as-cast K417G superalloy under different conditions were investigated. The results exhibit that tensile cracks tend to initiate at MC carbide and γ/γ′ eutectic structure and then propagate along grain boundary. As the temperature for tensile tests increases from 21°C to 700°C, the yield strength and ultimate tensile strength of K417G superalloy decreases slightly, while the elongation to failure decreases greatly because of the intermediate temperature embrittlement. When the temperature rises to 900°C, the yield strength and ultimate tensile strength would decrease significantly. The creep deformation mechanism varies under different testing conditions. At 760°C/645MPa, the creep cracks initiate at MC carbides and γ/γ′ eutectic structures, and propagate transgranularly. While at 900°C/315MPa and 950°C/235MPa, the creep cracks initiate at grain boundary and propagate intergranularly. As the creep condition changes from 760°C/645MPa to 900°C/315MPa and 950°C/235MPa, the γ′ phase starts to raft, which reduces the creep deformation resistance and increases the steady-state deformation rate.

The effect of aging on the serrated yielding and intermediate temperature embrittlement of nickel-base C-276 alloy

The influence of aging on serrated yielding and intermediate temperature embrittlement (ITE) of nickel-based C-276 alloy has been investigated by microstructural characterization and tensile tests at temperatures ranging from 300 to 950°C with strain rate of 3 × 10–4s–1. The precipitated phases in the C-276 alloy aged at 870°C for various times were identified as μ phase and M6C. Tensile ductilities obtained at 700°C were found to increase with extending aging time until 12h, and thereafter, it remained almost unaffected by the aging time. Serrated flow was noticed for all temperatures for the solutionized alloy, and the alloy presented the typical ITE. Aging the solutionized alloy at 870°C/36h resulted in a disappearance of serrated yielding in the temperature range of 600–750°C, which is most possibly attributed to the very weak interaction between Mo atoms and mobile dislocation as a result of precipitation. Although aging for 36h at 870°C decreased the tensile ductilities of this alloy below 600°C, the aged alloy showed the absence of ITE and higher yield strength in comparison with solutionized alloy. The tensile ductilities in the aged alloy were insignificantly affected by increasing the temperature until 700°C, beyond which the tensile ductilities were found to increase drastically with temperature.