High-temperature Embrittlement Research Articles

High-temperature tensile properties and deformation behavior of a multi-phase FeNiCrAlTi alloy

Single-phase multi-principal elements alloys (MPEAs) within the elevated temperature experience particularly severe strength loss, and even a distinct ductile-to-brittle transition. Herein, we report on a non-equiatomic, multi-phase FeNiCrAlTi MPEA which possesses remarkable combinations of mechanical properties across a wide range of temperatures from 400 °C to 800 °C, especially its specific yield strength possesses nearly 100 MPa/g∙cm−3 at 600 °C. The excellent mechanical behavior benefits from the synergistic effect of the special phase structure and precipitation strengthening by the ordered Ni (Al, Ti) B2 nanoparticles. The transformation of deformation mode at 600 °C should be considered the dominant of the resistance to intermediate or high-temperature embrittlement. This work provides a new methodology for regulating mechanical properties of superior alloys that involve high-temperature conditions.

High temperature embrittlement of Inconel 625 alloy manufactured by laser powder bed fusion

The comprehensive mechanical behavior of Inconel 625 alloy fabricated by laser powder bed fusion (LPBF) and hot isostatic pressing (HIP) was studied. Tensile experiments were performed at six temperatures ranging from room temperature to 1000 °C to explore the effect of temperature on the mechanical properties, and the microstructure and fracture mechanism was investigated by combining electron backscatter diffraction, transmission electron microscopy, and atom probe tomography. In comparison with the properties of conventionally fabricated Inconel 625 alloys, the mechanical properties at room temperature are essentially the same. However, the plasticity of the Inconel 625 alloys fabricated by LPBF is drastically decreased with increasing temperature, especially when the temperature exceeds 700 °C due to the transformation from transgranular ductile fracture to intergranular fracture. Detailed characterization of cracks and grain boundaries (GBs) reveals significant segregation of non-metallic elements at GBs, which impedes the movement of GBs and decreases their strength, thus enhancing the intergranular embrittlement and resulting in the drastic reduction of ductility. Moreover, the irregular grain morphologies result in numerous GBs directional sharp changes and steps, as well as GBs triple junctions, in which the stress concentration is further enlarged, thus promoting the nucleation of voids and cracks and its associated high temperature embrittlement. The underlying mechanism of ductility deterioration of LPBF-fabricated Inconel 625 alloy at elevated temperatures would provide important insights into its performance at high temperatures.

Effect of Substrate Bed Temperature on Solute Segregation and Mechanical Properties in Ti–6Al–4V Produced by Laser Powder Bed Fusion

Titanium alloys are particularly sensitive to temperature during additive manufacturing processes, due to their dual phase microstructure and sensitivity to oxygen uptake. In this paper, laser powder bed fusion (LPBF) was used in conjunction with a heated substrate bed at 100 °C, 570 °C and 770 °C to produce specimens of Ti–6Al–4V, to investigate the change in mechanical properties and segregation of alloying elements. An initial increase in ductility was observed when increasing the temperature from 100 °C to 570 °C, followed by a significant loss in ductility when samples were produced at 770 °C. A suite of multi-scale characterisation techniques revealed that the as-printed microstructure was drastically different across the range of temperatures. At 100 °C, α + α′ phases were identified. Deformation twinning was extensively observed in the a phase, with Al and V segregating at the twin interfaces. At 570 °C (the most ductile sample), α′, α and nano-particles of β were observed, with networks of entangled dislocations showing V segregation. At 770 °C, no martensitic α′ was identified. The microstructure was an α + β microstructure and an increased volume fraction of tangled dislocations with localised V segregation. Thermodynamic modelling based on the Gibbs-free energy of formation showed that the increased V concentration at dislocations was insufficient to locally nucleate β phase. However, b-phase nucleation at grain boundaries (not dislocations) caused pinning of grain boundaries, impeding slip and leading to a reduction in ductility. It is likely that the increased O-content within specimens printed at increased temperatures also played a key role in high-temperature embrittlement. Building operations are therefore best performed below sub-transus temperatures, to encourage the growth of strengthening phases via solute segregation, and the build atmosphere must be tightly controlled to reduce oxygen uptake within the samples.

Increasing machinability by cutting workpieces of parts made of heat-resistant stainless steels by high-temperature embrittlement

Heat-resistant alloyed stainless steels are increasingly being used for critical machine parts. The limited scope of application of heat-resistant stainless steels is due to their high strength and toughness that make them difficult to machine, allowing these materials to be classified as difficult to machine. It is proposed to solve the problem of poor machinability by means of high-temperature embrittlement of material arising at achievement of a certain temperature interval in the machining zone. When high-temperature embrittlement phase appears, strength and toughness of heat-resistant stainless steels decrease, machinability of material improves, which is accompanied by reduction of cutting force and power consumption during machining, reduction of tool wear intensity and transformation of drain type of chips into element type.

Effect of Ti-Hf-Zr-Cu-Ni high entropy alloy addition on laser beam welded joint of Ti2AlNb based intermetallic alloy

• In this study, LBW of Ti-22Al-27Nb was experimented through addition of an interlayer of the filler, Ti-Hf-Zr-Cu-Ni HEA, into the joint and a comparison between the LBWJs of Ti-22Al-27Nb either prepared with or without addition of the filler was established through microstructural analysis by corelating its impact on mechanical properties at room temperature. Followings are the highlights of this experimental work. • The addition of Ti-Hf-Zr-Cu-Ni HEA into FZ of LBWJ of Ti-22Al-27Nb activated heterogenous nucleation due to difference of chemical composition between the BM and the filler, resultantly, fine grained B2 phase with greater fraction of HAGBs was evolved in the FZ. Grain refinement and higher misorientation angle between GBs in the FZ contributed an increase in hardness, tensile strength and percentage elongation of HEA adulterated LBWJ of Ti-22Al-27Nb. • The average hardness of the FZ (343.5 HV) and UTS (1062 MPa) of the HEA added LBWJ was found in close agreement with that of BM, 345 HV and 1060 MPa, respectively. • The percentage elongation of the joint, prepared with an interlayer of HEA, was measured up to 11.2%, which was 83.6% of the BM. • The LBWJ of Ti-22Al-27Nb, prepared with the addition of HEA, fractured from the junction of FZ and HAZ in a ductile mode while the LBWJ prepared without addition of Ti-Hf-Zr-Cu-Ni filler was ruptured from FZ in quasi-cleavage mode. Ti 2 AlNb based intermetallic alloys, a potential competitor for next-generation super alloys, are susceptible to high-temperature embrittlement due to nucleation of a metastable single B2 phase in the fusion zone (FZ) during laser beam welding (LBW). In this study, a high entropy alloy (HEA), Ti-Hf-Zr-Cu-Ni, was self-developed and introduced as an interlayer into laser beam welded joint (LBWJ) of Ti-22Al-27Nb to analyze its impact on the evolution of microstructure in the weld zone (WZ) and subsequently on joint performance. Microstructural examination was carried out through electron probe micro analysis (EPMA), electron backscattered diffraction (EBSD) analysis, high-resolution scanning transmission electron microscopy (HRSTEM) comprising bright field (BF), selective area electron diffraction (SAED) and high angle annular dark-field (HAADF) imaging. Addition of the HEA into FZ of LBWJ triggered heterogenous nucleation during solidification, resultantly, fine-grained B2 with a greater proportion of high angle grain boundaries (HAGBs) was developed. FZ of Ti-22Al-27Nb LBWJ, prepared with an interlayer of HEA, was composed of planar, cellular, columnar and equiaxed dendritic grains; a solidification mode which was different from that observed in LBWJ prepared without adulteration of the HEA. The impact of heterogenous nucleation during epitaxial solidification on mechanical properties was established through micro vickers hardness mapping and tensile test, conducted at room temperature. The average hardness, 343.5 HV, in the FZ of LBWJ prepared with an interlayer of HEA, was compatible with that of base material (BM), 345 HV. The ultimate tensile strength (UTS), 1062 MPa, and percentage elongation, 11.2%, of the HEA tempered LBWJ were found in close approximation with that of BM, 1060 MPa and 13.4%, respectively. A ductile mode of failure was observed during tensile test of the Ti-Hf-Zr-Cu-Ni supplemented LBWJ of Ti-22Al-27Nb, while quasi-cleavage mode of fracture was apparent in the joint of Ti-22Al-27Nb welded without addition of the HEA.

Structural characterization of a composite joint prepared during laser welding of Ti–22Al–27Nb intermetallic alloy with an interlayer of Cu-Hf-Ni-Ti-Zr high entropy bulk metallic glass

The advent of high entropy alloys (HEAs) has provided scientific solutions to material related problems of grave importance. High-temperature embrittlement (HTE) in the laser welded joint (LWJ) of titanium aluminides is one of the challenges, known to impede the prevalent applications of this novel alloy system. In the current research work, a sandwich-structured composite joint of an intermetallic based alloy (Ti–22Al–27Nb) with a high entropy bulk metallic glass (HE-BMG) composed of Cu, Hf, Ni, Ti and Zr was welded through laser beam. The adulteration of LWJ with HE-BMG resulted into evolution of finite microregions in the fusion zone (FZ) through heterogeneous nucleation. The FZ comprised fine-grained B2 phase with greater fractions of high angle grain boundaries (HAGBs) and lower degree of local area misorientations (LAM). The crystal texture of B2 phase in the FZ abetted the plastic deformation at room as well as high temperature (650 °C), as the Schmid factor “m” for ≥ 99% of {1 1 0} <1 1 1> slip system of B2 remained greater than 0.38. The high-resolution transmission electron microscopy revealed the presence of nano-sized composite regions (NCRs), composed of crystalline/non-crystalline structure. These NCRs remained stable at 650 °C and inhibited the nucleation of O phase along B2 grain boundaries (GBs). The percentage elongation of HE-BMG supplemented LWJ of Ti–22Al–27Nb was improved by 28.19 and 31.34% at room temperature and 650 °C, respectively. The dilution of FZ with HE-BMG contributed a significant increase in tensile ductility at room as well as high temperature.

Chip Breaking During Automated Processing by Cutting Hard-to-Work Steels by Integrated Use of Chip Breaking Tools and High-Temperature Embrittlement

Purpose of research is to increase productivity when cutting hard-to-work steels by integrated use of chip breaking tool geometry and cutting modes, which reduce the viscous properties of the chips and the processed material. The article discusses the problems of mechanical processing of parts of oil production equipment at enterprises in Western Siberia. It is presented the main condition for ensuring chip crushing and revealed the problem of low efficiency of existing methods in modern industry, to one degree or another guiding, curling and chip-cutting method.Methods. Simulation modeling, calculation of chip direction and crushing were performed; effective methods for solving the problem of chip crushing during the machining of parts made of materials with corrosion-resistant, heat-resistant properties were identified; laboratory, persistent tests of the complex method were carried out.Results of studying the mechanism of chip formation during processing by cutting heat-resistant and corrosionresistant alloys are given here. To solve the problem, we have formulated two tasks that were solved using information taken from the theory of material resistance, tensile diagrams when testing samples of corrosionresistant, heat-resistant materials with high operational properties. A chip breaking exchangeable insert with a variable rake angle is designed and presented. The results and analysis of laboratory test data, in which a plate with chip breaking geometry of the front surface is used, are presented, the operating conditions of effective chip breaking are shown. The conclusions on the solution of the first problem are formulated, it is shown that the determining factor in chip breaking is the physical and mechanical characteristics of the processed material, which vary with different cutting conditions.Conclusion. The work shows that chip removal from the cutting zone prevents it from entering the cutting edge area, reducing impact loads on it and improving the quality of machining. It is possible to form stress concentrators in the chips, leading to chip self-destruction even when processing viscous heat-resistant alloys. The hypothesis is used that the temperature conditions for the maximum workability of heat-resistant and corrosion-resistant steels and alloys correspond to the conditions of high-temperature embrittlement, with the help of which it is possible to create conditions for effective chip crushing and conditions for maximum workability when cutting heat-resistant steels and alloys.

High-temperature Embrittlement in Si-added Austenitic Stainless Steel

High-temperature Embrittlement in Si-added Austenitic Stainless Steel

Microstructure and tensile fracture behavior of three-stage heat treated inconel 718 alloy produced via laser powder bed fusion process

In this study, Laser Powder Bed Fusion (LPBF) Inconel 718 is subjected to various heat treatments, namely double aging, solid solution + double aging, and homogenization + solid-solution + double aging, to investigate the effect of heat treatment on room- and high-temperature tensile properties. The results show that all three heat treatments increase hardness and room-temperature tensile strength, but greatly reduce ductility. With an increase in tensile test temperature, the stress-induced Portevin-Le Chatelier (PLC) effect can effectively prevent the occurrence of oxidation at grain boundaries and maintain a certain ductility in the range of room temperature to 600 °C. However, when the temperature is 650 °C, the PLC effect disappears, resulting in a high-temperature embrittlement effect.

Analyzing the causes for the dispersion of the fast reactor spent fuel rod cladding properties

The swelling, corrosion and high-temperature embrittlement behavior of the fast-neutron sodium-cooled reactor standard and test fuel rod claddings was studied following the operation up to a damaging dose of 55 to 69 dpa. The tested characteristics were found to differ sensitively in conditions similar to irradiation for the claddings of the experimental tube conversion technology. Unlike the standard fuel rod claddings, the test rod claddings were additionally heated in the process of fabrication to homogenize the solid solution at different temperatures and austenitization times. On the whole, this led to an increased cladding resistance contrary the damaging factor of the reactor environment. The positive effect is explained by the influence of carbon and the morphology of swelling-reducing alloying elements, as well as by the nature of the carbide and intermetallide phase precipitation. However, the dispersion of the post-irradiation properties which remained significant and was also earlier observed in the standard rods is explained by potential differences in the heat treatment technology and the irradiation temperature in conditions of a hard-to-control coolant flow velocity. The swelling rate and the in-fuel corrosion depth for the test technology tubes were respectively 0.04 to 0.058%/dpa and 20 to 47 μm; similar values for the test material are 0.036 to 0.056%/dpa and 15 to 35 μm respectively. The short-term mechanical properties of the test fuel rods at a temperature of 600 °C showed a smaller tendency towards high-temperature embrittlement. The dispersion of the properties was caused by the chemical and structural heterogeneity as the result of the tube fabrication.

Investigation on the hot ductility of Q235 low-carbon steel with boron addition

We tested the mechanical properties of B-containing steel at 600–1300 °C using Gleeble 1500, and the hot ductility was investigated according to the reduction in area (RA). The fractured specimens were examined using scanning electron microscopy. The results indicate that boron strongly influences the hot ductility of B-containing steel in comparison with B-free Q235 steel. Boron could expand the low temperature embrittlement range from 750–850 °C to 700–1000 °C, and lead to a worse hot ductility at 1150 °C, related to its re-melting property. The reduction in area is as low as 18% at 1300 °C, and the high temperature embrittlement zone is inevitable given the existence of residual solute phase between dendrites.

Fabrication and properties of wide temperature domain pavement seaweed modified bio-bitumen

In this paper, seaweed incorporated modified bio-bitumen was prepared using seaweed oil as a modifier. Preliminary studies demonstrated that 20 wt% seaweed bio-bitumen of the total mass of bitumen accounted as the best ratio. Styrene butadiene styrene (SBS)/seaweed bio-bitumen modified petroleum bitumen was prepared by adding 4 wt% SBS modifier to seaweed bio-bitumen modified petroleum bitumen. Furthermore, via in-situ polymerization, reaction of polystyrene (PS) with graphene oxide (GO) resulted in PS-GO with a spring-like structure as confirmed by atomic force microscopy. A wide temperature domain modified bitumen was prepared by adding 0.02 wt% PS-GO of the total mass of bitumen to SBS/seaweed bio-bitumen modified petroleum bitumen. Macroscopic tests showed that adding bio-bitumen and PS-GO effectively improved the performance of pristine SBS modified bitumen mainly attributed to their similar chemistry as confirmed by infrared spectroscopy. Fluorescence microscopy revealed swelling of SBS upon the addition of seaweed bio-bitumen and formed a network like structure in the petroleum bitumen, thereby improving its high temperature rutting resistance and low temperature embrittlement resistance. PS-GO entered the SBS network structure, thereby increasing the toughness and awarding modified bitumen a wide temperature domain. The wide temperature domain modified bitumen can solve the problems of high temperature rutting and low temperature cracking on roads in harsh environments, and can provide good pavement materials for road construction.

Problems and Improvements on the Production of Large Casting with Hi–Si Ductile Iron

We have studied the problems and improvement on producing large casting with solid solution strengthened ferritic spheroidal graphite cast irons (hereafter Hi–Si). As the matrix of Hi–Si material becomes hardened with silico-ferrite, hardness, tensile strength and yield strength increase. But elongation and impact value decrease conversely. If silicon content increases or if the cooling rate decreases due to heavy section, as silicon segregation progresses, the elongation and impact value are remarkably reduced. When increasing the graphite nodule count with optimum inoculation and setting chiller, the segregation of silicon decreases and then the mechanical properties are recovered. In addition, when Hi–Si reaches around 400 °C, its elongation decreases largely, which is called 400 °C embrittlement. This high-temperature embrittlement can be prevented with setting the ratio of Mg/P less than 1.5 and reducing the free Mg, because the grain boundary fracture occurs due to the free Mg not fixed as Mg3P2 at the grain boundary.

Study of the Long-Term Strength of Neutron-Irradiated Austenitic and Ferrite-Martensite Steel

The results of a study of the long-term strength of austenitic and ferrite-martensite steel after irradiation in the BR-10, BOR-60, BN-350, and BN-600 reactors are presented. It is shown that irradiation of austenitic-class steel (EI-847, 0Kh18N10T, and ChS-68) greatly reduces the time to failure and the plasticity. This behavior of austenitic steel is associated with the phenomenon of high-temperature radiation embrittlement. Neutron irradiation does not degrade the long-term mechanical characteristics of ferrite-martensite steel, which remain either at the level of the unirradiated material (EP-823, 05Kh12N2M steels) or surpass it (EP-450 steel).

Effect of Damaging Irradiation on Low-Temperature Embrittlement of Metals

The effect of low-temperature damaging irradiation on the high-temperature embrittlement (cold shortness) in metals was investigated in a previously proposed model. The cold shortness state with possible brittle fracture in metals is not formed in the irradiation process because of the dynamical radiation processes arising.

Improved machining of heat-resistant materials by high-temperature embrittlement in cutting

The following hypothesis is verified: that heat-resistant materials are most easily machined at the temperature corresponding to their embrittlement. At that temperature, moreover, the mechanical characteristics ψ, δ of the material are least temperature-dependent. New methods are proposed for determining the conditions in which heat-resistant materials are most easily machined.

High temperature embrittled duplex stainless steels: influence of the chemical composition on the fatigue crack propagation

Abstract: Austenitic-ferritic (duplex) stainless steels are successfully used in chemical, nuclear, oil and gas industries, due to their good mechanical properties and excellent generalized and localized corrosion resistance in many environments and operating conditions (for example, chloride induced stress corrosion). The aim of this work is the analysis of the influence of the chemical composition and of the high temperature embrittlement processes in three different duplex stainless steels. Scanning electron microscope (SEM) fracture surface analysis was performed to investigate the fatigue crack propagation micromechanisms.

A comparative assessment of the fracture toughness behavior of ferritic-martensitic steels and nanostructured ferritic alloys

The Fe-Cr alloys with ultrafine microstructures are primary candidate materials for advanced nuclear reactor components because of their excellent high temperature strength and high resistance to radiation-induced damage such as embrittlement and swelling. Mainly two types of Fe-Cr alloys have been developed for the high temperature reactor applications: the quenched and tempered ferritic-martensitic (FM) steels hardened primarily by ultrafine laths and carbonitrides and the powder metallurgy-based nanostructured ferritic alloys (NFAs) by nanograin structure and nanoclusters. This study aims at elucidating the differences and similarities in the temperature and strength dependences of fracture toughness in the Fe-Cr alloys to provide a comparative assessment of their high-temperature structural performance. The KJQ versus yield stress plots confirmed that the fracture toughness was inversely proportional to yield strength. It was found, however, that the toughness data for some NFAs were outside the band of the integrated dataset at given strength level, which indicates either a significant improvement or deterioration in mechanical properties due to fundamental changes in deformation and fracture mechanisms. When compared to the behavior of NFAs, the FM steels have shown much less strength dependence and formed narrow fracture toughness data bands at a significantly lower strength region. It appeared that at high temperatures ≥600 °C the NFAs cannot retain the nanostructure advantage of high strength and high toughness either by high-temperature embrittlement or by excessive loss of strength. Irradiation studies have revealed, however, that the NFAs have much stronger radiation resistance than tempered martensitic steels, such as lower radiation-induced swelling, finer helium bubble formation, lower irradiation creep rate and reduced low temperature embrittlement.

Shear Melting and High Temperature Embrittlement: Theory and Application to Machining Titanium

ABSTRACTWe show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.

Effect of boundary conditions on the kinetics of helium release from structural materials

Gaseous products of nuclear reactions (specifically, helium) play a significant part in altering the material properties upon irradiation. It is known that atoms of inert gases promote the generation and growth of pores in irradiated materials and affect phenomena such as swelling, high-temperature irradiation embrittlement, etc. Therefore, a study of the behavior of helium (its production, accumulation, retention, and release) within structural materials is fairly topical. In order to validate the methods of express imitation of accumulation and retention of helium within structural materials under reactor irradiation, we perform a comparative analysis of the spectra of the rate of gas release from samples of austenitic steel 0Kh16N15M3B that were saturated with helium in different ways, i.e., through irradiation in a cyclotron, a magnetic massseparation setup, the IRT-2000 reactor, the BOR-60 reactor, and using the so-called tritium trick technique. The effect of the presence of dislocations and grain boundaries on the release of helium from materials is evaluated. The results of the research conducted show that the kinetics of helium release from samples saturated with helium through the bombardment with alpha particles of different energies, which ensures the simultaneous introduction of helium and radiation defects (in wide ranges of helium concentration and radiation damage) into the material lattice, is similar to the kinetics of helium release from samples irradiated in reactors.