Qingdong Liu | Shanghai Jiao Tong University (original) (raw)

Papers by Qingdong Liu

Research paper thumbnail of 3D ATOM PROBE CHARACTERIZATION OF ALLOYING ELEMENTS PARTITIONING IN CEMENTITE OF A Nb-V MICROALLOYED STEEL

Acta Metallurgica Sinica, 2008

ABSTRACT

Research paper thumbnail of Effect of tempering temperature on the microstructure and mechanical properties of a reactor pressure vessel steel

Journal of Nuclear Materials, 2016

The microstructure and mechanical properties of reactor pressure vessel (RPV) steel were investig... more The microstructure and mechanical properties of reactor pressure vessel (RPV) steel were investigated after tempering at different temperatures ranging from 580 to 700 °C for 5 h. With increasing tempering temperature, the impact toughness, which is qualified by Charpy V-notch total absorbed energy, initially increases from 142 to 252 J, and then decreases to 47 J, with a maximum value at 650 °C, while the ultimate tensile strength varies in exactly the opposite direction. Comparing the microstructure and fracture surfaces of different specimens, the variations in toughness and strength with the tempering temperature were generally attributed to the softening of the bainitic ferrite, the agminated Fe 3 C carbides that resulted from decomposition of martensite/austenite (M/A) constituents, the precipitation of Mo 2 C carbides, and the newly formed M/A constituents at the grain boundaries. Finally, the correlation between the impact toughness and the volume fraction of the M/A constit...

Research paper thumbnail of Effect of Temperature on Microstructure and Mechanical Properties of Fe-9Ni-2Cu Steel during the Tempering Process

Materials

In this paper, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray ... more In this paper, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray stress meter (XRSA), atom probe tomography (APT), hardness, and tensile tests were used to study the effect of tempering temperature on the microstructure and properties of Fe-9Ni-2Cu steel. The results show that after the quenched samples were tempered at 460 °C for 2 h, the hardness values increased from 373 to 397 HV, and elongation also increased from 13% to 16%. With the tempering temperature increasing from 460 to 660 °C, the hardness firstly decreases from 397 to 353 HV and then increases to 377 HV, while the elongation increases to 17% and then decreases to 11%. The variation of the mechanical properties greatly depends on the evolution of the Cu-rich phase and carbides. The precipitation strengthening of the Cu-rich phase and carbides leads to the increase of hardness, but when the precipitate is coarsened, the precipitation strengthening weakens, and th...

Research paper thumbnail of Microstructure Evolution of a Reactor Pressure Vessel Steel During High-Temperature Tempering

The electron microscopy techniques were employed to analyze the microstructure evolution during h... more The electron microscopy techniques were employed to analyze the microstructure evolution during high-temperature tempering of a reactor pressure vessel steel. The results show that carbon enriched martensite/austenite (M/A) constituents decomposed into ferrite laths and accumulated carbides during initial stage of tempering. Simultaneously, the carbon atoms in the constituents diffused into the matrix continuously. With further prolonging of tempering, Mo2C carbides were found to be distributed uniformly in bainitic ferrite. In case of longer tempering, bainitic ferrite would combine and broaden, and grain boundary carbides grew up sequentially and coarsened. The newly formed austenite was detected during tempering at 660 °C for 5 h, and at 650 °C for 100 h,which shown that the Ac1 is time-related. This phenomenon may be depend on the component fluctuation of M/A constituents and segregation of carbides.

Research paper thumbnail of Decomposition of Supercooled Austenite in Continuous Cooling Transformation Process of a Mn-Mo-Ni Low Alloy Steel

Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni... more Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni low alloy steel was evaluated by dilatometric measurements, light microscopy, electron backscatter diffraction, and microhardness testing and other methods. The results show that at the cooling rates of 1°C/s or below, ferrite initially formed and continuously rejecting C into the untransformed austenite, which transforms to C-rich lower bainite at lower temperature, resulting in ferrite-bainite dual-phase microstructures. At the cooling rates between 1 °C/s and 5 °C/s, the successive transformation products are bainite ferrite, upper bainite and lower bainite, and islands of carbon-enriched austenite transform to martensite (plus retained austenite) at low temperatures. The upper bainite and martensite dual-phase microstructures are formed at the range of 5 °C/s to 50 °C/s with a lower Ms. When cooling rates greater than approximately 50 °C/s, the microstructure are martensite and reta...

Research paper thumbnail of A precipitate-free AlCoFeNi eutectic high-entropy alloy with strong strain hardening

Journal of Materials Science & Technology

Research paper thumbnail of Effect of Cyclic Intercritical Tempering on the Microstructure and Mechanical Properties of a Low-Carbon Cu-Bearing 7Ni Steel

Metallurgical and Materials Transactions A

High strength and toughness are usually hard to obtain simultaneously because of the trade-off. I... more High strength and toughness are usually hard to obtain simultaneously because of the trade-off. In this research, cyclic intercritical tempering (IT) was applied to a low-carbon Cu-bearing 7Ni steel to pursue a better strength-toughness balance than what conventional single intercritical tempering can achieve. The mechanical properties and microstructure of cyclic IT and single IT were studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in addition to dilatometry. It was found that cyclic IT can significantly improve the strength without much sacrifice of toughness. The additional strength comes from dislocation and precipitation strengthening. The mechanism of reverse transformation was studied, and it was found that the mechanism changes from diffusional at single IT or first-cycle IT to a combination of interface-dominated and diffusional at the following cyclic IT. It was suggested that enrichment of Ni after the first cyclic IT is responsible for the mechanism change by thermodynamic calculation. Furthermore, although the Ni content is higher in fresh martensite (FM) after following cyclic IT, no distinct decrease of Ms was found, which is related to the inhomogeneous elemental distribution of FM.

Research paper thumbnail of Strengthening of Ni-Mn-Cu-Al-Co steel by nanoscale Cu and β-NiAl co-precipitated couples

Materials Characterization

Abstract A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without ad... more Abstract A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without adding any strong carbide-forming element was designed to investigate the co-precipitation behaviors of Cu-rich (CRPs) and β-NiAl precipitates by using electron back-scatter diffraction (EBSD) and atom probe tomography (APT). In the light of phase transformation and hardness-temperature curves, it is estimated that the nanoscale clusters and precipitates formed at 450 and 500°C tempering respectively are responsible for the strong precipitation strengthening effect. The number density of the co-precipitated couples is significantly increased by an order of magnitude while the size almost keeps constant. The limited growth of the dispersed small precipitates is in association with the co-precipitation reactions itself and the addition of Co that possibly hinders fast coarsening of the co-precipitate couples. The strength contributions from dislocated martensite, solid solution and precipitation were roughly calculated and compared with the experimental values. It is found the precipitation hardening is dominant in strengthening and the solid solution hardening exhibits a non-negligible effect as well.

Research paper thumbnail of Improved impact toughness by multi-step heat treatment in a 1400 MPa low carbon precipitation-strengthened steel

Materials Science and Engineering: A

Research paper thumbnail of Nano-sized austenite and Cu precipitates formed by using intercritical tempering plus tempering and their effect on the mechanical property in a low carbon Cu bearing 7 Ni steel

Materials Science and Engineering: A

Abstract High toughness, especially cryogenic toughness, is usually hard to be obtained in precip... more Abstract High toughness, especially cryogenic toughness, is usually hard to be obtained in precipitation strengthening steels. In this report, intercritical tempering plus tempering were applied on a low carbon Cu-contained 7Ni steel. Results reveal that high strength (819MPa, 25 °C) combined with high cryogenic toughness (138 J, −196 °C) can be realized simultaneously by two steps tempering. The microstructural evolution during intercritical tempering and tempering was investigated systematically by scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was found that both Ni-rich regions and Ni-depleted regions exist after first step intercritical tempering and no stable austenite was found, whereas nano-sized austenite (with average thickness of 20 nm) was produced on the Ni-rich regions after following second tempering process. Thermo-dynamic calculation and DICTRA simulation was applied to systematically study formation of austenite and partition of Ni during two steps tempering process. Simulation results coincides well with experiments. Meanwhile, it is suggested that high cryogenic toughness is ascribed to the stable austenite because of its nanometer thickness and high Ni content. Precipitation of Cu-rich phases at second tempering process maintained high strength during final tempering.

Research paper thumbnail of The corrosion behavior of M5 (Zr–1Nb-0.12O) alloy in 360 °C water with dissolved oxygen

Journal of Nuclear Materials

Abstract The corrosion behavior of M5 alloy in 360 °C/20 MPa water with 1.5 ppm dissolved oxygen ... more Abstract The corrosion behavior of M5 alloy in 360 °C/20 MPa water with 1.5 ppm dissolved oxygen (DO) was studied. The test was conducted by using a circulating autoclave loop system. The results indicated that DO accelerated the corrosion of M5 alloy in high temperature water. The corrosion behavior of β-Nb phase in the barrier layer and the post-transition layer in oxygenated water is completely different from that in deoxygenated water. For materials exposed to oxygenated water, β-Nb phases are sequentially oxidized to crystalline NbO (Nb2+) and crystalline NbO2 (Nb4+), then part of it is completely oxidized to Nb2O5 (Nb5+) in the barrier layer. In the post-transition layer, these crystalline oxidation products of β-Nb phase change to amorphous state, wherein Nb is totally oxidized to Nb5+ state. In the outer layer, the dissolution of β-Nb oxide is not completed when exposed to oxygenated water for even 100 days. Oxygenated water can accelerate corrosion of β-Nb SPPs, which could be responsible for high corrosion rate of Nb-containing Zr alloy in DO water.

Research paper thumbnail of High-resolution Transmission Electron Microscopy Characterization of the Structure of Cu Precipitate in a Thermal-aged Multicomponent Steel

Chinese Journal of Mechanical Engineering

High-dispersed nanoscale Cu precipitates often contribute to extremely high strength due to preci... more High-dispersed nanoscale Cu precipitates often contribute to extremely high strength due to precipitation hardening, and whereas usually lead to degraded toughness for especially ferritic steels. Hence, it is important to understand the formation behaviors of the Cu precipitates. High-resolution transmission electron microscopy (TEM) is utilized to investigate the structure of Cu precipitates thermally formed in a high-strength low-alloy (HSLA) steel. The Cu precipitates were generally formed from solid solution and at the crystallographic defects such as martensite lath boundaries and dislocations. The Cu precipitates in the same aging condition have various structure of BCC, 9R and FCC, and the structural evolution does not greatly correlate with the actual sizes. The presence of different structures in an individual Cu precipitate is observed, which reflects the structural transformation occurring locally to relax the strain energy. The multiply additions in the steel possibly ma...

Research paper thumbnail of Pre-transition Corrosion Behavior of SZA-4 and ZIRLO Alloys in Dissolved Oxygen Aqueous Condition at 360 ℃

Journal of Mechanical Engineering

Research paper thumbnail of Compositional Variants of Cu-rich Precipitate in Thermally Aged Ferritic Steel

Acta Metallurgica Sinica (English Letters)

Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy s... more Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500°C for 1, 4, 16, and 64 h after water-quenching from 900°C. With prolonged aging time, the Curich precipitates (CRPs) increased in size and decreased in number density, and gradually evolved from spheroidal to elliptical in morphology. The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage. The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage. Additionally, Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces, and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size. KEY WORDS: High-strength low-alloy steel; Thermal aging; Cu-rich precipitate; Atom probe tomography * 400°C [7-13]. Therefore, it is important to tailor the precipitation morphology of Cu-rich precipitates (CRPs) for attaining desired strength and toughness balance. Actually, the characteristics of CRPs in aspect of size, number density, shape, etc., that determine final mechanical properties are largely dependent of the compositional and resultant structural evolution during heat treatment process, and the segregation of Ni and Mn at the precipitate/matrix interface significantly prohibits the growth of CRPs to large size [14-21]. In additional, the compositional evolution of CRPs provides important information for composition modification, processing optimization, and property improvement of Cu-containing steels. However,

Research paper thumbnail of Fretting wear behavior of zirconium alloy in B-Li water at 300 °C

Journal of Nuclear Materials

Abstract The tangential fretting wear of three kinds of zirconium alloys tube mated with 304 stai... more Abstract The tangential fretting wear of three kinds of zirconium alloys tube mated with 304 stainless steel (SS) plate was investigated. The tests were conducted in an autoclave containing 300 °C pressurized B-Li water for tube-on-plate contact configuration. The worn surfaces were examined with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and 3D microscopy. The cross-section of wear scar was examined with transmission electron microscope (TEM). The results indicated that the dominant wear mechanism of zirconium alloys in this test condition was delamination and oxidation. The oxide layer on the fretted area consists of outer oxide layer composed of iron oxide and zirconium oxide and inner oxide layer composed of zirconium oxide.

Research paper thumbnail of Hierarchical Cu precipitation in lamellated steel after multistage heat treatment

Philosophical Magazine

Abstract The hierarchical distribution of Cu-rich precipitates (CRPs) and related partitioning an... more Abstract The hierarchical distribution of Cu-rich precipitates (CRPs) and related partitioning and segregation behaviours of solute atoms were investigated in a 1.54 Cu-3.51 Ni (wt.%) low-carbon high-strength low-alloy (HSLA) steel after multistage heat treatment by using the combination of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and atom probe tomography (APT). Intercritical tempering at 725 °C of as-quenched lathlike martensitic structure leads to the coprecipitation of CRPs at the periphery of a carbide precipitate which is possibly in its paraequilibrium state due to distinct solute segregation at the interface. The alloyed carbide and CRPs provide constituent elements for each other and make the coprecipitation thermodynamically favourable. Meanwhile, austenite reversion occurs to form fresh secondary martensite (FSM) zone where is rich in Cu and pertinent Ni and Mn atoms, which gives rise to a different distributional morphology of CRPs with large size and high density. In addition, conventional tempering at 500 °C leads to the formation of nanoscale Cu-rich clusters in α-Fe matrix. As a consequence, three populations of CRPs are hierarchically formed around carbide precipitate, at FSM zone and in α-Fe matrix. The formation of different precipitated features can be turned by controlling diffusion pathways of related solute atoms and further to tailor mechanical properties via proper multistage heat treatments.

Research paper thumbnail of Interactive formation of Cu-rich precipitate, reverted austenite, and alloyed carbide during partial austenite reversion treatment for high-strength low-alloy steel

Journal of Materials Research

We address the competitive precipitation and coprecipitation of three types of secondary phases, ... more We address the competitive precipitation and coprecipitation of three types of secondary phases, i.e., Cu-rich precipitates (CRPs), reverted austenite (RA), and alloyed carbide, in a high-strength low-alloy steel with austenite reversion treatment at 675 °C by using electron back-scatter diffraction, transmission electron microscopy, and atom probe tomography. There is a strong competitive diffusion of Ni and Cu participating in austenite reversion and Cu precipitation with the fact that no CRPs are detected in and around the RA. Meanwhile, there is also a strong competitive diffusion of austenite stabilizing element Ni and carbide-forming elements Cr and Mo into the pre-existing C-rich zone, leading to the formation of nonequilibrium alloyed carbide deviating from the stoichiometric composition. On the other hand, the alloyed carbide and CRPs provide constituent elements for each other and make the coprecipitation thermodynamically favorable. The knowledge on the interactive formation of these three features provides versatile access to tailor the distributional morphology of CRPs, RA, and alloyed carbide via a multistage heat treatment and thus realize their beneficial effect on strength and toughness.

Research paper thumbnail of Effect of Multistage Heat Treatment on Microstructure and Mechanical Properties of High-Strength Low-Alloy Steel

Metallurgical and Materials Transactions A

The influence of Cu-rich precipitates (CRPs) and reverted austenite (RA) on the strength and impa... more The influence of Cu-rich precipitates (CRPs) and reverted austenite (RA) on the strength and impact toughness of a Cu-containing 3.5 wt pct Ni high-strength low-alloy (HSLA) steel after various heat treatments involving quenching (Q), lamellarization (L), and tempering (T) is studied using electron back-scatter diffraction, transmission electron microscopy, and atom probe tomography. The QT sample exhibits high strength but low impact toughness, whereas the QL samples mostly possess improved impact toughness but moderate strength, but the QLT samples again have degraded impact toughness due to additional tempering. The dispersion of nanoscale CRPs, which are formed during tempering, is responsible for the enhanced strength but simultaneously leads to the degraded impact toughness. The RA formed during lamellarization contributes to the improved impact toughness. Based on the present study, new heat treatment schedules are proposed to balance strength and impact toughness by optimizing the precipitation of CRPs and RA.

Research paper thumbnail of Microstructural Evolution and Carbide Precipitation in a Heat-Treated H13 Hot Work Mold Steel

Metallography, Microstructure, and Analysis

The microstructure and carbide precipitation for a H13 hot work mold steel after heat treatment a... more The microstructure and carbide precipitation for a H13 hot work mold steel after heat treatment and stabilization treatment at 620°C for up to 20 h were investigated by optical microscopy, electron backscatter diffraction, and transmission electron microscopy. After solutionization at 1020°C for 40 min followed by oiling cooling and double tempering at 610°C for 2 h, lathlike martensitic structure was obtained with various alloyed carbides including Cr-rich M 23 C 6 carbide located at prior austenite grain boundaries or lath boundaries and V-rich MC and Mo-rich M 2 C carbides distributed in grain interiors. The heat-treated sample had high strength surpassing 1000 MPa at room temperature and 500 MPa at 650°C. After stabilization treatment at 620°C for 20 h, the carbides underwent limited coarsening, particularly for the MC and M 2 C carbides, which resulted in excellent resistance to tempering softening for the microstructure as the hardness surpassed 300 HV.

Research paper thumbnail of Time-dependent temper embrittlement of reactor pressure vessel steel: Correlation between microstructural evolution and mechanical properties during tempering at 650 °C

Journal of Nuclear Materials

Abstract The microstructural evolution of reactor pressure vessel (RPV) steel and its effect on t... more Abstract The microstructural evolution of reactor pressure vessel (RPV) steel and its effect on the mechanical properties during tempering at 650 °C were studied to reveal the time-dependent toughness and temper embrittlement. The results show that the toughening of the material should be attributed to the decomposition of the martensite/austenite constituents and uniform distribution of carbides. When the tempering duration was 5 h, the strength of the investigated steel decreased to strike a balance with the material impact toughness that reached a plateau. As the tempering duration was further increased, the material strength was slightly reduced but the material impact toughness deteriorated drastically. This time-dependent temper embrittlement is different from traditional temper embrittlement, and it can be partly attributed to the softening of the matrix and the broadening of the ferrite laths. Moreover, the dimensions and distribution of the grain carbides are the most important factors of the impact toughness.

Research paper thumbnail of 3D ATOM PROBE CHARACTERIZATION OF ALLOYING ELEMENTS PARTITIONING IN CEMENTITE OF A Nb-V MICROALLOYED STEEL

Acta Metallurgica Sinica, 2008

ABSTRACT

Research paper thumbnail of Effect of tempering temperature on the microstructure and mechanical properties of a reactor pressure vessel steel

Journal of Nuclear Materials, 2016

The microstructure and mechanical properties of reactor pressure vessel (RPV) steel were investig... more The microstructure and mechanical properties of reactor pressure vessel (RPV) steel were investigated after tempering at different temperatures ranging from 580 to 700 °C for 5 h. With increasing tempering temperature, the impact toughness, which is qualified by Charpy V-notch total absorbed energy, initially increases from 142 to 252 J, and then decreases to 47 J, with a maximum value at 650 °C, while the ultimate tensile strength varies in exactly the opposite direction. Comparing the microstructure and fracture surfaces of different specimens, the variations in toughness and strength with the tempering temperature were generally attributed to the softening of the bainitic ferrite, the agminated Fe 3 C carbides that resulted from decomposition of martensite/austenite (M/A) constituents, the precipitation of Mo 2 C carbides, and the newly formed M/A constituents at the grain boundaries. Finally, the correlation between the impact toughness and the volume fraction of the M/A constit...

Research paper thumbnail of Effect of Temperature on Microstructure and Mechanical Properties of Fe-9Ni-2Cu Steel during the Tempering Process

Materials

In this paper, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray ... more In this paper, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray stress meter (XRSA), atom probe tomography (APT), hardness, and tensile tests were used to study the effect of tempering temperature on the microstructure and properties of Fe-9Ni-2Cu steel. The results show that after the quenched samples were tempered at 460 °C for 2 h, the hardness values increased from 373 to 397 HV, and elongation also increased from 13% to 16%. With the tempering temperature increasing from 460 to 660 °C, the hardness firstly decreases from 397 to 353 HV and then increases to 377 HV, while the elongation increases to 17% and then decreases to 11%. The variation of the mechanical properties greatly depends on the evolution of the Cu-rich phase and carbides. The precipitation strengthening of the Cu-rich phase and carbides leads to the increase of hardness, but when the precipitate is coarsened, the precipitation strengthening weakens, and th...

Research paper thumbnail of Microstructure Evolution of a Reactor Pressure Vessel Steel During High-Temperature Tempering

The electron microscopy techniques were employed to analyze the microstructure evolution during h... more The electron microscopy techniques were employed to analyze the microstructure evolution during high-temperature tempering of a reactor pressure vessel steel. The results show that carbon enriched martensite/austenite (M/A) constituents decomposed into ferrite laths and accumulated carbides during initial stage of tempering. Simultaneously, the carbon atoms in the constituents diffused into the matrix continuously. With further prolonging of tempering, Mo2C carbides were found to be distributed uniformly in bainitic ferrite. In case of longer tempering, bainitic ferrite would combine and broaden, and grain boundary carbides grew up sequentially and coarsened. The newly formed austenite was detected during tempering at 660 °C for 5 h, and at 650 °C for 100 h,which shown that the Ac1 is time-related. This phenomenon may be depend on the component fluctuation of M/A constituents and segregation of carbides.

Research paper thumbnail of Decomposition of Supercooled Austenite in Continuous Cooling Transformation Process of a Mn-Mo-Ni Low Alloy Steel

Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni... more Decomposition of supercooled austenite in continuous cooling transformation process of a Mn-Mo-Ni low alloy steel was evaluated by dilatometric measurements, light microscopy, electron backscatter diffraction, and microhardness testing and other methods. The results show that at the cooling rates of 1°C/s or below, ferrite initially formed and continuously rejecting C into the untransformed austenite, which transforms to C-rich lower bainite at lower temperature, resulting in ferrite-bainite dual-phase microstructures. At the cooling rates between 1 °C/s and 5 °C/s, the successive transformation products are bainite ferrite, upper bainite and lower bainite, and islands of carbon-enriched austenite transform to martensite (plus retained austenite) at low temperatures. The upper bainite and martensite dual-phase microstructures are formed at the range of 5 °C/s to 50 °C/s with a lower Ms. When cooling rates greater than approximately 50 °C/s, the microstructure are martensite and reta...

Research paper thumbnail of A precipitate-free AlCoFeNi eutectic high-entropy alloy with strong strain hardening

Journal of Materials Science & Technology

Research paper thumbnail of Effect of Cyclic Intercritical Tempering on the Microstructure and Mechanical Properties of a Low-Carbon Cu-Bearing 7Ni Steel

Metallurgical and Materials Transactions A

High strength and toughness are usually hard to obtain simultaneously because of the trade-off. I... more High strength and toughness are usually hard to obtain simultaneously because of the trade-off. In this research, cyclic intercritical tempering (IT) was applied to a low-carbon Cu-bearing 7Ni steel to pursue a better strength-toughness balance than what conventional single intercritical tempering can achieve. The mechanical properties and microstructure of cyclic IT and single IT were studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in addition to dilatometry. It was found that cyclic IT can significantly improve the strength without much sacrifice of toughness. The additional strength comes from dislocation and precipitation strengthening. The mechanism of reverse transformation was studied, and it was found that the mechanism changes from diffusional at single IT or first-cycle IT to a combination of interface-dominated and diffusional at the following cyclic IT. It was suggested that enrichment of Ni after the first cyclic IT is responsible for the mechanism change by thermodynamic calculation. Furthermore, although the Ni content is higher in fresh martensite (FM) after following cyclic IT, no distinct decrease of Ms was found, which is related to the inhomogeneous elemental distribution of FM.

Research paper thumbnail of Strengthening of Ni-Mn-Cu-Al-Co steel by nanoscale Cu and β-NiAl co-precipitated couples

Materials Characterization

Abstract A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without ad... more Abstract A novel ultralow carbon precipitation-hardening Ni-Mn-Cu-Al-Co ferritic steel without adding any strong carbide-forming element was designed to investigate the co-precipitation behaviors of Cu-rich (CRPs) and β-NiAl precipitates by using electron back-scatter diffraction (EBSD) and atom probe tomography (APT). In the light of phase transformation and hardness-temperature curves, it is estimated that the nanoscale clusters and precipitates formed at 450 and 500°C tempering respectively are responsible for the strong precipitation strengthening effect. The number density of the co-precipitated couples is significantly increased by an order of magnitude while the size almost keeps constant. The limited growth of the dispersed small precipitates is in association with the co-precipitation reactions itself and the addition of Co that possibly hinders fast coarsening of the co-precipitate couples. The strength contributions from dislocated martensite, solid solution and precipitation were roughly calculated and compared with the experimental values. It is found the precipitation hardening is dominant in strengthening and the solid solution hardening exhibits a non-negligible effect as well.

Research paper thumbnail of Improved impact toughness by multi-step heat treatment in a 1400 MPa low carbon precipitation-strengthened steel

Materials Science and Engineering: A

Research paper thumbnail of Nano-sized austenite and Cu precipitates formed by using intercritical tempering plus tempering and their effect on the mechanical property in a low carbon Cu bearing 7 Ni steel

Materials Science and Engineering: A

Abstract High toughness, especially cryogenic toughness, is usually hard to be obtained in precip... more Abstract High toughness, especially cryogenic toughness, is usually hard to be obtained in precipitation strengthening steels. In this report, intercritical tempering plus tempering were applied on a low carbon Cu-contained 7Ni steel. Results reveal that high strength (819MPa, 25 °C) combined with high cryogenic toughness (138 J, −196 °C) can be realized simultaneously by two steps tempering. The microstructural evolution during intercritical tempering and tempering was investigated systematically by scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was found that both Ni-rich regions and Ni-depleted regions exist after first step intercritical tempering and no stable austenite was found, whereas nano-sized austenite (with average thickness of 20 nm) was produced on the Ni-rich regions after following second tempering process. Thermo-dynamic calculation and DICTRA simulation was applied to systematically study formation of austenite and partition of Ni during two steps tempering process. Simulation results coincides well with experiments. Meanwhile, it is suggested that high cryogenic toughness is ascribed to the stable austenite because of its nanometer thickness and high Ni content. Precipitation of Cu-rich phases at second tempering process maintained high strength during final tempering.

Research paper thumbnail of The corrosion behavior of M5 (Zr–1Nb-0.12O) alloy in 360 °C water with dissolved oxygen

Journal of Nuclear Materials

Abstract The corrosion behavior of M5 alloy in 360 °C/20 MPa water with 1.5 ppm dissolved oxygen ... more Abstract The corrosion behavior of M5 alloy in 360 °C/20 MPa water with 1.5 ppm dissolved oxygen (DO) was studied. The test was conducted by using a circulating autoclave loop system. The results indicated that DO accelerated the corrosion of M5 alloy in high temperature water. The corrosion behavior of β-Nb phase in the barrier layer and the post-transition layer in oxygenated water is completely different from that in deoxygenated water. For materials exposed to oxygenated water, β-Nb phases are sequentially oxidized to crystalline NbO (Nb2+) and crystalline NbO2 (Nb4+), then part of it is completely oxidized to Nb2O5 (Nb5+) in the barrier layer. In the post-transition layer, these crystalline oxidation products of β-Nb phase change to amorphous state, wherein Nb is totally oxidized to Nb5+ state. In the outer layer, the dissolution of β-Nb oxide is not completed when exposed to oxygenated water for even 100 days. Oxygenated water can accelerate corrosion of β-Nb SPPs, which could be responsible for high corrosion rate of Nb-containing Zr alloy in DO water.

Research paper thumbnail of High-resolution Transmission Electron Microscopy Characterization of the Structure of Cu Precipitate in a Thermal-aged Multicomponent Steel

Chinese Journal of Mechanical Engineering

High-dispersed nanoscale Cu precipitates often contribute to extremely high strength due to preci... more High-dispersed nanoscale Cu precipitates often contribute to extremely high strength due to precipitation hardening, and whereas usually lead to degraded toughness for especially ferritic steels. Hence, it is important to understand the formation behaviors of the Cu precipitates. High-resolution transmission electron microscopy (TEM) is utilized to investigate the structure of Cu precipitates thermally formed in a high-strength low-alloy (HSLA) steel. The Cu precipitates were generally formed from solid solution and at the crystallographic defects such as martensite lath boundaries and dislocations. The Cu precipitates in the same aging condition have various structure of BCC, 9R and FCC, and the structural evolution does not greatly correlate with the actual sizes. The presence of different structures in an individual Cu precipitate is observed, which reflects the structural transformation occurring locally to relax the strain energy. The multiply additions in the steel possibly ma...

Research paper thumbnail of Pre-transition Corrosion Behavior of SZA-4 and ZIRLO Alloys in Dissolved Oxygen Aqueous Condition at 360 ℃

Journal of Mechanical Engineering

Research paper thumbnail of Compositional Variants of Cu-rich Precipitate in Thermally Aged Ferritic Steel

Acta Metallurgica Sinica (English Letters)

Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy s... more Atom probe tomography was utilized to investigate Cu precipitation in a high-strength low-alloy steel isothermally aged at 500°C for 1, 4, 16, and 64 h after water-quenching from 900°C. With prolonged aging time, the Curich precipitates (CRPs) increased in size and decreased in number density, and gradually evolved from spheroidal to elliptical in morphology. The small CRPs were rich in a high amount of Fe and a certain amount of Ni and Mn at their early nucleation stage. The large CRPs with increased size due to extensive aging contained less Fe and more Cu at their later growth stage. Additionally, Ni and Mn were both readily to segregate at the CRP/matrix heterophase interfaces, and Mn was higher in content than Ni in the precipitate interior especially when the CRPs were large in size. KEY WORDS: High-strength low-alloy steel; Thermal aging; Cu-rich precipitate; Atom probe tomography * 400°C [7-13]. Therefore, it is important to tailor the precipitation morphology of Cu-rich precipitates (CRPs) for attaining desired strength and toughness balance. Actually, the characteristics of CRPs in aspect of size, number density, shape, etc., that determine final mechanical properties are largely dependent of the compositional and resultant structural evolution during heat treatment process, and the segregation of Ni and Mn at the precipitate/matrix interface significantly prohibits the growth of CRPs to large size [14-21]. In additional, the compositional evolution of CRPs provides important information for composition modification, processing optimization, and property improvement of Cu-containing steels. However,

Research paper thumbnail of Fretting wear behavior of zirconium alloy in B-Li water at 300 °C

Journal of Nuclear Materials

Abstract The tangential fretting wear of three kinds of zirconium alloys tube mated with 304 stai... more Abstract The tangential fretting wear of three kinds of zirconium alloys tube mated with 304 stainless steel (SS) plate was investigated. The tests were conducted in an autoclave containing 300 °C pressurized B-Li water for tube-on-plate contact configuration. The worn surfaces were examined with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and 3D microscopy. The cross-section of wear scar was examined with transmission electron microscope (TEM). The results indicated that the dominant wear mechanism of zirconium alloys in this test condition was delamination and oxidation. The oxide layer on the fretted area consists of outer oxide layer composed of iron oxide and zirconium oxide and inner oxide layer composed of zirconium oxide.

Research paper thumbnail of Hierarchical Cu precipitation in lamellated steel after multistage heat treatment

Philosophical Magazine

Abstract The hierarchical distribution of Cu-rich precipitates (CRPs) and related partitioning an... more Abstract The hierarchical distribution of Cu-rich precipitates (CRPs) and related partitioning and segregation behaviours of solute atoms were investigated in a 1.54 Cu-3.51 Ni (wt.%) low-carbon high-strength low-alloy (HSLA) steel after multistage heat treatment by using the combination of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and atom probe tomography (APT). Intercritical tempering at 725 °C of as-quenched lathlike martensitic structure leads to the coprecipitation of CRPs at the periphery of a carbide precipitate which is possibly in its paraequilibrium state due to distinct solute segregation at the interface. The alloyed carbide and CRPs provide constituent elements for each other and make the coprecipitation thermodynamically favourable. Meanwhile, austenite reversion occurs to form fresh secondary martensite (FSM) zone where is rich in Cu and pertinent Ni and Mn atoms, which gives rise to a different distributional morphology of CRPs with large size and high density. In addition, conventional tempering at 500 °C leads to the formation of nanoscale Cu-rich clusters in α-Fe matrix. As a consequence, three populations of CRPs are hierarchically formed around carbide precipitate, at FSM zone and in α-Fe matrix. The formation of different precipitated features can be turned by controlling diffusion pathways of related solute atoms and further to tailor mechanical properties via proper multistage heat treatments.

Research paper thumbnail of Interactive formation of Cu-rich precipitate, reverted austenite, and alloyed carbide during partial austenite reversion treatment for high-strength low-alloy steel

Journal of Materials Research

We address the competitive precipitation and coprecipitation of three types of secondary phases, ... more We address the competitive precipitation and coprecipitation of three types of secondary phases, i.e., Cu-rich precipitates (CRPs), reverted austenite (RA), and alloyed carbide, in a high-strength low-alloy steel with austenite reversion treatment at 675 °C by using electron back-scatter diffraction, transmission electron microscopy, and atom probe tomography. There is a strong competitive diffusion of Ni and Cu participating in austenite reversion and Cu precipitation with the fact that no CRPs are detected in and around the RA. Meanwhile, there is also a strong competitive diffusion of austenite stabilizing element Ni and carbide-forming elements Cr and Mo into the pre-existing C-rich zone, leading to the formation of nonequilibrium alloyed carbide deviating from the stoichiometric composition. On the other hand, the alloyed carbide and CRPs provide constituent elements for each other and make the coprecipitation thermodynamically favorable. The knowledge on the interactive formation of these three features provides versatile access to tailor the distributional morphology of CRPs, RA, and alloyed carbide via a multistage heat treatment and thus realize their beneficial effect on strength and toughness.

Research paper thumbnail of Effect of Multistage Heat Treatment on Microstructure and Mechanical Properties of High-Strength Low-Alloy Steel

Metallurgical and Materials Transactions A

The influence of Cu-rich precipitates (CRPs) and reverted austenite (RA) on the strength and impa... more The influence of Cu-rich precipitates (CRPs) and reverted austenite (RA) on the strength and impact toughness of a Cu-containing 3.5 wt pct Ni high-strength low-alloy (HSLA) steel after various heat treatments involving quenching (Q), lamellarization (L), and tempering (T) is studied using electron back-scatter diffraction, transmission electron microscopy, and atom probe tomography. The QT sample exhibits high strength but low impact toughness, whereas the QL samples mostly possess improved impact toughness but moderate strength, but the QLT samples again have degraded impact toughness due to additional tempering. The dispersion of nanoscale CRPs, which are formed during tempering, is responsible for the enhanced strength but simultaneously leads to the degraded impact toughness. The RA formed during lamellarization contributes to the improved impact toughness. Based on the present study, new heat treatment schedules are proposed to balance strength and impact toughness by optimizing the precipitation of CRPs and RA.

Research paper thumbnail of Microstructural Evolution and Carbide Precipitation in a Heat-Treated H13 Hot Work Mold Steel

Metallography, Microstructure, and Analysis

The microstructure and carbide precipitation for a H13 hot work mold steel after heat treatment a... more The microstructure and carbide precipitation for a H13 hot work mold steel after heat treatment and stabilization treatment at 620°C for up to 20 h were investigated by optical microscopy, electron backscatter diffraction, and transmission electron microscopy. After solutionization at 1020°C for 40 min followed by oiling cooling and double tempering at 610°C for 2 h, lathlike martensitic structure was obtained with various alloyed carbides including Cr-rich M 23 C 6 carbide located at prior austenite grain boundaries or lath boundaries and V-rich MC and Mo-rich M 2 C carbides distributed in grain interiors. The heat-treated sample had high strength surpassing 1000 MPa at room temperature and 500 MPa at 650°C. After stabilization treatment at 620°C for 20 h, the carbides underwent limited coarsening, particularly for the MC and M 2 C carbides, which resulted in excellent resistance to tempering softening for the microstructure as the hardness surpassed 300 HV.

Research paper thumbnail of Time-dependent temper embrittlement of reactor pressure vessel steel: Correlation between microstructural evolution and mechanical properties during tempering at 650 °C

Journal of Nuclear Materials

Abstract The microstructural evolution of reactor pressure vessel (RPV) steel and its effect on t... more Abstract The microstructural evolution of reactor pressure vessel (RPV) steel and its effect on the mechanical properties during tempering at 650 °C were studied to reveal the time-dependent toughness and temper embrittlement. The results show that the toughening of the material should be attributed to the decomposition of the martensite/austenite constituents and uniform distribution of carbides. When the tempering duration was 5 h, the strength of the investigated steel decreased to strike a balance with the material impact toughness that reached a plateau. As the tempering duration was further increased, the material strength was slightly reduced but the material impact toughness deteriorated drastically. This time-dependent temper embrittlement is different from traditional temper embrittlement, and it can be partly attributed to the softening of the matrix and the broadening of the ferrite laths. Moreover, the dimensions and distribution of the grain carbides are the most important factors of the impact toughness.