Precipitation Of Cu-rich Phase Research Articles

EFFECT OF LONG-TERM THERMAL AGING AT 475 °C ON EMBRITTLEMENT OF Fe-32Cr-4Ni-4Cu MODEL ALLOY

The microstructure evolution and mechanical properties of a Fe-32Cr-4Ni-4Cu model alloy after long-term thermal aging at 475 °C was investigated in this study. The spinodal decomposition of the model alloy during thermal aging and the crystal structure evolution of Cu-rich precipitates were observed and analyzed using high-resolution transmission electron microscopy (TEM). It was revealed that after long-term thermal aging, the crystal structure of the precipitated Cu-rich phases in the Fe-32Cr-4Ni-4Cu model alloy transformed from a BCC structure to a multi-twin 9R or 2H structure, ultimately transforming into a stable FCC structure. During long-term thermal aging at 475 °C, the precipitated Cu-rich phase and the Cr-rich ’-phase produced by spinodal decomposition cause an increase in the hardness of the model alloy. The hardness of the model alloy in the early stage of thermal aging greatly depends on the Cu-rich precipitates. After continuous thermal aging for 1000 h, the hardness of the alloy depends on the Cr-rich ’-phase. The results of an oscillographic impact test indicate that the fracture mechanism of the model alloy after thermal aging at 475 °C is brittle fracture, which is mainly influenced by the precipitation of Cu-rich phases during the thermal aging process.

Co-precipitation mechanism of Cu-rich phase and κ-carbide precipitates in Fe-28Mn-10Al-1C-3Cu austenitic low-density steel

The excellent comprehensive properties of Fe-28Mn-10Al-1C-3Cu austenitic low-density steel are related to its main precipitates of Cu-rich phase and κ-carbide particles. The co-precipitation mechanism of Cu-rich phase and κ-carbide particles in Fe-28Mn-10Al-1C-3Cu austenitic low-density steel are investigated. Adding Cu to Fe-28Mn-10Al-1C austenitic low-density steel can promote the precipitation of κ-carbide. The κ-carbide can also promote the precipitation of Cu-rich phase with FCC structure. The κ-carbide preferentially precipitate compared to Cu-rich phase. The κ-carbide and Cu-rich phase have a cube-on-cube orientation relationship with the austenitic matrix: [110]γ//[110]κ//[110]Cu and (111)γ//(111)κ//(111)Cu.

Spinodal decomposition and radiation damage of a FeCuMnNi high-entropy alloy

The precipitation of Cu-rich phases during the irradiation process will reduce radiation resistance in Fe-Cu-Mn-Ni reactor pressure vessel steels. Therefore, high-entropy alloys with excellent radiation resistance are expected to replace reactor pressure vessel steels and become structural materials for fusion and generation IV fission reactors. This study investigated the phase formation, spinodal decomposition tendency, and radiation resistance of the FeCuMnNi high entropy alloy using a multi-scale method. The results show that FeCuMnNi alloy has a dual-phase face-centered-cubic structure. Here, for the first time, we found that the precipitation phase of the FeCuMnNi high-entropy alloy during the irradiation process is a Mn-rich phase rather than a Cu-rich phase found in conventional Fe-Cu-Mn-Ni reactor pressure vessel steels. Mn separates from the Cu-Mn-rich phase and forms Fe-Mn-Ni clusters with a Fe-Ni phase during irradiation.

Effect of heat treatment on precipitation behavior of second phase and property evolution of martensitic stainless steel

The microstructural optimization for 15–5PH stainless steel by heat treatment to enhance toughness and corrosion resistance were investigated. The results indicate that Cu-rich nanoparticles with a size of less than 30 nm are homogeneously dispersed within the matrix, exhibiting a K-S orientation relationship with the martensite matrix during the aging treatment. Additionally, reversed austenite is found to be distributed at the martensite lath boundaries, with the maximum content of 1.62% being observed at 550 °C. With increasing aging temperature, the dispersion strengthening effect caused by the precipitation of Cu-rich phases gradually weakens. After immersion in a 6% FeCl3 solution for 12 h, the greatest weight loss is observed in the 600 °C treated samples, followed by those treated at 450 °C, 550 °C, and 1050 °C.

Dissolution and precipitation of copper-rich phases during heating and cooling of precipitation-hardening steel X5CrNiCuNb16-4 (17-4 PH)

Continuous heating transformation (CHT) diagrams and continuous cooling transformation (CCT) diagrams of precipitation-hardening steels have the drawback that important information on the dissolution and precipitation of Cu-rich phases during continuous heating and cooling are missing. This work uses a comparison of different techniques, namely dilatometry and differential scanning calorimetry for the in situ analysis of the so far neglected dissolution and precipitation of Cu-rich phases during continuous heating and cooling to overcome these drawbacks. Compared to dilatometry, DSC is much more sensitive to phase transformation affecting small volume fractions, like precipitation. Thus, the important solvus temperature for the dissolution of Cu-rich phases was revealed from DSC and integrated into the CHT diagram. Moreover, DSC reveals that during continuous cooling from solution treatment, premature Cu-rich phases may form depending on cooling rate. Those quench-induced precipitates were analysed for a broad range of cooling rates and imaged for microstructural analysis using optical microscopy, scanning electron microscopy and transmission electron microscopy. This information substantially improves the CCT diagram.

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

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.

Effect of Cobalt on the Microstructure and Corrosion Behavior of Martensitic Age-Hardened Stainless Steel

To evaluate the influence of cobalt on the microstructure and pitting behavior of martensitic age-hardened stainless steel (MASS), the microstructures and electrochemical behaviors were characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, potentiodynamic curve, electrochemical impedance spectroscopy, and Mott–Schottky analysis. Results indicated that adding cobalt reduced the grain size of the original austenite, refined the lath martensite, inhibited the precipitation of Cu-rich phases, and increased the content of reversed austenite. Compared with the cobalt-free steel, the cobalt-containing steel has a higher pitting potential, lower corrosion current density, and higher pitting resistance. The primary components of the passive film grown on the cobalt-free steel included (Cr, Fe) oxides, such as Cr2O3, Cr(OH)3, FeOOH, and Fe2O3. The cobalt-containing steel included the aforementioned components as well as Co, which was attributed to the formation of the passive film in the forms of CoFe2O4, Co(OH)2, and CoO. In addition, the synergy of the refined lath martensite and the precipitation of the Cu-rich phase induced an increase in corrosion resistance in MASS containing cobalt.

Phase-field modeling of microstructure evolution of Cu-rich phase in Fe–Cu–Mn–Ni–Al quinary system coupled with thermodynamic databases

Quinary phase-field model was extended and constructed, and the diffusion-controlled phase decomposition and morphology in thermal aging Fe–Cu–Mn–Ni–Al quinary system coupled with CALPHAD thermodynamic databases were successfully studied. The effects of Manganese composition on the morphology, volume fraction, number density, particles size and growth and coarsening of Cu-rich precipitates were investigated systematically. The simulation results showed that Cu-rich α phase was firstly formed, and the Ni, Al and Mn atoms were partitioned to the Cu-rich phase, leading to the formation of a Ni–Al–Mn-rich intermetallic ring/Cu-rich core precipitate morphology and final conversion of Cu-rich phase to Cu-rich γ phase. The curves of volume fraction and free energy variation with simulation time indicated that Mn element can promote nucleation driving force to overcome the nucleation barrier in initial stage and can also accelerate the precipitation of Cu-rich phase and promote the growth and coarsening processes. Through the analysis of number density (ND) and average particle size (APS), we found that the values of ND and APS of Cu-rich particles increase with the rising of Mn content, which indicates that the higher Mn content will boost the nucleation and growth rate. Moreover, the time exponent at the later precipitation was 0.41, 0.42 and 0.37, respectively, which had deviations with the 0.33 of the LSW’s value, and the growth and coarsening of Cu-rich precipitates were under the mixed mechanisms of Ostwald ripening and coalescence coarsening of neighboring precipitates. These results provided useful information for the preparation of multi-component alloyed steel with excellent mechanical properties to some extent.

Effect of electromagnetic interaction on microstructure and corrosion resistance of 7075 aluminium alloy during modified indirect electric arc welding process

Abstract The effects of applying an electromagnetic interaction of low intensity (EMILI) on the microstructure and corrosion resistance of 7075-T651 Al alloy plates (13 mm in thickness) during modified indirect electric arc (MIEA) welding were investigated. The welding process was conducted in a single pass with a heat input of ∼1.5 kJ/mm. The microstructural observations of the welds were correlated with the effect of EMILI on the local mechanical properties and the corrosion resistance in natural seawater by means of microhardness measurements and electrochemical impedance spectroscopy, respectively. Microstructural characterization of the welds revealed a grain refinement in the weld metal due to the electromagnetic stirring induced by EMILI of 3 mT during welding. In addition, observations in the scanning electron microscope showed that the precipitation of Cu-rich phases and segregation of eutectics were reduced in the heat affected zone (HAZ) also as an effect of EMILI. The high corrosion dissolution of the 7075-T651 welds in natural seawater and the extent of overaging in the HAZ were reduced when welding with EMILI of 3 mT. Thus, EMILI along with the MIEA technique may lead to welded joints with better microstructural characteristics, improved mechanical properties in the HAZ and reduced electrochemical activity.

Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys

Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFexNi (x = 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFexNi (x = 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (ΔT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (ΔTcrit) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe1.5Ni and 293 K for CoCrCuFe2Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFexNi alloys prepared from melts with a small undercooling (ΔT < ΔTcrit). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, ΔT in excess of the critical value, ΔTcrit. A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquid–solid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFexNi (x = 1.0, 1.5, 2.0) alloys.

Experimental Investigation Of Modified Heat Treatment Of AK64-Type Al-Si-Cu Sand Cast Alloy

Abstract An experimental investigation was conducted to observe and analyze the microstructural evolution of phases present in the AK64 Al-Si-Cu alloy subjected to a modified T6 heat treatment (HT). The AK64 alloy1 is the Polish alternative of the A319.0 ASM standard aluminium alloy. The modified T6 HT schedule with a higher temperature and shorter duration was applied in the solutioning process and lower quenching and higher artificial ageing temperature than the prescribed by the ASM standard were used. The cooling curves registered during the liberating of overheating and solidifying processes give important information on nucleation temperatures for the Al dendrite network, Al-Si eutectic reaction and precipitation of Cu-rich phases. Comparison of the as-cast and heat treated microstructures revealed predicted microstructural changes and also partial fragmentation of the Fe-rich phases was observed after the application of the modified HT programme.

An Investigation on Precipitation Behaviour of Cu-Rich Phase in Super304H Heat-Resistant Steel by Three Dimensional Atom Probe

Super304H, a Cu-containing 18Cr-9Ni-3CuNbN heat-resisting steel is wildly used as an superheater/reheater tube material for ultra-super-critical (USC) power plants all over the world. It is recognized that the Cu-rich phase is an important strengthening phase for Super304H. However, the detail precipitation behaviour and its strengthening effect are still not very clear. Investigated material was taken from routine production and was aged at 650°C for different times. The precipitation of Cu-rich phase in Super304H was studied by three dimensional atom probe (3DAP) and TEM. Experimental results show that Cu-rich clusters have been formed at very early stage of 650°C aging. The Cu-rich particle images have been clearly caught just after 650°C aging for 5h. The Cu atoms gradually concentrate to Cu-rich particles and the other elements (such as Cr, Ni etc) diffuse away from Cu-rich particles to γ-matrix with the increasing of aging time. The Cu-rich particle size and its density have been determined as a function of aging time. Cu-rich particles still keep nano-size and distribute homogenously in grains even after long time (1,000h) aging, which is one of the most important reasons for keeping good strength of Super304H heat-resistant steel at high temperatures.

17-4PHステンレス鋼の析出挙動と二段時効処理

The precipitation behavior of 17-4PH stainless steel, aged at 400-900°C for 0.1-4 h, was studied to understand the strengthening effect of mainly Cu and Nb. MC, M6C, and Cu-rich phases were identified through the x-ray diffraction of the extracted residue, the measurement of electric resistivity, and TEM observation. Among these, the Cu-rich phase was most finely (around 20 nm) dispersed in the matrix, and was supposed to have a dominating effect to strengthen the steel. The precipitation of Cu-rich phase was considerably rapid to start, showing the nose of a c-curve with a shorter aging condition than 700°C ×0.1 h, and the high temperature limit of precipitation being about 700°C.The Cu-rich phase was supposed to have the largest effect for strengthening at the earlier stage of precipitation, which was clarified using the tempering parameter separating the age hardening and the matrix softening by martensite recovery. A quantitative discussion, based on a morphological study of microstructure, was carried out through the mean free path calculation for dislocation. Also, several combinations of high-temperature and low-temperature aging (2-step aging) was tried for effective age hardening, and conditions for a better-balance of tensile strength, hardness and ductility than the H900 heat treatment in JIS was obtained. The difference between single and double-step aging was discussed.

微量Ｚｎを添加した２０９０および２０９１アルミニウム合金の孔食挙動

Effects of Zn addition, stretching prior to aging and aging condition on the corrosion behavior of 2090 and 2091 alloy sheets have been studied using electrochemical measurements and total immersion-corrosion tests. The stretching prior to the peak-aging, which results in a uniform precipitation of Cu rich phases within grains, reduces the susceptibility to intergranular corrosion, having a more marked effect in 2090 alloy. The combination of stretching and Zn addition of 0.7% or over facilitates pitting dissolution within grains and enhances well general corrosion. The corrosion rates obtained from the total immersion test increase with the progress of aging. 2090 alloy has a lower corrosion rate and a higher resistance to localized pit than 2091 alloy. The results of the immersion test show a good correlation with the polarization properties.

17Cr-4Ni-4Cu型不銹鋼に関する研究（第2報）冷間圧延材の諸性質に及ぼすNi, Cu, Nの影響

The effect of Ni,Cu and N content on the properties of 17Cr-4Ni-4Cu type stainless steels were studied from a point of view giving the wrought steels a better combination of strength and corrosion resistance by the co-operation of cold-rolling and age-hardening. The following results were obtained. (1) The Ms temperature is lowered and the retained austenite is increased with increasing content of Ni, Cu or N in the steels. (2) Age-hardening in the tempering of the steels is larger as the Ni content is decreased or the Cu content is increased. Cold-rolling promotes the age-hardening and gives higher hardness. (3) Especially the steel containing N hardens significantly by the rolling and the aging treatment, which probably owes to the superimposition of the large work-hardening intensified by both the formation of martensite containing N and the hardening caused by the precipitation of Cu-rich phase and nitride from the martensite. (4) Increase of Cu- and Ni-content in the steels modifies considerably the resistance of the steels to boiling H2SO4 solution. Solution-quenching at 1000° followed by cold-rolling and tempering at below 440° does not show any deteriorating effect on the corrosion resistance, a result differing from that with 17-7 and 18-8 steels. (5) In the range of the present work the best results were obtained in the steel bearing 17Cr-4Ni-4Cu-0.17N.