Evolution mechanism of recrystallization in a nickel-based single crystal superalloy under various cooling rates during heat treatment

The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200℃ for 4 h with various cooling rates were studied.Results show that the thickness of recrystallization layer decreases with the increase of cooling rate.In addition,the microstructures ofγ′phase in the recrystallization region are different in various cooling rates.In the high cooling rates(70,100℃·min^-1),small size and high volume fraction ofγ′phases are formed in the recrystallization region.It is also found that irregular fine secondaryγ′phases are precipitated between matrix channels with an average size of 150 nm in the original matric(100℃·min^-1).The sizes of the secondaryγ′phase decrease with the increase of cooling rate.In contrast,large size and small volume fraction ofγ′phases are formed in the recrystallization region,and a grain boundary layer is formed under a low cooling rate(10℃·min^-1).The evolution mechanism of recrystallization at various cooling rates during heat treatment is analyzed.     

Influences of Cooling Rate After Solution Treatment on Microstructural Evolution and Mechanical Properties of Superalloy Rene 80

Influences of the cooling rate after solution treatment on microstructural evolution and mechanical properties of Rene 80 nickel-based superalloy were investigated by optical microscopy,scanning electron microscopy,transmission electron microscopy and mechanical test.Results showed that the high cooling rate decreased the size of secondary γ′particles in the supersaturated matrix,but led to a high coarsening rate of γ′ particles during subsequent aging treatment.Despite various coarsening rates,the size and morphology of γ′ particles in the final microstructures of all samples were close due to the long enough holding time to an equilibrium state.During the aging of 870 ℃/2 h,primary MC started to decompose with the carbide reaction:MC + γ→ M_6C or M_(23)C_6 + γ′.And a number of observations showed that the coarsening of γ′ particles on grain boundaries resulted in the depletion of γ′ during aging treatment.The test results indicated that high cooling rate resulted in the presence of quench crack,and the air cooling method following solution treatment was an optimum heat treatment method for Rene 80.     

Simulation of interphase boundary of Ni75AlxV25-x alloys using microscopic phase-field method

The evolution of ordered interphase boundary （IPB） of Ni75AlxV25-x alloys was simulated using the microscopic phase-field method. Based on the atomic occupation probability figure on 2D and order parameters, it was found that the IPB formed by different directions of θ phase has great effect on the precipitation of γ′ phase. The γ′ phase precipitated at the IPB that is formed by [100]θ direction where the （001）θ plane is opposite, and then grows up and the shape is strap at fmal. The IPB structure between γ′phase and θ phase is the same. There is no γ′ phase precipitate at the IPB where the （002）θ and （001）θ planes are opposite, the ordered IPB is dissolved into disordered area. There is γ′ phase precipitation at the IPB formed by the [001]θ and [100]θ directions, and the IPB structure is different between γ′ phase and the different directions of θ phase. The IPB where （001）γ′, and （100）θ plane opposite does not migrate during the γ′ phase growth, and γ′ phase grows along [100]θ direction.     

Active Eutectoid Decomposition of α → γ + τ _1 and the Morphological Evolution in a Ru-Containing TiAl Alloy

The active eutectoid decomposition and its morphological evolution during slow cooling and isothermal holding were investigated in a Ru-containing Ti Al alloy. The fine τ 1/γ active eutectoid nodules precipitated at α grain boundary and interior during water quenching. The active eutectoid microstructure evolved from sheaf-like, granular to bulky net-like sluggish eutectoid morphology gradually with the decrease in quenching/holding temperatures. The active eutectoid reaction occurs from 1220 to 1290 ℃, while the beginning temperature of sluggish eutectoid locates at 1305 ± 5 ℃. The combination of the intact τ_1 phase and immature nano-sized γ laths suggests a short incubation period of τ_1 phase in the active type. Furtherly, the semiquantitative estimation shows that the growth velocity of active eutectoid is about ninety times higher than sluggish type. In addition, a representative feature of γ phase abruptly ripening along {111} crystallographic plane was also observed in the active eutectoid.     

Characterization of γ′ precipitates in a nickel base superalloy quenching from aging temperature at different rates

The characteristics of γ′ precipitates in a superalloy quenched from 1050°C at different rates were investigated using field emission scanning electron microscope(FESEM).When quenched from 1050°C, the size of primary aging γ′ precipitates has a small increase in the specimens that experienced iced-brine-quenching, oil-quenching, and air-cooling-quenching conditions and a drastic increase in the specimen that experienced a furnace-cooling-quenching condition.The cooling γ′ precipitates have unimodal distributions after quenching at the air-cooling rate and bimodal distributions after quenching at the furnace-cooling rate, but there are not these distributions in the specimens that experienced iced-brine-quenching and oil-quenching conditions.When aging at 760°C, the size of primary aging γ′ precipitates appears unaffected in the specimens that experienced iced-brine-quenching, oil-quenching, and air-cooling-quenching conditions.However, it has a drastic increase in the specimen that experienced a furnace-cooling-quenching condition, and it is interesting that the bigger cooling γ′ precipitates have a coalescence and octodendritic shape.The microhardness study indicates that the hardness has no variation in the specimens that experienced iced-brine-quenching, oil-quenching, and air-cooling-quenching conditions and has a drastic decrease in the specimens that experienced a furnace-cooling-quenching condition and obtains the minimum microhardness value 390.8 HV.     

Gas Quenching during Age Hardening of the Aluminium Casting Alloy Al-7Si-0.3Mg

For quenching of age hardenable aluminium alloys today predominantly aqueous quenching media are used,which can lead due to the Leidenfrost phenomenon to a non-uniform cooling of the parts and thus to thermal stresses.Particularly at thin-walled or complex shaped parts local plastic deformations can occur by the uneven thermal stresses. In relation to the conventional quenching procedures in aqueous media, gas quenching exhibits a number of technological,ecological and economical advantages. In comparison to liquid quenching media, gas does not change its phase during quenching. Moreover, the cleaning problem of the parts can be avoided. The quenching intensity can be adjusted by the variable parameters gas pressure and gas speed as well as the kind of gas and thus can be adapted to the requirements of the alloy. By the higher uniformity and the better reproducibility, gas quenching offers a high potential to reduce distortion. The goal of these investigations is to clarify, if the cooling rate during gas quenching is sufficient to obtain the specific required strength after age hardening of the alloy Al-7Si-0.3Mg. For this purpose different tests in high-pressure gas quenching facilities, gas nozzle fields and water quenching baths were performed.     

ON THE ORDERING TRANSFORMATIONS IN Ti_3AL-Nb ALLOY

Study was made of the behaviour of ordering transformation in Ti_3AI-Nb alloy,including the ordering at high temperatures,the transformation of high temperature β-phase during cooling,and the decomposition of metastable β-phase during aging.The results show that the ordered primary α_2 and high temperature β in alloy form at 1060℃.The transformation of high temperature β-phase proceed by β→α_2+ω type during cooling,and the decomposition of metastable β and ω type proceeded by(β+ω)_(metustabte)→(α_2+β)_(stable)during aging at 700℃.     

Effect of tempering temperature on microstructure and mechanical properties of high boron white cast iron

The effect of different tempering temperatures on the microstructure and mechanical properties of airquenched high boron white cast iron was studied.The results indicate that the high boron white cast iron comprises dendritic matrix and inter-dendritic M 2 B boride;and the matrix comprises martensite and pearlite.After quenching in the air,the matrix is changed into lath martensite;but only 1-μm-size second phase exists in the matrix.After tempering,another second phase of several tens of nanometers is found in the matrix,and the size and quantity increase with an increase in tempering temperature.The two kinds of second precipitation phase with different sizes in the matrix have the same chemical formula,but their forming stages are different.The precipitation phase with larger size forms during the austenitizing process,while the precipitation phase with smaller size forms during the tempering process.When tempered at different temperatures after quenching,the hardness decreases with an increase in the tempering temperature,but it increases a little at 450 ℃ due to the precipitation strengthening effect of the second phase,and it decreases greatly due to the martensite decomposition above 450 ℃.The impact toughness increases a little when tempered below 300 ℃,but it then decreases continuously owing to the increase in size and quantity of the secondary precipitate above 300 ℃.Considered comprehensively,the optimum tempering temperature is suggested at 300 ℃ to obtain a good combination of hardness and toughness.     

PHASE TRANSFORMATION IN RAPIDLY SOLIDIFIED AI-Cu-Fe ALLOY

By means of rapid solidification, two Al_(65)Cu_(20)Fe_(15) powders were prepared with water and liquid N_2 as the respective cooling agent. Both powders are composed of a qnasicrystalline icosahedral phase and a crystalline hexagonal phase, with the water-cooled alloy having a higher crystalline phase content. In the isothermal an nealing process, the crystalline phase in the water-quenched alloy begins to decrease at 500℃ and then disap pears at 600～700℃. At about 800℃, new crystalline phases form, and at 900℃, the quasicrystalline phase disappears. Conversely, in the liquid N_2 quenched alloy, the quasicrystalline phase starts to decrease at about 500℃. and the hexagonal phase decomposes into new crystalline phases. At 700～800℃, the quasicrystalline phase disappears. For the water-cooled sample, the quenching at 100～200C makes the crystalline to quasicrystalline phase transformation start at a lower temperature and the crystallization of the quasicrystal occur at a higher temperature. For the liquid N_2 quenched alloy, the quenching at 100～400℃, did not affect its phase transformation at high temperature.     

Cooling γ precipitation behavior and strengthening in powder metallurgy superalloy FGH4096

Two cooling schemes (continuous cooling and interrupted cooling tests) were applied to investigate the cooling γ precipitation behavior in powder metallurgy superalloy FGH4096.The effect of cooling rate on cooling γ precipitation and the development of γ precipitates during cooling process were involved in this study.The ultimate tensile strength (UTS) of the specimens in various cooling circumstances was tested.The experiential equations were obtained between the average sizes of secondary and tertiary γ precipitates,the strength,and cooling rate.The results show that they are inversely correlated with the cooling rate as well as the grain boundary changes from serrated to straight,the shape of secondary γ precipitates changes from irregular cuboidal to spherical,while the formed tertiary γ precipitates are always spherical.The interrupted cooling tests show that the average size of secondary γ precipitates increases as a linear function of interrupt temperature for a fixed cooling rate of 24℃/min.The strength first decreases and then increases against interrupt temperature,which is fundamentally caused by the multistage nucleation of γ precipitates during cooling process.     

Microstructures and Hardness of 8CrWMoV Steel with Multiple Types of Ultra Fine Carbides

The structure and hardness of 8CrWMoV steel with multiple types of ultra fine carbides are studied after annealing, quenching and tempering in this paper. The results show that multiple types of carbides M3C, M7C3, M23C6, M6C and MC were observed in the annealed steel. Nucleation and coalescence of new carbides, partial dissolution of original carbides in 7 phase region during annealing at 800-840℃, result in ultra-fine carbides. Average size of the carbides is 0.33～0.34μm in the steel annealed at 800～840℃. Because M3C and M23C6 dissolve easily in austenite, the high hardness HRC63～65 can be obtained by quenching at 840～860℃. Un-dissolved carbides M6C and MC (VC) can effectively prevent the coarsening of austenitic grain, and conduce to obtain very fine martensite. The retained austenite can be easy to decompose during tempering at low and middle temperature due to the precipitation of multiple types of carbides and the good tempering-resistance of the steel is obtained. The microstructure and property of the steel after heat treatment can be accurately explained by calculating based on phase equilibrium thermodynamic.     

Influence of heat treatment on fracture and magnetic properties of radially oriented Sm2Co17 permanent magnets

The quenching, fracture and aging treatment of radially oriented Sm2Co17 ring magnets were investigated. The results indicate that the ring magnets have obvious anisotropy of thermal expansion, which easily leads to the splits of the magnets during quenching. The fracture is brittle cleavage fracture. The difference （Aa） of the expansion coefficient reaches the maximum value at 800-850 ℃. So, various quenching processes at different steps are adopted in order to reduce the splits. When the magnets are aged, 1：5 phase precipitates from the 2：17 matrix phase and forms a cellular microstructure with 2：17 phase. BHmax and JHc reach the maximum value 226 kJ/m^3 and 2 170 kA/m after being aged at 850 ℃ for 4 h and 8 h, respectively. The aging treatment at 850 ℃ has little influence on remanence（Br）, which can always keep a high value （≥1.0 T）. Through appropriate heat treatment, the ring magnets have uniform cellular microstructure and excellent magnetic properties： Br ≥ 1.0T, JHc ≥2 100 kA/m, BHmax ≥ 220 kJ/m^3.     

Computer simulation of γ＇ and θ phase precipitation of Ni-Al-V alloy using microscopic phase-field method

The precipitation processes of γ‘ and θ phases in Ni75Al6.5 V18.5 alloy were simulated at different temperatures and the precipitation sequence of two phases and morphological evolution were investigated. The simulation demonstrates that the two phases precipitate simultaneously at high temperature and γ‘ phase precipitates earlier than θ phase at 1000 K and 1120 K. With the temperature decreasing, the velocity of precipitation quickens, the quantity of θ phase increases and the size reduces; but the volume fraction increases, the quantity of phase increases and the size reduces as well. The two phases nucleate and grow independently at high temperature and the θ phase precipitates from the boundaries of γ‘ phase at 1000 K and 1120 K. We also find that there are many kinds of domain boundaries between the same and different phases. The results of average deviation of composition and average absolute long range order parameter show that the ordering and compositional clustering of γ‘ phase happen simultaneously at high temperature, the congruent ordering occurs prior to spinodal decomposition at 1 000 K and 1 120 K and the ordering advances and quickens as the temperature decreases. Ordering and compositional clustering of θ phase occur simultaneously at each temperature and are quickened with temperature decreasing.     

Effect of melt superheat on microstructure of Al4Fe2Mn1.5 Monel alloy

The effect of melt superheat on microstructure of Al4Fe2Mn1.5 Monel alloy made by vacuum melting method was studied. The results show that the alloy consists of dendritic γ matrix and γ′ phase, wherein γ′ phase has two morphologies at different melt superheat. One is divorced eutectic γ′ which distributes in the interdendritic area, the other distributes dispersedly in single particle on the dendritic arm and exists in the petalform shape in the transition area between dendritic arm and interdendritic area. With the increase of superheat, the dendrite becomes finer, the primary dendritic arm is melted off and the secondary dendritic arm spacing decreases. The size of γ′ phase distributed on the dendritic arm becomes smaller and the divorced eutectic γ′ phase increases.     

Microstructure Evolution of a Single Crystal Nickel-Base Superalloy During Heat Treatment and Creep

Microstructure evolution of a single crystal nickel-base superalloy during heat treatment and tensile creep at 1010℃ and 248 MPa for 30h was observed and analyzed. Internal stresses because of lattice mismatch between γ and γ‘ phase provided the driving force for γ‘ shape evolution during heat treatment. More than 65 vol. % distorted cubic γ‘ phase keeping coherency with the γ matrix precipitated after solution at 1295℃ for 32h. The shape of γ‘ phase was perfectly cubic with increasing precipitate size during the two-step aging treatment. Due to the applied stress and intemal stress field the continuous γ-γ‘ lamellar structure perpendicutar to the apptied stress was fonmed after 30h tensile creep.     

Effect of directional solidification process on microstructure and stress rupture property of a hot corrosion resistant single crystal superalloy

The influences of different directional solidification processes, i.e., the high rate solidification(HRS) and liquid metal cooling(LMC), on microstructure and stress rupture property of DD488 alloy were investigated. The DD488 alloy was directional solidified by both HRS and LMC processes. The microstructure and stress rupture properties at 980 ℃/250 MPa were investigated by using optical microscopy(OM), scanning electron microscopy(SEM), electron microprobe analyzer(EPMA), transmission electron microscopy(TEM) and stress rupture testing. The results indicated that the LMC process refined the primary dendrite arm and decreased the microporosity volume fraction and solidification segregations of Cr and Co in as-cast DD488 alloy. After standard heat treatment of 1,260 ℃/4 h, AC(air cooling) + 1,080 ℃/4 h, AC + 870 ℃/24 h, AC, the γ′ morphology in LMC alloy was more cuboidal than that in HRS alloy, and the γ′ volume fraction of LMC alloy was higher than that of HRS alloy. The stress rupture life at 980 ℃/250 MPa of HRS alloy was 76.8 h, and it increased to 110.0 h in LMC al oy. The LMC process increased the stress rupture life due to the higher γ′ volume fraction, more perfect rafting structure and finer interfacial dislocation networks.     

Precipitation process and its effects on properties of aging Cu–Ni–Be alloy

The precipitation process of aged Cu-Ni-Be alloy was investigated by X-ray diffraction （XRD）, trans- mission electron microscopy （TEM）, and high-resolution transmission electron microscopy （HRTEM）. The tensile strength, yield strength, and electronic conductivity of this alloy after aging were also studied. The precipitation sequence of the C17510 alloy aged at 525 ℃ is supersat-urated solid solution→G.P zones→ γ″-γ′→ γ. This transformation can be achieved by the accumulation of Be-atom layers. The G.P zones are composed of disk-shaped monolayers of Be atoms, which are formed on （001） matrix planes. The intermediate γ″ precipitate is nucleated in the G.P zones. The γ″ and γ′ precipitates have the same orientation relationship with matrix, e.g., （110）p｜｜（100）M,[001]p｜｜[001]M. The tensile strength of specimen shows a maximum during the aging process and then continuously decreases if the specimen is over aged. The strengthening effect of γ′ phase precipitated in aging at 525 ℃ for 4 h is calculated to be 436 MPa according to the Orowan strengthening, which is quite consistent with the experimental data.     

Computer simulation of γ''and θ phase precipitation of Ni-Al-V alloy using microscopic phase-field method

The precipitation processes of γ'and θ phases in Ni75 Al6.5 V18.5 alloy were simulated at different temperatures and the precipitation sequence of two phases and morphological evolution were investigated.The simulation demonstrates that the two phases precipitate simultaneously at high temperature and γ'phase precipitates earlier than θ phase at 1 000 K and 1 120 K.With the temperature decreasing,the velocity of precipitation quickens,the quantity of θ phase increases and the size reduces;but the volume fraction increases,the quantity of phase increases and the size reduces as well.The two phases nucleate and grow independently at high temperature and the θ phase precipitates from the boundaries of γ'phase at 1 000 K and 1 120 K.We also find that there are many kinds of domain boundaries between the same and different phases.The results of average deviation of composition and average absolute long range order parameter show that the ordering and compositional clustering of γ'phase happen simultaneously at high temperature,the congruent ordering occurs prior to spinodal decomposition at 1 000 K and 1 120 K and the ordering advances and quickens as the temperature decreases.Ordering and compositional clustering of θphase occur simultaneously at each temperature and are quickened with temperature decreasing.     

The microstructure of a single crystal superalloy after different aging heat treatments

To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated.Results show that for the experimental alloy, even the homogenization-solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ' phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ' cubes. For specimens with primary aging heat treated at 1120 ℃,γ' morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 ℃,γ'phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ'phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ' phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ' phase. As a result, the γ'-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ' size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then,high-temperature properties can be improved.