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.     

Computer simulation for precipitation process of θ（Ni3V） and γ＇（Ni3Al） with microscopic phase-field method

With the microscopic phase-field model, the precipitation process of aged alloys was explored by computer simulation, which could clarify some discussional views on the precipitation mechanisms of alloys. The precipitation process of Ni75A12.5V22.5 alloy was studied. From the simulated atomic pictures, calculated order parameters and volume fraction of different precipitates, it＇s found that the θ ordered phase precipitates earlier than γ＇ ordered phase by congruent ordering＋spinodal decomposition mechanism and thus produces a nonstoicheometric θ single ordered phase. Then, the nonstoicheometric γ＇ phase is precipitated by a non-classical nucleation and growth mechanism at the APBS of θ phases. Meanwhile, both of them transform to stoicheometric ordered phases.     

Precipitation behavior of γ′ phase in superalloy FGH96 under interrupted cooling test

The precipitation behavior of γ′ phase,under various interrupt cooling tests after 1170℃,solution treatment was examined.The results indicate that the size of secondary γ′ precipitates increases with the decrease of interrupt temperature,and the shape changes from spherical to butterfly like.The fine tertiary γ′ can form either during the post cool air quenching at high interrupt-temperatures,or during the specified 5℃ min-1cooling.Air quenching at high temperatures cannot suppress further nucleation of tertiary γ′ phase.     

MICROSTRUCTURES AND THE STRUCTURE STABILITY OF INCONEL 725,A NEW AGE—HARDENABLE CORROSION RESISTANT SUPERALLOY

INCONEL725 is a highly corrosion resistant nickel based alloy capable of being age-hardened to high strength levels. The micro structure observations and the phase identification after a standard heat treatment were investigated. The results show that the precipitation phases include the strengthening phasesγ', γ", and 8 phase, primary carbide phase TiC, as well as M6 C carbide and a little extent MC (mainly TiC) precipitates which nucleate mainly at grain boundaries. An isothermal aging study was carried out on this alloy for up to 10 000 hours at 593℃. This additional aging did not affect the tensile strength. However, micro structures show that the thermal exposure has a little additional effect. With increasing the exposure time, the size of γ" phase slightly increases, almost no change for γ' phase, while M6C carbides precipitated at grain boundaries have an increased and complex tendency on a few grain boundaries. The experimental results illustrate the excellent structure stability of the ag     

Effect of Zr addition on precipitates in K4169 superalloy

In order to investigate the effect of Zr addition on the precipitations of K4169 superalloy,a manual electric arc furnace was used to prepare the superalloy with different Zr addition from 0.03wt.% to 0.07wt%.After standard heat treatment and long-time aging,the microstructures of the alloys were observed using XRD,SEM and TEM.The results show that Zr not only inhibits the precipitation of Laves phase at the grain boundary,but also significantly promotes the precipitation of earlobe-like γ’ and γ″ phases.After long time aging at 680℃ for 500 h,the γ″ phase grows up obviously and forms a γ’/γ’’ clad microstructure when the Zr addition is 0.03 wt.%.A large number of fine orbed γ’ particles precipitate in the grains and some γ″ phase transforms to disk-like δ phase when the Zr addition increases to 0.05wt.%.The nano-polycrystalline γ’ phase precipitates in the grains and there is a little δ phase when the Zr addition is 0.07wt.%.As the Zr addition increases,the amount of Laves phase at the grain boundary decreases at first,and then increases and forms flaky morphology.     

Effect of heat treatment on microstructure and high temperature tensile properties of cast nickel-base superalloy with high W, Mo and Nb contents

The microstructural features and high temperature tensile properties of M963 superaUoy at as-cast, as-solutioned and as-aged conditions were investigated in detail. The results show that the solution treatment at 1220℃ for 4 h,AC causes an increase in high temperature yield strength but a drastic drop in high temperature ductility due to the precipitation of both the secondary carbide M6C along grain boundaries and at the interdendritic regions and very fine γ‘ particles in the dendrite cores. Aging treatment following the solution treatment can improve the high temperature tensile properties of M963 superalloy due to the coaraing of the γ’ precipitate. One stage aging at 850℃ for 16 h following the solution treatment causes an increase in both strength and ductility d alloy M963, and two-stage aging of 1089 ℃/2 h, AC plus 850℃/16 h, AC following the solution treatment further increases the ductility d alloy M963 but slightly decreases its strength.     

Aging Precipitation Behavior and Mechanical Properties of Inconel 617 Superalloy

Aging precipitation behavior and mechanical properties of Inconel 617 superalloy aged at 760 ℃ for up to 10000 h were investigated. The results showed that the precipitates of the aged alloy are M23C6 and M6C carbides and γ phase. The carbide particles precipitated both at the grain boundaries and within grains, and the γ phase particles were situated at intragranular sites in the process of aging. The carbide particles were discontinuously dispersed at grain boundaries after aging for 3000 h, while after aged for 5000 h the carbide particles are merged. The precipitates inside grains remained stable even after aging for 10000 h. The hardness was increased for the alloy aged for 300 h up to 3000 h, which was resulted primarily from the precipitation of carbides as discrete particles both at the grain boundaries and inside grains. Small quantity γ precipitates were formed inside grains, to some extent, which contributed to an enhanced hardness. However, a decrease of the hardness was observed after aging for 5000 h. A significant drop in toughness of the alloy aged for 300 h was attributed to the reduction of the bonding interface strength when carbides precipitated at grain boundaries. Thereafter, the toughness decreased slowly with the prolonged aging time. The high temperature tensile properties of the aged alloy are rather stable even aged for 300-3000 h.     

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.     

CREEP CRACK GROWTH BEHAVIOR OF ALLOY 718 DURING HIGH TEMPERATURE EXPOSURE

Alloy 718 is a precipitation strengthened nickel-hased superalloy based on the precipitation of γ’’-Ni3Vb(DO22 structure) and γ‘-Ni3(Al, Ti) (Ll2 structure) phases. Creep crack growth rate (CCGR) was investigated after high temperature exposure at 593, 650 and 677℃ for 2000h in Alloy 718. In addition to the coalescence of γ‘/γ‘‘ and the amount increasing of 6 phase, the existence of a bcc chromium enriched α-Cr phase was observed by SEM, and the weight fraction of α-Cr and other phases were determined by chemical phase analysis methods. The CCGR behavior and regulation have been analyzed by means of strength and structure analysis approaches. The experimental results show higher the exposure temperature and longer the exposure time, lower the CCGR. This is probably attributed to the interaction of material softening and brittling due to complex structure changes during high temperature exposure. Therefore,despite α-Cr phase formation and amount enhancement were run in this test range. It seems to us a small amount of α-Cr will be not harmful for creep crack propagation resistance, which is critical for disk application in aircraft and land-based gas turbine.     

Microstructure and properties of modified and conventional 718 alloys

Continuing the effort to redesign IN718 alloy in order to provide microstructural and mechanical stability beyond 650 ℃, IN718 alloy was modified by increasing the Al, P and 13 contents, and the microstructure and mechanical properties of the modified alloy were compared with those of the conventional alloy by SEM and TEM. The precipitation of the grain boundaries of the two alloys is different. The Cr-rich phase, Laves phase and α-Cr phase are easily observed in the modified alloy. The γ＂ and γ＇ phases in the modified alloy are precipitated in a ＂compact form＂. The tensile strengths of the modified alloy at room temperature and 680 ℃ are obviously higher than those of the conventional one. The impact energy of the modified alloy is only about half of that of the conventional alloy. Ageing at 680 ℃ up to 1000 h lowers the tensile properties and impact energy of both the conventional and modified 718 alloys, except increasing the ductility at 680 ℃. It is concluded that the modified alloy is more stable than the conventional one.     

Precipitation behavior and effect of new precipitated β phase in AZ80 magnesium alloy

Granular precipitate that was a new kind of β-Mg17Al12 phase found in aged AZ80 wrought Mg alloy at all aging temperature was studied. The structure and precipitation behavior of this granular β-Mg17Al12 precipitate were studied by environmental scanning electron microscopy （ESEM） and transmission electron microscopy （TEM）. The effect of the granular precipitate on mechanical properties of AZ80 alloy was also studied. The new precipitate that was granular and nucleated both on grain boundaries （GBs） and twin boundaries, has the same crystal structure and lattice parameter as those of the continuous or discontinuous precipitated fl-Mgl7All2. And the nucleation and growth of the granular precipitate are faster than those of the other two precipitates at higher temperatures （above 583 K）, but are suppressed at lower temperatures （below 423 K）. At lower temperatures, the discontinuous β-Mg17Al12 precipitates firstly and the granular β-Mg17Al12 precipitates after aged more than 40 h. The crack is easily nucleated on the phase boundaries of granular phase and matrix because of the weak binding force. As a result, the strength and ductility ofAZ80 Mg alloy are decreased by the granular β-Mg1TAl12 precipitate.     

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.     

Effect of step-quenching on microstructure of aluminum alloy 7055

The effect of step-quenching on the microstructure of aluminum alloy 7055 after artificial aging was studied by hardness testing and transmission electron microscopy (TEM). Step-quenching leads to decomposition of solid solution and heterogeneous precipitation of equilibrium phase mainly on dispersoids and at grain boundaries; thus lower hardness after aging. Prolonging isothermal holding at 415 ℃ results in coarser and more spaced η phase particles at grain boundaries with wider precipitates free zone, and lower density of larger η′ hardening precipitates inside grains after aging. Isothermal holding at 355 ℃ results in heterogeneous precipitation of η phase both on dispersoids and at grain boundaries. Isothermal holding at 235 ℃ results in heterogeneous precipitation of η phase first, and then S phase. Precipitates free zones are created around these coarse η and S phase particles after aging. Prolonging isothermal holding at these two temperatures leads to fewer η′ hardening precipitates inside grains, larger and more spaced η phase particles at grain boundaries and wider grain boundary precipitates free zone after aging.     

Re-recognition of the Effects of Phosphorus and Boron on the γ’’ and γ’ Phases in IN718 Alloy

The effects of P and B on the matrix strength and precipitations of γ’ and γ’’ phases in the grain interior were re-recognized in this study. The combination addition of P and B markedly accelerated the precipitations of γ’ and γ’’ phases and strengthened the matrix of IN718 alloy when air-cooled from high temperature, while made no difference when waterquenched from high temperature. The effect of single addition of P on the precipitations of γ’’ and γ’ phases was the same with that of the combination addition of P and B, while the single addition of B had no effect on the precipitations of the two phases. Therefore, it was P rather than B which accelerated the precipitations of γ’ and γ’’ phases. P could take part in the precipitations of γ’ and γ’’ phases, which was revealed by electrochemical extraction and quantitative analysis of chemical composition. It also revealed that P atoms were dissolved in the c matrix to a relatively high degree at the temperature that γ’ and γ’’ phases began to precipitate, and consequently the precipitations of γ’ and γ’’ phases were accelerated. The first-principle calculation indicated that P decreased the formation enthalpies of γ’ and γ’’ phases when it occupied the Ni lattice sites in the two phases, which explained the effect of P on the γ’ and γ’’ phases.     

Computer Simulation of Ordering and Atom Clustering in Aging Binary AI-Li Alloy

Ordering and atom clustering in aging binary Al-Li alloy has been investigated by computer simulation through calculating the long range order (lro.) parameter and composition deviation order parameter from single-site occupation probabilities of Li atom. The results show that when the alloy lies in metastable region in the phase diagram ordering and atom clustering occur simultaneously. As the composition of the alloy increases ordering occurs earlier than atom clustering gradually. When the alloy lies in instable region atom clustering takes place after the congruent ordering completes. It has also been found that the incubation period of the phase transformation is shortened as the composition increases.     

Effect of second phase precipitation behavior on mechanical properties of casting Al-Cu alloys

The effect of the second phase precipitation behavior on the mechanical properties and fracture behavior of the modified casting Al-Cu alloys was investigated. The tensile strength of the alloys increases firstly and then decreases due to the appearance of θ＇ precipitation phases, which increases firstly and then become coarser with the aging time increasing from 10 h to 20 h at 155 ℃. The strength of the alloys reaches the peak, resulting from ,Ω and θ＇ precipitation phases, and decreases due to ,Ω phases becoming coarser and θ＇ precipitation decreasing with the aging time increasing from 10 h to 20 h at 165 ℃. ,θ phase becoming coarser and θ＇ precipitation decreasing result in the strength of the alloys drastically decreasing after aging at 175 ℃ for 20 h. The ductility remains high level with increasing aging time at 155 ℃. The ductility irregularly changes as aging time prolongs at 165 ℃. The ductility is very low and at the same time gradually decreases with increasing aging time at 175 ℃. The Al-Cu alloy with a promising combination of tensile strength and ductility of about 474 MPa and 12.0% after aging at 165℃ for 10 h is due to a dense, uniform distribution of,Ω precipitation phases together with a heterogeneous distribution of θ＇ precipitations.     

Microstructure and properties of A1-Cu-Mg-Ag alloy exposed at 200℃ with and without stress

The effect of stress on the microstructure and properties of an Al-Cu-Mg-Ag alloy under-aged at 165℃ for 2 h during thermal exposure at 200℃ was investigated. The tensile experimental results show that the remained tensile strength of both specimens at room temperature after being exposed at 200℃ with and without applying stress rises firstly, and then drops with the increasing of exposure time. The peak value of the remained strength reaches 439 MPa for non-stress-exposure for l0 h, and 454 MPa after being exposed with stress loaded for 20 h at 220 MPa. The elongation change is similar to that of strength. After being exposed for 100 h, specimen exposed at 220 MPa still remains a tensile strength of 401 MPa, larger than that exposed without applying stress. TEM shows that the microstructure of under-aged alloy is dominated by Ω phase mainly and a little θ＇ phase. The θ and Ω phases are believed competitive with increasing exposure time. The width of precipitation free zone（PFZ） increases and the granular second phase precipitates at grain-boundary correspondingly. It is shown that the mechanical properties of alloy decrease slightly and present good thermal stability after thermal exposure at 200 ℃ and 220 MPa for 100 h.     

EFFECT OF W ON MECHANICAL PROPERTIES OF 12%Cr HEAT-RESISTANT STEEL

The effect of W on mechanical properties of 12% Cr-W-V-Nb heat resistant steel at hightemperatures and room temperature is reported.The experimental results indicated that if theW content was about 2.2—3.0 wt-%,there was no obviously change of R.T.tensile strength,but impact toughness decreased with the rise of W content.On the other hand,the increase ofW content enhanced the short time stress rupture strength,but did not for the long time one.The increase of W have two effects on the precipitation behavior,promoting Laves formationof type Fe_2W,increasing the precipitated phase amount and speeding up the coarsening pro-cess of precipitated phase at high temperatures.The effect of W on the mechanical propertiesis closely associated with precipitation behaviors.When the rupture life is short,there has noenough time to coarsen the precipitated phases,so the increase of precipitated phases results instrengthening effect,i.e.the W increases the high temperature strength.After prolonged expo-sure,the evident coarsening took plaee,that decreased the effect of precipitation.     

Microscopic phase-field simulation for precipitation behavior of Ni-Al-Cr alloy during two-step aging

Based on the microscopic phase-field model, the pre-aging temperature effects on the precipitation mechanism and microstructure evolution during two-step aging for Ni75A19Cr16 alloy were simulated. The results show that the early precipitation mechanism of Ll2 phase is the mixed mechanism of spinodal decomposition and non-classical nucleation growth, whereas the early precipitation mechanism of DOzz phase is spinodal decomposition when the pre-aging temperature is 873 K. The early precipitation mechanism of LIz phase is non-classical nucleation growth, whereas the early precipitation mechanism of DOzz phase is spinodal decomposition when the pre-aging temperature is 973 K. Under the effects of elastic strain energy, the cubic particles exhibit directional alignment along [ 100] and [010] directions during the late precipitation, which is more obvious at lower pre-temperature. DOzz phases appear earlier than Llz phases under these two kinds of precipitation processes; and the nucleation incubation time becomes long with the increase of pre-temperature.     

Mechanical properties and microstructures of hot extruded AE42 alloy with addition of zinc

Mechanical properties and microstructures of AE42 magnesium alloy with addition of Zn and subjected to hot extrusion at 370 ℃ and an extrusion ratio of 8：1 were investigated. The results show that for the AE42 alloy, the addition of Zn can obviously improve its elongation as well as the ultimate tensile and yield strengths below 150 ℃. The addition of Zn can refine the mierostructure of the AE42 alloy, and result in the precipitation of Mg17Al12 and MgZn2 phases. Due to the addition of Zn to the AE42 alloy, the amount of Al11RE3 phase decreases, while the Al11RE3 phase becomes short rod-shaped from acicular and block, and distributes along the grain boundaries, which will have a stronger effect on the tensile properties of the alloy at elevated temperature. In addition, with the presence of MgZn2 precipitates, the sliding of grain boundaries is restrained and the strength of the alloy gets enhanced.