γ/γ＇ interface strengthening of Re in single crystal superalloys

Based on analysis of three alloys with various Re contents, the effects of Re on the interfacial structure characteristic and interracial dislocations of γ/γ＇ in single crystal superalloys were investigated. A high resolution transition electron microscopy （I-IRTEM） technique was used to detect the structure characteristic of γ/γ＇ interface. The interfacial dislocations of γ/γ＇ were analyzed with TEM. The results show that as Re content increases, the interfaces of γ/γ＇ become orderly, the atomic arrangements at interfaces are more uniform, the number of mismatch dislocations increases and transition areas between γ, and γ＇ phases become narrow. That Re changes the interracial structure promotes the formation of dense., regular and homogeneous interfacial dislocation networks in short time during creep at 1 100 ℃ and 140 MPa, which results in strengthened γ/γ＇ interface at elevated temperature.     

Turning machining induced microstructural stability of a high Nb-containing TiAl alloy during high temperature exposure

Turning machining induced microstructural instability was investigated in a fully lamellar Ti-45Al-8.5Nb-(W,B,Y) alloy during high temperature exposure.After turning machining followed by thermal exposure at900 or 1000℃ for 100,300 and 500 h,a depth-dependent gradient microstructure with random orientations was produced in the region close to the machining surface.Two typical layers,a fine-grained(FG) layer with equiaxed grains and a coarse-grained(CG) layer with elongated grains,are formed in this region in transversal direction.The thickness of the two layers is up to 120 urn after thermal exposure at 1000℃ for 500 h,which is less than the depth of the hardened region(200 μm) after turning machining.Most of the new grains in FG and CG layers are constituted of γ single phase,while short α_2 segments and few B2 particles are precipitated at the γ/γ interface or inside the γ grains.Recrystallization and phase boundary bulging are found to be the major mechanisms responsible for lamellar degradation in FG layer and CG layer,respectively.The residual deformation energy stored is considered to be the main driving force of this process.     

Interfacial Characteristic of as-Deformed SiCp-Reinforced Magnesium Matrix Composite

The （submicron ＋ micron） SiCp-reinforced magnesium matrix composite was fabricated by stir casting. After the application of forging and extrusion, the interface between SiCp and Mg in the composite was investigated by transmission electron microscopy. Results show that the interfacial characterization was different at the interfaces of micron-SiCp/Mg and submicron-SiCp/Mg. While most interfaces between micron-SiCp and Mg were clean, the precip- itated Mg17A112 phase as well as dispersedly distributed nano-MgO particles was observed at some interfaces. Unlike the interface between micron-SiCp and Mg, no interfacial reaction product was found at the interface between submicron-SiCp and Mg in the present study. Besides, the specific orientation relationships were found at the interfaces between submicron- SiCp and Mg, which was thought to have developed during hot deformation process. At the fracture surface of the composite, the microcracks were found at the interface between micron-SiCp and Mg, while the interfacial bonding between submicron-SiCp and Mg was very well.     

Effect of V and Nb additions on microstructure,properties,and deformability of Ti-45Al-9 (V,Nb,Y) alloy

Ti-45Al-9(V, Nb, Y) alloys with four different x=V/Nb (atomic ratio x = 1, 1.5, 2 and 3.5) have been prepared, and the microstructures, properties and hot deformation behaviors were investigated. SEM, XRD and TEM results showed that Ti-45Al-9(V, Nb, Y) alloys were mainly composed of γ, α 2 , and β phase, and the volume fraction of β phase increased with the increase of the atomic ratio of V/Nb. The alloys were featured with lamellar microstructure with β and γ phases locating at the colony boundaries, and some β precipitates appearing at γ/γ interfaces. It was found that the colony size decreased with the increase of x. The alloys exhibited moderate mechanical properties at room temperature, with a yield strength of over 600 MPa, and fractures showed mainly translamellar character. The alloy with x=3.5 exhibited the best deformability at elevated temperature and that with x=1 had superior oxidation resistance at 800 ℃.     

Molecular Dynamics Simulation of the Evolution of Interfacial Dislocation Network and Stress Distribution of a Ni-Based SingleCrystal Superalloy

The evolution of misfit dislocation network at γ/γ' phase interfaces and the stress distribution characteristics of Ni-based single-crystal superalloys under different temperatures of 0,100 and 300 K are studied by molecular dynamics(MD) simulation.It was found that a closed three-dimensional misfit dislocation network appears on the γ/γ' phase interfaces,and the shape of the dislocation network is independent of the lattice mismatch.Under the influence of the temperature,the dislocation network gradually becomes irregular,all[110]dislocations in the y matrix phase emit and partly cut into the γ′ phase with the increase in temperature.The dislocation evolution is related to the local stress field,a peak stress occurs at γ/γ' phase interface,and with the increase in temperature and relaxation times,the stress in the γ phase gradually increases,the number of dislocations in the y phase increases and cuts into γ' phase from the interfaces where dislocation network is damaged.The results provide important information for understanding the temperature dependence of the dislocation evolution and mechanical properties of Ni-based single-crystal superalloys.     

Evolution of interfacial dislocation networks during long term thermal aging in Ni-based single crystal superalloy DD5

Interfacial dislocations found in single crystal superalloys after long term thermal aging have an important effect on mechanical properties. Long term thermal aging tests for DD5 single crystal superalloy were carried out at 1,100 ℃ for 20, 100, 200, 500 and 1000 h, and then cooled by air. The effect of long term thermal aging on the dislocation networks at the γ/γ' interfaces was investigated by FE-SEM. Results showed that during the long term thermal aging at 1,100 ℃, misfit dislocations formed firstly and then reorientation in the(001) interfacial planes occurred. Different types of square or rectangular dislocation network form by dislocation reaction. Square dislocation networks consisting of four groups of dislocations can transform into octagonal dislocation networks, and then form another square dislocation network by dislocation reaction. Rectangular dislocation networks can also transform into hexagonal dislocation networks. The interfacial dislocation networks promote the γ' phase rafting process. The dislocation networks spacings become smaller and smaller, leading to the effective lattice misfit increasing from-0.10% to-0.32%.     

Transformation Mechanism of (γ+γ') and the Effect of Cooling Rate on the Final Solidification of U720Li Alloy

The transformation mechanism of(γ+γ') was studied by analyzing the microstructure and elemental distribution of the U720 Li samples heated at 1250℃ and cooled at the rates in the range of 1–100℃/s. Although the(γ+γ') is deemed to be formed by a eutectic reaction and has been called eutectic(γ+γ'), it was found in the present study that the(γ+γ') precipitation begins with a peritectic reaction of(L+γ) ? γ', and develops by the eutectic reaction of L ?(γ+γ'). The energy for the γ' nucleation is low because the interfacial energy for the γ/γ' interface is about one-tenth of the solid/liquid interface, and hence, the nucleation rate is high and the fine structure of(γ+γ') is formed at the initial precipitation stage. The γ and γ'in(γ+γ') tend to grow into a lamellar structure because it is difficult for them to nucleate directly from the residual liquids, and hence, the γ' precipitates naturally tend to grow divergently direction of the regions rich in Al and Ti, forming a fan-like structure of the(γ+γ'). As a result, the γ' precipitates will coarsen finally because the space between them is enlarged. The solidification of the final residual liquids is a diffusion dependent process. When cooled at a higher rate, a higher degree of super cooling is reached and finally the solidification is finished by the pseudoeutectic reaction of L ?(γ+ boride) and L ?(γ+η), which can absorb Zr and B. When cooled at a rate low enough,most of the residual liquids are consumed by the(γ+γ') growth due to the sufficient diffusion, and the boride and Zrbearing phase are precipitated at a quasi-equilibrium state. Under this condition, Ti is depleted at the(γ+γ') growth front.However, the g-Ni_3 Ti phase is formed there occasionally due to the boride precipitation, because the compositions of the two phases are complementary.     

Fatigue properties of a medium-strength γ-TiAl alloy with different surface conditions

Effects of surface condition on fatigue properties of a medium-strength γ-TiAl alloy Ti-45Al-5Nb-lW(at%) were investigated.It is found that the maximum stresses of fatigue samples are lower than the yield stresses of the medium-strength γ-TiAl alloy.Meanwhile,the local plastic deformation is unconspicuous to occur at the crack tip.In this case,the fatigue strength is mainly decided by surface conditions of maximum-stressed surface,but compressive stress and deformation especially resulted from shot peening play an important role in the improvement of the condition fatigue strength.The affecting depth of shot peening is about 250 μm.As a result,the relatively weak microstructures and phases become the preferential initiation sites and propagation routes.They are observed to be equiaxed γ grains,B2 + ω grains,and α_2-γ lamellar interface in soft orientations.The existence of V-notch can significantly reduce the fatigue properties of the samples.     

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior,microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach.The true stress–strain curves exhibited typical work hardening and flow softening,in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon(Z)parameter,and the activation energy was calculated to be 446.9 k J/mol.The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation.The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy.The coarse recrystallized γ and several massive γ phase were observed at grain boundaries.While in the case of Nb segregation,β/B2 phase harmonized bending of lamellae,combined with the growth of recrystallized γ grains and α+β+γ→α+γ transition under conditions of temperature and stress,leading to the breakdown of α_2/γ lamellar colony.During the hot compression process,gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism,leading to extensive c twins and cross twins in α/γ lamellae and at grain boundaries.In general,homogeneous microstructure during the hot deformation process can be obtained in Ti Al alloy with high Nb addition and low Al segregation.The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.     

TENSILE CREEP DEFORMATION AND DAMAGE BEHAVIOR IN A NICKEL-BASE SUPERALLOY AT 900℃

The tensile creep deformation and damage evolution in a Ni-base superalloy at 900℃/170MPa were investigated. At the first creep stage, abnormal creep occured due to the resolution of fine particles, and the deformation initiated from grain boundary areas. It is evident that nearly all of the dislocations were in γ matrix channels in form of dislocation pairs and the dislocations were impeded at γ /γ‘ interfaces, thus the dislocation networks developed deformation. At the steady creep stage, impeded dislocations at γ/γ‘ interfaces climbed over γ‘ phases by diffusion-dominant mechanism. At the last creep stage, voids were formed around carbides at grain boundary which leaded to accumulated damage and caused creep rate accelerated. With the dislocation networks being broken, the voids connected and grew into micro-cracks gradually. Finally the cracks propagated along grain boundary area and resulted in failure.     

Analysis of electron structure of γ/α2 phase boundaries in ternary TiAl intermetallics

The electron structure of γ/α2 phase boundaries in lamellar colonies in Ti-47Al-2M（M=Nb, Cr, V） （mole fraction, %） alloys was theoretically investigated by Empirical Electron Theory of Solid and Molecules （EET） and the bond-length-difference （BLD） method. Average-Atom-Model was employed to calculate valence electron structure of TiAI intermetallics containing site substitution elements. On this basis, the boundary condition of electron movement was employed in the improved Thomas-Fermi-Dirac （TFD） theory to decide the continuity of the electron density of the lamellar colonies interface and it is found that of γ/α2 interface is continuous（△ρ〈10%）. Furthermore, it is found that adding alloying elements （including Nb, Cr, and V） can improve the electron density （ρ） of ）γ/α2 interfaces, and decrease the electron density difference（△ρ） of γ/α2 interfaces. Adding V element decreasing △ρ is more remarkable than other site substitution elements. According to electron structure study of γ/α2 interfaces in Ti-47Al-2M alloys, the added elements improve mechanical properties of the alloy in the following order： V〉Cr〉Nb.     

Asymmetry in interface and bending property of Al/Cu/Al bimetallic laminates

This work was aimed to study the interfacial microstructures and three-point bending properties of Al/Cu/Al bimetallic laminates produced by the asymmetrical roll bonding and annealing. It is found that the microstructure and bonding strength of the Al/Cu interface are different with those of the Cu/Al interface. The interfacial microstructure of Cu/Al interface is improved due to the large interfacial plastic deformation caused by the different rotation speeds of roll in the asymmetrical roll bonding process. The bonding strength between Al and Cu layer can be enhanced by the moderate atomic diffusion, but is dramatically depressed by the formation of intermetallic compounds in the interface.The bending strength of bimetallic laminates is enhanced when the Cu/Al interface is loaded in tension because of the improvement of stress transition and damping by the Cu/Al interface during the three-point bending deformation. The bending fracture reveals that the interfacial cracks can be inhibited from the restricted stress concentration in the improved Cu/Al interface.     

FINE STRUCTURES OF BOTH INTERFACES AND INTERFACIAL REACTION PRODUCTS

The characteristic of interface depending on the atomic structure exerts an inportant,andsometime controlling,influence on performance of the interacial materials.The present paper reviews the main studies on fine structure of both the materials inter-faces and interfacial reaction products in semiconductor uperlattice,metal multilayer ceram-ics and composite materials by mean of selected area electron doffraction patterns and highresolution electron microscopy.The following features of interfaces are reviewed:the orientation relationships;the char-acteristic of steps,facets and ronghness of interfaces;atomic bonding across the interface;thedegree of coherency,the structure of misfit dislocations and elastic relaxations at the inter-faces:the presence of defects at the onterfaces:the structure of the interfacial reaction prod-ucts as well as the reaction kinetics and reaetion mechanism.     

PHASE INTERFACE SEGREGATION OF Mg IN A Ni-BASE SUPERALLOY IN100

The segregation of Mg to phase interfaces in a nickel base superalloy IN 100 has been investi-gated using EPT(Electron Microprobe Technique).AES(Auger Electron Spectroscopy)and EDS analyses on thin TEM film.The results show that Mg segregates to the phase inter-faces of MC/γ and γ′/γ.The segregation concentration and layer thickness of Mg onMC/γ phase interface are larger than that on γ′/γ phase interface.Mg is not only a grainboundary segregation element,but also a phase interface segregation one.     

CREEP OF POLYCRYSTALLINE NEAR γ-TiAl WITH A LAMELLAR MICROSTRUCTURE

Creep of a polycrystalline near γ-TiAl alloy in two fully lamellar conditions is presented. A lamellar structure with fine interface spacing and planar grain boundaries provides improved creep resistance. The lamellar structure with wide interface spacing and interlocked grain boundaries has <1/2 the creep life, five times the minimum strain rate and greater tertiary strain.Creep strain is accommodated by dislocation motion in soft grains, but the strain rate is controlled by hard grains. The resistance to fracture is controlled by the grain boundary morphology, with planar boundaries causing intergranular fracture.To maximize the creep resistance of near γ-TiAl with a lamellar microstructure requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

INFLUENCE OF DISPERSIVELY PRECIPITATED PARTICLES ON MARTENSITE MORPHOLOGY IN Fe-Ni-Co-Ti ALLOYS

The Martensite formed at low temperatures in Fe-Ni-Co-Ti alloys is featured as thin plate when the size of γ' dispersively precipitated particles was about 20—50 nm.The interface of martensite plate is not smooth.If the γ'particles are sized less than 5 nm,the thin plate martensite with thermoelastisity may be found at low temperatures,whi greater than 5 nm, intermittent martensite may occur,and the plate martensite increases both in size and quantity with the coarsening of the γ particles.The lenticular or butterfly martensite forms at low tem- peratures until the γ' particles are up to 10—5 nm.     

Effect of Interactions Among Elements on Diffusion Process Associated with γ′ Coarsening in a Ni-Based Single-Crystal Superalloy

Coarsening of cuboidal γ' precipitates and relevant diffusion process in Ni-based single-crystal superalloy CMSX-4 were investigated at 1000,1020 and 1040℃ for specific times.The y' coarsening kinetics followed a cubic rate law with time and was presumably controlled by bulk diffusion of elements in y matrix.The associated diffusion activation energy was experimentally determined to be about 300 kJ/mol when it is considered the temperature-dependent thermo-physical parameters in modified Lifshitz-Slyozov-Wagner theory.The influence of temperature on γ/γ' microstructure is briefly discussed based on pseudo-binary [Ni]-[Al] phase diagram.Interactions among elements can effectively raise the local vacancy formation and vacancy-atom exchange barriers close to γ-and γ'-partitioning elements,respectively.Thus,it can significantly reduce the inter-coupling migrations of atoms during the macroscopic cross-diffusion process associated with γ' coarsening of Ni-based superalloys.     

Microstructural evolution of a PM TiAl alloy during heat treatment in α+γ phase field

In this study,the effect of temperatures and cooling rates of heat treatment on the microstructure of a powder metallurgy (PM) Ti-46Al-2Cr-2Nb-(B,W) (at.％) alloy was studied.Depending on the cooling rate and temperature,the different structures were obtained from the initial near-γ (NG) microstructures by heat treatment in the α+γ field.The results show that the microstructures of samples after furnace cooling (FC) consist primarily of equiaxed γ and α2 grains,with a few grains containing lamellae.Duplex microstructures consist mainly of γ grains and lamellar colonies were obtained in the quenching into another furnace at 900℃ (QFC) samples.However,further increasing of the cooling rate to air cooling (AC) induces the transformation of α→α2 and results in a microstructure with equiaxed γ and α2 grains,and no lamellar colonies are found.     

Effect of microstructure on impact toughness of TC21 alloy

The impact toughness of TC21 alloy after different types of forging and heat treatments was studied. The results show that heat treatment at 915℃ for 1 h followed by air-cooling can achieve the highest impact toughness. The crack propagation path of bimodal microstructure is different from that of lamellar microstructure. Boundaries of primary a grain are observed to be preferential sites for microcrack nucleation. With the increase of heat treatment temperature, the volume fraction of primary a phase decreases and the nucleation sites of microcrack at the primary α phase boundaries also decrease, the impact toughness value is effectively improved. The microcracks of lamellar microstructure are located on α/β interface, or the boundary of colony, and/or grain boundary a phase. The crack propagates cross the colony, or along the colony boundary, and/or along β grain boundary. The crack propagation path of lamellar microstructure is dependent on the size, direction of colony. The crack path deflects at grain boundaries, colony boundaries, or arrests and deviates at α/β interface because of crisscross a lamellar. Therefore the impact toughness value of basket microstructure is higher than that of Widmanstatten microstructure.     

Phase-field simulation of phase separation in Ni75AlxV25-x alloy with elastic stress

The phase separation in Ni75AlxV25-x alloys incorporated with the elastic stress was investigated using the microscopic phase-field model. The final morphology of γ＇ and 0 is similar in spatial alignment, but the volume fraction of γ＇ phase increases and that of 0 decreases as the AI concentration increases. For the small elastic interactions of early-stage phase separation, the coarsening of 7＇ and 0 can be approximated by a linear growth law as predicated by Lifshitz and Slyozov and Wangner （LSW） theory. As the elastic interactions increase at late-stage coarsening, the growth rate decreases, and the growth presents quick increase at early-stage and slows down at late-stage.