The effects of Co and Ti additions on microstructures and compressive strength of Udimet710

The effects of Co and Ti additions on the microstructures and compressive strength of the Ni-based superalloy Udiemt710 (U710) were investigated. The preliminary results showed that the Ni₃Ti-type (η) phase was observed in the alloys with low Co and Ti content; while a (Ni_xCo_1?x)₃Ti phase with a hexagonal structure was detected in the alloys with high Co and Ti content. The γ′ → η → (Ni_xCo_1?x)₃Ti phase transformation was discussed in terms of Co and Ti contents. The γ′ morphology changed from spherical to cuboidal with increasing Co and Ti content. Compressive tests showed that the alloys with Co and Ti addition possessed higher yield strength than the base alloy, U710. The strength increase could be ascribed to solid–solution strengthening of the γ and γ′ phases as a result of the Co and Ti additions, and the higher volume fraction of γ′ in the new alloys.     

Electron diffraction study of the low temperature phase in lead orthovanadate

Using electron diffraction and transmission electron microscopy it is shown that lead orthovanadate can follow two different transformation paths on cooling. The high temperature γ-phase transforms according to the scheme γ → β → α. The metastable γ′ phase, which results from frozen-in γ, transforms into a different low temperature phase, the α′-phase. In the literature, two different structures, both called α, were proposed for the low temperature phase. It turns out that both structures (here called α and α′) occur, but under different conditions.     

Stereological characterization of γ′ phase precipitation in CMSX-6 monocrystalline nickel-base superalloy

The purpose of this investigation was to study in detail the means to quantitatively evaluate γ′ phase precipitation. Many of the mechanical properties of superalloys are directly influenced by the presence of the γ′ (gamma prime) precipitate phase dispersed in a γ matrix phase. The γ′ precipitates act as effective barriers to dislocation motion and restrict plastic deformation, particularly at high temperatures. Due to this, it is essential to accurately quantify the γ′ precipitate size, volume fraction and distribution. Investigations based on quantitative metallography and image analysis were performed on a monocrystalline nickel-base superalloy taking into consideration various γ′ precipitate sizes present in that alloy microstructure. The authors of the present paper propose a new method of quantifying the total volume fraction of the γ′ phase applying images of the microstructure with γ′ phase precipitates registered using light microscopy, scanning electron microscopy (at two different magnifications) and scanning transmission electron microscopy.     

GH742中γ′相和γ基体成分随时效时间和温度的变化规律研究

利用透射电镜能谱法(TEM-EDX)研究了GH742合金中γ′和γ基体两相成分随温度和时效时间的变化规律.结果表明:合金在1050℃时效时,γ′相和γ基体的成分在时效初期变化较大,当时效时间超过1440min后,γ′相和γ基体的成份基本稳定.合金在750～1100℃时效时,γ′相和γ基体的成分均随着温度的升高而发生变化,其中γ基体的成分随温度变化较明显.合金中各元素在γ′和γ两相中的偏析率Cγ′/ Cγ变化规律研究表明:Ti,Al,Nb,Ni等γ′形成元素的偏析率均随着时效温度的升高而降低,而Cr,Co,Mo等γ形成元素的偏析率均随着时效温度的升高而增大.     

Effects of Al on microstructural stability and related stress-rupture properties of a third-generation single crystal superalloy

To examine the influences of minor modification of Al content on the microstructural stabilities and stress rupture properties,two alloys with minor difference in Al content were exposed isothermally at 1100 ℃ for 100 h,500 h,and 1000 h,respectively.The microstructures were characterized before and after thermal exposure.It was found that when Al content was decreased by 0.4 wt %,the volume fractionγ' decreased by 4 %,the size of γ' increased by 40 nm,the matrix channel width increased by 5 nm,and the misfit degree of γ/γ' phases increased by 0.006 % after heat treatment(HT).During thermal exposure,the alloy with low Al content had a better resistance to coarsening of γ' phase and precipitation of μphase.The γ' particles of the alloy with high AI content tended to connect each other and coarsened along 100directions;however,the γ' particles of the alloy with low Al content remained cubic after 500 h.A serious coarsening behavior took place in the two alloys after aging for 1000 h.The structural stabilities were significantly improved.However,the change of 0.4 wt % Al content was found to have little effect on the high temperature stress-rupture properties.     

Elastic behavior of composites containing Boolean random sets of inhomogeneities

The overall mechanical response as well as strain and stress field statistics of an heterogeneous material made of two randomly distributed, linear elastic phases, are investigated numerically. The Boolean model of spheres is used to generate microstructures consisting of either porous or rigid inclusions, at any volume fraction of the phases. Stress and strain field integral ranges, or equivalently the representative volume element, are computed and linked to features of the field statistics, and to the microstructure geometry.     

Formation of η and σ phase in three polycrystalline superalloys and their impact on tensile properties

The formation and phase transformation mechanism of η and σ phases in three experimental polycrystalline superalloys were studied. It was shown that a high (Ti + Al) content in the alloys would favor the formation of η and σ phases in the interdendritic region. Different as-cast microstructures resulted in different phase transformation processes during heat treatment and thermal exposure. Influence of η and σ phase on tensile properties had been investigated as well. The tensile properties of the alloys were sensitive to γ′ volume fraction of the alloys, as well as morphologies of η and σ phases in the interdendritic area. Formation of plate-like η phase had negative impact on the low and intermediate temperature tensile properties of the polycrystalline superalloy.     

Simulation of rafting type and stress distribution in nickel-based superalloys with different volume fractions of γ′ phase

A representative volume element is introduced to represent the microstructure of γ/γ′ morphology with periodic boundary conditions to formulate the full mode of the micromechanical analysis. [0 0 1]-oriented alloys with γ′ volume fractions from 60 to 70% are simulated under tensile loading. A raft criterion is implemented into the user subroutine to predict the rafting type. The misfit stress is considered by different thermal expansion coefficients of the two phases. It is very high in γ phase and slightly decreases with the increasing of γ′ volume fraction. The stress distributions in the two phases change during creep deformation. The creep crack initiation time slightly increases with the increase of γ′ volume fraction. The stress components decrease with the increasing of γ′ volume fraction at the beginning and change due to the stress redistribution during creep loading.     

Phase Relations in TiAl+Nb System

The microstructures and phases of ternary TiAl+Nb alloys containing 50-60 at.-%Al, 0-21 at.-%Nb have been studied using transmission electron microscopy (TEM) and X-ray powder diffraction. The phases present in the alloys and their distribution were found to be a sensitive function of composition. The phase relations between γ-TiAl and γ1 (a new ordered ternary intermetallic compound based on γ-TiAl) were determined. Essentially single γ phase was determined for alloys with relativety low Nb content (≤10 at.-%Nb). the γ1 phase was determined to exist in the composition range containing higher Nb contents (15-21 at.-%Nb). Between γ and γ1 phases, with intermediate Nb contents there is a transitional phase γ1 (a superstructure of γ-TiAl). As for the influence of Al concentration on the phase relations. the γ1 phase was inclined to form in the alloys with relatively high Al contents. The ordering transformation of γ, to γ1 is a continuous ordering process and the transition may be second order.     

Microstructure and creep behavior of a new developed nickel‐base single‐crystal superalloy

A new developed nickel‐base superalloy is employed to prepare single‐crystal castings. The as‐cast and heat‐treated microstructure, the creep behavior in 900 °C/ 500 MPa and 1100 °C / 140 MPa are investigated. The dendrites are well developed in the as‐cast microstructure. Segregation and γ′ precipitation morphology difference exist between dendrite core and γ/γ′ eutectic pool. After heat treatment the segregation is significantly decreased and the γ′ precipitations of all regions become much more uniform and cubic. Creep curves in varied conditions both present three stages, while the specimen in 1100 °C exhibits lower creep rate and longer life span. The fracture analysis shows that fracture style changes from mixed fracture to ductile when the temperature increases from 900 °C to 1100 °C. Porosity is the crack resource to form the facet, which is observed in both specimens. The γ′ precipitations raft in both specimens, while the further development of γ′ rafting is found in that of 1100 °C / 140 MPa, as well as the surface recrystallization with coarser rafted γ/γ′ microstructure.     

Delayed cracking of low‐nickel austenitic stainless steel studied with constant load tensile testing

Delayed cracking in unstable low‐Ni austenitic stainless steel 204Cu was studied by constant load tensile testing. The developed testing arrangement enabled a systematical examination on the effect of applied stress, strain‐induced α′‐martensite and internal hydrogen content on time to fracture. Volume fraction of strain‐induced α′‐martensite was shown to affect cracking kinetics, except at a very high stress level. Hydrogen content had a marked effect on time to fracture, also at the highest applied stress level. When hydrogen content was reduced by annealing, delayed cracking kinetics and susceptibility were suppressed, and cracking required a considerably higher stress level. The apparent critical hydrogen content, below which delayed cracking was not observed, was about 0.85 wppm. According to scanning electron microscope and electron backscattering diffraction examination, fracture mechanism in the constant load test specimens was mainly transgranular quasi‐cleavage, and cracking propagated along α′‐martensite.     

Effects of precipitation phases on the hydrogen embrittlement sensitivity of Inconel 718

We investigated the effects of precipitation phases on the hydrogen embrittlement (HE) sensitivity of Inconel 718 by means of tensile tests. Hydrogen was charged into the test specimens via a cathodic charging process prior to the tensile tests. Various heat treatments were applied to conventionally aged specimens to fabricate specimens with different precipitation conditions for the γ″ phase and the δ phase. For each precipitation condition, we fabricated two specimens, one of which was charged with hydrogen before the tensile test. All specimens were tensioned under identical tensile conditions. The percent loss of the reduction of area (RA) caused by pre-charged hydrogen was used to assess HE sensitivity. Both the δ phase and the γ″ phase were found to play significant roles in altering HE sensitivity of Inconel 718. When these phases were totally dissolved, the HE sensitivity of the alloy was very low. The percent loss of RA decreased along with a decrease in the fractional volume of γ″. The δ-free aged alloy had greatly enhanced HE resistance, the same level as that of conventionally annealed alloy, and its strength was equal to that of the conventionally aged alloy. Fracture origins noted on the specimens were located on the surface layers and displayed brittle cleavage when pre-charged hydrogen was utilized. Local transgranular cleavages initiated from the δ/matrix were also observed in conventionally aged specimens, where there was a presence of pre-charged hydrogen. Therefore, the δ phase was considered to promote HE by initializing micro-cracks from δ/matrix interfaces. Since the δ-free aged alloy has both good strength and good ductility, we propose that it is advantageous for fabricating some hydrogen-containing parts.     

Identifying Phase‐Dependent Electrochemical Stripping Performance of FeOOH Nanorod: Evidence from Kinetic Simulation and Analyte–Material Interactions

Metal hydroxide nanomaterials are widely applied in the energy and environment fields. The electrochemical performance of such materials is strongly dependent on their crystal phases. However, as there are always multiple factors relating to the phase‐dependent electrochemistry, it is still difficult to identify the determining one. The well‐defined crystal phases of α‐ and β‐FeOOH nanorods are characterized through the transmission electron microscopy by a series of rotation toward one rod, where the cross‐section shape and the growth direction along the [001] crystalline are first verified for 1D FeOOH nanostructures. The electrosensitivity of the two materials toward Pb(II) is tested, where α‐FeOOH performs an outstanding sensitivity whilst it is only modest for β‐FeOOH. Experiments via Fourier transform infrared spectroscopy, X‐ray absorption fine structure (XAFS), etc., show that α‐FeOOH presents a larger Pb(II) adsorption capacity due to more surficial hydroxyl groups and weaker Pb? O bond strength. The reaction kinetics are simulated and the adsorption capacity is found to be the determining factor for the distinct Pb(II) sensitivities. Combining experiment with simulation, this work reveals the physical insights of the phase‐dependent electrochemistry for FeOOH and provides guidelines for the functional application of metal hydroxide nanomaterials.     

Influences of Re on low-cycle fatigue behaviors of single crystal superalloys at intermediate temperature

Low-cycle fatigue behaviors of Ni-base single crystal superalloys containing different Re contents have been investigated at 760 °C. During heat treatment, Re retards γ′ phases coarsening and equalizes the distribution of γ′ phases. As Re content increases, fatigue life increases and slip bands distribute more inhomogeneously. Moreover, adding Re not only reduces stacking fault energy of the matrix, but also promotes the element segregation to increase the lattice misfit. However, the larger lattice misfit does not lead to the formation of dislocation networks, but which activates dislocation movement and promotes dislocations cross-slip and climbing movement under high temperature and applied stress. On the other hand, with the addition of Re, cyclic deformation behaviors change from cyclic hardening to cyclic stability, mainly depending on a transformation of deformation mechanisms from slip bands cutting through γ and γ′ phases to stacking faults shearing.     

The effects of ruthenium additions on tensile deformation mechanisms of single crystal superalloys at different temperatures

The tensile behavior of two experimental nickel-base single crystal superalloys has been studied from room temperature to 1100 °C. Emphasis is placed on elucidating the effects of ruthenium (Ru) additions on the deformation mechanisms using transmission electron microscopy (TEM). Furthermore, the partitioning behavior of the alloy elements between the γ and γ′ phases for both experimental alloys has been studied using three-dimensional atom probe (3DAP). Detailed analysis demonstrates that at low and medium temperature ranges, the stacking faults present in the γ matrix of the 3Ru alloy but no trace of stacking fault in the γ matrix of the 0Ru alloy have been observed; during high temperature range, as a result of Ru additions, the γ/γ′ interfacial dislocation space of the 3Ru alloy is smaller than that of the 0Ru alloy due to further decreasing the lattice misfit. Apart from that, Ru additions result in more Re partitioning to the γ′ phase, and thus the solution strengthening for the γ phase is decreasing. Thus, during tests below and at the temperature corresponding to the peak strength, the yield strength of the 3Ru alloy is lower than that of the 0Ru alloy. At last, in the light of the TEM observations, the changing trends of the stacking fault energy in the γ matrix and the transformation points (the temperature related to the stacking faults formation) for the two experimental alloys have been drawn. The temperature range of the stacking faults formation in the γ matrix is expanded after Ru additions. The energy conditions of the stacking faults formation of the 0Ru and 3Ru alloys have been analyzed in detail. The changing of lattice misfit with temperature can be considered as one of the principal reasons for the stacking faults formation.     

Influence of thermal exposure on γ′ precipitation and tensile properties of DZ951 alloy

The effect of long-term exposure on the γ′ phase and the tensile behavior of a directionally solidified nickel-base superalloy DZ951 was investigated. Alloys after standard heat treatment (SHT) were isothermally aged at 900 °C up to 2000 h and tensile tests were performed in both SHT and aged conditions at various temperatures. The morphology of the γ′ phase changes from cuboid to rafting and the size increases from 300 nm at SHT to 930 nm, and the volume fraction of the γ′ phase decreases from 70% at SHT to 65% during aging at 900 °C for 2000 h. The changing trend of yield stress at different test temperatures is similar. The yield stress decreases slightly at 600 °C. This arises from few dislocations shearing the γ′ precipitates. There is a peak yield stress value at 760 °C, which is attributed to the high strength of the γ′ phase, the homogeneous deformation structure, and dislocation-γ′ precipitate and dislocation–dislocation interactions. The yield stress then decreases rapidly with increased temperature. The low strength of the γ′ phase and γ′ rafting at high temperature contribute to the drop of yield stress. The change of tensile elongation is inverse to that of yield stress. The yield stress continuously decreases with the increase of aging time at 900 °C. This arises from the coarsening of γ′ and a decrease in the γ′ volume fraction.     

Ternary alloy films of Ni-Cr-Al for thin film resistors

Multicomponent thin alloy films based on Ni-Cr with additions of aluminium have been used in the G.D.R. for the production of discrete thin film resistors for more than 10 years. Additions of aluminium stabilize the layer and reduce the temperature coefficient of resistance (TCR) to values of about zero. Three regions have been found up to now in the system Ni-Cr-Al which result in highly stable resistive films with a very low TCR. The properties of these films are determined by the content and the distribution of the phases which are present in the amorphous film. The following phases have been found in structural investigations of vacuum-annealed films: α phase (chromium), β phase (NiAl), γ phase (Ni₃Cr), γ′ phase (Ni₃Al) and γ-γ′ phase (Ni₃(Cr, Al)). Of the three regions, one with less than 3 at.% Al, one with 28–32 at.% Al and one with 45–60 at.% Al, the first two regions have been used for the production of thin film resistors up to now. Films of the third region, which has only been found during the last few years, can be obtained by sputtering.     

Measurements of γ/γ' Lattice Misfit and γ' Volume Fraction for a Ru-containing Nickel-based Single Crystal Superalloy

A conventional X-ray diffractometer has been used to determine the γ/γ lattice misfit and γ volume fraction for a Ru-containing nickel-based single crystal superalloy at room temperature.The rocking curve was used to characterize the distribution of subgrains.The diffraction peaks obtained by ω-2θ scan were used to determine the γ/γ lattice misfit and γ volume fraction.A three peaks fitting model was proposed.The peak fitting results are in good agreement with the model.The X-ray diffraction results indicate that the nickel-based single crystal superalloy was not a perfect monocrystalline material,which is comprised of many subgrains;and each subgrain also consists of large numbers of mosaic structures.In addition,two anomalous reflection phenomena were found during the experiment and discussed with respect to their occurrence and impact on the measurement.The experimental results show that the γ/γ lattice misfit and γ volume fraction will be various at the different regions of its dendritic microstructure.The average γ/γ lattice misfit and γ volume fraction of the experimental alloy are approximately-0.2% and 70%,respectively.Furthermore,the γ volume fraction calculated by atom microprobe(AP) data is also basically consistent with the experimental results.     

Microstructure and tensile properties of superalloy IN100 fabricated by micro-laser aided additive manufacturing

Layer by layer fabrication using micro-laser aided additive manufacturing (micro-LAAM) was successfully implemented on nickel-base superalloy IN100. It is known that IN100, a type of superalloy having high titanium and aluminum contents, has poor weldability due to weld liquation cracking in the heat-affected zone (HAZ) and strain age cracking. In this study, micro-LAAM process was optimized through a set of designed experiments to eliminate crack formation and reduce porosity. It was found that the crack-free deposition can be achieved owing to the fact that micro-LAAM process used in this study had very low heat input. Three distinct sizes of γ′ precipitates were observed on the post heat-treated samples. The volume fractions of γ to γ′ phases were found to be approximately 60–40%. Microstructure and chemical analysis results showed that γ′ phase was embedded within γ-Ni matrix while various carbides (MC, M₂₃C₆ and M₆C) were observed as precipitates at grain boundaries or within grains. Electron backscatter diffraction (EBSD) was used to compare grain morphologies and size distribution of three distinctly different regions on each layer. The achieved ultimate tensile strength and yield strength are much better than the minimum requirements specified in aerospace material specification 5397 for cast IN100.     

Microstructure evolution and analysis of a single crystal nickel-based superalloy during compressive creep

During compressive creep, the cubical γ′ phase in [0 0 1] orientation single crystal nickel-based superalloy is transformed into the rafted structure along the direction parallel to the applied stress axis. By means of the elastic stress-strain finite element method (FEM), the von Mises stress distributions of the cubical γ′/γ phases are calculated for investigating the influence of the applied stress on the stress distribution and the directional coarsening regularity of γ′ phase. Results show that the stress distribution of the cubical γ/γ′ phases may be changed by the applied compressive stress, and the coarsening orientation of γ′ phase is related to the von Mises stress distribution of the γ matrix channel. Thereinto, under the action of applied compressive stress, the bigger von Mises stress produced on (0 0 1) plane of the cubical γ′ phase is thought to be a main reason of the microstructure evolution. The expression of the driving force for the elements diffusion and the directional growing of γ′ phase during compressive creep are also proposed.