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The contribution of crystallography-dependent metallurgical factors, such as supersaturation of liquid aluminum and minimum dendrite tip undercooling, to solidification behavior and microstructure development is numerically analyzed during Ni-Cr-Al ternary single-crystal superalloy molten pool solidification to better understand thermodynamic and kinetic driving forces behind solidification cracking resistance. The variation of supersaturation of liquid aluminum and minimum dendrite tip undercooling with location of solid/liquid interface is symmetrically consistent in (001)/[100] welding configuration. By comparison, the variation is asymmetrically consistent in (001)/[110] welding configuration. The different distribution is attributed to growth crystallography and dendrite selection. Significant increase of supersaturation of liquid aluminum and dendrite tip undercooling from [010] dendrite growth region to [100] dendrite growth region preferentially aggravates microstructure development as result of nucleation and growth of stray grain formation with the same heat input on each half of the weld pool in (001)/[110] welding configuration. High heat input (both increasing laser power and decreasing welding speed) exacerbates supersaturation of liquid aluminum and dendrite tip undercooling by faster diffusion to incur stray grain formation with severity of contributing thermometallurgical factors for susceptibility to solidification cracking, while low heat input (both decreasing laser power and increasing welding speed) ameliorates microstructure development and increases resistance to solidification cracking. Weld microstructure of optimum welding conditions, such as combination of low heat input and (001)/[100] welding configuration, is less susceptible to solidification cracking to suppress asymmetrical microstructure development and improve weld integrity potential rather than insidious welding conditions, such as combination of high heat input and (001)/[110] welding configuration. Severer supersaturation of liquid aluminum and wider dendrite tip undercooling occur in the [100] dendrite region as consequence of alloying enrichment, while smaller supersaturation of liquid aluminum and narrower dendrite tip undercooling occur in the [001] dendrite region as consequence of alloying depletion to spontaneously facilitate epitaxial growth of single-crystal essential. Symmetrical (001)/[100] welding configuration decreases growth kinetics of dendrite tip with smaller overall supersaturation of liquid aluminum and dendrite tip undercooling than that of asymmetrical (001)/[110] welding configuration regardless of combination of laser power and welding speed. Mitigation of supersaturation of liquid aluminum and dendrite tip undercooling simultaneously alleviate crack-susceptible microstructure development and solidification cracking. Additionally, the appropriate mechanism of solidification cracking resistance improvement through modification of crystallography-dependent supersaturation and undercooling of dendrite tip is proposed. Calculation analyses are sufficiently explained by experiment results in a reasonable way. The additional purpose of this theoretical analysis is to evaluate solidification cracking susceptibility of similar nickel-based or iron-based single-crystal superalloys.

The interfacial reaction between alloys and ceramic materials is an important factor to influence the quality and service performance of the turbine blade. In this paper, three typical height sections of 120mm, 160mm and 210mm were selected, and the interface reactions between DD6 single crystal superalloy and silica based ceramic cores were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS). The results showed that the infiltration degree of the melt alloy increases with the increase of reaction time. The thickness of the reaction layer could be over 0.3mm when the reaction time increased up to 70min. The main reasons of forming the infiltration layer were the infiltration of the Al element and the interfacial reaction between the Al element and the ceramic core. There formed an aluminum deficient layer on the metal surface because of the interface reaction between the alloy and the ceramic core. The dense layer formed by interfacial reaction on the surface of the core will cause some difficulties for core leaching. Keywords: DD6 single crystal superalloy; Silica based ceramic core; Interface reaction

Numerical simulation and prediction of grain formation and defects, including the stray grain and high angle orientation deviation during directional solidification process of a single crystal superalloy hollow turbine blade are experimentally conducted by means of commercial software ProCAST and backscattering scanning electron microscope. The results show that the initial nucleation amount at the beginning section of the starter block is 104 of magnitude, and the number of grains decreases gradually with the competitive growth, and the number is about 100 at the spiral of the selector. And the orientation distribution of grains is close to direction, with the orientation deviation between 10° and 15°. Moreover, with the increase of withdrawal rate, the curvature of isoline of liquidus of single crystal blade increases, and the tendency to form stray grains defects increases. The grain with a large deviation from orientation blocks the growth of other grains at the first rotating transition site of the selector, and then gradually grows and solidifies to form the final blade.

The solidification temperature range was numerically analyzed to optimize nonequilibrium solidification behavior during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification with variation of laser welding conditions (either heat input or welding configuration). The distribution of solidification temperature range along the fusion boundary is beneficially symmetrical about the weld pool centerline in the (001)/[100] welding configuration. The distribution of solidification temperature range along the fusion boundary is detrimentally asymmetrical about the weld pool centerline in the (001)/[110] welding configuration. The stray grain formation and solidification cracking are preferentially confined to [100] dendrite growth region. [001] epitaxial growth region with columnar dendrite morphology is favored at the expense of undesirable [100] growth region with equiaxed dendrite morphology to facilitate essential single-crystal solidification with considerable reduction of heat input. The smaller heat input is used, the narrower solidification temperature range is thermodynamically promoted to reduce nucleation and growth of stray grain formation with decrease of constitutional undercooling ahead of dendrite tip and mitigate thermo-metallurgical factors for morphology instability and microstructure anomalies. Potential low heat input(both decreasing laser power and increasing welding speed) with (001)/[100] welding configuration decreases solidification temperature range to significantly minimize columnar/equiaxed transition (CET) and stray grain formation, and improve resistance to solidification cracking through microstructure control. On both sides of weld pool are imposed by the same heat input, while the solidification temperature range along the fusion boundary inside of [100] dendrite growth region on the right part of the weld pool is spontaneously wider than that of [010] dendrite growth region on the left part to increase solidification cracking susceptibility in the (001)/[110] welding configuration. Furthermore, another mechanism of solidification cracking as consequence of severe solidification behavior and anomalous microstructure with asymmetrical crystallographic orientation is therefore proposed. The theoretical predictions are well verified by experiment results. The useful and satisfactory numerical modeling is also available for other single-crystal superalloys during successful laser repair process without stray grain formation.

Experimental investigation on the influence of load interaction on the life of Single Crystal Nickel-based Superalloy is conducted. Three kinds of load spectrums considering single and coupled failure mode are designed. Life tests are carried out under creep, thermo-mechanical fatigue (TMF), and creep-TMF interaction loading. The test results show that test lives, under the creep-TMF interaction loading, are lower than the life predictions given by the linear damage accumulation (LDA) rule, indicating that the load coupling can accelerate the damage evolution process. The microstructure of fractured specimens shows that under the creep-TMF interaction loading, rafting cause more dislocations to accumulate in the at γ/γ′ phase boundary, which could be the evidence of life decrease.

After solid solution treatment at 1335°C for 4 hours and cooling to room temperature at different rate, the nickel-based single crystal superalloy were made into three kinds of nickel-based single crystal superalloy materials containing different size γ′ phases, respectively. The tensile test of I-shaped specimens was carried out at 980°C, and their effect of γ′ phase microstructure on the tensile properties was studied. The results show that the yielding strength of the material air-cooled to room temperature was lower than that with cooling rate at 0.15°C/s, but both of them were lower than the yielding strength of original material. Little difference was found on the elastic modulus of I-shaped specimens made of three kinds of materials. When the cubic degree of the γ′ phase is higher and the size is larger, the tensile properties of the material is better, which can be attributed to the larger size and narrower channel of the matrix phase that lead to higher dislocation resistance.

A thermal metallurgical coupling model was further developed for multicomponent dendrite growth of primary γ gamma phase during laser welding nickel-based single-crystal superalloys. It is indicated that welding configuration has a predominant role on the overall dendrite trunk spacing than heat input throughout the weld pool, and modifies the dendrite growth kinetics. The dendrite trunk spacing in (001) and [100] welding configuration is finer than that of in (001) and [110] welding configuration. In (001) and [100] welding configuration, the bimodal distribution of dendrite trunk spacing is symmetrical about weld pool centerline, the dendrite trunk spacing in [100] growth region near the weld pool center is coarser than [010], [0ī0] and [001] dendrite growth regions. In (001) and [110] welding configuration, the distribution of dendrite trunk spacing is crystallographically asymmetrical, and the dendrite trunk spacing in [100] growth region is severely coarser than that of [010] and [001] dendrite growth regions. (001) and [110] welding configuration is of particular interest, because dendrite trunk spacing decreases in [100] dendrite growth region and dendrite trunk spacing increases in [010] dendrite growth region from the maximum weld pool width to the end due to crystallography-dependent growth kinetics. Moreover, strict control of low heat input (low laser power and high welding speed) beneficially promotes fine dendrite trunk spacing and reduces the size of dendrite growth regions. High heat input (high laser power and low welding speed) monotonically coarsens dendrite trunk spacing. The dendrite trunk spacing is refined and [100] dendrite growth is suppressed by the optimum low heat input and (001) and [100] welding configuration to improve weldability. An alternative mechanism of solidification cracking because of asymmetrical dendrite trunk growth is proposed. The useful results facilitate understanding of single-crystal superalloys microstructure development and solidification cracking phenomena. The theoretical predictions agree well with the experiment results. Moreover, the model is also applicable to other single-crystal superalloys with similar metallurgical properties by feasible laser welding or laser cladding.

The thermal metallurgical modeling of alloying aluminum redistribution was further developed through couple of heat transfer model, dendrite selection model, multicomponent dendrtie grwoth model and nonequilibrium solidification model during three-dimensional nickel-based single-crystal superalloy weld pool solidification over a wide range of welding conditions (laser power, welding speed and welding configuration) to facilitate understanding of solidification cracking phenomena. It is indicated that the welding configuration plays more important role than heat input in aluminum redistribution. The bimodal distribution of solid aluminum concentration along the solid/liquid interface is crystallographically symmetrical about the weld pool centerline for (001) and [100] welding configuration, while the distribution of solid aluminum concentration along the solid/liquid interface is crystallographically asymmetrical throughout the weld pool for (001) and [110] welding configuration. The size of vulnerable [100] dendrite growth region is beneficially suppressed in favor of epitaxial [001] dendrite growth region through optimum low heat input (low laser power and high welding speed) to facilitate single-crystal dendrite growth for successful crack-free weld at the expense of shallow weld pool geometry. The overall aluminum concentration in (001) and [100] welding configuration is significantly smaller than that of (001) and [110] welding configuration regardless of heat input. Severe aluminum enrichment is confined to [100] dendrite growth region where is more susceptible to solidification cracking. Heat input and welding configuration are optimized in order to minimize the solidification cracking susceptibility and improve microstructure stability. The relationship between welding conditions and alloying aluminum redistribution are established for solidification cracking susceptibility evaluation. The higher heat input is used, the more aluminum enrichment is monotonically incurred by diffusion with considerable increase of metallurgical driving forces for morphology instability and microstructure anomalies to deteriorate weldability and vice versa. The mechanism of asymmetrical solidification cracking because of crystallography-dependent alloying redistribution is proposed. The theoretical predictions agree well with the experiment results. Moreover, the useful modeling is also applicable to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

The thermal metallurgical modeling by coupling of heat transfer model, dendrite selection model, columnar/equiaxed transition (CET) model and nonequilibrium solidification model was further developed to numerically analyze stray grain formation and solidification temperature range on the basis of three criteria of constitutional undercooling, marginal stability of planar front and minimum growth velocity during multicomponent nickel-based single-crystal superalloy weld pool solidification. It is indicated that the primary γ gamma phase microstructure development and solidification cracking susceptibility along the solid/liquid interface are symmetrically distributed throughout the weld pool in (001) and [100] welding configuration. The microstructure development and solidification cracking susceptibility along the solid/liquid interface are asymmetrically distributed in (001) and [110] welding configuration. Appropriate low heat input (low laser power and high welding speed) simultaneously minimizes stray grain formation, grain boundary misorientation and solidification temperature range in the vulnerable [100] dendrite growth region and beneficially maintains single-crystal nature of the material in the [001] epitaxial dendrite growth region to improve the cracking resistance, while high heat input (high laser power and low welding speed) increases the solidification cracking susceptibility to deteriorate weldability and weld integrity. The solidification temperature range in (001) and [110] welding configuration is detrimentally wider than that of (001) and [100] welding configuration due to crystallographic orientation of dendrite growth regardless of heat input. The mechanism of asymmetrical crystallography-dependant solidification cracking because of nonequilibrium solidification behavior is proposed. The elliptical and shallow weld pool shape is less susceptible to solidification cracking for successful crack-free laser welding. Moreover, the promising theoretical predictions agree well with the experiment results. The useful modeling is also applicable to other single-crystal superalloys with similar metallurgical properties during laser welding or laser cladding.

The thermal-metallurgical model of primary γ gamma phase through couple of heat transfer model, dendrite selection model, columnar/equiaxed transition (CET) model, multicomponent dendrite growth model and nonequilibrium solidification model is further developed on the basis of criteria of minimum growth velocity, constitutional undercooling and marginal stability of planar front during nickel-based single-crystal weld pool solidification. It is clearly indicated that crystallographic orientation plays more important role than heat input in microstructure development and solidification behavior. The dendrite trunk spacing and solidification temperature range along the solid/liquid interface are symmetrically distributed about the weld pool centerline in (001) and [100] welding configuration, while they are asymmetrically distributed in (001) and [110] welding configuration. The dendrite size and solidification temperature range are beneficially smaller in (001) and [100] welding configuration than that of (001) and [110] welding configuration regardless of heat input. The mechanism of asymmetrical solidification cracking because of crystallography-dependent growth kinetics and solidification behavior is proposed. Optimum low heat input (low laser power and high welding speed) refines dendrite size and suppresses the solidification temperature range to minimize the solidification cracking susceptibility and ameliorate the weldability through microstructure control, while high heat input (high laser power and low welding speed) deteriorates the weldability and weld integrity. It is therefore imperative to optimize the welding conditions for successful defect-free laser welding. Moreover, the promising theoretical predictions agree well with the experiment results. The useful model is also applicable to other single-crystal superalloys with similar metallurgical properties by laser welding or laser cladding, and provide a more accurate and reliable way of solidification cracking susceptibility evaluation.

The thermal metallurgical modeling of liquid aluminum supersaturation was further developed through couple of heat transfer model, dendrite selection model, multicomponent dendrite growth model and nonequilibrium solidification model during three-dimensional nickel-based single-crystal superalloy weld pool solidification. The welding configuration plays more important role in supersaturation of liquid aluminum, morphology instability and nonequilibrium partition behavior. The bimodal distribution of liquid aluminum supersaturation along the solid/liquid interface is crystallographically symmetrical about the weld pool centerline in (001) and [100] welding configuration. The distribution of liquid aluminum supersaturation along the solid/liquid interface is crystallographically asymmetrical throughout the weld pool in (001) and [110] welding configuration. Optimum low heat input (low laser power and high welding speed) with (001) and [100] welding configuration is more favored to predominantly promote epitaxial [001] dendrite growth to reduce the metallurgical factors for solidification cracking than that of high heat input (high laser power and slow welding speed) with (001) and [110] welding configuration. The lower the heat input is used, the lower supersaturation of liquid aluminum is imposed, and the smaller size of vulnerable [100] dendrite growth region is incurred to ameliorate solidification cracking susceptibility and vice versa. The overall supersaturation of liquid aluminum in (001) and [100] welding configuration is beneficially smaller than that of (001) and [110] welding configuration regardless of heat input, and is not thermodynamically relieved by gamma prime γˊ phase. (001) and [110] welding configuration is detrimental to weldability and deteriorates the solidification cracking susceptibility because of unfavorable crystallographic orientations and alloying aluminum enrichment. The mechanism of asymmetrical solidification cracking because of crystallography-dependent supersaturation of liquid aluminum is proposed. The eligible solidification cracking location is particularly confined in [100] dendrite growth region. Moreover, the theoretical predictions agree well with the experiment results. The useful modeling is also applicable to other single-crystal superalloys with similar metallurgical properties for laser welding or laser cladding. The thorough numerical analyses facilitate the understanding of weld pool solidification behavior, microstructure development and solidification cracking phenomena in the primary γ phase, and thereby optimize the welding conditions (laser power, welding speed and welding configuration) for successful crack-free laser welding.

The thermal-metallurgical modeling of microstructure development was further advanced during single-crystal superalloy weld pool solidification by coupling of heat transfer model, columnar/equiaxed transition (CET) model and multicomponent dendrite growth model on the basis criteria of minimum dendrite velocity, constitutional undercooling and marginal stability of planar front. It is clearly indicated that heat input (laser power and welding speed) and welding configuration simultaneously influence the stray grain formation, columnar/equiaxed transition and dendrite growth. For beneficial (001) and [100] welding configuration, the microstructure development along the solid/liquid interface is symmetrically distributed about the weld pool centerline throughout the weld pool. Finer columnar in [001] epitaxial dendrite growth region is kinetically favored at the bottom of the weld pool. For detrimental (001) and [110] welding configuration, the microstructure development along the solid/liquid interface is asymmetrically distributed. The dendrite trunk spacing along the solid/liquid interface from the beginning to end of solidification morphologically increases on the left side of the weld pool, while it spontaneously decreases on the right side. The vulnerable location of solidification cracking is confined in the [100] dendrite growth region on the right side of the weld pool because of increasing metallurgical contributing factors of severe stray grain formation, centerline grain boundary formation and coarse dendrite size. The mechanism of crystallography-dependent asymmetrical solidification cracking due to microstructure anomalies is proposed. It is crystallographically favorable for predominant morphology instability to deteriorate weldability. Active [100] dendrite growth region is diminished in the shallow elliptical weld pool by optimum low heat input (low laser power and high welding speed) with (001) and [100] welding configuration to essentially facilitate single-crystal solidification conditions and provide enough resistant to solidification cracking. Moreover, the theoretical predictions agree well with the experiment results. The reliable weldability maps are therefore established to determine the prerequisite for successful crack-free laser welding or cladding. The useful model is also applicable for other single-crystal superalloys with similar metallurgical properties.

In order to improve the composition and microstructure of nickel-base single crystal superalloys, equilibrium phases of third generation single crystal superalloys RenéN6 and CMSX-10 have been researched by using thermodynamic calculation software JMatPro. The calculated results indicated that the two superalloys have the same equilibrium phases, such as liquid phase, γ phase, γ’ phase and TCP (topologically close-packed phases), however, there are differences in the quantity and temperature range. RenéN6 alloy has higher content of μ phase. And CMSX-10 alloy has higher γ’ phase precipitation temperature and more γ’ phase precipitates.

Intermediate temperature creep properties are considered a key indicator of single crystal superalloys used for turbine blades of aircraft engines. The interrupted and ruptured creep tests were carried out in a second generation single crystal superalloy under the conditions of 760°C/785MPa. The creep rupture life as well as minimum creep rate were also in the same level of those in CMSX-4 and PWA1484. The microstructural evolution at different creep stages were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed the γ’ phases kept the cuboid morphology mostly until the creep rupture, and super lattice stacking faults (SSFs) extended along [-1 1 0] and [-1-1 0] orientations within the γ’ precipitate were the typical dislocation configuration.

The second generation single crystal superalloy DD6 after standard heat treatment was respectively overheated at 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1320°C for 1h and air cooled. The effect of overheating on the microstructure and stress rupture properties at 980°C/250MPa of the alloy was investigated. The results showed that the size of γ′ phase was slightly increased overheating at 1100°C, 1150°C and 1200°C. The size of γ′ phase had a big increase and its size distribution was very uneven after overheating at 1250°C. The small part of γ′ phase has serrated γ′/γ phase surface as a result of un-completely solution and the irregular small γ′ phase was in the majority when overheated at 1300°C. While all the irregular small γ′ phase precipitated again after completely solution when overheated at 1320°C. There was no fine second γ′ phase in the γ matrix channel of the alloy after standard heat treatment and overheating at 1320°C. But the fine second γ′ phase precipitated in the γ matrix channel after overheating at every temperature of 1100 °C~1300°C. No obvious change of the stress rupture life was found after overheating at 1100°C, 1150°C, 1200°C and 1250°C. The stress rupture life considerably reduced after overheating at 1300°C, whereas slightly reduced after overheating at 1320°C. The appearance of the raft had almost no change after overheating at 1100°C. With increasing of overheating temperature from 1100°C to 1250°C, the length of raft became shorter and the width thickening. The γ phase formed the wavy raft after overheating at 1300°C and 1320°C and the thickness of latter was larger than that of the former. Finally, the relationship between the microstructural evolution and stress rupture properties of the alloy after overheating was discussed.

Optimization design and simulation of the spiral selector in the process of single crystal preparation were studied. Based on the analysis of simulation and experiment results, the grain structure and orientation in different section height at starter block and spiral part have been discussed. The influence and mechanism of spiral selector parameters on the behavior of the crystal selection were proposed. The design standard of the spiral selector was put forward.

The stress rupture properties, fracture behavior and microstructure evolution of the second generation single crystal superalloy DD6 at different conditions were investigated. The results show that the alloy has excellent stress rupture properties. The fracture mechanism of stress rupture of the alloy at 760°C/785MPa, 760°C/750MPa and 760°C/720MPa shows quasi-cleavage mode and the fracture mechanism at 980°C/250MPa, 1070°C/160MPa and 1100°C/140MPa shows dimple mode, while the fracture mechanism at 850°C/550MPa shows quasi-cleavage and dimple mixture mode. Only a little change in the morphology of γ′ phase occurred at 760°C/750MPa. The γ′ rafts form at temperature above 760°C and the thickness of rafts increases with increasing temperature. The dislocation shear mechanism including stacking fault formation is operative at lower temperature and high stress. The dislocation by-passing mechanism occurs to form networks at γ/γ′ interface under the condition of high temperature and lower stress.

The coarsening behavior of γ' phase and precipitation behavior of TCP phase in a Ni3Al based single crystal superalloy were investigated by experimental and calculation. The Re-containing and Mo-rich single crystal superalloy used in this study exhibit good high-temperature strength and oxidation resistance. After fully heat-treated, the experimental superalloys were aged at 1050°C and 1150°C for different time periods from 50h to 1000h. The microstructure after thermal exposure was examined by field emission-scanning electron microscopy (FE-SEM). The results indicated that the coarsening behavior of γ' phase agrees well with the typical Lifshitz-Slyozov-Wagner (LSW) theory. After thermal exposure, both the size and the volume fraction of the TCP phase changed perceptibly.

The single crystal superalloy with [001] orientation were prepared by screw selecting method in the directionally solidified furnace. Three different cooling method, water cooling (WC), air cooling (AC) and furnace cooling (FC) were used after same solution treatment. Then these specimens received same two-step aging treatment. Influence of solution cooling method on the microstructure and stress rupture properties of the alloy under the test condition of 980 °C and 300 MPa was investigated. The microstructures of the samples were examined by scanning electron microscope (SEM). The results showed that the solution cooling method of heat treatment played an important role in the microstructure and stress rupture properties of the alloy. The size of γ′ phase and the width of the γ matrix channel of the alloy increased with decreasing cooling rate. The stress rupture properties of the alloy increased at first and decreased afterwards with decreasing cooling rate. The alloy with air cooling (AC) has the best stress rupture properties. The γ′ phase changed into a perfect raft structure during the stress rupture process of the specimens with AC method. However, the γ′ phase changed into a very irregular raft microstructure in the specimens with the water cooling (WC) and furnace cooling (FC) method. The micro-cracks in the specimen with irregular raft make the initiation and interconnection easier than that in the specimen with regular raft. Therefore, the alloy with AC method has optimum microstructure and stress rupture property.

Competitive growth and grain selection were simulated and analyzed during the directional solidification with the conditions for single crystal superalloy DD6 castings using Commercial software, ProCAST. A three dimensional cellular automaton (CA) model coupled with finite-element (FE) heat flow calculation was applied. Measurements at the grain scale were made using the EBSD method at the cross sections in the starter block and grain selector at an interval of 4 mm from the chill surface. The grain characteristics and the rules of competition growth were obtained. The validity of the simulation results were compared with those of the experiment. It concluded that the model-predicted tendency shows satisfactory agreement with the experiment. Increasing the distance from the chill decreases the number of grains, increases the radius of grains and drives the crystallization orientation of grains to principal stress orientation.

We present a computer inexpensive method to calculate the profile of the high resolution diffraction pattern of a single crystal superalloy. The displacement field is taken as the sum of the individual fields of cuboidal inclusions with uniform eigenstrains. Early simulations are compared to experimental data.

The directional solidification behavior of a first generation single crystal superalloy CMSX-6 was investigated. The solidification rate range in 25μm/s to 100μm/s and a thermal gradient G of 30K/cm were used for the present study. The experimental results show that the primary dendrite arm space (PDAS) decreased from (432±8) μm to (369±4) μm as the solidification rate increased, and the sizes of the eutectic pools also decreased as the solidification rate increased. And the volume fractions of eutectic γ/γ' were about 7% to 9% with different solidification rate. The γ/γ'- eutectic was comprised with coarse γ' phase and fine γ/γ' network. The morphology of the γ/γ’ eutectic supported the possibility that the solidification of γ/γ’ eutectic initiates with the formation of fine γ/γ’.

The effects of pouring temperature on the microstructure and the tensile properties of single crystal superalloy DD6 were investigated. The results show that with the decrease of pouring temperature, the primary dendrite arm spacing increases, and the segregation ratio of main elements decreases obviously. DD6 alloy has the similar tensile behavior under the conditions of the pouring temperatures of 1520°C and 1570°C. The pouring temperature has little influence on the yield and ultimate strengths of DD6 alloy. The yield and ultimate tensile strengths of the specimens with the pouring temperature of 1520°C is little lower than the specimens with the pouring temperature of 1570°C under the testing temperatures at room temperature and 650°C, while the specimens with the pouring temperature 1520°C have higher yield and ultimate tensile strength when the testing temperature is higher than 760°C. The pouring temperature did not have an obvious influence on tensile fracture behavior. It has been observed that the tensile fracture surface belongs to quasi-cleaveage fracture mode at testing temperature of 760°C, but the mix characteristic of quasi-cleaveage fracture mode and dimple fracture mode at the testing temperature of 980°C.

An observation was conducted on the microstructures of as-cast as well as solid solution treated the third generation single crystal superalloy DD9 using optical microscope and SEM. The effects of solid solution temperature and time on the eutectic fractions of γ/γ and size of γ of the alloy were investigated. The results showed that the microstructure of DD9 was uniform. W and Re segregated to the dendrite cores while Al, Ta and Nb were enriched in interdendritic regions during solidification. The eutectic fractions of γ/γ reduced with the solid solution temperature raising and the solid solution time prolonging. When solid solution temperature increased to 1340, the eutectic in the alloy was entirely dissolved. The size of γ in dendritic cores was consistent, however the size of γ in interdendritic regions was gradually decreased with the increase of solid solution treatment and time, eventually the sizes of γ were completely uniform at the temperature of 1340

The formation mechanism of stray grains in cross-section change region of casting clusters for single crystal superalloy DD483 were investigated experimentally during the conventional directional solidification. The results showed that the stray grains formation was different on the two sides of the platform. Indeed, stray grains formation in the region of the cross-section change on the shadow side showed a high probability, while the heater side, no stray grains appeared. Additionally, the cooling curves at the cross-section change region also showed that the distribution of temperature was different on the two side of the platform. The formation of stray grain can be attributed to the local undercooling at the cross-section change region is exceed the critical undercooling for nucleation, led to the heterogeneous nucleation, while the asymmetric temperature distribution should be responsible for asymmetrical formation of stray grains on the two sides of the platform.

The tensile properties of a low-cost first generation single crystal superalloy DD16 have been investigated. The results show that values of the tensile strength and yield strength of DD16 alloy were similar at typical temperatures; from room temperature to 760, the yield strength of DD16 alloy increases; However, above 760, the yield strength of DD16 alloy decreases remarkably, and the maximum of the yield strength was 1145.5MPa at 760. From room temperature to 760, the fracture mode was cleavage fracture; But above 760, the fracture characteristics changed from cleavage to dimple.

The third generation single crystal superalloy DD9 was processed with different withdrawal rates and the effect of withdrawal rate on the tensile and stress rupture properties of the alloy was investigated. The relation between the mechanical properties and microstructure of the alloy with different withdrawal rates was discussed. The results showed that the withdrawal rate had a little effect on the tensile properties at 25 of the alloy. The tensile strength at 980 and stress rupture life at 1120/140MPa of DD9 alloy all increased with the increasing of withdrawal rate. The γ precipitates of specimen tensile at 25 had a little extension in the stress orientation. The extension of γ precipitates in the stress orientation at 980 was much more than that at 25. The vertical γ matrix at 980 became thinner and horizontal γ matrix became thicker slightly. The γ precipitates of those had rafted in a direction transverse to the applied stress.

The grain competitive growth and elimination during the directional solidification of a Ni3Al-base single crystal superalloy IC6SX prepared by spiral grain selection method was studied systematically. The experimental results revealed that there were 5 kinds of mechanism during the grain competitive growth and elimination. The grains with preferred growth direction and smaller deviation angle to growth direction have stronger competitiveness, and the mutual thwarting of dendrites played an important role in the processing of grains competitive growth. The results can explain the competitive growth mechanism during the directional solidification and can be used to optimize processing parameters to lay an important foundation for improving preparation processes of single crystal superalloys.

An Al-Si coating was prepared on IC21 alloy by powder pack cementation. The cyclic oxidation tests were carried out at 1150 in air for up to 100 h. The results indicate that the oxidation resistance of IC21 alloy is significantly improved by the Al-Si coating due to the presence of Ni2Al3and β-NiAl enriched outer layer, and Si can effectively supress the outward diffusion of Mo. The oxide scales mainly consist of α-Al2O3, which is the favorite to the oxidation resistance. Phase transformation occurred from β-NiAl to γ-Ni3Al and γ-Ni in the coating during oxidation. The coating still remained a certain amount of β phase after oxidation for 100h, which indicate a good protection. The microstructure change evolution was characterized, and the oxidation behavior of the coating was discussed.

The tensile properties of DD6 alloy with twist low angle boundaries were investigated at 760°C, 850°C and 980°C, and the misorientation angle of twist low angle boundaries of the specimens of the alloy varies from 0º to 12º. The results showed that when the angle of low angle boundaries is relatively small, it has a little effect on the tensile properties of the alloy. It is notable that effects of low angle boundaries on the tensile strength are weaker than those on the tensile elongation. The characteristic of quasi-cleavage, intergranular and dimple fracture were observed on the tensile rupture surfaces of the specimens, and the tensile fracture mechanism was discussed.

The effect of forging pressure on linear friction welding (LFW) behaviour of a single crystal Ni-based superalloy was investigated. Crystal orientations of the specimens were controlled and results indicated that welding success is dependent on proximity of the oscillation direction to the direction. Characterization of the welds included optical examination of the microstructural features of the weld and thermomechanically affected zones (TMAZ) in relation to the parent material. Mechanical properties of the welded material examined via microhardness testing showed an increase in strength in the weld zone (WZ). Microstructural examination indicated that some recrystallization occurred in the WZ, as well as a small amount of distortion of the dendrites in the TMAZ. With increased forge pressure, recrystallized grains remaining in the weld were minimized. © 2012 Trans Tech Publications, Switzerland., 7th International Conference on Processing and Manufacturing of Advanced Materials, THERMEC'2011, 1 August 2011 through 5 August 2011, Quebec City, QC

The behaviour of surface recrystallization and its effect on rupture life of SRR99 single-crystal superalloy are studied. Results show that the recrystallization at lower temperature (such as 1240'C) is of a cellular mode and the recrystallization rate is slower. At higher temperature (such as 1300°C), the recrystallized structure is consisted of grains. γ' particles retard migration of recrystallizing grain boundary. The depth of recrystallized layer exhibits a linear relationship with shot peening intensity. At 950°C, when the nominal stress is lower than 320MPa, the rupture life is not affected seriously, but is reduced when the nominal stress is 400MPa. Deep local recrystallization leads to surprised reduction of rupture life due to the stress concentration resulted from recrystallized volume.

The effects of high temperature exposure simulating service conditions on stress rupture properties were studied for the second generation single crystal superalloy DD6. The specimens with [001] orientation were exposed in air at temperatures of 980°C and 1070°C for 100h to 1000h. They were then tested using conventional mechanical tests at 1070°C/140MPa to determine the effects of exposure on stress rupture properties. The analysis indicated that stress rupture life decreased with increasing exposure time. At the temperature of 980°C, the stress rupture life is more than 180h after exposure for 1000h. When the test temperature increased to 1070°C, the stress rupture life exceeds 100h after 800h exposure. The morphology of γ prime phase after exposure was observed by using scanning electron microscopy (SEM). Morphologies evaluations have shown that alloy DD6 exhibits excellent microstructure stability after exposure. TCP (Topologically Closed Packed) phases have not been observed. It has been also found that the morphology and size of γ prime affected stress rupture life of the alloy. The decrement in stress rupture life with increasing exposure is a result of γ prime rafting.

Directional solidification technique permits materials to grow along specific orientation, in order to obtain mechanical and/or physical anisotropy. The present research attempts to introduce the research work in the field of processing of some advanced materials by innovative directional solidification techniques performed at State Key Laboratory of Solidification Processing and with author’s intended research work. The paper deals with the specific topics on state of the art of directional solidification: single crystal superalloy and Nd-Fe-B alloys under high thermal gradient, Cu-Ni alloys under deep supercooling of the melt. The relevant solidification phenomena, such as morphological evolution, crystal growth for multi-phases in the processing of directional solidification, are discussed briefly. The trends of developments of directional solidification techniques are also prospected.

The tensile creep properties of the second generation single crystal superalloy DD6 with [001], [011] and [111] orientations were investigated under different stress levels at 760°C and 980°C, and the creep mechanism of DD6 alloy was discussed. The results show that DD6 alloy possesses good creep properties, and obviously exhibits anisotropic behavior. The deviation degree between stress axis and [001] crystallizing orientation gradually deceases, while deviation degree between stress axis and [011], [111] crystallizing orientations increases due to lattice rotation during creep. The alloy with [001] orientation provides geometry hardening tendency, but the alloy with [011] and [111] orientations supplies geometry softening tendency. The creep deformation of DD6 alloy with three orientations mainly concentrates on the matrix channels and y' phase is stable at 760°C. The alloy demonstrates complicated creep behavior and the creep deformation with three orientations is situated at both matrix channels and y' phases at 980°C, and the directional coarsening of γ' phase occurs.

Ru coating prior to aluminizing is one of the effective methods to reduce the harmful intermediate layer that forms under the coating (SRZ) on a 4th generation Ni-base SC superalloy. This study examined the short-term isothermal oxidation behavior of this Ru-modified coating at 1373 K in air. Surface observation by SEM showed that the scale becomes flat and uniform in comparison to simple aluminide coating. XRD and cross-sectional analysis results also showed that phase transformation from β-NiAl to γ’-Ni3Al seldom occurs in the Ru-modified coating layer leading to the prevention of local oxidation.

Three unusual Ru-rich phases have been identified in a multicomponent Ni-base single crystal superalloy, including a L21 Ru2AlTa Heusler phase, a B2 RuAl phase and a hcp Re(Ru)-rich δ phase. These phases have their own preferential precipitation location within the dendritic structure. No conventional topologically-close-packed (TCP) phases have been observed with thermal exposure at 950oC for 1500 hours.

Stress rupture and tensile properties of a single crystal superalloy DD32 are investigated comparing with the alloy SRR99. It is shown that the alloy DD32 offers an improved creep temperature capability of more than 60°C at higher stresses. The g¢ precipitates in the stress ruptured samples were rafted to P-N type directional coarsening. The fracture mechanism of the stress ruptured samples was initiated from the micropores.

In order to reveal the effect of oxidation on thin blade walls, a new machine allowing tests up to 1250°C under controlled atmosphere has been designed. Creep tests were performed on MC2 single crystal superalloy at 1150°C, under hydrogenated argon and dry air, but also with a switch from one atmosphere to the other after reaching the steady state creep stage. The results point out the decrease of the minimum creep rate in case of tests performed or at least started under hydrogenated argon, compared with the value obtained under synthetic dry air. This effect of oxidation was attributed to the protective oxide scale formed under hydrogenated argon. The low growth rate of alumina layer leads to a thinner zone affected by metal consumption, which is assumed to be non bearing, and prevents from vacancy flux toward the alloy. The second point results in slowing down creep mechanisms controlled by diffusion and therefore dislocation motion and microstructure evolution. Thermogravimetric tests confirm the difference of oxidation kinetics regards to environment (hydrogenated argon and dry air). However, oxide scales have different microstructures on thermogravimetric and creep samples when tested under air.

Des essais d'oxydation isotherme a 1100°C sous oxygene ont ete realisees sur des versions commerciales et modifiees de plusieurs superalliages base-Ni monocristallins (CMSX2, CMSX4, CMSX6, AM3, MC2, SRR99). Les couches formees a 900°C sont adherentes et revelent la structure dendritique du substrat sous-jacent. Ces couches sont composees de trois sous-couches riches respectivement en (Ti,Co,Ni), Cr et (Ta,W,Ti). Une zone appauvrie en γ' due a l'oxydation interne d'Al est aussi identifiee. Les couches d'oxydes formees a 1100°C sont peu adherentes et s'ecaillent quelque soit la vitesse de refroidissement. La difference AT entre la temperature d'oxydation et la temperature d'initiation de l'ecaillage depend de l'epaisseur ξ de la couche d'oxyde et de l'energie de rupture interfaciale γ F . Pour un couple oxyde/metal, (ΔT) 2 ξ/γ F est constant (modele d'EVANS et LOBB). Les alliages AM3 et CMSX4 suivent ce modele alors que les alliages AM3 et MC2 modifies s'en ecartent. L'addition de Si et Hf et la substitution de Mo par Re ou Pt et de Ti par Ta ameliorent la resistance a l'oxydation a 900°C et l'adherence des couches d'oxydes a 1100°C.