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MICROSTRUCTURE EVOLUTION AND INTERFACIAL FORMATION MECHANISM OF WIDE GAP BRAZING OF DD5 SINGLE CRYSTAL SUPERALLOY
Yuan SUN,Jide LIU,Xingyu HOU,Guanglei WANG,Jinxia YANG,Tao JIN,Yizhou ZHOU
Acta Metall Sin    2016, 52 (7): 875-882.   doi:10.11900/0412.1961.2015.00622
Accepted: 29 April 2016

Abstract67)   HTML2)    PDF (1174KB)(521)      

Ni-based single-crystal superalloy DD5 has excellent high temperature properties, which is the preferred raw material for aero-engine turbine blade in recent year. In this research, DD5 superalloy was brazed by different contents of Ni-Co-Cr-W-B+DD99 mixed powder filler alloy. The microstructure evolution and interfacial formation mechanism of DD5 superalloy brazing joint were analyzed by SEM and EPMA. The mechanical properties of joint after solid solution treatment and aging treatment were tested. The results show that γ-Ni primary phase formed firstly in the Ni-Co-Cr-W-B/DD99 interface during the brazing process, and then B element segregated and precipitated to fine granular M3B2 type boride. The residual liquid phase solidified and formed lastly to the M3B2 phase, γ+γ′ eutectic phase and γ-Ni+Ni3B+CrB eutectic phase during cooling. With increasing the ratio of DD99 in mixed powder filler alloy, the low melting point eutectic phase and borides in the joint decrease and the uniformity of composition and microstructure of joint improve. When the ratio of DD99 increased to 70% (mass fraction) in the mixed powder filler alloy, it can be observed that element of B diffused to DD99 additive powder which result ed in the decrease of low melting point eutectic phases and brittle compounds. The high temperature tensile properties of joint is 1010 MPa at 870 ℃.

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CONVERGING COMPETITIVE GROWTH IN BI-CRYSTAL OF Ni-BASED SUPERALLOY DURINGDIRECTIONAL SOLIDIFICATION
Songsong HU,Lin LIU,Qiangwei CUI,Taiwen HUANG,Jun ZHANG,Hengzhi FU
Acta Metall Sin    2016, 52 (8): 897-904.   doi:10.11900/0412.1961.2015.00631
Accepted: 10 May 2016

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Ni-based single crystal superalloy has been widely used in turbine blades due to its excellent high temperature mechanical behavior. In order to completely exhibit high temperature mechanical properties, the seed method has been used to produce Ni-based single crystal components for [001] orientation paralleling to main force direction. Stray crystals, which unexpectedly nucleate in the melt-back region, will competitively grow with seed during directional solidification. It is important to profoundly understand the mechanism of competitive growth to find ways of overgrowing stray crystal during producing Ni-based single crystal components. However, within the published research there are conflicting views on the mechanism of competitive growth at converging case. Bi-crystal converging competitive growth was investigated in Ni-based single crystal superalloy with different pulling rates using seed technology. A series of polishing and imaging quenching interface were done for the positional relationship of dendrites near grain boundary in 3D reference. It was found that solidification microstruc tures were different with different crystal orientations. Unfavorable oriented dendrite tilting to heat flux restrained favorable oriented dendrite aligning to heat flux mainly thought inserting into the favorable oriented dendrites channel, and this resulted in unfavorable oriented dendrite overgrowing favorable oriented dendrite at low pulling rate. However, at high pulling rate the unfavorable oriented dendrites mainly blocked by grain boundary favorable oriented dendrite and the grain boundary grew paralleling to favorable oriented dendrite core. Favorable oriented dendrite being depressed and vanished, owning to that unfavorable oriented dendrite inserting into favorable oriented dendrites channel result in adjusting primary dendrite spacing, is the main factor to favorable oriented grain overgrew by unfavorable oriented grain. According to above mechanism, effect of pulling rate on competitive growth at converging case was interpreted. This could broaden our understanding of competitive growth at converging case in 3D reference.

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RECENT PROGRESS ON EVOLUTION OF PRECIPI-TATES IN INCONEL 718 SUPERALLOY
Yongchang LIU, Qianying GUO, Chong LI, Yunpeng MEI, Xiaosheng ZHOU, Yuan HUANG, Huijun LI
Acta Metall Sin    2016, 52 (10): 1259-1266.   doi:10.11900/0412.1961.2016.00290
Accepted: 05 September 2016

Abstract204)   HTML16)    PDF (1049KB)(1688)      

For the manufacture of complicated metallic structural components in power plants, aerospace and defense industry, Inconel 718 superalloy has been widely employed. High-temperature fatigue resistance and creep rupture strength of Inconel 718 superalloy are susceptible to the microstructure evolution in manufacture processing. Previous research work is generally focused on the parameter optimization of hot working processes (directional solidification, heat treatment, forging and welding). Relationships between the cold deformation, hot working, welding and the high-temperature mechanical performance, are seldom discussed, especially in the light of precipitate control . In this work, various types of secondary phases in Inconel 718 alloy are reviewed, including the primary strengthening phase (γ'' phase), secondary strengthening phase (γ' phase), equilibrium phase of γ'' phase (δ phase), MX-type carbonitride and Laves phase. Precipitation mechanisms of secondary phases in Inconel 718 alloy are also reviewed, as well as the effects of different types of precipitates on high-temperature performance of the Inconel 718 alloy. With respect to the high-energy electron beam welding of Inconel 718 alloys, factors contributing to the cracking in heat affected zone are indicated.

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EVALUATING SERVICE TEMPERATURE FIELD OF HIGH PRESSURE TURBINE BLADES MADE OF DIRECTIONALLY SOLIDIFIED DZ125 SUPERALLOY BASED ON MICRO-STRUCTURAL EVOLUTION
Yadong CHEN, Yunrong ZHENG, Qiang FENG
Acta Metall Sin    2016, 52 (12): 1545-1556.   doi:10.11900/0412.1961.2016.00170
Accepted: 18 October 2016

Abstract59)   HTML5)    PDF (9498KB)(527)      

To get the actual service temperature distribution of turbine blades in aeroengines is very important for the design and maintenance. However, the acquisition of service temperature distribution has always been a challenge due to the complex and severe working condition of turbine blades. In this work, one turbine blade made of directionally solidified DZ125 superalloy was investigated after the service in air for 900 h. The microstructural evolution of DZ125 superalloy after thermal exposure at 900~1100 ℃ without the stress in different time period was also investigated, for comparison. According to microstructural degradation behaviors in the dendritic region, interdendritic region, carbides and grain boundary of DZ125 superalloy before and after service, the volume fraction of γ’ precipitates in the dendritic region was determined as the quantitative characterization parameter. A method to evaluate the service temperature of turbine blades was developed, based on the quantitative characterization of microstructural evolution, such as the relationship between the thermally exposured temperature and volume fraction of γ’ precipitates. The equivalent average service temperature (Tave) and the equivalent maximum service temperature (Tmax) were proposed based on the assumption of the constant temperature during service and the nearly service condition with variable temperature of blades, respectively. The results indicate that the service temperature was higher in the middle of the blade, and became lower at the locations closer to the tip or the root. For each cross-section, the service temperatures of the serviced blade in the descending order were leading edge, pressure side, trailing edge and suction side. The highest service temperature of 1050~1100 ℃ appeared at the leading edge in the middle of the blade. The distribution trend of Tave agreed well with that of Tmax, but Tmax was higher than Tave in some locations of the blade. This work suggests that the evaluation results of Tmax were more reasonable than those of Tave. This method would be helpful to establish the assessment method of the service-induced microstructural damage in turbine blades made of directionally solidified superalloys.

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Effects of W on Microstructural Stability of the Third Generation Ni-Based Single Crystal Superalloys
Bo WANG,Jun ZHANG,Xuejiao PAN,Taiwen HUANG,Lin LIU,Hengzhi FU
Acta Metall Sin    2017, 53 (3): 298-306.   doi:10.11900/0412.1961.2016.00379
Abstract27)   HTML2)    PDF (6712KB)(957)      

Ni-based single crystal superalloys are widely used in the manufacture of aero engine turbine blades because of the excellent mechanical properties at high temperature. With the development of single crystal superalloys, the content of refractory elements is constantly increased (especially Re) to improve the high temperature capability, which in turn leads to the decrease in microstructural stability of alloys, such as the TCP phase precipitation. It is important to find one element which not only can maintain high temperature performance but also does not evidently promote TCP phase precipitation and is very cheap in price to replace Re partially. W is one of the most important solution strengthening elements in superalloys, its diffusion rate in Ni matrix is close to Re and far below the other alloying elements, meanwhile, the advantage of low price make it to be the most suitable substitute of Re. However, there is little work about the effect of W on microstructural stability in Re contained third generation superalloys. In this work, the effects of W on the elemental segregation, elemental partitioning ratio of γ /γ′, microstructure evolution and TCP phase precipitation during thermal exposure at 950, 1000 and 1050 ℃ have been investigated in a third generation Ni-based single crystal superalloys with varied contents of W (6%~8%, mass fraction). The results show that the addition of W has no obvious effect on segregation of the alloying elements of as-cast alloys as well as the morphology, size and volume fraction of γ′ phase after heat treatment. During the thermal exposure at 950 ℃, the connection and deformation of γ′ phase are accelerated, but its coarsening rate is decreased with increasing W content. The TCP phases precipitated in three alloys during thermal exposure are mainly μ phase and σ phase. The area fraction of TCP phases is increased slightly with the W addition during thermal exposure, which is the largest at 1000 ℃, less at 950 ℃ and the least at 1050 ℃.

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Effect of Ru on the Solidification Microstructure of a Ni-Based Single Crystal Superalloy with High Cr Content
Likui NING,Jian TONG,Enze LIU,Zheng TAN,Huisi JI,Zhi ZHENG
Acta Metall Sin    2017, 53 (4): 423-432.   doi:10.11900/0412.1961.2016.00291
Abstract73)   HTML3)    PDF (10721KB)(1156)      

Ni-based single crystal superalloys have been widely used in manufacturing the critical components of aero-engines, such as turbine blades and vanes. Improvements in phase stability on the addition of Ru are well known in the field of Ni-based superalloy development. Cr is beneficial to hot co-rrosion resistance of Ni-based superalloys. Generally, superalloys which used under easy corrosion conditions should contain high levels of Cr. Early researches about the influence of Ru on solidification microstructures in Ni-based single crystal alloys are mostly focused on low-Cr systerms (<6%). Since Cr has complex interactions with Ru, it is meanful to study the effects of Ru on solidification microstructures in high-Cr (>10%) Ni-based single crystal superalloy systems. The materials used in this work are Ni-based single crystal superalloy with high Cr content. Three superalloys by changing Ru addition (0, 1.5%, 3%, mass fraction) were designed. By observing the as-cast structure, the effect of Ru on the elements distribution and the precipitation characters of different phases in these alloys were studied. It is found that as the Ru content increases, the primary and secondary dendrite arm spacings decrease gradually; the volume fraction of (γ+γ′) eutectic increases firstly and then decreases; the γ′ size is reduced progressively. The addition of 3%Ru leads to the formation of β-NiAl phase, which contain a certain amount of Cr, Co and Ru except the basic elements Ni and Al. The typical "reverse partitioning" of other alloying elements is exhibited with the addition of Ru, while the formation of β-NiAl phase can reduce the "reverse partitioning" of other alloying elements. The addition of Ru could enhance the segregation of positive segregation elements Ta, Al and negative segregation element Re while reduce the segregation of positive segregation elements Mo and Cr.

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Incipient Melting Behavior and Its Influences on the Mechanical Properties of a Directionally Solidified Ni-Based Superalloy with High Boron Content
Hongwei ZHANG,Xuezhi QIN,Xiaowu LI,Lanzhang ZHOU
Acta Metall Sin    2017, 53 (6): 684-694.   doi:10.11900/0412.1961.2016.00495
Accepted: 01 April 2017

Abstract154)   HTML14)    PDF (19817KB)(675)      

A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 γ′ phase. To achieve a uniform distribution of precipitated γ′ particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of γ′/γ eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and η phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in γ matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 ℃ and 1170 ℃. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical γ dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, γ matrix and γ′/γ eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes γ matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200 ℃ and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes γ matrix outside the incipiently melted circle melt when the temperature is higher than 1210 ℃. Especially, IMR swallows up plenty of γ matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 ℃ which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.

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Precipitation Behavior of δ Phase of Deformation Induced GH3625 Superalloy Hot-Extruded Tube
Yutian DING,Yubi GAO,Zhengyi DOU,Xin GAO,Dexue LIU,Zhi JIA
Acta Metall Sin    2017, 53 (6): 695-702.   doi:10.11900/0412.1961.2016.00508
Accepted: 17 February 2017

Abstract72)   HTML0)    PDF (6704KB)(1028)      

GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the γ'' is a metastable phase, easily transformed into stable δ phase, or δ phase directly formed in the γ matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of δ phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 ℃ for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of δ phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that δ phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of δ phase at the deformation bands increases with the increase of cold deformation. The morphologies of δ phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of δ phase increases which grows according to LSW theory. At 800 ℃, the relationship between the precipitation content of δ phase and ageing time follows Avrami equation. As cold deformation increases, the content of δ phase increases, the time index n decreases, whereas the δ phase precipitation rate increases. Cold deformation promotes the precipitation of δ phase. The solute drags of Nb in soild solution and pinning of δ phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at ε =35% but no obvious change at ε ≥50%.

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Effects of Re-Melting Processes on the Tensile Properties of K452 Alloy at High Temperature
Jinxia YANG,Futao XU,Donglin ZHOU,Yuan SUN,Xingyu HOU,Chuanyong CUI
Acta Metall Sin    2017, 53 (6): 703-708.   doi:10.11900/0412.1961.2016.00286
Accepted: 16 March 2017

Abstract58)   HTML4)    PDF (3849KB)(2112)      

K452 alloy is a nickel-based cast superalloy having the good tensile properties at high temperature and excellent corrosion resistance. It has been applied as a blade material of engines when environmental temperature is not above 950 ℃. It is found that the tensile properties of the alloy have become more scattered and unstable although its chemical compositions are not changed. Hence, the tensile properties of the alloy were studied in order to increase its stability at high temperature and improve its applied properties. Tensile specimens were prepared using the different re-melting processes. Tensile tests were done at 900 ℃. When the pouring temperature was 1430 ℃, tensile properties were not only lower than expected, but also had great degree of dispersion, i.e., the vales of ultimate strengths changed in the range of 410 MPa and 510 MPa, and the elongations changed in the range of 3.5% and 22.0%, the average contents of O and N were the highest among three tested conditions. The highest N content was 0.0028%. And the shrinkage area was higher than those in other two re-melting processes. When the pouring temperature was 1500 ℃, the tensile properties were improved, and their changing scopes became small, the average contents of O and N decreased, the shrinkage area decreased. When the refining temperature was 1590 ℃ and the holding time was 5 min, both average contents of O and N were decreased greatly, the shrinkage was not seen in the fracture surfaces. And the tensile properties were improved. Furthermore, their changing scopes were very small.

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Effects of Nitrogen Addition on Microstructure and Grain Boundary Microchemistry of Inconel Alloy 690
Bo CHEN, Xianchao HAO, Yingche MA, Xiangdong CHA, Kui LIU
Acta Metall Sin    2017, 53 (8): 983-990.   doi:10.11900/0412.1961.2016.00545
Accepted: 14 June 2017

Abstract132)   HTML4)    PDF (938KB)(688)      

Inconel alloy 690 is an austenitic nickel-based corrosion resistant alloy with about 30%Cr, which is considered as the most ideal steam generator tubing materials in nuclear power plants because of its superior resistance to intergranular attack (IGA). However, the existence of impurities and the addition of minor alloying elements cause significant difference of carbide morphology, microstructure and chromium depletion of Inconel alloy 690. In this work, the microstructure and grain boundary chemistry of Inconel alloy 690 with four different nitrogen contents have been investigated by SEM and TEM. Stacking fault probability (SFP) and IGA with respect to the microstructure was tested and analyzed. The results indicated that thermal treatment at 715 ℃ following solution annealing (SA) at 1080 ℃ caused a wide range of intergranular carbide morphology with the associated chromium depletion in the vicinity of grain boundaries. With the increasing of nitrogen content, the characters of the carbides ranged from thin continuous bands along boundaries to coarse discrete particles. Stacking fault probability was increased with the increasing of nitrogen content, and the value reached the peak at 100×10-6 of nitrogen content, then it dropped. The corrosion tests showed that moderate nitrogen content alloy performed favorable intergranular attack correlated with the presence of semi-continuous grain boundary carbide and chromium depletion was mitigated. The consequent nitrides were appeared in high nitrogen alloy. So, about 100×10-6 contents of nitrogen in alloy 690 is suitable by synthesis considering of carbides, nitrides and chromium depletion.

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INFLUENCE OF Re ON MICROSTRUCTURESOF A DIRECTIONALLY SOLIDIFIEDNi-BASED SUPERALLOY
Siqian ZHANG,Dong WANG,Di WANG,Jianqiang PENG
Acta Metall Sin    2016, 52 (7): 851-858.   doi:10.11900/0412.1961.2015.00600
Accepted: 21 March 2016

Abstract86)   HTML0)    PDF (1805KB)(504)      

Up to now, considerable effort has been expended in attempts to investigate the influences of Re on Ni-based single crystal superalloys. However, few study had elucidated the influences of Re on carbide, boride and grain boundary. Therefore, the influence of a 2%Re (mass fraction) addition on the as-cast and heat-treated microstructures of a Ni-based directionally solidified superalloy was investigated by comparison with Re-free alloy using SEM, EPMA and TEM. The results show that Re accelerates the precipitation of μ phase in the periphery of eutectic and at grain boundary for as-cast microstructure. After heat treatment, Re also accelerates the precipitation of phase in the vicinity of primary MC carbide and at grain boundary. For 0Re alloy, there are small number of M6C carbide in the vicinity of primary MC carbide and M23(C, B)6 boro-carbide at grain boundary. For 2Re alloy, a large amount of blocky μ phase enveloped by thick γ′-film is found in the vicinity of primary MC carbide and at grain boundary. Enrichment of B along the grain boundary is observed in 0Re alloy. On the contrary, relatively uniform distribution of B is found in 2Re alloy. The precipitation mechanism of μ phase during the process of heat treatment is also analyzed.

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TRANSIENT LIQUID PHASE BONDING OF SECOND AND THIRD GERNERATION Ni-BASED SINGLE CRYSTAL SUPERALLOY WITH Hf-CONTAININGINTERLAYER ALLOY
Zhengrong YU,Xianfei DING,Lamei CAO,Yunrong ZHENG,Qiang FENG
Acta Metall Sin    2016, 52 (5): 549-560.   doi:10.11900/0412.1961.2015.00408
Accepted: 05 January 2016

Abstract68)   HTML4)    PDF (2138KB)(579)      

A Hf-containing Ni-based alloy was used as the interlayer alloy of TLP bonding for the 2nd (CMSX-4, as-cast condition) and 3rd (SXG3, standard heat treatment condition) generation Ni-based single crystal superalloys containing Re in this work, and the microstructure, composition and micro-hardness of bonding zone were characterized. The results show that the TLP bonding of CMSX-4 and SXG3 alloy were completed after bonded at 1290 ℃ in vacuum for 24 h. These TLP bonding process of CMSX-4 and SXG3 alloys can be explained well using classical TLP model. The diffusion affected zone was not observed during the TLP bonding process. In addition, the heat treatment process of CMSX-4 is shortened by 24 h resulted from the solid solution heat treatment of CMSX-4 alloy has been completed after the process of TLP bonding. The isothermal solidification stage of SXG3 alloy was also accelerated due to the precipitation of HfC at the bonding temperature, resulting in the reduced Hf concentration of Hf in the melting zone. This work also indicates that the interfacial stability of low angle grain boundaries can be investigated by the TLP bonding. The critical misorientation value for discontinuous precipitation of SXG3 alloy along TLP bonding grain boundaries by Hf-containing interlayer alloy was in between 10° and 17° after heat treatment at 1150 ℃.

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EFFECT OF Re AND W ON RECRYSTALLIZATION OF AS-CAST Ni-BASED SINGLE CRYSTAL SUPERALLOYS
Sheng PU,Guang XIE,Li WANG,Zhiyi PAN,Langhong LOU
Acta Metall Sin    2016, 52 (5): 538-548.   doi:10.11900/0412.1961.2015.00456
Abstract88)   HTML3)    PDF (1916KB)(316)      

Ni-based single crystal (SX) superalloys have been used as blades in aero-space industry and land-based applications due to their excellent high-temperature properties. However, residual strain is introduced into as-cast SX superalloy blades during the manufacturing process, such as casting, grinding or shot peening, and so on. Recrystallization (RX) occurs easily during subsequent high temperature heat treatment. In previous work, it is believed that RX has detrimental effect on the mechanical properties of SX superalloy. Furthermore, in order to improve the mechanical properties, more and more refractory elements, such as W, Re, Mo, Ta, are added into SX superalloys. However, so far, few reports about the effect of refractory elements on the RX in as-cast SX superalloys have been available. In the present work, the effect of Re and W on the RX behavior of as-cast Ni-based SX superalloy was studied. Single crystal superalloys with different Re and W contents were annealed at 1230~1330 ℃ after indened using Brinell hardnesstester. It is found that RX grains form at the surface under indentation and grow preferentially along the dendritic cores. Subsequent growth of RX is impeded by the residual coarse γ' and γ +γ' eutectics in the interdendritic regions. Both the volume fraction of γ +γ' eutectics and γ' solvus temperature are increased with the addition of Re and W, which are attributed to the increase of RX threshold temperature. For all SX superalloys studied in this work, RX area increases with the increase of annealing temperature due to the dissolution of γ' and γ+γ' eutectics. At the same annealing temperature, in comparison to Re, W shows more effect to inhibit RX growth. Additionally, SX superalloy containing both Re and W has the smallest RX area in the present experiments.

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DEGENERATION PROCESS AND MECHANISM OF PRIMARY MC CARBIDES IN A CAST Ni-BASED SUPERALLOY
Wen SUN,Xuezhi QIN,Jianting GUO,Langhong LOU,Lanzhang ZHOU
Acta Metall Sin    2016, 52 (4): 455-462.   doi:10.11900/0412.1961.2015.00399
Accepted: 25 January 2016

Abstract72)   HTML2)    PDF (1328KB)(526)      

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During long-term thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+γM6C+γ′, MC+γM6C + M23C6+ γ′ and MC+γM6C + M23C6+η. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of γ′ phase, coarsening of grain boundaries and precipitation of topologically close-packed (TCP) phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 ℃ for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field (HAADF) mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the γ matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the γ matrix, while Ti, W and Mo come from both primary MC and γ matrix. The precipitation of η phase is determined by the atomic fraction of Ti+Nb+Ta+Hf and atomic ratio of (Ti+Nb+Ta+Hf)/Al and its amount is affected by the degeneration degree of primary MC carbide. The higher the degeneration degree, the larger the tendency for the precipitation of the η phase.

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EFFECTS OF TOPOLOGICALLY CLOSE PACKED μ PHASE ON MICROSTRUCTURE AND PROPERTIES IN POWDER METALLURGY Ni-BASED SUPERALLOY WITH Hf
Yiwen ZHANG,Benfu HU
Acta Metall Sin    2016, 52 (4): 445-454.   doi:10.11900/0412.1961.2015.00389
Abstract53)   HTML0)    PDF (1349KB)(376)      

It is widely acknowledged that topologically close packed (TCP) phases are detrimental to comprehensive properties of superalloys, as TCP phases deplete strengthening elements from matrix and easily become crack initiations. In this work, the precipitation kinetics and morphology of topologically close packed μ phase in FGH4097 powder metallurgy (PM) superalloy with (0~0.89%)Hf and the effect of μ phase on the mechanical properties of FGH4097 PM superalloy billet with 0.30%Hf has been investigated. The results showed that μ phase precipitated obviously in the alloys with 0.30%Hf and 0.89%Hf after long-term ageing at 750~900 ℃, the amount and size of μ phase increased as the ageing temperature, ageing time and Hf content increasing. μ phase mainly precipitated in grains with strip and flake shapes. After long-term ageing at 550~650 ℃, no μ phase precipitated in FGH4097 PM superalloy billet with 0.30%Hf and the tensile properties and stress-rupture properties at high temperature were not decreased, which showed excellent microstructure stability. After long term ageing at 750 ℃, precipitated μ phase had little effect on tensile strength at high temperature, however, the tensile ductility increased and high temperature stress rupture life reduced, and the stress rupture ductility increased by about 20%. In this work, the precipitation behavior of μ phase, the redistribution of elements in γ solid solution and the FGH4097 PM superalloy fracture morphology characteristics have been discussed in detail. The mechanism of the brittle and ductile dual effect of μ phase on the mechanical properties has been explained. The methods of controlling and avoiding excessive μ phase precipitation which leaded to performance deterioration have been proposed.

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FRECKLE FORMATION DURING DIRECTIONALSOLIDIFICATION OF COMPLEX CASTINGSOF SUPERALLOYS
Dexin MA
Acta Metall Sin    2016, 52 (4): 426-436.   doi:10.11900/0412.1961.2015.00379
Abstract109)   HTML1)    PDF (1300KB)(491)      

Freckles are a detrimental grain defect formed during directional and single crystal solidification of superalloy components leading to a high rejection rate. Based on the experimental and theoretical studies over the past forty years, the occurrence of freckles is generally believed to be mainly dependent on the alloy chemistry and process parameters, while the geometrical factor of castings was hardly taken into account. In the present work, a series of superalloy castings with complex geometry were directionally solidified in a production-scale Bridgman furnace. Some new features of freckle appearance have been observed, indicating new aspects of freckle formation. The freckles are preferably formed on the edges instead of on the plane surfaces of the castings. Correspondingly, freckles were found exclusively on the casting surface having positive curvature, whereas no freckles formed on the surface with negative one. The casting portions having inward sloping surfaces are very freckle prone, while those with outward sloping surfaces are absolutely freckle free. Therefore, as an independent factor the geometrical feature of the castings can more effectively affect the freckle formation than the local thermal conditions. It was also observed that freckles could occur not only on the external surfaces, but also inside the castings where a core was inserted, because both the shell and the core wall can provide very high permeability for freckling convection in the mushy zone. Based on this wall effect, all the important phenomena observed in the present work, such as the edge effect, the step effect, the sloping effect and the curvature effect on freckle formation in complex castings of superalloys, can be reasonably explained.

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EFFECTS OF IN718 GRAIN SIZE ON ULTRASONICBACKSCATTING SIGNALS AND ITS NONDE-STRUCTIVE EVALUATION METHOD
Yongfeng SONG, Xiongbing LI, Haiping WU, Jiayong SI, Xiaoqin HAN
Acta Metall Sin    2016, 52 (3): 378-384.   doi:10.11900/0412.1961.2015.00367
Abstract68)   HTML2)    PDF (3253KB)(287)      

Superalloy In718 enjoys wide application in such crucial parts as turbine engine disks due to high strength, great toughness and corrosion resistance in different temperature environment. Since the mechanical properties of superalloy In718 are greatly influenced by the grain size, a nondestructive detection method is studied in order to determine the grain size quickly and effectively. In this work, superalloy In718 samples of different grain sizes were produced and the empirical mode decomposition (EMD) method was employed to find the characteristics of the time-frequency domain of the ultrasonic backscattering signals. Then the effects of the grain size over the intrinsic mode function (IMF) of different frequency bands were analyzed to seek the relations between the grain size and the power of the IMF signals. The original backscattering signals and IMF1 (the first IMF) signals barely respond to the change of the grain size because of their wide frequency bandwidths; the distribution of the frequency domain of the IMF2 signals is centralized and the amplitude of the peak frequency increases with the grain size, and the correlation coefficient between the power and the grain size is 0.995, much higher than that of other modes. This method eliminates the components irrelative to the grain size and takes the IMF2 components which fully reflect the intensity of the grain scattering as the characteristic signals of the grain size evaluation to build an ultrasonic backscattering EMD model evaluating the grain size of superalloy In718. The actual measurement results of the grain size show that the sensitivity of this method is 3.7 times over the traditional backscattering method; the evaluation errors over the two verification test samples are -3.72% and 2.87%, apparently more accurate than the ultrasonic velocity method; compared with the attenuation method, this method requires no information of the thickness so that the evaluation results are independent of the thickness measuring error; compared with the metallographic method, this method is more efficient and requires no damage on the components to be evaluated.

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HIGH-CYCLE FATIGUE BEHAVIOR OF K416B Ni-BASED CASTING SUPERALLOY AT 700 ℃
Jun XIE, Jinjiang YU, Xiaofeng SUN, Tao JIN
Acta Metall Sin    2016, 52 (3): 257-263.   doi:10.11900/0412.1961.2015.00281
Abstract65)   HTML0)    PDF (9879KB)(526)      

Ni-based speralloys have been widely used to make the blade parts of the advanced aeroengines for their high temperature tolerance and good mechanical property. During high temperature service, the materials endure the effects of temperature and alternating load, causing high-cycle fatigue deformation on the hot-end components. Meanwhile, the fatigue behaviors of the alloy are closely related to the deformation mechanisms and its microstructure characteristics, such as the size, distribution and morphology of γ' phase and carbides, and the fatigue fracture of the using materials possesses unpredictability. Therefore, investigating fatigue behaviors of the material is of significance in alloy design and life prediction. But the high-cycle fatigue behavior of K416B superalloy with high W content is still unclear up to now. For this reason, by means of high-cycle fatigue property measurement and microstructure observation, the high-cycle fatigue behavior of K416B Ni-based superalloy at 700 ℃ has been investigated. The results show that at 700 ℃ and stress ratio R=-1, the high-cycle fatigue life of K416B superalloy decreases with the stress increasing, and high-cycle fatigue strength of the alloy is 175 MPa. At the condition of low stress amplitude, the deformed dislocations may slip along different orientations in the matrix. With the stress amplitude increasing, the dislocations may shear into γ' phase and form the stacking fault. During tension and compression high-cycle fatigue, multiple slip systems are activated in the alloy, and the distortion occurs along various directions, resulting in stress concentration on the regions of γ +γ' eutectic and carbides. The crack sources may be initiated at the eutectic and blocky carbide near the surface of the alloy. As high-cycle fatigue goes on, the cracks propagate along the inter-dendrite in expansion region, and the typical cleavage fracture occurs in the final rupture region.

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STUDY ON THE PRECIPITATION AND DISSOLUTION OF σ PHASE IN A HOT CORROSION RESISTANCE CAST NICKLE BASE SUPERALLOY
Jieshan HOU,Jianting GUO,Chao YUAN,Lanzhang ZHOU
Acta Metall Sin    2016, 52 (2): 168-176.   doi:10.11900/0412.1961.2015.00358
Abstract50)   HTML2)    PDF (10914KB)(180)      

The experimental alloy is designed and employed in high-performance industrial gas turbines as low-pressure turbine blades, working in temperature range of 750~900 ℃. The alloy contains high levels of refractory elements in order to increase the high-temperature mechanical properties. However, this can make the alloy prone to the formation of σ phase during service, which could deteriorate the properties further if the fraction of σ phase exceeds the safety allowances. In this study, the formation of σ phase during long-term thermal exposure, dissolution of the σ phase during rejuvenation process and their influence on stress-rupture properties of a hot-corrosion resistant nickel base superalloy have been investigated. During long-term thermal exposure at 800~900 ℃ for up to 1×104 h, the σ phase formation is mainly in dendrite cores with a few at interdendritic regions. As the aging temperature increases, the precipitation rate of σ phase increases and the incubation time for nucleation of σ phase decreases. From the kinetic analysis, the σ phase form firstly in the vicinity or on the M23C6 in dendrite cores with the strong segregation of W, Cr and Co. The calculated activation energies of σ formation show that the early stage is related to Co and Cr diffusions and the steady stage is related to Mo diffusion. During solid solution process at 1000~1170 ℃, the σ phase precipitated during long-term thermal exposure dissolves to γ matrix. As the solid solution temperature is higher, the dissolution of σ phase becomes faster. Moreover, the σ phase does not embrittle the alloy. The reheat treatment of the alloy leads to the dissolution of precipitated σ phase and further prolongs the stress-rupture life efficiently.

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