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
DZ444合金是一种低成本、抗热腐蚀定向凝固镍基高温合金,在K444等轴晶合金基础上设计开发而成。该合金的使用温度较K444提高30 ℃左右,力学性能与国外同类型合金MGA1400DS相当,优于GTD111DS和IN792DS等合金,主要用于制造现代船舶和地面燃气轮机中工作温度不高于930 ℃的涡轮工作叶片等高温零部件[15,16]。由于该合金包含微量B元素(0.01%, 质量分数,下同)和少量Hf元素(0.5%),在偏析较严重的情况下,理论上有形成低熔点相的倾向性,因此本工作向合金中添加较高含量的B (0.09%),考察合金在固溶处理期间的初熔规律、初熔与B或硼化物的关系以及对合金力学性能的影响,对铸造镍基高温合金的化学成分设计和热处理工艺选择具有一定的指导意义。
实验合金的化学成分(质量分数,%)为：C 0.07,B 0.09,Cr 15.8,Co 10.7,W 5.3,Mo 2.0,Al 3.0,Ti 4.7,Ta 0.5,Hf 0.5,Ni余量,这相当于在DZ444合金中加入了过量的B。在真空感应炉中冶炼母合金,然后在定向凝固炉中制备定向凝固试棒。从试棒上切取小块试样,在不同温度(1150、1160、1170、1180、1190和1200 ℃)下进行不同时间(5、10和30 min)的固溶处理,分别采用水淬(WQ)和空冷(AC) 2种方式冷却试样。
将试棒分为3组,分别在1210、1230和1250 ℃ 3个温度下固溶处理2 h,接着空冷至室温,然后进行两级时效(1080 ℃、4 h、AC和850 ℃、24 h、AC)处理。根据由低到高的固溶温度,以上3个完整热处理制度对应表示为HT1、HT2和HT3。热处理后,在AG-X250kN型电子拉伸试验机上测试试棒在900 ℃下的拉伸性能,在RJ-50型持久试验机上测试试棒在930 ℃、275 MPa下的持久性能。
使用GX51型光学显微镜(OM)、配有能谱仪(EDS)的JEM 6340型场发射扫描电镜(FESEM)和Tecnai G2 F20透射电镜(TEM)观察显微组织并测定成分,使用EPMA-1610型电子探针仪(EPMA)测定元素面分布。试样腐蚀采用电解方法：一种电解液(A)是体积比为1∶3∶5的HNO3+HCl+甘油溶液,腐蚀掉的是
铸态试样在1150和1160 ℃保温不同时间(5、10和30 min)后,没有发生初熔现象。在1170 ℃处理5 min,也未发生初熔(
当处理温度从1180 ℃增加到1200 ℃、保温时间从5 min延长到30 min时,初熔圈内的基体首先熔化,接着各团聚相先后溶解或熔化,最后整个初熔圈内的原始组织都变成了金属液,即初熔池形成(
DZ444合金(B含量为0.01%)的标准热处理工艺为1210 ℃、2 h、AC+1080 ℃、4 h、AC+850 ℃、24 h、AC。在本实验中,对B含量为0.09%的DZ444合金进行相似的热处理,但是固溶温度选取1210、1230和1250 ℃ 3个温度,以研究初熔在热处理期间的变化及其对力学性能的影响。
统计表明,当温度从1210 ℃升至1230 ℃时,初熔区变化不是很大,但是当温度升至1250 ℃时,其尺寸和面积分数都显著增大(
研究结果表明,在高温时效(1080 ℃、4 h、AC)期间,初熔区的尺寸、形貌和分布等不发生明显变化。但是,团聚相中蜂窝状的Ni5Hf相消失,取而代之的是富Hf的颗粒状
经完整热处理(HT1、HT2和HT3)后,测试了合金在900 ℃下的拉伸性能和在930 ℃、275 MPa条件下的持久性能,结果列于
(2) 较高的B含量,使得合金具有较低的初熔温度(在1160~1170 ℃之间)。提升温度或延长保温时间,初熔更加严重。采用水淬冷却方式,初熔凝固为典型的
(3) 完整热处理时,固溶温度由1210 ℃提高到1230 ℃,初熔略微增加;而当温度达1250 ℃时,初熔吞噬大量周围基体,初熔区尺寸和面积分数剧烈增大,对合金的微观组织连续性造成严重破坏。由于温度较低,合金的高、低温时效对初熔组织的影响不是很大。
The authors have declared that no competing interests exist.
The elemental characteristics of incipient melting (or lower melting) regions in as-cast DS200 + Hf single crystal were investigated in detail. In addition to the Ni 5 Hf phase, the Ni 7 Hf 2 -type compound was observed to exist in the incipient melting regions. Ni 7 Hf 2 and Ni 5 Hf showed considerable solubilities for chromium, cobalt and aluminium. Chromium and cobalt replace nickel in the INi 7 Hf 2 phase. A large variability of composition was observed in the incipient melting regions. For example, nickel and hafnium contents varied between 27.35 and 59.94wt% and 14.47 and 54.87 wt%, respectively.
Analytical transmission electron microscopy study of intergranular regions in directionally solidified (DS) RENE 80 superalloy was performed. In addition to the generally reported M 3 B 2 -type boride particles that form in the alloy during ingot solidification, considerable presence of a different type of boride particles, M 5 B 3 , which has not been generally reported in the alloy, was observed. Extensive formation of these particles along the grain boundaries in the DS alloy is pertinent to high-temperature performance of the material.
The microstructure of directionally solidified Rene 80 (DS Rene 80) superalloy in the standard solution heat treated condition was examined. Sub-solidus incipient melting was observed, which was found to be caused by the liquation of terminal solidification reaction products present in front of the γ–γ′ eutectic in the as-cast alloy. Based on the differential scanning calorimetry (DSC) measurements, and microstructural examination of heat-treated and water-quenched specimens, sub-solidus incipient melting in the alloy is likely to occur below 1,160°C, and this is a key factor to be considered in the development of a suitable solution heat treatment scheme for DS Rene 80.
Abstract Incipient melting of IN939 superalloy was investigated during solution annealing. In addition, the effect of this phenomenon was studied on hot workability of the alloy. The incipient melting was detected to be within the temperature range 1150–1160°C, which has been reported to be the standard temperature range of solution annealing of this material. The local melted regions were enriched in Zr, Nb and B elements, which appeared in the form of three different phases. The results also showed that even small amounts of local melted regions impair hot workability of the alloy. The use of long time solution annealing at 1150°C or a two-step solution heat treatment has been recommended to delay the incipient melting of the alloy. These treatments enhance hot workability of the alloy.
Something exciting has been happening during these last several years. Structural components of a major engineering device were first constructed so that each one consisted of an individual metal crystal. These were alloy single crystal turbine blades for advanced aircraft jet engines. A new precision casting technique, based on directional solidification, which imparts significantly improved ductility and thermal shock resistance to high temperature creep resistant, nickel-base superalloys, has been carried through from research to production. This controlled solidification technique has been used to produce both columnar grain and alloy single crystal gas turbine components. The improvement in physical properties is achieved by controlling the solidification process to produce either columnar grains throughout a cast-to-size part, or a complete single crystal throughout a cast-to-size part, with a preferred  crystallographic orientation. This orientation is established parallel to the major stress axis of the part without the use of separate “seeding”. These parts have exhibited superior structural strength and stability in the severe operating environments associated with gas turbine engine operation. A comparison is made between the properties of superalloys having conventional equiaxed grains, directionally solidified columnar grains and  oriented single crystals. The evolution of this new process is traced from its beginning in columnar grain directional solidification experiments through the pilot-plant operation. The feasibility of producing parts using the “directional solidification process” has been demonstrated in production foundry facilities where several thousand gas turbine blades and vanes have been cast-to-size in various complex shapes.
The as-cast and heat-treated microstructures and high temperature creep properties have been investigated in four experimental Ni-based single crystal superalloys containing various levels of Hf addition (0–0.4wt%) and B addition (0–0.02wt%). The experimental results indicated that the creep rupture life showed an improvement with individual addition of Hf, but it was decreased with individual addition of B. The elemental partitioning ratio and interfacial dislocation spacing of γ/γ′ were obviously changed with individual Hf or B additions. Meanwhile, the formation of secondary phases, such as the blocky MC carbide, script-like shape M3B2phases, was observed in the creep samples, which was also closely related to the high temperature creep behaviors. The high volume fraction of residual (γ+γ′) eutectics was mainly attributed to the significant decrease of creep rupture life for the present experimental alloy containing both Hf and B additions. This study is helpful to better understand Hf and B's role of strengthening mechanism and to optimize Hf and B additions in single crystal superalloys.
Nickel-based superalloys are complex alloys that contain ten to 15 elements that are widely used in industries where high-temperature strength and corrosion resistance are required. Alloy additions commonly include Cr, Co, W, Ta, Al, Ti, Re, Mo, and, in some alloys, Ru. Each of these additions can affect the as-cast microstructure due to differences in elemental segregation. A better understanding of the effects of typical additions to nickel-based superalloys on the segregation of the elements in the alloy can help identify potential improvements in the processing of current alloys and the development of new alloys. Therefore, the effects of several common alloying additions on solidification segregation and defects were evaluated. In general, an increase in the degree of elemental segregation was observed with increases in each of the elements listed except cobalt and molybdenum. Increased levels of cobalt and molybdenum resulted in reductions in the segregation of most of the elements in the alloy.
The effects of an atom-for-atom substitution of Mo for Ta, Nb, or W on the mechanical properties of three typical nickel-base superalloys are discussed relative to the changes in the composition of the γ, γ′, and carbide phases. W partitions to the y phase in the IN738 alloy system, but to the γ′ in the U710 alloy. In addition, Mo was found to be more soluble in either the γ or γ′ phase when W partitioned to that phase. Ta, and possibly Nb, decreases the solubility of Mo in the γ′ phase. Mo was found to be a greater antiphase boundary strengthener than W. However, initial data indicate that Mo does not strengthen the γ′ phase as much as Ta. The substitution of Mo for W had detrimental effects on the elevated temperature yield strengths of the modified U710 and IN738 alloys as well as on the creep-stress rupture resistance of the IN738 alloy. The substitution of Mo for Ta also decreased the creep-stress rupture resistance of the modified B1900 alloy.
The effects of minor refractory elements (Re, W and Ta) on the structure and dynamics of molten Ni are studied by ab initio molecular dynamics simulations. The results show that minor additions of refractory elements would not significantly change the structure of liquid Ni. However, the local structure around Ta is quite different from that around Re or W. In the liquid Ni alloys, the 15lm-type bond pairs dominate. As temperature increases, the numbers of 16lm- and 15lm-type bond pairs would decrease while those of 14lm- and 13lm-type bond pairs would increase. The Ni self-diffusion coefficient of pure Ni shows a temperature dependence described by the Arrhenius law. With the addition of Re and W, the Ni self-diffusion coefficients still exhibit an Arrhenius relationship. However, the self-diffusion of Ni deviates from the Arrhenius law with the addition of Ta.
采用水淬法测定出铸态及采用不同温度阶段处理后的K417G合金γ＋γ′共晶相的初始熔化温 度,对铸态合金及水淬处理后的合金的微观组织采用扫描电镜进行观察,确定出阶段热处理对合金中γ＋γ′共晶相的初始熔化温度的影响.研究结果表明：铸态 K417G合金中的γ＋γ′共晶相主要由细小的γ＋γ′共晶组织和粗大的γ′相组成,在共晶相的前沿存在少量的硼化物共晶组织.铸态K417G合金中的 γ＋γ ′共晶组织的熔化主要发生在共晶相的前沿,熔化后出现大量的白色组织相.电子探针对熔化区的分析表明,熔化后形成的激冷组织主要为γ＋γ′共晶,熔化区主 要富集B、Mo和Cr元素.铸态合金中的γ＋γ′共晶的初熔温度为1 220～1 230℃,而经1 090℃、1 100℃和1 110℃阶段处理10h后合金中的γ＋γ′共晶的初熔温度升高至1 280～1 290℃.
The solidification path of single-crystal nickel-base superalloys containing minor carbon was investigated under various laser rapid directional solidification (LRDS) conditions. By controlling the solidification rate, LRDS processing can provide the evidence whether some diffusion-controlled phase transformations occur because such transformations will be suppressed under high cooling rates. Results show that the solidification path and final solidification microstructure depend upon the cooling rate; the microstructure without γ-γ′ eutectic can be obtained as long as the cooling rate is high enough. A peritectic transformation in carbon-containing single-crystal superalloys was first experimentally verified by controlling the cooling rate during LRDS processing.
The melt superheating treatment is performed on a third-generation Ni-based single crystal superalloy during directional solidification, aiming to investigate the influence of melt thermal history (melt superheating temperature, superheating time and cooling rate) on solidification behavior and microstructural characteristics. The results show that the nucleation undercooling increases nonlinearly with the increase of melt superheating temperature from 1450 to 1780°C, but decreases when the melt superheating temperature further increases up to 1800°C. The nucleation undercooling first has little change and then increases sharply with increasing the melt superheating time. The critical superheating time is obtained to be about 15min. Moreover, it is found that the higher the cooling rate is, the larger the nucleation undercooling and liquidus-solidus range are. Additionally, the segregation is reduced, and the dendrite and γ′ phase are obviously refined when the superheating temperature increases from 1500 to 1700°C. The influence mechanism of melt superheating treatment on the nucleation undercooling and solidification microstructure is discussed, which is helpful for optimizing the process parameters and improving the metallurgical quality of Ni-based single crystal superalloy.
The solidification characteristics and segregation behavior of K418 alloy were investigated by the DSC measurement and the isothermal solidification quenching process. Phase transformation temperatures, together with the solid fraction with temperature were measured by experiments and comparatively calculated by Thermo-Calc. It is demonstrated that the solidification begins with the formation of primary γ dendrite and terminates with the γ/γ′ eutectic reaction, being accompanied by the formation of MC carbide and M(C, B) carboboride. During the solidification, interdendritic region is significantly enriched with Nb, Ti, Zr, B, C and Mo, which results in the formation of carbide, carboboride and γ/γ′ eutectic. The slow increase of solid fraction at the final stage of solidification is also caused by the strong segregation of the positive segregation elements in interdendritic area. In the case of K418, equilibrium model is shown to be a valuable tool to predict the solidification sequence and phase transition temperatures. Scheil calculation can be used to investigate the segregation behavior during solidification.
采用真空感应熔炼和定向凝固重熔技术制备了不同Re含量的Ni基 高温合金试样,分析了合金微观组织,测试了热处理后其室温拉伸和高温持久性能.结果表明:Re主要分布于γ基体中,在γ’强化相中分布很少.随着Re含量 的增加,γ+γ’共晶体积分数略有增多,枝晶杆γ’细化明显.热处理后γ’立方化程度增加,共晶相含量明显减少.Re的加入显著提高了合金的事温拉伸屈服 强度和高温持久寿命,但室温和高温塑性有所降低.Re主要通过固溶在γ基体中来阻碍γ’粗化,增大错配度,从而提高合金综合性能.
利用光学显微镜、扫描电子显微 镜、定量金相分析、差示扫描量热仪等研究了微量元素对一种镍基高温合金共晶数量的影响。结果表明,添加B、Zr、Y等元素都使合金的共晶数量有了明显的增 加,而且同时加入两种微量元素对共晶数量的影响更加显著。分析表明,Zr、Y等元素由于具有较大的原子半径,阻碍了元素的扩散,导致糊状区偏析加剧而促进 共晶组织的形成。而添加B增大了凝固区间,从而促进元素偏析导致共晶数量增加。
ABSTRACT The cause and the nature of the limiting incipient melting, which should aid development of appropriate higher solution heat treatment schedule, was investigated. The lamellar morphology of the solidification product suggests that it was formed by eutectic transformation involving at least ternary and quaternary eutectic reactions. The results show that the constituent phases of the terminal solidification product in the alloy could respond differently to heat treatment. Eutectic melting of the terminal solidification constituents during fabrication welding will contribute to sub-solidus intergranular liquation, which is known to reduce resistance to HAZ cracking.
The evolution of microstructures at a wide range of solidification cooling rate in a Ni-based superalloy was investigated by employing conventional casting, spray casting and melt spinning processes. Depending on solidification cooling rate, microstructures sequentially show planar, cellular, dendritic, the dendritic growth suppressed features (in melt spinning process) and the shapes of γ′ precipitates progressively exhibit irregular (planar and cellular growth), cuboidal (dendritic growth) and spherical (dendritic growth suppressed) patterns. Moreover, in dendritic growth conditions, γ′ shapes experience irregular cuboidal, regular cuboidal and near cuboidal patterns with increasing cooling rate. γ′ precipitates in dendritic cores show more and less regular morphology than those in interdendritic regions in lower and higher cooling rate range, respectively. The size scale of γ′ precipitates decreases with solidification cooling rate in cellular and dendritic growth conditions and γ′ precipitates are obviously smaller in crystallizing (cellular or dendritic) cores than those in interval regions.
61Single-crystal CMSX-6 was processed by a novel directional solidification process.61The microstructures were characterized as functions with increasing withdrawal rate.61Some functional relationships did not agree with that in LMC and Bridgman processes.61The comparison of PDAS theoretical models with the experimental data was conducted.61The microsegregation will be suppressed with the increasing withdrawal rate.
A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN79202+02Hf. Both eta and Cr-rich phases were found to be the final solidification products developed from the remaining liquid after γ/γ′ eutectic reaction. The (Ti02+02Ta02+02Hf)/Al ratio in the residual liquid played a significant role in the nucleation of eta phase. During the solidification of γ/γ′ eutectic, the continual increase of (Ti02+02Ta02+02Hf)/Al ratio in the residual liquid eventually led to the completion of γ/γ′ eutectic reaction and caused the nucleation of eta phase. The results of electron probe microanalysis and transmission electron microscopy revealed that the Cr-rich phase was Cr, Mo, and W containing M 5 B 3 and M 3 B 2 type borides. The formation of these boride phases was found to be strongly influenced by the formation of γ/γ′ eutectic. Because of the limited solubility of Cr, Mo, and W in γ′ phase, these elements were enriched in the residual liquid during the solidification of γ/γ′ eutectic. In addition, boron would preferentially segregate into liquid due to its very limited solubility in both γ and γ′ phases so that the possibility of boride formation in the residual liquid ahead of the γ/γ′ eutectic was increased. A modified Scheil model was adopted to explain the influence of solidification rate on the formation of eta phase and borides, and the results were discussed.
The results of investigations of boron and carbon segregations, boride and carbide phases forming in various elements of the structure of ingots of the Ni-base superalloy EP962 have been presented. The methods of light metallography (LM), scanning electron microscopy (SEM), X-ray spectral microanalysis (XRSMA), high sensitive track autoradiography on boron via the nuclear reaction 10B(n, α) 7, Li and activation autoradiography on carbon via the reaction 12C(d, n) 13N were used. Boron segregation and precipitation of boride phase in interdendritic regions and on the grain boundaries were established. Composition of boride phase was found to be (Ni 0.32Cr 0.22Nb 0.14Mo 0.11Co 0.13W 0.02Al 0.02Ti 0.01)B 0.67, which is of M 3B 2 type. Formation of the non-equilibrium γ′–γ eutectic in interdendritic regions causes microliquation of Cr, Nb, Mo, Co, W and B in the crown of γ′–γ eutectic that results in formation of boride particles in the crown of γ′–γ eutectic. Carbon was found mainly forming carbide particles in interdendritic regions. Composition of carbide particles was found by the XRSMA as (Nb 0.61Ti 0.22Ni 0.10Cr 0.02W 0.01Hf 0.01)C.
这研究集中于硼增加的效果，在到 0.03wt% 的 0.0007wt% 的范围，在微观结构和方向性地团结的 superalloy 的压力破裂性质上。与在当演员组合金增加硼内容，最容易溶解的 / 的部分有增加，块硼化物在 / 附近猛抛最容易溶解。在 0.03wt% 的一个高硼内容，有薄片状的硼化物的降水。在热处理之上，好块硼化物趋于与增加硼内容在谷物边界猛抛。总的来说，方向性地团结的 superalloy 的破裂生活显著地与硼的名字的内容的增加被改进。然而，当硼内容超过 0.03wt% 时，破裂生活减少。