Tensile strengthening in the nickel-base superalloy IN738LC

The tensile properties of superalloy IN738LC with different precipitate microstructures are evaluated at room temperature, 650 °C, 750 °C, and 85 °C at two different strain rates. The properties can be presented in two groups based on the comparable closeness of the values obtained—those of microstructures C and M, with coarse and medium size precipitates, and those of microstructures F and D, with fine and duplex size (medium + fine) precipitates. Preferred orientations, lattice parameters, and metallography are used to characterize the microstructure and tensile testing to determine the yield strength, tensile strength, and strain hardening coefficients. An anomalous increase in yield strength is observed, which occurs at temperatures about 100 °C higher with higher strain rate than with lower strain rate applied. The experimental results show that the yield strength is influenced by preferred orientations and precipitate size, while the tensile strength is effected by the size and morphology of precipitates.     

Influences of gap size and cyclic-thermal-shock treatment on mechanical properties of TLP bonded IN-738LC superalloy

Influences of gap size and cyclic-thermal-shock treatment on the mechanical properties of transient liquid phase (TLP) bonded IN-738LC superalloy were investigated. For this purpose, TLP bonding of IN-738LC superalloy was carried out in a vacuum furnace using powdered AMS 4777 as the filler metal. The results showed that isothermal solidified zone (ISZ) consisted of Ni solid-solution and the distribution of alloying elements was homogeneous. High hardness of HV 409 and high shear strength of 506 MPa were observed in 40 μm gap sample. Alloying elements formed γ′ precipitates and the solid-solution in the ISZ. Hardness and shear strength of bonds were reduced with increasing the gap size (in range of 40–120 μm). The fractured surfaces of complete isothermal solidified bonds showed dimpled rupture, but athermal solidified bonds showed cleavage fracture surface. 10, 20, 30 and 40 thermal-shock cycles were applied to 80 μm gap samples, respectively. The shear strength of the bond was measured to be 268 MPa after the 40th thermal-shock cycle. The sample with gap size of 80 μm was failed due to crack nucleation on faying surface at 45th thermal-shock cycle. The amount of the produced brittleness due to quenching the samples in water bath was attributed to the number of thermal-shock cycles.     

Studies on the Surface Chemistry of Oxide Films Formed on IN-738LC Superalloy at Elevated Temperatures in Dry Air

The oxidation behavior of IN-738LC was studied to develop high-temperature materials for low cost and highly efficient turbine systems. The present study was undertaken to investigate the kinetics and the surface chemistry of the oxide films formed during isothermal oxidation of IN-738LC superalloy in the temperature range 1123–1223 K in dry air. The oxidation kinetics followed the parabolic law. The activation energy of oxidation was 264 kJ mol–1. The scaling process is controlled mainly by the diffusion of chromium ions through the intermediate chromia layer in the scale. The surface morphology and the oxide phases of the scale were characterized by SEM, XRD, and EDS studies. XRD analysis revealed the presence of NiO, NiAl₂O₄, NiCr₂O₄ spinel, Al₂O₃, and Cr₂O₃ on the top-scale surface. The scale surface and cross section were further characterized using X-ray photoelectron spectroscopy (XPS), which revealed the presence of NiO, Ni₂O₃, NiAl₂O₄, Al₂O₃, and TiO₂ on the top-oxide surface. The chromia layer was found to be underneath the top scale. The chromia layer also contains NiCr₂O₄ and NiAl₂O₄ spinels along with Al₂O₃. Application of XPS was found to be successful to understand the oxide-scale chemistry in terms of the oxide-growth mechanism of IN-738LC at elevated temperatures.     

Surface Chemistry of Oxide Scale on IN-738LC Superalloy: Effect of Long-Term Exposure in Air at 1173 K

The oxidation kinetics of IN-738LC at 1173 K in dry air up to 1500 hr followed parabolic law. Surface morphology and the oxide phases present in the scale were characterized by SEM, XRD, EDS, FIB, and XPS. FIB investigation exhibited a compact and adherent oxide layer. XRD analysis revealed the presence of NiO, NiAl₂O₄, NiCr₂O₄ spinel, and Al₂O₃ on the top scale surface formed at 1173 K in dry air. Extensive XPS analyses revealed the presence of Cr₂O₃, CrO₂, and CrO₃ on the top scale surface formed on IN-738LC after 10 hr of exposure. The presence of TiO₂, Al₂O₃, Cr₂O₃, NiO, and NiAl₂O₄ and NiCr₂O₄ spinels along with the oxides of Ta at the top surface of the scale was observed after 100 hr of oxidation. The TiO₂ content was high on the surface and the entire scale cross section was composed mostly of Cr₂O₃, NiO, TiO₂, and Al₂O₃ after 100 hr of exposure to dry air at 1173 K. The concentration of Al₂O₃ on the surface of the oxide scale was found to increase after 100 hr of exposure and remained constant thereafter. After 300 and 1500 hr of exposure, the surface oxide was mainly Al₂O₃ along with oxides of Ni, Ti, and Cr. The oxide scale cross section consisted mostly of Al₂O₃ along with other oxides such as Cr₂O₃, NiO, and TiO₂. The oxide-scale composition was found to vary significantly with the duration of exposure to dry air at 1173 K.     

Microstructure of diffusion-brazing repaired IN738LC superalloy with uneven surface defect gap width

Slots with uneven width were cut by femtosecond laser in small plates of IN738LC superalloy to imitate service cracks. The ‘cracks’ were repaired by diffusion brazing using BNi-1a or a mixed filler alloy at 1100°C. The joint region was composed of isothermal solidification zone (ISZ), diffusion affected zone (DAZ) and precipitate zone (PZ). The compositions were different between the upper and lower ISZ due to the variation of gap width. The sample, repaired with two kinds of filler metals, had similar DAZ microstructure. PZ of mixed filler alloy bonded sample had a similar microstructure with that of BNi-1a, but less borides. The maximum gap sizes of complete isothermal solidification were almost the same for different filler alloys, followed a square root relationship with time. However, PZ of BNi-1a bonded was larger, resulting from a more base metal dissolution. The relationship between the PZ, ISZ and crack width is discussed.     

Microstructural characterization of a Pt-aluminide coated IN738LC

In this paper, the microstructural and chemical characterization of the IN738LC superalloy, coated with a Pt-Cr modified aluminide coating, is presented. The effects of aging in air at 850 °C on superalloy and coating microstructures were investigated. The growth of γ′ precipitates in IN738LC follows the Wagner-Lifshits model. The positive effect of Pt in preventing refractory element diffusion into the outer coating is not influenced by the aging. A moderate precipitation of TCP phases has been noted at the coating-superalloy interface.     

标准热处理对采用BNi-9焊料的瞬时液相连接IN-738LC高温合金的影响

研究标准热处理对扩散焊IN-738LC高温合金显微组织和力学性能的影响。对连接样品进行全固溶退火、部分固溶退火和时效处理3个不同的热处理。结果表明,在1120℃下焊接5 min,会导致不完全等温凝固,在焊缝处形成富Ni、Cr的硼化物共晶相。当保温时间延长到45 min时,接头中发生完全等温凝固,形成镍的先共晶固溶体γ相。等温凝固和非等温凝固样品的标准热处理能完全消除扩散影响区的硼化物相,并在等温凝固区形成γ’析出相。然而,在非等温凝固样品的接头区观察到不连续的再凝固产物。等温凝固样品经标准热处理后,剪切强度最高(约801 MPa),为基材剪切强度的99%。     

Plasma transferred arc welding—modeling and experimental optimization

Plasma transferred arc (PTA) welded coatings are used to improve surface properties of mechanical parts. Advantages are the high reliability of the process and the low dilution of substrate and coating material. Processing of surfaces by PTA welding is restricted at the time to flat horizontal position. Furthermore, industry is interested in the development of strategies for coating with PTA in constraint position as complex three-dimensional (3D) parts could be then easily processed as well. Under commercial aspects, the process design can be optimized to increase process efficiency and to reduce heat input during the welding process. Process optimization involves the determination of guidelines for PTA welding in constraint positions as well. Modeling the process gives an alternative to reduce the experimental effort to optimize the welding process. Results of simulation studies of the PTA welding process are given in the present work. It will be shown that coating conditions can be optimized by varying plasma gas flow, heat input and heat flow, process speed, or powder injection with regard to welding in constraint positions. The defined controlling of the PTA welding allows modification of process management with less experimental effort and to develop coating strategies for processing in different positions. In experimental investigations, the developed coating strategies are confirmed by producing PTA coatings in constraint position as well as complex 3D parts. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Laser-assisted friction stir processing of IN738LC nickel-based superalloy: stir zone characteristics

Friction stir processing (FSP) of high softening-temperature materials such as nickel-based superalloys is considered to be difficult. Laser heating of a localised area ahead of the FSP tool was used to provide sufficient plasticity during the FSP of IN738LC nickel-based superalloy. The stir zone (SZ) microstructure of the friction stir processed and laser-assisted friction stir processed were characterised. Laser-assisted friction stir processing (LAFSP) produced a defect-free pass, but FSP resulted in generation of a discontinuity in the SZ. Both lower volume fraction of partially dissolved γ′ precipitates and coarser grain structure of SZ in LAFSP led to more ductility of the SZ material and elimination of the defects.     

激光沉积修复GH536/GH738合金的组织及力学性能

采用GH536合金粉末对GH738合金损伤试样进行激光沉积修复试验研究,通过正交试验法优化工艺参数,得到较小熔深、无缺陷的修复试样;测试分析了修复试样的显微组织、室温拉伸性能及显微硬度。结果表明:合金修复区为外延生长的柱状晶组织,修复区边缘柱状晶取向较一致,修复区中心柱状晶出现一定角度转向;合金修复区枝晶干上主要为富含Mo、Cr的M6C碳化物,晶界处出现少量细小的M23C6碳化物。相比于基体,合金热影响区中γ′相数量减少并呈粗化趋势,MC碳化物发生分解。激光沉积修复试样室温抗拉强度为GH738锻件的66.5%,高于GH536锻件强度;断后伸长率分别低于GH738锻件及GH536锻件的;修复区域的显微硬度分别低于GH738基体的显微硬度,高于GH536锻件的显微硬度。