Materials Science Forum Vols. 449-452

Abstract: The present study examined the effects of heat treatment and the addition of Cu-Ni alloy on the corrosion resistance of the matrix of spheroidal graphite cast iron in aqueous environments. Test materials of white cast iron and carbon steel were used for comparison with spheroidal graphite cast iron. The alloy spheroidal graphite cast iron that added Cu and Ni was prepared. The spheroidal graphite cast iron was subjected to three kinds of heat treatment to adjust the matrix: annealing, oil quenching, and austemper heat treatment. In electrochemical tests, measurements of corrosion electrode potential and cathode and anode polarization were used. The following was clarified from the relationship between the electrode potential and current density of each of the materials in each of the solution. The alloy spheroidal graphite cast iron had a high corrosion electrode potential owing to the addition of Cu-Ni, and tended to have a low corrosion current density. This demonstrates that in any of the materials having a matrix adjusted by heat treatment, the addition of Cu-Ni increased the corrosion resistance. The corrosion current density was highest in a sulfuric acid environment.

Abstract: X20CrMoV12 1 steels were exposed at three different high temperatures and various durations. Aging parameter, LMP _s, was used to describe long-term thermal history. Creep rupture, hardness and tensile tests for aged X20CrMoV12 1 steels were carried out. Yield strength was slightly decreased and hardness was rapidly decreased afterLMP _s reached 20.6. Creep strength of aged X20CrMoV12 1 steel decreased rapidly after the LMP_s reached 20.6. It was shown that the creep strength decreased due to the coarsening of acicular type M₂₃C₆ at subgrain boundaries and the agglomeration of M₂₃C₆ at prior austenite grain boundaries. After LMP_s reached 20.6, the assumed linear regression line of stress and Larson-Miller parameter,LMP_f, also moved toward a lower LMP_f region as aging parameter increased. The dependency of creep rupture life on the microstructural degradation could be represented by C_LMPs, which is LMP_f at stress of 1 kg/mm². It was found that the creep rupture life of X20CrMoV12 1 steel could be predicted by considering the material degradation during and prior to creep test.

Abstract: Microstructural evolution during high temperature exposure and its effects on tensile and stress rupture properties of the Ni-base superalloy B1900 have been studied. Tensile deformation of the as-cast specimen was concentrated in the localized slip bands in general. Stacking faults and deformation twins were observed in the as-cast tensile specimen tested at 871°C where the alloy exhibited the lowest ductility. Dense dislocation network formed at γ/γ´ interface during thermal exposure caused homogenous deformation in the thermally exposed tensile specimen. Thermal exposure did not have significant effect on the stress rupture lives of the alloy at 760°C and at and above 871°C but it reduced stress rupture life of the alloy at 816°C γ´ coarsening and coherency loss at the γ/γ´ interface during thermal exposure were primarily responsible for the deterioration of mechanical properties and characteristic deformation behavior of the alloy.

Abstract: Investigation of γ -channel widening behavior in a nickel-base single crystal superalloy during uniaxial tensile creep tests was carried out. Scanning electron microscopy (SEM) was adopted to reveal the microstructural evolution of γ -channel and the dimension of γ -channel. It was found that different tests condition such as temperature, time and stress in tensile creep tests all influenced the γ -channel widening behavior, and a parabolic rate law governs the mean increase value of γ -channel width. The stress and temperature enhanced the γ -channel widening. The γ -channel widening was the result of directionally diffusion of multiatom.

Abstract: The tensile deformation behavior of M963 superalloy treated by various heat treatments has been studied at both room and high temperature (1173K). The result shows that the alloy treated by the standard solution treatment at 1483K for 4h followed by air-cooling has low ductility especially at 1173K. The additional aging treatment at 1123K for 16h followed by furnace cooling can recover the ductility of the alloy at 1173K, but further decrease its room temperature ductility. The TEM observation shows that the deformation mechanism varied with both the testing temperature and heat treatment. Finally, the mechanism of the aging treatment on the deformation behavior of the M963 superalloy is discussed.

Abstract: The effect of melt treatment on the microstructure and creep-rupture behavior of M963 superalloy at 1248K under 225MPa has been investigated. The microstructure of the as-cast superalloy without melt treatment consists of γ solid solution matrix, γ´precipitate, coarse blocky MC carbide and (γ+γ´)eutectic. The striking difference in microstructure is that the melt treatment changes the MC carbide from the coarse blocky morphology into the fine script-like morphology. After heat-treated at 1483K for 4h followed by air-cooling, both the creep life and rupture elongation of the melt-treated alloy are all doubled those of the alloy without melt treatment. The mechanism of the melt treatment on the creep-rupture behavior of the M963 superalloy is discussed.

Abstract: The tensile deformation behavior of M963 superalloy treated by various heat treatments has been studied at both room and high temperature (1173K). The result shows that the alloy treated by the standard solution treatment at 1483K for 4h followed by air-cooling has low ductility especially at 1173K. The additional aging treatment at 1123K for 16h followed by furnace cooling can recover the ductility of the alloy at 1173K, but further decrease its room temperature ductility. The TEM observation shows that the deformation mechanism varied with both the testing temperature and heat treatment. Finally, the mechanism of the aging treatment on the deformation behavior of the M963 superalloy is discussed.

Abstract: Low cycle fatigue (LCF) behavior of coarse and fine grained superalloy CM247LC at 760°C has been investigated. Both coarse and fine grained CM247LC showed similar cyclic stress response, however, the fine grained CM247LC specimen exhibited relatively uniform and superior fatigue properties to the coarse grained one. It was found that fatigue crack initiation of the alloy was keen to the applied strain range. Fatigue crack initiated at the surface of the specimen with high strain range (∆ε_t≥( 0.7%) while the initiation site moved to the internal defects at low strain range (∆ε_t≤0.6%).

Abstract: Electrodeposited Ni films were produced by using four types of electrodeposition methods, direct current without additive (DC), direct current with additive (AD), square pulse without additive (SP) and square pulse with additive (AS). The <100> texture was obtained by using the DC method at about 20 °C and by using the AD and the AS methods at about 80°C. The <110> texture was obtained by using the DC and the SP methods at about 80 °C. When using the AS method, the surface roughness of Ni deposit was lower than that of Ni deposits after using the other electrodepositon methods and the microstructure of Ni deposits showed nanocrystalline grain size. The <100> textures of Ni deposits using the DC and the AS methods remained unchanged after the recrystallization process. The <110> texture of Ni deposits using the SP method at about 80 °C changed to < 3 1 0> orientation after the recrystallization process.

Abstract: It was attempted to evaluate nondestructively the creep damaged IN738LC Ni based superalloys with different degree of rafting using ultrasonic wave. Microstructural parameters (length and width of γ' precipitates) and ultrasonic attenuation were measured in order to clarify the relationship between them. Both the mean length of γ' precipitates and the ultrasonic attenuation increased with creep time. Ultrasonic attenuation was found to have a linear correlation with the mean length of γ' precipitates. It is suggested that the ultrasonic attenuation can be used as a potential nondestructive parameter for assessing the degree of creep damage of IN738LC superalloy.