Acoustic Properties Of Nickel-based Superalloys With Characteristical Crystal Orientations

Ultrasonic testing of nickel-based superalloys,especially single crystal and directionally solidified nickel-based superalloys shows strong anisotropy due to their special structure and various geometric shapes of practical components.Most of the alloys in service are heat treated,But the microstructures of single crystal and directional solidified nickel-based superalloys before and after heat treatment are different.Therefore,the acoustic propagation behavior is influenced by the crystal orientation and microstructure of the materials,and the evolution of acoustic properties is more important for defect and damage detection in service.In order to elucidate the mechanism of elastic anisotropy of grains and the influence of microstructure evolution on acoustic properties during heat treatment.In this paper,as the object of study,the Rene N4 single crystal and DZ444 directionally solidified nickel-based superalloys were studied.the crystal orientation of Rene N4 single crystal and DZ444 directionally solidified as cast nickel-based superalloy were measured by X ray diffraction and electron backscatter diffraction,The microstructures of the alloys were observed by metallographic microscope and scanning electron microscopy.Finally,the acoustic characteristics of specimens with characteristic crystal orientations and at different heat treatment stages were studied by ultrasonic pulse echo technique,trying to establish the relationship between microstructure and macroscopic acoustic properties.The main contents are as follows:1.The microstructures of the Rene N4 and DZ444 directionally solidified as cast nickel-based superalloys Specimens with an included angle φ of 0o,45o,90o respectively between the normal direction of the sheet plane and the solidified direction were obtained.,and the as cast microstructure along the dendrites in different directions was observed in each of them.The primary and secondary dendrite arm spacing of Rene N4 single crystal alloys are 220 ?m and 70 ?m respectively.the size of dendritic γ’ phase is 300-880 nm.the interdendritic γ’ phase is about 580 nm;the primary and secondary dendritic arm spacing of the DZ444 directionally solidified as cast alloys were different from Rene N4 single crystal,but the size of γ ’ phase was smaller.2.The acoustic properties of Rene N4 and DZ444 as cast nickel-based superalloys were analyzed based on ultrasonic pulse echo technique and spectrum analysis.The longitudinal velocity,attenuation coefficient,amplitude spectrum main frequency offset and amplitude change of ultrasonic wave in different orientation samples were calculated and compared.The results showed that there were significant differences in acoustic parametersof different orientations,showing obvious anisotropy.the longitudinal velocity of 45o Rene N4 was the largest and acoustic attenuation coefficient was the smallest.the values were about 6260m/s and 0.33 db/mm,respectively.the longitudinal velocity of the 0°sample was about 5411 m/s,which was the smallest.the acoustic attenuation coefficient of the 90o specimen was the maximum,the difference between whose value and 45o specimen’s was about 0.51 dB/mm.which was mainly due to different crystal orientations.The variation of longitudinal velocity of DZ444 was the same as that of Rene N4.However,the variation of the main frequency offset,amplitude and apparent integral reflection coefficient in frequency domain of the two alloys were not exactly the same.The scattering attenuation characteristic of specimens in 45 °and 90° were similar to polycrystal.3.The solution,primary aging and secondary heat treatment were performed on two kinds of specimens of Rene N4 and DZ444 nickel-based superalloys,and the microstructures of different heat treatment stages were analyzed.The results showed that the segregation of two alloys gradually decreased and the dendrite morphology became more and more blurred.The size of the γ’ phase increased firstly and then decreased.The morphology,type and quantity of carbide in Rene N4 superalloys changed.The grain boundary coarsening also occurred in DZ444 superalloys.4.There were some differences in acoustic properties of the same orientation sample at different heat treatment stages.The velocity and attenuation coefficient of Rene N4 reached the maximum value after primary aging,then decreased slightly.The attenuation coefficient increased to the maximum value because of the increase of carbides and the average size of the γ’ phase.The longitudinal wave velocity of DZ444 did’t change a lot in different heat treatment stages,which was mainly affected by γ ’phase size,morphology,grain boundary coarsening and dendrite homogenization.