Modified 9Cr-1Mo ferritic steel (T91/P91) has been subjected to a series of heat treatments consisting of soaking for 5 minutes at the selected temperatures, starting from the ␣ -phase region (1073 K) to the ␥ ϩ ␦-phase region (1623 K), followed by oil quenching. Hardness measurements, microstructural features, and grain-size measurements by the linear-intercept method have been used for correlating them with the ultrasonic parameters. Ultrasonic velocity and attenuation measurements, and spectral analysis of the first backwall echo have been used for characterization of the microstructures obtained by various heat treatments. As the soaking temperature increased above Ac 1 , the ultrasonic velocity decreased because of the increase in the volume fraction of martensite in the structure. There were sharp changes in the ultrasonic velocities corresponding to the two critical temperatures, Ac 1 and Ac 3 . Ultrasonic longitudinal-and shear-wave velocities were found to be useful in identifying the Ac 1 and Ac 3 temperatures and for the determination of hardness in the intercritical region. However, ultrasonic attenuation and spectral analysis of the first backwall echo were found to be useful to characterize the variation in the prior-austenitic grain size and formation of ␦ ferrite above the Ac 4 temperature. The scattering coefficients have been experimentally determined for various microstructures and compared with the theoretically calculated value of the scattering coefficients for iron reported in literature.

The deformation and damage mechanisms in wrought, double-aged, Inconel718 superalloy (AMS 5663D) tested under monotonic tensile strains of 2% and lo%, fully-reversed fatigue, and tensile strain (2% or 10%) followed by fully-reversed fatigue conditions were investigated by examining the microstructures of representative specimens. All tests were conducted in air at room temperature. The specimens were sectioned and examined by transmission electron microscopy to reveal typical microstructures as well as the active deformation and damage mechanisms. Specific mechanistic features addressed include the type of slip, interaction of dislocations with y", y' and the carbides (precipitated during solidification and the subsequent heat treatment received by the superalloy), twinning, and microcracking. In all cases the microstructure of the as-received superalloy is employed as the reference to establish the nature and distribution of the secondary phases before the superalloy is subjected to different types of mechanical loading. Results of the investigation and comparisons of the mechanisms of deformation and damage observed under monotonic tensile strain, fully-reversed fatigue, and tensile strain followed by fully-reversed fatigue in Inconel 718 superalloy are reported.Superalloys 718,625,706 and Various Der~at~ves

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Ultrasonic velocity measurements have been carried out across the weld line in two perpendicular sections of modified 9Cr–1Mo ferritic steel weldments, in the as welded and post-weld heat treated (PWHT) conditions. The ultrasonic velocity plot is correlated with the weld profile in both the sections and with the associated microstructural features in different regions of the weldments for the two conditions investigated. The present study reveals that the weld profile can be imaged and the adequacy of the PWHT can also be assessed in modified 9Cr–1Mo ferritic steel weldments using ultrasonic velocity measurements.

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A detailed assessment of the macroscopic and microstructural cyclic deformation behavior of Haynes 188" has been conducted under various isothermal and thermomechanical conditions over the temperature range 26 to 1000°C.A fully reversed mechanical strain range of 0.8% was examined with constant mechanical strain rates of 109s-' and 104s-'. Particular attention was given to the effects of dynamic strain aging (DSA) on the stress-strain response. Detailed transmission electron microscopy was conducted to examine the deformation substructures and establish correlations with the cyclic macroscopic behaviors. Although DSA was found to occur over a wide temperature range between approximately 300 and 700°C the microstructural characteristics and deformation mechanisms responsible for DSA varied considerably and were dependent upon temperature. In general, the operation of DSA processes led to a maximum of the cyclic stress amplitude at -65O"C, and was accompanied by pronounced planar slip, the generation of stacking faults and high dislocation density. DSA was evidenced through a combination of phenomena, including serrated yielding, an inverse dependence of maximum cyclic hardening with strain rate (6), and an instantaneous inverse 8 sensitivity.The TMF cyclic hardening behavior exhibited unique behaviors in comparison to the isothermal response, predominantly at the minimum TMF temperature extremes.

The deformation mechanisms and fatigue behavior of prestrained Inconel 718 superalloy under fully-reversed fatigue were investigated at room temperature. Uniform gage section specimens were monotonically strained initially in either tension or compression up to 2% strain. Fullyreversed fatigue tests were subsequently conducted both on as-machined (no prestrain) and monotonically prestrained specimens in total strain control at a strain range of 0.8%. All tests were conducted in a computer-controlled servohydraulic test system at room temperature. Evolution of stress ranges and mean stresses observed during the fatigue tests on as-machined and prestrained specimens are reported. The specimens were sectioned and examined in a transmission electron microscope to reveal the deformation mechanisms under different types of loading.Results of the investigation and comparisons of the mechanisms of deformation observed under monotonic tensile strain, monotonic compressive strain, fully-reversed fatigue, tensile strain followed by fully-reversed fatigue, and compressive strain followed by fullyreversed fatigue are reported. Fatigue lives of the prestrained specimens are compared with life predictions by two different methods.