Surface damage from machining operations is a potential source of failure in metallic components. The ability to quantitatively characterize the depth and extent of the damage layer is critical to controlling the machining process. Electron back scattered diffraction and synchrotron high energy x-ray diffraction were applied to the measurement of machining surface damage in a Ni-based super alloy. Both techniques clearly showed a plastic deformation profile below the surface as a function of the machining conditions used. Using the average intragrain misorientation parameter, the electron back scattered diffraction was able to quantify the amount of surface damage from one surface treatment to another. In addition, the x-ray diffraction measurements were able to simultaneously measure the elastic strain as a function of depth from the surface.

Achieving a balance between low cycle fatigue and creep in alloy 718 critical rotating hardware has become challenging as application conditions become more aggressive and geometries more complex. Optimization of both forge practice and of minor chemistry variations are required to meet this property balance. A review of potential factors influencing creep variation in direct age alloy 718 forgings identified phosphorus level as a statistically significant factor over the range of 50 ppm to 125 ppm. Verification of the findings was accomplished using a series of full scale heats with phosphorus levels targeted to a 90 ppm minimum. Data from the 12 full scale VIM heats, including 104 creep tests from multiple forgings, exhibited an average creep life of 111 hours with Q1 and Q3 distributions of 80 hours and 145 hours, respectively, when tested at 1100F/120 KSI and 0.2% strain. This is an approximate 2X increase in mean life from historical levels. The results of the evaluation show that with an optimized well controlled forge process phosphorus levels above 80 ppm, but below 150 ppm, will improve creep performance of DA alloy 718.

Most machinery components require some amount of machining during their construction. In a general sense, machining involves cutting, grinding, and polishing the surface of the metal component. The mechanical properties and service life of the component can be greatly affected by residual surface damage from the machining process. It is difficult to know how much residual damage is left in the surface of the metal as a result of these processes. In this paper, we have applied electron backscattered diffraction (EBSD) and synchrotron X-ray diffraction to quantify the level of residual damage (plastic strain) as a function of distance from the surface of the component.The measurements were made on components fashioned from a Fe-Ni-based superalloy. The components were machined using a set of increasingly aggressive parameters to intentionally increase the amount of damage on the surface of the material. Cross-sectional samples were cut from these components and polished down to a 0.05µm silica finish on the cross-sectional face. EBSD measurements were performed at the GE Global Research Center using a CamScan CS44 SEM with an HKL-Channel 5 EBSD system. EBSD data was collected from the samples in lines parallel to the surface of the component (Figure 1). The average intragrain misorientation (AMIS) was calculated for each line and plotted as a function of distance from the surface as has been done previously for studies on welds [1] (Figure 2). X-ray diffraction (XRD) measurements were made using high-energy (50 keV) synchrotron X-rays at the National Synchrotron Light Source. The diffraction was performed in transmission using the same samples as the EBSD measurements but sliced to be ~1 mm thick (Figure 3) [2]. The sample was translated through the beam (0.015 x 2 mm in cross-section) perpendicular to the surface to achieve a sampling of points as a function of depth from the surface. A CCD camera was used to record the diffraction rings from each point on the sample. From this data, the width of the diffraction lines (sensitive to plastic strain (Figure 4)) and the positions of the diffraction lines (sensitive to elastic strain) were measured. Both EBSD and XRD were able to measure distinct distributions of plastic strain for the medium and highly aggressive conditions. In both EBSD and X-ray diffraction data, the plastic strain is observed to decay from significant levels at the surface to baseline levels about 300-400um from the surface for the most aggressive machining conditions. For the least aggressive conditions, no change in EBSD-AMIS value was observed as a function of depth. XRD measurements also suggest that these plastic strain distributions give rise to significant levels of elastic residual stress that depend upon the surface machining procedures.