Nondestructive Quantification of Induced Surface Treatment and Relaxation Effects in Metals

Photon Induced Positron Annihilation (PIPA) and related portable technologies’ have demonstrated their ability to nondestructively quantify shot peening/surface treatments and relaxation effects in single crystal superalloys and steels with a single measurement as part of a National Science Foundation SBIR program. PIPA measurement of surface treatment effects provides a demonstrated ability to quantitatively measure initial treatment effectiveness along with the effect of operationally induced changes over the life of the treated component. Test specimens of steel and CMSX-4, a single crystal, nickel-based superalloy, were prepared and measured at incremental shot peening intensities ranging from 0-20A. The CMSX-4 samples were then subjected to a range of fatigue and thermal conditions to assess relaxation effects and to assess subsurface residual stresses. The PIPA technology proved to be highly sensitive to the induced changes in surface treatment intensity and accurately measured the relaxation of the shot peening intensity induced by varying degrees of simulated operational conditions. In addition, the tests demonstrated PIPA and related technologies’ ability to quantify subsurface residual stresses in test specimens. Use of PIPA to nondestructively quantify surface and subsurface residual stresses in turbine engine materials and components, especially single crystal and complex geometry components, will lead to improvements in current, conservative engineering designs and maintenance procedures. Introduction: Single crystal superalloys are used in a broad variety of applications in industry today, including critical turbine applications for power plants and commercial jet engines. Because of the high consequence potential of failure of these components, many are subjected to surface treatments that induce a compressive stress at the surface of the component to improve resistance to fatigue crack initiation and growth. Single crystal materials, such as nickel-based superalloys, provide increased fatigue and thermal resistance and extended life to turbine engine components, but due to their unique atomic structure, no nondestructive technology currently exists that can detect and accurately quantify surface and subsurface residual stresses/strains in these materials. Many of the components manufactured from nickel-based superalloys are not only costly, but may have potentially catastrophic consequences if failure occurs. Quantification of the effects of near-surface residual stresses induced by surface treatments and the ability to quantify the effect of interactive damage mechanisms that result in long term relaxation of the surface effects are critical issues in developing ways to improve the use of the nickel-based superalloys and other metal components. Unfortunately there is limited knowledge about surface treatment effects on operational components because only destructive methods are available for evaluation that prevent additional use of the component or only provide limited surface information on the effects (e.g., x-ray diffraction analysis). Shot peening and other surface treatments have been used for years as a means of inducing compressive residual stresses in a component’s surface, which is considered to improve fatigue life. 2,3,4,5,6 Shot peening is considered effective at inducing surface residual stress in many metals to a depth of 300-400 micron depending on the hardness of the material. The intensity of the induced residual stress is typically quantified using the Almen intensity, which involves peening a strip of given dimensions and material (typically SAE1070 spring steel) with a given intensity and with a coverage of 200%. The Almen intensity is related to the shot velocity or pressure used to fire the peening pellets at the surface of the material. The Almen intensity is quantified by measuring the deflection in the arc of the strip caused by the change in subsurface residual stress distribution over a fixed length. However, this approach results in a number of problems as the Almen intensity is specific to the material type, stress profile and material hardness. Consequently, the Almen intensity level provides little information on materials other than the test material, and more specifically, on the effectiveness of shot peening on actual components. Photon Induced Positron Annihilation (PIPA) and Distributed Source Photon Annihilation (DSPA) have demonstrated the capability to detect and quantify induced surface treatments and operational damage effects in single crystal materials. These technologies have the capability to vastly improve the understanding of microstructural states, failure mechanisms, and annealing treatments that can be used to improve critical component designs. The PIPA and DSPA technologies used in this research program were recently developed at the Idaho National Engineering and Environmental Laboratory (INEEL). The PIPA technology is a volumetric measurement technology that can measure microstructural damage at depths up to 4 inches in materials, whereas the DSPA technology uses high-energy positrons (nominally 3 MeV) that can be used to penetrate depths of up to about 2 mm in metals over large areas up to 100 cm. The DSPA process can be used to penetrate into greater depths in less dense materials such as plastics. These technologies are new additions to current material characterization technologies that when fully developed are expected to have specific applications to almost all materials industries. Specific improvements developed at INEEL and incorporated in this technology include the use of high-energy photons (15-20 MeV) to generate neutron deficient nuclei in materials (e.g., most metals, composites and polymers) that produce positrons within the bulk material for a few minutes to hours allowing bulk fatigue or lattice structure change/ damage to be accurately measured (<1% uncertainties). Other more portable approaches, based on neutron induced prompt gamma rays are under development. The PIPA process generates positrons inside the bulk material through the application of highenergy X-ray bombardment of the target material by a linear accelerator. Positrons are formed when the X-ray interactions result in a photo-neutron reaction. The short-lived isotope decays into a more stable material through positron decay within a few minutes or hours. Positrons annihilate with electrons and produce gamma rays at 0.511 MeV with small momentum induced changes that are indicative of the quantity and type of defects present in the material. Positron annihilation occurs when a positron encounters an electron and their mass is converted into pure energy in the form of two gamma rays. If the positron and the electron with which it annihilates were both at rest or with little momentum at the time of decay, (i.e., in a defect), the two gamma rays would be emitted in exactly opposite directions (180 degrees apart), with an energy of 0.511 MeV, whereas if the annihilation occurs in a location without defects where electrons have momentum energy, the gamma ray measured has incremental, measurable differences from 0.511 MeV (Conservation of Energy). Figure 1 shows the PIPA process and Figure 2 depicts the formation and subsequent thermalization of the positron as it travels through the lattice structure, searching for a lower charge density region (defected area), where it becomes trapped and then annihilates with an electron. The positrons created by the PIPA process are formed throughout the bulk material, achieving better sensitivity and accuracy of defect detection than historical surface positron beam spectroscopy. The depth of defect detection for PIPA is only limited by the attenuation of the annihilation gammas to be measured by the germanium detector; related to the material density. Additional information on the measurement processes used can be obtained from a number of references. 7,8,9,10,11,12,13