Residual Stresses VII, ECRS7

Abstract: A novel surface treatment has been developed in the present work to enhance the tribological properties of 316L Stainless Steel. This Technique involves the formation of a nanocrystalline layer ascribable to a grain refinement mechanism induced by repeated impact loadings supported by the surface. The resultant system has a layered structure, comprising nanometric grains (less than 100 nm) at the top and a strain hardened transition layer in the subsurface. Such a microstructural feature has the potential to significantlty enhance the surface hardness and to create a high compressive residual stress state. The tribological properties of the stainless steel are thus improved in terms of lower friction coefficient and increased wear resistance. Detailed studies on the response of the nanocrystalline surface layer to annealing at temperatures between 400°C and 600°C showed that an annealing at high temperature can offer much better tribological enhancement than low temperature annealings due to enhanced martensitic transformation.

Abstract: The advance of the XRD technique allows us to reach the properties of each coarse grain. This paper has demonstrated a new method to determine stress in a single crystal for multicrystal material and this new method could be specially applied for any symmetric crystalline systems. The strain tensor ε is determined by the change of the metric tensor G before the initial state and after the deformed state in the crystal reference system. Then stress tensor at grain scale is calculated by the Hooks law. The stress evaluations are carried out in coarse grains of a thin galvanized coating on a steel substrate during tensile loading. This study allows us to link the microstructure evolution to the elastic heterogeneity at grain scale or between the grains.

Abstract: The purpose of this study is to examine the effect of crystallite preferred orientation on the mechanical strength of TiCN thin films in highly compressive residual stress. TiCN thin films were deposited by PVD on JIS-SKH55 (AISI M35) steel. The applied substrate bias voltages were set for –50, -80, -100, -120 and –150V. Subsequently, residual stress and crystalline preferred orientation of these specimens were investigated by X-ray diffraction methodology. The crystalline preferred orientation in thin films was evaluated by the ODF calculated from pole figures. On the other hand, dynamic hardness test (DH) and scratch test were executed to evaluate the mechanical strength of thin films. In our study, it was observed that negative bias voltages had an effect on the preferred orientation. The orientation density at –120V was the highest of all specimens. In addition, the value of scratch section area at –120V was the largest of all specimens. As a conclusion, the relation between the scratch area and the negative bias voltages corresponded to the relation between the preferred orientation and the bias voltages.

Abstract: X-ray diffraction is used in combination with tensile testing for measuring elastic properties of metallic thin films. Size effect, elastic anisotropy and grain morphologies are considered in all these experiments and supported by different kind of numerical simulations operating at different length scales. Such instrumental studies are time consuming even if synchrotron sources are used. New experiments are under progress for reducing acquisition data and improving precision on strain measurements. After introducing briefly the main principles and results of our techniques, first promising measurements on nanometric W/Cu multilayers using 2D CCD detectors and high monochromatic flux at the Advanced Light Source Berkeley (USA) on beam line 11.3.1 are presented. In addition, simulation experiments for analyzing elasticity in textured gold film are discussed.

Abstract: Diffraction of penetrating radiation such as neutrons or high energy X-rays provides a powerful non-destructive method for the evaluation of residual stresses in engineering components. In particular, strain scanning using synchrotron energy-dispersive X-ray diffraction has been shown to offer a fast and highly spatially resolving measurement technique. Synchrotron beamlines provide best available instruments in terms of flux and low beam divergence, and hence spatial and measurement resolution and data collection rate. However, despite the rapidly growing number of facilities becoming available in Europe and across the world, access to synchrotron beamlines for routine industrial and research use remains regulated, comparatively slow and expensive. A laboratory high energy X-ray diffractometer for bulk residual strain evaluation (HEXameter) has been developed and built at Oxford University. It uses a twin-detector setup first proposed by one of the authors in the energy dispersive X-ray diffraction mode and allows simultaneous determination of macroscopic and microscopic strains in two mutually orthogonal directions that lie approximately within the plane normal to the incident beam. A careful procedure for detector response calibration is used in order to facilitate accurate determination of lattice parameters by pattern refinement. The results of HEXameter measurements are compared with synchrotron X-ray data for several samples e.g. made from a titanium alloy and a particulate composite with an aluminium alloy matrix. Experimental results are found to be consistent with synchrotron measurements and strain resolution close to 2×10-4 is routinely achieved by the new instrument.

Abstract: For industrial applications concerning the nondestructive characterization of the nearsurface material condition in terms of residual stresses, work hardening, phase transformation and formation of reaction compounds there is a strong demand for X-ray diffraction measurements on large components with complex geometry. Because many regions of interest on these components are not accessible with conventional laboratory or even mobile X-ray diffractometers, a novel center- free diffractometer with two cooperating robots named "Charon XRD" has been developed at MTU Aero Engines. Using a special optical measuring system to synchronize the two six-axis robots it was possible to achieve positioning accuracies that are comparable to those of conventional stationary diffractometers. This paper describes the design and functionality of Charon XRD and presents calibration and reference measurements, along with first measurements on aero-engine components.

Abstract: To enhance the fatigue resistance of mechanical components, different surface treatment processes are often applied to put the near surface layer into compression. Surface treatment processes are typically associated with deformation and work-hardening of the material. When applying x-ray diffraction techniques to the characterization of such surfaces, the work-hardening will cause the x-ray diffraction peak width to increase. When peak widths reach high values, the peak tail may extend beyond the active area or window of the multichannel x-ray detector, in which case the peak is truncated. Subsequent analytical treatment of broad diffraction peaks is troublesome and advanced numerical methods are required to accurately determine the peak position. The following work indicates that when a wider detector is used it is possible to collect the full, non-truncated peak, determine the peak position with a high level of confidence and subsequently, to calculate the residual stress with much improved repeatability and reproducibility.

Abstract: For a few years, new kinds of setups for residual stress analysis by X-ray diffraction have been commercialised by manufacturers with two linear Position Sensitive Scintillation Detectors (PSSD) including Charge-Coupled Device (CCD) sensors. Although these equipments allow an important reduction of acquisition time, some questions subsist on their measurement reliability, especially on the raw profile corrections and on the associated statistical uncertainty. One of them concerns the required gain correction because of the weak spatial homogeneity of these detectors. In fact, a bad knowledge of sensor noise does not permit a good correction of the raw patterns which can significantly affect the results of stress measurements. In this study, an original statistical analysis is proposed to visualize and analyse the various kinds of intrinsic noise of PSSD detectors, and especially the most important one: the dark noise. Based on the results of this investigation, a method for gain correction is then proposed. The method, easy to apply, permits a better correction of the sensors defects without increasing acquisition time. This analysis also allows a better understanding of the sensors behaviour and thus an optimisation of the acquisition.

Abstract: In this paper, results are presented of the development of internal stresses in 2124 aluminium alloy reinforced with 25% of particulate silicon carbide. Measurements have been made during in situ low-cycle fatigue loading of this composite, using neutron diffraction. The neutron measurements were made using the ENGIN-X diffractometer at the ISIS neutron facility, UK. The results show how the internal stresses evolve with fatigue cycling. Eshelby-based modelling has been used to allow separation of the internal stress components in the MMC.

Abstract: Variations in the lattice parameters of γ and γ' phases perpendicular to the [001] tensile axis were recorded in situ at ~10 minutes intervals using the Triple Axis Diffractometer of the High Energy (ID15) beamline at ESRF. Testing was carried out on an AM1 superalloy specimen with a raft microstructure at high temperature (1072°C) under load steps between 0 MPa and 300 MPa. These data were used to evaluate the Young modulus and the effective (Von Mises) stresses within the γ' rafts and γ corridors, as well the average plastic strain rates of each phase. The recorded stress data scatter was within the MPa range, and should be good enough to probe the elementary mechanisms of plasticity.