X-ray Diffraction Residual Stress Analysis Research Articles

Tin whisker growth on immiscible Al–Sn alloy

Spontaneous growth of Sn whiskers has seriously affected the reliability of electronic systems and caused many major accidents. Its growth mechanism and mitigation strategy have been the focus of research in the past decades. In this paper, a large number of Sn whiskers were found to grow quickly on Al–Sn alloys prepared by mechanical alloying when treated at 300 °C. Different from the large randomness of the whisker growth on metallic coatings and platings as in major research work of this topic, the Sn whisker growth from ball-milled Al–Sn substrate shows good repeatability. To demonstrate the compressive stress in the samples, X-ray diffraction residual stress analysis was detected. An average compressive stresses in Al phase after heat treatment was 81.2 MPa, which may have been responsible for the whiskers growing directly on the surface. Meanwhile, the interfacial energy caused by high-energy ball milling may have played a positive role in the growth of whiskers. Sn whiskers formed on Al–Sn alloy samples have shorter incubation period and faster growth rate, which makes it much more convenient to research the mechanism.

Impact of the Process Parameters, the Measurement Conditions and the Pre-Machining on the Residual Stress State of Deep Rolled Specimens

Mechanical surface treatments, e.g., deep rolling, are widely spread finishing processes due to their ability to enhance the fatigue strength of the treated materials with means of cold working and inducement of favorable compressive residual stresses. Despite of the clear advantages of deep rolling, the controlled generation of compressive residual stresses is still a challenging task, as the process can be influenced by the pre-machining stress state of the treated material. Additionally, the exact characterization of the induced residual stress field is impacted by the specific characteristics of the applied measurement technique. Therefore, this paper is focused on the X-ray diffraction residual stress analysis of deep rolled specimens, pre-machined to achieve rough or polished surface. The deep rolling process was realized as a single-trace to avoid the influence of the other process parameters and the resulted residual stress field on the surface and in depth was investigated. Additionally, the surface residual stress profiles were determined using two different measuring devices to analyze the impact of the different measurement conditions.

Effect of severe shot peening on ultra-high-cycle fatigue of a low-alloy steel

It is well known that shot peening is able to increase the fatigue strength and endurance of metal parts, especially with a steep stress gradient due to a notch. This positive effect is mainly put into relation with the ability of this treatment to induce a compressive residual stress state in the surface layer of material and to cause surface work hardening. Recently the application of severe shot peening (shot peening performed with severe treatment parameters) showed the ability to obtain more a remarkable improvement of the high cycle fatigue strength of steels. In this paper severe shot peening is applied to the steel 50CrMo4 and its effect in the ultra-high cycle fatigue regime is investigated. Roughness, microhardness, X-ray diffraction residual stress analysis and crystallite size measurement as well as scanning electron microscopy (SEM) observations were used for characterizing the severely deformed layer. Tension–compression high frequency fatigue tests were carried out to evaluate the effect of the applied treatment on fatigue life in the ultra-high cycle region. Fracture surface analysis by using SEM was performed with aim to investigate the mechanism of fatigue crack initiation and propagation. Results show an unexpected significant fatigue strength increase in the ultra-high cycle region after SSP surface treatment and are discussed in the light of the residual stress profile and crystallite size.

Characterization of local residual stress inhomogeneities in combined wire drawing processes of AISI 1045 steel bars

Cold formed semi-finished products face an increasing demand from industry as they can be manufactured to dimensional precision and high surface quality. Products from cold formed bar, on the other hand, may contain inhomogeneous distributions of mechanical properties and residual stresses which arise from the elastic response of the material to an inhomogeneous distribution of elastic–plastic strains. These material properties may cause distortion in further manufacturing operations, and consequently, precision of components then could be reestablished at higher costs only. X-ray diffraction residual stress analysis, for example, is misleading if only slices or cross sections of a bar or a component are considered and residual stress fields and their variations are neglected. Automotive products generally are cut from drawn bars, and local differences in microstructure, mechanical properties and residual stress states are increasing the danger of dimensional changes out of a specified range. Cold-drawn bars were manufactured with different drawing angles and uncoated and TiCN-coated dies. Surface and subsurface properties were investigated along the length and around the periphery of drawn bars. Differences in material states, those affected by the contact zone and those related to the elastic–plastic deformation of the drawing process, were observed. The variation of surface residual stresses of up to a factor of 2 can be correlated with locally different friction coefficients and slip stick effects. As subsurface material states (residual stresses and strain hardening) do not show a significant variation around the periphery and along the length of bars, the effects of geometry variations of hot rolled bar, the effect of a not perfectly concentric bar at the die entrance and/or the level of pre-straightening ahead of drawing are assumed to be of minor importance, compared to the high level of plastic strain involved in cold drawing. The local properties identified here will lead to a higher degree of dimensional scatter of individual automotive components cut from these long cold-drawn bars.

Influence of Shot-Peening Parameters on the Sub-Surface Residual Stress Profiles in Al-7075 Alloy Components

Aluminum alloy (Al-7075-T6) samples were analyzed to determine the in-depth residual stress profile induced by a shot-peening treatment. The influence of coverage degree and Almen intensity on the surface residual stress and on the sub-surface residual stress gradient was investigated. Residual stress profiles were obtained using three different techniques: (i) standard laboratory X-ray diffraction (XRD) residual stress analysis with progressive chemical layer-removal; (ii) XRD residual stress analysis with synchrotron radiation using different X-ray energies, thus changing the penetration depths, and (iii) Blind Hole Drilling (BHD). A comprehensive comparison of the results given by the used techniques is shown.

Extremely anisotropic, direction-dependent elastic grain interaction: The case of ultrathin films

The elastic grain interaction in a 50-nm-thick Pd thin film sputter deposited on a single-crystalline Si substrate has been investigated employing x-ray diffraction residual stress analysis. The occurrence of pronouncedly nonlinear sin2 ψ-plots (i.e., plots of the lattice strain for a particular hkl reflection versus the squared sine of the inclination angle of the diffraction vector with respect to the surface normal ψ) in diffraction stress analysis for the 200 and 222 reflections revealed anisotropic grain interaction of severity not observed before near free surfaces. Application of a direction-dependent elastic grain-interaction model showed that the grain interaction perpendicular to the surface is of Reuss character, whereas the in-plane grain interaction is of Voigt character.

Non-ambient X-ray diffraction residual stress analysis of thin films: tracing nanosize-related effects on thermoelastic constants and identifying sources of residual stresses

The stress evolutions at non-ambient temperatures in Ni and Pd layers (the thickness of each metal layer was 50 nm) were investigated employingin-situX-ray diffraction measurements in a temperature range between 173 and 523 K. Stress determinations were performed by analysing the X-ray diffraction data according to the sin2ψmethod on the basis of the crystallite group variant. The stress evolution in the Ni layer was correlated with the change of its coherently diffracting domain size and its microstrain with temperature. The advantage of measurements at temperatures below ambient to distinguish the thermal stresses from the effects of stress relaxation and emerging secondary stresses, arising from thermally activated processes (such as grain growth), is highlighted in this work for the first time. The possibility of using thein-situX-ray diffraction stress measurement to obtain coefficients of thermal expansion and thermoelastic slopes of thin films (with nanosized grains) is discussed in detail.

Direct X-Ray Measurement of Residual Strains in Textured Steel

One of the most detrimental effects on the accuracy of an X-ray diffraction residual stress analysis, XRDRSA(l), is found in the examination of textured materials. The degree of elastic anisotropy and texture is in general agreement with the extent of the error in the residual stress. Several approaches have been made to correct for the effects of texture, particularly involving experimental techniques. Reviews of such efforts are given by H. D811e(2), v.M. Hauk﹛3) and G. Maeder, J.L. Lebrun and J.M. Sprauel (4), just to mention a few.A brief chronology of the texture corrections involved in XRDRSA follows. With isotropic materials the d spacing of a crystal lattice, d, is assumed to vary linearly with sin2ψ. With textured materials the d vs sin2ψ relationship is nonlinear. This is due to the anisotropy of the elastic constants and their departure from a random distribution, or taking on a preferred orientation.