Focused Ion Beam Digital Image Correlation Research Articles

Multiscale residual stress and mechanical behavior analysis in machining of Ti-6Al-4V alloy with advanced microscopic characterization

Machining of polycrystalline metal materials results in metallurgical modifications in the machined surface, and it can introduce multiscale residual stresses (macroscale and microscale residual stress), which may be detrimental to the mechanical properties thereby interfering with the service performance of machined parts. However, most of the current studies only focus on the macroscale residual stress, the microscale residual stresses have always been neglected since the limitations of the commonly used techniques for residual stress measurement in machined surfaces. In this study, the recently developed Focused Ion Beam-digital image correlation (FIB-DIC) ring core method was employed to evaluate the time-resolved strain relaxation and multiscale residual stress of the machined surface under different cutting conditions. The quantitative evaluation of residual stress and microstructural effects on hardness of the machined surface was conducted by combining with FIB incremental milling and nanoindentation technique. It has been observed that although the global tensile strain relaxation was captured in the ring-core region, local microscale compressive strain relaxation also occurred. The hardness of the machined surface was determined by the combined effects of microstructure and residual stress induced by machining. Specifically, the grain refinement and compressive residual stress contributed to hardening, while tensile residual stress resulted in softening of the machined surface. Furthermore, it was also observed the hardening/softening effects were enhanced with increasing intensity of tension and compression.

Quantitative evaluation of residual stress and microstructural effects on the surface hardness of machined Ti-6Al-4 V alloy with microscopic characterization techniques

The surface hardness of machined Ti-6Al-4 V components has a significant influence on the service performance and fatigue life. It’s acknowledged that the hardness value of machined surface is dependent on microstructure and residual stress, however, the quantification of their influence levels on surface hardness is still an unresolved issue, which limits the potential for further enhancing the mechanical property of machined parts. Furthermore, it’s difficult for current studies to fully reveal the mechanism of hardness fluctuation due to release of residual stress. To address these gaps, this study presents a novel method for quantitatively evaluating residual stress and microstructural effects on the machined surface hardness using microscopic characterization techniques. The microstructures of machined Ti-6Al-4 V workpiece were characterized by electron backscatter diffraction (EBSD), and the microstructural hardening effects were analyzed. The residual stresses release and evaluation were performed by the Focused Ion Beam-Digital Image Correlation (FIB-DIC) ring core method, then the surface hardening originated from residual stress and microstructural effects can be assessed by nano-indentation measurements. The results show that the surface hardness increase induced by the combined effects of compressive residual stress and microstructure, where the microstructural hardening can be strengthening by obtaining smaller grain size and high proportion low grain boundary, and the higher magnitudes of compressive residual stress can also enhance the hardness increase. The surface softening induced by compressive residual stress relaxation with no obvious microstructural alterations was observed and the maximum softening reached 14.54% in this study. The findings are beneficial for understanding the mechanical property strengthening mechanism of the machined parts.

Effect of CrAIN coating properties on impact fatigue of tool steel

The tools in forming processes are subjected to cyclic impact loads. Physical vapor deposition (PVD) coatings can potentially improve the service life of such tools, given the effect of coating properties on impact fatigue of coated substrate is well-understood. Previous investigations on the cyclic impact loading of PVD coated tool steels mainly correlate the plastic deformation of the substrate to the resulting coating cracks. Hereby, the effect of underlying elastic-plastic coating deformation mechanism in combination with coating properties needs further clarification. Therefore, the current work aims to investigate the combined effect of thickness, morphology, elastic-plastic deformation and residual stress state of the coating on the impact fatigue behavior of coated tool steel substrates. For this purpose, (Cr90Al10)N and (Cr66Al34)N coatings, each with a thickness of s = ∼ 1.7 μm and s = ∼3.5 μm, were deposited on HS6–5-2C substrates. In addition to coating characterization, the residual stresses were measured by focused ion beam-digital image correlation (FIB-DIC) ring-core method. The elastic-plastic deformation of the coatings was studied by nanoindentation. The coated samples were subjected to cyclic impact loading with an initial hertzian contact pressure pH = ∼ 9.7 GPa and frequency f = 50 Hz for N = 0.1, 0.5 and 1 million impacts. The impact imprints were analyzed using high resolution electron microscopy to study the coating deformation mechanism in combination with fatigue cracks initiation. Columnar inclination and shear gliding along the column boundaries resulted in the initiation of fatigue cracks in the coating around the boundary region of impact imprint. The chemical composition, morphology, residual stress state and elastic-plastic deformation behavior of the coating cumulatively affected the impact fatigue as thick coatings showed reduced resistance against initiation of fatigue cracks. The investigation contributes to adjusting the PVD coating thickness and resulting coating properties for higher tool service life in applications involving cyclic impact loading.

Grain-level residual stress distribution at dwell fatigue crack tips in a nickel-based superalloy

Dwell fatigue crack growth in nickel-based superalloys is influenced by a complex interplay between mechanical and chemical phenomena. The stress distribution at the crack tip is closely related to these phenomena and to the local microstructure. In this study we evaluate experimentally the microscale residual stress distribution ahead of crack tips in RR1000 alloy samples after interrupted high temperature dwell fatigue testing. In the crack opening direction the results show a compressive residual stress zone, the extent of which is linked to the plastic zone size and crack growth retardation behaviour. In the direction of crack propagation the residual stress is also influenced by crystallographic grain orientation. The results demonstrate a novel approach of experimentally evaluating crack tip stress distribution via focused ion beam – digital image correlation (FIB-DIC) ring-core measurements at the grain-level. This provides new insights into fatigue crack growth in microstructurally heterogeneous materials.

An experimental and numerical analysis of residual stresses in a TIG weldment of a single crystal nickel-base superalloy

The Tungsten Inert Gas (TIG) welding technique is extensively used to join various automobile and aerospace components, such as control arms, rotating blades, and vanes. Highly localized heating followed by rapid cooling during welding exert complex thermal and mechanical loading on the components and give rise to significant residual stress fields which may increase the likelihood of time-dependent failure by promoting crack initiation. In the context of engineering design for structural integrity and reliability of operation, quantitative residual stress evaluation in the finished parts needs to be carried out in a reproducible manner. Samples investigated in this study were TIG fill-in weldments in single crystal superalloy components with nearly cylindrical geometry. The present research employed Focused Ion Beam – Digital Image Correlation (FIB-DIC) micro-ring-core technique for stress evaluation, and a sequentially coupled thermo-mechanical finite element model to assess the residual stress state near the weldment surface in the radial and hoop directions. Good agreement was obtained between experimentally evaluated residual stresses and the refined numerical predictions. The highest hoop and radial tensile residual stresses were both observed near the boundary between the filler metal and base metal, whilst a compressive region was found for hoop stress in the parent metal at the component edge. These observations were discussed in conjunction with the temperature history and residual stress self-equilibration. This research provides the foundation for further investigations of Post Weld Heat Treatment (PWHT) and surface treatment to improve the fatigue performance of weldments.

Enhance Fatigue Resistance of Nanocrystalline NiTi by Laser Shock Peening

A pre-strain laser shock peening method is proposed to fabricate fatigue-resistant nanocrystalline NiTi with graded nanolayers and compressive residual stress layers. Grain size gradient surface layer of about 100 μm thick is fabricated on a 1.5-mm-thick bulk nanocrystalline NiTi plate. It is found that the nanostructure shows a gradient distribution from middle region of the plate to its laser-treated surface, and the grain size at the treated surface is about 5 nm. B2, B19′ phase and Ni4Ti3, Ni3Ti precipitates are found at the treated surface. The nanohardness at the laser-treated surface reaches 10 GPa. Residual stress profile on the laser-treated plate cross-section is measured by a focused ion beam-digital image correlation technique. The measured maximum residual compressive stress is about 1.2 GPa at the laser-treated top surface, while there is residual tensile stress of about 200 MPa in the middle region. Four-point bending displacement-controlled experiments show that the fatigue life of the NiTi sample increases about seven times after laser treatment. The work demonstrates that pre-strain laser shock peening without surface coating is an effective method to fabricate fatigue-resistant nanocrystalline NiTi with gradient grain size and compressive residual stress layers.

Residual stresses in single particle splat of metal cold spray process – Numerical simulation and direct measurement

This report provides the first quantitative evaluation and prediction of residual stress arising from a single particle deposition in the metal cold spray of Ti-6Al-4V. Micro-ring-core Focused Ion Beam–Digital Image Correlation (FIB-DIC) technique is employed to determine the residual stress variation experimentally, while finite element simulation with Johnson-Cook plasticity and dynamic failure model is employed to numerically predict the residual stress distribution within single particle, and they show good agreement with each other for different impact velocities. This provides a tight link between the validated description of microscopic phenomena and the ensuing macroscopic properties and processes of the deposit.

Evaluation of macro- and microscopic residual stresses in laser shock-peened titanium alloy by FIB-DIC ring-core milling with different core diameters

The residual stresses in a laser shock-peened (LSP) plate of titanium alloy were measured by means of the ring-core Focused Ion Beam–Digital Image Correlation (FIB-DIC) technique using pillar diameters of 10 μm and 5 μm. A cross-section of the Ti-6Al-4V laser shock peened plate was electrochemically polished to avoid induced residual stress modification due to grinding. FIB-DIC measurements with pillar diameters of 10 μm show that a near-surface compressive residual stress of approximately 350 MPa is present in the direction parallel to the peened surface. This changes to a tensile residual stress of approximately 100 MPa at the depth of 2 mm, and then changes back to a small compressive residual stress on the opposite face. These results are in good agreement with the previously obtained set of measurements using high-energy synchrotron X-ray diffraction. However, FIB-DIC measurements with pillar diameters of 5 μm show strong deviations from this general macroscopic trend that corresponds to the macroscopic average (Type I) residual stresses. The reason for the apparent discrepancy lies in the smaller gauge volume used in the second set of measurements, which causes the evaluated residual stress values to be dominated by the microscopic (Type II + III) residual stress components. These Type II + III residual stresses are the result of the local microstructure and intra- and intergranular interactions. The results demonstrate that the FIB-DIC ring-core technique can be tailored to determine either macro- or microscopic residual stress, by tuning the core diameter with respect to the grain size. Type II + III stresses have significant magnitude sufficient to produce local tensile stress in the region that is subjected to macroscopic compression due to LSP treatment, and vice versa. The implications of this finding are discussed.

Separating macro- (Type I) and micro- (Type II+III) residual stresses by ring-core FIB-DIC milling and eigenstrain modelling of a plastically bent titanium alloy bar

A novel approach to separating macroscopic (Type I) from microscopic (Type II + III) residual stress is presented, based on Focused Ion Beam – Digital Image Correlation (FIB-DIC) ring-core stress evaluation and eigenstrain modelling. This approach was applied to study the residual stresses for a titanium alloy bar following plastic four-point bending. It was found that electrochemical polishing is a surface preparation technique that is very well suited for FIB-DIC ring-core measurements, in the sense that it removes the influence of prior sample grinding and polishing, leads to a stress profile that satisfies force and moment equilibrium, and thus enables the evaluation of absolute values of total residual stress. The obtained relief strain profile across the bar width is asymmetric, highlighting the difference in the alloy's response to tension and compression. Total experimental residual stress values were calculated using (i) the assumption of material elastic isotropy, with an average Young's modulus, and (ii) under the assumption of elastic anisotropy, taking into account the crystallographic orientation of each investigated grain. Based on the measured relief strain values, the eigenstrain distribution in the bar was reconstructed and used to obtain the macroscopic (Type I) residual stress profile. The differences in the residual stress between the eigenstrain reconstruction values and the individual experimental results were ascribed to the local microscopic (Type II + III) residual stresses. This conclusion was substantiated by revealing the correlation between the residual stress values in individual grains in the elastic zone and their respective Young's moduli in the loading direction, as well as the correlation between the residual stress values in grains located in the plastic zone and their respective Schmid factors for basal slip.

Experimental and modelling characterisation of residual stresses in cylindrical samples of rapidly cooled bulk metallic glass

Quench processing is widely used in industry to impart the desired structural and mechanical properties by controlling microstructure and compositional gradients, e.g. to obtain supersaturated solid solutions in aluminium alloys, or to achieve martensitic hardening in steels. Rapid cooling, also referred to as quenching or tempering, is also the principal production route for bulk metallic glasses that exhibit high hardness and strength due to their amorphous structure that precludes plastic deformation by easy crystal slip. Importantly, rapid cooling is accompanied by the creation of residual stresses that also have a strong effect on the deformation behaviour.The present study aims to obtain insight into the residual stresses in cylindrical samples of Zr-based bulk metallic glass (BMG) by combining analytical modelling of thermal and mechanical problems with experimental measurements using Focused Ion Beam–Digital Image Correlation (FIB-DIC) ring-core milling. The results show good agreement between the two approaches, providing improved confidence in the validity of the two approaches considered here.

A study of overload effect on fatigue crack propagation using EBSD, FIB–DIC and FEM methods

Abrupt increase in the maximum load during fatigue cycling modifies the deformation conditions at the crack tip, causing plastic flow that may lead to crack closure, introducing residual stress and hardening. The net consequence of these effects is notable crack growth retardation. Despite decades of research in the field, controversy persists regarding the role of each specific mechanism and their interaction. Resolving these issues with the help of experimental observation is related to the difficulty of obtaining local residual stress information at appropriate resolution. The present study examines the effect of overload on fatigue crack grown in a Compact Tension (CT) specimen of aluminium alloy AA6082 (BS HE30). Fatigue crack was grown in the sample under cyclic tension (R=0.1). After the application of a single overload cycle, fatigue loading was recommenced under previous cycling conditions. The crack morphology was investigated using Scanning Electron Microscopy (SEM). Electron Backscattered Diffraction (EBSD) was used to map grain orientation and crystal lattice distortion (pattern quality) in the vicinity of the crack. EBSD analysis of intra-granular misorientation allowed the qualitative analysis of the region around the crack tip location at the time of the overload application. Observations are discussed with a view to identify the roles of crack closure and residual stress effects. Residual stress was evaluated at salient locations around the crack retardation site using the FIB–DIC method which combines the use of Focused Ion Beam (FIB) and Digital Image Correlation (DIC) for residual stress measurement at the (sub)micron-scale. The residual stress field due to overload occurrence was also simulated using Finite Element Method (FEM), and the results compared with experimental observations.

A review of micro-scale focused ion beam milling and digital image correlation analysis for residual stress evaluation and error estimation

Abstract In the past decade several versions of micro-scale residual stress analysis techniques have been developed based on Focused Ion Beam (FIB) milling at sample surface followed by Digital Image Correlation (DIC) for the determination of the resulting strain relief. Reliable and precise estimation of the error bounds on these measures is critical in determining the usefulness and accuracy of residual stress evaluation. Here we present an overview of the steps necessary for effective outlier removal, error propagation and estimation in order to provide reliable confidence limits for the stress value obtained. Error propagation analysis begins with DIC marker tracking errors that depend on imaging contrast and magnification, and can be improved with sub-pixel tracking and marker shift averaging. We demonstrate how the outliers and poorly tracked markers ought to be removed from the data set using correlation coefficient thresholding and/or correlation peak confidence intervals. Markers showing large displacements relative to their neighbours can also be identified as aberrant, and removed. By performing careful error propagation throughout the analysis chain we quantify the displacement and strain fields, and qualify them with the associated confidence intervals. These values, in combination with the elastic modulus confidence limits, are then used to provide the final confidence intervals for the determined residual stress values. The generic nature of the methodology presented ensures its suitability for all residual stress analysis techniques based on FIB milling and image correlation analysis. An example of implementation is presented for the micro-scale ring-core FIB–DIC approach.

Residual micro-stress distributions in heat-pressed ceramic on zirconia and porcelain-fused to metal systems: Analysis by FIB–DIC ring-core method and correlation with fracture toughness

ObjectivesThe production of fixed partial dentures (FPDs) induces complex residual stress profiles, due to both the thermal expansion coefficient mismatch between the veneering ceramic and the framework and to the thermal gradients occurring during the final cooling. Detailed knowledge of residual stress distributions in the veneering ceramics is important to understand the interface phenomena with the framework and the consequences of the different firing systems. The first objective of this study was to analyse the residual stress distribution in heat-pressed ceramic on zirconia core with micrometer spatial resolution, with also a focus on the stress at the interface versus porcelain-fused-to-metal samples. The second purpose was to correlate the residual stress with the fracture toughness. MethodsThe micron-scale focused ion beam (FIB) ring-core method was used to map the residual stress over the cross-sections of the veneering ceramics. The methodology is based on FIB micro-milling of annular trenches, combined with high-resolution in situ scanning electron microscope (SEM) imaging, a full field strain analysis by digital image correlation (DIC) and numerical models for residual stress calculation. Fracture toughness was evaluated by using high load Vickers indentation and hardness/modulus were measured by nanoindentation testing also across the interfaces. ResultsBoth prosthetic systems showed a compressive stress at the ceramic surface on a micron-scale. The stress profile for porcelain fused to metal (PFM) showed a transition to tensile stress at the half of the layer, whilst the stress in proximity of the interface was more compressive in both the cases. Residual stress on a micron scale are higher in magnitude than the corresponding macro-scale values reported in the literature, due to the stress relaxation given, at larger scales, by micro-voids and cracks. The stress field was directly correlated with the indentation fracture toughness, which was higher in those areas where the compressive stress is greater. Stress analysis in correspondence of interfacial porosity for the zirconia sample also showed that micro-defects could induce local modifications of the residual stress field, which may even locally generate a tensile stress state. SignificanceThe interfacial stress in dental systems was analysed on a micron scale and can give further insights into the process/property/performance correlation for this class of materials. In particular, interfacial and/or local modifications of the residual stress are expected to have a significant influence on crack nucleation mechanism in correspondence of micro-defects. A direct correlation between residual stress distribution and fracture toughness was proposed. It is noteworthy that the method can be used to study real crowns and bridges. In fact, complex geometries can be easily analysed by this procedure.

A state-of-the-art review of micron-scale spatially resolved residual stress analysis by FIB-DIC ring-core milling and other techniques

Quantification of residual stress gradients can provide great improvements in understanding the complex interactions between microstructure, mechanical state, mode(s) of failure and structural integrity. Highly focused local probe non-destructive techniques such as X-ray diffraction, electron diffraction or Raman spectroscopy have an established track record in determining spatial variations in the relative changes in residual stress with respect to a reference state for many structural materials. However, the interpretation of these measurements in terms of absolute stress values requires a strain-free sample often difficult to obtain due to the influence of chemistry, microstructure or processing route. With the increasing availability of focused ion beam instruments, a new approach has been developed which is known as the micro-scale ring-core focused ion beam-digital image correlation technique. This technique is becoming the principal tool for quantifying absolute in-plane residual stresses. It can be applied to a broad range of materials: crystalline and amorphous metallic alloys and ceramics, polymers, composites and biomaterials. The precise nano-scale positioning and well-defined gauge volume of this experimental technique make it eminently suitable for spatially resolved analysis, that is, residual stress profiling and mapping. Following a summary of micro-stress evaluation approaches, we focus our attention on focused ion beam-digital image correlation methods and assess the application of micro-scale ring-core methods for spatially resolved residual stress profiling. The sequential ring-core milling focused ion beam-digital image correlation method allows micro- to macro-scale mapping at the step of 10–1000 μm, while the parallel focused ion beam-digital image correlation approach exploits simultaneous milling operation to quantify stress profiles at the micron scale (1–10 μm). Cross-validation against X-ray diffraction results confirms that these approaches represent accurate, reliable and effective residual stress mapping methods.

Depth-resolved residual stress analysis of thin coatings by a new FIB–DIC method

Abstract A new methodology for the measurement of depth sensitive residual stress profiles of thin coatings with sub-micrometer resolution is presented. The two step method consists of incremental focused ion beam (FIB) ring-core milling, combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained by comparison of the experimentally measured surface strain with finite element modeling using Schajer's integral method. In this work, a chromium nitride (CrN) CAE-PVD 3.0 μm coating on steel substrate, and a gold MS-PVD 1.5 μm on silicon were selected for the experimental implementation. Incremental FIB milling was conducted using an optimized milling strategy that produces minimum re-deposition over the sample surface. Results showed an average residual stress of σ = −5.15 GPa in the CrN coating and σ = +194 MPa in the Au coating. These values are in reasonable agreement with estimates obtained by other conventional techniques. The depth profiles revealed an increasing residual stress from surface to the coating/surface interface for both coatings. This observation is likely related to stress relaxation during grain growth, which was observed in microstructural cross sections, as predicted by existing models for structure–stress evolution in PVD coatings. A correlation between the observed stress gradients and the in-service mechanical behavior of the coatings is proposed. Finally, critical aspects of the technique and the influence of microstructure and elastic anisotropy on stress analysis are analyzed and discussed.