Microscopic Internal Stresses Research Articles

Correlations Between XRD Peak Broadening and Elastic and Plastic Deformation for Mild Steel Sheets Under Tensile Loading

The polycrystalline nature of steel sheets plays a fundamental role in their plastic behavior during forming, especially under complex stresses, such as the deep drawing process. Heterogeneous microstructures cause variable internal stress levels, which strongly impact the material’s macroscopic mechanical properties. X-ray diffraction (XRD) technique can be applied to determine microscopic internal stresses by peaks broadening analysis, which can be related to the degree of cold work at the microscopic level. As deep drawing is a complex deformation mode, an important motivating factor to perform such a study is to find the correlation between anisotropy and XRD peak broadening through tensile deformation tests. Therefore, the initial mechanical state and different deformed states were analysed by in-situ XRD peak broadening under tensile tests in various directions in the rolling plane of mild steel sheet, for both the elastic and plastic deformation domains. The results showed that the XRD peak broadening gives accurate estimations of deformation degrees. In the elastic domain, under realistic assumptions, a relation between the relative variation of peak broadening and the applied stress σ, like Hooke’s law, was obtained by exploiting the linearity between the peak broadening and the elongation measured by the strain gauge. This formula may be used to deduce The Young’s modulus. Interesting empirical relations between peak broadenings, macro deformations and hardness have been also established for the studied sheet in the plastic deformation region. Moreover, XRD peak broadenings were able to characterize the anisotropy of deformation in the rolling plane of the sheet. They are like the anisotropy coefficients in indicating the expected behavior under complex processes such as deep drawing by using ordinary tensile tests.

Formation of microscopic internal stresses in rails during long-term operation

Formation of microscopic internal stresses in rails during long-term operation

Relation between Microscopic Internal Stress Caused by Phase Transformations and Mechanical Properties in Steels

Relation between Microscopic Internal Stress Caused by Phase Transformations and Mechanical Properties in Steels

Effect of Titanium Matrix Structure on Growth Morphology of Anodized TiO2 Nanotube Arrays for Applications in Photoelectrochemical Performances

In the application of a solar cell and photocatalytic system, anodized titanium dioxide nanotube arrays (TNTs) have one-dimensional highly ordered nanotube structure, so they have attracted much attention. In order to form ordered TNTs, many factors, such as voltage, anodizing time, solvent type, and so on, need to be controlled. However, these factors have mainly focused on the effect of external conditions, while the effect of internal factors such as the crystal structure of the titanium substrate on the growth of nanotubes have been less investigated. In this paper, the titanium substrate tissue structure with a (002) preferred orientation was obtained by a rolling treatment. On this basis, TNTs with highly ordered structure were constructed, and the effect of the titanium substrate structure on the nanotube growth morphology was investigated. The changes in morphology were characterized by scanning electron microscopy, and the relationship between the changes in the crystal structure and morphology of TNTs was systematically analyzed by combining metallographic structure and X-ray diffraction data. The changes in the electrochemical behavior of the titanium substrates after rolling were examined through electrochemical methods. The above results show that there are a large number of dislocations and microscopic internal stresses in the grain structure of the titanium substrate after rolling, which leads to improvement of the corrosion resistance of the materials. The changes in the microstructure of these materials make the nanotube arrays have highly ordered structure and better photoelectrochemical properties, which provides an idea for improving their applications in photoelectrochemistry.

Texture and microstructural changes after thermal cycling of 6061Al-20vol%SiCw metal matrix composite: The role of microscopic internal stresses

The dramatic texture and microstructural changes observed in 6061Al-20vol%SiC w metal matrix composite undergoing severe thermal cycles, under the absence and the simultaneous action of an external tensile stress, are studied. Under only thermal cycles (100-450°C) homogenization and disorientation of the SiC whisker reinforcement and crystallographic texture randomization occurs. However, when a simultaneous tensile stress is applied, the whiskers rotate so that their long direction aligns with the tensile axes. Furthermore, a strong texture and large deformations (superplasticity), higher than 1000%, are achieved. These results are explained on the basis of the microscopic stress fields generated at the different microstructural scales (stresses of type II and type III) and well-known observation of dislocation generation at the SiC-metal interface during the cooling period of the cycles. We propose that moving dislocations (responsible of type III stresses) operate differently under the absence or the presence of the external stress. Under no stress, dislocation motion (occurring mainly during the heating period) is driven only by the type II internal stress. However, dislocation motion is improved when an external stress is applied, leading to texture changes and large elongations. Despite that low external stresses are applied, they overcome the effect of the internal stresses for dislocation motion. • The changes related with the homogeneity/orientation of the reinforcement in 6061Al-20vol%SiC w composite undergoing thermal cycles are discussed. • Thermal cycling produces homogenization of the SiC w reinforcement. • Dramatic SiC w reorientation with the tensile axis direction occurs when samples are strained under thermal cycling conditions. • The microstructural and texture changes are explained form the evolution/interaction of microscopic internal stresses developed during the thermal cycles. • The cooling period of the cycle generates massive moving dislocations which are driven by the internal stress fields or the external stress.

Anisotropic cleavage fracture caused by transformation-induced internal stress in an as-quenched martensite

The effect of microscopic internal stresses (Type-II) generated via martensitic transformation from austenite (A) to martensite (M) on {001} M cleavage fracture was investigated in an as-quenched 0.1C–5.0Mn steel (mass%). Crystallographic orientation analysis using the electron backscatter diffraction technique revealed that cryogenic {001} M cleavage fracture was promoted predominantly in mode I fracture by Charpy impact testing, and that the Bain group, where martensite variants belong to the same Bain variants, acted as an effective unit to prevent fracture. It was also discovered that the cleavage crack preferentially propagates on (001) M rather than on (100) M in the Bain group, where (001) M was nearly parallel to (001) A in the Bain lattice correspondence ([−110] A // [100] M , [110] A // [010] M , (001) A // (001) M ). In addition, by using a combination of a micro-scale focused ion beam and high-precision digital image correlation techniques, it was found that microscopic internal stresses were anisotropically developed in each Bain group. The principal axes of the internal stresses corresponded to the <001> M coordinate axes in the Bain lattice correspondence, and the principal stress parallel to [001] M is much higher than that of the other two. After the measurement of the internal stresses, it was proved that the cleavage fracture behavior obeyed the effective normal stress in the normal direction of {001} M consisting of the resolved bending stress in the impact test and the anisotropic internal stress generated via martensitic transformation. This suggests that the transformation-induced internal stress originating from the Bain strain leads to the anisotropy of the cleavage fracture in lath martensite. • The effect of microscopic Type-II internal stress on the macroscopic cleavage fracture was investigated in lath martensite. • FIB-DIC technique revealed that martensitic transformation induces the anisotropic elastic strain in each Bain group while maintaining <001> as the principal axes. • EBSD analysis proved that mode-I cleavage crack preferentially propagates on (001) rather than on (100) in a Bain group regardless of carbon contents. • Transformation-induced internal stress originating from Bain strain leads to the anisotropy of the cleavage fracture in lath martensite.

Internal Stresses and their Sources in BCC and FCC Steels

The present work summarizes and presents separate results obtained by the authors when investigating mesoscopic and microscopic internal stresses formed under the conditions of thermal and mechanical treatment of martensitic, pearlitic and austenitic steels. Internal stresses were investigated using the method based on the analysis of bend extinction contours. The results obtained on industrial steels were presented. The sources were described and examples of internal stresses induced by these sources were given. The nature of bending-torsion of the crystal lattice depending on the averaging volume was determined. It has been shown that in martensitic steels along with the increase in the averaging volume (carbide particle → separate martensitic lath → martensite packet→ martensitic plate → grain) the amplitude of bending-torsion of the crystal lattice decreases. The nature of distortions also changes. At large amplitudes and low volumes of averaging they are completely or partly elastic, at large volumes of averaging they are completely plastic. Thereby, distortions are fully driven by the excess dislocation density.

Effects of carbon nanotubes on hardness and internal stress in Ni–P coatings

Nickel–phosphorus carbon nanotube (CNT) composite coatings were prepared via brush electroplating. The CNTs were added in amounts of 0, 0·5, 1·0 and 2·0 g L−1. Half samples of these coatings were heated at 400°C for 1 h to obtain crystalline Ni–P CNT composite coatings. The hardness and internal stress of two kinds of composite coatings were studied. The results showed that the addition of CNTs improves the microhardness of crystalline Ni–P coatings; the higher the added CNT, the greater increase in microhardness. However, the effects are completely different for amorphous coatings. Heat treatment greatly increased the microhardness of amorphous Ni–P platings, but the microhardness of Ni–P based CNT composite coatings only slightly increased. The microhardness of the crystalline Ni–P based CNT composite coatings was lower than that of the crystalline Ni–P platings and was not insensitive to the changes in the added amounts of CNTs. The addition of CNTs significantly reduced the microscopic internal pressure stress of crystalline coatings; the higher the added CNT, the lower the microscopic internal pressure stress.

XRD peak broadening characterization of deformed microstructures and heterogeneous behavior of carbon steel

The XRD peak broadening is in direct relation with the structural anisotropy and heterogeneity in the rolling plane during deformation by deep drawing in mild soft steel. The inhomogeneous structures give internal stresses at different scales, which strongly influence the superficial mechanical properties. The XRD method is a local, quantitative and selective technique, which made particularly possible to evaluate intensity and distribution of microscopic internal stresses in deformed microstructures. In that study different deformed samples (<2%, 3%, 6%, 9%, 12%, 15%) were analyzed by DRX peak broadening. The evolution of peak broadening for deformation’s ratio is compared to evolution of the yield strength R0.2% obtained by macroscopic mechanical tests with variable angle to the rolling direction, for a simulation of drawing behavior. It may be noted that the peak broadening has good linear relationship with the yield strength. As well the anisotropy of macroscopic behavior has been observed through XRD peak broadening. The relationship between structure heterogeneity and microscopic internal stresses seems well established.

Effect of precipitate–precipitate interaction on residual stress in welded structure

The process of assembly by welding led to the creation of micro-structural heterogeneities zones. Generally these zones can be discontinuities macroscopic geometrical (macro-geometry of the weld nugget) or/and geometrical or metallurgical defects. At the origins of stress concentration, these regions are favorable sites where fatigue cracks can initiate and propagate. Our work concerns the role of the existence of the precipitate during the process of welding on the level of the microscopic internal stress thermal origin. The distribution of this stress in the steel matrix around these defects was the subject of a numerical analysis by the finite element method. The originality of this study lies in the angle of attack in the volume fraction of the precipitate and the effect of precipitate–precipitate interaction on the distribution and the level of this stress. The precipitate in the steel matrix resulting from a process of welding generates thermal stress whose level reaches its maximum value with the interface matrix–precipitate. Far from this interface the matrix is completely released of this stress. This stress can constitute a risk of damage to the welding joint.

Infrared birefringence imaging of residual stress and bulk defects in multicrystalline silicon

This manuscript concerns the application of infrared birefringence imaging (IBI) to quantify macroscopic and microscopic internal stresses in multicrystalline silicon (mc-Si) solar cell materials. We review progress to date, and advance four closely related topics. (1) We present a method to decouple macroscopic thermally-induced residual stresses and microscopic bulk defect related stresses. In contrast to previous reports, thermally-induced residual stresses in wafer-sized samples are generally found to be less than 5 MPa, while defect-related stresses can be several times larger. (2) We describe the unique IR birefringence signatures, including stress magnitudes and directions, of common microdefects in mc-Si solar cell materials including: β-SiC and β-Si3N4 microdefects, twin bands, nontwin grain boundaries, and dislocation bands. In certain defects, local stresses up to 40 MPa can be present. (3) We relate observed stresses to other topics of interest in solar cell manufacturing, including transition metal precipitation, wafer mechanical strength, and minority carrier lifetime. (4) We discuss the potential of IBI as a quality-control technique in industrial solar cell manufacturing.

The Nondestructive Estimation of Mechanical Properties of a Carbon Steel by X-ray Diffraction Peak Broadening

Abstract Carbon steel is largely used in automotive sheet works by deep drawing. The nondestructive determination of the superficial mechanical properties is searched for qualification of the fabrication processes and for service duration estimation. X-ray diffraction (XRD) technique is nondestructive and can be used to determine microscopic internal stresses by broadening of the XRD peaks. The average magnitude of XRD peak broadening is related directly to the hardness, the elastic limit, or degree of cold work of the material. So it can be used as a means of quantifying the surface mechanical properties. With an empirical relationship between the XRD peak broadening and the conventional mechanical properties (hardness, elastic limit, and plastic deformation) for carbon steel, the superficial mechanical characteristics can be estimated from diffraction peak broadening for finished stages of the manufacture cycle or for maintenance pieces.

Effect of 0.770 wt%H addition on the microstructure of Ti–6Al–4V alloy and mechanism of δ hydride formation

Ti–6Al–4V alloy was hydrogenated at 750 °C below the eutectoid transformation temperature with 0.770 wt%H, and its microstructural evolution and phase transformations were investigated by the use of optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that, in comparison to the as-received alloy, the contrasts of equiaxed α phase and transformed β phase under optical microscope in the hydrogenated one were reversed. The XRD peaks of β phase after hydrogenation moved to lower angles obviously because of the lattice expansion of β phase with the solution of hydrogen atoms, and the XRD peaks of α phase were broadened due to microscopic internal stress. In addition, XRD and TEM analysis revealed that the face-centered cubic (FCC) δ hydride could precipitate not only in α phase but also in β phase. Furthermore, the mechanisms of δ hydride formation were discussed, and the results showed that the hydrogen diffusion has a significant effect on the δ hydride formation.

Plastic Deformation of Irradiated Zirconium Alloys: TEM Investigations and Micro-Mechanical Modeling

Abstract TEM investigations have been performed on irradiated samples after deformation covering various testing conditions for different recrystallized Zr alloys. It is shown that for transverse tensile tests and internal pressure tests performed at 350°C, only basal channels are observed for strain levels up to uniform elongation, whereas only prismatic and pyramidal channels are observed for axial tensile test. Then, focusing on internal pressure testing, irradiation hardening and strain hardening behavior have been analyzed in detail and compared to TEM observations. It is proposed that dislocation channeling phenomenon leads to high strain incompatibility between channels and surrounding grains and therefore to high microscopic internal stresses. These high microscopic internal stresses are believed to induce strong kinematic hardening at the macroscopic scale, as suggested by the mechanical behavior analysis. Finally, a micro-mechanical model based on microscopic deformation mechanisms and using homogenization techniques is proposed, taking into account the observed channeling phenomenon.

Stress Changes and Stability of Sputter-Deposited Mo/B4C Multilayer Films for Extreme Ultraviolet Mirrors

Periodic Mo/B4C multilayer films (MLs) for extreme ultraviolet mirrors were deposited by an rf-enhanced plasma magnetron sputtering system with changing the sputtering Ar pressure and layer thickness ratio. Internal stresses in the entire film were measured by Newton's rings method. Microscopic internal stresses in Mo crystallites in Mo/B4C MLs and in Mo single layer film were also determined by wide-angle X-ray diffraction from which lattice spacing in Mo (110) planes was measured. The internal stresses changed from a compressive value of about 1.1 GPa to a tensile value of about 0.6 GPa with increasing Ar pressure from 1.3 to 6.0 mTorr. The transition point of changing the stress from compressive to tensile shifted to lower Ar pressure with increasing the Mo layer thickness ratio. From atomic force microscope observation, the surface roughness of Mo/B4C multilayer films was found to increase monotonically from 0.11 to 0.23 nm (rms) with increasing Ar pressure. We examined the effect of the storage conditions of the films under vacuum, in low-humidity air and in air saturated with water vapor. Deterioration of film quality in Mo/B4C MLs was observed within a few weeks in the samples stored in open air with high humidity.

The non-destructive estimation of the superficial mechanical properties of components in the INCONEL 600 alloy by X-ray diffraction peak width

INCONEL 600 alloy is largely used as nuclear power center components. The non-destructive determination of the superficial mechanical proprieties is indispensable for the qualification of the fabrication process and for the service duration estimation in aggressive environment. X-ray diffraction (XRD) technique is non-destructive and can be used to determine microscopic internal stresses from the broadening of the XRD peaks. The average magnitude of XRD peak broadening is related directly to the hardness, the elastic limit or degree of cold work of the material which can be used as a means of quantifying the surface mechanical properties. With an empirical relationship between the {311} XRD peak width and the conventional mechanical proprieties (hardness, elastic limit, hard work degree) for INCONEL 600 alloy, the superficial mechanical characteristics can be estimated from diffraction peak width for just finished parts or for maintenance pieces. Several TEM observations are also realized for microstructure study.

Electromechanical Fatigue of Lead Zirconate Titanate Ceramics

Mechanical fatigue of lead zirconate titanate ceramics when both an ac electric field and mechanical stress were applied to the specimen was investigated. Fatigue strengths under an ac electric field degraded compared with the fatigue strengths under short-circuit conditions while the slope of the S-N curve was the same as that under the short-circuit condition. The reason for the degradation of fatigue strength under an ac electric field is considered to be the microscopic internal stress caused by the electric-field-induced strain. Degradation of fatigue strengths under an ac electric field became larger when the electric field became bipolar even though the amplitude of the field was the same. The reason for the dependence of fatigue strength under an ac electric field on the applied field polarity is considered that the internal stress became large when the ac electric field became bipolar.

Effects of dc Electric Field on Mechanical Properties of Piezoelectric Ceramics

The effects of dc electric field on mechanical properties such as hardness, fracture toughness, flexural strength and flexural fatigue strength of piezoelectric ceramics were investigated. Hardness and fracture toughness were not influenced by the dc electric field, where as the flexural strength and the fatigue strength degraded with the absolute value of the electric field. The degradation of the flexural strength and the acceleration of fatigue were greater under an electric field whose direction was opposite to the poling direction than an electric field whose direction was the same as the poling direction. The main cause of strength degradation and fatigue acceleration in piezoelectric ceramics is estimated to be the microscopic internal stress which is generated at the grain boundary by the piezoelectric and domain switching deformations.

Hardness, stress relaxation and microstructure of TiBN multilayer coatings

Multilayer coatings of a TiBN layer sequence have been prepared by sequential sputtering from a Ti cathode and a h-BN cathode. These layers were subjected to thermal treatment at 400°C by which a hardening process as consequence of interdiffusion was observed. Hardness values of up to 60 GPawere obtained. In contrast to most other hard coatings these TiBN coatings display only small macroscopic stress of the order of ±1 GPa as measured by the substrate bending method. Tensile or compressive stress can be engineered by correctly choosing the repeat thickness and the individual layer ratio. TEM investigations suggest that these low stress values are due to an internal stress relaxation process. The individual multilayers are subjected to a high microscopic internal stress resulting in fragmentation into very small pieces of about 50 nm. This is supported by the strong bending of these fragments as observed in the TEM micrographs.

Harmonic Generation for Measurement of Internal Stresses as Produced by Dislocations

Absrmcr-The present knowledge on the state of microscopic internal stresses, as produced by dislocations, and the relation of these internal stresses to materials properties such as flow stress and the state of fatigue are briefly reviewed. It will then be shown that ultrasonic harmonic generation is sensitive to the dislocation density and loop lengths and thus to microscopic internal stresses Some experimental data on harmonic generation in a deformed and fatigued aluminum specimen, which are in agreement with the above theory, are presented. Thus the technique of ultrasonic harmonic generation is shown to be a new and potentially powerful technique for the nondestructive evalua- tion of technologically important materials. N A RECENT meeting on nondestructive evaluation of internal stresses (l) the need for additional work in this area became quite apparent. The presence of either a compres- sional or a tensile macroscopic internal stress in a structural component is a very important parameter, in particular in de- sign work, which very effectively determines crack initiation and crack propagation under external loads. Strong efforts are being made at present to determine this kind of internal stress, mainly using X-ray techniques and, lately, acoustic birefringence and magnetic techniques. The role of micro- scopic internal stresses on mechanical properties, on the other hand, seems to be not quite clear to the designer or mainte- nance. The present paper, therefore, is addressed specifically to this subject. In particular, the internal stresses due to dis- locations which are the defects responsible for plasticity ef- fects in metals will be discussed. A literature survey as well as some preliminary experiments to be reported in this paper seem to indicate that the flow stress as well as a part of the fatigue life of a material are reflections of the state of the microscopic internal stress. These experiments have been per- formed using acoustic harmonic generation, and it is this tech- nique that is proposed in the present paper as a possible non- destructive tool to study the microscopic internal stress due to dislocation or, in other words, plasticity effects. The work reported here by no means is complete, and many basic questions remain to be answered. It is hoped, however, that the present work will lead to a better understanding of the materials' reliability and product lifetime.