Monotonic Tensile Deformation Research Articles

Quantification and Characterization of Microdamage Resistance in Metals for Designing High-Strength Ductile Microstructures

ConspectusFractures in metallic materials such as ductile austenitic, ferritic, and dual-phase steels often occur after significant plastic deformation. The dislocation-driven plasticity enhances the microscopic stress concentration and induces vacancies (e.g., by the jogged screw dislocation motion), resulting in microstructure-scale cracks and voids at specific crystallographic planes or microstructure boundaries. (Hereafter, cracks and voids are referred to as damage.) In addition, plasticity plays important roles in microscopic damage growth in terms of damage tip blunting and coalescence. In fact, plasticity-related damage evolution in various fracture phenomena such as metal fatigue and hydrogen embrittlement still contains many uncertainties, particularly in high-strength materials containing fine and complex microstructures. Therefore, microscopy-based damage quantification and characterization are required for designing damage-resistant microstructures. In this context, because damage evolution is a multiscale phenomenon from atomistic to over millimeter scales, the target size of damage in the measurements is important to realize reliable analyses. The intrinsic origin of damage evolution is an atomistic/nanometer scale phenomenon, which requires transmission electron microscopy for its analysis. However, from mechanical viewpoints, revealing damage nucleation behavior is not necessary, because general mechanical analyses are performed for oversubmicrometer-sized damages that can be observed by optical microscopy and scanning electron microscopy. Hence, submicrometer/micrometer-sized damage is the major target in this damage quantification. As a post-mortem analysis method, the damage area fraction, the number of damages, the damage size, and the damage shape at various plastic strains can be quantified by observing micrometer-sized damages. In the case of monotonic tensile deformation, the number density of damages plotted against strain indicate the damage initiation probability. In addition, when damages stop growing, a strain range where the average damage size remains nearly constant appears. The strain range quantitatively indicates damage arrestability. For instance, dual-phase steel consisting of soft and hard body-centered cubic phases clearly shows three strain-dependent damage evolution regimes: (1) damage incubation regime (no damage appears), (2) damage arrest regime (damage initiates, but stops growing immediately), and (3) damage growth regime. Hydrogen uptake increases damage initiation probability and decreases damage arrestability. Specifically, a comparison of the quantified damage parameters between specimens with and without hydrogen revealed that large amounts of hydrogen in the dual-phase steel degrades the resistance to microscopic damage growth, which critically decreases the macroscopic ductility. In this case, the damage arrestability (damage growth resistance) is more important than crack initiation resistance, which is dependent on the dislocation-driven stress accommodation capability of soft microstructure components surrounding the crack initiation site. Thus, damage quantification enables the evaluation and classification of microscopic problems in plasticity-related fractures. After the classification of the problems, the underlying mechanisms for damage initiation and growth must be clarified separately. To reveal the mechanisms, full-field micrometer-scale strain mapping and site-specific dislocation characterization are required for damage initiation and growth. To fulfill these demands, new techniques were developed, such as micrometer-scale replica digital image correlation and dislocation-resolved in situ electron channeling contrast imaging. Based on the deformation/dislocation characterization results, metallurgical models of damage evolution can be developed, enabling to propose a mechanism-based microstructure-design strategy for creating damage-resistant high-strength materials.

A synchrotron X-ray and electron backscatter diffraction based investigation on deformation and failure micro-mechanisms of monotonic and cyclic loading in titanium

Synchrotron X-ray diffraction technique has been used to estimate defect structure in terms of dislocation density, crystallite size and micro-strain in commercially pure titanium subjected to tension and cyclic deformation in stress and strain control mode. Statistical analysis of micro-texture data collected from electron backscatter diffraction approximately from the same region as that of synchrotron X-ray has been used to correlate orientation dependent micro-strain and dislocation density with deformation microstructure and micro-texture. Two different orientations, namely, A with prismatic-pyramidal and B with basal orientation along the loading axis has been considered. Weak initial texture yet significant anisotropy in hardening/softening response and failure mode for monotonic tension and cyclic loading paths has been observed. Higher strain hardening response of orientation A during monotonic tensile deformation can be attributed to the evolution of lower micro-strain on basal orientation grains i.e, 〈0002〉ǁND along with extensive multi-variant twinning that also restricts crack propagation and delays failure in stress control mode. On the other hand, in strain control mode, orientation B shows higher fatigue life due to the generation of lower micro-strain in the basal orientation grains and single variant twinning that can undergo detwinning easily is responsible for delayed crack nucleation and subsequent failure.

Effects of carbon content on the tensile and fatigue properties in hydrogen-charged Fe-17Mn-xC steels: The opposing trends

This study investigated the effects of carbon content on the tensile and low-cycle fatigue (LCF) properties of hydrogen-charged high-Mn steels. Fe-17Mn-xC (x = 0.5, 0.7, and 0.9 wt%) steels were electrochemically hydrogen-charged for this purpose. Interestingly, the carbon content gave rise to opposite hydrogen embrittlement (HE)-resistance trends between monotonic and cyclic loadings; with increasing carbon content, tensile properties deteriorated, while LCF life showed a positive correlation. Such differences were interpreted in terms of primary deformation mechanisms under each loading condition. Increasing carbon contents accompanied an increasing number of thin mechanical twins as well as a higher density of sessile dislocations in monotonic tensile deformation. However, stacking fault energy, which is closely related to carbon content, changed the slip mode during cyclic deformation, thus resulting in the opposing trends found in this work.

A new design concept for prevention of hydrogen-induced mechanical degradation: viewpoints of metastability and high entropy

‟How crack growth is prevented” is key to improve both fatigue and monotonic fracture resistances under an influence of hydrogen. Specifically, the key points for the crack growth resistance are hydrogen diffusivity and local ductility. For instance, type 304 austenitic steels show high hydrogen embrittlement susceptibility because of the high hydrogen diffusivity of bcc (α´) martensite. In contrast, metastability in specific austenitic steels enables fcc (γ) to hcp (ε) martensitic transformation, which decreases hydrogen diffusivity and increases strength simultaneously. As a result, even if hydrogen-assisted cracking occurs during monotonic tensile deformation, the ε-martensite acts to arrest micro-damage evolution when the amount of ε-martensite is limited. Thus, the formation of ε-martensite can decrease hydrogen embrittlement susceptibility in austenitic steels. However, a considerable amount of ε-martensite is required when we attempt to have drastic improvements of work hardening capability and strength level with respect to transformation-induced plasticity effect. Since the hcp structure contains a less number of slip systems than fcc and bcc, the less stress accommodation capacity often causes brittle-like failure when the ε-martensite fraction is large. Therefore, ductility of ε-martensite is another key when we maximize the positive effect of ε-martensitic transformation. In fact, ε-martensite in a high entropy alloy was recently found to be extraordinary ductile. Consequently, the metastable high entropy alloys showed low fatigue crack growth rates in a hydrogen atmosphere compared with conventional metastable austenitic steels with α´-martensitic transformation. We here present effects of metastability to ε-phase and configurational entropy on hydrogen-induced mechanical degradation including monotonic tension properties and fatigue crack growth resistance.

Effect of film thickness on the stretchability and fatigue resistance of Cu films on polymer substrates

Abstract

Stability of Fatigued Dislocation Wall Structure in Coarse-Grained and Ultrafine-Grained Aluminum against Monotonic Tensile Deformation

Coarse-grained (CG) and ultrafine-grained (UFG) pure aluminum samples fabricated by equal channel angular pressing (ECAP) were cyclically deformed at 77K under constant plastic strain amplitude. Monotonic tensile tests were performed at 300 and 77K soon after the fatigue tests. In spite of the increase in the tensile strength of fatigued CG Al, tensile ductility decreased remarkably in comparison to that of as-annealed CG Al. On the other hand, the ultimate tensile strength (UTS) and tensile ductility of UFG Al were much higher at 77K than those at 300K. Furthermore, UFG Al at 77K maintained high UTS and high tensile ductility even after fatigue tests. Microstructural observation has revealed that dislocation wall structure formed in fatigued CG Al persists after the monotonic tensile tests. However, dislocation wall structure formed in fatigued UFG Al disappeared during early stages of monotonic tensile tests at both 300 and 77K. These results indicate that the dislocation wall structure in UFG Al is unstable against succeeding monotonic tensile deformation. [doi:10.2320/matertrans.M2012280]

Variations of Mechanical Parameters and Strain Energy Dissipated During Tension-Torsion Loading

Variations of Mechanical Parameters and Strain Energy Dissipated During Tension-Torsion LoadingThe paper presents behaviour of materials under complex loading being combinations of torsion-reverse-torsion cycles superimposed on monotonic tensile deformation. The 2024 aluminium alloy, P91 steel and M1E copper were investigated under plane stress state using thin-walled tubular specimens. All tests were strain controlled and a total strain was less than 1%. An influence of torsion cycles on tensile characteristic was manifested by lowering of the proportional limit and yield point. This effect was increased with magnification of cyclic strain amplitude and in the case of copper a reduction of yield point was equal around 90%. A character of this effect was checked using the yield surface concept after each test. The papers also presents, variations of tangential hardening modulus and plastic strain energy dissipation.

OS1306 破壊様式が異なるCu-10%Zn合金の破面近傍の微視的組織

The purpose of the present study is to show possibility of a new scheme which can clarify fracture process by means of microstructure observation near fracture surface. In the present study, strip specimens of Cu-10%Zn alloy were fractured under monotonic tensile deformation, fatigue, and corrosive conditions. Microstructure near fracture surface was investigated by electron channeling contrast imaging (ECCI) technique and by electron backscatter diffraction (EBSD) analysis. For the specimen fractured under tensile deformation, the microstructure imaged by the ECCI technique consisted of blurred patches, which would be caused by heavily distorted grains. In the ECC image of the fatigued specimen, dislocation wall structure - which was self-organized by cyclic plastic deformation - was observed. No specific structure was detected for the stress-corrosion cracking. Through the EBSD analysis, distortion of a grain near fracture surface was investigated quantitatively. For the tensile deformation test, the maximum deviation angle from average grain orientation was amounted to about 10°, while those for the fatigue and the stress-corrosion cracking were less than 2°. It is hence suggest that fracture mode could be identified by the ECCI and EBSD analyses of the microstructures developed near fracture surface.

On the Effects Associated with Control Parameters Delay during Biaxial Cyclic Loading of Engineering Materials

The paper presents experimental results of investigations carried out on the P91 steel and 2024 aluminium alloy under complex stress states due to various combinations of an axial force and twisting moment. An influence of out-of-phase sinusoidal and trapezoidal strain signals on the mechanical behaviour of tested materials was considered. The experiments enabled identification of the second order effects connected with the non-proportional cyclic loadings such as the phase shift between stress and strain signals during the deformation along the circular strain path, and a significant stress drop of the one of loading components applied in the case of deformation enforced by the trapezoidal signals. The experimental programme also contained the tests of monotonic tensile deformation realized simultaneously with delayed torsional cycles. They enabled to observe a drastic variations of the proportional limit and yield point of the materials in the tensile direction. This fact manifests an important material feature which can be applied to the optimal designing of some metal forming processes like an extrusion or forging for example.

Quantitative Evaluation of Strain Induced Martensite in STS 316L Stainless Steel

In the present work, the strain induced martensite in 316L stainless steel was quantitatively characterized by X-ray diffraction, the measurement of the magnetic coercivity and the AE technique during the monotonic tensile deformation of plate specimens. Plate specimens subjected to different heat treatments (i.e. having different initial microstructures) were tensile-deformed and the AE counts obtained during tensile deformation were correlated with the microstructural development. The AE count was observed to increase with increasing amount of strain induced martensite phase, as determined by X-ray diffraction analysis. The potential of the AE technique and the measurement of the magnetic coercivity to be used for the evaluation of the tensile deformation was discussed in relation to the existence of strain-induced martensite.

Deformation-induced phase development in a cobalt-based superalloy during monotonic and cyclic deformation

ULTIMET ® alloy is a metastable face-centered-cubic (FCC) material that undergoes a deformation-induced phase transformation to a hexagonal-close-packed (HCP) phase. In-situ neutron-diffraction measurements were made at room temperature to study the phase transformation during monotonic tensile deformation to a total strain of 16% (890 MPa) and also during strain-controlled cyclic deformation with a total strain range of Δ ε=2.5% up to 500 fatigue cycles. The HCP phase developed with increasing strain during the monotonic-loading experiment, while, during the fatigue deformation, the HCP phase continuously developed with each loading cycle. Transmission electron microscopy was also performed on monotonically loaded specimens in order to complement the neutron-diffraction results.

Reinforcement Stresses during Deformation of Sphere- and Particulate-Reinforced Al-Matrix Composites

Recent experimental results showed that reinforcement fracture is very often the dominant damage mechanism during ambient temperature deformation of discontinuously reinforced Al-matrix composites (DRAC). The reinforcements are broken by cracks perpendicular to the loading axis, and the fraction of broken reinforcements increases during plastic deformation as the load transferred from the matrix to the ceramic particulates increases. This process continues until a criteria fraction of broken reinforcements is reached, and then fracture takes place suddenly by a ductile mechanism involving localized necking of the intervoid matrix. The rate of reinforcement fracture depends on the strength of the ceramic reinforcements and on the stresses acting on them, and the modeling of the processes of damage accumulation in DRAC requires the knowledge of both. In particular, the stresses acting on the reinforcements during monotonic tensile deformation are a function of the applied strain, of the matrix and reinforcement properties, and of the volume fraction and shape of the ceramic reinforcements. A parametrical study is presented in this article of the influence of these factors on the stress acting on the reinforcements. The numerical results covered the whole range of matrix strengths which are typical in DRAC and were fitted to simple analyticalmore » expressions to facilitate their use by other researchers.« less

Tensile Deformation-Induced Microstructures in Freestanding Copper Thin Films

ABSTRACTExisting work in the mechanical behavior of thin films focuses mainly on measurement of macroscopic properties without strong correlation to microstructural features. We used transmission electron microscopy (TEM) to characterize the microstructures of free-standing copper thin films both before and after monotonic tensile deformation in an ex-situ thin film tensile testing system, as well as during in-situ loading in the TEM. The defect structures contributing to plastic deformation were investigated with an emphasis on comparison to mechanisms known to operate in bulk copper. The thin film exhibited much lower ductility (approximately 1%) than that normally observed in bulk form (greater than 40%). The predominant plastic deformation mechanisms did not include the typical dislocation activity that occurs in bulk copper, but rather greatly inhibited dislocation interactions typical of stages I and II hardening only. The absence of those structures normally found in tensile-deformed bulk copper is attributed to the differences in characteristic sizes of features within the microstructure available for deformation in bulk versus evaporated thin film material, that is, grain size and film thickness. The thin film ductility is an order of magnitude lower than what has been observed in bulk, ultrafine-grained copper, implying that a pure thin film effect on ductility exists and is significant. Microstructural features both near to and far from the fracture surface regions will be presented.

An experimental and numerical study of cyclic deformation in metal-matrix composites

The cyclic stress-strain characteristics of discontinuously reinforced metal-matrix composites are studied both experimentally and numerically. The model systems used for investigation are aluminum alloys reinforced with SiC particulates and whiskers. Finite element analyses of the fatigue deformation of the composite are performed within the context of axisymmetric unit cell formulations. Two constitutive relations are used to characterize the matrix of the composite: the fully dense Mises model of an isotropically hardening elastic-viscoplastic solid and the Gurson model of a progressively cavitating elastic-viscoplastic solid (to simulate ductile matrix failure by the nucleation and growth of voids). The brittle reinforcement phase is modeled as elastic, and the interface between the ductile matrix and the reinforcement is taken to be perfectly bonded. The analyses provide insights into the effects of reinforcement shape and concentration on (1) constrained matrix deformation under cyclic loading conditions, (2) cyclic hardening and saturation, (3) the onset and progression of plastic flow and cavitation within the matrix, and (4) cyclic ductility. The numerical predictions of flow strength, strain hardening, evolution of matrix field quantities, and ductility under cyclic loading conditions are compared with those predicted for monotonic tensile deformation and with experimental observations.

Fatigue deformation accompanying dynamic strain aging in a pearlitic eutectoid steel

The effect of dynamic strain aging on the cyclic stress response and the fatigue life in a pearlitic eutectoid steel was studied. The temperature ranges at which dynamic strain aging takes place during strain-controlled fatigue deformation and monotonic tensile deformation were also examined. Marked cyclic hardening due to dynamic strain aging took place at temperatures between 473 and 573 K, whereas cyclic softening occurred at room temperature. At 523 K and at a strain rate of 3.3 × 10 −3 s −1, the cyclic hardening was the most prominent and the fatigue life was shorter than that at room temperature by a factor of more than 2. At the given strain rate, dynamic strain aging during fatigue deformation occurred at lower temperatures in comparison with that during tensile deformation.

Dynamic strain aging during fatigue deformation in type 304 austenitic stainless steel

The test conditions (i.e. the test temperature and the strain rate) at which dynamic strain aging takes place during strain-controlled fatigue deformation in type 304 austenitic stainless steel were studied. When the strain rate was low (i.e. 5 × 10 −4 − 1.7 × 10 −3 s −1), the lower critical temperature for the appearance of serrations during fatigue deformation was much lower than that during monotonic tensile deformation. This result was considered to be caused by the longer arrest time of dislocations in fatigue deformation in comparison with that in monotonic tensile deformation. In such an arrest time the dislocations are locked by solute atoms. The longer arrest time in fatigue deformation is considered to be closely related to the cell-shuttling motion of dislocations in the saturated stress stage of the fatigue process.