Development Of Slip Bands Research Articles

Micro-mechanical investigation of maraging steel during in-situ tensile test

ABSTRACT The research aimed to examine the development of slip bands in 12Cr-9Ni maraging stainless steel through in-situ tensile testing in scanning electron microscopy (SEM) and atomic force microscopy (AFM). A comprehensive approach was employed, which included assessing strain distribution and the grain boundary character distribution (GBCD) at the intermediate plastic strain steps. During plastic deformation, dislocation pile-up leads to the formation of slip bands in the material. It was discovered that slip band formation, as well as slip line spacing, exhibit crystallographic orientation dependence. Finally, fractography was employed to unveil the likely fracture mechanism and further link it to the formation of slip bands at higher strain levels.

Influence of microstructure evolution on hot ductility behavior of austenitic Fe–Mn–Al–C lightweight steels during hot tensile deformation

Four alloys based on the Fe–30Mn–(8.5–12)Al–(1.0–1.3)C (wt%) system were prepared to investigate the effects of microstructure evolution and κ-carbide precipitation behavior on hot ductility behavior of austenitic lightweight steels. Hot tension tests were carried out at temperatures of 500–1230 °C using a Gleeble simulator. At high temperatures above 1000 °C, dynamic recrystallization occurred in all alloys, leading to high tensile ductility. At temperatures of 700–900 °C, the ductility decreased in all alloys due to the intragranular precipitation of κ-carbide, with increases in the amounts of Al and C contents then leading to a greater loss of ductility due to the formation of coarse intergranular κ-carbides. The addition of Cr and Mo suppressed the precipitation of κ-carbide, reducing the extent of ductility loss. At 500 °C, the ductility was recovered due to a reduction of inter-/intragranular κ-carbide precipitation and the development of slip bands caused by planar gliding of dislocations through κ-carbide shearing. The spacing among slip bands then became coarse with an increase in the Al and C contents, resulting from the coarsening of κ-carbide. Meanwhile, dynamic strain aging (DSA) behavior was observed in all alloys deformed at 500 °C. This occurred because the hot tensile tests were carried out under a high strain rate condition; therefore, the mobility of the dislocations was fast and thus solute atoms pinned the dislocations despite deformation at a high temperature. With a coarsening of κ-carbide, the extent of serration was reduced, resulted from the fact that the content of solute C decreased due to the greater precipitation of κ-carbide; i.e., the amounts of solute C atoms to cause the DSA behavior were reduced.

Anisotropic tensile behavior of laser powder-bed fusion made Ti5553 parts

Directional heating during metal additive manufacturing promotes the growth of a columnar-grained microstructure along the building direction with a preferred crystal orientation, which results in anisotropy of properties. In this work, the anisotropy of laser powder bed fused Ti-5553 metastable β titanium alloy components are investigated. Samples printed in the normal (vertically printed), and parallel (horizontally printed) orientations to the building direction were subjected to quasi-static uniaxial tensile tests. In comparison, samples printed parallel to the building direction exhibit a significantly higher ultimate strength of 846 ± 6 MPa, whereas the samples printed normal to the building direction reached 780 ± 10 MPa. Fracture initiation and propagation were investigated using interrupted tensile testing. The fracture path of partially fractured specimens reveals the development of slip bands oriented at a 45 ± 5° angle with respect to the loading direction, which promoted fracture by coalescence of aligned pores. Electron backscatter diffraction results indicate that the grain boundaries act as favorable nucleation sites for fracture initiation, particularly when they align perpendicular to the loading direction. Conversely, fracture in samples printed parallel to the building direction was predominantly transgranular, which is expected to be a major contributing factor to the higher strength observed.

Acoustic Birefringence Changes in the Base Metal and Heat Affected Zone of ASTM 1020 Steel under Plastic Deformation and Fatigue

This paper deals with metallographic studies and ultrasonic investigations of acoustic birefringence in the base metal and heat affected zone of welded specimens of ASTM 1020 steel under uniaxial tension and low-cycle fatigue. We examined microstructural changes on the surface using optical microscope. It was found that development of slip bands differs in the base metal and heat affected zone. The ultrasonic pulse echo method was used for the determination of acoustic birefringence and velocities of ultrasonic shear waves. The differences in the change in velocities in the base metal and in heat affected zone were revealed. Acoustic birefringence was found to change monotonously in the base metal under fatigue as well as in the base metal and heat affected zone under uniaxial tension. For heat affected zone, during fatigue, acoustic birefringence was found to change non-monotonously up to cycle ratio of 0.3 and decrease monotonously after cycle ratio of 0.3. Based on the relationship between relative number of loading cycles, or plasticity resource, and acoustic birefringence, ultrasonic techniques were proposed for assessing damage.

Metallographic and ultrasonic studies of fatigue failure of ASTM 1020 steel in the base metal and heat affected zone

This paper deals with optical and ultrasonic investigations in the base metal and heat affected zone of welded specimens of ASTM 1020 steel under low-cycle fatigue. Development of persistent slip bands on the surface was observed using optical microscope. Differences in development of slip bands in the base metal and heat affected zone were analyzed. Acoustic birefringence of shear waves was measured by the ultrasonic pulse echo method. For the base metal, acoustic birefringence was found to decrease monotonously. For heat affected zone, acoustic birefringence was found to change non-monotonously up to cycle ratio of 0.3 and decrease monotonously after cycle ratio of 0.3. Based on the linear dependence of acoustic birefringence on the relative number of loading cycles, an ultrasonic technique is proposed for assessing fatigue damage.

Correlation between microstructure evolution and mechanical response in a moderately low stacking-fault-energy austenitic Fe–Mn–Si–Al alloy during low-cycle fatigue deformation

Through in-depth microstructural and mechanical characterization, a mechanism for low-cycle fatigue (LCF) deformation of an Fe-30.7Mn-4.3Si-1.8Al (in wt%) austenitic transformation-induced-plasticity (TRIP) steel at a total strain range of 2% was investigated. The LCF deformation of the steel was performed through two main deformation modes which were planar slip of Shockley partials, and ε-martensitic transformation and its reverse transformation. The varying significance of each deformation mode as well as the relevance between the two modes determined the microstructures evolved during fatigue deformation and resulted in a four-stage cyclic hardening behavior. With increasing fatigue cycles, the LCF deformation was dominated sequentially by the multiplication of partial dislocations, development of slip bands, reversible ε-martensitic transformation, and formation of more thick and crossed ε-martensite along with enhancement of planar dislocation slip, thus generating the characteristic deformation stages of primary cyclic hardening, cyclic softening, cyclic saturation, and secondary cyclic hardening correspondingly. In general, the superior LCF resistance of the steel was attributed to appreciably high plasticity reversibility during the entire fatigue deformation which was caused by reversible planar dislocation slip and ε-martensitic transformation.

A multiscale study on the morphology and evolution of slip bands in a nickel-based superalloy during low cycle fatigue

Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ′ precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700 ∘C for 1, 40 and 500 cycles. The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ′-etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.

Interpretation of the size effects in micropillar compression by a strain gradient crystal plasticity theory

This work presents a study of the behavior of micropillars compressed with a flat punch, considering a continuum model based on a higher-order strain gradient crystal plasticity theory. Hardening models are associated with the density of geometrically necessary dislocations. Size effects observed in the present calculations are governed by the development of slip-bands in the micropillar and the geometrically necessary dislocations that build up around them. Two critical factors are identified as defining the slip-band behavior. One is the aspect ratio considered for the micropillar. Shorter micropillars introduce geometrical restrictions to the free path for dislocations in the slip-band, which may result in the development of a substantial amount of geometrically necessary dislocations and size dependent hardening (strain hardening and strengthening). In longer micropillars the effect is not observed and then the proposed model is incapable of capturing size effects in these cases. The other factor is the hardening model used. Two possible models are considered: a dissipative model directly related to slip rate gradients and an energetic model derived from a defect energy. Qualitative comparisons with experiments indicate that dissipative hardening is a more adequate representation of micropillar compression than the energetic model used. Finally, it is shown that a large friction between punch and crystal may inhibit strengthening converting it into strain hardening. Low friction coefficients also tend to promote the concentration of plastic strains in one slip system.

A Multiscale Study on the Morphology and Evolution of Slip Bands in a Nickel-Based Superalloy During Low Cycle Fatigue

Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ′ precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700˚C for 1, 40 and 500 cycles. The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ′-etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.

Measurements of plastic localization by heaviside-digital image correlation

In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.

Dislocation model of nucleation and development of slip bands and their effect on service life of structural materials subject to cyclic loading

Most of the destructions of machine parts are of fatigue character. Under cyclic loading, the surface layer, in which hardening–softening processes rapidly occur, is formed almost at once after its beginning. The interaction of plastic-deformation traces with each other and with other structural elements, such as grains, results in the formation of a characteristic microstructure of the machine-part surface subject to cyclic loadings. The character of accumulation of slip bands and their shape (narrow, wide, twisting, and broken) depends on the conditions under which (under what factors) the cyclic loading occurs. The fatigue-resistance index expressed in terms of the slope of left portion of the fatigue curve linearized in logarithmic coordinates also depends on the set of relevant factors. The dependence of the surface damageability on the fatigue resistance index makes it possible to implement the method of predicting the fatigue curve by the description of the factors acting on a detail or construction. The position of the inflection point on the curve in the highcycle fatigue region (the endurance limit and the number of loading cycles, the ordinate and abscissa of the inflection point on the fatigue curve, respectively) also depends on the set of relevant factors. In combination with the previously obtained value of the slope of the left portion of the curve in the high-cycle fatigue region, this makes it possible to construct an a priori fatigue curve, thus reducing the scope of required fatigue tests and, hence, high expenses because of their long duration and high cost. The scope of tests upon using the developed method of prediction may be reduced to a minimum of one or two samples at the predicted level of the endurance limit.

Experimental investigation and numerical description of the damage evolution in a duplex stainless steel subjected to VHCF-loading

The present study documents how the irreversible fraction of cyclic plastic strain, induced by loading amplitudes close to the durability limit, causes fatigue damage such as (i) slip band development, (ii) fatigue crack initiation and (iii) short fatigue crack propagation. The damage evolution of the austenitic–ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) was investigated up to one billion load cycles by means of high resolution electron microscopy (HR-SEM, TEM), focused ion beam (FIB) cutting, confocal laser scanning microscopy (CLSM), in-situ far field microscopy and high-energy (87.1keV) X-ray diffraction (XRD) experiments. The experimentally identified damage mechanisms were implemented into three-dimensional finite element simulations, which consider crystal plasticity. These simulations enable fatigue life predictions of real microstructures obtained for instance by means of, e.g. automated electron back scatter diffraction (EBSD) analysis. The simulations allow for determining whether microcracks (i) initiate in a microstructure, (ii) arrest in the midst of the first grain, (iii) are permanently, (iv) temporary or (v) not at all blocked by grain or phase boundaries. Moreover, this concept is capable to contribute to the concept of tailored microstructures for improved cyclic-loading behavior.

The kinematics of deformation and the development of substructure in the particle deformation zone

The details of the deformation near undeformable particles influence the recrystallization behaviour of commercial aluminium alloys. It has been shown that the size of the particle deformation zone (PDZ) and the local lattice rotation decrease with particle size and that a uniform distribution of very small closely spaced particles makes the deformation more homogeneous. Although a number of models have been proposed to describe the development of the PDZ, an explanation for this particle size effect remains elusive. In this paper, high resolution digital image correlation was used to map the deformation around particles of different sizes in a model Al-Si alloy. Deformation maps were compared to maps of local lattice rotation obtained using electron back scattered diffraction (EBSD) to elucidate the link between the kinematics and the resultant substructure. These results were also compared to crystal plasticity finite element predictions. The deformation maps revealed that deformation is concentrated in intense slip bands that have a characteristic spacing and which are strongly correlated to the deformation substructure. Whereas particles larger than this characteristic spacing interact strongly with the slip bands, causing large local rotations, smaller particles either interact more weakly or not at all. Since the strain between the bands is only a fraction of that in the bands, the rotations associated with the smaller particles are invariably smaller. Very small, closely spaced particles change the slip band pattern, decreasing the band spacing and decreasing the amount of shear within the bands. CFEM modelling was able to reproduce general features of the particle deformation zone but because it does not predict the development of slip bands and their characteristic spacing it fails to predict an effect of size on the local rotation.

Low-Cycle Fatigue Behavior and Microstructural Evolution of the Fe–30Mn–4Si–2Al Alloy

Superior fatigue life of 8000 cycles at low-cycle fatigue with a total strain Δε=2% was found in the Fe–30Mn–4Si–2Al high-Mn alloy, as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys with fatigue life of 2×103 cycles. Examination of microstructural evolution and cyclic hardening/softening behavior was shown that high fatigue resistance of Fe–30Mn–4Si–2Al alloy associated with delayed development of the deformation induced martensite and inhibited dislocation slip as compared to Fe–30Mn–6Si–0Al and Fe–30Mn–3Si–3Al alloys, respectively. Cyclic strain softening followed by secondary strain hardening was observed in the Fe–30Mn–4Si–2Al alloy after primary hardening. Primary hardening to about 40 cycles was associated with continuous increase in density of planar dislocations and the development of slip bands. The cyclic softening manifesting as the drop of the stress amplitude in the range of the cycles from 40 to 400 was accompanied by development of deformation induced ε-martensite in place of the slip bands. At the N>400 cycles further increase in the volume fraction of deformation ε-martensite leads to continuous hardening up to the failure. In the presentation we will discuss the details of microstructural evolution during LCF of the Fe–30Mn–4Si–2Al alloy.

Investigation on In Situ Tensile Behavior of Superalloy Bicrystals with Different GB Misorientations

The in situ tensile behavior of nickel-base superalloy bicrystals with different grain boundary (GB) misorientations of 2 deg, 10 deg, and 16 deg was preliminarily studied. Among three kinds of bicrystals, the bicrystal with 2 deg misorientation was characterized with continuous slow strain hardening as well as crack initiation and propagation along the SB-matrix interface due to the full development of slip bands (SBs). In contrast, GBs in 10 deg and 16 deg bicrystals could effectively impede the SBs. Thus, the crack initiation occurred along the carbide/matrix interface, and more specifically cracks in 16 deg bicrystal fully propagated along the GB. Irregular GB in superalloy bicrystals consists of three types of GB segments. Among them, parallel GB is distinctly beneficial to GB strengthening and improving the yield strength of bicrystal. The statistical results showed that the proportion of parallel GB in the 10 deg bicrystal is the highest. Meanwhile, the GB in 10 deg bicrystal possesses higher resistance to crack extending, which can be attributed to its highest carbide fraction at the GB. Thus, its fracture exhibited a mixed mode of both GB and SB cracking.

Investigation of crack initiation and propagation behavior of AISI 310 stainless steel up to very high cycle fatigue

Fatigue behavior up to very high cycles for AISI 310 stainless steel has been investigated. The fatigue crack initiated from the surface of the material. It was found that up to 106 cycles, cracks initiated from the carbide precipitates at grain boundaries. However, above 106 cycles, the cracks initiated from persistent slip bands found at the surface of the specimen. At lower stress levels, slip bands were developed without initiating the cracks. The horizontal asymptote S–N curve from 106 to 109 cycles was attributed to the development of slip bands all over the surface of the specimen, before crack initiation.

Quantitative study of surface roughness evolution during low-cycle fatigue of 316L stainless steel using Scanning Whitelight Interferometric (SWLI) Microscopy

This paper describes the preliminary study on the evolution of the surface roughness in polycrystalline 316L austenitic stainless steel during low-cycle fatigue using an optical interferometric surface profiling technique. The arithmetic mean surface roughness of four regions with different stress levels was measured at incremental fatigue cycles until the fracture of the specimen. We discovered that there are two different mechanisms contributing to the surface roughness increases in the fatigued specimen. At the initial fatigue stages, the surface roughness increases were dominated by the development of the slip bands. However, the heights of the slip bands stopped growing at certain fatigue cycles. At later fatigue cycles, after the slip band height is saturated, the surface roughness increases are mainly contributed by the out-of-plane grain displacement. Both the slip band development and out-of-plane displacements are correlated to the local stress levels. Local permanent plastic strains were measured from the micrographic images. Analysis on the plastic strains along the loading direction and the transverse direction indicated that the out-of-plane grain displacement is likely to be contributed by the strain differences among individual strains.

Numerical simulations of void growth near a notch tip in ductile single crystals

The objective of this work is to study the growth of a cylindrical void ahead of a notch tip in ductile FCC single crystals under mode I, plane strain, small scale yielding (SSY) conditions. To this end, finite element simulations are performed within crystal plasticity framework neglecting elastic anisotropy. Attention is focussed on the effects of crystal hardening, ratio of void diameter to spacing from the notch and crystal orientation on plastic flow localization in the ligament connecting the notch and the void as well as their growth. The results show strong interaction between shear bands emanating from the notch and angular sectors of single slip forming around the void leading to intense plastic strain development in the ligament. Further, the ductile fracture processes are retarded by increase in hardening of the single crystal and decrease in ratio of void diameter to spacing from the notch. Also, a strong influence of crystal orientation on near-tip void growth and plastic slip band development is observed. Finally, the synergistic, cooperative growth of multiple voids ahead of the notch tip is examined.

Deformation and Damage Evolution of the Microstructure around Ti-Particles in IF Steel during Tensile Deformation

The microstructure evolution in the deformation zone around second phase particles of IF steel sheets subjected to tensile deformation has been investigated in order to correlate it to the damage at microstructural scale. The experimental set-up consisted of a series of interrupted tensile tests which were carried out at different tensile deformations up to fracture. The microstructure of the deformed samples was investigated by EBSD analysis in which the EBSD scans were focused on the areas containing Ti (C, N) particles of cubical shape. It was found that at tensile strains below 25%, the ferrite matrix exhibited the evolution of slip bands inside specific grains depending on their crystallographic orientation although no special strain localization around the particles was observed. After 35% of tensile strain, the strain concentrates around the particles and particle-matrix decohesion was observed. The lattice rotations of the matrix surrounding the particles as well as the selective deformation of the grains are analyzed and discussed.

Effect of notch orientation on the evolution of plasticity in superalloy single crystals

The effect of notch orientation on the evolution of slip bands in double-notched tensile samples of a single crystal superalloy loaded in a 〈0 0 1〉 orientation was experimentally investigated and compared to the results of a three-dimensional linear elastic anisotropic finite element analysis (FEA). The analysis was conducted for two orientations with notches parallel to a 〈0 1 0〉 and a 〈1 1 0〉 orientation, respectively. The observed slip traces agreed well with the predicted dominant slip systems based on the FEA. The experimental results suggest that the dominant slip planes activated at low load levels persist at higher load levels and the activation of other slip bands within a domain is initially inhibited.