Propagation Of Slip Bands Research Articles

Dual precipitate simultaneous enhancement of tensile and fatigue strength in (FeCoNi)86Al7Ti7 high-entropy alloy fabricated using selective laser melting

Recent studies have indicated that precipitation-strengthened high-entropy alloys (HEAs) show superior mechanical performance and have been successfully fabricated by additive manufacturing. However, the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs. This work explored a dual precipitation-strengthened (FeCoNi)86Al7Ti7 HEA with excellent tensile and fatigue strength, prepared through selective laser melting and heat treatment. Compared with the as-built samples, the tensile properties and fatigue endurance limit improved through aging by 48.7% and 30%, respectively. The strengthening mechanism and enhanced fatigue performance were clarified in detail. The improvement in fatigue strength was attributed to the improved resistance of the L12 and L21 precipitates. During deformation, the dislocation shear coherent L12 precipitates reduced slip band energy and inhibited slip band expansion, while the L21 particles acted as obstructions for further slip band propagation, severely limiting the rapid formation and propagation of crack growth. In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior, demonstrating that crack deflection due to L21 precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips. This work offers implications for a new strategy to develop additive manufacturing HEAs.

Time-Resolved Digital Image Correlation in the Scanning Electron Microscope for Analysis of Time-Dependent Mechanisms

Background: Advancements in the Digitial Image Correlation (DIC) technique over the past decade have greatly improved spatial resolution. However, many processes, such as plastic deformation, have a temporal component spanning from fractions of a second to minutes that has not yet been addressed in detail, particularly for DIC conducted in-situ in the scanning electron microscope (SEM). Objective: To develop a methodology for conducting time-resolved digital image correlation in the SEM for analysis of time-dependent mechanical deformation phenomena. Methods: Microscope and electron beam scanning parameters that influence the rate at which time-resolved DIC information is mapped are experimentally investigated, providing a guide for use over a range of timescales and resolutions. Results: Time-resolved DIC imaging is demonstrated on a Ti-7Al alloy, where slip band propagation is resolved with imaging dwell times of seconds. The limits of strain resolution and strain collection speeds are analyzed. Conclusions: The new developed methodology can be applied to a wide range of materials loaded in-situ to quantify time-dependent plastic deformation phenomena.

Micromechanical properties and deformation behavior of Al3BC/6061 Al composites via micropillar compression

Al3BC has been proved to be a promising candidate as reinforcement of Al alloy. In this work, Al3BC reinforced 6061 Al composites were in-situ fabricated through a liquid-solid reaction method followed by hot extrusion, and the micromechanical properties as well as deformation behavior of the composites were investigated via micropillar compression. The Al3BC/6061 composites show a significant improvement of compression strength than the matrix alloy, indicating an outstanding strengthening effect of Al3BC. Compared with the large parallel local slip bands on the post-compressed pillars of unreinforced matrix materials, the wrinkled slip bands were observed and homogenously distribute on the post-compressed pillars of the composites, indicating an impeding effect on the propagation of slip bands and a better slip homogenization caused by Al3BC particles. The strengthening mechanisms were also discussed and microstructural characterizations reveal that the high strength of the composites is mainly attributed to its high dislocation density, grains refinement and load transfer effects caused by Al3BC.

Additive manufacturing of high-strength CrMnFeCoNi high-entropy alloys-based composites with WC addition

Laser melting deposition with WC addition has been developed to fabricate high-strength CrMnFeCoNi-based high-entropy alloys-based composites. By this technique, a microstructure of compact refined equiaxed grains can be achieved, and the tensile strength can be remarkably improved. The sample with 5 wt% WC addition shows a promising mechanical performance with a tensile strength of 800 MPa and an elongation of 37%. The improvement in mechanical property may be attributed to the formation of Cr23C6 reinforcement precipitates, which could promote the heterogeneous nucleation of grains and hinder the propagation of slip bands.

Continuous Electron Beam Post-Treatment of EBF3-Fabricated Ti–6Al–4V Parts

In the present study, the methods of optical, scanning electron, and transmission electron microscopy as well as X-ray diffraction analysis gained insights into the mechanisms of surface finish and microstructure formation of Ti–6Al–4V parts during an EBF3-process. It was found that the slip band propagation within the outermost surface layer provided dissipation of the stored strain energy associated with martensitic transformations. The latter caused the lath fragmentation as well as precipitation of nanosized β grains and an orthorhombic martensite α″ phase at the secondary α lath boundaries of as-built Ti–6Al–4V parts. The effect of continuous electron beam post-treatment on the surface finish, microstructure, and mechanical properties of EBF3-fabricated Ti–6Al–4V parts was revealed. The brittle outermost surface layer of the EBF3-fabricated samples was melted upon the treatment, resulting in the formation of equiaxial prior β grains of 20 to 30 μm in size with the fragmented acicular α′ phase. Electron-beam irradiation induced transformations within the 70 μm thick molten surface layer and 500 μm thick heat affected zone significantly increased the Vickers microhardness and tensile strength of the EBF3-fabricated Ti–6Al–4V samples.

Additive manufacturing of high-strength CrMnFeCoNi-based High Entropy Alloys with TiC addition

High-strength CrMnFeCoNi-based High Entropy Alloys have been developed by laser melting deposition, in which tensile properties of them could be adjusted by controlling the quantity of the TiC addition. The CrMnFeCoNi high entropy alloy with 5 wt% TiC addition has exhibited 723 MPa tensile strength and 32% tensile strain. The improvement in mechanical property is attributed to the introduction of micron TiC reinforced phase, which could promote hinder the propagation of slip bands.

Strips of localization of plastic deformation

Model of slip band propagation in materials with yielding plateau is introduced. Development of a slip band is modeled in a form of loss of stability during transition of the material from an elastic state to hardening. This transition is the generalization of crack mode I development model by Novozhilov (J Appl Math Mech 3:201–210, 1969) in elastic solids. Possibility for slip bands of limited length is shown in the model in contrast to ideal plasticity model that only leads to infinite slip bands. Problems of localization in a form of slip bands in a state for the pure shear and for intermaterial layer are considered. For different external loads and various mechanical properties of the interlayer, the lengths of the localization zone of plastic deformations, the graphs of the tangential displacement jump in this zone, and the shear stress on their continuation are found.

ECCI based characterization of dislocation shear in polycrystalline arrays during heterogeneous deformation of commercially pure titanium

A new electron channeling contrast imaging (ECCI) based approach for the characterization of heterogeneous plastic deformation across multiple grains in a polycrystal is presented. The nature of the shear accommodation at grain boundaries has been assessed by evaluating the spatial distribution of dislocations and slip system activities near the boundaries and relating these observations to standard slip transfer criteria and the global state of stress. The direction of slip band propagation, and consequently the location of dislocation slip band nucleation, was determined by characterizing the slip band widths across individual grains. This assessment leads to a better understanding of the factors involved in polycrystalline deformation, including the grain boundary shear accommodation associated with both dislocation pile-ups and grain boundary dislocation nucleation. Because ECCI allows study of dislocation structures over large surface areas, and thus larger overall volumes than typically are accessible by other techniques, the overall approach allows the dislocation distribution across individual grains to be understood in terms of the broader polycrystalline constraints.

Ultrasonic fatigue testing in the scanning electron microscope

Abstract The continuing trend to increase the performance and durability of machines requires the use of materials, whose structural integrity must be ensured far beyond the classical fatigue limit. In this so-called „very high cycle fatigue“ regime (VHCF), the materials show a strong scatter in fatigue life, which makes life service assessment difficult. Present studies on austenitic-ferritic duplex steel 1.4462 (X2CrNiMoN22-5-3) have shown that the reason of this scatter in fatigue life can be found in the microstructural length scale mainly due to the barrier effect of grain and phase boundaries. The integration of an ultrasonic fatigue testing system in a high-resolution scanning electron microscope allows observing and evaluating the active microstructural mechanisms of fatigue crack initiation and early crack propagation, such as the formation and propagation of slip bands, during the majority of VHCF life. Additional micro texture measurements by electron backscatter diffraction (EBSD) provide data for the development of a mechanism-oriented numerical short crack simulation. In the context of crack initiation and propagation in the VHCF regime in duplex stainless steel, the focus of the present paper is put on the technical realization of the in-situ ultrasonic fatigue testing in the scanning electron microscope.

In situ micro tensile testing of He+2 ion irradiated and implanted single crystal nickel film

Abstract The effect of ion irradiation on the tensile properties of pure Ni single crystals was investigated using an in situ micro-mechanical testing device inside a scanning electron microscope. A 12.8 μm-thick Ni film with {0 0 1} plane normal was irradiated with 6 MeV He +2 ions to peak damage of 10 and 19 displacements per atom (dpa). Micro-tensile samples were fabricated from the specimens parallel to the plane of the film using a focused ion beam (FIB) instrument, and tested in tension along [1 0 0] direction, up to fracture. The peak strength increased from ∼230 MPa for the unirradiated material to about 370 MPa and 500 MPa for the 10 dpa and 19 dpa samples respectively, while the ductility decreased with increasing dose. The surface near the peak damage regions fractured in a brittle manner, while the regions with smaller dose underwent significant plastic deformation. Slip bands extended to the peak-damage zone in the sample with a dose of 19 dpa, but did not propagate further. Transmission electron microscopy confirmed the stopping of the slip bands at the peak-damage region, just before the high He concentration region with voids or bubbles. By removing the peak damage region and the He bubble region with FIB, it was possible to attain propagation of slip bands through the entire remaining thickness of the sample. This material removal also made it possible to calculate the irradiation hardening in the region with peak hardness – thus enabling the separation of hardening effects in the high and low damage regions.

Ultrafine grained Ti-based composites with ultrahigh strength and ductility achieved by equiaxing microstructure

Abstract Ultrafine grained Ti-based composites with equiaxed microstructure were fabricated by sintering and crystallizing from glassy powder precursors. The optimized composites exhibit ultimate strength of 2585 MPa and plasticity of exceeding 30%, occupying an unprecedented regime in the strength and ductility space of advanced titanium alloys. Dislocation pile-ups induced by precipitated phases and slip band propagation in the β-Ti matrix are the main reasons for the ultrahigh strength and plasticity respectively. The results provide an innovative route for fabricating titanium alloys with higher mechanical properties.

Effect of orientations on in situ tensile deformation and fracture behaviours of nickel-base single-crystal superalloys

After systematically investigating in situ tensile deformation and fracture behaviours of nickel-base single-crystal superalloys with four different orientations, their slip and fracture modes in relation to the orientations are obtained and shown as follows. In [0 0 1] oriented specimen, four slip systems were activated and the crack initiated along the interactive location between the third slip bands (SBs) and deformation bands in priority. The crack propagation involves all three crack opening types of I, II and III. [0 1 1] oriented specimens exhibited obvious low strain hardening rate and high ductility, which can be attributed to the balance between the softening caused by the propagation of SBs and the latent hardening caused by the propagation of deformation bands. Except for case, the critical resolved shear stress (CRSS) of the other oriented superalloys at room temperature is approximate, which is due to non-Schmid behaviour in superalloys. Lall–Chin–Pope model is used to explain the orientation dependence of CRSS. Furthermore, the fracture modes of different oriented specimens depend on the intensity of intrusion and extrusion and the degree of match between normal stress and shear stress. The shear stress is beneficial to the crack initiation and the normal stress contributes to the crack propagation.

Nano-network mediated high strength and large plasticity in an Al-based alloy

A novel Al-based Al–La–Ni–Co–Y composites consisting of the soft α-Al grains surrounded by a rigid frame of nanosized Al3Y/Al3Ni intermetallic phases was developed, and such nano-network structured composite not only showed a high fracture strength of ∼1.1GPa but also a large plasticity of ∼11%. It was found that the nanosized rigid frame can arrest and suppress propagation of slip bands. In addition, after the severe plastic deformation, splitting of the α-Al grains occurred as a result of the large applied stress, leading to the formation of subgrains. Consequently, plastic deformation of the α-Al grains is confined, giving rise to the unique macroscopic properties.

Molecular dynamics simulations of quasi-brittle crack development in iron

The paper presents the results of molecular dynamics (MD) simulations of three-dimensional kinetics of micro-crack propagation in alpha-iron and the accompanying lattice transformations at the crack tips. We show that crack initiation on {0 0 1} planes in iron is preceded with the emission of compact slip bands from the pre-crack tips, in agreement with the predictions of the earlier quasi-two-dimensional simulations. The application of Voronoi decomposition technique for atomic short-range order processing has allowed us to clarify the kinetics of structural transformations at the tips of nucleating and propagating cracks for three most common systems of crack propagation in iron. It is demonstrated that the compact slip bands emanating from the crack tips not only accompany crack nucleation, but remain an essential feature of the crack propagation on {0 0 1} planes. Due to the strong coupling between the crack tip and slip band propagation, the crack propagation can be limited by slip band interaction with microstructural obstacles, abundantly created in ferritic–martensitic steels in radiation environment of nuclear facilities.

Dislocation sources and slip band nucleation from indents on silicon wafers

The nucleation of dislocations at controlled indents in silicon during rapid thermal annealing has been studied byin situX-ray diffraction imaging (topography). Concentric loops extending over pairs of inclined {111} planes were formed, the velocities of the inclined and parallel segments being almost equal. Following loss of the screw segment from the wafer, the velocity of the inclined segments almost doubled, owing to removal of the line tension of the screw segments. The loops acted as obstacles to slip band propagation.

Grain Boundaries and Propagation of Slip Bands

Deformation experiments were performed on three different Fe-4at%Si bicrystals. Synchrotron radiation topography in situ and post mortem was used to investigate the influence of grain boundaries (GB) on slip transmission and to study the interaction of slip dislocations with the GB. It was found that the GB forms a strong barrier in all cases and the slip transfer cannot be fully explained by geometric relations of Burgers vectors and slip planes in the two grains. To explain the results of experiments the internal stresses near the GB due to accumulated slip dislocations produced in one grain and stopped at the GB and image forces due to the grain boundary were calculated. The structure of GB has been studied by conventional TEM.

Slip band propagation and slip vector transition in B2 FeAl single crystals

At the peak temperature, 823K, of the yield strength anomaly of B2 FeAl, slip band propagation and slip vector transition were investigated using Fe–39mol% Al single crystals. The single crystal oriented along a compression axis close to the [123] direction showed serrated flow during work-hardening at small strains (<2%). Coarse slip bands propagate in the single crystal specimens, like Lüders band propagation in polycrystals, in the initial strain range. The slip vector was identified by TEM to be parallel to 〈111〉 in the early stage of strain corresponding to yielding. Beyond plastic strains of about 3%, serrations disappeared and significant work-softening occurred. The slip vector responsible for the later stage of deformation was observed to be 〈100〉. That is, the slip vector changes from 〈111〉 to 〈100〉 as the plastic strain increases. Because the density of 〈111〉 superdislocations is found to be very low after the slip transition, glide decomposition of 〈111〉 superdislocations is believed to the primary source mechanism for 〈100〉 dislocations. The slip band propagation in B2 single crystals is discussed in comparison with that of other intermetallic single-crystalline materials.

Interaction of microbands with grain boundaries in mild steel

The interaction of microbands with grain boundaries in mild steel have been studied by TEM. Microbands were observed to propagate across the interface or stop at the grain boundary. In both cases the microbands correspond to a (110) slip system and no non-crystallographic propagation was observed. Microbands and slip bands behave similarly in front of an interface, and it is therefore suggested that the interaction of microbands with grain boundaries depends on the same crystallographic orientation conditions as those proposed for slip band propagation.

Slip-band propagation at the tip of an edge crack during plastic deformation

Experimental investigations indicate that in many cases under plane deformation, the initial plastic strains in the vicinity of a separation crack are localized along two narrow bands at an angle of approximately 45° to the line of the crack [9]. Methods of solving elastoplastic problems in which these plasticity bands (slip bands) are modeled by fracture lines of tangential displacements have been developed in fracture mechanics; in that case, tangential stresses equal to the yield point in shear τy are assigned to these lines [2]. The problem of the development of slip bands then reduces to solution of the plane problem of the theory of elasticity for a branched notch; in that case, the dimensions and orientation of lateral branching corresponding to plasticity bands are determined during the solution. A series of results [1, 5, 6, 8, 10–16] for an infinite plane with a separation crack, where the slip bands are located symmetrical bout the line of the crack, have been obtained in this segment. In our study, we solve the plane elastoplastic problem (plane deformation) for a half space with an arbitrary oriented edge crack under an arbitrary load. Numerical results are obtained for a constant pressure on the crack or tensile forces at infinity.

カドミウム単結晶の{11\bar22}&amp;lang;\bar1\bar123&amp;rang; 2次錐面すべりによる降伏応力の異常温度依存性の支配機構

The mechanism of anomalous temperature dependence of yield stress by {11\bar22}〈\bar1\bar123〉 second order pyramidal slip in Cd single crystals in proposed. (c+a) edge dislocations are immobilized as a result of thermally activated dissociation, i.e., (c+a)→>(c sessile dislocation)+(a basal dislocation). Increase in frequency of the immobilization with increasing temperature decreases the mean moving distance of (c+a) edge dislocations and consequently the amount of strain in slip bands. In order to maintain the constant strain rate at higher temperature, the velocity of slip band propagation by double cross slip of (c+a) screw dislocation to the next near slip plane must be increased by higher stress. Therefore, the yield stress increases with increasing temperature. The mean moving distance of (c+a) edge dislocation is nearly constant when the strain rate is increased. Then, the higher stress for increasing the slip band propagation velocity is needed to compensate for the increased strain rate. The yield stress, therefore, increases with increasing strain rate.The negative temperature dependence of the yield stress below 133 K is considered to be caused by the deformation controlled by the Peierls mechanism.