Dislocation Cell Structure Research Articles

Microstructure evolution of 24CrNiMoY alloy steel parts by high power selective laser melting

24CrNiMoY alloy steel used for brake disc was prepared by high power selective laser melting (HP-SLM) in order to increase the productivity. In this work, the microstructure and mechanical properties of as-fabricated part were characterized. Effects of the location of as-fabricated 24CrNiMoY part (top, middle and bottom) and scanning speed on microstructure evolution were studied. The results of this work indicated that the as-fabricated part microstructure mainly consisted of granular bainite and lath bainite. Demonstrating that bainite was obtained by a large temperature gradient and sufficient holding time by experiment. With increasing scanning speed, the size dB of the bainitic ferrite lath beam spacing and the size ∅B of the mixed structure of polygonal ferrite and quasi-polygonal ferrite were gradually refined and larger, respectively. The HP-SLM-produced parts were dominated by high-angle (>15°) grain boundaries (HAGBs) and the contents of HAGBs decreased at first while increased (62.3%, 63.5%, 50.1%) when scanning speed varied from 5 to 9mm/s. Supersaturated solid solution, dislocation, and subgrain cell structure, which results in a high microhardness and fine strength.

Assessment of the impact of hydrogen on the stress developed ahead of a fatigue crack

The microstructure generated in a low carbon steel under cyclic loading in air and a 40 MPa gaseous hydrogen environment has been compared as a function of distance from the crack tip. The presence of hydrogen resulted in the formation of a smaller and more equiaxed dislocation cell structure that extended further from the crack tip than the one generated in air. This enhancement and extension of the dislocation structure by hydrogen is consistent with it modifying the generation rate and mobility of dislocations as well as dislocation interactions. Qualitative assessment of the dislocation structure ahead of the crack tip found the stress ahead of the crack tip to vary linearly as ln(1/x), where x is the distance from the crack tip irrespective of the test environment. Hydrogen caused a shift to higher stresses, implying the critical damage level for crack propagation will be achieved more rapidly with a concomitant increase in the crack propagation rate.

Deformation behavior of Al–Cu–Mn alloy sheets under biaxial stress at cryogenic temperatures

A research on the deformation behavior of Al–Cu–Mn alloy sheets was conducted under a biaxial stress state at cryogenic temperatures to determine fundamentals of a novel forming method for complicated components. The strength, ductility, strain hardening exponent and work hardening rate of an Al–Cu–Mn alloy sheet were compared with uniaxial tension tests at different cryogenic temperatures. The deformation of a dome specimen was analyzed by using a newly-designed bulging device and an optical three-dimensional (3D) deformation measuring system at temperatures of 293 K and 113 K. The limiting dome height (LDH) at 113 K was 37.1% higher than that at 293 K. The strain and thickness distribution of the bulged specimens were more uniform at 113 K owing to a higher strain hardening capacity. Compared to room temperature, the cryogenic bulging results revealed that the formability of the Al–Cu–Mn alloy could be enhanced under a biaxial stress state. Likewise, the resistance to the micro-deformation of the aluminum alloy was increased due to the harder formation of the slip bands and fewer slip localizations at cryogenic temperatures. The increased dislocation density and strength of the alloy were resulted from numerous netted dislocation substructures with interactive dislocation walls (DWs) and dislocation tangles (DTs). Furthermore, more uniformly distributed dislocation cell structures with continuous ridges could facilitate multiple dislocation glides and diminish localized deformation, which resulted in the improvement of the ductility of the Al–Cu–Mn alloy during the cryogenic deformation process.

THE DEFORMATION OF AZ31 MAGNESIUM ALLOY DURING WARM CONSTRAINED GROOVE PRESSING

A magnesium alloy AZ31 as plate of dimensions (60 x 60 x 3) mm has been constrained groove pressed (CGP) four deformation passes (16 pressings) at 250 oC by simulation and experiments. On the basis of the analysis of calculation results about the deformation distribution in the alloy AZ31 workpieces, the mechanism for its microstructure evolution during the severe plastic deformation (SPD) process was partly clarified. On the other hand, deformation heterogeneity distribution developed in the workpieces by applying CGP caused the evolution of a non-uniform microstructure. The TEM microstructure analysis results provided clear evidence that across the plate both the banded deformed microstructure where dislocation cell structure and/or partially or fully recovered polygonized subgrain microstructure are present. The recovering dynamic and local polygonization process contributes significantly to the formation of ultra-fine materials (UFG) microstructure.

A Hybrid Model for Studying the Size Effects on Flow Stress in Micro-Forming with the Consideration of Grain Hardening

Size effects extremely exist in the metal micro-forming process. When a deformation process scales down to micro scale, the appearances of geometry size and single grain size start to play a major role in deformation. Generally, the size effects are unavoidable in the experimental work and cannot be neglect in the optimization of micro-forming processes. In this paper, size effect on flow stress is investigated in the form of the coupled effect of workpiece geometry (sample thickness) and grain size, (T/D) by the micro tensile test of pure copper foil. Following the previous approaches, a new hybrid material model is projected to describe the hardening behavior of grains in polycrystalline material. Tensile tests performed on the copper foil with constant thickness and width, while to get dissimilar grain sizes, the foil annealed for different times. The ratio of thickness to grain size (T/D) is limited to larger than 1 (T/D˃1). A hybrid material model is proposed and established based on grain heterogeneity and sample thickness. The hybrid material model builds a relationship between the surface layer and sheet interior. The hybrid material model developed by the strain gradient theory in which the dislocation cell structure, cell densities (interior and wall) engaged to define the polycrystalline aggregate and calculated the dislocations in a grain (grain interior and grain wall). The results show that flow stress varies with the different values of T/D, but with an increase of the share of the grains flow stress start to decreases. After applying the hybrid material model of flow stress, the micro-tensile test of copper foil is simulated by finite element method. The simulation outcomes well matched with experimental results.

Strain hardening behavior of Ni-carbonyl Chemical Vapor Deposited (CVD) material with bimodal grain structures: Ultrafine (UF) grains and large grains with UF/nano twins

Through Ni-carbonyl CVD process, bulk Ni material (hereafter referred as CVD Ni) with bimodal grain structures, i.e. UF grains and large grains with UF/nano twins, can be produced. In tensile test, this material demonstrated a very good combination of strength and ductility as well as a special strain hardening behavior. Upon investigating the material's hardening behavior, a critical threshold strain, εc, of 0.02 was identified for the distinct change in the strain hardening behavior. Prior to this strain level, the strain hardening rate of the CVD Ni was found always higher than that of its counterpart coarse-grained (CG) Ni; but it dropped, after this strain, to the same level as the CG material. Furthermore, through the analysis of results from Tension Unloading Reloading-in-Tension (TURT) tests, this specific strain value was also found to be significant in the change of the hysteresis loop width as well as the back stress strain hardening responsible for the material's Bauschinger behavior. Microstructural evolution examination suggests that most of the twin boundaries (TBs) initially acted as obstacles to dislocation movement providing strong dislocation back stress and hence contributing to the strong hardening behavior. From the strain level of 0.02 and onwards, however, massive detwinning was detected which is responsible for the drop in the hardening rate. Finally, dislocation cell structures, that were typically evolved from entanglements of dislocation inside CG Ni, were also found inside the detwinned large grains of CVD Ni in the high strain regime, which yielded similar strain hardening behavior as in the CG Ni material.

Nucleation of twins and dislocations in V-Ti alloys under various straining conditions

The features of plasticity nucleation in V-4Ti and V-16Ti crystallites under uniaxial tension and bilateral compression are studied. It is shown that the nucleation of plasticity in crystallites is associated with the formation of twins under uniaxial tension. During the development of plasticity, a screw dislocation cell structure is formed between twin plates. The strain and stress at which plasticity nucleates in the material decreases with increasing Ti concentration. It was found that the distribution pattern of Ti atoms in the initial structure has a significant effect on the elastic limit of the simulated crystallites. The plastic deformation of crystallites with 16% Ti under bilateral compression is realized only by the dislocation mechanism. This behavior of the material is due to the low value of the stress at the elastic limit, which is insufficient for the formation of twins.

Microstructure and mechanical properties of severely deformed Al-Mg-Sc-Zr alloy and their evolution during annealing

The ultrafine-grained structure developed here after severe plastic deformation of Al-Mg and Al-Mg-Sc-Zr by equal channel angular pressing (ECAP) and cold rolling (CR) is presented to consist of dense dislocation walls and cell structure of dimension of ~ 300 nm. Detailed characterization by traditional transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD) has been carried out to reveal microstructure evolution during deformation and annealing. A unique bimodal grain structure, which consists of ultrafine grains (less than 150 nm) and fine grains (in the range of 200–500 nm) having high dislocation density, was developed after 16 ECAP passes and CR. It was also found that microstructure inhomogeneity exits even after high ECAP passes and annealing at high temperature (280 °C) for 1 h. High yield strength (YS) of ~ 550 MPa and tensile strength (TS) of ~ 560 MPa were obtained after 16 ECAP passes and CR, however, the corresponding elongation dropped to ~ 8.2%. Subsequent optimum annealing was found to be 240 °C × 1 h, by which comprehensive mechanical properties were maintained (~ 481 MPa for YS, ~ 512 MPa for TS and ~ 13.7% for elongation) in Al-Mg-Sc-Zr (16 ECAP passes). The reasons of inhomogeneous microstructure and the origins of high strength, i.e. high solute Mg content, dense dislocations, ultrafine grains and nanoscale Al3(Sc, Zr) particles are discussed.

On the evolution of dislocation cell structures in two Al-alloys (Al-5Mg and Al-11Zn) during reciprocal sliding wear at high homologous temperatures

The formation of dislocation substructures in up to 10 µm deep subsurface regions of two aluminium alloys, Al-5Mg and Al-11Zn, was investigated under conditions of high homologous temperature reciprocal sliding wear (HT-RSW). Under creep conditions, Al-5Mg shows a solid solution type of inverse primary creep. In contrast, Al-11Zn creeps obstacle controlled and exhibits normal primary creep. These two materials were subjected to reciprocal sliding wear at 200 and 300 °C for 100 and 1000 cycles. Flat polished disks were exposed to the 1 mm reciprocal movements of a spherical aluminium oxide counterbody under normal forces of 5 and 10 N at an oscillation frequency of 1 Hz. Using focused ion beam (FIB) micromachining thin electron transparent foils were prepared from the surface regions of the as received and worn material states. Transmission electron microscopy (TEM) was used to study the evolution of nano and micro grain sizes in the surface regions. Despite the different creep behavior, the two materials behave similar under conditions of reciprocal sliding wear. The results obtained in the present work show that subgrain sizes decrease with increasing numbers of wear cycles and increasing normal forces. Subgrain sizes also increase with increasing temperature. At 300 °C, dynamic recrystallization was observed in both Al-alloys. The results of the present work are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted.

Effect of cyclic plastic deformation on microstructure and mechanical properties of weld metals used for reel-lay pipeline steels

Reel-lay process is an efficient and economic method for installing offshore pipelines. During reel-lay process, cyclic plastic deformation (CPD) is introduced into the pipeline and it will modify the mechanical properties of the pipeline steels. In this research, cyclic tension-compression plastic deformation was conducted on the weld metals of X60 pipeline to simulate the strain experienced in reel-lay process. Then the dislocation configurations and mechanical properties of the weld metals were investigated after CPD. The total dislocation density was increased by the CPD process. The dislocation configurations evolved from dislocation lines and tangles into dislocation walls under 2% CPD and dislocation cells upon 5% CPD, respectively. The formation of dislocation walls and cells was a dynamic process, in which dislocation multiplication and intersection competed with dislocation annihilation. The yield platform of the weld metals disappeared and the yield strength decreased after CPD, but the tensile strength had no obvious change with CPD. The work hardening ability and uniform elongation rate reduced with the CPD level. The reduction of work hardening ability was mainly associated with the disintegration of dislocation wall or cell structure during tensile tests.

Microstructure and strengthening model of Al-3%Mg alloy in a heat treated state subjected to ECAP process

Purpose: The aim of this paper is to investigate the microstructure evolution of a heat treated Al-3%Mg aluminium alloy subjected to the ECAP (equal channel angular pressing process). Design/methodology/approach: Commercial Al-3%Mg alloy subjected to the solution treatment followed by an artificial aging was subjected to the 6 ECAP passes using a processing route Bc. Then the microstructure investigations using a light microscopy, scanning electron microscopy and transmission electron microscopy were carried out. Additionally the XRD technique was used to calculate lattice micro strain and dislocation density. Findings: The experimental results showed that the obtained microstructure is refined by mutual interactions of shear and microshear bands. The TEM investigation revealed typical for deformed aluminium alloys structure constituents such as dislocation-free grains, nonequilibrium grain boundaries, dislocation cell and (sub)grain structures. Research limitations/implications: The presented investigation results were carried out on samples, not on final products. Practical implications: Current research is moving towards to develop high strength material having a ultra-fine grained microstructure and increased mechanical strength. Originality/value: The paper focuses on the microstructure characterization of ECAP processed Al-3%Mg aluminium alloy. The relationship between the obtained microstructure and its contribution to the Yield strength is investigated.

Numerical study of grain refinement induced by severe shot peening

Three-dimensional finite element models of single shot impact and multiple shot impacts are developed to study grain refinement induced by severe shot peening. A multiscale constitutive model coupling the microscopic dislocation density and the macroscopic plasticity theory is proposed and implemented into the finite element models to characterize the formation and evolution of dislocation cell structure. The random distribution of the indentations under probability control is achieved by parametric modeling to simulate multiple shot impacts with respect to 100% peening coverage. Severe shot peening of AISI 4340 steel under various coverages is simulated and the cell sizes predicted by the finite element models are in good agreement with the experimental data. Parametric analysis of severe shot peening is carried out by using different size shots with the same kinetic energy. The numerical results indicate that: (1) the larger shot causes a larger depth of grain refinement, (2) the smaller shot is beneficial for grain refinement of the peened surface and subsurface, and (3) the shot oblique impact results in a finer dislocation cell structure. Moreover, the interactive effect of multiple shot impacts can effectively improve grain refinement of near surface materials.

Dynamic Recrystallization Behavior and Critical Strain of 51CrV4 High-Strength Spring Steel During Hot Deformation

Single-pass compression experiments have been performed on 51CrV4 spring steel using a Gleeble 3800 thermomechanical simulator at temperatures in the range of 800–1000°C and strain rate of 0.01 s−1 or 0.1 s−1; the maximum deformation degree was 50%. By considering the inflection of the ln θ–e curve and minimum value of the − ∂(ln θ)/∂e–e curve, the relationship between the critical strain (ec) of dynamic recrystallization (DRX) and the deformation temperature was determined. The results showed that steady flow behavior could be observed during low-temperature (800°C, 850°C) deformation, and dynamic recovery (DRV) regulated the trend of the stress–strain curve. Dislocation cell structures and polygonization were formed during the DRV stage. DRX of the alloy occurred when the deformation temperature reached a higher value (900°C, 1000°C). The amount of ec required for DRX decreased with increase in the deformation temperature, and the relationship between ec and the peak strain (ep) was determined as ec = 0.49ep. Discontinuous DRX was clearly favored when the strain was lower than the critical value.

Fatigue life and mechanistic modeling of interior micro-defect induced cracking in high cycle and very high cycle regimes

Axially loaded push-pull cyclic tests of a precipitation-hardened stainless steel with different sampling orientations were conducted in high cycle and very high cycle fatigue regimes. Results showed apparent fatigue anisotropy with non-metallic inclusions dominating crack initiation behavior. A fatigue lifing model was developed by combining size, location and shape of inclusions into a new form of Z parameter to rationalize the orientation effect. Using a multi-scale and full-field approach, the inclusion-induced interior cracking mechanisms were found to be associated with inclusion-microstructure interaction resulted plasticity. Micro-hardness at the cracking site was the lowest on the fracture surface, and surrounding microstructures showed formation of small grains with clear interfaces. The fine granular area was characteristic of several nano-scale fine grains formed in terms of dislocation cell structures by martensitic laths breakdown. The coalescence of interfaces or micro-crackings finally became interior early fatigue cracks. The mechanistic modeling of “fragmentation of martensitic laths and formation of dislocation cells” revealed a microstructure-dependent crack initiation and stage I growth for interior fatigue cracking. All these inform the significance of combining metallurgical and processing factors in designing against fatigue of engineering materials.

Transient creep behavior and dislocation cell structure development during creep-fatigue deformation of fully annealed Cu–Cr–Zr alloy

Creep-fatigue tests of Cu–0.7Cr–0.09Zr (mass%) alloy, which include stress-holding-type creep and strain-controlled fatigue, were conducted at elevated temperatures. A simple creep test was also carried out in order to compare the results with the results of the creep-fatigue test. The creep strains accumulated during creep deformation in creep-fatigue and simple creep tests were 310% and 12%, respectively. Such a large difference was due to the stacking of normal/inverse-transient creep in the creep-fatigue test. A dislocation cell structure smoothly developed in the simple creep test, whereas the cell intermittently developed in the creep-fatigue test because compressive plastic deformation immediately after creep partly broke apart the cell walls and prevented the cell structure from smoothly developing. Compressive stresses necessary for −1.5% strain were changed as cycling progressed in the creep-fatigue test. A much higher compressive stress would introduce a much higher dislocation density immediately before creep deformation. At the start of creep, these dislocations recovered and their number gradually decreased, which resulted in the appearance of inverse-transient creep.

Evolution of Nanostructure Through Thickness of Deoxidized Low-Phosphorous Copper by Three-Layer Stack Accumulative Roll-Bonding.

A nanostructured deoxidized low-phosphorous copper (DLPC) was fabricated by three-layer stack accumulative roll-bonding process. Three sheets of 1 mm in thickness, 30 mm in width and 300 mm in length were stacked up and roll-bonded to thickness of 1 mm by two-pass cold rolling. The bonded sheet was cut in three pieces of same length, then stacked up and roll-bonded to the thickness of 1 mm again. The evolution of nanostructure through thickness with three-layer stack ARB were investigated in detail. It was found that the microstructure has been evolved from a dislocation cell structure to a nano grained structure with the proceeding of ARB cycles. The average grain thickness of 45 μm in initial decreased to 170 nm after 7 cycles of the ARB. The heterogeneity in microstructure through thickness was also largely decreased by the ARB. These results suggest that three-layer stack ARB is an effective process for a formation of nanostructure of DLPC alloy.

Formability and fracture behaviour of cryorolled Al-3 Mg-0.25 Sc alloy

The rolling of an Al-3Mg-0.25Sc alloy at room and cryogenic temperature to 50% and 75% reduction in thickness resulted in the formation of a bimodal microstructure. The grain size distribution based on electron backscattered diffraction (EBSD) analyses showed an enhanced fraction of ultra-fine grains with nearly 100 nm in cryorolled samples whereas; room temperature rolled samples exhibited sub-micron grains with 300 nm. The transmission electron microscopy (TEM) studies revealed the dense dislocation cell structures for cryorolled samples due to the restriction of dynamic recovery. The better forming and fracture limit strains were noted for cryorolled samples compared to room temperature rolled ones in the forming and fracture limit diagrams. A good correlation of formability with void coalescence parameters based on larger void size, lower ligament thickness and lower d-factor were obtained. This indicated better accumulation of plastic deformation and improved formability of cryorolled samples. Further, the reduced aspect ratio (L/W) of void signified delayed fracture behaviour of cryorolled samples compared to room temperature rolled samples and showed better fracture resistance. The improved fracture limit strain in combined forming and fracture limit diagram exhibited by cryorolled samples was consistent with the void coalescence parameters.

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

The present work offers both a complete, quantitative model and a conservative acceleration factor expression for the life span of SnAgCu solder joints in thermal cycling. A broad range of thermal cycling experiments, conducted over many years, has revealed a series of systematic trends that are not compatible with common damage functions or constitutive relations. Complementary mechanical testing and systematic studies of the evolution of the microstructure and damage have led to a fundamental understanding of the progression of thermal fatigue and failure. A special experiment was developed to allow the effective deconstruction of conventional thermal cycling experiments and the finalization of our model. According to this model, the evolution of damage and failure in thermal cycling is controlled by a continuous recrystallization process which is dominated by the coalescence and rotation of dislocation cell structures continuously added to during the high-temperature dwell. The dominance of this dynamic recrystallization contribution is not consistent with the common assumption of a correlation between the number of cycles to failure and the total work done on the solder joint in question in each cycle. It is, however, consistent with an apparent dependence on the work done during the high-temperature dwell. Importantly, the onset of this recrystallization is delayed by pinning on the Ag3Sn precipitates until these have coarsened sufficiently, leading to a model with two terms where one tends to dominate in service and the other in accelerated thermal cycling tests. Accumulation of damage under realistic service conditions with varying dwell temperatures and times is also addressed.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part II: Accelerated regime manifested by quasi-cleavage fracture at relatively high stress intensity range values

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Influence of gaseous hydrogen on plastic strain in vicinity of fatigue crack tip in Armco pure iron

A multi-scale characterisation of the crack tip plasticity has been investigated in a fatigue crack propagation under gaseous hydrogen at gas pressure of 35 MPa in a commercially pure iron, Armco iron. The dislocation structure beneath a fracture surface was observed by a Transmission Electron Microscopy(TEM), and the cyclic and monotonic plastic zones were evaluated by an Out-of-Plane Displacement (OPD) measurement. By the TEM observation in a non-accelerated regime (ΔK = 11 MPa×m1/2), a dislocation cell structure was observed even in the brittle intergranular fracture in hydrogen. This result indicates a certain amount of plastic strain is introduced into the grains in front of an intergranular crack in hydrogen, and this may explain the mechanism of hydrogen-induced intergranular fatigue crack propagation. On the other hand, in an accelerated regime (ΔK = 18 and 20 MPa×m1/2), a distribution of scattered dislocation tangles without any cell or vein structure was observed in hydrogen. Besides, the inhibition of the cyclic plasticity near the crack path in hydrogen was confirmed by the OPD measurement. These results are clear evidences of hydrogen-induced localization of cyclic plasticity in the vicinity of a crack tip, which suggests a mechanism model of hydrogen-enhanced fatigue crack growth based on the plasticity localization.