Pure Aluminum Single Crystals Research Articles

Effect of crystal orientation on fatigue crack initiation life in pure aluminum single crystals

The crystal orientation dependence of fatigue crack initiation life was studied on 99.99 % pure aluminum single crystals. The fatigue crack initiation life is not ruled only by Primary slip system, but also affected by the second, the third and the fourth slip systems. Fatigue damage formation is accelerated in the crystal orientation which has equally large Schmid factor both on Primary and Critical slip planes, but it is restrained as Schmid factor on Conjugate slip plane increases. X-ray micro-beam back reflection Laue observation showed that the subgrain structure was remarkably developed. The configuration of crystallographic fatigue damage depends on the crystal orientation. The secondary slip system plays an important role in it. The total misorientation observed at the fatigue crack initiation takes about 5.5 × 10-2 rad, not depending on the crystal orientation and the shear stress amplitude. This suggests that the fatigue crack is initiated when the dislocation density at the subgrain boundary reaches a certain value.

Effects of cross-slip activity on low-cycle fatigue behavior and dislocation structure in pure aluminum single crystals with single-slip orientation

Fully reversed tension-compression low-cycle fatigue tests were performed at room temperature (RT) and 77 K on single-slip-oriented pure aluminum single crystals having different Schmid factor (SF) values (0, 0.14 and 0.24) for the cross-slip system under constant plastic shear-strain amplitudes. Cyclic deformation curves showed no stress saturation at RT, regardless of the applied strain amplitude. The cyclic deformation behavior at RT was as follows: initial hardening, softening and secondary hardening in the specimens with non-zero SF, whereas the specimens with SF = 0 exhibited monotonic hardening. However, all the specimens formed and developed the cell structure from the early stage of fatigue. On the other hand, no differences were observed in the cyclic deformation of the three specimens at 77 K. Shear stress amplitudes revealed initial hardening followed by saturation at any shear strain amplitudes applied, and a plateau stress region with a value of 42 MPa appeared in the cyclic stress-strain curve obtained at 77 K. Dislocation structures in the tested specimens were almost identical to those observed in single-slip-oriented pure Cu single crystals fatigued at RT.

Evolving structure–property relationships in metals with nonequilibrium concentrations of vacancies

Here, we use molecular dynamics simulations as a tool to investigate vacancy clustering in pure aluminum single crystals. A 1% superconcentration of single vacancies are randomly introduced into an otherwise perfect lattice, and the system is allowed to evolve for 500 ns at an elevated temperature of 728 K. Under these conditions, the individual vacancies rapidly agglomerate into larger clusters to reduce their overall energy. The systems are then subject to mechanical deformation to failure. The results of a total of 35 molecular dynamics simulations are reported. The mechanical behavior of these systems is found to be highly sensitive to the vacancy cluster microstructure, with the largest cluster size being most closely correlated with the cavitation strength. Since the largest cluster size evolves, an interesting time–structure–property coupling governs the behavior of these supersaturated metals. Despite the idealizations of the microstructure and loading conditions, we find a remarkably favorable agreement with laser-driven spall experiments.

Rearrangement of fatigue dislocation structure in pure aluminum single crystals associated with an increment in deformation temperature

Rearrangement of fatigue dislocation structure in pure aluminum single crystals associated with an increment in deformation temperature

(Low-Cycle Fatigue Behavior and Dislocation Structure of Pure Aluminum Single Crystals)

(Low-Cycle Fatigue Behavior and Dislocation Structure of Pure Aluminum Single Crystals)

Finite element implementation of dislocation-density-based crystal plasticity model and its application to pure aluminum crystalline materials

A new time-integration algorithm is presented for a dislocation-density-based crystal plasticity model. This includes a monolithic iterative scheme in which stresses were solved simultaneously with dislocation densities based on the fully backward Euler method for the integration of the constitutive model. Furthermore, the heuristic convergence criterion based on the slip resistance, instead of dislocation densities, together with the stresses is employed to circumvent stability issues. A closed form of the consistent tangent moduli is also derived. The constitutive models and time-integration procedures have been implemented as a user-subroutine UMAT of a finite element program Abaqus. Representative comparisons between the numerical predictions and experimental data are then given to verify the robustness and stability of the current implementation of the dislocation-density-based crystal plasticity model. It is demonstrated that finite element calculations reproduce well the anisotropic hardening response of differently oriented pure aluminum single crystals under tension and the variation of pile-up patterns according to different initial crystallographic orientations during nanoindentation. Furthermore, the numerical procedure was used to predict the deformation response of the aluminum multicrystalline specimen under simple shear. Non-uniform deformation fields by the digital image correlation method and surface profiles by the shadow Moiré method were well reproduced by numerical predictions. In addition, the effect of the initial crystallographic orientation and the interaction effects among different crystals on the deformation response of the multicrystal are addressed.

A study of plastic deformation behavior during high pressure torsion process by crystal plasticity finite element simulation

High pressure torsion (HPT) is an efficient technique of producing ultrafine grained materials with exceptional small grain size. In this study, a crystal plasticity finite element method (CPFEM) model has been developed to investigate the plastic deformation behavior of pure aluminum single crystal during the HPT process. The simulation results show that, the distribution and evolution of the macroscopic plastic strain and the accumulative shear strain are similar. The value increases with the increase of the distance from the center as well as the number of revolution. The simulation is capable of reflecting the anisotropic characteristics of HPT deformation, a non-homogenous deformation along the circumference of the sample could be observed. At the early stage of HPT deformation, the critical resolved shear stress (CRSS) along the radial direction presents a rapid increase, followed by a moderate increase and then reaches the near-saturate state. As the HPT deformation proceeds, there is a relatively weak increase in the quasi-saturate value and the near-steady region expands gradually towards the sample center. The orientation changes during the HPT process with increasing applied strain predicted by the developed CPFEM model are also presented.

Texture evolution in {112} <111> aluminum single crystals processed by severe plastic deformation

Abstract A pure aluminum single crystal with (112) [1 1 − 1] orientation was deformed by nine cycles of conventional rolling and accumulative roll bonding (ARB) which corresponded to a rolling strain (e) of 7.18. (4 4 11) [11 11 − 8] was the main component of the deformation texture. Between the third and seventh cycles (e = 2.4–5.6), (4 4 11) [11 11 − 8] oriented areas existed stably in the center layer, and this stability could be rationalized by the crystal rotation mechanism reported by Kamikawa et al. and Heason et al. The deformation texture after the ninth cycle included (4 4 11) [11 11 − 8] and (− 4 − 4 11) [11 11 8] components. The formation of the (− 4 − 4 11) [11 11 8] orientation was attributed to crystal rotation from (001) [110], which was formed in the lower surface layer after the seven cycle under lubrication conditions.

Development of a 3D Crystal Plasticity Model that Tracks Dislocation Density Evolution

A dislocation density based crystal plasticity finite element model (CPFEM) is developed for aluminum in which dislocation densities evolve on all octahedral slip systems. Based upon the kinematics of crystal deformation and dislocation interaction laws, dislocation generation and annihilation are modeled. The CPFEM model is calibrated for pure aluminum using experimental stress-strain curves of pure aluminum single crystal from literature. Crystallographic texture predictions in plane-strain compression of aluminum are validated against experimental observations in the literature. The framework is implemented in ABAQUS with user interface UMAT subroutine. Dislocation densities evolve and are tracked as state variables in the model, leading to spatially inhomogeneous dislocation densities that show patterning in the dislocation structures.

A dislocation-density-based 3D crystal plasticity model for pure aluminum

A dislocation-density-based crystal plasticity finite-element model (CPFEM) is developed in which different dislocation densities evolve. Based upon the kinematics of crystal deformation and dislocation interaction laws, dislocation generation and annihilation are modeled. The CPFEM model is calibrated for pure aluminum using experimental stress–strain curves of pure aluminum single crystal from the literature. Crystallographic texture predictions in plane-strain compression of aluminum are validated against experimental observations in the literature. The framework is implemented in ABAQUS with user interface UMAT subroutine.

Springback properties of single- and poly-crystalline pure aluminum processed by severe plastic deformation

A bending tester with an incorporated high-precision angle sensor was developed in this study. Pure aluminum single crystal and polycrystalline materials were used. Bending properties, especially the springback characteristics of materials as-received and fine-grained using AR and ECAP processes, were investigated. The springback characteristics are discussed based on various mechanical properties that were elucidated by tensile and hardness tests. Consequently, the following results were obtained. (1) AR and ECAP cold-processing refined grains of both single crystal and polycrystalline materials with almost no work hardening exhibited increased Vicker's hardness Hv without a decrease in elongation δ, at an equivalent strain ε of not less than 1.0. (2) The ECAP sample, which underwent many cycles of high strain processing, i.e., being subjected to large equivalent strain ε, indicated a large springback angle δSB. It also turned out that springback angle increased with an increment of δSB=0.1° per cycle of ECAP processing. (3) Both nECAP-s [110] and nECAP-p samples demonstrated a constant springback angle δSB, irrespective of equivalent strain ε at a bending angle of not less than δ=95°. (4) Fine-grained materials show a bigger springback angle δSB than as-delivered materials. Nevertheless, AR and ECAP samples, processed with different strain loading mode, showed little difference in the trend of spring back angle δSB against bending angle δ. (5) Steel materials over a wide range tend to indicate springback angle δSB in direct relation to the n value. However, fine-grained pure aluminum material of this study turned out not to conform to this trend. The result of this series can constitute fundamental data that are useful toward the establishment of a plastic deformation processing method of fine-grained materials.

Deformation Induced Microtwins and Stacking Faults in Aluminum Single Crystal

Microtwins and stacking faults in plastically deformed aluminum single crystal were successfully observed by high-resolution transmission electron microscope. The occurrence of these microtwins and stacking faults is directly related to the specially designed crystallographic orientation, because they were not observed in pure aluminum single crystal or polycrystal before. Based on the new finding above, we propose a universal dislocation-based model to judge the preference or not for the nucleation of deformation twins and stacking faults in various face-centered-cubic metals in terms of the critical stress for dislocation glide or twinning by considering the intrinsic factors, such as stacking fault energy, crystallographic orientation, and grain size. The new finding of deformation induced microtwins and stacking faults in aluminum single crystal and the proposed model should be of interest to a broad community.

Dependence of Localized Deformation on Point Defect Development Caused by Intersected Cross Slip among Dislocations

In this paper, via experimental results of ultrasonic wave velocity changes and FEPM simulations of slip system under the pure shear plastic deformation of a pure Aluminum single crystal, the authors verify that the ultrasonic longitudinal wave velocity depends upon the development of point defects caused by intersected cross slip among dislocations and also examine the correlation of the point defect dependence of the longitudinal wave velocity with the elastic stiffness degradation due to plastic deformation damage which is closely related to the deformation instability mechanism.

Correlation of Ultrasonic Wave Velocity with Microstructural Change due to Point Defects Caused by Plastic Deformation

In the authors' previous studies, the dependence of ultrasonic wave velocity on the microstructural properties of solid materials under plastic deformation was investigated both theoretically and experimentally. From the results obtained, it has been found that the changes of longitudinal wave velocity under simple and pure shear states are quite different, on the other hand transverse wave velocity changes under both shear states are almost same. These results suggest clearly that the propagating character of ultrasonic waves is dependent upon the interaction between the propagating mode of ultrasonic waves and micro-structural material changes induced by damages due to plastic deformation. In this paper, therefore, to identify the dependence of longitudinal wave velocity on the point defect, the longitudinal wave velocity propagating in a pure Aluminum single crystal under the pure shear plastic deformation is examined for several combinations of crystal orientations and shear slip directions which are determined by the difference of the amount of point defect calculated by the finite element polycrystal model (FEPM).

Reshock response of shock deformed aluminum

The effects of shock loading on compressive yield strength are presented for several aluminum materials. These include commercially pure aluminum 1060, ultrapure aluminum, pure aluminum single crystal of [100] and [111] orientations, and aluminum alloy 6061-T6. The yield strength of these materials was measured by a cyclic shock loading technique in which the sample was first shocked to longitudinal stresses ranging from 4to22GPa, followed by complete release and then reshocked to final states of 4–31GPa. The results reveal that the yield strength of 6061-T6 alloy, as determined through measurement of its Hugoniot elastic limit during reshocking, remains unchanged after first shock loading and release. In contrast, the yield strength of all pure aluminum materials was found to increase during reshocking. In addition, plastic strain hardening was observed in all reshock wave profiles after the initial elastic response, as observed through substantially higher reshock wave speeds. This results in substantially higher reshock Hugoniot stresses than predicted by the original Hugoniot of aluminum at the same compression. The compressive yield strength at the reshock Hugoniot stress was estimated by comparing the first and second Hugoniot states and correcting for thermal effects. The results suggest that strain hardening during second shock compression is independent of first shock amplitudes over the range of 6–22GPa and appears to saturate for a first shock stress greater than 6GPa. The temperature correction for the Hugoniot mean stress shows that the increased temperature produced by second shock compression significantly reduces the flow strength during reshocking and makes the resultant strength strongly dependent on loading history.

Pure Aluminum Single Crystal Study on Point Defect Dependence of Ultrasonic Longitudinal Wave Velocity

In the authors' previous study, velocity changes of ultrasonic waves under simple and pure shear states were studied both theoretically and experimentally. The research work revealed the quite different longitudinal wave velocity changes between under the simple and pure shear states and also distinguished longitudinal velocity change under the pure shear state, on the other hand transverse wave velocity changes under simple and pure shear states showed almost the same change tendencies similar to the texture development under both shear states. Therefore, to clear the different effects due to plastic deformation on transverse and longitudinal waves velocity changes under simple and pure shear states the microstructural changes of polycrystal solids were investigated via finite element polycrystal model (FEPM). The analyzed results suggested that the transverse wave velocity depends upon texture developments mainly, whereas the longitudinal wave velocity depends heavily upon point defects induced by intersected cross slips among dislocations. The principal object of this paper is experimental verification of point defects dependence of the longitudinal wave velocity using a pure Aluminum single crystal. As a result, no point defects occurring under single slip stage was simulated by FEM analysis for crystal plasticity and experimental evidence of no longitudinal wave velocity changes under same stage was shown. It was confirmed that the longitudinal waves velocity depends under the point defects.

Evolution of crystallographic orientations in an aluminum single crystal during tensile deformation

The evolution of the surface relief and the crystallographic orientations were investigated in a pure aluminum single crystal during in situ uniaxial tensile deformation inside a scanning electron microscope using an electron backscattered diffraction (EBSD) system. The operative slip systems were determined from the EBSD measurement in the deformation bands. Domains of two types form during inhomogeneous deformation showing a different orientation rotation behavior developing with increasing strain. The formation of the domains is caused by the rotation of local crystal areas due to the intersection of the primary and secondary slip systems.

Cyclic deformation of pure aluminum single crystals with double-slip orientations

Abstract Fully reversed tension–compression fatigue tests were performed on pure aluminum single crystals with the [1 1 2], [1 1 4] and [1 1 2 0] double-slip orientations under constant plastic strain amplitudes in the range of 5 × 10−4 and 1 × 10−2. No stress saturation was obtained at any strain level in the cyclic hardening curves (CHCs). The CHCs were characterized as: initial hardening, intermediate softening and secondary hardening. The degree of the softening was found to be strongly dependent on the stress-axis: the softening stages were significant for the [1 1 2 0] and [1 1 4] specimens, and conversely, indefinable at any plastic strain amplitude for the [1 1 2] specimens. The changes in surface topography were also found to be dependent on the crystal orientation. Characteristic macrobands in the order of several hundred micrometers were formed in the [1 1 2 0] specimens. The appearance of cyclic softening during fatigue deformation in aluminum single crystals is explained in terms of both the activity of cross-slip and the strain localization in the macrobands.

541 純アルミニウム単結晶の準単純せん断変形における寸法効果の検討

541 純アルミニウム単結晶の準単純せん断変形における寸法効果の検討

543 SEM・EBSDによる純アルミニウム単結晶の塑性変形挙動の実験解析(OS7-6 微視的挙動・効果I)(OS7 微視構造を有する材料の力学)

543 SEM・EBSDによる純アルミニウム単結晶の塑性変形挙動の実験解析(OS7-6 微視的挙動・効果I)(OS7 微視構造を有する材料の力学)