Pure Polycrystalline Aluminum Research Articles

Determination of Constitutive Parameters of Crystal Plasticity using Micro-pillar Testing with Pure Aluminum

The uniaxial compression test using the micrometer-sized specimen is employed to evaluate elastic-plastic deformation behavior of the selective single crystal from polycrystalline materials. Through reflecting these properties into the crystal plasticity finite element method (CPFEM), elastic-plastic analyses of the polycrystalline materials can be performed based on the single crystalline properties of real materials. In this study, the micro-pillar compression tests were conducted in the some single crystals of recrystallized polycrystalline pure aluminum. A method identifying CRSS, work hardening parameters and anisotropic elastic constants was proposed. The predicted stress-strain curves of single crystals obtained by CPFEM agree well with the original experimental data.

Unraveling the implications of finite specimen size on the interpretation of dynamic experiments for polycrystalline aluminum through direct numerical simulations

Normal and Pressure-shear plate impact (NPI and PSPI) experiments are popular experimental techniques for studying the mean-field macroscopic behavior of polycrystalline metals under high-rate dynamic loading. However, since both configurations rely upon geometry for subjecting the specimen to high strain rates, these experiments often involve a limited specimen size. Moreover, because of the inherent heterogeneities present within polycrystalline metals, it is difficult to ascertain if the size of the specimen and/or regions where measurements are made are sufficiently large for making representative inferences about the mean-field macroscopic properties from single-point velocity measurements. In the present study, we quantify the expected measurement variability on observable point measurements in NPI and PSPI experiments by carrying out direct numerical simulations (DNS) of statistically representative polycrystalline microstructures subjected to dynamic compression and compression-shear loading. In particular, we consider the role of specific material heterogeneities (e.g. the grain-to-grain difference in size, crystallographic orientation) on dispersion in the normal and transverse particle velocity records and on local fluctuations in key state variables (e.g. velocity, accumulated plastic strain) by incorporating these effects directly into a representative synthetic microstructure geometry and crystalline description of pure polycrystalline aluminum. The form of the present study is a large parametric investigation, consisting of ten ensembles of one hundred simulations. Each of the thousand simulations reflects a randomly realized synthetic microstructure in one of five cases of decreasing average grain size for the two loading configurations. Our analysis of the DNS results demonstrates that for both of these experimental configurations, the grain size directly correlates with the coefficient of variation (CV) in simulated point measurements, showing a convergent decrease in CV to zero (i.e. particle velocity record approaches the mean-field value) with decreasing grain size. Remarkably, the magnitude of variations in the particle velocity record is shown to be largest where the deviatoric stresses are most significant. In the case of NPI, this occurs at the elastic and plastic wavefront, whereas, in the case of PSPI, the magnitude of fluctuations are approximately constant throughout the experimental window time. The reasoning for the scatter in particle velocity due to the heterogeneous microstructure is demonstrated to be dependent on the mechanisms for accommodating deformation and on the interaction of reflection waves generated at sites of heterogeneities occurring at the scale of grains. Lastly, we develop a power-law description for the magnitude of scattering versus characteristic length, which provides a statistical framework for assessing the required number of grains per characteristic specimen dimension for minimizing scatter within these two experimental configurations (NPI, PSPI).

Influence of Load Modes on the Characteristics of Severe Plastic Deformation Based on Crystal Plasticity Finite Element Method

A three-dimensional Voronoi polycrystal model with 125 grains was developed for analyzing the heterogeneous phenomena and microplasticity in polycrystalline solids. Using the crystal plasticity, finite element method, the role of the load modes in the response characteristics of severe plastic deformation was investigated. Among the tested load modes, uniaxial tension and plane compression are responsible for the presence of the shear band, while pure shear leads to uniform strain distribution, and simple shear leads to the concentration of deformation. The higher shear strain was achieved by torsion, followed by simple shear. At the start-up of the sliding system, torsion and pure shear have the strongest influence, while the effects of uniaxial compression and plane compression are relatively small. After experiencing the same strain, simple shear causes lower damage than torsion. In terms of the texture, after tensile strain, polycrystalline pure aluminum shows the texture of //ND, after compression, the texture type is {110}//ND, and after torsion deformation, the texture type is //TD. Under small strain, plane compression includes copper texture, brass texture, and S-texture. Under high strain, the {111} (annealed) texture was found in simple shear deformation. Experimental observation verified the high accuracy of the simulation results based on the excellent agreement between experiment and simulations for the stress–strain curve and texture evolution, and slip bands. Based on the principle of maximum cumulative plastic strain and minimum damage, simple shear is determined to be the optimal fine grain mode in the SPD process.

Dynamic flow stress of pure polycrystalline aluminum: Pressure-shear plate impact experiments and extension of dislocation-based modeling to large strains

The dynamic thermo-mechanical behavior of pure aluminum has attracted renewed interest lately due to experimental observations of an anomalous increase in Hugoniot Elastic Limit (HEL) at incipient plasticity and elevated temperatures in polycrystalline pure metals. In context of current dislocation-mediated plasticity models for metals, this increase in dynamic strength is indicative of a transition in the rate controlling mechanism for dislocation glide from being thermally assisted to phonon-drag restricted due to increase in phonon viscosity at elevated temperatures. Though these studies have helped to shed light on these important mechanisms operative in FCC metals at incipient plasticity, the extent to which they contribute to flow stress, particularly at larger strains, remains unclear. In the present study, we address these questions through a combined experimental and modeling effort focused on investigating the evolution of dynamic flow stress in polycrystalline aluminum using experimental data gathered from a series of combined pressure-and-shear plate impact (PSPI) experiments designed to reveal the flow stress of pure aluminum at strain rates ~ 105/s, plastic strains of up to 40% and temperatures ranging from room to 866 K. In all cases, the flow stress of aluminum, as inferred from the measured transverse particle velocity histories at the free surface of an elastic tungsten carbide target plate, reveals saturation with increasing plastic strains at stress levels that decrease with increasing test temperatures. Numerical simulations are performed to correlate the experimentally observed temperature and strain rate dependence of flow stress at small and large plastic strains using the Austin-McDowell dislocation-mediated plasticity model parametrized to normal plate impact experiments conducted in an earlier study by Zaretsky and Kanel (2012). Extensions to the model are made to better represent dynamic behavior of pure aluminum at larger plastic strains as observed in elevated temperature split Hopkinson Pressure bar (SPHB) experiments of Samanta (1971) and Lindholm and Yeakly (1965), and the combined pressure-and-shear plate-impact experiments conducted in the present study. The main theoretical extension to the Austin-McDowell model made in this paper is the introduction of a new rate- and temperature- dependent dynamic recovery function, which can potentially allow for an effective reduction in the rate of accumulation of dislocations at large plastic strains. The numerical predictions of the revised and re-calibrated plasticity model are brought into agreement with the experimental observations, correlating sufficiently well with dynamic yield stress at incipient plasticity and the flow stress levels at larger plastic strains, plastic strain rates in the range 103 – 106 /s, and elevated temperatures up to near melt. The model, in concert with the experimental measurements, suggests that at incipient plasticity the rate governing mechanism for plastic flow is phonon-drag restricted dislocation glide, whereas, at higher magnitudes of plastic strain, it transitions to stress-assisted thermally activated glide. The transition between these two rate governing mechanisms is controlled by the evolution of dislocations throughout the deformation process.

Monitoring microstructural evolution in-situ during cyclic loading with high-resolution reciprocal space mapping

High-resolution reciprocal space mapping using high-energy hard x-rays has been developed to investigate the microstructure of grains located in the bulk of a metallic sample in a non-destructive way in-situ during different loading conditions. The technique allows to identify and follow individual grains and subgrains during ongoing deformation such as loading in tension and compression, during repeated cyclic deformation or even individual load cycles while simultaneously monitoring macroscopic stress and strain. Insight in the structural reorganization within single grains is gained by in-situ monitoring of the characteristic intensity distribution of Bragg reflections from individual grains during cyclic deformation of commercially pure polycrystalline aluminium. By reciprocal space mapping with high angular resolution and combined analysis of the radial and azimuthal information, individual subgrains are tracked during single load cycles with different strain amplitudes. Additionally, changes in mean peak position, peak width and asymmetry of integrated radial profiles from individual grains are analyzed as well as their orientation spread. In this manner, the microstructural evolution in grains embedded in the bulk of polycrystalline specimens is traced and linked to the changing mechanical loads during cyclic deformation.

Influence of constant magnetic field on plastic characteristics of paramagnetic metals

The reported study analyses the influence of weak constant magnetic fields (up to 0.6 T) on plastic characteristics of paramagnetic materials, e.g. titanium and aluminum. Such characteristics as microhardness, plasticity parameter and creeping process were investigated. In the process of work, it has been established that weak magnetic fields can essentially change plastic characteristics of the materials under consideration. The effect of magnetic field is linearly dependent on induction, becoming evident in changing microhardness of commercially pure polycrystalline titanium and aluminum.

Determination of positron annihilation lifetime spectroscopy instrument timing resolution function and source terms using standard samples

The extraction of material positron lifetime components from positron annihilation lifetime spectroscopy measurements, performed using conventional unmoderated radionuclide positron sources, requires accurate knowledge of both the spectrometer instrument timing resolution function (IRF) and annihilation events extrinsic to the material, the source correction terms. Here we report the results from study of spectrometer performance made using two reference samples, high purity polycrystalline aluminium, and stainless steel supplied by the National Metrology Institute of Japan (NMIJ RM 5607-a). Both prepared with directly deposited 22NaCl positron sources. The IRFs obtained by fitting spectra from both reference samples were monitored with time to evaluate spectrometer stability and to compare methods of IRF determination. Using the aluminium IRFs the analysis of spectra from the NMIJ stainless steel reference samples yielded a single lifetime component with value 106.9(9) ps.

Microstructure and texture development in a polycrystal and different aluminium single crystals subjected to hydrostatic extrusion

A hydrostatic extrusion (HE) process was applied to commercial pure polycrystalline aluminium (99.9%) and two aluminium single crystals \(\langle {111}\rangle \) and \(\langle {110}\rangle \). On comparison, the results obtained from single crystals and polycrystalline aggregates are unique. Microstructure and crystallographic texture investigations were performed by transmission electron microscopy, electron backscatter diffraction and X-ray diffraction (XRD). Significant differences in grain refinement and texture formation were noticed depending on the starting orientation. The deformed single crystal with \(\langle {110}\rangle \) starting orientation features an average grain size value of 150% higher than the second investigated single crystal (\(0.5\ \upmu \hbox {m}\) for the \(\langle {111}\rangle \) single crystal and \(1.3\ \upmu \hbox {m}\) for the second crystal). In turn, the average grain size obtained for polycrystalline aluminium is \(0.9\ \upmu \hbox {m}\). The deformation process causes a difference in the grain sizes, while a fraction of the high angle grain boundaries have a comparable volume percentage in all the deformed microstructures—reached about 35%. The qualitative and quantitative XRD texture results proved that the HE process leads to the formation of a characteristic fibrous texture.

Strengthening mechanisms and Hall-Petch stress of ultrafine grained Al-0.3%Cu

An ultrafine grained Al-0.3 wt %Cu has been produced by cold rolling to a thickness reduction of 98% (εvM = 4.5). The deformed structure is a typical lamellar structure with a boundary spacing of 200 nm as characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Coarsening of the deformed structure to recrystallization is achieved by heat treatment in the range of 100–300 °C. Good thermal stability has been observed up to 175 °C with some segregation of Cu to the boundaries as observed by 3D atom probe characterization. Tensile tests have shown a flow stress (0.2% offset) of 198 MPa with continuous flow with no yield drop and Lüders elongation. To quantify the contribution of boundary strengthening to the flow stress, dislocation strengthening and solid solution hardening have been calculated and subtracted from the flow stress. It has been found that boundary strengthening can be expressed by a Hall-Petch relationship and that the constants in this equation are in very good agreement with previous observation of recrystallized pure polycrystalline aluminium with a grain size in the tens of micrometer range. Thereby the Hall-Petch relationship of aluminium can be extended an order of magnitude from the micrometer to the sub-micrometer range, which is of both scientific and technical importance.

Mechanical response of 99.999% purity aluminum under dynamic uniaxial strain and near melting temperatures

A series of reverse geometry normal plate impact experiments are employed to investigate the incipient plastic behavior of commercial purity polycrystalline aluminum under ultra-high strain-rates (∼105 /s) and test temperatures up to melt (i.e. 643 °C). The applied stress at the aluminum/target interface, as inferred from the measured normal free-surface particle velocity record, shows progressive weakening with increasing test temperatures ranging from 23–620 °C; at higher temperatures (470–620 °C) this rate of weakening is observed to decrease, and at the highest test temperatures employed in the study, i.e. temperatures approaching the melt point of aluminum (643 °C), a net increase in the longitudinal stress is observed. Additionally, estimates of longitudinal elastic impedance of the sample material, as inferred via impedance matching from the particle velocity states at shock plateau show similar monotonic decrease with temperature, except for the highest test temperature case, in which reversal of this trend is observed. Though slight, these noticeable changes in the impedance, and stress/velocity states likely indicate variances in the strengthening/hardening response of the material, which can be linked to transition in the dominant plastic flow mechanisms as the sample approaches the melting point. Scanning electron microscopy of the impact surface of the post-test samples reveal coarsening of the sample grains due to static recrystallization during the heating phase of the experiment. Additional experiments are performed on annealed specimens to probe the effects of microstructural changes, i.e. grain size, and/or relief of residual strains in the interpretation of the measured response of aluminum. The results, which show a decrease in the longitudinal acoustic impedance, and decrease in the levels of stress/particle velocity under shock loading, indicate that the effects of annealing may play a role in the observed response by promoting softening of the dynamic strength, and/or lessening of the rate of hardening in a post-yield regime.

Variations in defect substructure and fracture surface of commercially pure aluminum under creep in weak magnetic field**Project supported by the Ministry of Education and Science of Russian Federation (State Task No. 3.1283.2017/4.6).

Commercially pure polycrystalline aluminum of grade A85, as a test material, is investigated. Using scanning and transmission electron microscopy the aluminum fine structure and fracture surface are analyzed. Fractures are studied in the regime of creep with and without a simultaneous effect of 0.3-T magnetic field. It is found that the application of a magnetic field in a linear stage of creep leads to substructure imperfection increasing. Furthermore, the magnetic field effect on aluminum in the process of creep causes the average scalar density of dislocations to increase and induces the process of dislocation loop formation to strengthen. Fractographic investigation of the fracture surface shows that in the fibrous fracture zone the average size of plastic fracture pits decreases more than twice under creep in the condition of external magnetic field compared with in the conventional experimental condition. In a shear zone, the magnetic field causes the average size of fracture pits to decrease. Experimental data obtained in the research allow us to conclude that the magnetic field effect on aluminum in the process of creep leads to the fracture toughness value of the material decreasing, which will affect the state of defect substructure of the volume and surface layer of the material. The influence of the magnetic field is analyzed on the basis of the magneto-plasticity effect.

A Novel Approach for Plate Impact Experiments to Determine the Dynamic Behavior of Materials Under Extreme Conditions

This paper presents a unique approach to plate impact experiments for characterization of dynamic material behavior under extreme conditions, i.e., ultra-high strain-rates (~106/s) and test temperatures up to 1000 °C. Strategic modifications are made to the existing single-stage gas-gun facility at CWRU to enable this approach. These include an extension of the gun barrel at the breech end of the gas-gun, which now incorporates a precision machined steel housing that carries a vertical heater well to accommodate an 800 W resistance coil heater with axial and rotational degrees of freedom. This custom designed heater assembly enables specimens placed on the flyer plate carried by the sabot to be heated uniformly across the diameter in a 100 mTorr vacuum prior to impact at the breech end of the gun barrel. A new sabot design is utilized to minimize heat transfer from the heated flyer plate to the sabot body. Using this modified gas-gun facility, the dynamic strength of pre-heated commercial purity polycrystalline aluminum samples is investigated at the onset of plastic flow in response to weak normal shock compression at test temperatures ranging from room to near melt point of aluminum. The dynamic strength of aluminum samples, as inferred from the measured normal particle velocity history at the free (rear) surface of the target plate, show progressive weakening with increasing specimen temperatures in the temperature range 23–643 °C; at higher test temperatures, however, the rate of softening in dynamic strength is observed to weaken and even reverse as the sample temperatures approach the melt point of aluminum (test temperature ~643 °C).

Interactions between grain size and geometrical defects in pure aluminium in the high cycle fatigue regime

In this study, the influence of geometrical defects on the High Cycle Fatigue (HCF) resistance of aluminium is investigated, with emphasis on the impact of local microstructure on fatigue crack initiation. In order to meet this objective, an experimental approach, using a commercial purity polycrystalline aluminium alloy, is proposed.First, different thermomechanical treatments are applied to the aluminium alloy to obtain two homogeneous microstructures with respective mean grain sizes of 100 and 1000 µm. Then, fatigue specimens with an artificial hemispherical surface defect of diameter 1000 µm are subjected to fully reversed stress-controlled cyclic loading conditions. In-situ observations are carried out to monitor the crack length during fatigue tests. It is noted that, for a higher grain size, the number of cycles needed for the initiation of a 100 µm-long surface crack is lower.A study of the influence of the defect size relative to the grain size is also conducted. Two sizes of defects are used, and the influence of characteristic sizes seems to be explained by the role of cyclic plasticity in the crack initiation process.

Interfacial energies and mass transport in the Ni(Al)–Al2O3 system: The implication of very low oxygen activities

Adhesion and capillary-driven mass transport at ceramic–metal interfaces play a very important role in the performance and durability of materials for many applications, and the influence of the oxygen activity is a critical issue. This work systematically investigates the variation of interfacial energies and atomic transport mechanisms at metal–oxide interfaces at very low oxygen activities by bonding Ni–Al alloys and pure polycrystalline alumina under controlled conditions in sessile drop experiments. The angles and the evolution of the grain boundary grooves were analyzed by scanning electron microscopy, atomic force microscopy and focused ion beam milling to calculate the interfacial and grain boundary energies and the transport rates at the metal–Al2O3 interface. In parallel, high-resolution structural and chemical analysis of selected grain boundaries was performed using advanced transmission electron microscopy. Our results confirm that all the interfacial energies (metal–Al2O3, Al2O3 surface and grain boundary energy) are smaller at reduced p(O2) than those of stoichiometric interfaces. The atomic transport at the metal–Al2O3 interface was found to decrease initially with decreasing p(O2) but increased significantly with a further decrease in the oxygen activity.

An EPR and NMR study on Mo/HZSM-5 catalysts for the aromatization of methane: Investigation of the location of the pentavalent molybdenum

EPR and 27Al NMR were used to probe the nature of the phases that formed during the conversion of methane at high temperature on the Mo/HZSM-5 catalyst. It was shown that ammonium 6-molybdoaluminate III supported over silica gave rise, when reduced, to an EPR signal due to Mo5+ ions with the odd electron interacting with the nonzero nuclear spin of aluminum in much the same way as in HZSM-5. Therefore, the occurrence of this particular spin interaction should not be taken as evidence of the location of the Mo5+ ions at the exchange sites of the zeolites. Rather, combined 27Al NMR and EPR investigation of the reduction of a pure polycrystalline aluminum molybdate suggested that the observed signal in the case of the zeolite composite catalyst was due to Mo5+ ions formed within the Al2(MoO4)3 extra phase that formed upon dealumination of the zeolite in the presence of molybdenum.

Hydrogen Diffusivity During Corrosion of High-Purity Aluminum

The effective diffusivity of hydrogen in as-annealed (550°C for 5 h) 99.999% purity polycrystalline aluminum was determined using Al/Pd bilayer membranes following the procedure of K.R. Hebert. Bulk diffusion control was verified by the foil thickness variation method. The room temperature diffusivity of hydrogen was determined to be 1.6 ± 0.4 × 10−10 cm2/s during initial hydrogen charging in 0.01 M sodium hydroxide (NaOH) solution. The effective diffusivity of hydrogen increased by two orders of magnitude during the second hydrogen charging and approaches the literature for gas phase egress from precharged material. This difference was interpreted to be the result of extensive trapping in freshly annealed foils and high residual trap site occupancy after initial precharging. The activation energy for lattice hydrogen egress was analyzed through constant heating rate thermal desorption spectra. This activation energy was 17.0 ± 0.6 kJ/mol. The experimental results also demonstrated that the measured hydrogen diffusivity is sensitive to the NaOH concentration speculated to be related to its influence on hydrogen production rate, hydrogen overpotential, and solid-state hydrogen concentration.

Formation Mechanism of Micropores on the Surface of Pure Aluminum Induced by High-Current Pulsed Electron Beam Irradiation

The mechanism of micropores formed on the surface of polycrystalline pure aluminum under high-current pulsed electron beam (HCPEB) irradiation is explained. It is discovered that dispersed micropores with sizes of 0.1–1 μm on the irradiated surface of pure aluminum can be successfully fabricated after HCPEB irradiation. The dominant formation mechanism of the surface micropores should be attributed to the formation of supersaturation vacancies within the near surface during the HCPEB irradiation and the migration of vacancies along grain boundaries and/or dislocations towards the irradiated surface. It is expected that the HCPEB technique will become a new method for the rapid synthesis of surface porous materials.

Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials

Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods to predict small crack fatigue behavior. In this paper small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and was used (i) to determine the bridging stress profile for the material and (ii) to make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, it has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.

Universal strain–temperature dependence of dislocation structure evolution in face-centered-cubic metals

The combined effect of strain and temperature on the microstructural evolution of plastically deformed face-centered-cubic (fcc) metals is explored systematically. In particular, the detailed nanoscale, internal structure of dislocation boundaries is determined in pure polycrystalline aluminum, nickel and gold and compared to earlier results in copper. In all the metals studied, dislocations within the boundaries tend to rearrange themselves with increasing strain in the same sequence from tangles into dislocation cells with tangled boundaries, followed by dislocation boundaries consisting of wavy, parallel dislocations and finally into arrays of parallel dislocations. The strain at which rearrangement occurs decreases with increasing temperature. The results are represented by microstructural maps on the strain–temperature plane. The topology of the microstructural maps is found to be similar for all metals studied, suggesting a universal strain–temperature dependence in deformed fcc metals.

Fatigue Threshold R-Curve Behavior of Grain Bridging Ceramics: Role of Grain Size and Grain-Boundary Adhesion

To better understand the role of grain size and grain-boundary adhesion on the fatigue threshold R-curve behavior of grain bridging ceramics, a study was conducted on the fatigue threshold behavior of 99.5% pure polycrystalline alumina with two different microstructures (fine and coarse) and in two different environments (moist air and dry N2). The fine-grained microstructure showed higher fatigue thresholds at short crack sizes, while the coarse-grained microstructure demonstrated higher fatigue thresholds at long crack sizes. The former effect lead to slightly higher calculated fatigue strengths and was attributed to the crack stalling process that leads to earlier elastic bridge formation in that microstructure. The latter effect is attributed to toughening that is dominated by frictional and mechanical interlocking bridges at longer crack sizes where the larger grains are able to give more bridging. By testing the coarse microstructure in a dry environment, a higher K0 was achieved for the glassy grain boundaries giving a higher R-curve at short crack sizes and higher calculated fatigue strengths.