Plastic Deformation Mode Research Articles

Development of the rotational mode of plastic deformation upon drawing of pearlitic steels of various alloying systems

Transmission electron microscopy has been used to study the evolution of the microstructure of fine pearlite in the process of drawing of a number of steels of eutectoid composition of different natural alloying complexes. A relation has been established between the development of a rotational mode in lamellar pearlite and the formation of an axial texture upon the deformation of steels containing microadditives of alloying elements.

Crystallographic orientation-dependent pattern replication in direct imprint of aluminum nanostructures.

In the present work, we perform molecular dynamics simulations corroborated by experimental validations to elucidate the underlying deformation mechanisms of single-crystalline aluminum under direct imprint using a rigid silicon master. We investigate the influence of crystallographic orientation on the microscopic deformation behavior of the substrate materials and its correlation with the macroscopic pattern replications. Furthermore, the surface mechanical properties of the patterned structures are qualitatively characterized by nanoindentation tests. Our results reveal that dislocation slip and deformation twinning are two primary plastic deformation modes of single-crystalline aluminum under the direct imprint. However, both the competition between the individual deformation mechanisms and the geometry between activated dislocation slip systems and imprinted surface vary with surface orientation, which in turn leads to a strong crystallographic orientation dependence of the pattern replications. It is found that the (010) orientation leads to a better quality of pattern replication of single-crystalline aluminum than the (111) orientation.

Stress-driven migration of deformation-distorted grain boundaries in nanomaterials

Stress-driven migration of low-angle grain boundaries (LAGBs) having deformation-distorted structures is theoretically described as a special mode of plastic deformation in nanocrystalline and ultrafine-grained materials. Equilibrium migration distances, energy and critical stress characteristics for stress-driven migration of deformation-distorted LAGBs are calculated as those depending on their structural and geometric parameters. It is theoretically revealed that deformation-distorted LAGBs tend to split under shear stresses, and these splitting processes lead to formation of new nanoscale (sub)grains. The role of migrating, deformation-distorted LAGBs in diminishing grain size in ultrafine-grained metallic materials during severe plastic deformation is discussed. Also, widening of deformation-distorted LAGBs in the absence of external stresses is theoretically described. Generalization of the suggested model to the case of high-angle grain boundaries is discussed. The results of our theoretical examination are compared with the corresponding experimental data and computer simulations reported in the literature.

Mineralogical Characteristics of Garnet in Garnet Mica Schist and Its Tectonic Significance in the Tongbai Section of the Shangdan Fault Zone

Studies on the metamorphism and deformation conditions of rocks in the suture zone are good ways to discuss the orogenic process and mechanism. The microstructure and ultramicrostructure features of minerals are true embodiment of formation environment of the orogenic belt. Based on the study of microstructure, ultramicrostructure deformation characteristic and compositional zonation of garnets in garnets-mica schists in the Tongbai mountain, east section of the Shangdan fault zone, the results show that the three types of garnets have suffered various states of plastic deformation. The dynamic recrystallization of garnets is due to the subgrain growth and boundary migration. The dislocations are mainly free dislocation and dislocation walls. The free dislocation density ρ = 6.14 × 108/cm2, and dislocation movement are mainly slips; slip planes are 1/2 {110} and {001}. Garnet microprobe analysis shows that it belongs to almandine, and reflects it has undergone epidote-amphibolite to amphibolite facies metamorphism. Compositional zonation of garnet shows that the rocks had experienced progressive metamorphism. First metamorphic environment was continuous temperature-pressure and in mid-term there were two non-synchronous transient cooling and decompression processes, and it finally underwent decompression and warming process of the thermal relaxation environment. The formation condition of garnet-mica schist is estimated: T = 562°C - 617°C, and P = 0.77 - 1.02 GPa. The differential stress is 0.511 GPa and strain rate is 4.97536 × 10-10 m/s. After systemic analysis, a conclusion is drawn that the plastic deformation mode, deformation mechanism and formation environment of garnets are closely related to the formation and development of the Shangdan fault zone. It truly reflects that the Shangdan fault zone, as the suture zone of Yangtze and north China plate, has been subjected to early medium-grade metamorphism. With the continuous compression after the collision, the left-lateral shearing happened and caused the formation of high density dislocations and subgrains of the garnets; finally the plastic deformation happened and the bulging recrystallization formed in the period of heat relaxation had a relative low stress.

Mechanical Resonance Dispersion Revisited

Mechanical resonance dispersion is the inelastic response of a solid to a periodic shear stress. Instead of the elastic Young's Modulus, the phenomenon is described by both a real J', and an imaginary J'' component of complex shear compliance, corresponding to in phase and out of phase strain responses, respectively. The experimental results are plots of J' and J'' vs. frequency, which are typically in the audiofrequency range of 10 - 5600 Hz. Resonances are observed as maxima in J'' and inversions in J' at frequencies corresponding to modes of plastic deformation, which are much lower frequencies (audiofrequency range) than elastic normal modes. The theoretical explanation of Edwin R. Fitzgerald involves particle waves and momentum transfer and leads to a particle-in-a-box frequency formula for these inelastic modes. Unfortunately, most of his and other published raw data were never analyzed by this model. The purpose of this article is to apply this formula to previously uninterpreted resonance dispersion curves and to address some of the earlier criticism of Fitzgerald's work. Results of these calculations support the Fitzgerald Theory to a high degree, demonstrate the importance of impurities and chemical analysis, largely mollify previous criticisms, and suggest the possibility of a new particle wave mass spectroscopy at great distances.

Robust scaling of strength and elastic constants and universal cooperativity in disordered colloidal micropillars.

We study the uniaxial compressive behavior of disordered colloidal free-standing micropillars composed of a bidisperse mixture of 3- and 6-μm polystyrene particles. Mechanical annealing of confined pillars enables variation of the packing fraction across the phase space of colloidal glasses. The measured normalized strengths and elastic moduli of the annealed freestanding micropillars span almost three orders of magnitude despite similar plastic morphology governed by shear banding. We measure a robust correlation between ultimate strengths and elastic constants that is invariant to relative humidity, implying a critical strain of ∼0.01 that is strikingly similar to that observed in metallic glasses (MGs) [Johnson WL, Samwer K (2005) Phys Rev Lett 95:195501] and suggestive of a universal mode of cooperative plastic deformation. We estimate the characteristic strain of the underlying cooperative plastic event by considering the energy necessary to create an Eshelby-like ellipsoidal inclusion in an elastic matrix. We find that the characteristic strain is similar to that found in experiments and simulations of other disordered solids with distinct bonding and particle sizes, suggesting a universal criterion for the elastic to plastic transition in glassy materials with the capacity for finite plastic flow.

Is Wetting Collapse an Unstable Compaction Process?

This paper provides an interpretation of the phenomenon of wetting collapse in fine-grained soils based on principles of material stability. For this purpose, classic experiments displaying the accumulation of irreversible compaction have been reinterpreted through a plasticity model for unsaturated soils. The resulting simulations have been inspected mathematically, with the goal of detecting a possible loss of control of the wetting process. The reanalysis of the experiments suggests that, if the wetting-induced deformations are interpreted as homogeneous, the considered compaction events do not tend to be affected by a loss of material stability. As a result, such well-known phenomena have been explained as controllable modes of plastic deformation. This feature has been found to be valid regardless of the mode of saturation (i.e., it is valid for both controlled suction removal and water volume injection). Indeed, parametric analyses have shown that the compaction process tends to become unstable only in the presence of highly water-sensitive media exposed to a drastic loss of suction (e.g., soaking or injection), i.e., circumstances during which the soil undergoes large strains and suffers a marked deterioration of its mechanical properties. In these cases, the theory predicts a limit threshold of the water content at which the control of the injection process is lost, the applied stress is no longer sustainable, and even infinitesimal alterations of the moisture content can cause a sharp accumulation of compaction. These findings impact the interpretation of wetting-induced settlements predicted via computational models, and are of general relevance for the design, maintenance, and safety assessment of unsaturated earthen systems interacting with hydrologic processes.

Plastic Deformation Modes in Mono- and Bimodal-Type Ultrafine-Grained Austenitic Stainless Steel

An attempt was made to track plastic tensile deformation modes operating in bulk ultrafine-grained austenitic stainless steel with mono- (maximum at ~0.6 μm) and bimodal-type (minimum at ~0.5 μm and maximum at ~1.65 μm) grain size distributions produced by cyclic thermal processing. Post tensile deformation electron backscatter diffraction studies were conducted to analyse the impact of grain size distribution on plastic deformation characteristics. The study revealed extensive strain localisation in the monomodal-type grain size distribution, leading to poor strain-hardening behaviour. On the other hand, the bimodal-type distribution disclosed a conventional dislocation-mediated deformation mechanism operating in the coarse grains, while it was restricted to initial small strains in ultrafine austenite grains. The subsequent deformation process in these ultrafine austenite grains was dictated by nucleation and autocatalytic growth of strain-induced α′-martensite. The observed martensitic transformation of ultrafine austenite grains in preference to coarse grains was attributed to activation of local ‘grain to grain’ interactions.

Compressive Creep Behavior of Spark Plasma Sintered 8 mol% Yttria Stabilized Cubic Zirconia

The present paper describes compressive creep behavior of cubic 8 mol% Yttria-stabilized Zirconia, fabricated by spark plasma sintering, in the temperature range of 1300-1330 °C at a stress level of 78-193 MPa in vacuum. The pre- and post-creep microstructures, along with the values of the stress exponent (n = 1.7-2.7) and the activation energy (Q = 711-757 kJ/mol) suggest that a mixed mode of plastic deformation, dominated by grain boundary sliding, occurred in this material. The relatively high activation energy observed was related to the pinning of the grain boundaries by voids during creep, leading to microcrack formation, shear strain-induced grain exfoliation, and finally creation of new voids at grain boundaries.

Nanostructuring of Ni by Various Modes of Severe Plastic Deformation

Various modes of severe plastic deformation (SPD), such as high-pressure torsion (HPT) at cryogenic temperature, equal channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP), have been applied for nanostructuring of Ni, and the thermal stability of the structure obtained has been studied. The nanocrystalline structure with average grain sizes of 80 nm and the microhardness of 6200 MPa is produced by HPT in liquid nitrogen. DCAP and ECAP result in the submicrocrystalline structure of a mixed type, with ultra-fine grains separated by high-angle boundaries along with deformation bands and coarse cells with low-angle dislocation boundaries. The thermal stability of the structures obtained by ECAP and DCAP is approximately the same, and it is higher than after the HPT at cryogenic temperature.

Nano- and microvoid formation in ultrafine-grained martensitic Fe-Ni-Mn steel after severe cold rolling

Severe cold-rolling was applied on solution annealed Fe-Ni-Mn steel with fully lath martensite structure to obtain ultrafine-grained structure. Field emission scanning electron microscopy and high resolution transmission electron microscopy (HRTEM) were employed to investigate the microstructural evolution after severe cold-rolling. HRTEM images showed the typical deformed structure consisting of lamellar dislocation cell blocks. HRTEM study also revealed strain-induced reverse martensitic transformation (activated during grain refinement). It was assumed that severe plastic deformation route and related deformation mode were responsible for microstructural evolutions. X-ray diffraction (XRD) diagram revealed 7% (volume fraction) reverted austenite after final deformation pass. Moreover, HRTEM images revealed nano-void nucleation at the interface of severely deformed martensite and reverted austenite presumably due to high strain energy of misfit and molar volume difference between the austenite and the martensite. It seems that the coalescence of nano-voids could lead to the formation of microvoids in the microstructure.

Twinning-like lattice reorientation without a crystallographic twinning plane

Twinning on the plane is a common mode of plastic deformation for hexagonal-close-packed metals. Here we report, by monitoring the deformation of submicron-sized single-crystal magnesium compressed normal to its prismatic plane with transmission electron microscopy, the reorientation of the parent lattice to a ‘twin’ lattice, producing an orientational relationship akin to that of the conventional twinning, but without a crystallographic mirror plane, and giving plastic strain that is not simple shear. Aberration-corrected transmission electron microscopy observations reveal that the boundary between the parent lattice and the ‘twin’ lattice is composed predominantly of semicoherent basal/prismatic interfaces instead of the twinning plane. The migration of this boundary is dominated by the movement of these interfaces undergoing basal/prismatic transformation via local rearrangements of atoms. This newly discovered deformation mode by boundary motion mimics conventional deformation twinning but is distinct from the latter and, as such, broadens the known mechanisms of plasticity.

Cascading collapse of honeycomb columns under compression along the long axis

Axial compression tests were performed on columns with a low-density honeycomb structure made of gluing drinking straws together. Columns with slenderness ratios higher than about 7 failed by global buckling, and those with smaller slenderness ratios failed by a peculiar plastic deformation mode in which the low-density structure collapsed successively one layer following another with a rather constant periodicity, beginning from one end of the column. A theoretical model is proposed to describe the onset of the cascading collapse based on energetic considerations, and this explains the observed Young's modulus, periodicity of the collapse and the yield strength in a self-consistent way.

Synergy of grain boundary sliding and shear-coupled migration process in nanocrystalline materials

An attempt has been made to illustrate a new physical mode of plastic deformation in nanocrystalline materials that synergizes two processes, i.e. grain boundary (GB) sliding and stress-driven shear-coupled GB migration. The latter process incorporates a rotational and a translational plastic flow, in which a normal migration is coupled with a shear. The energy change resulting from the above-mentioned mode was calculated, and showed that the new deformation mode was much more energetically favorable than both the pure GB sliding mode and the cooperative process of GB sliding and migration without a coupling shear. In addition, the new deformation mode considerably enhances the ductility of nanocrystalline materials compared with the other two above-mentioned processes.

Effect of material inhomogeneity on the cyclic plastic deformation behavior at the microstructural level: micromechanics-based modeling of dual-phase steel

The microstructure of dual-phase (DP) steels typically consists of a soft ferrite matrix with dispersed islands of hard martensite phase. Due to the composite effect of ferrite and martensite, DP steels exhibit a unique combination of strain hardening, strength and ductility. A microstructure-based micromechanical modeling approach is adopted in this work to capture the tensile and cyclic plastic deformation behavior of DP steel. During tensile straining, strain incompatibility between the softer ferrite matrix and the harder martensite phase arises due to a difference in the flow characteristics of these two phases. Microstructural-level inhomogeneity serves as the initial imperfection, triggering strain incompatibility, strain partitioning and finally shear band localization during tensile straining. The local deformation in the ferrite phase is constrained by adjacent martensite islands, which locally results in stress triaxiality development in the ferrite phase. As the martensite distribution varies within the microstructure, the stress triaxiality also varies in a band within the microstructure. Inhomogeneous stress and strain distribution within the softer ferrite phase arises even during small tensile straining because of material inhomogeneity. The magnitude of cyclic plastic deformation within the softer ferrite phase also varies according to the stress distribution in the first-quarter cycle tensile loading. Accumulation of tensile/compressive plastic strain with number of cycles is noted in different locations within the ferrite phase during both symmetric stress and strain controlled cycling. The basic mode of cyclic plastic deformation in an inhomogeneous material is cyclic strain accumulation, i.e. ratcheting. Microstructural inhomogeneity results in cyclic strain accumulation in the aggregate DP material even in symmetric stress cycling.

Vortex matter in the type-II superconductor La3Ni2B2N3−x in the light of NMR

A quantitative analysis of spin-echo NMR measurements of the local field distribution and the spin–spin relaxation time T2 in the field-cooled superconducting state of the type-II superconductor La3Ni2B2N3−x shows evidence for two fluctuation modes of the weakly pinned vortex matter dominating T2 at different temperatures: long wavelength diffusive modes in the vortex solid at low temperature, and plastic deformation of the viscous liquid close to the critical temperature Tc, with a broad crossover region in between where a more complex theory is required. We determine the melting temperature Tm = 11.5 K of the vortex matter as the temperature where the characteristic time of the plastic deformation slows down to the time scale set by the spin-echo experiment. This is preferred to the conventional estimates from a Lindemann criterion for the melting vortex solid since the quantitative description of T2 near Tm by the viscous vortex liquid appears to be more consistent. It indicates that the plastic deformation mode might be important for T2 in the mixed state of other type-II superconductors in the weak pinning regime near Tc as well.

Deformation mechanisms in ferritic/martensitic steels and the impact on mechanical design

Structural steels for nuclear applications have undergone rapid development during the past few decades, thanks to a combination of trial-and-error, mechanism-based optimization, and multiscale modeling approaches. Deformation mechanisms are shown to be intimately related to mechanical design via dominant plastic deformation modes. Because mechanical design rules are mostly based on failure modes associated with plastic strain damage accumulation, we present here the fundamental deformation mechanisms for Ferritic/Martensitic (F/M) steels, and delineate their operational range of temperature and stress. The connection between deformation mechanisms, failure modes, and mechanical design is shown through application of design rules. A specific example is given for the alloy F82H utilized in the design of a Test Blanket Module (TBM) in the International Thermonuclear Experimental Reactor (ITER), where several constitutive equations are developed for design-related mechanical properties.

Nanoindentation on the doubler plane of KDP single crystal

The nanohardness is from 1.44 to 2.61 GPa, the Vickers hardness is from 127 to 252 Vickers, and elastic modulus is from 52 to 123 GPa by the nanoindentation experiments on the doubler plane of KDP crystal. An indentation size effect is observed on the doubler plane in the test as the nanohardness and elastic modulus decreases with the increase of the maximum load. Slippage is identified as the major mode of plastic deformation, and pop-in events are attributed to the initiation of slippage. And the variation of unloading curve end is the result of stick effects between the indenter and the contact surface. The depth of the elastic deformation, which is between 40 and 75 nm, is responsible for the elastic deformation. The doubler plane of KDP crystal has anisotropy, and the relative anisotropy of nanohardness is 8.2% and the relative anisotropy of elastic modulus is 8.0%.

The quasi-static mechanical properties of extruded binary Mg–Er alloys

The binary Mg–Er alloys with respective concentration of 2.0wt%Er, 4.0wt%Er and 6.0wt%Er were prepared via extruding and annealing, and their quasi-static mechanical properties were investigated. It was found that after complete recrystallization, the “Rear Earth-texture” (RE-texture) develops with Er incremental addition. Under both the tensile and compressive loadings, Mg–4.0wt%Er and Mg–6.0wt%Er alloys had flow stresses lower than that of Mg–2.0wt%Er alloy. However, their ductility was higher than that of Mg–2.0wt%Er alloy. The elongation and reduction of Mg–6.0wt%Er alloy reached to 29.4% and 42.3%, respectively, much higher than the usual elongation and reduction of pure magnesium. The dependence of the mechanical properties on Er addition could be interpreted by the RE-texture development and the reduction of the c/a axial ratio allowing for the activation of the additional plastic deformation modes.

Suppression of shear banding in amorphous ZrCuAl nanopillars by irradiation

Using molecular dynamics simulations, model Zr50Cu40Al10 metallic glass (MG) nanopillars were subjected to simulated irradiation processes followed by uniaxial compression tests. As the intensity or dosage of irradiation increases, the plastic deformation mode of the MG nanopillars transits from localized shear banding to homogeneous shear flow. The suppression of shear banding in MG nanopillars is due to irradiation-induced structural disordering. Furthermore, a correlation is found between the average potential energy of MG nanopillars and their deformation modes, common to both irradiation processing and thermal processing. Our results imply that the homogeneous shear flow observed in experimental MG nanopillars carved by focused ion beam may be due to irradiation damage instead of size effect.