Occurrence Of Plastic Instability Research Articles

Effect of solute clusters on plastic instability in magnesium alloys

The interaction between solute atoms and matrix dislocations during plastic deformation of materials strongly influences the flow behavior and can lead to plastic instability at the macroscopic scale. In this study, Lüders band formation and dynamic strain aging phenomena were investigated in a Mg–1%Mn-1%Nd (wt.%) alloy, deformed in uniaxial tension at ambient temperature and 150 °C. The investigated alloy was studied in the initial as-extruded state and after a subsequent heat treatment at 275 °C for 120 min, which allowed for a variation of solute concentration in the microstructure, and thus for a different type/level of solute – dislocation interaction. Microstructure investigations at near-atomic resolution using three dimensional atom probe tomography were carried out in order to visualize the spatial distribution of alloying elements and reveal potential solute clustering trends in connection with the occurrence of plastic instability. Identified nanoscale solute clusters were analyzed on the basis of their size distribution, chemical composition, and statistical significance. Results demonstrate that the number, as well as the types of solute clusters, play an essential role in triggering plastic instability by interacting with deformation carriers at the atomic scale.

High Temperature Plastic Instability and Dynamic Strain Aging in the Tensile Behavior of AISI 316 Stainless Steel

The plastic instability in the stress-strain curve and work hardening of a type AISI 316 austenitic stainless steel was investigated in the high temperature range of 150°C to 800°C for two strain rates. The results indicate the occurrence of plastic instability and serrations between 200°C and 650°C with peaks in both the tensile strength and work hardening rate. These plastic instability/serrations are associated with dynamic strain aging behavior. As a consequence, within the temperature interval of plastic instability, a negative value for the strain rate sensitivity was found. Based on the activation energy, it is proposed that a mechanism of substitutional solute atoms interaction with dislocations be responsible for the plastic instability.

Particle stabilization of plastic flow in nanostructured Al-1 %Si Alloy

A nanostructured Al-1 %Si alloy containing a dispersion of Si particles in ultrapure aluminum (99.9996 %) was produced by heavy cold rolling to study the effect of second-phase particles on the occurrence of plastic instability during tensile testing of a nanostructured metal. Tensile tests were conducted on the as-deformed sample and on samples after recovery annealing treatments. Work hardening and strain rate sensitivity were studied by tensile test at different strain rates, and deformed and annealed samples were characterized by transmission electron microscopy. By comparing the observed behavior with that of nanostructured commercial purity Al, a significant increase in the tensile elongation was found. This is related to a homogenization of the deformation microstructure due to the presence of finely dispersed Si particles. As a result localized deformation is reduced and tensile ductility increased.

Kinematics of Portevin–Le Chatelier bands in simple shear

The mechanical behavior of 5000 series aluminium alloy sheets has been investigated in simple shear, in order to highlight the influence of strain rate and temperature on the occurrence of plastic instabilities. Indeed, as in tension, Portevin–Le Châtelier effect is also observed in simple shear and the aim of this study is to investigate the characteristics and kinetics of the bands of localized deformation under such a strain/stress state. Jerky flow and localized deformation are observed from room temperature up to 100°C but disappear at 150°C or above. A critical shear strain for the occurrence of jerky flow is evidenced at 100°C but it starts from the beginning of the test at room temperature. Concerning the bands, there are initially aligned with the applied load direction and rotate as the shear strain increases, similarly to the material frame. Finally, both McCormick and Johnson–Cook models were used to perform the numerical simulation of the shear tests, in order to compare the predicted load drops and kinetics of the bands with experiments.

Dose Dependence of Strength After Low-Temperature Irradiation in Metallic Materials

This study intends to review and characterize the low-temperature (<473 K [200 °C]) irradiation-hardening behaviors in metallic materials and to propose new interpretations on the dose dependence of strength, particularly in the prehardening and saturation regimes. The analysis of results of yield stress-dose curves indicate that four dose-dependence regimes exist: the prehardening, main hardening, saturation, and embrittlement regimes. The semilog plots of yield stress vs dose data revealed that the prehardening regime displaying zero hardening or softening was common at least for the alloys with low-dose data available. It was observed that the dose range of the prehardening regime increased with the strength of material, which indicates that slower initiation in irradiation hardening is expected when strength is higher. For the majority of the metallic materials analyzed, it was reconfirmed that the exponent of the power-law hardening function was evaluated to be about 0.5 in the main hardening regime and about 0.1 in the saturation regime. In these positive hardening regimes, the low strength pure metals such as Fe, Ta, Cu, and Zr displayed lower hardening exponents. The minimum dose to the saturation of irradiation hardening was in the range of 0.003 to 0.08 dpa, depending on the category of materials. It was also reaffirmed that there exists a strong relationship between the saturation in irradiation hardening and the occurrence of plastic instability at yield.

Evaluation of the Work-Hardening of Brass Sheets Following Strain Path Changes

The strain paths followed by metals during sheet forming can be quite complex, especially when successive forming steps are involved. The work hardening of metals associated with these strain paths differs from that caused only by monotonic straining, such as simple tension or compression. It is important to have an adequate description of the work hardening of the material under processing, especially when numerical simulations of the forming are used. The experimental evaluation of the effect of strain path changes on the material work hardening is usually performed through tensile testing following the strain path changes. This technique, however, demands complex machining operations of the formed sheets and the imposed strain is severely limited by impending necking. The present paper utilizes simple shear as a tool for the determination of the work hardening of CuZn34 brass sheets following various strain path changes associated with combinations of different modes of deformation such as rolling, tension, cyclic and forward shears. The results indicate that the cyclic shearing delays the occurrence of plastic instabilities for brass previously tensioned, occurring the opposite for final monotonic shearing. These phenomena were correlated with the probable microstructural evolution of the CuZn34 brass.

Trichroic infrared analysis of the strain-induced structural changes in the PA6 layer of PA6/PE multilayer films under biaxial drawing

This work deals with the structural evolution under biaxial drawing of PA6/tie/PE multilayer films as a function of composition. The occurrence of plastic instabilities in the PA6 layer in the multilayers is strongly reduced with respect to the PA6 monolayer film and the maximum biaxial drawability of PA6 is considerably improved with increasing PE layer thickness. Quantitative evaluation by trichroic infrared analysis of the various crystal forms of the PA6 component shows that the strain-induced β–α crystalline phase transition is delayed in the multilayers, i.e. equivalent level of phase transformation is reached at higher strains. Similar phenomenon is observed regarding the orientation within the film plane of the PA6 chains in the strain-induced α-crystals, i.e. equivalent chain orientation is reached at higher strains in the multilayers. The deformation synergism between the PA6 and PE layers is discussed in terms of benefiting effect from the PE layer that prevents plastic instabilities and promotes homogeneous deformation accompanied with orthotropic extension of the chains in the PA6 layer. This is the driving force to the delay of the detrimental β–α phase transition of PA6, the mechanically anisotropic α-crystals being highly crack-prone in contrast to the ductile isotropic β-crystals. Film rupture is shown to occur when the PA6 strain-induced α-crystals reach a constant content of about 10%, irrespective of film composition and draw temperature in the range 80–110°C. This kind of critical phenomenon is assigned to a percolation process of the crack-prone α-crystals.

Investigations on serrated flow in 8090 Al alloy and 8090 Al–SiC p composites occurring during dynamic ultra low load micro hardness (DUH) indentation

Occurrence of plastic instabilities during depth sensing indentation in 8090 Al alloy and 8090 Al–SiC p composites containing 8 and 15 vol.% of SiC particles have been investigated by dynamic ultra low load micro hardness (DUH) technique. The unreinforced alloy shows instabilities (serrations) in the load–displacement curve in the as-extruded (AE) and solution-treated conditions. However, the magnitude of serrations becomes negligible after ageing. Al–Li–8% SiC composite shows signs of serrations in the solution-treated condition. On the other hand, Al–Li–15% SiC composite shows almost no or negligible serrated flow in all the conditions (as-extruded, solution-treated, and aged).

Plastic deformation of yttria stabilized cubic zirconia single crystals I. Activation parameters of deformation

Cubic zirconia containing 10 mol% yttria was deformed along the hard 〈100〉 direction and materials with 15 and 20 mol% yttria along the soft 〈112〉 orientation between 1400 °C and the lowest possible temperatures without brittle failure. The 10% material shows along 〈100〉 a strong increase in the flow stress at decreasing temperature, while the materials with 15 and 20 mol% yttria along 〈112〉 exhibit a higher flow stress with a weak temperature dependence. The activation parameters of deformation were measured by stress relaxation and temperature change tests. The microstructures are studied by high-voltage transmission electron microscopy. The results are interpreted by long-range interactions between the dislocations, the Peierls mechanism at the lowest temperatures, precipitation hardening between 500 °C and 800 °C, and by recovery-controlled deformation at high temperatures. The occurrence of plastic instabilities are described in Part II [phys. stat. sol. (a) 201, No. 1 (2004)] of this paper. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Occurrence of plastic instabilities in dynamic microhardness testing

Plastic instabilities were observed to appear during dynamic ultramicrohardness testing of a solid solution Al–3.3 wt.% Mg alloy. The tests were carried out at room temperature with a Vickers hardness indenter in a computer-controlled dynamic ultramicrohardness testing machine. During the tests the applied load was increased from 0 to 2000 mN at constant loading rate. The instabilities appear as characteristic steps in the continuously recorded load-indentation depth curves. The physical basis for the occurrence of the instabilities is the interaction between moving dislocations and solute atoms, a phenomenon termed in the literature as serrated yielding, jerky flow, or Portevin-Le Châtelier effect. The instabilities start at a critical load, Fc, in the depth-load curve. Varying the loading rate, μ, by two orders of magnitude Fc was found to increase linearly with the loading rate.

Spatio-temporal aspects of low-temperature thermomechanical instabilities: A model based on dislocation dynamics

The occurrence of plastic instabilities which are accompanied by a significant heat release is a typical feature of the plastic behaviour of metals deformed at sufficiently low temperature. This phenomenon may be studied within the framework of a dislocation-dynamical model. The influence of the heat which is released by the deformation process on the dislocation velocity, and thus on the deformation dynamics, is taken into account. In particular, the influence of the spatial coupling which arises from heat conduction on the spatio-temporal behaviour of the deformation process is studied.

Evolution of dislocation substructures during monotonic deformation of low-carbon steel

It has long been established that in many metals, dislocation substructures (cell structures) develop in the grains during plastic deformation. More recently, it has been shown that, in low carbon steel, the dislocation substructures are closely related to active slip planes, i.e. to the grain orientation and loading conditions. Such a correlation gives rise to an effect of the strain history. This correlation may have an influence on the plastic behavior of prestrained samples and on their plastic stability. In fact, depending on the amplitude of the change of loading conditions, the subsequent stress can be deduced by the use of a generalized latent hardening concept accounting qualitatively for the substructures. On the other hand, during the reloading, some microbands may develop in the grains ; and the appearance of the microbands may be connected with the early occurrence of plastic instability. It is not known however, whether or not the dislocation substructure has an inherent importance, that is whether a heterogeneous distribution of dislocations is required to induce these features. Up to now, this point has not yet been made clear since the models used have only assumed changes in the critical shear stress values or qualitatively described the occurrence of ‘avalanche’ processes. The role of the formation and presence of such substructures on the subsequent behavior of materials requires better understanding.