Deformation Mechanisms In Metals Research Articles

Twin Boundary Superstructures Assembled by Periodic Segregation of Solute Atoms.

Twinning is a common deformation mechanism in metals, and twin boundary (TB) segregation of impurities/solutes plays an important role in the performances of alloys such as thermostability, mobility, and even strengthening. The occurrence of such segregation phenomena is generally believed as a one-layer coverage of solutes alternately distributed at extension/compression sites, in an orderly, continuous manner. However, in the Mn-free and Mn-containing Mg-Nd model systems, we reported unexpected three- and five-layered discontinuous segregation patterns of the coherent {101̅1} TBs, and not all the extension sites occupied by solutes larger in size than Mg, and even some larger sized solutes taking the compression sites. Nd/Mn solutes selectively segregate at substitutional sites and thus to generate two new types of ordered two-dimensional TB superstructures or complexions. These findings refresh the understanding of solute segregation in the perfect coherent TBs and provide a meaningful theoretical guidance for designing materials via targeted TB segregation.

Nucleation of Twinning Dislocation Loop in Pt: A Computational Approach

Twinning plays a critical part in the plastic deformation of the materials and the strengthening mechanisms and is hence considered as one of the most prevalent deformation mechanisms in metals. Because to the high stacking fault energy of the fcc metals like Al, Pd, and Pt, the extended dislocations are believed to be energetically favored over isolated partials, thereby rendering deformation twinning unfeasible. Nevertheless, some recent experimental researches have confirmed a potential deformation twinning pathway in nanocrystalline platinum. This alternate-shear mechanism has a much lower energy barrier than the usual layer-by-layer twinning. We utilize computations involving atomistic calculations and continuum modeling in this study to examine the genesis of deformation twins in Pt. Atomistic simulations provides the generalized planer fault energy using an EAM (embedded-atom-model) potential. Moreover, a potential energy-based method, namely; a nudged-elastic band (chain of states) has been used to compute the activation energy barrier for the nucleation of the twinning dislocation loop in the alternate-shear model. The critical stress needed for the nucleation of the twinning dislocation loop in platinum is estimated using some of the parameters acquired from atomistic calculations and using them as fitting parameters in the continuum model. The minimum-energy path between the two end states can be identified using this methodology. Through the unusual alternate-shear approach, the results provide a rudimentary but essential dislocationbased perspective of the occurrence of deformation twins in fcc metals.

The Effect of Electropulsing Treatment on Properties of Pre-compressed AZ31 Magnesium Alloy

The effect of electropulsing treatment (EPT) on quasi-static compression behavior and anisotropy of AZ31 Mg alloy were investigated based on quasi-static compression test. The results show that the orientation of the sample has a significant effect on the deformation mechanism of metal and this mechanism can be changed by pre-deformation and EPT. However, the strain rate sensitivity of the material is not affected by pre-deformation. Compared with the as-received plates, the anisotropy of AZ31 Mg alloy increased in ND-RD plane after pre-compression along the transverse direction, the ∆YS of specimens increased from 87 MPa to 98 MPa. After EPT, the anisotropy of AZ31 Mg alloy gradually decreased with the change of EPT temperature and EPT time, the ∆YS of the sample reached 14.2 MPa when EPT conducted at 200 ℃ for 15 min, which decreased 72.8 MPa compared with the as-received plates.

Improved embedded-atom model potentials of Pb at high pressure: application to investigations of plasticity and phase transition under extreme conditions

Local stress relaxation mechanisms of crystals are a long-standing interest in the field of materials physics. Constantly encountered inelastic deformation mechanisms in metals under dynamic loadings, such as dislocation, deformation twinning and phase transition, have been extensively discussed separately or as some of their combinations. Recently, virtual melting is found to be a dominant local stress relaxation mechanism under extreme strain rates. However, these deformation mechanisms have never been investigated in the same metal at an atomic level due to the lack of high pressure interatomic potentials. In this work, an embedded-atom model potential of Pb is developed and tested for high pressure applications. The developed potential of Pb could not only reproduce many energetic, elastic and defective properties at ambient conditions well, but also correctly describe face-centered cubic (fcc)-hexagonal close packed (hcp) and hcp-body-centered cubic phase transition of Pb under high pressures. Shock Hugoniot, as well as equation of states for fcc and hcp phase, also agrees well with the literature ones up to more than 100 GPa. With the developed potential, non-equilibrium molecular dynamic simulations are conducted to investigate dynamic behaviors of Pb single crystal under ramp-shock compressions. Depending on applied strain rates, dislocation-mediated plasticity, phase transition and virtual melting, constantly reported by experiments or theoretical investigations, are observed in our results. Additionally, a new phase transition mechanism of Pb subjected to the ramp compressions is uncovered.

High-velocity projectile impact induced 9R phase in ultrafine-grained aluminium

Aluminium typically deforms via full dislocations due to its high stacking fault energy. Twinning in aluminium, although difficult, may occur at low temperature and high strain rate. However, the 9R phase rarely occurs in aluminium simply because of its giant stacking fault energy. Here, by using a laser-induced projectile impact testing technique, we discover a deformation-induced 9R phase with tens of nm in width in ultrafine-grained aluminium with an average grain size of 140 nm, as confirmed by extensive post-impact microscopy analyses. The stability of the 9R phase is related to the existence of sessile Frank loops. Molecular dynamics simulations reveal the formation mechanisms of the 9R phase in aluminium. This study sheds lights on a deformation mechanism in metals with high stacking fault energies.

First-principles prediction of the stacking fault energy of gold at finite temperature

The intrinsic stacking fault energy (ISFE) γ is a material parameter fundamental to the discussion of plastic deformation mechanisms in metals. Here, we scrutinize the temperature dependence of the ISFE of Au through accurate first-principles derived Helmholtz free energies employing both the super cell approach and the axial Ising model (AIM). A significant decrease of the ISFE with temperature, −(36–39) % from 0 to 890 K depending on the treatment of thermal expansion, is revealed, which matches the estimate based on the experimental temperature coefficient dγ/dT closely. We make evident that this decrease predominantly originates from the excess vibrational entropy at the stacking fault layer, although the contribution arising from the static lattice expansion compensates it by approximately 60%. Electronic excitations are found to be of minor importance for the ISFE change with temperature. We show that the Debye model in combination with the AIM captures the correct sign but significantly underestimates the magnitude of the vibrational contribution to γ(T). The hexagonal close-packed (hcp) and double hcp structures are established as metastable phases of Au. Our results demonstrate that quantitative agreement with experiments can be obtained if all relevant temperature-induced excitations are considered in first-principles modeling and that the temperature dependence of the ISFE is substantial enough to be taken into account in crystal plasticity modeling.

Gradient Nano-Grained Cu and Cu-Zn Alloys Processed by Surface Mechanical Attrition Treatment

Cu and Cu-30wt.%Zn alloys with stacking fault energies (SFEs) of 78 mJ/m2 and 14 mJ/m2 were processed by surface mechanical attrition treatment (SMAT) at room temperature and liquid nitrogen (LN) temperature, respectively. The effect of SFE and deformation temperature on tensile properties of these samples was investigated. The tensile testing results indicated that the yield strength and uniform elongation of these samples enhanced simultaneously with decreasing SFE. Meanwhile, the LN-SMAT processed samples exhibited remarkably higher strength and slightly lower ductility compared to those processed at room temperature. The SFE affected the deformation mechanisms of metals greatly. X-ray diffraction (XRD) measurements indicated that the twin density increased while the average grain size decreased with SFE decreasing, and twinning became the dominant deformation mechanism. The relationship between microstructure and mechanical property is also discussed.

Molecular dynamics simulation of effect of grain on mechanical properties of nano-polycrystal -Fe

The nanocrystalline metals are widely investigated due to their unique mechanical properties. Currently, the available studies about deformation mechanisms of metals mainly focus on face-centered cubic metals such as Ni, Cu and Au. However, the body-centered cubic metals are still very limited, despite their industrial importance. Here, we investigate the effects of grain size and temperature on the mechanical behavior of nano-polycrystal -Fe under uniaxial tensile loading by using molecular dynamics (MD) simulation. The models of nanocrystalline -Fe with the grain sizes of 3.95, 6.80, 9.70, 12.50, 15.50, 17.50, 20.70 and 26.00 nm are geometrically created in three dimensions by using Voronoi construction, and these models are relaxed to reach an equilibrium state. Then, each of them has a strain of 0.001 along the Z-direction in each step, keeping zero pressure in the X- and Y-directions until the strain increases up to 0.2. A 1.0 fs time step is used in all of the MD simulations. Based on the data output, the stress-strain curves at different grain sizes are obtained. The results indicate that the peak stresses of nano-polycrystal -Fe decrease with the decrease of grain size, exhibiting a breakdown in the Hall-Petch relation when the grain size is smaller than a critical size. The major deformation mechanism is found to change from dislocation slips and twinning-mediated plasticity in a model with a larger grain size to grain boundary sliding in a model with a smaller grain size. It should be noted that twinning is formed by the emission of 1/6111 partial dislocations along the {112} slip plane. The results show that crack formation during tension is a cause of reducing the flow stress of nano-polycrystal -Fe with a large grain size and that the Young's modulus of nano-polycrystal -Fe decreases with the grain size decreasing. The main reason for the crack nucleation is here that grain boundaries perpendicular to the loading direction bear higher stress and the twin band interacts with grain boundaries at a larger grain size, causing the stress to concentrate at the intersections of grain boundaries. The results also show the detwinning behavior and migration of deformed twins in nano-polycrystal -Fe. The detwinning behavior occurs via the migration of the intersection of grain boundary and twin, and this intersection is incoherent boundary. The migration of deformed twins proceeds by repeating initiation and glide of 1/6111 partial dislocations on adjacent {112} planes. In addition, we find that the nucleation and propagation of dislocation become easier at higher temperature than at lower temperature.

Dislocation-kinetic approach to the analysis of deformation mechanisms of metals and alloys

This paper presents the results of the development and application of a dislocation–kinetic approach to the analysis of strength and ductility in metals and alloys. The influence of the temperature–strain rate conditions of deformation on the character of dislocation processes, taking place in some pure metals and alloys, has been investigated here as well. Special attention has been given to the analysis of the effect of grain boundaries, twins, impurity atoms and a non–equilibrium state of the grain boundaries on the activation of one or another deformation mechanisms in bulk ultrafine–grained metallic materials, obtained by the severe plastic deformation method.

Structural refinement and deformation mechanisms in nanostructured metals

Deformation mechanisms in metals deformed to ultrahigh strains are analyzed based on a general pattern of grain subdivision down to structural scales ∼10 nm. The materials analyzed are medium- to high-stacking fault energy face-centered cubic and body-centered cubic metals with different loading conditions. The analysis points to dislocation glide as the dominant deformation mechanism at different length scales supplemented by a limited amount of twinning at the finest scales. With decreasing deformation temperature and increasing strain rate, the contribution of twinning increases.

Internal state variable models for microstructure in high temperature deformation of titanium alloys

There exists an interaction between microstructural evolution and deformation behavior in high temperature deformation of titanium alloys. And the microstructure of titanium alloys is very sensitive to the process parameters of plastic deformation process. In this paper, on the basis of plastic deformation mechanism of metals and alloys, a microstructural model including dislocation density rate equation and grain growth rate equation is established with the dislocation density rate being an internal state variable. Applying the model to the high temperature deformation process of Ti60 titanium alloy, the average relative errors of grain sizes between the experiments and the predictions are 9.47% for sampled data, and 13.01% for non-sampled data.

A set of unified constitutive equations for modelling microstructure evolution in hot deformation

Recrystallisation and recovery are two competing processes. Metals showing high degrees of dynamic recovery are unlikely to recrystallise dynamically, since it is difficult to accumulate the dislocation density to a certain level to initiate recrystallisation. Recrystallisation and recovery may continue after hot deformation, such as during passes in multipass hot rolling processes. Both processes reduce dislocation density of materials and allow large plastic deformation to be achieved. The main aim of this research is to develop a set of mechanism-based unified viscoplastic constitutive equations, which models the evolution of dislocation density, recrystallisation and grain size during and after hot plastic deformation. Deformation mechanisms of metals at high temperatures and techniques of modelling the physical effects are analysed. The effects of dislocation density and recrystallisation on viscoplastic flow and grain size evolution of a micro-alloyed steel are rationalised. Optimisation techniques and procedures are developed to determine the unified viscoplastic constitutive equations from experimental data.

High-Temperature Deformation Mechanisms in Metals and Alloys

High-Temperature Deformation Mechanisms in Metals and Alloys

金属材料の高温変形機構

金属材料の高温変形機構