International Journal of Plasticity最新文献

Repetitive deep cryogenic soaking treatment (DCT) of laser metal deposition (LMD) processed CrMnFeCoNi high entropy alloy (HEA) significantly enhances its strength without compromising ductility. This is attributed to the compressive stress induced nanotwin formation, which in turn facilitates twin induced plasticity. In this work, a parametric study on the effect of the residual stress profile and the DCT cycles on the tensile properties of the HEA, along the build and scan directions is conducted. Towards this end, builds fabricated with 5 different laser powers, 1100, 1400, 1700, 2000 and 2300 W, are examined and the ones with highest and lowest residual stress gradient are considered for further DCT treatments. Results indicate that the build fabricated with 1400 W laser power, which has the highest gradient in initial residual stresses, exhibits a greater enhancement in dislocation and twin density with increasing number of DCT treatments. Compared to its as-built state the peak yield and tensile strength of the HEA (along the scanning direction) increases to 592 ± 13 and 778 ± 15 MPa, without significant decrease in its ductility after 12 DCT cycles. However, the enhancement in the dislocation, twin density and therefore, the strength, is minimal after it is treated to 15 DCT cycles. Anisotropy in both strength and ductility, which is of the order of 20-25 %, is also observed in the DCT treated builds along the build and scan directions. These observations were rationalized on the basis of dislocation and twin evolution and distribution during DCT and deformation of the build when deformed in different directions. Implications of these results in the context of employing DCT for strengthening LMD fabricated HEA components are discussed.

As deformation twins have a profound impact on the plastic flow and mechanical properties of metallic materials, enhancing deformation twinning in face-centered cubic (FCC) metallic materials has long served as a unique microstructure design strategy to attain an extraordinary strength-ductility synergy. Deformation twinning, however, rarely occurs in pure FCC Al and its alloys since its generalized planar fault energies (GPFEs) are almost unaffected by most soluble alloying elements such as Mg, Zn and Cu. Here we successfully tune the GPFEs of a nanocrystalline Al-Mg alloy by alloying with Zr, Fe or Y element, and enable deformation twinning in the Zr-, Fe- and Y-containing alloys. Based on a combined analysis of microscopic observations, modeling and ab initio calculations, we find a strong grain-size-dependent twinning (i.e., twinning occurs in preferable grains having sizes in the range ∼20–40 nm), as well as only one single twinning plane (i.e., twinning occurs in single, parallel atomic planes) for twin formation rather than intersecting twinning planes (i.e., twinning occurs in multiple, unparallel atomic planes) usually observed in coarse-grained FCC materials. This interesting twinning behavior is further observed to be accompanied by grain rotations, producing defective twin boundaries. Our experimental results extend the current understanding of the plastic deformation mechanisms in nanograined metallic materials, and will guide microstructure design of twinnable nanograined Al alloys with an improved strength-ductility synergy.

In this study, an innovative tri-functional co-nanoprecipitation strategy was employed to enhance the mechanical properties of a low stacking-fault energy (SFE) 17Mn steel for cryogenic applications. By combining severe cold deformation and subsequent annealing, a hierarchical structure emerged, featuring (Ti, Nb)C carbide (∼10 nm) and Cu-rich intermetallic (∼2 nm) in the austenitic matrix with heterogeneous grain size distributions. The co-precipitation (CP) sample exhibited superior performance compared to single-precipitation (SP) steel, with a yield strength of ∼1150 MPa, tensile elongation of ∼44.8 %, and an impact toughness of ∼110 J at liquid nitrogen temperature (LNT), even surpassing the base-17Mn steel. The CP-17Mn samples displayed a higher density and thinner nano-twins at larger strains, leading to a rapid increase in geometrically necessary dislocations (GNDs). The detrimental martensitic transformation was effectively suppressed during both tensile and impact tests. The observed inverse strength-ductility and strength-toughness trade-off can be attributed to the tri-functional co-precipitates’ roles: they provide disperse strengthening, induce structural heterogeneity, and act as effective barriers for twin thickening. The large-sized (Ti, Nb)C carbides facilitate grain refinement and pin boundary migration, while the smaller Cu-rich intermetallic inhibits the growth and thickening of nano-twins, preventing further dislocation movement due to their strong stress fields at the twin-precipitate interactions. This novel mechanism paves the way for developing higher-performance steels with fine and dense nano-twins at cryogenic conditions.

The Type-II Twin Boundary (TB) is a critical interface in functional materials whose irrational Miller-index identity has recently drawn significant research interest. This study establishes general structural characteristics of the Type-II twin interface, utilizing TBs in Shape Memory Alloys (SMAs) - TiPd, TiPt, and AuCd - as study targets. It is shown how the irrational identity of each TB is explained by the Terrace-Disconnection (T-D) structural topology. It is proposed that the terrace is the rational-plane nearest to the irrational TB in the reciprocal space, having integral Miller-indices of least magnitude. Crystallographic-registry on this terrace requires non-trivial coherence-strains. A novel kinematic-origin of the coherence-strain is proposed, coming directly from a transformation of the classical twinning deformation-gradient. This transformation revealed that the classical twinning-shear partitions into the coherence-strain and a new metric termed the “terrace-shear”. It is shown that the magnitude of shear relating the twin-structure to the matrix is the terrace-shear and not the twinning-shear, contrary to classical understanding. Furthermore, the Burgers vector of the twinning disconnection is shown to be related directly to the terrace-shear. The energy of each Type-II interface is determined from ab initio Density Functional Theory (DFT) calculations. It is shown that the energy-minimal atomic-structure on the terrace requires determination of a “lattice-offset” that is non-trivial and unknown apriori. In summary, this study expounds on T-D topological structure of Type-II twin interfaces, establishing methods to identify rational terraces, coherence strains, ab initio planar TB energies and revealing a unique partitioning of the twinning-shear exhibited by this class of interfaces.

In this work, a multiscale constitutive model is established to describe the deformation behaviors of magnesium-shape memory alloy (Mg-SMA) composite in a wide temperature range and reveal the strengthening mechanism of SMA reinforcement on Mg. The model is established at the grain scale firstly and gradually transited to the macroscopic scale by employing a newly developed three-level scale transition rule. At the grain scale, the thermodynamic-consistent constitutive models of Mg and SMA are, respectively, constructed by addressing different inelastic deformation mechanisms. The basal, prismatic, pyramidal, slip systems and extension twinning system are considered for the Mg phase, and the martensite transformation (MT) and austenitic plasticity are addressed for SMA reinforcement. Thermodynamic driving forces of each inelastic deformation mechanism are derived from the dissipative inequality and the constructed Gibbs free energies. At the polycrystalline scale, to evaluate the interactions among the grains and pores, and obtain the whole responses of the polycrystalline Mg and SMA, a thermo-mechanically coupled self-consistent homogenization scheme is employed. At the mesoscopic scale, a modified thermo-mechanically coupled Mori-Tanaka's homogenization scheme is adopted to evaluate the interaction between the Mg phase and SMA phase, and predict the whole responses for the representative volume element (RVE) of the composite. According to the geometrical features and mechanical loadings applied on the specimen, a hypothesis of homogeneous stress and strain fields at the macroscopic scale is adopted to achieve the scale transition from the RVE of the composite to the whole specimen. The capacity of the multiscale model is verified by comparing the predictions with the existing experimental data (Aydogmus, 2015). Moreover, the influences of characteristic information for the microstructures at different spatial scales on the deformation behaviors of the composite are predicted and discussed.

Strain gradients have been cast in the form of geometrically-necessary dislocations (GND) to relate the length-scale dependence of strength and to determine potential sites for failure initiation. The literature contains various different incompatibility measures, the main ones being: the total form ($\nabla \times F_p$), the rate form for large displacements ($\nabla \times {\dot{\gamma }}^a{n}^a{F}_p$), and the slip gradient form ($\nabla {\dot{\gamma }}^a$). Here, these different approaches are compared rigorously for the first time. Obtaining GND densities when using the total form is a rank-deficit linear problem, solved by singular value decomposition (SVD) known as the Least Squares Minimization (L2 method). Alternative methods for finding GND densities such as Karush–Kuhn–Tucker (KKT) optimization are also investigated. Both L2 and KKT methods predict unrealistic GND densities on inactive slip systems leading to excessive strain hardening; even for a single crystal single slip case. Therefore, the restriction of GNDs to the active slip systems by using a threshold based on the total slip is proposed. This restriction reveals relatively consistent results for various single crystal single slip cases including: simple shear, uniaxial tension, and four-point bending. In addition, the small numerical differences in the slip leads to large discrepancies in the flow stress due to error accumulation, even for strain-gradient-free uniaxial tension, hence a threshold for the GND density increment ($2\times 10^2$ m⁻²) is used in all models to avoid formation of erroneous GND densities. Finally, the proposed method is applied to the evolution of the GND density for a grain inside a polycrystal aggregate that posses a complex stress state. The lowest incompatibility error is obtained by both of the total forms that use the curl of the plastic deformation gradient with the active slip system restriction suggesting them to be the most reliable GND measures.

Annealing softening is commonly observed in traditional coarse–grained materials. Herein, an annealing–induced hardening mechanism in selective laser melted 316L stainless steel (SLM–ed 316L SS) was investigated. The SLM–ed 316L SS, without prior cold–working history, displayed evident hardening behaviour as the annealing temperature increased from 400 °C to 500 °C. Several dedicated scanning transmission electron microscope and quasi–in–situ/electron backscatter diffraction techniques were employed to investigate the intrinsic characteristics evolution of the samples, including cellular/wall dislocation structure, nano–particles/segregation, dislocation density, crystallographic orientations, and low–angle grain boundaries (LAGBs).This phenomenon primarily arises from unique guardrail–like dislocation walls decorated with nano–particles (O, Cr, Mo, and Si) and a high proportion of LAGBs, hindering movement of dislocations and leading to their accumulation. Furthermore, this structure and the stable configuration of columnar crystals can synergistically affect the 500 °C annealed sample, resulting in a high yield stress of 628 MPa. On the other hand, complex deformation substructures, such as stacking faults, Lomer–Cottrell locks, and forest dislocations, also proliferated during deformation. These substructures enabled multiscale plastic strain partitioning, intensifying strain hardening and realizing a strength–ductility combination of a comparable yield/ultimate tensile strength of 628 MPa/789 MPa and tensile ductility of 32%. Dislocation motion was the dominant deformation mechanism based on the strengthening mechanism model in this study.

Featured with various excellent mechanical properties, the high strength Al-Zn-Mg-Cu alloys (7xxx series) have become one of the most widely-used metal materials. During the hot processing of 7xxx alloys, the high stacking fault energy and alloying element concentration can lead to the simultaneous occurrence of several physical mechanisms including the dynamic recovery (DRV), dynamic recrystallization (DRX), dynamic precipitation (DPN), and their interactions. Such complex microstructural mechanisms are difficult to be characterized and modeled in both of the experimental and theoretical aspects. In present work, aiming at 7055 alloy subjected to uniaxial hot compression, the systematic experiment investigation is first conducted to reveal hot deformation behaviors. Based on experimental analysis, a unified crystal plastic (CP) model, incorporating the DRX and DPN, is proposed by combining the viscoplastic self-consistent (VPSC) framework. In the unified model, the DRX and DPN behaviors are described by a physical based continuous dynamic recrystallization (CDRX) model and the classical Kampmann-Wagner numerical (KWN) model, respectively, and the effect of DPN on CDRX is specially modeled. After identifying the model parameters, the unified model can reasonably capture the essential features of DPN, DRX, and mechanical response under different deformation temperatures. Therefore, this study offers an innovative modeling approach for the dynamic evolution of several physical processes, facilitating the further understanding for the hot deformation behaviors of aluminum alloys.

Grain refinement is an important mechanism to produce stronger alloys. Strain hardening is an essential phenomenon in metal forming processes. The interaction between grain size and strain hardening is evident: a decrease in grain size $\left(d_g\right)$causes an increase in ultimate tensile strength but a decrease in uniform elongation. The Kocks-Mecking (KM) model for strain hardening is based on the relationship between shear strain and the path length for dislocation slip. It provides good general estimates for stress-strain curves, and empirical modifications have been made to include $d_g$. Here, the empirical approach is substituted by theoretical probability calculations, accounting for the fact that the grain size imposes a bound on the mean slip distance, while strain compatibility defines a relationship between grain boundary-dislocation interaction and bulk storage and annihilation. The resulting differential only uses the two parameters inherent to KM. Fitting to published tensile curves for Al, Cu, and Ni produces excellent results. The fitting parameters allow to predict the tensile strength as a function of $d_g$to good approximation, for $d_g>1\mu m$. Below this limit, fundamental changes in dislocation statistics impose the activation of grain boundary dislocation sources and may induce dislocation density gradients, which seem to determine the flow stress in the sub-$\mu m$ range.

The transition from Hall-Petch (HP) to inverse Hall-Petch (IHP) behaviors associated with grain size reduction has been recognized for over two decades. However, the underlying mechanisms for such transition in high entropy alloys (HEAs) under dynamic loading, in which abundant deformation mechanisms could be activated either sequentially or simultaneously, remain unclear. Here, we investigate the HP to IHP transition in nanocrystalline CoCrFeMnNi HEAs under shock loading by examining their deformation mechanisms and flow stresses using large-scale molecular dynamics (MD) simulations. It is found that this transition is strongly dependent on the shock pressure as a result of the complex interplay among multiple competing deformation mechanisms, including the hardening mechanisms such as dislocations interactions and grain boundary (GB) blocking, as well as the softening mechanisms like phase formation, amorphization, GB thickening, and grain rotation. Moreover, there exists a critical shock pressure, which corresponds to the largest critical grain size for the HP-IHP transition. Below the critical shock pressure, the critical grain size increases with pressure due to a stronger hardening effect in grain interior (GIs), while above the critical pressure, the critical grain size first decreases and then undergoes a pressure-insensitive plateau before further decrease due to softening effects in GIs. A theoretical model that includes different deformation mechanisms is proposed for the first time to capture the shock pressure-dependent HP-IHP transition. Our work provides valuable guidance for optimizing the grain size of nanocrystalline HEAs for applications involving dynamic loadings.