Ductile Metallic Materials Research Articles

Study of the dynamic impact spalling of ductile materials based on Gurson-type phase-field model

The formation of void damage and spalling failure in ductile metallic materials under strong impact is a well-established phenomenon. In aerospace and defense technology engineering design, understanding the spalling failure process and related mechanisms is of utmost importance. This paper develops an explicit Gurson-type phase-field model that can simulate the void evolution and spalling damage of three-dimensional ductile metallic materials under high-velocity impacts based on the study of Aldakheel et al.. The model incorporates the Gurson-type void evolution equation and the phase-field approach while taking into account the pressure-dependent bulk modulus and inertia effects. This model is used to study the main processes and mechanisms of impacted layer cracking of metals in different dimensions. Meanwhile based on the study of complex spallation cracking processes in metals in two and three dimensions, observing and proposing the formation mechanism of complex spallation cracking modes in materials due to lateral and edge (base angle) rarefied effects.

Enhancing plasticity in laser additive manufactured high-entropy alloys: The combined effect of thermal cycle induced dissolution and twinning

Breaking the trade-off between strength and ductility and pursuing stronger and more ductile metal materials has been a long-standing dream for scientists. Interestingly, even for the same material, different manufacturing methods can lead to significantly different performance. In this study, we utilized different laser additive manufacturing techniques (laser direct energy deposition (LDED), and laser powder bed fusion (LPBF)) to fabricate FeCoCrNiMo0.5 high-entropy alloy (HEA). Through microstructural analysis, we discovered a novel thermal cycling-induced dissolution (TCID) mechanism. By controlling the cooling rate post thermal cycling, we achieved the dissolution of precipitate phases and increased the Mo content within the matrix. Based on this phenomenon and supported by theoretical calculations, we revealed a reduction in stacking fault energy (SFE), which gave rise to a twinning-induced plasticity (TWIP) mechanism. This led to a dramatical increase in plasticity (LDED: 1.6 %, LPBF: 17.7 %) in the LPBF alloy, while maintaining similar strength (LDED: 784.9 MPa, LPBF: 840.89 MPa). This study provides guidance for designing metals structures with balanced strength and plasticity.

Influence of the Plastic Deformation Process on the Residual Stresses and Hardness of an Al-5Mg Alloy.

The service behavior of ductile metallic materials, when they have previously undergone technological plastic deformation, depends on the deformation conditions. These are represented, among others, by the deformation rate, the process temperature, the applied pressures, and the introduced stresses, as well as other process variables. The investigation of the mechanical properties obtained after plastic deformation is an important means that contains two characteristics: on the one hand, to determine to what extent the parameters of the technological manufacturing process influence the main characteristics of the final component; and, on the other hand, on the basis of these characteristics, to analyze whether the component subjected to plastic deformation will be able to function reliably and safely. In the present work, an experimental study was made of the residual stresses developed and hardnesses obtained both in the immediate vicinity of a highly plastically deformed area and in an area previously obtained by rolling, without additional plastic deformation. For the determination of the residual stresses, the tensiometric rosette drilling method was used. By determining the same quantities in a non-plastically deformed area, significant changes in the values of the two quantities in the plastically deformed area were found. An increase in the maximum principal normal stresses by approx. 60 MPa and an increase in the Rockwel hardness by approx. 10 HRC was found. A sample was taken from the area under a plastic deformed circular shape, and was analyzed microscopically.

A continuum approach to multiaxial high-cycle fatigue modeling for ductile metallic materials

In this work we propose a generalization of the endurance function employed in the continuum approach to high-cycle fatigue modeling by Ottosen et al. (2008). The von Mises-type backstress-modified effective stress term of the original formulation is replaced by the non-quadratic Hershey-Hosford function, which introduces flexibility with respect to the predicted fatigue limit in pure shear. While the original model assumes a fixed ratio between the fatigue limits in fully reversed torsion and fully reversed bending of 0.5774 for all materials, the modification enables the ratio to take on values on the interval from 0.5 to 0.5852 by calibration to experimental data that is readily available in the literature. Like the original formulation simplifies to the invariant-based Mises-Sines fatigue limit criterion for proportional stress histories, the current proposal can be written as a Hershey-Hosford-Sines criterion. A systematic validation to experimental multiaxial infinite fatigue life data from eight different mild and semi-hard metals with fully reversed torsion and bending fatigue limit ratios lower than 0.6 demonstrates that the proposed modification increases the accuracy and reduces the number of non-conservative predictions.

Molecular dynamics simulation of effects of loading parameters on fatigue crack growth behavior in titanium single crystal

AbstractThe fatigue crack growth behavior in single‐crystal titanium with a prismatic crack was simulated using molecular dynamics (MD) considering the effects of applied temperature, strain ratio, and strain rate. The results indicate that the material exhibits transient cyclic hardening behavior and dominant cyclic softening behavior. The peak tensile stress decreases with increasing temperature or decreasing strain rate. The deformation mechanism for crack growth involves prismatic dislocation emission and the formation of vacancy defects at different loading conditions. Deformation twinning occurs at the highest temperature, and a secondary crack emerges at the highest strain rate. The Mises stress concentration at the twin boundary and the coalescence between the initial and secondary cracks may accelerate crack propagation. The ΔJ shows good linear relationships with fatigue crack growth rate (FCGR), indicating that ΔJ possesses the potential to assess fatigue crack growth behavior at the atomic scale for ductile metallic materials.

Adhesion, bonding and electronic properties of Ti/Zr doped c-BN(100)/Cu(100) interfaces: A first-principles study

Brazed monolayer cubic boron nitride (c-BN) tools are considered as promising tools for machining ductile metal materials. The interface between c-BN and filler alloys determines the properties of the tools to some extent. This paper uses first-principles calculations to study the atomic structures, work of adhesion, electronic properties, and density of states of c-BN(100)/Cu(100) clean and doped interfaces where the doping atoms are Ti/Zr. In this paper, four types of models are considered and calculated. The calculations reveal that Ti doping and Zr doping both increase the work of adhesion of the c-BN(100)/Cu(100) interface and the highest increase rate is 109 % and 83.4 %, respectively. The work of adhesion of c-BN(100)/Cu(100) interface doped with Ti is much higher than that doped with Zr and the electronic properties and density of states also prove this point of view. To sum up, Ti and Zr are both effective active elements to improve the performance of the c-BN/Cu interface, but, by comparison, Ti has better wettability to c-BN than Zr which provides an interpretation of the reported experimental results through the view of the atomic structures, work of adhesion, electronic properties and density of states.

Damage and failure of semi-rigid steel joints during progressive collapse

Ductile metal materials during a ductile failure will suffer from micro-void damage, simulation of which can contribute to the prediction of the macro fracture of steel structure. In this paper, the Gurson-Tvergaard-Needleman (GTN) constitutive model which describes the aggregate failure of metal voids is selected as the constitutive relation to investigating the damage evolution and fracture of semi-rigid joints in steel structures during a progressive collapse. During the process, specific parameters of the GTN model of steel are confirmed based on the combined results of the test and numerical simulation. The progressive collapse of two kinds of semi-rigid joints of steel pipe-to-H shaped steel were simulated by a nonlinear finite element method. A comparison between the simulation and test results verified that the GTN constitutive model can accurately simulate the fracture failure of the steel semi-rigid joints. Besides, a simplified joint failure model based on concentrated plastic hinge theory is proposed. Considering the damage and fracture of semi-rigid joints and the collision contact among different components, the progressive collapse of the semi-rigid steel frame structure is simulated by explicit dynamic analysis based on a finite particle method. Afterwards, the dynamic response of the structural progressive collapse can be predicted more accurately.

Lightweight Fe47Mn25Al13Cr7Ni5C3 medium-entropy alloy with enhanced mechanical properties

In this study, we designed a lightweight Fe47Mn25Al13Cr7Ni5C3 medium-entropy alloy (MEA) (calculated density of 6.803 g/cm3) with enhanced mechanical properties. The MEA samples were produced in three conditions with varying microstructures via thermomechanical processing. They achieved excellent tensile properties through strengthening mechanisms such as partial recrystallization, ultrafine grains, and M23C6 and B2 precipitates. Different strengthening mechanisms were applied depending on the annealing heat treatment conditions, resulting in three different strength-elongation combinations. Furthermore, the MEA, under all designed conditions, exhibited superior specific yield strength-uniform elongation and specific ultimate tensile strength-uniform elongation combinations compared to previously studied lightweight high-entropy alloys (HEAs) and lightweight steel. This was primarily attributed to the combination of benefits obtained from the low proportion of iron (47 at%) as a principal element and the large amount of aluminum addition (13 at%). The proposed MEA and its design strategy can satisfy the requirements for lightweight, cost-effective, strong, and ductile metallic materials, making a great contribution to the automotive industry in terms of crash resistance and fuel efficiency.

Pursuing ultrastrong and ductile medium entropy alloys via architecting nanoprecipitates-enhanced hierarchical heterostructure

Nanoprecipitate-strengthened medium/high entropy alloys (M/HEAs) with ultrahigh strengths and unusual physical properties represent promising candidates for structural applications. However, further strengthening these materials usually relies on increasing the volume fraction of nanoprecipitates and/or introducing harder intermetallics that inevitably sacrifice the ductility. Here, we report a strategy to break this trade-off in a Co-Cr-Ni-based MEA by architecting a hierarchical heterostructure that integrates high-volume fraction (up to ∼30%) D022-γ’’ nanoprecipitates, shearable and non-shearable intermetallic particles, with a low stacking fault energy (SFE) matrix with heterogeneous grain structure. This hierarchical heterostructure can arouse hybrid strengthening mechanisms and lead to cross-scale and dynamic strain partitioning during plastic deformation, contributing to a superior strain-hardening capability and an excellent combination of the yield / ultimate tensile strength of 1323 MPa / 1690 MPa and 29% tensile elongation. This study provides a promising paradigm to design ultrastrong and ductile metallic materials via controlling multiscale microstructural heterogeneities.

Finite element modeling of steady plastic shockwaves in porous metals: Role of size, shape, and spatial distribution of voids

This paper aims at developing a finite-element model, which simulates a planar impact experiment on a ductile metallic porous material. When the shockwave passes through a porous medium, the voids face a rapid collapse, and the material particles in the vicinity of the voids are subjected to very high acceleration. Acceleration forces generated at the microscale level may affect the overall response of the porous material. Analyzing the effect of the initial porosity, size, shape, and spatial distribution of voids on the plastic shock velocity and the shock structure was the main objective of this work. It is found that, for a given impact velocity, the plastic shock velocity depends solely on the material parameters of the dense material and the initial porosity. However, the size, shape and spatial distribution of voids, together with the initial porosity, play a significant role on the structure of the plastic shock.

Strengthening mechanism and plasticity deformation of crystalline/amorphous Cu3Fe/Fe3Cu nanolayered composite

Metallic glasses possess high strength and excellent elastic properties due to their amorphous atomic structure. However, they often display brittleness and suffer catastrophic failure because of shear localization, without exhibiting significant macroscopic plastic deformation under tension. In this paper, an effective method to enhance the strength and ductility of metallic glasses is presented by molecular dynamics simulation. The approach involves controlling the thickness of the crystalline Cu3Fe and amorphous Fe3Cu layers. The results indicate that increasing the thickness of the crystal layer improves the strength of the nanolayered composite. The crystalline/amorphous interface impedes stress propagation in the shear transformation zone, thereby enhancing the yield stress of the composite. Additionally, the presence of Hirth dislocations in the crystal layer strengthens the composite. The generation and annihilation of fixed dislocations in the crystal layer cause stress fluctuations in the composite, which further enhances its ductility. However, a small content of crystalline layer can cause stress concentration towards the shear band, leading to rapid fracture failure and reduced ductility. These findings shed light on the plastic deformation behavior of crystalline/amorphous composites at the nanoscale, providing valuable theoretical guidance for designing high-strength and ductile metallic materials.

Enhanced strength-ductility synergy in high-entropy alloys via architecting three-level gradient hierarchical nanostructure

Gradient microstructural design represents a new strategy for enhancing strength-ductility synergy of high/medium entropy alloys (H/MEAs). However, most of the reported gradient H/MEAs have only one type of gradient microstructure, which has limited ability to realize the full potential of gradient structure. Here, we report a three-level gradient hierarchical nanostructure (GHN) in a Al0.3CoCrFeNi HEA by ultrasonic surface rolling processing (USRP) followed by aging treatment that can result in a six-fold enhancement in yield strength with almost no loss in ductility compared to the coarse structured counterpart. The proposed three-level GHN is characterized with gradient variations in grain size, nano-twin and dislocation density, B2 and σ precipitates content. Systematic experiments indicate that strong hetero-deformation-induced (HDI) hardening associated with the activation of deformation twinning, are responsible for this exceptional strength-ductility synergy. This three-level GHN engineering strategy in this work can also be feasibly applied to many other H/MEAs, and offers an available pathway to develop high strong and ductile metallic materials.

Monitoring Elastoplastic Deformation in Ductile Metallic Materials Using Sideband Peak Count-Index Technique

Abstract Ductile metallic materials such as aluminum alloy, brass, and steel are widely used in engineering structures. Monitoring elastoplastic deformation in these materials is important for structural health monitoring (SHM) to ensure the safety of structures made of metallic materials. This article presents a newly developed and promising nonlinear ultrasonic (NLU) technique called sideband peak count-index (or SPC-I) for monitoring the early stages of elastoplastic deformation in ductile metallic alloy Al6061. Experimental results presented in this article show that the elastic range of the Al6061 SPC-I values shows slight changes due to the inherent inhomogeneities (imperfect grain boundaries or dislocations at the grain boundaries under loadings) of Al6061. Then the SPC-I value changes rapidly as the material enters the plastic range zone. Compared to the linear ultrasonic (LU) parameters (wave velocity and attenuation changes), the SPC-I shows a noticeable advantage (higher sensitivity) for monitoring the early stages of the elastoplastic deformation in these ductile metallic specimens investigated in this study. It is concluded that the SPC-I technique is useful for monitoring deformations in ductile metallic materials, especially in their plastic zone. This work extends the applicability of the SPC-I technique for monitoring elastoplastic deformations in metallic specimens that have not been reported in earlier works and can provide some guidelines for SHM related to elastoplastic deformation in metallic structures.

Modified plasticity constitutive model for extruded aluminum alloys

Aluminum alloys have been increasingly used in a wide range of construction applications, especially in important public buildings. A plasticity constitutive model that can accurately describe the elastoplastic behavior of aluminum alloys under complicated stress states is of great importance in precisely simulating aluminum alloy structures under complicated working conditions. Also, a precise plasticity model serves as the foundation for studying the ductile fracture of metals, and is therefore crucial for structural analysis involving large deformation, such as progressive collapse analysis. The classic J2 plasticity model based on the von Mises yield criterion is the most commonly utilized model for ductile metal materials. However, it ignores the effect of stress state on metal plasticity behavior, making it inapplicable to aluminum alloy materials under various complicated stress states. This study experimentally demonstrated that stress triaxiality and Lode angle have a noticeable effect on the plasticity behavior of extruded aluminum alloys (6061-T6, 6082-T6, and 6063-T5). Accordingly, a plasticity constitutive model suitable for extruded aluminum alloys under complicated stress states was proposed and validated, including a full-range hardening law and a new yield criterion. In addition, the parameter calibration method for this new model was detailed. Finally, comparisons between the experimental and numerical results proved the high accuracy of the proposed model in predicting the actual response of a structural extruded aluminum alloy under complicated stress states. Compared with previous studies, the proposed model can not only consider the effects of stress triaxiality and Lode angle, but it can also provide a more reasonable description of yield surface under considerably high or low stress triaxiality.

The effect of initial texture on multiple necking formation in polycrystalline thin rings subjected to dynamic expansion

In this paper, we have investigated, using finite element calculations, the effect of initial texture on the formation of multiple necking patterns in ductile metallic rings subjected to rapid radial expansion. The mechanical behavior of the material has been modeled with the elasto-viscoplastic single crystal constitutive model developed by Marin (2006). The polycrystalline microstructure of the ring has been generated using random Voronoi seeds. Both 5000 grain and 15000 grain aggregates have been investigated, and for each polycrystalline aggregate three different spatial distributions of grains have been considered. The calculations have been performed within a wide range of strain rates varying from 1.66⋅104s−1 to 3.33⋅105s−1, and the rings have been modeled with four different initial textures: isotropic texture, 001∥Θ Goss texture, 001∥ R Goss texture and 111∥ Z fiber texture. The finite element results show that: (i) the spatial distribution of grains affects the location of the necks, (ii) the decrease of the grain size delays the formation of the necking pattern and increases the number of necks, (iii) the initial texture affects the number of necks, the location of the necks, and the necking time, (iv) the development of the necks is accompanied by a local increase of the slip activity. This work provides new insights into the effect of crystallographic microstructure on dynamic plastic localization and guidelines to tailor the initial texture in order to delay dynamic necking formation and, thus, to improve the energy absorption capacity of ductile metallic materials at high strain rates.

Grain-size effect on dislocation source-limited hardening and ductilization in bulk pure Ni

Dislocation source-limited hardening and ductilization is an effective strategy to obtain superior strength-ductility synergy in some engineering structural metals. Recent works demonstrated that the synergy could be enhanced by grain-size reduction. However, the mechanism of grain-size dependence is still a mystery. In this work, bulk pure Ni produced by electrodeposition and subsequent annealing, with grain sizes ranging from ∼20 nm to ∼20 µm, were methodically investigated to unravel the mechanism of the grain-size effect on dislocation source-limited hardening and ductilization. The high-density nano-twinning in the as-electrodeposited nanograined specimens exhibited better thermodynamic stability than the peers with random high-angle grain boundaries, leading to fine recrystallized grains with low-density dislocations. The low dislocation density enabled extra hardening beyond grain boundary strengthening via yield-point behavior with grain sizes ranging from ∼110 nm to ∼10 µm and extra ductilization over ∼500 nm. This work demonstrated that the prerequisite for dislocation source-limited hardening was that the dislocation density of the specimen should be lower than the size-dependent critical value of ((1.1 × 107/d) m–2, d is the grain size in unit of the meter) where a transition from forest-dominated hardening to dislocation source-limited hardening could occur. On the other hand, dislocation source-limited ductilization only worked when the grain size was comparable to/larger than the theoretical dislocation mean slip distance. Dislocation source-limited ductilization resulted from more room in grains for accumulation of dislocations and deformation nano-stacking faults enabling the higher work hardening rate. This work offered an altogether new avenue to obtain stronger and more ductile metallic materials through utilizing grain-size dependent dislocation source-limited hardening and ductilization.

Cohesive Model Application in the Assessment of Plastic Zone Magnitude for One Particular Case of Crack Loading

The answer of the thin, infinite, central cracked plate, subjected to in-plane loading, is investigated. The plate is made from a ductile metallic material. The two pairs of the concentrate forces, F, act on the crack surface in the direction perpendicular to it. The other edges of the plate, at infinity, are free of loading. The forces, F, open the crack and they are monotonously increased. The small plastic zones around crack tips will appear. It is assumed an isotropic and non-linear strain hardening of a plate material which can be good described by the Ramberg-Osgood´s equation. The investigations were carried out for the several different values of the strain hardening exponent n which is changed among the discrete values n = 2, 3, 4, 5, 7, 10, 15, 25, 50 and 1000. One well-known cohesive model (Dugdale´s model) was applied in the crack tip plasticity investigating. It was assumed that the cohesive stresses within the plastic zone are changed according to non-linear law. Also, a new algorithm was established which enables direct calculation of plastic zone magnitude depending on the magnitude of external loads. The contemporary mathematical tools, like the software package Wolfram Mathematica, were used. The solutions are presented through the special, Gamma and the Hypergeometric functions.

Review of Techniques for Evaluating the Uniaxial Tensile Properties and Fracture Toughness of Ductile Metallic Materials Based on Micron Indentation

Review of Techniques for Evaluating the Uniaxial Tensile Properties and Fracture Toughness of Ductile Metallic Materials Based on Micron Indentation

A unified sharp indentation method for obtaining stress-strain relations, strength and Vickers hardness of ductile metallic materials

Combining dimensional analysis and the definition of Vickers hardness (HV), an integrated prediction model (IPM) for determining the stress-strain relation, strength and Vickers hardness of ductile metallic material is established. The dimensionless functions in the model are determined by using finite element analysis (FEA). The accuracy of the model predicted load-depth relations, Vickers hardness and stress-strain relations are verified within a wide range of imaginary materials via FEA. Moreover, on the basis of the experimental sharp indentation load-depth data for 10 ductile metallic materials from the references, the stress-strain relations, ultimate tensile strength and Vickers hardness of the materials, which predicted by IPM are similar to the uniaxial tensile or hardness testing results. Likewise, the simulated load-depth relations agree well with the sharp indentation experiments results.

In Situ Measurement of Cyclic Plastic Zone and Internal Strain Response of Q&P Steel near Fatigue Crack Tip Region Based on Micro-DIC.

The shape and internal dynamic response characteristics of the plastic zone near the fatigue crack tip region, especially the cyclic plastic zone (CPZ), are the main factors affecting the fatigue crack initiation and propagation behaviors of ductile metal materials. The existing methods for characterizing the CPZ have some problems, which include the complexity of the process, the difficulty of achieving in situ measurement, and the inability to characterize the dynamic response in the CPZ during the crack propagation process. Therefore, a novel method is proposed for the in situ measurement of the CPZ near the crack tip region based on image stitching and matching algorithms, a load–strain loop curve characteristic judgement algorithm, and the microscopic digital image correlation (DIC) method. A microscopic camera and a macroscopic camera are used to simultaneously capture the micro crack tip speckle images and the global crack image of the two sides of the Compact Tension (CT) specimen for calculating in situ crack length and crack tip strain fields. The proposed method was performed and verified by a fatigue crack growth (FCG) test and micro-hardness experiments with Quenching and Partitioning 980 (Q&P980) steel, and the results show that the method is feasible because the maximum error is less than 5%. A “butterfly wings” shape of the CPZ and a strain concentration phenomenon in the CPZ of the Q&P980 were observed. Moreover, as the fatigue crack propagates, the area of the CPZ and the degree of the strain concentration increase gradually. This method, which can obtain the in situ and tracking measurements of the crack tip CPZ, will help to increase our understanding of CPZ characteristics, the FCG mechanism, and the behavior of Q&P steel and the plastic metal materials similar to Q&P steel.