Intrinsic Ductility Research Articles

Precipitation prediction and strengthening investigation of the non-equimolar TiVNbMoCr high entropy alloys

This study designed a series of TiVNbMoCr alloys under the VEC constraint. Based on the thermodynamic calculation, the precipitation behavior was predicted, and the precipitation strengthening was experimentally investigated and discussed. The results showed that below 600°C, the designed alloys with VEC from 4.30 to 4.60 are metastable at a single β phase state. With low VEC, the HCP-structured α phase precipitates from the BCC-structured β phase with a coherent interface between them. With high VEC, TiCr2 (C15-Laves phase) precipitates from the β phase companying with the α phase. The prediction results are well consistent with the experimental results. Aged at 400°C for 60h, the alloys with VEC of 4.30 and 4.35 contained large volume fraction of the large-sized α phase, playing a great role in strengthening. The alloys with VEC of 4.55 and 4.60 exhibited small-sized precipitation particles acting in strengthening in terms of the Orowan mechanism, and the yield strength depends mainly on the volume fraction and size of precipitation particles, as well as the intrinsic strength of the β matrix that can be predicted by the Varvenne model based on the MD calculation. To some extent, the intrinsic ductility of the β matrix determines the tensile plasticity of the alloys with high VEC. Although favoring for strengthening, more TiCr2 precipitates are not conducive to tensile plasticity.

Electronic descriptors for dislocation deformation behavior and intrinsic ductility in bcc high-entropy alloys

Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is central to their application as structural materials. Materials discovery efforts for HEAs are therefore impeded by an incomplete understanding of the chemical factors governing this balance. Through first-principles calculations, this study explores factors governing intrinsic ductility of a crucial subset of HEAs—those with a body-centered cubic (bcc) crystal structure. Analyses of three sets of bcc HEAs comprising nine different compositions reveal that alloy chemistry profoundly influences screw dislocation core structure, dislocation vibrational properties, and intrinsic ductility parameters derived from unstable stacking fault and surface energies. Key features in the electronic structure are identified that correlate with these properties: the fraction of occupied bonding states and bimodality of the d-orbital density of states. The findings enhance the fundamental understanding of the origins of intrinsic ductility and establish an electronic structure–based framework for computationally accelerated materials discovery and design.

Achieving superior strength-ductility balance by tailoring dislocation density and shearable GP zone of extruded Al-Cu-Li alloy

Pre-stretching is commonly employed to accelerate ageing precipitation kinetics in wrought Al-Cu-Li alloys, but uneven precipitation resulting from dislocation pile-ups often degrades ductility. Herein, the strength and ductility of extruded Al-Cu-Li alloy are significantly improved through a novel thermomechanical treatment, involving pre-ageing and pre-stretching, followed by low-temperature interrupted ageing. A superior balance between high yield strength (∼ 657 MPa) and good ductility (elongation to fracture of ∼ 13.5 %) is obtained, with elongation increased by 105 % compared to the conventional T8 temper, while maintaining a respectable yield strength. Microstructure analysis reveals that dense Guinier–Preston (GP) zones induced by pre-ageing effectively dissipate energy from dislocation sliding, resulting in a uniform dislocation configuration even at 8 % pre-stretching. However, the GP zone density is greatly reduced due to their dissolution following pre-stretching. Upon interrupted ageing, the reprecipitation of GP zones forms a homogeneous mixture of δ′, GP zones, and T1 phases. This combination alleviates local stress concentrations and lengthens the dislocation mean free path during tensile testing by shearing the GP zones at multiple sites, thereby improving ductility. Simultaneously, T1 precipitates strengthen the alloy by pinning dislocations and promoting dislocation cross-slip, improving work hardening capacity. The dissolution of GP zones also redistributes the Cu atoms within the matrix, further enhancing the intrinsic ductility of the Al matrix. These findings offer valuable insights for developing high-performance wrought Al-Cu-Li alloys.

Understanding the Mechanical Behavior of Sulfide Solid Electrolyte Membranes for Practical Applications in All-Solid-State Batteries

All-solid-state batteries (ASSBs) hold great promise as next-generation energy storage systems due to their enhanced stability and high energy density compared to traditional lithium-ion batteries. This improvement stems from the use of a solid electrolyte (SE) instead of a highly volatile liquid electrolyte and the incorporation of a lithium metal anode. In general, sulfide-based SSBs require adequate pressurization conditions to establish an intimate interface between the solid particles, ensuring a continuous electron/ion conduction path. At the cell level, maintaining external stack pressure during operation is crucial to preserve conformal solid-solid contact. Furthermore, when scaling up, fabrication pressure should be carefully considered to enhance electrode densification without causing damage. In particular, the fabrication of thin, robust, and scalable SE membranes faces practical challenges due to a limited amount of polymeric binder and the low intrinsic ductility of SEs after densification. Understanding the mechanical behavior of these SE membranes is essential to ensure their structural stability during manufacturing.In this study, we systematically examined the mechanical properties of SE and SE membranes. The deformability of various types of sulfide SEs was assessed using a powder compaction test, departing from the commonly used indentation method. The yield pressure, which indicates the deformability of the SE powder, is influenced by the composition and synthesis conditions of the SEs. The stress-strain behaviors of the SE membranes were obtained through tensile and flexural tests for both as-prepared and pressed samples, respectively. The tensile and bending behaviors of the SE membranes strongly depend on the combination of the SE and polymeric binder. The utilization of a ductile SE and a rubbery polymer binder can enhance the flexibility and ductility of the membrane, leading to increased densification degree and low interfacial resistance even under lower fabrication pressure. The details of cell configuration, fabrication procedure, and the resulting performances of the cell will be discussed in this talk.

Intermetallic Compounds in Solder Alloys: Common Misconceptions

Intermetallic compounds (IMC) or intermediate phases are formed between two or more metallic elements in many metal alloy systems. During soldering, an IMC is formed at the soldered interface as the molten solder reacts with an element in the substrate. IMCs also can form within the bulk solder as the joint solidifies. IMCs have critical roles in the solder joint quality and reliability. Unlike most metal alloys, an intermetallic compound typically has a fixed stoichiometry and is in variance with the conventional phases or constituents in the metal system (e.g., alpha and beta). IMCs have a different crystal structure than any of its constituents and some but never all the characteristics and properties of its constituents. Ductility is an important solder joint property, and the low intrinsic ductility of IMCs is associated with brittle behavior and reliability risk in service. However, a review of published solder field failures shows little evidence that IMC properties or IMC evolution under service conditions reduce solder joint reliability. Most IMC-induced solder joint failures are found to result from incorrect material specification or uncontrolled soldering processes. This paper describes the IMCs that occur typically in eutectic, Sn63Pb37 solder and near-eutectic SAC305 or other tin-based Pb-free solder alloys, including how they impact solder joint reliability. The paper also describes the potential impact of IMCs on the solder joint reliability for the newest generation of Pb-free high-performance solder alloys.

Unveiling ductile, rare-earth-free structural materials: A DFT exploration of MnTi and MnZr

This paper presents a theoretical exploration of the electronic, structural, and mechanical attributes inherent in three rare-earth-free intermetallic compounds, namely, MnTi, MnZr, and MnHf. Employing density functional theory (DFT) calculations with the Implementation of projector augmented wave (PAW); our investigation adopts the supercell approach to meticulously determine the structural and mechanical properties of these materials. The findings reveal that MnTi and MnZr exhibit intrinsic ductility, positioning them as viable contenders for applications demanding high-strength structures. In contrast, MnHf demonstrates mechanical instability. This study provides promising insights into the mechanical characteristics of MnTi and MnZr, underscoring their potential as sustainable structural materials, given the abundance and non-toxic nature of their constituents. The research findings presented herein contribute to the understanding of rare-earth-free intermetallics, offering valuable information for applications in materials science and engineering.

Aluminium matrix composites reinforced with high entropy alloys: A comprehensive review on interfacial reactions, mechanical, corrosion, and tribological characteristics

Aluminium matrix composites (AMCs) reinforced with high entropy alloy particulates (HEAp) represent an innovative category of metal-matrix composites with considerable potential for fulfilling the stringent demands of emerging technological applications. Their appeal lies in the advantageous combination of toughness, strength, ductility, and enhanced workability, addressing acknowledged limitations associated with ceramic-reinforced AMCs. The heightened performance of these composites is attributed to improved wettability between the aluminium matrix and HEAp reinforcement, alongside the intrinsic ductility and hardness of HEAp. This review explores the suitability of high entropy alloys as substitutes for ceramic materials in reinforcing aluminium matrix composites. The mechanical, corrosion, thermal and wear properties of AMCs reinforced with HEAp are thoroughly examined, encompassing fabrication characteristics and interfacial reactions. The incorporation of HEAp is found to notably enhance the strength-ductility ratio of AMCs. Remarkably, HEAp bring about significant improvements in the wear and corrosion resistance of AMCs. The report accentuates the performance benefits and certain challenges associated with the application of HEAp reinforcement in AMCs. Ultimately, this review proposes potential avenues for future research in this domain, outlining directions for further exploration and development.

The enhanced toughness and growth kinetics in HfxTa2O2x+5 through modulated structure

Although HfxTa2O2x+5 shows great potential as a next-generation thermal barrier coating (TBC) material, the diversity of its potential modulated structures presents challenges in determining their mechanical properties. HfxTa2O2x+5 (x = 4, 6, 8) ceramics were successfully fabricated using the sol–gel method. Research on growth kinetics has indicated that grain boundary diffusion is the speed-controlling step. The activation energy increased with a larger modulated structure. In addition, the fracture toughness also increased from 1.98 MPa m1/2 for Hf4Ta2O13 to 3.30 MPa m1/2 for Hf8Ta2O21. Density functional theory calculations revealed that HfxTa2O2x+5 exhibited intrinsic ductility owing to the denser packing of atoms in its larger modulated structure. Thus, the tight-knit chemical bond net may contribute to a large surface energy. The results of this study provide deep insights into the toughening mechanism of modulated structures and pave the way for the design of next-generation TBCs.

Toughening mechanism in superstructure Hf6Ta2O17 thin film

Fracture toughness is a key indicator to estimate the anti-peeling performance of thermal barrier coatings (TBCs). A unique superstructure endows Hf6Ta2O17 with an outstanding phase stability and mechanical property, which also poses great challenges for studying its intrinsic toughening mechanism. Here, the superstructure Hf6Ta2O17 single crystal film was epitaxially grown on YSZ(011) substrate by pulsed laser deposition. An uncommon ripple pattern near the Vickers indentation crack tips was first reported. The pop-in signals in the load-displacement curve indicated it may be relevant to ferroelastic domain transformation, which was further confirmed by spontaneous strain distribution in HRTEM. The transformation path, predicted by DFT, was conducted in b→c ferroelastic domain through in-plane rotation of Hf–O/Ta–O polyhedrons. The corresponding critical stress for domain transformation was up to 5.83 GPa. In addition, an intrinsic ductility was confirmed with a high Pugh ratio B/G of 2.49, much larger than the critical value (1.75) for brittle-ductile transition. These two mechanisms make contribution to the superior fracture toughness of Hf6Ta2O17. This work provides a deep insight into the toughening mechanism of ceramics and paves the way to design of next-generation TBCs.

Substantially improved room-temperature tensile ductility in lightweight refractory Ti-V-Zr-Nb medium entropy alloys by tuning Ti and V content

Lightweight high/medium-entropy alloys (H/MEAs) possess attractive properties such as high strength-to-weight ratios, however, their limited room-temperature tensile ductility hinders their widespread engineering implementation, for instance in aerospace structural components. This work achieved a transformative improvement of room-temperature tensile ductility in Ti-V-Zr-Nb MEAs with densities of 5.4–6.5 g/cm3, via ingenious composition modulation. Through the systematic co-adjustment of Ti and V contents, an intrinsic ductility mechanism was unveiled, manifested by a transition from predominant intergranular brittle fracture to pervasive ductile dimpled rupture. Notably, the modulated deformation mechanisms evolved from solitary slip toward collaborative multiple slip modes, without significantly compromising strength. Compared to equimolar Ti-V-Zr-Nb, a (Ti1.5V)3ZrNb composition demonstrated an impressive 360% improvement in elongation while sustaining a high yield strength of around 800 MPa. Increasing Ti and V not only purified the grain boundaries by reducing detrimental phases, but also tailored the deformation dislocation configurations. These insights expanded the applicability of lightweight HEAs to areas demanding combined high strength and ductility.

Investigation of effects of process parameters on microstructure and fracture toughness of SLM CoCrFeMnNi

Selective Laser Melting (SLM) of multi-component alloy (MCA) such as CoCrFeMnNi is gaining significant attention due to its potential to exhibit an intriguing blend of mechanical properties, particularly strength-ductility combinations. However, limited attention has been directed toward the critical aspect of fracture toughness, which is indispensable for various engineering applications. This study delves into obtaining the fracture toughness of SLMed CoCrFeMnNi MCA, with a focus on the influence of microstructure, densification, and key printing parameters (volumetric energy density (VED), laser power, and scan speed). Experimental findings revealed that individual printing parameters rather portray better insights on the densification as opposed to the VED. Notably, slow scan speed of 800 mm/s significantly enhances densification irrespective of the VED. The alloy presents complex cellular structures with the morphology reflecting a non-linear dependence on VED. The presence of cellular dislocations analogous in scale to the cellular structures was observed with dark nanoprecipitates, recognized as Mn-rich oxides. Microhardness evaluations portrayed a non-linear relationship with varying VED, with values spanning between 230 HV and 280 HV. The fracture toughness did not exhibit a linear correlation also with VED, but better fracture toughness values were achieved at 200 W. Additionally, a notable observation of late crack initiation was observed on the load-displacement curves, indicative of the material's intrinsic ductility. The fracture toughness values ranged between 320 and 220 MPa.m−0.5. A correlation between the microstructure and densification with the fracture toughness was made. The paper also presents a comprehensive discussion on the mechanisms underpinning microstructure formation and their consequential effects on the fracture toughness of the material.

An intrinsic ductility parameter derived from anisotropic linear elasticity theory

A new indicator of intrinsic ductility (κ) is introduced based on linear elasticity. In the limit of elastic isotropy this parameter is equal to the Pugh ratio (B/G) plus a constant; but unlike the Pugh ratio, κ incorporates anisotropy and crystallography to improve predictive value. We identify a single ductile-to-brittle transition setpoint for κ that predicts crack-tip plasticity in atomistic simulations, experimental elongation to failure of polycrystalline elemental metals pulled in tension, and fracture energy measurements of glasses, suggesting commonality in the relationship between plasticity and fracture across all three of these cases. Statistical analysis supports the superiority of κ over B/G at predicting crack-tip plasticity in atomistic simulations and elongation to failure of polycrystalline elemental metals.

Cohesive, elastic and anisotropic properties of s-, p- and d-block fission metals substituted Fe–Zr intermetallics

Density functional theory (DFT) based calculations are performed to study cohesive, elastic and anisotropic properties of c-Fe2Zr, t-FeZr2 and o-FeZr3 phases in the presence of s-, p- and d-block fission metals (FMs), viz. Rb, Sr, Cs, Ba, In, Sn, Sb, Te, Y, Nb, Mo, Tc, Ru, Rh, Pd, Ag and Cd. The FM substituted intermetallics are found to be thermodynamically stable owing to their negative formation energy. The Gibbs free energy of mixing is calculated to find the solubility of these FMs in these intermetallics. The calculated single crystal elastic constants suggest that all the FM substituted phases are mechanically stable. We also report the change in polycrystalline elastic moduli, viz. bulk, shear and Young moduli of the pristine intermetallics on incorporation of these FMs. The substitution of FMs in the FeZr2 phase increases its intrinsic ductility; while that in FeZr3 phase decreases it. The degree of anisotropy exhibited by the intermetallics follow the order FeZr2> FeZr3> Fe2Zr. The substitution of FMs does not affect the anisotropic behavior of Fe2Zr and FeZr2 phases, but slightly changes that of FeZr3 phase. Finally, the potential of these intermetallics to incorporate the FMs is discussed.

Atomistic fracture in bcc iron revealed by active learning of Gaussian approximation potential

The prediction of atomistic fracture mechanisms in body-centred cubic (bcc) iron is essential for understanding its semi-brittle nature. Existing atomistic simulations of the crack-tip under mode-I loading based on empirical interatomic potentials yield contradicting predictions and artificial mechanisms. To enable fracture prediction with quantum accuracy, we develop a Gaussian approximation potential (GAP) using an active learning strategy by extending a density functional theory (DFT) database of ferromagnetic bcc iron. We apply the active learning algorithm and obtain a Fe GAP model with a converged model uncertainty over a broad range of stress intensity factors (SIFs) and for four crack systems. The learning efficiency of the approach is analysed, and the predicted critical SIFs are compared with Griffith and Rice theories. The simulations reveal that cleavage along the original crack plane is the atomistic fracture mechanism for {100} and {110} crack planes at T = 0 K, thus settling a long-standing issue. Our work also highlights the need for a multiscale approach to predicting fracture and intrinsic ductility, whereby finite temperature, finite loading rate effects and pre-existing defects (e.g., nanovoids, dislocations) should be taken explicitly into account.

Entropy-stabilized binary alloys

Alloying W with Re is an effective method for improving its intrinsic ductility. The solubility limit of Re in W is ∼27% at 851 K. However, this alloy possesses a high enthalpy of formation, which can result in phase-instability. Configurational entropy alone cannot stabilize this alloy as a single-phase solid solution. Recent research questions the importance of configurational entropy in determining the single-phase stability of “complex, concentrated solid solutions” and suggests that vibrational entropy may play a critical role. The question arises: Can vibrational entropy contribute significantly enough to ensure single-phase stability in a binary alloy? Our first-principles phonon calculations using density functional theory demonstrate that vibrational entropy is equally essential as the configurational entropy in promoting the formation of a single-phase solid solution in WRe binary alloys. This finding necessitates revisiting the role of vibrational entropy in the single-phase stability of binary alloys.

On the influence of enthalpy of formation on lattice distortion and intrinsic ductility of concentrated refractory alloys

Valence electron concentration (VEC), atomic size difference (δ), and Pugh’s ratio (B/G) are a few of the empirical parameters widely used to design ductile refractory alloys. Here, we used the intrinsic ductility parameter (D), which is the ratio of surface energy (γs) and unstable stacking fault energy (γusfe), to design ductile refractory alloy. We found that the D correctly captures the experimentally observed ductility in concentrated refractory alloys. Here, we studied the enthalpy of formation (ΔEf), lattice distortion, and D of 9 refractory metals and 36 equiatomic refractory alloys using density functional theory simulations. We found that the ΔEf strongly influences the D of concentrated refractory alloys. The positive ΔEf and δ lead to large lattice distortion in concentrated refractory alloys. However, we did not find a strong correlation between lattice distortion and D in the presently studied alloys. We found that the success of VEC and Pugh’s ratio in designing ductile refractory alloys has a strong dependence on the underlying ΔEf of the alloy. We have developed a bottom-up method, which drastically reduces the number of alloys to be studied, to design ductile concentrated refractory alloys that can be thermodynamically stable.

Insights into fracture behavior of Ni-based superalloy single crystals: An atomistic investigation

Comprehending the plastic deformation and failure mechanisms in Ni-based superalloys is vital to enhance their intrinsic ductility. In view of this, the present work intends to illustrate the complex deformation mechanisms ensued during tensile deformation of pre-cracked Ni-Ni3Al single crystals using atomistic simulations. Simulations of fracture, thermodynamic continuum treatments of crack initiation and detailed calculations of crack propagation toughness are performed synergistically to analyze the competition between dislocation emission and cleavage. This competition is evaluated under Mode-I loading for 18 different pre-cracked configurations (three different crack lengths, three phase orientations and two phases) in totality. Our results demonstrate that dislocation as well as twin plasticity eventuate and the competition between the two plastic modes depends on the crystallographic orientation of the abutting phases. Higher initial crack lengths can otherwise lead to cleavage like propagation in the material; however this does not imply inferior ductility of the system as long as the phase boundary lies in the path of advancing crack. Changes in configurations in terms of pre-cracked phase or initial crack lengths can engender appreciable differences in the elementary crack tip phenomena. The numerical computations of crack propagation toughness are compared favorably well with the available experiments in the literature. The calculated variations in overall toughness of the material are discussed and analyzed in light of the observed deformation landscapes.

Mining of lattice distortion, strength, and intrinsic ductility of refractory high entropy alloys

Severe lattice distortion is a prominent feature of high-entropy alloys (HEAs) considered a reason for many of those alloys’ properties. Nevertheless, accurate characterizations of lattice distortion are still scarce to only cover a tiny fraction of HEA’s giant composition space due to the expensive experimental or computational costs. Here we present a physics-informed statistical model to efficiently produce high-throughput lattice distortion predictions for refractory non-dilute/high-entropy alloys (RHEAs) in a 10-element composition space. The model offers improved accuracy over conventional methods for fast estimates of lattice distortion by making predictions based on physical properties of interatomic bonding rather than atomic size mismatch of pure elements. The modeling of lattice distortion also implements a predictive model for yield strengths of RHEAs validated by various sets of experimental data. Combining our previous model on intrinsic ductility, a data mining design framework is demonstrated for efficient exploration of strong and ductile single-phase RHEAs.

Toughening Thermoelectric Materials: From Mechanisms to Applications.

With the tendency of thermoelectric semiconductor devices towards miniaturization, integration, and flexibility, there is an urgent need to develop high-performance thermoelectric materials. Compared with the continuously enhanced thermoelectric properties of thermoelectric materials, the understanding of toughening mechanisms lags behind. Recent advances in thermoelectric materials with novel crystal structures show intrinsic ductility. In addition, some promising toughening strategies provide new opportunities for further improving the mechanical strength and ductility of thermoelectric materials. The synergistic mechanisms between microstructure-mechanical performances are expected to show a large set of potential applications in flexible thermoelectric devices. This review explores enlightening research into recent intrinsically ductile thermoelectric materials and promising toughening strategies of thermoelectric materials to elucidate their applications in the field of flexible thermoelectric devices.

The intrinsic high ductility in B2 intermetallic compound YAg: Causality clarified by ab initio study

The intrinsic high ductility in B2 intermetallic compound YAg: Causality clarified by ab initio study