Local order influences initiation of plastic flow in metallic glass: Effects of alloy composition and sample cooling history

Bulk metallic glasses (MGs) with tunable plasticity and strength have been reported recently. Using Cu–Zr and Cu–Zr–Al MG models, here we illustrate how and why alloy composition and cooling history influence the initial flow behavior in the early stage of plasticity. Starting from Cu₄₆Zr₅₄, either increasing the Cu concentration, or substituting Al for a few percent of Zr, increases the resistance to the initiation of plastic flow, the softening after the local yielding, and the propensity for strain localization. These effects are shown to be intrinsic to the uniform, fully amorphous MGs and rooted in their internal structure. Our quantitative monitoring of the local environment, especially the role of full-icosahedral clusters in shear transformations, identifies the fertile and resistant structural entities controlling deformation. The structural mechanisms have implications for macroscopic plasticity, and the alloy dependence of the MG structure reveals a microscopic origin underlying the varying mechanical properties.     

Stability,elastic and electronic properties of the Rh–Zr compounds from first-principles calculations

A systematic investigation on structural, elastic and electronic properties of Rh–Zr intermetallic compounds is conducted using first-principles electronic structure total energy calculations. The equilibrium lattice parameters, enthalpies of formation (E_for), cohesive energies (E_coh) and elastic constants are presented. Of the eleven considered candidate structures, Rh₄Zr₃ is most stable with the lowest E_for. The two orthogonal-type, relative to the CsCl-type, are the competing ground-state structures of RhZr. The result is in agreement with the experimental reports in the literature. The analysis of E_for and mechanical stability excludes the presence of Rh₂Zr and RhZr₄ at low temperature mentioned by .Curtarolo et al. [Calphad 29, 163 (2005)]. It is found that the bulk modulus B increases monotonously with Rh concentration, whereas all other quantities (shear modulus G, Young's modulus E, Poisson's ratio σ and ductility measured by B/G) show nonmonotonic variation. RhZr₂ exhibits the smallest shear/Young's modulus, the largest Poisson's ratio and ductility. Our results also indicate that all the Rh–Zr compounds considered are ductile. Furthermore, the detailed electronic structure analysis is implemented to understand the essence of stability.     

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔG_c, the shear band stability index k₀, and the critical shear band length l_c, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔG_c, large k₀, and small l_c. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.     

Using ab initio calculations in designing bcc Mg–Li alloys for ultra-lightweight applications

Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys, because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. In this paper, single crystal elastic constants of 11 bcc Mg–Li alloys are calculated using density functional theory (DFT) and compared with available experimental data. Based on DFT determined properties, engineering parameters such as the ratio of bulk modulus over shear modulus (B/G) and the ratio of Young’s modulus over mass density (Y/ρ) are calculated. Analysis of B/G and Y/ρ shows that bcc Mg–Li alloys with 30–50 at.% Li offer the most potential as lightweight structural material. Compared with fcc Al–Li alloys, bcc Mg–Li alloys have a lower B/G ratio, but a comparable Y/ρ ratio. An Ashby map containing Y/ρ vs B/G shows that it is not possible to increase both Y/ρ and B/G by changing only the composition of a binary alloy.     

Influence of Severe Plastic Deformation on the Structure and Properties of Al–Li–Cu–Mg–Zr–Sc–Zn Alloy

The structural and phase transformations in the Al–Li–Cu–Mg–Zr–Sc–Zn alloy have been studied by the electron microscopy after the aging for the maximum strength and in the nanostructured state after severe plastic deformation by high-pressure torsion. It has been shown that severe plastic deformation leads to the formation of a nanostructured state in the alloy, the nature of which is determined by the magnitude of deformation and the degree of completeness of the dynamic recrystallization. It has been established that deformation also causes a change in the phase composition of the alloy. The influence of the structural components of the severely deformed alloy on the level of mechanical properties, such as the hardness, plasticity, elastic modulus, and stiffness has been discussed.     

Structural, elastic and electronic properties of Mg(Cu1−xZnx)2 alloys calculated by first-principles

The structural, elastic and electronic properties of Mg(Cu_1−xZn_x)₂ alloys (x = 0, 0.25, 0.5,and 0.75) were investigated by means of first-principle calculations within the framework of density functional theory (DFT). The calculation results demonstrated that the partial substitution of Cu with Zn in MgCu₂ leaded to an increase of lattice constants, and the optimized structural parameters were in very good agreement with the available experimental values. From energetic point of view, it was found that with increase of Zn content the structural stability of Mg(Cu_1−xZn_x)₂ alloys decreased apparently. The single-crystal elastic constants were obtained by computing total energy as a function of strain, and then the bulk modulus B, shear modulus G, Young's modulus Y and Poisson's ratio ν of polycrystalline aggregates were derived. The calculated results showed that among the Mg(Cu_1−xZn_x)₂ alloys, MgCuZn exhibited the largest stiffness, while Mg₂Cu₃Zn showed the best ductility. Finally, the electronic density of states (DOSs) and charge density distribution were further studied and discussed.     

Peculiar elastic behavior of Ti–Nb–Ta–Zr single crystals

The cause of a low Young’s modulus was investigated in quaternary β-type Ti–Nb–Ta–Zr alloys, as the modulus is decreased to prevent bone absorption and degradation of bone quality when these alloys are implanted into human bones. This investigation was carried out using the alloys′ single crystals. Acoustic measurements and analysis by the Hill approximation revealed that a low Young’s modulus in a polycrystalline form is caused by the low shear modulus c′, related to the low β-phase stability, low c₄₄, and relatively low bulk modulus B compared with those of binary Ti-based alloys. Furthermore, it was found that the single crystals had strong orientation dependence on Young’s modulus, where that in the 〈1 0 0〉-direction E₁₀₀ is the lowest of all crystallographic orientations. For quaternary Ti–29Nb–13Ta–4.6Zr alloy (mass%), E₁₀₀ is only ∼35 GPa, which is similar to Young’s modulus of human cortical bones as a result of the low B and c′. These results indicate that decreases in c′, c₄₄ and B are essential for decreasing Young’s modulus of novel β-type Ti alloys which are expected to be developed in the near future.     

The dependence of shear modulus on dynamic relaxation and evolution of local structural heterogeneity in a metallic glass

Starting from the nanoscale structural heterogeneities intrinsic to metallic glasses (MGs), here we show that there are two concurrent contributions to their microscale quasi-static shear modulus G_I: one (μ) is related to the atomic bonding strength of solid-like regions and the other (G_II) to the change in the possible configurations of liquid-like regions (dynamic relaxation). Through carefully designed high-rate nanoscale indentation tests, a simple constitutive relation (μ = G_I + G_II) is experimentally verified. On a fundamental level, our current work provides a structure–property correlation that may be applicable to a wide range of glassy materials.     

Formation of amorphous phase with crystalline globules in Fe–Cu–Si–B and Fe–Cu–Zr–B immiscible alloys

The microstructure of rapidly solidified melt-spun ribbons of (Fe_0.75M_0.10B_0.15)_100−xCu_x (M = Si, Zr) alloys was investigated focusing on amorphous-phase formation and the solidification structure. In this study, Fe–Cu–Si–B and Fe–Cu–Zr–B alloys were designed to show amorphous-phase formation and liquid-phase separation simultaneously. Amorphous-phase formation was confirmed in both Fe–Cu–Si–B and Fe–Cu–Zr–B alloys. Minor exceptions in a combination map of mixing enthalpy and quaternary predicted phase diagram are acceptable range for designing a quaternary Fe–Cu-based alloy system that shows liquid-phase separation in Fe-based and Cu-based liquids and the formation of an Fe-based amorphous phase.     

Bulk metallic glasses with large plasticity: Composition design from the structural perspective

While most bulk metallic glasses (BMGs) appear brittle at room temperature, appreciable compressive plastic strains have been observed for some glass compositions. The origin of this behavior is not understood, and the compositions of such plastic BMGs remain difficult to predict. Here we explain the plasticity observed in a Zr–Cu(Ni)–Al BMG, based on a computational analysis of the composition-dependent internal structures that influence shear transformations and shear localization behavior under loading. A strategy is then proposed to design BMG compositions with the desired local order for significant compressive plasticity, and is demonstrated by the successful discovery of an Hf₆₂Ni₂₅Al₁₃ BMG capable of sustaining large compressive strains. The experimentally measured compressive strength, glass transition temperature and Poisson’s ratio, which are all composition dependent, are also shown to be macroscopic indicators that correlate well with the predictions from the atomic level structure.     

Structure and shear deformation of metallic crystalline–amorphous interfaces

The structure and shear properties of crystalline–amorphous laminar nanocomposites are studied in an atomistic model of face-centered cubic copper with amorphous Cu₄₆Zr₅₄ bulk metallic glass in the quasi-static limit. The plastic shear deformation response is determined by the production and motion of interface dislocations at the crystalline–amorphous interface, which is closely linked to the structural and chemical transition from crystalline Cu to the amorphous Cu/Zr phase. The implication of interfacial shear are discussed in context of dislocation–interface interactions and co-deformation of a crystalline–amorphous nanocomposite.     

Effect of addition of Be on glass-forming ability,plasticity and structural change in Cu–Zr bulk metallic glasses

The present study reports the effect of the addition of Be in Cu–Zr bulk metallic glass (BMG) on glass-forming ability (GFA), plasticity and structural change. Although Be has a negative enthalpy of mixing with all the constituent elements of these glasses, Cu_47.5Zr₄₀Be_12.5 alloy exhibits apparent double glass transitions (T_g) and enhanced plasticity as well as improved GFA. Intensive structural analysis using extended X-ray absorption fine structure suggests that a large difference in the enthalpy of mixing between atom pairs in multi-component BMGs can cause atomic scale structural inhomogeneity and/or locally favored structures in the amorphous matrix, resulting in enhanced compressive strains, although the enthalpies of mixing for atom pairs are all negative. This concept may shed light on the development of BMGs with large plasticity as well as high GFA.     

Theoretical investigation of typical fcc precipitates in Mg-based alloys

First-principles calculations were performed to study structural, elastic and electronic properties of typical face-centered cubic (fcc) precipitates of Mg-based alloys (Mg₃Gd, Mg₃Gd_0.5Y_0.5 and Mg₃Zn₃Y₂) within the generalized gradient approximation. The calculated results show that the substitution of part of the Gd with Y in Mg₃Gd leads to a slight decrease in the cell volume (0.35%), and the lattice parameters obtained after full relaxation of crystalline cells are in good agreement with the experimental data. The calculated negative formation enthalpies and the cohesive energies show that these typical fcc precipitates of Mg-based alloys have good alloying ability and structural stability. According to the calculated density of states of these phases, it is found that the highest structural stability of Mg₃Zn₃Y₂ is attributed to an increase in the bonding electron numbers below the Fermi level. In addition, the elastic constants C_ij of these phases were also calculated, and the bulk modulus B, shear modulus G, Young^’s modulus E, Poisson^’s ratio ν and anisotropy value A of polycrystalline materials were derived from the elastic constants. The mechanical properties are further discussed.     

Wear and Corrosion Behavior of Zr-Doped DLC on Ti-13Zr-13Nb Biomedical Alloy

Zirconium (Zr)-doped DLC was deposited on biomedical titanium alloy Ti-13Nb-13Zr by a combination of plasma-enhanced chemical vapor deposition and magnetron sputtering. The concentration of Zr in the films was varied by changing the parameters of the bi-polar pulsed power supply and the Ar/CH₄ gas composition. The coatings were characterized for composition, morphology, nanohardness, corrosion resistance in simulated body fluid (SBF) and tribological properties. X-ray photoelectron spectroscopy (XPS) studies on the samples were used to estimate the concentration of Zr in the films. XPS and micro-Raman studies were used to find the variation of I _D/I _G ratio with Zr concentration. These studies show that the disorder in the film increased with increasing Zr concentration as deduced from the I _D/I _G ratio. Nanohardness measurements showed no clear dependence of hardness and Young’s modulus on Zr concentration. Reciprocating wear studies showed a low coefficient of friction (0.04) at 1 N load and it increased toward 0.4 at higher loads. The wear volume was lower at all loads on the coated samples. The wear mechanism changed from abrasive wear on the substrate to adhesive wear after coating. The corrosion current in SBF was unaffected by the coating and corrosion potential moved toward nobler (more positive) values.     

Reaction behavior and formation mechanism of ZrC and ZrB2 in the Cu–Zr–B4C system

In this work, the formation mechanism of ZrC and ZrB₂ in the Cu–Zr–B₄C system was studied by differential scanning calorimeter and X-ray diffraction. Moreover, the effect of heating rate on the reaction behavior was also investigated. The results revealed that the heating rate did hardly influence the reaction process and product in the range of 10–30 °C/min. The formation mechanism of ZrC and ZrB₂ in the Cu–Zr–B₄C system could be ascribed to the solid-state reaction between Zr and B₄C particles, and the replacement reactions of B₄C with the Cu–Zr liquid and copper zirconium compounds. The addition of Cu in the Cu–Zr–B₄C reactants can change the phase evolution route via producing various Cu–Zr intermediate phases and promote the formation of ZrC and ZrB₂.     

First-principles investigations on structural,elastic, thermodynamic and electronic properties of Ni3X (X = Al,Ga and Ge) under pressure

The structural, elastic, thermodynamic and electronic properties of L1₂-ordered intermetallic compounds Ni₃X (X = Al, Ga and Ge) under pressure range from 0 to 50 GPa with a step of 10 GPa have been investigated using first-principles method based on density functional theory (DFT). The calculated structural parameters of Ni₃X at zero pressure and zero temperature are consistent with the experimental data. The results of bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio v, anisotropy index A^U and Debye temperature Θ_D increase with the increase of external pressure. In addition, the Debye temperature of these compounds gradually reduce as the order of Ni₃Al > Ni₃Ga > Ni₃Ge. The ratio of shear modulus to bulk modulus G/B shows that the three binary compounds are ductile materials, and the ductility of Ni₃Al and Ni₃Ga can be improved with pressure going up, while Ni₃Ge is opposite. Finally, the pressure-dependent behavior of density of states, Mulliken charge and bond length are analyzed to explore the physical origin of the pressure effect on the structural, elastic and thermodynamic properties of Ni₃X.     

Elastic constants of binary Mg compounds from first-principles calculations

Elastic constants (C_ij's) of 25 compounds in the Mg–X (X = As, Ba, Ca, Cd, Cu, Ga, Ge, La, Ni, P, Si, Sn, and Y) systems have been predicted by first-principles calculations with the generalized gradient approximation and compared with the available experimental data. Ductility and the type of bonding in these compounds are further analyzed based on their bulk modulus/shear modulus ratios (B/G), Cauchy pressures (C₁₂–C₄₄), and electronic structure calculations. It is found that MgNi₂ and MgCu₂ have very high elastic moduli. Mg compounds containing Si, Ge, Pb, Sn, and Y, based on their B/G ratios, are inferred as being brittle. A metallic bonding in MgCu₂ and a mixture of covalent/ionic bond character in Mg₂Si, as inferred from their electronic structures, further explain the corresponding mechanical properties of these compounds.     

Structural,half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb,As and Si) and IrMnAs from first-principles calculations

The structural, half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb, As and Si) and IrMnAs were investigated using first-principles calculations within the generalized gradient approximation (GGA) based on density function theory (DFT). The most stable lattice configurations about site occupancy are (Ni)_4a(Mn)_4c(Sb)_4d, (Ni)_4a(Mn)_4c(As)_4d, (Ni)_4a(Mn)_4c(Si)_4d and (Ir)_4a(Mn)_4c(As)_4d, respectively, and the exchange of elements in Wyckoff position 4c and 4d results in an identical (symmetry-related) phase. The half-Heusler compounds show half-metallic ferromagnetism with a half-metallic gap of 0.168 eV, 0.298 eV, 0.302 eV and 0.109 eV, respectively, and the total magnetic moments (M_tot) are 4.00 μ_B, 4.00 μ_B, 3.00 μ_B and 3.00 μ_B per formula unit, respectively, which agree well with the Slater–Pauling rule based on the relationship of valence electrons. The compound (Ir)_4a(Mn)_4c(As)_4d with half-metallic ferromagnetic character was reported for the first time. The individual elastic constants, shear modulus, Young's moduli, ratio B/G and Poisson's ratio were also calculated. The compounds are ductile based on the ratio B/G. The Debye temperatures derived from the average sound velocity (ν_m) are 327 K, 332 K, 434 K and 255 K, respectively. The predicted Debye temperature for NiMnSb agrees well with the available experimental value, and the Debye temperatures for the rest three compounds were reported for the first time.     

Formation and properties of Ti-based Ti–Zr–Cu–Fe–Sn–Si bulk metallic glasses with different (Ti + Zr)/Cu ratios for biomedical application

Ti-based Ti–Zr–Cu–Fe–Sn–Si bulk metallic glasses (BMGs) free from highly toxic elements Ni and Be were developed as promising biomaterials. The influence of (Ti + Zr)/Cu ratio on glass-formation, thermal stability, mechanical properties, bio-corrosion resistance, surface wettability and biocompatibility were investigated. In the present Ti-based BMG system, the Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy exhibited the highest glass forming ability (GFA) corresponding to the largest supercooled liquid region, and a glassy rod with a critical diameter of 3 mm was prepared by copper-mold casting. The Ti-based BMGs possess high compressive strength of 2014–2185 MPa and microhardness of 606–613 Hv. Young's modulus of the Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy was about 100 GPa, which is slightly lower than that of Ti–6Al–4V alloy. The Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy with high GFA exhibited high bio-corrosion resistance, and good surface hydrophilia and cytocompatibility. The mechanisms for glass formation as well as the effect of (Ti + Zr)/Cu ratio on bio-corrosion behavior and biocompatibility are discussed.     

Nature of atomic trajectories and convective flow during plastic deformation of amorphous Cu50Zr50 alloy at room temperature-classical molecular dynamics studies

To identify the atomistic mechanism of plastic deformation in metallic glasses (MGs), classical molecular dynamics (MD) simulation of plastic deformation of amorphous Cu₅₀Zr₅₀ alloy is carried out for the strain rates varying from 10⁻³ s⁻¹ to 10¹¹ s⁻¹ and for two states of stress viz. pure shear and uniaxial tension. Although MD studies of plastic deformation in MGs have been widely reported previously, atomic movements have seldom been studied. In the present study, random movement of atoms, as observed in diffusion, is observed irrespective of the strain rate and state of stress. The average ratio of distance traveled to the magnitude of net displacement by an atom, within a time interval, is found to increase with strain rate. It suggests that the randomness in the atomic trajectories, although present, decreases with the strain rate. At high strain rates, control masses are observed to translate (convective flow) and exchange atoms with their neighboring control masses, losing their individuality within a short time. These observations clearly suggest that there is no such thing as shear transformation zone (STZ), and the atomistic mechanism of plastic deformation in MGs is exactly like that in liquid.