Coarse-grained Molecular-level Analysis of Polyurea Properties and Shock-mitigation Potential

Several experimental investigations reported in the open literature clearly established that polyurea (PU), an elastic copolymer, has an unusually high ability to attenuate and disperse shock waves. This behavior of PU is normally attributed to its unique nanometer-scale two-phase microstructure consisting of (high glass-transition temperature, T _g) hydrogen-bonded discrete, hard domains dispersed within a (low T _g) contiguous soft matrix. However, details regarding the mechanism(s) responsible for the superior shock-wave mitigation capacity of PU are still elusive. In the present study, molecular-level computational methods and tools are used to help us identify and characterize these mechanism(s). Because the shock-wave front structure and propagation involve coordinated motion of a large number of atoms and nano-second to micro-second characteristic times, these phenomena cannot be readily analyzed using all-atom molecular-level modeling and simulation techniques. To overcome this problem, all-atom PU microstructure is coarse-grained by introducing larger particles (beads), which account for the collective degrees of freedom of the constituent atoms, the associated force-field functions determined and parameterized using all-atom computational results, and the resulting coarse-grained model analyzed using conventional molecular-level computational methods and tools. The results thus obtained revealed that a combination of different deformation mechanisms (primarily shock-induced ordering and crystallization of hard domains and coordinated shuffle-like lateral motion of the soft-matrix segments) is most likely responsible for the superior ability of PU to attenuate/disperse shock waves.     

Concept-Level Analysis and Design of Polyurea for Enhanced Blast-Mitigation Performance

Polyurea is an elastomeric co-polymer in which the presence of strong hydrogen bonding between chains gives rise to the formation of a nano-composite like microstructure consisting of discrete hard-domains distributed randomly within a compliant/soft matrix. Several experimental investigations reported in the open literature have indicated that the application of polyurea external coatings and/or internal linings can substantially improve ballistic penetration resistance and blast survivability of buildings, vehicles and laboratory/field test-plates. Recently, it was proposed that transition of polyurea between its rubbery state and its glassy state under high deformation-rate loading conditions is the main mechanism responsible for the improved ballistic-impact resistance of polyurea-coated structures. As far as the shock-mitigation performance of polyurea is concerned, additional/alternative mechanisms such as shock-impedance mismatch, shock-wave dispersion, fracture-mode conversion, and strain delocalization have been suggested (without validation). In this study, an attempt is made to identify the phenomena and processes within polyurea which are most likely responsible for the observed superior shock-mitigation performance of this material. Towards that end, computational methods and tools are used to investigate shockwave generation, propagation, dispersion, and transmission/reflection within polyurea and the adjoining material layers as present in the case of a blast-loaded assembly consisting of a head covered with a polyurea-augmented helmet. The results obtained show that for effective shock mitigation, the operation of volumetric energy-dissipating/energy-storing processes is required. Candidate processes of this type are identified and presented.     

Amorphous and nanocrystalline Al82Ni10Y8 alloy powder prepared by gas atomization

An Al–10Ni–8Y (at.%) alloy was atomized by Ar gas and the morphology, microstructure, thermal stability, phase composition and microhardness of the as-atomized powder were investigated. Most of the powders are spherical in shape, but the surface morphology was different for powder of different size. The cross-section microstructure of powder with size below 15 μm in diameter showed no detailed feature, indicating existence of amorphous phase or nanocrystalline structure. The as-atomized powder showed four distinct exothermic peaks when heated at 296, 340, 366 and 456 °C. The glass transition temperature T_g, crystallization temperature T_x, and the temperature interval of the supercooled liquid region ΔT_x (=T_x−T_g) were detected to be about 266, 288 and 22 °C. The Al₈₂Ni₁₀Y₈ alloy powder exhibits a high Vickers hardness of 230.6, and shows great potential for structural application.     

Identification of microstructural mechanisms during densification of a TiAl alloy by spark plasma sintering

This work aims at identifying, by coupled scanning and transmission electron microscopy (SEM and TEM) observations, the densification mechanisms occurring when an atomized Ti-47Al-1W-1Re-0.2Si powder is densified by spark plasma sintering (SPS). For this purpose, interruptions of the SPS cycle have been performed to follow the evolution of the microstructure step by step. The powder particles exhibit a classical dendritic microstructure containing a large amount of out-of-equilibrium α phase. During heating-up, the microstructure undergoes successive transformations. At T = 525-875 °C the α phase transforms into γ. The γ phase formed is supersaturated in W and Re. It de-saturates for T above 875 °C by discontinuous precipitation of W and Re-rich B2 phase. Densification takes place for T between 900 °C and 1150 °C by plastic deformation of the powder particles. TEM observations show that the repartition of the plastic deformation is correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occur. Microstructural phenomena directly resulting from the high currents involved in the SPS process have not been observed.     

Effect of LiSbO3 on the phase structure, microstructure and electric properties of Sr0.53Ba0.47Nb2O6 ceramics

LiSbO₃ doped Sr_0.53Ba_0.47Nb₂O₆ ceramics were synthesized by conventional mixed-oxide method. The phase structure, microstructure, dielectric and ferroelectric properties of obtained ceramics were investigated. Pure tungsten bronze structure could be obtained in all ceramics and LiSbO₃ additive could promote densification and reduce the sintering temperature. The dielectric characteristics showed diffuse phase transition phenomena for all samples, which was proved by linear fitting of the modified Curie-Weiss law with γ value varying between 1.65 and 1.92. With increasing LiSbO₃ content, the transition temperature T_c decreased gradually to near room temperature. Normal ferroelectric hysteresis loops could be observed in all compositions, but the remnant polarization (P_r) and coercive field (E_c) all decreased gradually. Besides, the underlying mechanism for variations of the electrical properties caused by LiSbO₃ doping was explained in this work.     

A new glass-forming ability criterion for bulk metallic glasses

A new indicator of glass-forming ability (GFA) for bulk metallic glasses (BMGs) is proposed based on crystallization processes during cooling and reheating of the supercooled liquid. The interrelationship between this new parameter and the critical cooling rate or critical section thickness is elaborated and discussed in comparison with two other representatives, i.e. reduced glass transition temperature T_rg (=T_g/T_l, where T_g and T_l are the glass transition temperature and liquidus temperature, respectively) and supercooled liquid range ΔT_xg (=T_x−T_g, where T_x is the onset crystallization temperature and T_g the glass transition temperature). Our results have shown that ΔT_xg alone cannot infer relative GFA for BMGs while the new parameter γ, defined as T_x/(T_g+T_l), has a much better interrelationship with GFA than T_rg. An approximation of the critical cooling rate and critical section thickness for glass formation in bulk metallic glasses is also formulated and evaluated.     

Embrittlement of Zr-based bulk metallic glasses

Embrittlement of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature T_g for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub-T_g annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above T_g. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (<T_g) is sufficient to embrittle the BMG significantly. Above T_g, plasticity is retained for annealing far beyond the relaxation time but, instead, embrittlement is caused by crystallization. The magnitude of the decrease in Poisson’s ratio is insufficient to explain the severe embrittlement within the framework of a critical value as previously suggested.     

Effect of Single and Double Sintering on Microstructure and Magnetic Properties of Gd-Substituted Ni-Cd Ferrites

Two different groups, I and II, which are fired one and two times, respectively, from Ni-Cd ferrites of chemical formula Ni_0.7Cd_0.3Gd _x Fe_2?x O₄ (x = 0 to x = 0.1 with step = 0.025) have been prepared by conventional ceramic method. The effect of Gd-substitution on the microstructure and magnetic properties of the two groups has been studied. X-ray patterns indicated the presence of a minor secondary phase with the spinel phase at Gd-concentrations with x = 0.075 and 0.1. SEM and VSM are used to investigate the microstructure and to measure the magnetization of the samples at room temperature, respectively. The initial permeability is measured as a function of temperature and Curie temperature is determined. It was found that there are considerable differences in the microstructure and the magnetic properties of the two groups. Although the Gd-substitution decreased the values of both saturation magnetization M _s and initial permeability μ_i, the value of Curie temperature T _C has been improved. Moreover, the double sintering process improved the magnetization and densification of samples.     

Enthalpy recovery and free volume relaxation in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass

We report on the free volume of the Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅ bulk metallic glass in terms of its enthalpy recovery and volumetric relaxation below the glass transition temperature, T_g. Glassy samples are isothermally annealed below T_g using differential scanning calorimetry and the resulting enthalpy recovery, ΔH_r, is measured upon re-heating into the supercooled liquid region. Volumetric changes below T_g are measured isothermally using Thermo-Mechanical Analysis. The total changes in the relative free volume, Δν_f/ν_m, between the initially glassy state and the equilibrium liquid are calculated from the volumetric relaxation. The measured values of ΔH_r and Δν_f/ν_m correlate well within the framework of free volume theory and a linear relationship is found between the two.     

Thermodynamics of La based La–Al–Cu–Ni–Co alloys studied by temperature modulated DSC

The onset and offset melting temperatures T_m and T_l, glass transition temperature T_g and heat of fusion ΔH_m of six La based La–Al–Cu–Ni–(Co) alloys were measured by DTA or DSC at a heating rate of 20 K/s. The absolute values of specific heat capacity for the undercooled liquid and the corresponding crystalline of these six alloys were obtained by means of temperature-modulated DSC (TMDSC). Entropy, enthalpy and Gibbs free energy differences between the undercooled liquid and crystalline of these alloys as a function of temperature have been calculated. Their glass forming ability was discussed from a thermodynamic point of view.     

Glass–rubber transition temperature of polyaniline: Experimental and molecular dynamic simulation

Available literature reports wide variation of glass–rubber transition temperature (T_g) of polyaniline (PAni). The present study determines the T_g of doped PAni using different techniques such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), temperature dependent DC conductivity, and dilatometry, and it is found to vary between 30 and 40 °C. Apart from measurement techniques the T_g of PAni is found to depend on the water and dopant present in it. The T_g of PAni is again determined through molecular dynamic (MD) simulation technique and found to be in agreement with the experimentally obtained results.     

Bulk metallic glass formation in binary Cu-rich alloy series – Cu100−xZrx (x=34, 36, 38.2, 40 at.%) and mechanical properties of bulk Cu64Zr36 glass

The compositional dependence of a glass-forming ability (GFA) was systematically studied in a binary alloy series Cu_100−xZr_x (x=34, 36, 38.2, 40 at.%) by the copper mold casting method. Our results show the critical casting thickness jumps from below 0.5 mm to above 2 mm when x changes from 34 to 36 while further increase in x reduces the critical casting thickness. The best glass former Cu₆₄Zr₃₆ does not correspond to either the largest undercooled liquid region (ΔT=T_x1−T_g, where T_g is the glass transition temperature, and T_x1 is the onset temperature of the first crystallization event upon heating) or the highest reduced glass transition temperature (T_rg=T_g/T_l, where T_l is the liquidus temperature). Properties of bulk amorphous Cu₆₄Zr₃₆ were measured, yielding a T_g∼787 K, T_rg∼0.64, ΔT∼46 K, H_v (Vicker's Hardness) ∼742 kg/mm², Young's Modulus ∼92.3 GPa, compressive fracture strength ∼2 GPa and compressive strain before failure ∼2.2%.     

Features of the thermal behavior of PMMA/C<Subscript>60</Subscript> film composites

Polymethylmethacrylate-based film composites containing small additives of fullerenes (up to 3 wt %) are obtained. The thermal behavior of the obtained materials is studied by DSC in the temperature range from 25 to 130°C. It is found that the character of the DSC curve depends on the composite composition. For films containing up to 0.1 wt % C₆₀, one glass transition temperature (T _g ^soft) is observed, while in the case of films with a higher concentration of the filler, two glass transition temperatures (T _g ^soft and T _g ^solid) are observed. It is found that the dependence of T _g ^soft value on the content of fullerenes is nonmonotonic with a minimum at 0.5 wt % of C60.     

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB₂ particles were investigated. The densification on-set temperature (T_d-micron) and grain growth on-set temperature (T_g-micron) for micron-sized ZrB₂ particles were about 1500 °C and 1800 °C, respectively. And the densification on-set temperature (T_d-nano) and grain growth on-set temperature (T_g-nano) for nano-sized ZrB₂ particles were about 1300 °C and 1500 °C, respectively. A bimodal micron/nano-ZrB₂ ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2 ± 35.8 MPa and an improved fracture toughness of 7.2 ± 0.4 MPa·m^1/2 were obtained. The measured critical thermal shock temperature difference (ΔT_c) for this bimodal micron/nano-ZrB₂ ceramic was as high as 433 °C.     

Effect of Te additions on the glass transition and crystallization kinetics of (Sb15As30Se55)100−xTex amorphous solids

Differential scanning calorimetry results under non-isothermal conditions of chalcogenide (Sb₁₅As₃₀Se₅₅)_100?xTe_x (where 0 ≤ x ≤ 10 at.%) glasses are reported and discussed. The dependence of the characteristic temperatures “glass transition temperature (T_g), the crystallization onset temperature (T_c) and the crystallization temperature (T_p)” on the heating rate (β) utilized in the determination of the activation energy for the glass transition (E_g), the activation energy for crystallization (E_c), the glass thermal stability (ΔT = T_c ? T_g) and the Avrami exponent (n). The composition dependence of the T_g, E_g, and E_c were discussed in terms of the chemical bond approach, the average heats of atomization and the cohesive energy (CE). The diffractogram of the transformed material shows the presence of some crystallites of AsSb, Sb₄Te₆, As₂Se₃ and Sb₂Se₃ in the residual amorphous matrix.     

Microstructure, mechanical, EPR and optical properties of lithium disilicate glasses and glass-ceramics doped with Mn ions

The microstructure, mechanical, EPR and optical properties of transparent MnO₂ doped lithium disilicate (LDS) glass-ceramics prepared by melt quenching and controlled crystallization, have been studied. The microstructure of the glass-ceramics has been characterized using FE-SEM, TEM, FT-IR and XRD techniques. FE-SEM micrographs show elongated, highly interlocked, dense (∼80 vol.%) nanocrystals of LDS with an average size ∼100 nm. XRD and FT-IR studies reveal that the only crystalline phase formed after heat-treatment at 700 °C for 1 h is LDS. A good combination of average microhardness ∼5.6 GPa, high fracture toughness ∼2.8 MPa m^1/2, 3-point flexural strength ∼250 MPa and moderate elastic modulus 65 GPa has been obtained. The EPR spectra of both LDS glasses and glass-ceramics exhibit resonance signals with effective g values at g = 4.73, g = 4.10, g = 3.3, and g = 1.98. The resonance signal at g = 1.98 is found to be more intense than the other signals and exhibits hyperfine structure at lower concentration of manganese. From the observed spectrum, the spin-Hamiltonian parameters have been evaluated. In glass samples the optical absorption spectrum exhibits a broad band around ∼20,320 cm⁻¹ which has been assigned to the transition ⁶A_1g(S) → ⁴A_1g(G) ⁴E_g(G)-of Mn²⁺ ions. The cerammed samples upon 394 nm excitation emit a green luminescence (565 nm, ⁴T_1g → ⁶A_1g(G) transition of Mn²⁺ ions), and a weak red emission (710 nm). From the ultraviolet absorption edges, the optical bandgap energies (E_opt) were evaluated and are discussed.     

High-strength Cu-based bulk glassy alloys in Cu–Zr–Ti and Cu–Hf–Ti ternary systems

New Cu-based bulk glassy alloys were formed in Cu–Zr–Ti and Cu–Hf–Ti systems by the copper mold casting method. The critical diameter is 4 mm for the Cu₆₀Zr₃₀Ti₁₀ and Cu₆₀Hf₂₅Ti₁₅ alloys which are larger than 1 mm for the Cu₆₀Zr₄₀ and Cu₆₀Hf₄₀ glassy alloys. The substitution of Zr or Hf for Ti causes an increase in the glass-forming ability (GFA). As the Ti content increases, the glass transition temperature (T_g), crystallization temperature (T_x), and the supercooled liquid region ΔT_x(=T_x−T_g) decrease for both Cu₆₀Zr_40−xTi_x and Cu₆₀Hf_40−xTi_x alloys. In contrast, the liquid temperature (T_l) has a minimum value of 1127 K for the Cu₆₀Zr₂₀Ti₂₀ alloy and 1175 K for the Cu₆₀Hf₂₀Ti₂₀ alloy, resulting in a maximum T_g/T_l of 0.63 and 0.62, respectively. The alloys with the highest T_g/T_l value showed the highest GFA for these Cu-based alloys. The bulk glassy alloys exhibit high tensile fracture strength of 2000–2160 MPa, compressive fracture strength of 2060–2150 MPa and compressive plastic elongations of 0.8–1.7%. The finding of the new Cu-based bulk glassy alloys with high GFA, high fracture strength above 2000 MPa and distinct plastic elongation is encouraging for the future development of a new type of bulk glassy alloy which can be used for structural materials.     

Simulation study on non-linear effects of initial melt temperatures on microstructures during solidification process of liquid Mg7Zn3 alloy

The non-linear effects of different initial melt temperatures on the microstructure evolution during the solidification process of liquid Mg₇Zn₃ alloys were investigated by molecular dynamics simulation. The microstructure transformation mechanisms were analyzed by several methods. The system was found to be solidified into amorphous structures from different initial melt temperatures at the same cooling rate of 10 × 10¹² K/s, and the 1551 bond-type and the icosahedron basic cluster (12 0 12 0) played a key role in the microstructure transition. Different initial melt temperatures had significant effects on the final microstructures. These effects only can be clearly observed below the glass transition temperature T_g; and these effects are non-linearly related to the initial melt temperatures, and fluctuated in a certain range. However, the changes of the average atomic energy of the systems are still linearly related with the initial melt temperatures, namely, the higher the initial melt temperature is, the more stable the amorphous structure is and the stronger the glass forming ability will be.     

Properties of the Ti<Subscript>40</Subscript>Zr<Subscript>10</Subscript>Cu<Subscript>36</Subscript>Pd<Subscript>14</Subscript> BMG Modified by Sn and Nb Additions

The results of investigation of the influence of additions of 2 and 3 at.% of Sn and simultaneously of Sn and 3 at.% Nb on microstructure and properties of the bulk metallic glasses of composition (Ti₄₀Cu_36?x Zr₁₀Pd₁₄Sn _x )_100?y Nb _y are reported. It was found that the additions of Sn increased the temperatures of glass transition (T _g), primary crystallization (T _x ), melting, and liquidus as well as supercooled liquid range (ΔT) and glass forming ability (GFA). The nanohardness and elastic modulus decreased in alloys with 2 and 3 at.% Sn additions, revealing similar values. The 3 at.% Nb addition to the Sn-containing amorphous phase decreased as well all the T _g, T _x , T _L, and T _m temperatures as ΔT and GFA; however, relatively larger values of this parameters in alloys containing larger Sn content were preserved. In difference to the previously published results, in the case of the amorphous alloys containing small Nb and Sn additions, a noticeable amount of the quenched-in crystalline phases was not confirmed, at least of the micrometric sizes. In the case of the alloys containing Sn or both Sn and Nb, two slightly different amorphous phase compositions were detected, suggesting separation in the liquid phase. Phase composition of the alloys determined after amorphous phase crystallization was similar for all compositions. The phases Cu₈Zr_3, CuTiZr, and Pd₃Zr were mainly identified in the proportions dependent on the alloy compositions.     

Phase formation and thermal stability in Cu–Zr–Ti(Al) metallic glasses

In the present study we investigate the phase formation and the thermal stability of Cu₅₀Zr_50 ? xTi_x (0 ≤ x ≤ 10) and (Cu_0.5Zr_0.5)_100 ? xAl_x glass-forming alloys. Parameters indicating the glass-forming ability (GFA) are calculated from isochronal and isothermal calorimetric experiments. A high Ti content in the Cu–Zr–Ti alloys causes the precipitation of a metastable ternary Laves phase (C15), which does not form in Cu–Zr–Al. Accompanied with it is a significant drop in the activation energy of crystallization. Also the supercooled liquid region (ΔT_x = T_x ? T_g), the reduced glass transition temperature (T_rg = T_g/T_liq), and the γ parameter (γ = T_x/(T_g + T_liq)) (T_x: crystallization temperature, T_g: glass transition temperature and T_liq: liquidus temperature) are sensitive to the change in the crystallization sequence. The fragility values calculated are believed to overestimate the GFA of the investigated alloys. Careful selection of the alloy composition enables the targeted precipitation of different crystalline phases.