Materials Science Forum Vol. 849

Abstract: High-entropy alloys of NbTaVTiNi, NbTaVTiCo, NbTaVHfCo, NbTaVHfNi, and NbTaVZrNi were synthesized by vacuum arc melting. The crystal structure was identified by an X-ray diffractometer analysis. Microstructures and element composition of the resulting alloys were investigated using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer. All alloys presented the dendritic structure which is mainly composed of body-centered cubic (NbTaV,X) (X=Ti, Hf, Zr) phases and eutectic-like phases such as TiNi, TiCo, HfCo, HfNi and ZrNi. (NbTaV,X) phases take the function of hydrogen permeability while eutectic-like phases bear the responsibility of resistance to hydrogen embrittlement. Taking the microstructure of alloys into account, NbTaV-based high entropy alloys is appropriate for improving hydrogen permeability.

Abstract: Metallic glasses have been expected to be used as structural materials since their high strength and high hardness. Unfortunately, their catastrophic brittle fracture behavior with poor plasticity becomes the major weakness for structural application. It has been recognized that the mechanism of plastic deformation in metallic glasses is through the formation of shear bands, which provide them with limited ductility. Laser shock peening (LSP) is an innovative surface treatment technique which can introduce deep compressive residual stress layer to materials for improving their mechanical behavior. In this work, a finite-element model has been developed to numerically simulate the pure bending process, LSP and subsequent bending process. An advanced constitutive equation was established based on the large deformation theory of nonlinear mechanics, the free volume model and the Coulomb-Mohr yield criterion. The model is able to capture the following results: (i) for a given bending deflection, the shear band spacing increases with increasing plate thickness; (ii) for a given plate thickness, the free volume increases with the bending deflection; (iii) for a given thickness and a given deflection, the shear bands increase under the effect of LSP.

Abstract: A Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (vit1) bulk metallic glass was processed by Nd: Glass laser pulses with duration 30ns and energy in the range 20 to 30J. The surface morphology and surface micro-hardness of the vit1 metallic glass, treated with varying laser energy, had been studied in detail. Laser shock peening induced plastic deformation and caused a micro-dent to be generated on the vit1 surface. The optical profiling tests showed that laser pulse energy greatly influenced the diameter and depth of the micro-dents. The surface roughness which was caused by various laser pulse energy was assessed and characterized. The three-dimensional surface topography of the laser treated region on vit1 surfaces had been characterized. In addition the plastic deformation features were also studied.

Abstract: Metallic glasses (MGs) exhibit extremely high strength and superior resistance to corrosion. They are also supposed to be resistant against displacive irradiation due to their inherent disordered structure, and thereby are viewed as potential candidates for applications in irradiation environments. However, the structures and properties evolution of metallic glasses, especially bulk metallic glasses (BMGs), under irradiation has not been fully understood up to now. In this work, the structural stability and damage characteristics of a Zr-based BMG under helium ions irradiation environment were investigated. Meanwhile, the effect of structural relaxation and crystallization on the irradiation response of the BMG was also studied. Results show that the BMG reserves the amorphous structure within the studied range of fluence, and exhibits better irradiation resistance compared to that of the crystalline alloys. In our opinion, the initial free volume concentration affects the damage morphology of the BMG, while partial crystallization will lead to significantly embrittlement under irradiation.

Abstract: The influence of direct current pulse on–off patterns on spark plasma sintering of [Fe_0.8Co_0.2B_0.05Si_0.2]₉₆Nb₄ on kinetics competition and coordination mechanism of the interface bonding and crystallization has been investigated systematically. No crystallization of metallic glassy matrix and good bonding state between the particles were responsible for good mechanical properties of the fabricated Fe-based bulk glassy alloy at 2:9 (on: off) pattern. The higher local micro area’s instantaneous power and larger cooling time caused by low pulse duty ratio played a vital role in consolidation of amorphous alloy powder and desired structural properties.

Abstract: A series of CoFeNi₂W_0.5Ta_x (x = 0-0.6) high entropy alloys (HEAs) were synthesized by arc melting to investigate the alloying effect of Ta element on the microstructure and mechanical properties of the CoFeNi₂W_0.5 alloy system. Phase constitution, microstructure and mechanical properties of the alloys were analyzed by X-ray diffraction (XRD), scanning electron microscopes (SEM), Vickers hardness and compressive test. It was found that when x = 0, the alloy consists of a single-phase face-centered cubic (FCC) solid solution structure and exhibit excellent ductility, the compressive plastic elongation of which can reach 80% without fracture. While with increasing Ta content, the brittle Co₂Ta-type Laves phase appears which leads to a decrease of the plastic strain and an increase of the yield strength, and the Vickers hardness shows an obvious increase from HV 179.5 to HV 753.2.

Abstract: The microstructures, phase composition and hardness of the AlCrFeNi_xMo_0.2 high entropy alloy (x=0.5, 0.8, 1.2 and 1.5, the x values refer to molar ratio) were reported. When the value of x was smaller than 1.2, the alloys consisted of BCC and B2 structures. The BCC and B2 phases were identified to be (Cr, αFe) solid solution and NiAl intermetallic compound, respectively. With the increase of x from 0.5 to 1.2, the microstructure transformed from dendrite/inter-dendrite to eutectic microstructures. When the x was equal to 1.5, besides BCC and B2 phases, another CrFe2.32MoNi phases formed and Net-like (Cr, αFe) phases distributed in the NiAl intermetallic compound matrix. The hardness first decreased then increased with the increase of Ni content. Generally, Ni element is a FCC stabilizer. However, in AlCrFeNi_xMo_0.2 alloys, Ni element promoted the formation of B2 and CrFe2.32MoNi phases. The influence mechanism of Ni element was discussed systematically.

Abstract: The properties of Iron-based metallic glasses, such as glass-forming ability and soft magnetic properties, (MGs) have been widely investigated. However, seldom reports are available concerning the properties of these iron-based melts. In the present work, the viscosity of superheated Fe-Si-B-Nb metallic glass forming liquids (MGFLs) was measured by a torsional oscillating viscometer. It has been found that the crucial condition to get viscosity data using graphite crucibles is to reach a superheated degree of at least 250 K for melts prior to measurements. The viscosity increases monotonically with a decrease in temperature before solidification, without distinct changes of dynamic mechanism during cooling. The present work indicates that these iron-based melts are heterogeneous even above the liquidus temperature. This finding helps to understand the melts of iron-based MGs and to improve the production process of iron-based glass ribbons.

Abstract: In this paper, the effects of annealing treatment on the microstructure, thermal property and magnetic properties of mechanical alloyed FeSiBAlNiC and FeSiBAlNiCe amorphous high entropy alloys has been systematically investigated using X-ray diffraction, differential scanning calorimetry, field emission scanning electron microscopy and alternation gradient magnetometry. The results showed that annealing at different temperature range induced the pronounced microstructural evolution from the amorphous phase to a mixture of amorphous phase and intermetallics but without any formation of solid solutions. In addition, the thermal stability and heat resistance were enhanced with increasing the annealing temperature. Similarly, annealing treatment also had significant effect on the particle morphology, making the particles evolve from near-round shape to plate-like shape. Furthermore, subsequent annealing of W6-Ce amorphous HEAs enhanced the saturation magnetization (Ms) remarkably (from 0.6 emu/g of 140 h as-milled W6-Ce to 20.17 emu/g). But for the Ms values of the 140 h as-milled amorphous W6-C HEAs, annealing in different temperature range all exhibited a slightly decreased effect.