Plasticity Of Bulk Metallic Glasses Research Articles

Synergistic effect of crystalline metal on the plasticity of bulk metallic glasses under uniaxial synchro-compression

A uniaxial synchro-compression test of a cylindrical bulk metallic glass (BMG) specimen along with a crystalline metallic ring having an inner diameter much larger than the specimen’s diameter and thereby without radial confinement on the BMG specimen was performed. The plastic deformation behavior of a conventional Zr65Cu17.5Ni10Al7.5 BMG under synchro-compression with a cupper ring was investigated. It was found that remarkably plastic deformation in the inherently brittle BMG could occur under the uniaxial stress state as the loading area of the cupper ring was sufficiently large, and multiplication of the shear band on the synergistically deformed BMG specimen could be accomplished. The synergistic effect of the crystalline metal on the plastic deformation behavior of the BMG was attributed to the reduced thermal softening extent inside the shear bands that resulted from the restraining effect of the copper ring on the spring-back action of the testing machine.

Stress-induced structural inhomogeneity and plasticity of bulk metallic glasses

The correlation between stress-induced structural inhomogeneity and plasticity in bulk metallic glasses (BMGs) was studied. It was found that the structural inhomogeneity induced by an appropriate stress field can markedly enhance macroscopic compressive plasticity in various monolithic BMGs. The origin for the plasticization is correlated to the size of the plastic zone of BMGs.

On factors influencing the ductile-to-brittle transition in a bulk metallic glass

An experimental study to ascertain the ductile-to-brittle transition (DBT) in a bulk metallic glass (BMG) was conducted. Results of the impact toughness tests conducted at various temperatures on as-cast and structurally relaxed Zr-based BMG show a sharp DBT. The DBT temperature was found to be sensitive to the free-volume content in the alloy. Possible factors that result in the DBT were critically examined. It was found that the postulate of a critical free volume required for the amorphous alloy to exhibit good toughness cannot rationalize the experimental trends. Likewise, the Poisson’s ratio–toughness correlations, which suggest a critical Poisson’s ratio above which all glasses are tough, were found not to hold good. Viscoplasticity theories, developed using the concept of shear transformation zones and which describe the temperature and strain rate dependence of the crack-tip plasticity in BMGs, appear to be capable of capturing the essence of the experiments. Our results highlight the need for a more generalized theory to understand the origins of toughness in BMGs.

Effect of Pre-Introduced Shear Bands Direction on Deformation Behavior in Zr<SUB>55</SUB>Al<SUB>10</SUB>Ni<SUB>5</SUB>Cu<SUB>30</SUB> Bulk Metallic Glass

We investigated the effect of pre-introduced shear bands direction on compressive deformation behavior in Zr55Al10Ni5Cu30 bulk metallic glass. The compressive deformation behavior varied with the difference of the direction of pre-introduced shear bands with respect to loading direction. The specimen in which shear bands were introduced at an angle of 45 degree to the loading direction showed largest plastic strain. This large plasticity was achieved due to the activity of pre-introduced shear bands and formation of multiple shear bands. The direction of preintroduced shear bands is an important factor to improve the plasticity in bulk metallic glasses. [doi:10.2320/matertrans.MAW200920]

Geometry Constrained Plasticity of Bulk Metallic Glass

effect even with the sample aspect ratio to be larger than 1. Based on this experiment, we have shown that the extraordinary plasticity of BMGs of more that 50% is mostly due to the geometry constrained effect. Such compressive plastic behavior under geometrically constrained conditions as observed in the present case should be distinguished from the inherent ductility of BMGs. 2. Experimental Zr58Cu22Co4Fe4Al12 and Zr55Cu30Ni5Al12 alloy ingots were prepared by arc melting high purity Zr (99.9%), Cu (99.99%), Co (99.9%), Fe (99.9%), Ni (99.9%) and Al (99.99%) under an Ar atmosphere. The former alloy shows relatively large unconstrained plasticity, i.e., inherently plastic BMG, while the latter was selected as a typical alloy that rupture without plastic deformation, i.e., inherently brittle BMG. 2 mm diameter rods of 50 mm length metallic glass samples were prepared using copper mold casting from the ingots in an Ar-atmosphere. The casting unit was equipped with a radiation temperature recorder. Melt temperatures before casting were varied from 1253 to 1323 K for the Zr58Cu22Co4Fe4Al12 alloy to provide two different levels of unconstrained plasticity. 18) The Zr55Cu30Ni5Al12 alloy was cast from 1253 K. The macroscopic amorphous nature of the cast alloys was confirmed using x-ray diffraction (XRD) before use. Compression tests were performed at a quasi-static strain rate of 1 � 10 � 2 s � 1 in an Instron machine using rod samples of 2 mm diameter at room temperature. Both the contact surfaces of the sample with jig are lubricated with silicon grease to minimize the friction. Clip gauge was used during mechanical testing. The minimum length up to which the clip gauge can work was 1.6 mm. The length of the samples has been varied in such a way to get AR from 2 to 1.2. 3. Results and Discussion

Designing bulk metallic glass and glass matrix composites in martensitic alloys

To circumvent the limited plasticity of bulk metallic glasses (BMGs), heterogeneous materials with glassy matrix and different type and length-scale of heterogeneities (micrometer-sized second phase particles or fibers, nanocrystals in a glassy matrix, phase separated regions, variations in short-range order by clustering) have been reported. We developed bulk metallic glasses and glass matrix composites in martensitic Zr–Cu-base alloys. Large plasticity can be obtained from microstructure consisting of either a glassy structure, or for alloys with martensitic second phase embedded in a glassy matrix. This type of glasses and glass–matrix composites are able to achieve high strength together with pronounced work-hardening. We explore the possibilities to synthesize such in situ composite microstructures based on shape memory alloys (“M-Glasses”) through metal mold casting.

The effect of nanocrystallization and free volume on the room temperature plasticity of Zr-based bulk metallic glasses

To understand the mechanism of the room temperature plasticity of bulk metallic glasses (BMGs), microstructure observations, density measurements and positron annihilation studies were carried out for Zr-based BMGs cast at various temperatures and post-annealed under different conditions. We found that higher casting temperatures cause partial crystallization, which enhance the plasticity as long as the volume fraction of the crystalline phase is low. However, a similar nanocrystalline microstructure produced by post-annealing often leads to a large loss of plasticity, while certain conditions enhance the plasticity. Based on density measurements and positron annihilation lifetime spectroscopy, we conclude that the relative contribution of free volume and nanocrystallization is important for acquiring plasticity in metallic glasses.

Plasticity of bulk metallic glasses improved by controlling the solidification condition

Different bulk metallic glasses (BMGs) were prepared in ductile Cu47.5Zr47.5Al5, Zr62Cu15.4Ni12.6Al10, and brittle Zr55Ni5Al10Cu30 alloys by controlling solidification conditions. The achieved microstructures were characterized by x-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and synchrotron- based high-energy x-ray diffraction. Monolithic BMGs obtained by high-temperature injection casting are brittle, while BMGs bearing some nanocrystals with the size of 3 to 7 nm and 2 to 4 nm, obtained by low-temperature injection casting and in situ suction casting, respectively, exhibit good plasticity. It indicates that the microstructures of BMGs are closely affected by the solidification conditions. Controlling the solidification conditions could improve the plasticity of BMGs.

A study on the evolution of subsurface deformation in a Zr-based bulk metallic glass during spherical indentation

Due to its several advantages, including a simple testing procedure and the use of a small volume of material, the instrumented indentation technique has been widely performed to elucidate the characteristics of plasticity in bulk metallic glasses (BMGs) which typically show very different deformation behaviour from that of crystalline counterparts. As an effort to shed additional light on the topics, here we examined the inhomogeneous plastic deformation of a Zr–Cu–Ni–Al–Ti BMG by performing both macroscopic instrumented indentation (with a spherical indenter) and nanoindentation (with a three-sided pyramidal Berkovich indenter) on the interface-bonded sample as well as on a bulk sample. Especially, we put emphasis on analysing the evolution of shear-band-ruled deformation generated underneath the indenter during spherical indentation. It was revealed that the inter-band spacing and the shear band density are independent of indentation load and thus stress level. Furthermore, subsequent performance of nanoindentation showed that the subsurface region under the indenter was indeed softened and had quite different deformation characteristics from that of the un-deformed region.

Slithering of Shear Bands Can Improve Plasticity in Bulk Metallic Glasses

Slithering of Shear Bands Can Improve Plasticity in Bulk Metallic Glasses

Free-volume-induced enhancement of plasticity in a monolithic bulk metallic glass at room temperature

The limited plasticity of bulk metallic glasses (BMGs) is commonly circumvented by introducing micrometer- or nanometer-sized heterogeneities (crystallites and/or phase separation) into the BMGs. In this work, we report the improvement of plasticity in a ternary monolithic BMG caused by a large amount of randomly distributed free volume induced during solidification using a high cooling rate. This indicates that introducing large amounts of free volume into BMGs might be another promising way of improving the plasticity of these materials.

Improvement of plasticity by tailoring combination of constituent elements in Ti-rich Ti–Zr–Be–Cu–Ni bulk metallic glasses

The effect of replacement of Ti with Y or Nb in Ti-rich Ti–Zr–Be–Cu–Ni bulk metallic glasses (BMGs) has been investigated. The minor addition (MA) of Y (Y–Ti: +58 kJ/mol) induced phase separation into Y-rich crystalline particles and Ti-rich amorphous matrix, while the MA of Nb (Nb–Ti: +10 kJ/mol) led to nanocrystallization in Ti-rich BMGs with icosahedral nuclei. This result indicates that MA of elements having positive enthalpy of mixing can induce a different degree of instability in the single amorphous matrix depending on the amount of repulsive interaction energy. In particular, MA of Nb (up to 4 at.%) significantly increased the compressive fracture strain (ϵf) up to ∼9.35 ± 0.2%, which indicates that the plasticity of BMGs can be enhanced by the size-modulated icosahedral phase embedded in the amorphous matrix.

Plasticity in bulk metallic glasses investigated via the strain distribution

We measured the atomic-scale elastic strain in order to investigate the yielding of ${\mathrm{Zr}}_{55}{\mathrm{Cu}}_{20}{\mathrm{Ni}}_{10}{\mathrm{Al}}_{10}{\mathrm{Ti}}_{5}$ and ${\mathrm{Cu}}_{47.5}{\mathrm{Zr}}_{47.5}{\mathrm{Al}}_{5}$ bulk metallic glasses (BMGs) by x-ray synchrotron radiation at room temperature. High resolution strain scanning reveals a deviation from the linear stress-strain relationship at the onset of macroplastic flow. Similar to polycrystalline metals, a saturation of the elastic strain components has been revealed in the case of the ``plastic'' ${\mathrm{Cu}}_{47.5}{\mathrm{Zr}}_{47.5}{\mathrm{Al}}_{5}$ BMG. The results show that the atomic-level elastic strains of the plastic ${\mathrm{Cu}}_{47.5}{\mathrm{Zr}}_{47.5}{\mathrm{Al}}_{5}$ BMG are more homogeneous compared to the ``brittle'' ${\mathrm{Zr}}_{55}{\mathrm{Cu}}_{20}{\mathrm{Ni}}_{10}{\mathrm{Al}}_{10}{\mathrm{Ti}}_{5}$ glass.

Enhance plasticity of bulk metallic glasses by geometric confinement

We report that bulk metallic glasses (BMGs) with large plasticity can be obtained in conventional brittle BMGs by a shrink-fit metal sleeve. The mechanical performance especially the plasticity in the Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG with a shrink-fit copper sleeve is much enhanced. The approach results in the formation of the highly dense and frequent interacting and arresting events of shear bands and is the origin of the observed large global plasticity. The results present another simple step toward toughening the inherently brittle BMGs.

Stress–strain behaviors of Ti-based bulk metallic glass and their nanostructures

We have investigated the compression stress–strain behaviors of Ti40Zr25Cu12Ni3Be20 bulk metallic glasses prepared by Cu mold casting from various melt temperatures. Plastic strain was found to vary sensitively on the temperature of melts and subsequent annealing conditions. To understand the origin of the plasticity change, the microstructures were characterized using transmission electron microscopy and a laser-assisted three-dimensional atom probe. The fully amorphous sample cast from 1273 K showed 0.6% plastic strain, and it was enhanced to 1.3% after isothermal annealing at 573 K. The sample cast from 1423 K showed 3.0% plastic strain, from which the presence of nanocrystals with a volume fraction of about 12% was confirmed. The sample cast from a higher temperature (1573 K) contained a larger fraction of crystals, which showed limited plastic strain. The effect of the volume fraction of the nanocrystals on the plasticity of bulk metallic glasses is discussed based on the experimental results.

Phase separation and improved plasticity by modulated heterogeneity in Cu–(Zr, Hf)–(Gd, Y)–Al metallic glasses

The effect of replacement of Zr/Hf with Gd or Y in Cu46Zr47Al7 and Cu50Hf42.5Al7.5 alloys has been investigated. Phase separation into Cu–Gd-rich (or Cu–Y-rich) and Cu–Zr-rich (or Cu–Hf-rich) amorphous phases occurs during cooling when the Gd (or Y) content is above 15 at.% in Cu–(Zr, Hf)–(Gd, Y)–Al alloys. In particular, it was realized that the plasticity in bulk metallic glass with heterogeneity can be enhanced either by tailoring the combinations of the constituent elements or by controlling the cooling rate.

Critical size and strength of the best bulk metallic glass former in the Mg–Cu–Gd ternary system

We have developed a Mg61Cu28Gd11 bulk metallic glass (BMG) that has the highest glass-forming ability among all the known ternary Mg-based BMGs. We also used a microcompression test to determine the intrinsic strength of such brittle Mg-based BMGs. The strength and plasticity of BMGs at small sample sizes are discussed in comparison with their behavior in conventional compression tests. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A parameter governing the plasticity of Cu–Zr containing bulk metallic glasses

Experiments were conducted on bulk metallic glasses (BMGs) to identify a crucial factor that governs the degree of the plasticity shown by these alloys. It was found that in situ formation of nanocrystallites, which is known as the deformation-induced nanocrystallization, play a key role in providing initiation sites of the local deformation, indicating that a high degree of plasticity could be achieved from BMGs in which deformation facilitates nanocrystallization. The degree of this deformation-induced nanocrystallization was found to be strongly dependent on the activation energy for crystallization. In this paper, we report that a factor governing the plasticity of BMGs lies in the activation energy for crystallization.

Influence of relaxation and crystallization on micro-hardness and deformation of bulk metallic glass

The development of super-plastic precise forming technology, utilizing viscous flowing property exhibited by bulk metallic glasses (BMGs) in super-cooled liquid region, will lead to wider applications of these alloys. During the process of super-plastic forming in high temperature of BMGs, relaxation is inevitable and crystallization will occur if adopting unsuitable forming process. So it is very meaningful to realize how relaxation and crystallization influence the mechanical properties of BMGs. In this paper, the samples of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 BMG are annealed under different conditions. The relaxation and crystallization behaviors, the micro-hardness of the annealed samples are analyzed and measured. The deformation behaviors of the annealed samples in super-cooled liquid region are investigated by small punch test (SPT). The results show that the structure relaxation caused by annealing at the temperature lower than glass translation temperature T g makes the micro-hardness increase slowly, but the crystallization caused by annealing at the temperature higher than T g makes the micro-hardness increase rapidly and along with the crystallization proportion arising. Structure relaxation engenders a little influence on the plasticity of BMG in super-cooled liquid region, but slightly crystallizing makes the deformation load increase rapidly. The bulk metallic glass displays different degrees of brittleness as the crystallization proportion increases. The suitable super-plastic forming time of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22 BMG at 390 °C is 10 min. The deformation force of the alloy increases rapidly when the forming time exceeds 10 min and the plasticity decreases rapidly when the forming time exceeds 30 min.

Design of Bulk metallic glasses with high glass forming ability and enhancement of plasticity in metallic glass matrix composites: A review

To overcome some of the limits of existing metallic alloys, a new alloy design concept has been introduced recently in order to control the crystallinity, i.e. to utilize crystalline, quasicrystalline, and amorphous structures. In particular, bulk metallic glasses (BMGs) receive great attention because of their unique properties due to their different atomic configuration. Recently, significant progress in enhancing glass forming ability (GFA) has led to the fabrication of BMGs having potential for application as structural and functional materials. Moreover, successful design of BMG matrix composite microstructure suggests that the plasticity of BMGs can be controlled properly. In this review article, we introduce recent research results on the design of BMGs with high GFA and on the enhancement of plasticity in metallic glass matrix composites.