and roughness of these jagged patterns are in good agreement with the measured fracturetoughness values. This jagged region will be termed as ‘plastic zone’ hereafter. As fracture progresses these plastic zones (jagged patterns) disappear, and the rest of the fracture surface (region “B” in Figure 3-2(a)) shows the typical glassy metal dimple pattern, which is shown in Figure 3-3 and reported in many other studies [5-8,14-17]. The fracture surfaces of low fracturetoughness specimens shown in Figure 3-2(f) and (g) don’t exhibit the characteristic rough topography in front of the pre-crack, and they appear to be uniformly filled with dimples created by mode 1 opening (Figure 3-3). The severe embrittlement caused by annealing the specimens for 2.5 days at 50°C below the glass transition temperature (T g , 340°C measured by 20K/min DSC
Residual stress is known to affect fracturetoughness significantly . Residual stress develops during the casting process due to the high temperature gradients which arise during sample cooling and solidification. Compressive stress develops in the surface while tensile stress develops in the interior. The development of the plastic zone could be influenced by the residual stress. Generally, heat treatment below glass transition temperature is used to anneal out residual stress. In this study, the surfaces of all the as-cast test specimens were ground off in an attempt to remove the compressive region of the residual stress and produce a certain degree of relaxation. According to the viscoelastic model of Aydiner et al. , an 8.25mm thick Vitreloy 1 plate is estimated to develop up to -230MPa surface compression and +90MPa interior tension. However, an actual experiment by these authors revealed that a copper mold cast piece with the same thickness exhibited only -25 to -30MPa surface compression and +10 to +13 interior tension. In addition, the model suggested significant residual stress decreases with decreasing casting thickness. The casting thickness used in this study is 2.5mm. Their results also indicate that the compressive surface stresses are confined to a relatively thin surface layer. Therefore, removing ∼10% of the surface layer is believed to reduce the residual stress to an insignificant level. Indeed,
study are given in Table 1. The eﬀects of friction-welding conditions on weldability were investigated, where the friction time, rotational speed and upsetting pressure were in the range of 0.05 to 0.4 s, 1500 to 6000 min 1 and 50 to 150 MPa, respectively, as shown in Table 2. The welded specimens were investigated by optical microscopy, scanning electron microscopy (SEM) of their polished cross-section. The glassy structure was examined by micro-focused X-ray diﬀractometry using CuK radiation. The diameter of the X- ray beam was 100 mm. The joining strength of the welded BMGs was estimated by tensile tests. Tensile test specimens with a gauge length of 3.0 mm and a diameter of 2.1 mm were prepared by machining the welded BMGs. The tensile tests were carried out using an Instron tensile test machine at a strain rate of 5 10 4 s 1 . The fracture surface was observed
tension-tension test because of the blockage of un-fractured part on the fracture part. The blunting and re-sharpening of the crack propagation is not signiﬁcant, either. Thus, the fatigue striation could not be formed under the compression- compression test (Fig. 5(a)) (Mode II). Second, there is a clear boundary between the crack-propagation and fast- fracture regions under the tension-tension test. The corre- sponding fracture morphologies are the fatigue striation and vein pattern, respectively. However, under the compression- compression fatigue, although there is also a clear boundary, the fracture morphologies of both the crack-propagation near the boundary and fast-fracture regions are vein patterns except with diﬀerent vein pattern sizes. The smaller vein pattern size in the crack-propagation region could suggest a relatively slow fracture velocity, compared with the larger vein pattern size in the fast-fracture region (Figs. 5(b)–5(d)). This result could indicate that the transition from the crack- propagation region to fast-fracture region is very slow in the compression-compression fatigue, which could probably be the reason of the longer fatigue life above the fatigue- endurance limit, relative to the tension-tension fatigue. Third, the melting region under tension-tension fatigue tests usually consists of large vein patterns combined with some droplets. However, nearly no vein pattern was observed in the melting region under the compression-compression fatigue (Figs. 5(e) and 5(f)). The whole melting region is almost covered by the solidiﬁed liquid. The energy could be stored in the materials during fatigue test as the fatigue cycles increase. When the materials fracture, the stored energy de- absorb, which could lead to the increase of the temperature. Since the fatigue life under the compression-compression fatigue test is much longer than tension-tension fatigue tests. More energy could be de-absorbed in the materials, which cause more materials melt under compression-compression fatigue.
In order to find diffusion processes unique to glasses and liquids, let us discuss how to define the structure of glasses in a way meaningful in discussing their atomic transport. In the present work we define the structure topologically as a net- work in terms of atomic connectivity. Such a definition is a standard method to describe covalently bonded glasses, 15) but it can be extended to metallicglasses as well, since the neg- ative curvature of the interatomic potential tends to separate the nearest and second nearest neighbors clearly, as argued by Turnbull and Cohen in their discussion on the free-volume theory. 5) Let us consider an atom interacting via a two-body
behavior was investigated by volume relaxation, where the volume of glass was determined from the physical density measured at room temperature after relaxing the sample under a given condition. The bulk sample, which had a weight of about 2 g, was put into the evacuated quartz tube with pure Ar gas. The tube was placed into a pre-heated salt- bath for annealing the sample, and it was quickly quenched into water held at room temperature just after accomplishing a given relaxation. We believe that the structure of the relaxed glass was quenched during such an operation except for the thermal expansion effect. The surface of sample was mechanically polished with ﬁ ne abrasive papers to remove the oxide layer formed during annealing prior to the density measurement. The detail of the density experiment is shown in Refs. 6, 7).
adopting a pneumatic cylinder and gripper based on a conventional lathe, and a welding process which provides a suﬃcient cooling rate after welding were devised. Friction time and friction pressure were chosen as the control parameters in the friction welding process. Their inﬂuences on the shape and volume of the protrusion formed from the welded interface were investigated. In addition, the temperature distribution around the interface during friction welding was measured using an infrared thermal imager. In order to characterize the friction welded interface, X-ray diﬀraction (XRD) and micrographic observation of welded sections were carried out. A successful joining of Zr 55 Al 10 Ni 5 Cu 30 alloy was
interface between the BMG matrix and the inclusion determines how the composite will fail during unconfined loading. In the case of particulate reinforced BMGs, brittle materials such as carbides are often used to arrest shear band growth. Since compression tests are typically used to evaluate the mechanical properties of the composites, apparent toughening seems to occur. However, as we have seen, frictional forces combined with closing stresses on shear bands lead to plasticity in compression that is not present in unconfined loading geometries. As we know now, ex situ particle-reinforced BMGs are far more brittle that monolithic BMGs in bending or tension tests because of the interface between the glass and the particles. During unconfined loading, the particles simply separate from the matrix and shear bands grow uninterrupted. Additionally, the particles often act as stress nucleation sites for shear bands to form, which lowers the overall strength of the material. Recent work in our own group has shown that in the case where soft particles are used instead of carbides, oxide layers on the particles cause brittle interfaces between the particles and the glass matrix, and similar catastrophic failure is observed.
Gardon  reviewed the thermal tempering of silicate glasses and terms such as tem- perature equalization and solidification stresses are taken from this work. Unlike the trivial temperature equalization stresses, the analysis of solidification stresses and the stress evo- lution in the glass transition region is a complicated problem and necessitate various levels of viscoelastic phenomenology. Gardon classifies the varius models developed for silicate glasses as instant-freezing theories (e.g., Aggarwala and Saibel ), the viscoelastic theory (Lee et al. ) and the structural theory (Narayanaswamy [63, 64]). The first is a sim- plistic approach to estimate residual stresses, the second is the linear thermoviscoelastic treatment with time-temperature-superposition principle and the last one is nonlinear vis- coelastic accounting for the temperature history dependence of the glass structure. The latter theory is accepted to be the definitive theory of thermal tempering in silicate glasses since it achieved reasonable agreement with experiments for both the evolution and mag- nitude of temper stresses. In silicate glasses, photoelasticity could be used to monitor both the evolution and final value of temper stresses whereas for BMGs no known method will avail to monitor the stress evolution in situ.
In this paper, the deformation behaviour of BMGs was investigated using FEM in conjunction with the ﬁctive stress constitutive model for applying BMGs to micromachine parts. The peak stress and steady state stress values predicted by the ﬁnite element solutions on the Pd-based BMG at 573 K during the die compression and the nano-imprinting process were investigated. Implementation of the model into the MARC software has shown its versatility and good predictive capability. The emerging generation of the ﬁctive stress- based constitutive model provides an excellent tool for computer simulations of microforming operations guided by
Actually, diﬀerences between fractographs shown in Figs. 3c–3e illustrate such fracture development from region-A to region-C. Fractographs of the three regions are examined in details as follows. Figure 4 displays the fractograph of region-A. An enlarged image of the squared region in the insert is shown in Fig. 4a, where vein- like patterns smear almost all over the region. Figure 4b shows the details of the squared zone in Fig. 4a. The vein-like pattern is consisted with dimple cells of a nearly equiaxial shape at a scale of less than 10 µm, the cells’ characteristic size. In particular, the dimple cell structure in this region seems to be built by micro-voids’ stacking, so that under high magnifications this dimple structure is a porous structure with interconnected voids (Fig. 4b). Meanwhile, some small white particles, less than 100 nm in size, can be found out embedded in dimple cell walls. These dimple cells in equiaxial shape existed in region- A verifies that this region is under hydro-tensile stresses subjecting to a peak tensile stress along the sample’s thickness. The porous micro-dimple cell structure in this region indicates that, under hydro-tensile stress in the spallation plane, the resultant microdamage should be microvoids, which would result in nucleation, growth and coalescence. As a consequence, spallation occurs.
There is not currently a mathematical framework presented in the literature for modelling transformation toughening in metallicglass composites. However, there exists a body of literature surrounding the transformation toughening behaviour in zirconia-based ceramics . Developed in its original form by McMeeking and Evans , the method considers the surface tractions required to compress the transformed particle into its original space in the matrix in an Eshelby-type model.
In order to enhance the ductility of metallicglasses, the formation of heterogeneous microstructures in a composite-like manner has been found recently essential and employed in a variety of procedures combining a glassy matrix with second phase crystalline particles [85,86]; the deformation-induced nanocrystallization [87,88] is one of the possible ways illustrating this approach. One of the ways is to impede the propagation of shear bands through the sample by interaction with the phases (or microscopic pores) embedded into a glassy matrix. This enables multiplication, branching and termination of the shear bands similar to the composites where cracks are either blocked by reinforcements or blunted in ductile phases or matrix.
m ≤ 30 [4,5].Whereas, fragile liquids like ionic melts and polymers display large values ( m ≥ 100) [4,5].Bulkmetallicglasses typically have m values in the range of 32 ≤ m ≤ 70 and hence are classified in the intermediate category according to Angell’s classification..It is clear from the equation (3) and (5) the fragility parameter , m depends on T g .In order to make uniform comparison with fragility data found in the literature, m, has been evaluated at T g
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 metallicglass 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 tem- peratures before casting were varied from 1253 to 1323 K for the Zr 58 Cu 22 Co 4 Fe 4 Al 12 alloy to provide two diﬀerent
The cutting is considered one of the most basic methods in the mechanical working. Recently, Bakkal et al. had reported the chip light emission and morphology, cutting forces, surface roughness and tool wear through the investigation of the turning of Zr-based BMG together with an Al alloy and SUS304 stainless steel under the same cutting condition using the diﬀerent tool materials. 10–12) Authors also had
Metallicglasses have some amazing properties that captured my imagination and determined my path for grad school. In APh 110, Winter term 2003-2004, Dr. William L. Johnson gave a seminar lecture describing his research with an enthusiasm that was intoxicating. He described amorphous metals, materials having a random arrangement of atoms that were frozen in a liquid configuration because of a clever choice of alloying elements. These elements were chosen to have large negative heats of mixing and near eutectic compositions meaning the elements were much happier mixed than separate. The atomic sizes of alloying elements were also chosen so that many different sized spheres can log jam efforts of the mixture to crystallize upon cooling from the molten state.
The fracture topography of metallicglass has been investigated by Pampillo and Reimschuessel  and classified by features that include tributaries (T), voids (V), well- developed vein patterns (W), undeveloped vein patterns (U), and triple ridge point (R). These unique topography features, as marked in Figs. 5.11(c) and 5.11(d), are results of the highly inhomogeneous shear deformation which occurs prior to fracture and defines a plane on which cracks nucleate and propagate. The vein patterns are produced by the collision of cracks. The V in Fig. 5.11(d) indicates the site of void nucleation. In the shear plane, voids nucleate and initiate the propagating cracks. Along the line where two cracks meet, due to the reduction in stress concentration and increase in temperature, a necked ridge is generated. These ridges form the tributaries and vein patterns on the shear fractured BMG surface.
Metallicglasses have typically been treated as a “one size fits all” type of material. Every alloy is considered to have high strength, high hardness, large elastic limits, corrosion resistance, etc. However, similar to traditional crystalline materials, properties are strongly dependent upon the constituent elements, how it was processed, and the conditions under which it will be used. An important distinction which can be made is between metallicglasses and their composites. Charpy impact toughness measurements are performed to determine the effect processing and microstructure have on bulkmetallicglass matrix composites (BMGMCs). Samples are suction cast, machined from commercial plates, and semi-solidly forged (SSF). The SSF specimens have been found to have the highest impact toughness due to the coarsening of the dendrites, which occurs during the semi-solid processing stages. Ductile to brittle transition (DTBT) temperatures are measured for a BMGMC. While at room temperature the BMGMC is highly toughened compared to a fully glassy alloy, it undergoes a DTBT by 250 K. At this point, its impact toughness mirrors that of the constituent glassy matrix. In the following chapter, BMGMCs are shown to have the capability of being capacitively welded to form single, monolithic structures. Shear measurements are performed across welded samples, and, at sufficient weld energies, are found to retain the strength of the parent alloy. Cross-sections are inspected via SEM and no visible crystallization of the matrix occurs.