After all the confusion and perplexity, joy and elation, finally it comes to the stage when we are humble and grateful, as the last thing to learn from the doctoral studies.
I find an ordinary "thank you" entirely inadequate to express my gratitude to my advisor, Prof. Guruswami Ravichandran, who knows the students better than themselves and has never been parsimonious in encouraging them to explore their scientific enthusiasm and giving them his full support. I am indebted to him for introducing me to the enchanting world of experimental mechanics and providing the maximum freedom to let me pursue my interest in metallicglasses. In my moments of straying and wandering along the way, I especially thank him for bringing me back to focus, and helping me to distinguish important issues and interesting problems with a discerning and candid manner. Over four and half years I learned from him the great qualities of being not only a good scientist but also a responsible and considerate person. I wish I could bring his integrity, generosity, and sharpness with me in my future path.
banding operations in front of the plastic zone, rather than the shear banding repeatedly at pre-existing shear bands.
3.3.2 Effect of composition on the serration behavior
For readily observed, we choose P-h curves with the loading rate of 0.1 mNs -1 for three kinds of Zr-based BMGs and plotted in one pattern, as shown in Figure 30. In the same loading rate, the serrated plastic flows of the three kinds of BMGs are different, the Zr 50 Cu 40 Al 10 is more pronounced than that for Zr 65 Cu 18 Ni 7 Al 10 , and however, Zr 50 Cu 30 Al 10 Ni 10 do not have the serrated plastic flows. Another phenomenon is that the curve on the top of the Zr 65 Cu 18 Ni 7 Al 10 is very close to the curve of Zr 50 Cu 40 Al 10 and far away of the Zr 50 Cu 30 Al 10 Ni 10 . Due to the P-h curves are displaced on the same axes with the origins offset 150 nm, the final load depth of the Zr 65 Cu 18 Ni 7 Al 10 is greater than Zr 50 Cu 40 Al 10 and Zr 50 Cu 30 Al 10 Ni 10 . This trend means that the hardness of the Zr 65 Cu 18 Ni 7 Al 10 is bigger than Zr 50 Cu 40 Al 10 and Zr 50 Cu 30 Al 10 Ni 10 , which corresponding the results in Figure 19. The significant difference of the three Zr-based metallicglasses reflects their microstructures, in terms of free volume, and short and middle-range ordering are dissimilarity. As shown in Figure 19 and Figure 30, the hardness and reduced modulus of Zr 65 Cu 18 Ni 7 Al 10 is smaller than Zr 50 Cu 40 Al 10 , and that the plastic flow of former alloy shows less serration than the latter. The basic principle for this phenomenon is not well understood at this moment, but will be deep research in the future. Liu et al. 
7. Conclusive Remarks
In general, bulkmetallicglasses (glassy alloys) have high strength about two times higher than that of their crystalline counterparts but poor plasticity. However, the selected bulkmetallicglasses and crystal-glassy composites on their base exhibit both high strength and good room-temperature plasticity. Moreover, in the case of the Pd-based alloys their plasticity can be drastically improved by fluxing with B 2 O 3 . On deformation, bulkmetallicglasses exhibit periodic drops of stress indicating so-called serrated flow, while no such a behavior is seen on testing at cryogenic temperature and in case of various composite samples. Disappearance of the stress drops may be connected with a more homogeneous deformation. On the other hand, submicron-size Zr 65 Ni 10 Cu 5 Al 7.5 Pd 12.5 glassy samples demonstrate homogeneous deformation at room temperature. Some metallicglasses which have pre-existing nuclei demonstrate deformation-induced nanocrystallization within heavily deformed areas while alloys which exhibit nucleation and growth behavior on heating are less predisposed to demonstrate such a behavior. The nanocrystallization was also observed upon cyclic loading.
Integrating with related investigations, the present manuscript further generalizes the coupled thermo- mechanical shear model into the multi-axial stress status, and takes all the contributions of free volume, temperature and hydrostatic stress into account. Besides, based on the coalescence mechanism of free volume in BMGs, a failure criterion of critical free volume concentration, ξ c , is introduced. Furthermore, the generalized multi- axial-stress-status model is extended to describe the mechanical behavior of BMGs at hightemperature (T ≥ T g ) by estimating related material parameters. Then the constitutive model and failure criterion are implemented into LS-DYNA commercial software through the user material subroutine (UMAT), and FEM simulations for different initial material temperatures are conducted integrated with the related test data. The evolutions of parameters as well as their effects on the mechanical behavior of material are analyzed, and corresponding mechanism of the macroscopic behavior is discussed.
For the excellent physical and mechanical properties such as high strength and hardness, BMGs have potential of wide applications in the engineering areas, and has raised much attention in recent years. Especially, their fracture mechanism is widely investigated. As the atomic structures are disordered, plastic deformation of this kind of materials is prone to be localized into thin shear bands. Hence the fracture process of BMGs is usually undergoing highly localized deformation via formation and rapid propagation of shear bands .
Recently, the study of such high-temperature plastic ﬂow of bulkmetallicglasses in supercooled liquid have been conducted, and an interesting result that the strain hardening occurs due to the crystallization during deformation has been reported. 9–13) Takigawa et al. have been reported that pre- annealed Zr 65 Al 10 Ni 10 Cu 15 bulkmetallic glass exhibits large homogeneous elongation in a supercooled liquid region. 13) The reported results are shown in Figs. 1 and 2. 13) As- received specimen exhibits signiﬁcant necking, but uniform deformation occurs and large elongation is obtained in the specimen annealed at 673 K for 1.8 ks before tensile test as shown in Fig. 1. As shown in Fig. 2, apparent strain hardening occurs in the specimen annealed at 673 K for 1.8 ks, but, on the contrary, initial stress increase compared with in as-received in the specimen annealed at 673 K for 2.7 ks. They concluded that the strain hardening due to precipitation and growth of icosahedral phase, which is enhanced by deformation in a supercooled liquid region
Many early results on metallicglasses provide important
information for the characterization of inhomogeneous ﬂow behavior in amorphous metals. 19–22) One of the ﬁrst experi- ments tackling the inhomogeneous ﬂow kinetics in metallicglasses has shown preferential etching of shear bands in deformed samples — a clear indication of a modiﬁcation of the atomic structure during straining, 19) as was also suggested recently in Refs. 23–25. Serrated ﬂow was recognized in stress-strain diagrams, and each measured stress drop correlated with a shear event. It was also noted that pre- existing shear bands are reactivated more easily than new shear bands are formed, which is further evidence for the change in the atomic structure of shear bands. 19,20) In addition, a change in the serration amplitude with increasing strain was observed and associated with diﬀerent stages of deformation. 20) The temperature dependence of the appear- ance and the disappearance of serrations observed in a stress strain diagram 19–22) was explained with an adiabatic shear model proposed by Chen and Leamy. 20,26) In this model a suﬃciently hightemperature is needed for the shear event to initiate a local decrease in viscosity and result in an overall drop in the stress, i.e. serrated ﬂow. 20)
The second stage of the thermal tempering analysis is the mechanical problem which de- termines the stress evolution in the sample given the temperature solution. The mechanical problem requires modeling of the material behavior in addition to the experimental deter- mination of stresses. The constitutive behavior is modeled at three levels of viscoelastic phenomenology using the instant freezing, viscoelastic, and structural models. The first is a simple analytical estimate for residual stresses and requires only the glass transition tem- perature as rheological input. The second is the temperature dependent thermoviscoelastic treatment that employs time-temperature superposition. The third accounts for the tem- perature history dependence of the glass structure. The constitutive laws for the viscoelas- tic and structural models are incorporated into the finite element method (ABAQUS TM software package), allowing the application of these models to complex geometries. For implementation of the structural model, and to investigate the advanced rheological fea- tures of the material, structural volume recovery experiments are conducted with a Perkin Elmer TMA 7. The in-situ measurement of transient stresses during BMG casting is, yet, not possible. In fact, high-resolution, accurate measurement of residual stresses in these opaque and amorphous materials is challenging. The crack compliance method is deter- mined as the ideal method to accomplish this goal, after evaluating few other methods such as the hole-drilling strain gauge method and the layer removal method. Also, the use of the non-destructive neutron diffraction method on a crystalline phase that is cast with a BMG sample is assessed in a model composite.
Metallicglasses, i.e. amorphous alloys produced during solidiﬁcation from the melt, possess various excellent properties, compared with their crystallized state, such as high strength and toughness, high corrosion resistance and good soft magnetic properties for ferromagnetic alloys. Until 1980’s, however, most metallicglasses have been produced only as thin foil samples, which restricts the usefulness for application of the excellent properties of the metallicglasses, particularly of their high strength. Since the end of 1980’s, Inoue et al. have succeeded in producing bulkmetallicglasses with a thickness of several millimeters to a few centimeters for various multi-component alloys. 1) The syn- thesis of bulkmetallicglasses has expanded the application ﬁelds of metallicglasses. 2)
A wide range of superior mechanical properties of both chemically and structurally inhomogeneous bulkmetallicglasses renders a unique opportunity for developing new advanced materials, which can be exploited for a variety of engineering applications. Monolithic bulkmetallicglasses do not show macroscopic plastic deformability but recent advances in the area of structurally inhomogeneous glasses with high Poisson’s ratio open a new door for ‘processing for unique properties’ of bulkmetallicglasses. Even though the deformation mechanisms of BMGs are rather poorly under- stood, high elastic strain and high strength, good corrosion resistance and high fracture toughness of BMGs and their composites bring a intriguing perspective for replacing conventional crystalline metals and alloys. Clustered glasses based on martensitic alloys exhibit high compressive strength and large plastic deformability providing an unique oppor- tunity to develop a large number of ductile BMGs or ‘M- Glasses’ in Ni-Zr, Ni-Ti, Fe-Ni, Fe-Pt, Ti-Pt and several other near-martensitic alloys.
Another useful part that can be made using semi-solid induction forging is a thin plate. We have observed that there is high demand for BMG composite plates in dimensions less than or equal to 1 mm and yet these parts are difficult to die-cast. Thermoplastic forming has been used for many years to flatten ingots of BMGs into submillimeter thicknesses, but this has not been accomplished with toughened composites. Semi-solid induction forging was used successfully to produce low-oxygen content plates of BMG matrix composites in thickness from 0.25-10 mm. To assure that the thin plates are two-phase composites, x-ray scans were performed on two 1 mm plates of DH1 and DV1, shown in Figure 4-5a-b. In both scans, b.c.c. peaks are superimposed on a glassy background, indicating the thins plates have been processed without heterogeneous nucleation of another phase or without total vitrification. A 0.5 mm plate with diameter ~5 cm is shown in Figure 4-5c. To illustrate the high elastic limit of the thin plate, Figure 4-5d-e shows the 0.5 mm thick plate of DH1 in a three-point bending fixture undergoing elastic deformation. The plate can be bent substantially, but when the force is removed, it elastically returns to a flat plate.
On the other hand, Li et al 10 state that by increasing the sliding velocity there is an increase in wear rate for as cast, crystallized, and annealed Vitreloy 1. Wear rate has also been shown to directly relate to the sliding distance for Cu-based BMGs. 11
Most tests tend to be performed on the lower end of the force and sliding velocity spectrum. A reason for this is, as a wear test is performed, heat is generated. If the sliding velocity was high enough, the material wouldn’t have enough time to diffuse heat away from the wear track, and temperatures could rise to the glass transition range (or higher). Likewise, for higher pressures, more work is being done at the point of contact, so more heat should be generated. Therefore, at high pressures and sliding speeds, the most heat will be generated, and a breakdown to the linear nature of sliding velocity and load can be expected. At this point, crystallization or displacement of material through flow instead of plasticity can be expected. In order to probe this region, a few experiments have been performed. Huang et al found a decrease in wear resistance along with hardness as temperature increased in a Fe-based BMG. 14 Liu et al performed wear tests near the glass transition temperature in Vitreloy 1. They found near T g both µ and wear was reduced. As temperatures went higher, friction was reduced more as well. Yet another study found conflicting results, with the µ increasing at higher temperatures. 12
Each fracture specimen was produced via vacuum injection casting into copper mold from separately prepared ingots. Injection temperature during the casting process was not strictly controlled so the cooling rate could vary between specimens. In turn, this will leads to differences in the configurational state of the sample and associated free volume distribution . Residual stress develops during the casting process due to the hightemperature gradients which arise during sample cooling and solidification. Residual stress is known to affect fracture toughness significantly . According to Aydiner et al. , an 8.25mm thick Vitreloy 1 plate cast in a copper mold exhibited -25 to -30MPa surface compression and +10 to +13 interior tension. Their model suggested significant residual stress decreases with decreasing casting thickness. The casting thickness used in this study is 2.5mm. Aydiner et al. also showed that the compressive surface stresses were confined to a relatively thin surface layer. To reduce residual stress effects in the present work, ~10% of the surface layer was removed by grinding. This should
Bulkmetallicglasses (BMGs) possess a unique set of mechanical properties that make them attractive structural materials: yield strength > 2 GPa, fracture toughness ~20 MPa.m 1/2 and elastic strain limit ~2%. BMGs can also be cast into intricate shapes which retain their dimensional integrity and require no further machining. Unfortunately, monolithic BMGs fail catastrophically under unconstrained loading by forming shear bands. To overcome this problem, BMG matrix composites with fiber and dendritic reinforcements were proposed. The former type includes metallic fibers of Ta, Mo and stainless steel. The latter composites develop precipitates during casting and are thus called in-situ composites. Here, the reinforcements form an interpenetrating dendritic structure and enhance the ductility of the composite.
2 National Microgravity Laboratory, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, P. R. China
3 State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, P. R. China
The thermal stability and the mechanical behavior of Zr 65 Al 10 Ni 10 Cu 15 and Zr 52:5 Al 10 Ni 10 Cu 15 Be 12:5 bulkmetallicglasses (BMGs) were investigated by diﬀerential scanning calorimetry, uniaxial compressive test, ultrasonic method, and nanoindentation. The substitution of Zr by Be signiﬁcantly improved the thermal stability of the amorphous phase, exhibited by a wide supercooled liquid region of 116 K. The Be containing BMG exhibited a compressive strength of 1780 MPa, and in particular a high plastic strain of about 6%. The simultaneous operation of multiple shear bands during plastic deformation in Zr 52:5 Al 10 Ni 10 Cu 15 Be 12:5 BMG is proved by the less pronounced serrated ﬂow during the loading process in the compression and nanoindentation, as well as the fracture surface morphologies. A high Debye temperature derived from the ultrasonic measurements indicates a condensed atomic arrangement in the Be containing BMG, and may responsible for the high thermal stability.
Chapter 1: Introduction 11 low-loss magnetic cores in transformers, magnetic readheads or as components in electronic article surveillance systems. BMGs have also found applications in sports equipment, such as golf club heads, baseball bats or parts of tennis rackets due to their unique impact properties compared to conventional metals . BMGs are also capable of storing high densities of elastic energy, which led to utilisation of their efficient energy transfer characteristics this spheres for shot- peening purposes are ideal. A high elastic strain limit of BMGs resulted in their use for Coriolis mass flowmeters, pressure sensors and strain gauges with properties superior to conventional crystalline parts. Metallicglasses are also suitable for medical applications, such as implants and surgical instruments, due to biocompatibility and self-sharpening effect of BMGs. Generally good hardness and peculiar tribological properties of metallicglasses make them attractive as coating materials. And, indeed, there have been efforts to fabricate amorphous coatings, for example by low-temperature spraying or radio-frequency magnetron sputtering.
In this work, an alloy series was created by collaborators by systematically decreasing the β-stabilizing vanadium content. Changes of only 2% in the vanadium concentration resulted in drastic changes in the tensile behavior of the composite, as well as the deformation mechanisms in the dendrites. Nanoindentation was used to ascertain the properties of the glass and crystalline phases, and showed that some of the tensile behavior could be attributed to differences in the elastic properties of the two phases, although it could not explain the bulk scale behavior of the complete series. Modulus mapping was also performed on three of the alloys in the series that exhibited differing behavior and properties. The V2 alloy, which was the only one that displayed significant strain hardening as well as high strength and ductility, was also the composite that had the widest modulus distribution. As with the monolithic metallicglasses, the glass heterogeneity is an important factor in the deformation of the glass. Consideration of both the interphase heterogeneity between the glass and the crystal as well as the intraphase heterogeneity within the glass could together lead to the design of better BMGCs.
1 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2 RIMCOF Tohoku University Laboratory, R&D Institute of Metals and Composites for Future Industries, Sendai 980-8577, Japan
3 Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
We have studied the elastic and anelastic behavior of fully structural-relaxed Pd 40 Ni 40 P 20 and Zr 55 Al 10 Ni 5 Cu 30 bulkmetallicglasses using high-frequency ultrasound vibrations at elevated temperature, and found that the structural anomaly is induced by ultrasound vibrations in a supercooled liquid region. The electromagnetic acoustic resonance and resonant ultrasound spectroscopy methods were employed to measure the resonant spectra and ultrasonic attenuation coeﬃcients. When the glassy samples are subjected to sub/low-MHz ultrasound vibrations during heating process, the crystallization is accelerated around their glass transition temperatures, and with this abrupt structural change, irregular - shaped internal friction peaks appear. From the standpoint of ultrasonic echography, the glass transition and crystallization temperatures are considerably lowered by ultrasound vibrations in the present measurements.
3.2 Magnetic yoke for linear actuator
It is well known that conventional Fe- and Co-based amorphous alloys exhibit good soft magnetic properties, in particular, high saturated magnetic ﬂux density for the former alloys and high permeability with zero saturated magneto- striction for the latter alloys. 9) Although these amorphous alloys have been formed in Fe-Si-B, Fe-P-C and Co-Fe-Si-B systems, the production of their amorphous alloys requires high cooling rates exceeding 10 5 K/s and the resulting material thickness is usually limited to less than 50 mm. 9) Since 1995, Inoue and his colleagues have succeeded in ﬁnding a number of Fe-based glass-forming systems. 10–14) By use of these ﬁndings, we slightly modiﬁed the composition of Fe-Si-B-Nb 15) system in order to enhance glass-forming ability and ductility. Figure 3 shows I-H loop of glassy ribbons for (Fe 0:6 Co 0:4 ) 72 Si 4 B 20 Nb 4 . Saturation magnetiza- tion (I s ) and maximum permeability ( max ) for the (Fe 0:6 - Co 0:4 ) 72 Si 4 B 20 Nb 4 glassy ribbon are 1.15 T and 73000, respectively. These values indicate that the alloy has relatively good soft magnetic properties. Using prepared BMG plate as a set of yokes, the trial product of linear actuator was constructed. Figure 4 shows the outer appear- ance of linear actuator system including power source, AC wave generation circuit, power ampliﬁer and actuator. The actuator is assembled by two pieces of SmCo permanent