ABSTRACT

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ABSTRACT

In the present thesis study, we combined molecular dynamics (MD) simulations and experiments to systematically investigate the Debye-Grüneisen thermal expansion effect and configurational potential energy dependence of elastic properties for glassy states of amorphous metals, and utilized the correlation between elastic properties and fragility as a guiding tool to design and to develop novel bulk metallic glasses.

It is extremely important to create appropriate interatomic potentials, generate glassy configurations, and study the local structures of the system before moving to the next step.

An effective tight-bonding, RGL-type, n-body force field for the binary Cu-Zr alloy system was constructed and employed in MD simulations. Partial radial distribution functions, coordination numbers, and Honeycutt Andersen (HA) indices have been calculated to analyze the local structures of Cu46Zr54 metallic glass.

We report the strong dependence of elastic properties on configurational changes in a Cu-Zr binary metallic glass assessed by molecular dynamics simulations. By directly evaluating the temperature dependence and configurational potential energy dependence of elastic constants, we show that the shear modulus dependence on the specific configurational inherent state of metallic glasses is much stronger than the dependence on Debye-Grüneisen thermal expansion.

We present the isothermal equation of state (EOS) in a wide range of temperatures and pressures by carrying out molecular dynamics simulations on a simple binary model metallic glass. A universal form of EOS proposed by Vinet et al. is utilized to fit the data,

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assuming no phase transitions. Pressure-induced cavitation was observed in glassy states and liquids from our simulations. The thermodynamic limit of instability and kinetic limit of instability of the cavitation behavior were analyzed. Negative pressure is critical to trigger the cavitation. The cavitation barrier height was estimated using the classical nucleation theory. The intrinsic origin of why and how Poisson’s ratio or the ratio of G/B is involved in the deformation and fracture behavior of glasses is discussed.

The compositional dependence of thermal and elastic properties of Cu-Zr-Be ternary bulk-metallic-glass-forming alloys was systematically studied. There exists a linear relationship between the glass transition temperature, Tg, and the total Zr concentration.

Shear modulus G decreases linearly with increasing Zr concentration as well. The results also show that Tg, G, and Poisson’s ratio, ν, are very sensitive to changes in compositions.

Low Tg, low G, and relatively high ν can be achieved with high Zr and Ti concentration.

Lightweight Ti-based bulk amorphous structural metals with more than double the specific strength of conventional titanium alloys have been discovered. Thermal, elastic, and mechanical properties of these metallic glasses were studied and are presented. These amorphous alloys exhibit good glass-forming ability, exceptional thermal stability, and high strength. The research results have important implications for designing and developing low-density bulk metallic glasses. The technological potential of this class of lightweight Ti-based glassy alloys as structural metals is very promising.

The exceptional processability and large supercooled liquid region of bulk amorphous metals makes them highly promising candidates for thermoplastic processing. We report a

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lightweight (ρ = 5.4 g/cc) quaternary glass-forming alloy, Zr35Ti30Cu8.25Be26.75, having the largest supercooled liquid region, (ΔT = 159 K at 20 K/min heating rate) of any known bulk-glass-forming alloy. The alloy can be cast into fully amorphous rods of diameter ~ 1.5 cm. The undercooled liquid exhibits an unexpectedly high Angell Fragility of m = 65.6.

Based on these features, it is demonstrated that this alloy exhibits “benchmark”

characteristics for thermoplastic processing. We report results of mechanical, thermal, rheological, and crystallization (TTT-diagrams) studies on this new material. The alloy exhibits high yield strength and excellent fracture toughness, and a relatively high Poisson ratio compared with other Zr- or Ti-based glasses. Simple micro-replication experiments carried out in open air using relatively low applied pressures demonstrate superior thermoplastic processability for engineering applications.

Starting from the two binary bulk-glass formers in the Cu-Zr system, we systematically investigated the compositional dependence of glass formation, and thermal, elastic and mechanical properties in the Cu-Zr-Ag ternary alloys. Both the Cu(50-x)Zr50Agx and Cu(64- x)Zr36Agx series alloys show a good combination of high glass-forming ability and high Poisson’s ratio.

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LIST OF TABLES

Number Page Table 1.1 Summaries of bulk metallic glass alloys with critical

size ≥ 10 mm... 4 Table 1.2 Expected qualitative correlations of values of selected

properties with the kinetically strong or fragile behavior of the supercooled liquid in bulk-metallic-glass–forming

systems... 10

Table 2.1 Rosato-Guillope-Legrand (RGL)-type force field parameters .. 37 Table 2.2 A comparison between RGL-type force field and

QM results for Zr... 37 Table 2.3 A comparison between RGL-type force field and

QM results for Cu... 38 Table 2.4 A comparison between RGL-type force field and

QM results for Cu-Zr system ... 38 Table 2.5 First peak positions from different techniques for amorphous Cu-Zr alloys... 45 Table 2.6 The numbers of near-neighbour correlations obtained

from different techniques ... 52

Table 5.1 Thermal and elastic properties of representative Cu-Zr-Be

and Vitreloy-type glassy alloys... 98

Table 6.1 Density, thermal, and elastic properties of representative

lightweight Ti-Zr-Be and Vitreloy-type glassy alloys ... 111

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Table 7.1 Thermal, mechanical, and rheological properties of

various BMG-forming alloys... 127

Table 8.1 Thermal and elastic properties of representative

Cu(50-x)Zr50Agx bulk glassy alloys... 143 Table 8.2 Thermal and elastic properties of representative

Cu(64-x)Zr36Agx bulk glassy alloys... 143

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LIST OF FIGURES

Number Page Figure 1.1 Record-size bulk metallic glasses developed in various alloy

systems and the year of their discovery... 5 Figure 1.2 Elastic limit plotted against Young’s modulus for bulk metallic glasses and over 1500 conventional materials... 8 Figure 1.3 Experimental shear stress at yieldingvs. shear modulus at

room temperature for 30 bulk metallic glasses... 11 Figure 1.4 Angell plot comparing the viscosities of different types of

glass-forming liquids... 12 Figure 1.5 Correlation between fragility of liquids and the ratio of

instantaneous bulk to shear modulus of respective glasses... 13 Figure 1.6 The correlation between m and B/G for metallic glasses ... 14 Figure 1.7 The correlation of fracture energy with Poisson’s ratio for

all the collected data on metallic glasses ... 15 Figure 1.8 Potential energy density function in the vicinity of an

inherent state... 19

Figure 2.1 Volume as a function of temperature of Cu46Zr54 during

heating and cooling at a rate of 5 K/ps ... 41 Figure 2.2 Volume vs. temperature curves for Cu46Zr54 obtained

from three different quenching rates... 42 Figure 2.3 Partial radial distribution function of Cu46Zr54 for different bond pairs at 400 K during the heating process (5 K/ps) ... 46 Figure 2.4 Partial radial distribution function of Cu46Zr54 for different bond pairs at 2000 K during heating and cooling cycles ... 47 Figure 2.5 Partial radial distribution function of Cu46Zr54 for different

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bond pairs at 400 K during the cooling process (5 K/ps)... 48 Figure 2.6 PRDF of Cu46Zr54 for different bond pairs during the cooling cycle (5 K/ps), Cu-Cu pair ... 49 Figure 2.7 PRDF of Cu46Zr54 for different bond pairs during the cooling cycle (5 K/ps), Zr-Zr pair ... 50 Figure 2.8 PRDF of Cu46Zr54 for different bond pairs during the cooling cycle (5 K/ps), Cu-Zr pair ... 51 Figure 2.9 Partial, total, and average coordination numbers of Cu46Zr54 calculated for the cooling cycle (5 K/ps)... 53 Figure 2.10 Variation of the fractions of Honeycutt-Andersen indices... 57

Figure 3.1 Temperature dependence of shear modulus G for the Cu46Zr54

metallic glass prepared at two different cooling rates... 67 Figure 3.2 Temperature dependence of bulk modulus B for the Cu46Zr54

metallic glass prepared at two different cooling rates... 68 Figure 3.3 Volume dependence of B and G for the Cu46Zr54

metallic glass prepared at a cooling rate of 2.5 K/ps... 69 Figure 3.4 Strong configurational potential energy dependence of

shear modulus G for the Cu46Zr54 metallic glass when

varying the cooling rates to obtain different configurations ... 72 Figure 3.5 The shear and bulk Modulii of Vit-4 as measured in-situ

from -78 to 298 K and measured from samples quenched

from the equilibrium liquid around Tg... 73

Figure 4.1 Pressure change with atomic volume at different temperatures of the Cu46Zr54 binary model metallic glass and liquid ... 82 Figure 4.2 Pressure evolution vs. time at 300 K during the whole

simulation process ... 86 Figure 4.3 Cavitation pressure and spinodal pressure as a function

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of temperature... 87 Figure 4.4 Cavitation time vs. pressure at T=1200 K and the fitting

curve obtained from classical nucleation theory ... 88

Figure 5.1 Bulk metallic glass formation map in the Cu-Zr-Be ternary alloy system ... 97 Figure 5.2 Glass transition temperature as a function of the total Zr and Ti content in Cu-Zr-Be ternary and Vitreloy-type bulk

metallic glasses... 101 Figure 5.3 Shear modulus G vs. the total concentration of Zr and Ti

in Cu-Zr-Be ternary and Vitreloy-type bulk metallic glasses . 102 Figure 5.4 Poisson’s ratio vs. the total Zr and Ti concentration in

Cu-Zr-Be ternary glassy alloys and Vitreloy-type glasses... 103

Figure 6.1 Pictures of bulk amorphous samples of lightweight Ti-based bulk metallic glasses... 113 Figure 6.2 XRD patterns verifying the amorphous nature of the

corresponding samples: amorphous 6 mm diameter rod of Ti45Zr20Be35 (S1), 7 mm diameter rod of Ti45Zr20Be30Cr5 (S2), and 8 mm diameter rod

of Ti40Zr25Be30Cr5 (S3)... 114 Figure 6.3 DSC scans of the amorphous Ti45Zr20Be35 (S1),

Ti45Zr20Be30Cr5 (S2), and Ti40Zr25Be30Cr5 (S3) alloys

at a constant heating rate of 0.33 K/s... 115

Figure 7.1 DSC scans of three typical metallic glasses with excellent glass-forming ability and extremely high thermal stability .... 123 Figure 7.2 A closer view of the glass transition temperatures of the

three bulk metallic glasses from DSC curves... 124

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Figure 7.3 Temperature dependence of equilibrium viscosity of several

metallic-glass-forming liquids... 128 Figure 7.4 Angell fragility plot of several bulk-metallic-glass-forming liquids... 129 Figure 7.5 TTT diagrams for several amorphous alloys ... 132 Figure 7.6 Demonstration of the strong thermoplastic processability

of the Zr35Ti30Cu8.25-Be26.75 metallic glass... 133 Figure 7.7 Demonstration of the strong thermoplastic processability

of the Zr35Ti30Cu8.25-Be26.75 metallic glass... 134

Figure 8.1 X-ray patterns of amorphous 4 mm rod of Cu45Zr50Ag5, 4 mm rod of Cu43Zr50Ag7, 5 mm rod of Cu40Zr50Ag10, and 7 mm rod of Cu43Zr40Ag7Ti10 prepared by the

copper-mold casting method... 144 Figure 8.2 DSC scans of the amorphous Cu45Zr50Ag5, Cu43Zr50Ag7,

Cu40Zr50Ag10, and Cu43Zr40Ag7-Ti10 alloys at a constant

heating rate of 0.33 K/s ... 145 Figure 8.3 X-ray patterns of 3 mm cast rod of Cu60Zr36Ag4,

4 mm glassy rod of Cu57Zr36Ag7, 6 mm glassy rod of Cu54Zr36Ag10, 7 mm cast rod of Cu52Zr36Ag12,

and 8 mm rod of Cu49Zr36Ag10Ti5... 147 Figure 8.4 DSC scans of the amorphous Cu57Zr36Ag7,

Cu54Zr36Ag10, and Cu49Zr36Ag10Ti5 alloys at a constant

heating rate of 0.33 K/s ... 148 Figure 8.5 Poisson’s ratio, ν, as a function of the Ag concentration

for both Cu(50-x)Zr50Agx and Cu(64-x)Zr36Agx series ... 149 Figure 8.6 Compressive stress-strain curves of 2 mm amorphous

rods for Cu(50-x)Zr50Agx series alloys. (a), Cu46Zr54; (b), Cu48Zr50Ag2; (c), Cu43Zr50Ag7; (d), Cu40Zr50Ag10;

(e), Cu Zr Ag ; and (f), Cu Zr Ag Ti ... 150

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Figure 8.7 Compressive stress-strain curves of 2 mm amorphousrods for Cu(64-x)Zr36Agx series alloys. (a), Cu64Zr36; (b), Cu57Zr36Ag7; (c), Cu54Zr36Ag10; and (d), Cu40Zr45Ag10Ti5... 151

ABSTRACT

ABSTRACT

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES