.

The present work aims at evaluating and characterizing the high density Ta based bulk metallic glass with high Glass Forming Ability (GFA). Initially, alloying elements were chosen based on inoue criteria of bulk GFA and subsequently Ta-(W/Hf)-Ni-B alloy system has been selected. The GFA of all various alloy systems have been calculated based on thermodynamic parameter PHSS and a ternary phase diagram has been evaluated. It has been observed that the GFA is maximum, when composition of Ta and Ni are almost equi-atomic composition. Further increasing at.% of W/Hf, PHSS value gradually increases with minimal amount of B addition. Based on this criteria, a series of ternary and quaternary alloys of TaxNiyBz (where x = 46, 47, y = 44, 45, and z = 8, 9, 10), Ta50Ni49.5-

xWxB0.5 (x = 1, 2, 3, 4), Ta43Ni42Hf14.5B0.5 and Ta41Ni40Hf18.5B0.5 have been prepared using vacuum arc melting furnace. Rapidly solidified ribbons have been obtained using vacuum melt spinning process. The thermal characterizations of amorphous ribbons were carried out using DSC. These ribbons have been characterized using different techniques such as XRD and micro-hardness analysis. Detailed characterization revealed that amorphous phase exists in alloys with low B content. It has further observed that the amorphous phase retains even if W and Hf contents increase up to 5 at.%. Spark plasma sintering technique has been carried out to obtain different diameter Ta based BMG compacts. Detailed characterization of compacts viz. XRD, SEM, density measurement and micro-hardness test have been carried out to study densification, microstructural behavior and mechanical properties of the composite.

composition HardnessOf master alloys (Hv)

Hardness of melt spun ribbons (Hv)

Hardness of compacted samples (Hv)

Ta50W1Ni48.5B0.5 617 634 1147

Ta50W2Ni47.5B0.5 616 626 1031

Ta50W3Ni46.5B0.5 602 609 786

Ta50W5Ni44.5B0.5 617 662 885

Ta48W5Ni46.5B0.5 635 644 866

Ta43Hf14.5Ni42B0.5 621 629 901

Ta41Hf18.5Ni40B0.5 594 704 834

Sample SPS Temp (oC) & time

Density of master alloy (g/cc)

Density of SPS compact (g/cc)

% deviation in density

Ta50W1Ni48.5B0.5670-

5min

13.51 13.5 -0.0007

Ta50W2Ni47.5B0.5670-

5min

13.76 13.72 -0.0029

Ta50W3Ni46.5B0.5670-

5min

13.87 13.76 -0.0079

Ta50W5Ni44.5B0.5670-

5min

14.01 13.97 -0.0028

Ta48W5Ni46.5B0.5675-

5min

13.94 13.31 -0.047

Ta43Hf14.5Ni42B0.5630-

5min

13.58 12.88 -0.054

Ta41Hf18.5Ni40B0.5635-

5min

13.52 12.78 -0.057

The experiments indicated that glass would form if PHSS of the alloy is between -4.5 to -6.5 kJ/mol. Glass forming composition ranges calculated in the study are in good agreement with the experimentally determined composition range of amorphous phase formation in the system.

Ta50Ni48.5W1B0.5 and Ta50Ni46.5W3B0.5 have the best glass forming ability, which was obtained as 10 mm pellets by SPS. These BMGs exhibit high thermal stability, high glass transition temperature (Tg) and high crystallization temperature (Tx) compare to other BMGs like Fe and Pd based alloys.

The glasses also have excellent mechanical properties such as high density ranging from 12.7 g/cc to 14 g/cc and high micro-hardness ranging from 780 VHN to 1200 VHN. The combined properties of the Ta-based BMGs may be a probable candidate of the effective kinetic energy penetrators.

Mechanical properties such as ultimate tensile strength, fatigue and creep should be evaluated to generate a comprehensive understanding on life of the components made of metallic glasses.

CONCLUSION

FUTURE WORK

THEORETICAL CALCULATIONS Based on thermodynamic calculations proposed by Bhatt et al. have been used

to evaluate the glass forming ability (GFA) of Ta based alloys, i.e.,

PHSS = (PHS*∆Sc/R)

Where PH-S = Hchem (ΔS /kB)

Where ΔHchem = Enthalpy of chemical mixingΔS = Mismatch entropy

kB = Boltzmann’s constant ∆Sc = Configurational entropy

R = Universal gas const. The density of all compositions were calculated theoretically by using the

formula,Density (ρth) = 100/((W1/ ρ1 ) + (W2/ ρ2 ) + (W3/ ρ3 ) +............ (Wn/ ρn ))

Where, n = Number of element in alloy composition

W = weight % of element in alloy composition ρ = density of pure element

The theoretical melting points of all compositions have been calculated using Thermo-Calc software version 4.

composition PHSS (kJ/mol) Theoretical melting temperature (oC)

Theoretical density(g/cc)

Ta47Ni45B8 -7.63 1553 13.18

Ta46Ni45B9 -7.74 1556 13.24

Ta46Ni44B10 -7.81 1572 13.35

Ta50W1Ni48.5B0.5 -5.91 1627 13.78

Ta50W2Ni47.5B0.5 -6.03 1631 13.88

Ta50W3Ni46.5B0.5 -6.22 1634 13.97

Ta50W5Ni44.5B0.5 -6.35 1639 14.16

Ta48W5Ni46.5B0.5 -5.92 1625 14.02

Ta43Hf14.5Ni42B0.5 -5.06 1635 13.62

Ta41Hf18.5Ni40B0.5 -6.00 1578 13.54

ABSTRACT

INTRODUCTION According to the elastic moduli criterion, the Ta

based bulk metallic glasses could have unique mechanical and physical properties.

The relation between the elastic moduli and properties are guidelines to produce good BMGs by appropriate composition selection of components.

The Ta element content can effectively modulate the properties of the Ta based BMGs, such as elastic moduli and density.

Glass Forming Ability (GFA)• Reduced glass transition temperature (Trg ):

Trg= Tg/ Tl GFA α 1/Tl

Where Tg = glass transition temperature Tl = liquidus temperature

• Undercooled liquid regime (∆Tx):

∆Tx =Tx-Tg GFA α ∆Tx

Where Tx = crystallization temperature

Tg= glass transition temperature

The main application of Ta based BMGs are used as kinetic energy penetrators.

Selected compositions based on the high GFA, high density and low melting point

RESULTS Characterization of Melt Spun Ribbons

Characterization of SPS Compacted Samples

XRD patterns of Ta-based ternary melt spun ribbons

XRD patterns of W added Ta -based BMG melt spun ribbons

XRD patterns of Hf added Ta-based BMG melt spun ribbons

DSC plots of W added Ta-based BMG melt spun ribbons

DSC plots of Hf added Ta-based BMG melt spun ribbons

XRD patterns of W added Ta-based BMG sintered samples

XRD patterns of Hf added Ta-based BMG sintered samples

SEM images of SPS compacts

(a) Ta50Ni48.5W1B0.5, (b) Ta50Ni46.5W3B0.5, (c) Ta43Ni42Hf14.5B0.5, (d) Ta50Ni44.5W5B0.5, (e) Ta50Ni47.5W2B0.5, (f) Ta41Ni40Hf18.5B0.5.

Density deviation between master alloys and SPS compacted samples ACKNOWLEDGEMENT

REFERENCES H. S. Chen and D. Turnbull, Appl. Phys., 10

(284) & 38 (4646) (1967). A. Inoue, T. Nakamura, T. Zhang, and T.

Masumoto, Mater. Trans., JIM, 34, 351 (1993). B. Ramakrishna Rao, M. Srinivas, A. K. Shah, A.

S. Gandhi and B. S. Murthy, Intermetallics (2012),1-9.

I am very thankful to Director, Defence Metallurgical Research Laboratory (DMRL), Hyderabad giving me an opportunity to do my work and provide all resources required for my project work at DMRL.

PROCESS FLOW CHARTTa based Alloy system

Calculation of Theoretical Density

Calculation of theoretical Melting Point

Calculation of Glass Forming Ability

Elemental mixer of Ta, (W/Hf), Ni, B elements

Vacuum arc melting Chemical analysis

Alloy of Ta (W, Hf)x

NiyBzMelt spinning

Amorphous ribbons

XRD

DSC

Micro hardness

Spark Plasma Sintering Density

measurement

Ta based BMG composite compacts

XRD

SEM

Mechanical properties

Density measurement

Compression test

Micro hardness test

Selection of alloy composition based on high GFA, high density and low melting point

Ta50Ni48.5W1B0.5 Ta50Ni46.5W3B0.5

Ta43Ni42Hf14.5B0.5 Ta50Ni44.5W5B0.5

Ta50Ni47.5W2B0.5 Ta48Ni46.5W5B0.5

Ta41Ni40Hf18.5B0.5

SIXTH INTERNATIONAL CONFERENCE ON SOLIDIFICATION SCIENCE AND PROCESSING (ICSSP6 – 2015)

Evaluation of high density Ta based Bulk Metallic Glasses and their structural characterization

K. Bharath, J. Arout Chelvanel, Mahesh Kumar Kumavat, B. Majumdar

ACKNOWLEDGEMENT