Study Of Mechanical Deformation Of a Zr-Cu Based BMG: Experiments And Numerical Studies ANISH ROY VAHID NEKOUIE GAYAN ABEYGUNAWARDANE-ARACHCHIGE VADIM SILBERSCHMIDT 1 Mechanics of Advanced Materials Research Group
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1. ANISH ROY VAHID NEKOUIE GAYAN ABEYGUNAWARDANE-ARACHCHIGE
VADIM SILBERSCHMIDT 1 Mechanics of Advanced Materials Research
Group
2. What is a Bulk Metallic Glass? 2 amorphous material: atoms
frozen in non-crystalline form first formed in 1957 by Duwez by
rapid quenching gold-silicon alloy only very thin, small samples
could be produced (order or micrometers) first believed atoms were
randomly packed together densely like hard spheres in a liquid
solvent atoms randomly arranged with solute atoms fitting into open
cavities now believe short-range, even medium-range order exists in
materials Sheng et al. (2006), Nature
3. What is a Bulk Metallic Glass? 3 BMG Compared to metals in
general, BMGs have high strength, f and low stiffness, E Unusually
high Elastic Strain, f/E From: Material selection in mechanical
design, MF Ashby (1999) Very high Elastic stored energy
4. Applications 4 Digital light processor, hinges made of Ti-Al
metallic glass with no fatigue failure after 1012 cycles. Micro
components in MEMS devices LENGTH SCALES
5. Introduction & Motivation Deformation mechanisms of
metallic glass are unique plastic shear flow in the micro scale,
but brittle fracture in macro scale At ambient temperatures/high
stress: flow localization in shear bands (SB) At high
temperatures/low stress: homogeneous viscous flow Research
Objectives Experiments: study SB initiation and evolution under
loads. Characterise SBs mechanically. Modelling: Develop a
continuum model of SB initiation and propagation, which can then be
used to study component deformations across length scales 5
6. What is a Shear band? Localised thin bands (~ 10 - 20 nm).
Cohesion is maintained across the planes. Propagation is
inhomogeneous Propagation depends on loading conditions, sample
imperfection. Origin of SB is controversial: structural change?
Temperature rise? Localised melting? 6 Source nature materials
7. BMG alloy and experiments BMG alloy manufactured at
IFW/Dresden Zr48Cu36Al8Ag8 Samples: 70 mm 10 mm 2 mm ; 40 mm 30 mm
1.5 mm 7 Characterisation (is it actually amorphous?) X-ray
diffraction (XRD) No obvious presence of crystalline phases
8. Experiment : 3 point bending E (GPa) y (MPa) 95.4 0.345 930
3mm TensionCompression 100 m Vein like structures on the
surface
9. Experiment : 3 point bending E (GPa) y (MPa) 95.4 0.345 930
400 m 10 m Shear Bands are evident
10. Nano-indentation studies 10 Fracture surface is noticeably
weaker than the bulk material There is a large variation of the
mechanical properties on the fractured surface Objective: To assess
if there is any difference in the mechanical characteristics of the
fracture surface in comparison to the bulk material Vickers
indentation Total load 100 mN Loading rate 2 mN/s
12. Wedge indentation studies Why wedge? Observing shear bands
in Nano/Microindentation is difficult o Shear bands initial in the
material volume o Bonded interface method is not ideal With a Wedge
we have a 2D plane-strain scenario Observe shear bands terminating
on the surface as they initiation and evolve. Relatively easy to
setup Easy to model 12
13. 1. Sample cut and polished 2. Loaded into a custom rig
Wedge indentation: Experimental steps 13 Zygo Talisurf Ra = 2 to 3
nm BMG Spring
14. Wedge indentation: details Incremental loading: 1 KN to 3
KN Deformation mode: Compression Displacement rate: 0.5 mm/min
Indenter: HSS 14 60 m 1kN 22 m 400 m 50 m
15. Wedge indentation: Results (SEM) 60 m 1kN 22 m 1-2kN 60 m
50 m 1-2-3kN 60 m 85m 400 m 400 m 400 m 85 m 130m 50 m
16. Wedge indentation: Load-Displacement Curve Single load,
different locations Incremental load, same location ~ 50m ~22 m
Area under the curve will give us work done for plastic
deformation
17. Shear Bands: XRD results 17 XRD results are inconclusive
since crystalline phases < 5% is hard to detect
18. Shear Band analysis/ TEM + SAED 18 Virgin Sample Shear Band
Crystalline material FIB milling TEM/SAED sample Shear Bands are
fully amorphous
19. Nano-indentation studies on a Shear Band Vickers
indentation Total load 100 mN Loading rate 2 mN/s 19
20. Nano-indentation studies on a Shear Band 20
21. MODELLING OF WEDGE INDENTATION /Finite Element Modelling
21
22. Microscale modelling Bulk material Drucker Prager :
hydrostatic stress component is considered. Captures the rise of
shear strength with the increase of hydrostatic pressure increase.
Major cause for adoption. = 2 1 J2 second deviatoric stress
invariant constant for a given material I1 first stress invariant
hardening and softening function ABAQUS 6.12 is used to model
Linear Drucker - Prager criterion is used: = Here: = and = To
calculate, and : = 1 2 q 1 + 1 1 1 3 and = 1 1 3 = , = = 22
23. Microscale modelling Shear band Cohesive Zone Elements with
traction separation law. Shear band thickness lies in the ~nm
scale. This fact prompt to employ traction separation laws. 23
Linear elastic behaviour = 0 , = 0 , = 0 Traction Separation
response Damage initiation criterion 0 2 + 2 + 2 = 1 Nominator
calculated by the solver, Denominator is user input dependent.
Linear damage evolution = 0 0 effective displacement at complete
failure, 0 effective displacement at damage initiation effective
traction at damage initiation, maximum value of the effective
displacement
24. 24 Wedge Indenter Radius: 43 m FE Model Dimension: (2000
2000 ) m Displacement Given to Indenter: 4 m to 10 m Element type:
Bulk Specimen and indenter CPE4R Shear bands COH2D4 Wedge Indenter:
Deformable Body FE model 2D Plain Strain BC: bottom rigid
25. 25 FE model Material Properties Drucker-Prager parameters
Hardening Angle of friction() Flow stress ratio Dilation angle ()
0.01 1 0.02 Shear damage parameters Yield stress (MPa) Plastic
strain Fracture strain Shear stress ratio Strain rate ( s-1 ) 930 0
0.05 1 0.016 Material Properties for bulk metallic glass E (GPa)
95.4 0.345 Material Properties for deformable indenter (HSS) E
(GPa) 231 0.30 Material properties for CZE were chosen by
sensitivity analysis.
26. 26 FE model: Results Damage initiation and propagation
through the shear band
27. Outlook & Future Work SB and Fracture surface are
weaker than bulk material SB are amorphous rules out melting
Cohesive Zone Elements can be used to determined the propagation
along the shear band. A gradient plasticity based approach is
currently being developed to capture the nucleation and the effect
of the local shear bands. 27
28. 28 Wedge Indenter Radius: 21 m FE Model Dimension: (2000
2000 ) m Element type: Bulk Specimen and indenter CPE4R Shear bands
COH2D4 Wedge Indenter: Deformable Body FE model 2D Plain Strain BC:
bottom rigid