1. 1. ANISH ROY VAHID NEKOUIE GAYAN ABEYGUNAWARDANE-ARACHCHIGE VADIM SILBERSCHMIDT 1 Mechanics of Advanced Materials Research Group
2. 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. 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. 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. 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. 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. 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. 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. 9. Experiment : 3 point bending E (GPa) y (MPa) 95.4 0.345 930 400 m 10 m Shear Bands are evident
10. 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
11. 11. Fractured surface analysis 11 Indentation Load = 500 mN
12. 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. 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. 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. 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. 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. 17. Shear Bands: XRD results 17 XRD results are inconclusive since crystalline phases < 5% is hard to detect
18. 18. Shear Band analysis/ TEM + SAED 18 Virgin Sample Shear Band Crystalline material FIB milling TEM/SAED sample Shear Bands are fully amorphous
19. 19. Nano-indentation studies on a Shear Band Vickers indentation Total load 100 mN Loading rate 2 mN/s 19
20. 20. Nano-indentation studies on a Shear Band 20
21. 21. MODELLING OF WEDGE INDENTATION /Finite Element Modelling 21
22. 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. 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. 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. 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. 26 FE model: Results Damage initiation and propagation through the shear band
27. 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. 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