| Project Team | Institution | Role | ||
| Mr Pranesh Dayal | The University of New South Wales | Postgraduate Student | ||
| Professor Yuri Estrin | Monash University | Chief Investigator | ||
| Profesor Peter Hodgson | Deakin University | Chief Investigator | ||
| Professor Mark Hoffman | The University of New South Wales | Chief Investigator | ||
| Dr Tania Vodenitcharova | The University of New South Wales | Research Fellow | ||
| Dr Cui'e Wen | Deakin University | Associate Investigator | ||
| Mr Dengke Yang | Deakin University | Postgraduate Student | ||
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Executive Summary
At the micro-scale, micro-multilayer materials were synthesized by cold roll/heat treatment/cold roll and accumulative roll bonding of commercially available Al/Ti, Al/Nb, Al/Fe, Cu/Ti, Cu/Fe, and Cu/Al foils. It was found that the bonding between the layers was provided by shear bands developed at the interface. The bond strength of the composites was assessed for different thermal conductivity of the individual layers. TEM was used to study the variations in the microstructure inside the shear bands, in order to reveal the mechanism of grain refining.
At the nano-scale, multilayered aluminium/palladium (Al/Pd) thin films were deposited on silicon (Si) substrates using magnetron sputtering in the laboratory of CSIRO. The resulting composite structures were characterised for their structure, hardness, and elastic modulus applying cross-sectional transmission electron microscopy (XTEM) and nano-indentation.
At the micro-scale, micro-multilayer materials were synthesized by cold roll/heat treatment/cold roll and accumulative roll bonding of commercially available Al/Ti, Al/Nb, Al/Fe, Cu/Ti, Cu/Fe, and Cu/Al foils. It was found that the bonding between the layers was provided by shear bands developed at the interface. The bond strength of the composites was assessed for different thermal conductivity of the individual layers. TEM was used to study the variations in the microstructure inside the shear bands, in order to reveal the mechanism of grain refining.
At the nano-scale, multilayered aluminium/palladium (Al/Pd) thin films were deposited on silicon (Si) substrates using magnetron sputtering in the laboratory of CSIRO. The resulting composite structures were characterised for their structure, hardness, and elastic modulus applying cross-sectional transmission electron microscopy (XTEM) and nano-indentation.
Project Aims/Targets
The main aim of the project is the in-house development of metallic multi-layered structures with improved mechanical properties over their monolithic constituents. Both micro- and nano-scale structures are targeted.
At previous stages of the project, the multi-layered structures at both micro- and nano- levels were developed and their superior mechanical properties were proved. The current stage of the project is characterized with in-depth study of the mechanisms of interfacial bonding and thus, of property improvement.
The main aim of the project is the in-house development of metallic multi-layered structures with improved mechanical properties over their monolithic constituents. Both micro- and nano-scale structures are targeted.
At previous stages of the project, the multi-layered structures at both micro- and nano- levels were developed and their superior mechanical properties were proved. The current stage of the project is characterized with in-depth study of the mechanisms of interfacial bonding and thus, of property improvement.
At the micro-scale, titanium-based multi-layered metal composites were produced by accumulative roll bonding (ARB) without lubrication, using a high rolling mill with 365 mm diameter rolls in a rotating speed of 16 m min-1 and at ambient temperature. The titanium plate was rolled at a strain rate of 3 S-1 from 12 mm to 2 mm in thickness with a reduction of 16.7 % per pass. Foils of Al/Ti, Al/Nb, Al/Fe, Cu/Ti, Cu/Fe, and Cu/Al, possessing different thermal conductivities, were also used.
The resulted structures were mechanically characterized, and the bond strength was determined using a modified ASTM C-633 method. A square of 10 mm × 10 mm sample was cut from the centre of the rolled multilayer and then, both sides were glued to cylindrical stainless steel shafts with a diameter of 10 mm and length of 20 mm using strong glue (Selleys Quick Fix). A tensile load was applied to shafts with a servo hydraulic material testing machine (MTS insight 100) at a crosshead speed of 1 mm/min, at room temperature, until fracture occurred. The mean tensile bond strength was calculated from the fracture load and the surface area.
The great reduction in thickness after cold rolling inevitably leads to adiabatic heating. The significant increase in temperature at internal interfaces during roll bonding deformation influences the bond strength between the ARB components. The effects of deformation-induced adiabatic heating on the roll bond strength were investigated in this study. The bond strength of the composites was affected by the thermal conductivity of the individual layers. The results from the bond strength test indicate that the metallic multilayer system with a low thermal conductivity shows a greater temperature rise due to deformation-induced localized adiabatic heating, and exhibits relative high bond strength. On the contrary, the high thermal conductivity metal system might fail to be roll-bonded altogether.
Examination of the cold rolled titanium films showed localized shear bands with nanostructure formation in the titanium foils. The evolution of the shear bands have been of a particular interest as it reveals the mechanism of grain refining. This has been studied by using TEM, to investigate the microstructure variations inside shear bands.
Micro-regions with localized shear deformation are first initiated at low strains. These micro-regions contain fine twin/matrix lamellae, thin laths and elongated subgrain structures interspersed with the deformed matrix comprised of dislocation cells. Further shear localization with increasing strain leads to the formation and multiplication of approximately parallel, distinct microscopic shear bands inclined to the rolling direction at an angle of about 40°. These bands contain a mix of thin lath structures and elongated subgrains. The microscopic shear bands gradually grow, as a result of strain becoming increasingly concentrated in the sheared regions, and finally coalesce to form a macroscopic shear band. At large strains, the macroscopic shear band represents a complex composite structure containing thin lath structures in the boundary regions, fine elongated subgrains in the outer areas and ultrafine, roughly equiaxed (sub)grains in the core region. This suggests that there is a significant strain gradient across the macroscopic shear band, resulting from the band gradual development, the core region experiencing larger shear strain compared to the outer regions. Thus, the roughly equiaxed, ultrafine (sub)grains found in the shear band core are a final product of the process of progressive splitting and breakdown of the thin elongated structures, which originate from the matrix/twin lamellar structure aligned approximately parallel to the shear direction.
The findings contained several elements of novelty, i.e., the metallic multilayer system with low thermal conductivity has relative high bond strength, while high thermal conductivity metal system may fail to be roll bonded. The deformation-induced localized heating in the low thermal conductivity metal multilayer systems may provide opportunities for achieving a successful accumulative roll bonding or a “cold roll/heat treatment/cold roll” process to synthesize metallic multilayer materials.
The study conducted during this report period had sufficient merit for publication, and was published in reputed international journals: (1) Yang DK, Hodgson PD and Wen CE (2009), ‘The kinetics of two-stage formation of TiAl3 in multilayered Ti/Al foils prepared by accumulative roll bonding’, Intermetallics, 17, 727–732; in (2) Yang DK, Hodgson PD and Wen CE (2009), ‘Nucleation and Growth During Reactions in Accumulative Roll Bonding of Ti/Al Multilayers’, Material Science Forum, 618-619, 429-432; (3) Yang DK, Hodgson PD and Wen CE (2009) ‘Influence of deformation-induced heating on the bond strength of rolled metal multilayers’, Materials Letters, 63, 2300-2302, 2009; (4) Yang DK, Cizek P, Hodgson P and Wen C ‘Ultrafine equiaxed-grain Ti/Al composite produced by accumulative roll bonding’, Scripta Materialia, 62, 321-324, 2010. A conference presentation entitled ‘Ultrafine equiaxed grain Ti/Al composite produced by accumulative roll boning’ by Yang DK, Hodgson PD and Wen CE was also made at the Fourth International Light Metals Technology Conference 2009, 1 July, Gold Coast, Queensland, Australia.
In addition, a manuscript is currently being summarized: Yang DK, Cizek P, Hodgson P and Wen C: ‘Nanostructure formation during shear localization in cold-rolled titanium’.
The resulted structures were mechanically characterized, and the bond strength was determined using a modified ASTM C-633 method. A square of 10 mm × 10 mm sample was cut from the centre of the rolled multilayer and then, both sides were glued to cylindrical stainless steel shafts with a diameter of 10 mm and length of 20 mm using strong glue (Selleys Quick Fix). A tensile load was applied to shafts with a servo hydraulic material testing machine (MTS insight 100) at a crosshead speed of 1 mm/min, at room temperature, until fracture occurred. The mean tensile bond strength was calculated from the fracture load and the surface area.
The great reduction in thickness after cold rolling inevitably leads to adiabatic heating. The significant increase in temperature at internal interfaces during roll bonding deformation influences the bond strength between the ARB components. The effects of deformation-induced adiabatic heating on the roll bond strength were investigated in this study. The bond strength of the composites was affected by the thermal conductivity of the individual layers. The results from the bond strength test indicate that the metallic multilayer system with a low thermal conductivity shows a greater temperature rise due to deformation-induced localized adiabatic heating, and exhibits relative high bond strength. On the contrary, the high thermal conductivity metal system might fail to be roll-bonded altogether.
Examination of the cold rolled titanium films showed localized shear bands with nanostructure formation in the titanium foils. The evolution of the shear bands have been of a particular interest as it reveals the mechanism of grain refining. This has been studied by using TEM, to investigate the microstructure variations inside shear bands.
Micro-regions with localized shear deformation are first initiated at low strains. These micro-regions contain fine twin/matrix lamellae, thin laths and elongated subgrain structures interspersed with the deformed matrix comprised of dislocation cells. Further shear localization with increasing strain leads to the formation and multiplication of approximately parallel, distinct microscopic shear bands inclined to the rolling direction at an angle of about 40°. These bands contain a mix of thin lath structures and elongated subgrains. The microscopic shear bands gradually grow, as a result of strain becoming increasingly concentrated in the sheared regions, and finally coalesce to form a macroscopic shear band. At large strains, the macroscopic shear band represents a complex composite structure containing thin lath structures in the boundary regions, fine elongated subgrains in the outer areas and ultrafine, roughly equiaxed (sub)grains in the core region. This suggests that there is a significant strain gradient across the macroscopic shear band, resulting from the band gradual development, the core region experiencing larger shear strain compared to the outer regions. Thus, the roughly equiaxed, ultrafine (sub)grains found in the shear band core are a final product of the process of progressive splitting and breakdown of the thin elongated structures, which originate from the matrix/twin lamellar structure aligned approximately parallel to the shear direction.
The findings contained several elements of novelty, i.e., the metallic multilayer system with low thermal conductivity has relative high bond strength, while high thermal conductivity metal system may fail to be roll bonded. The deformation-induced localized heating in the low thermal conductivity metal multilayer systems may provide opportunities for achieving a successful accumulative roll bonding or a “cold roll/heat treatment/cold roll” process to synthesize metallic multilayer materials.
The study conducted during this report period had sufficient merit for publication, and was published in reputed international journals: (1) Yang DK, Hodgson PD and Wen CE (2009), ‘The kinetics of two-stage formation of TiAl3 in multilayered Ti/Al foils prepared by accumulative roll bonding’, Intermetallics, 17, 727–732; in (2) Yang DK, Hodgson PD and Wen CE (2009), ‘Nucleation and Growth During Reactions in Accumulative Roll Bonding of Ti/Al Multilayers’, Material Science Forum, 618-619, 429-432; (3) Yang DK, Hodgson PD and Wen CE (2009) ‘Influence of deformation-induced heating on the bond strength of rolled metal multilayers’, Materials Letters, 63, 2300-2302, 2009; (4) Yang DK, Cizek P, Hodgson P and Wen C ‘Ultrafine equiaxed-grain Ti/Al composite produced by accumulative roll bonding’, Scripta Materialia, 62, 321-324, 2010. A conference presentation entitled ‘Ultrafine equiaxed grain Ti/Al composite produced by accumulative roll boning’ by Yang DK, Hodgson PD and Wen CE was also made at the Fourth International Light Metals Technology Conference 2009, 1 July, Gold Coast, Queensland, Australia.
In addition, a manuscript is currently being summarized: Yang DK, Cizek P, Hodgson P and Wen C: ‘Nanostructure formation during shear localization in cold-rolled titanium’.
(a) 33.3 %, (b) 50.0 %, (c) 66.7 % and (d) 83.3.
(a) A montage of a partial shear band; (b) DF image showing the fine grains in the localized band;
(c) Corresponding BF image and SADP; (d) Mean grain size distribution in the localized band;
(e) Statistic distribution for the measured intersection angles between grain longitude and RD.
At the nano-scale, two different sets of multilayered aluminium/palladium (Al/Pd) thin films were fabricated in the laboratory of CSIRO, using magnetron sputtering on silicon (Si) substrates. In Set I, the Al layer thickness was equal to the Pd layer thickness, with individual layer thickness varying from 1 nm to 40 nm. In Set II, the Al layer thickness was fixed at 27 ± 2 nm while the Pd layer thickness was varied from 2 to 10 nm. The total number of bi-layers was varied according to the bi-layer thickness λ (the sum of Al layer thickness and Pd layer thickness) for each sample, in order to maintain a total film thickness of approximately 1 μm for all the multilayered samples.
Additionally, monolayer Al and Pd films, each having a thickness of approximately 1 μm, were also deposited under the same conditions, in order to compare their mechanical properties with those of the multilayered Al/Pd thin films. To observe the cross-sectional microstructure and to re-confirm the individual layer thicknesses of the deposited films, the multilayered samples were characterized by XTEM, using a Philips CM200 field emission gun transmission electron microscope operating at 200 kV. The XTEM samples were prepared by focussed ion beam milling using an xT Nova NanoLab 200 DualBeam (FEI Company).
The XTEM micrographs indicated a sharp but not flat Al-Pd interface as shown in Figure B4 (a, b). Nano-indentation results showed that with just 6.5 % (v/v) Pd, a hardness enhancement of ~200 % can be achieved for multilayered Al/Pd compared to the hardness of pure Al film (Figure B4 (e)). A maximum hardness enhancement of up to 350 % was found in multilayered Al/Pd samples compared to the hardness of pure Al film, when the bi-layer thickness was 2 nm and Pd was 50 % (v/v) (Figure 28 (c)). It was also observed that the modulus of the multi-layered thin films was enhanced compared to the modulus of pure Al film (Figure B4d, f)).
The results on the above findings were reported on an International Conference: P. Dayal, M. Z. Quadir, C. Kong, N. Savvides, M. Hoffman: ‘Uniaxial compression of sub-micron sized pillars milled in nanolayered Al/Pd thin films displaying layer-thickness dependent strengthening’, Nanomechanical Testing in Materials Research & Development Conference, October 11-16, 2009, Il Ciocco Hotel and Conference Center, Barga (Tuscany), Italy (poster presentation). A paper was also published in a reputable journal: Dayal P, Savvides N, and Hoffman M (2009), ‘Characterisation of nanolayered aluminium/palladium thin films using nanoindentation’, Thin Solid Films, 517, No. 13, 3698-3703. In addition, presentations were made on workshops: (1) P Dayal, N Savvides and M Hoffman (2009), ‘Deformation behaviour of nano-layered Al/Pd thin films’, Third Australian Nano-indentation Workshop, 5-7th July 2009, The Australian National University, Canberra, Australia; (2) P. Dayal., Z. Quadir, C. Kong, N. Savvides and M. Hoffman: ‘Understanding strengthening in nanolayered Al/Pd thin films’, ARC Centre of Excellence for Design in Light Metals Annual Workshop 2009, 9-10 December 2009, Deakin University, Geelong, Australia; (3) Pranesh Dayal, Nick Savvides and Mark Hoffman: ‘Microstructure and mechanical properties of multilayered Al/Pd thin films’, Research Poster Symposium – 2009, 01 October 2009, The University of New South Wales, Sydney, Australia.
To understand this extraordinary enhancement of material properties, testing was undertaken in uniaxial compression of submicron-sized pillars milled in multilayered Al/Pd thin films with different bi-layer thicknesses, λ = 2, 20 and 80 nm. It was observed for the first time that pillars with λ= 80 nm and λ= 2 nm exhibited two different deformation modes: extrusion (involving flow within layer boundaries) and shear (involving deformation across layer boundaries), respectively. However, pillars with λ= 20 nm showed a mixture of these two deformation behaviors. Those results are currently being summarized in a journal paper: P. Dayal, M. Z. Quadir, C. Kong, N. Savvides, M. Hoffman: ‘Layer-thickness dependent deformation behaviors of submicron sized pillars milled in nanolayered Aluminium/Palladium thin films’.
Additionally, monolayer Al and Pd films, each having a thickness of approximately 1 μm, were also deposited under the same conditions, in order to compare their mechanical properties with those of the multilayered Al/Pd thin films. To observe the cross-sectional microstructure and to re-confirm the individual layer thicknesses of the deposited films, the multilayered samples were characterized by XTEM, using a Philips CM200 field emission gun transmission electron microscope operating at 200 kV. The XTEM samples were prepared by focussed ion beam milling using an xT Nova NanoLab 200 DualBeam (FEI Company).
The XTEM micrographs indicated a sharp but not flat Al-Pd interface as shown in Figure B4 (a, b). Nano-indentation results showed that with just 6.5 % (v/v) Pd, a hardness enhancement of ~200 % can be achieved for multilayered Al/Pd compared to the hardness of pure Al film (Figure B4 (e)). A maximum hardness enhancement of up to 350 % was found in multilayered Al/Pd samples compared to the hardness of pure Al film, when the bi-layer thickness was 2 nm and Pd was 50 % (v/v) (Figure 28 (c)). It was also observed that the modulus of the multi-layered thin films was enhanced compared to the modulus of pure Al film (Figure B4d, f)).
The results on the above findings were reported on an International Conference: P. Dayal, M. Z. Quadir, C. Kong, N. Savvides, M. Hoffman: ‘Uniaxial compression of sub-micron sized pillars milled in nanolayered Al/Pd thin films displaying layer-thickness dependent strengthening’, Nanomechanical Testing in Materials Research & Development Conference, October 11-16, 2009, Il Ciocco Hotel and Conference Center, Barga (Tuscany), Italy (poster presentation). A paper was also published in a reputable journal: Dayal P, Savvides N, and Hoffman M (2009), ‘Characterisation of nanolayered aluminium/palladium thin films using nanoindentation’, Thin Solid Films, 517, No. 13, 3698-3703. In addition, presentations were made on workshops: (1) P Dayal, N Savvides and M Hoffman (2009), ‘Deformation behaviour of nano-layered Al/Pd thin films’, Third Australian Nano-indentation Workshop, 5-7th July 2009, The Australian National University, Canberra, Australia; (2) P. Dayal., Z. Quadir, C. Kong, N. Savvides and M. Hoffman: ‘Understanding strengthening in nanolayered Al/Pd thin films’, ARC Centre of Excellence for Design in Light Metals Annual Workshop 2009, 9-10 December 2009, Deakin University, Geelong, Australia; (3) Pranesh Dayal, Nick Savvides and Mark Hoffman: ‘Microstructure and mechanical properties of multilayered Al/Pd thin films’, Research Poster Symposium – 2009, 01 October 2009, The University of New South Wales, Sydney, Australia.
To understand this extraordinary enhancement of material properties, testing was undertaken in uniaxial compression of submicron-sized pillars milled in multilayered Al/Pd thin films with different bi-layer thicknesses, λ = 2, 20 and 80 nm. It was observed for the first time that pillars with λ= 80 nm and λ= 2 nm exhibited two different deformation modes: extrusion (involving flow within layer boundaries) and shear (involving deformation across layer boundaries), respectively. However, pillars with λ= 20 nm showed a mixture of these two deformation behaviors. Those results are currently being summarized in a journal paper: P. Dayal, M. Z. Quadir, C. Kong, N. Savvides, M. Hoffman: ‘Layer-thickness dependent deformation behaviors of submicron sized pillars milled in nanolayered Aluminium/Palladium thin films’.
(c)-(f) Hardness and modulus of multilayered Al/Pd thin films compared with that of monolayer Al and Pd thin films.
Future Activity Plan
The project will continue to deepen the understanding of the interaction at the interface between the layers in the metal-based laminated structures at both nano- and micro-scales. At the micro-scale, the interface between the commercially pure titanium sheets subjected to cold rolling will be further studied. A detailed TEM investigation of the microstructure evolution within the areas of localized shear will be undertaken, in order to elucidate the grain refining mechanisms.
At the nano-scale, modeling of the uniaxial pillar-compression data and of nano-indentation will be attempted.
The project will continue to deepen the understanding of the interaction at the interface between the layers in the metal-based laminated structures at both nano- and micro-scales. At the micro-scale, the interface between the commercially pure titanium sheets subjected to cold rolling will be further studied. A detailed TEM investigation of the microstructure evolution within the areas of localized shear will be undertaken, in order to elucidate the grain refining mechanisms.
At the nano-scale, modeling of the uniaxial pillar-compression data and of nano-indentation will be attempted.