news & views METALLIC GLASSES
Fast track to production
A high-throughput approach combining combinatorial deposition of materials with parallel blow-forming speeds up the discovery rate of bulk metallic glasses that can be easily formed into complex shapes.
Dan B. Miracle
H
eating and blowing oxide glasses (such as silicate glass) into a shaped cavity has been used for centuries to make bottles and jars. Blow moulding and other thermoplastic-forming methods such as injection moulding have also been used for decades to form plastic objects quickly and cheaply. For conventional metals, however, their crystalline structure rules out thermoplastic forming, so it is much more difficult — and expensive — to make metals into complex shapes. Although metallic glasses lend themselves to moulding by thermoplastic forming (Fig. 1), this has only been achieved in the past several years1. One of the reasons is that metallic glasses are less stable than oxide glasses and plastics. Indeed, metallic glasses have narrower processing windows of time, temperature and strain that require better understanding and more careful control of the forming process. Understanding is usually obtained by performing many experiments that vary process conditions for each alloy, which is a tedious effort. Hence, most often the production of metallic glasses has been limited to thin pieces and simple shapes. Now, Jan Schroers and colleagues report in Nature Materials an accelerated approach to
develop bulk metallic glasses (BMGs) that can be formed into thick, complex shapes quickly and cheaply 2. Metallic glasses have important benefits over traditional glassy materials. They are stronger, stiffer and more resistant to harsh chemicals and environments (including ultraviolet radiation and temperature) than plastics. They have better fracture resistance than oxide glasses. Their surface finish is atomically smooth, and so thermoforming of metallic glasses can be used for nanoforming and nanoimprinting 1. Parts can be made with metallic properties (thermal and electrical conductivity) and have a metallic lustre with a visual appeal that plastics sometimes try to mimic with surface treatments. The magnetic properties of metallic glasses give them an edge in a wide range of applications: they are commonly used in power electronics, telecommunications, high-efficiency (up to
99.3%) power transformers and as anti-theft devices (such as the shiny metal foil under the plastic label in DVD cases)3,4. They are also a mainstay in consumer electronics, including video-recording heads and the thin metallic layer of rewriteable compact discs. Metallic glasses also have exceptional mechanical properties, which enable farranging applications, such as surgical knives, microhinges in digital light projectors5 and golf club heads6, for example. However, the main barriers to widespread consumer applications of BMGs are the ability to shape them quickly and affordably while preserving their amorphous structure and to discover and develop new BMGs that are inexpensive and have the desirable properties. At present, glasses based on palladium and zirconium have the best glass-forming ability (that is, to retain an amorphous atomic structure in thicknesses greater than 1 mm), but these
a 7 1 2
6
3 4 5 1. Composition library 2. Adhesion layer 3. Silicon 4. Back-up layer 5. Heater 6. Gas releasing agent 7. Camera
2020K Km −1 inin −1 m
c
b 120 °C
Glass
130 °C
145 °C
Tg
160 °C
165 °C
Supercooled liquid
Onset of blow forming
Figure 1 | Metallic-glass objects formed by thermoplastic forming. Image courtesy of Yanhui Liu and Jan Schroers. 432
175 °C
185 °C
Tx Crystal End of blow forming
Figure 2 | Parallel blow-forming approach2. a,b, Gas released through 0.5-mm holes can cause the formation of bubbles out of materials from a compositional library deposited as a thin film by co-sputtering (a). Bubbles form through plastic deformation above the glass transition temperature (Tg) of the material, and stop growing at its crystallization temperature (Tx; b). c, Close-up of a region of the film where bubbles have formed. Scale bar, 1 mm. NATURE MATERIALS | VOL 13 | MAY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
news & views elements are too expensive to be practical in large volumes. Schroers and collaborators used a common combinatorial approach based on co-sputtering 7,8 to make a library of materials consisting of thousands of compositions on a single thin film (Fig. 2a). Thousands of miniature shapeforming experiments were then prepared by etching circular holes about 0.5 mm in diameter beneath the film. A compound that generates gas at a fixed temperature was placed in each cavity before the temperature was steadily increased for the entire materials library. The gas is produced at a temperature where the glass is still relatively solid, thus pressurizing the cavity. The metallic glass begins to thermoplastically deform at a slightly higher temperature (the glass transition temperature), and the process ends at the crystallization temperature, locking in the final bubble size. By simply measuring the height of each miniature metallic glass bubble (Fig. 2b,c), Schroers and co-authors were able to characterize the shape-forming ability of each metallic-glass composition with reproducible precision. The same test also gives data, such as viscosity, that is difficult to measure by other techniques,
and may give essential new insights into the fundamentals of metallic glass stability. Therefore, with a simple measurement — the bubble height after forming — this massively parallel synthesis and shapeforming approach allowed the authors to dramatically reduce sample preparation time and to determine which BMGs can most easily be shaped into complex, useful shapes. Moreover, the same combinatorial experiments that tell which BMGs can be shaped most easily also indicate which compositions are the most stable, thus overcoming another major barrier to commercialization. In fact, there is no way at present to predict which alloys will have good glass-forming ability, and so laborious trial-and-error experiments are used to discover and develop new BMGs. The approach of Schroers and colleagues establishes the glass-forming ability of new alloys more than 100 times faster than current methods, so that studies that used to take a year can now be done in a single day. To put this in perspective, over 50 years of extensive trial-and-error were required to identify all the metallic glasses known at present; the new approach has the potential to give a similar amount of information in under two months.
Still, there is more work to be done. Commercial metallic glasses can have more than six elements, and deciding which elements to add is still more art than science. The number of possible compositions for alloys with six or more elements is vast, and tackling such a compositional space would be a challenge, even with the accelerated blow-forming approach of Schroers and co-authors. And producing controlled composition gradients of four to six elements, although possible in theory, is rarely done. The authors’ highthroughput approach should thus fast-track efforts into tapping the practical benefits of metallic glasses. ❐ Dan B. Miracle is at the AF Research Laboratory, Materials and Manufacturing Directorate, Dayton, Ohio 45433, USA. e-mail: [email protected] References Schroers, J. Adv. Mater. 22, 1566–1597 (2010). Ding, S. et al. Nature Mater. 13, 494–500 (2014). Hasegawa, R. Mater. Sci. Eng. A 375–377, 90–97 (2004). Hilzinger, H. IEEE Trans. Magnetics 21, 2020–2025 (1985). Tregilgas, J. H. Adv. Mater. Proc. 162, 40–41 (2004). Salimon, A. I., Ashby, M. F., Brechet, Y. & Greer, A. L. Mater. Sci. Eng. A 375–377, 385–388 (2004). 7. Zhao, J.‑C. Prog. Mater. Sci. 51, 557–631 (2006). 8. Green, M. L., Takeuchi, I., Hattrick-Simpers, J. R. J. Appl. Phys. 113, 231101 (2013). 1. 2. 3. 4. 5. 6.
BIOINSPIRED CERAMICS
Turning brittleness into toughness Nacre-like bulk ceramics with a unique combination of high toughness, strength and stiffness can be produced from brittle constituents by an ice-templating approach.
André R. Studart
A
lthough seemingly different, the seashells of molluscs and the ceramics that are used for kitchen plates, bathroom sinks and turbine coatings have much in common. They are made primarily of brittle building blocks (95% in volume); carbonates in the case of seashells and (most often) oxides in advanced ceramics. However, whereas the latter are prone to shatter and sudden fracture, seashells are surprisingly tough. This is something that has fascinated materials scientists for decades. By means of their beautiful brick-and-mortar-like (nacreous) internal architecture (Fig. 1a) — which drives cracks through a long and tortuous path (thereby dissipating energy) before the material can be broken apart — seashells achieve a remarkable resistance to crack growth1. In fact, inorganic bridges (Fig. 1b)
present in between the submicrometre-thick bricks help distribute the externally applied forces over a larger number of bricks, thus reducing the driving force for cracking. Also, organic matter (less than 5 vol%; Fig. 1c) deforms plastically during fracture to absorb part of the energy of the propagating crack. Replicating the design principles underlying the microscopic architecture of seashells in macroscopic synthetic materials has been a long-standing challenge. Although impressive enhancements in fracture toughness combined with high strength and plasticity have been achieved by assembling strong inorganic ‘bricks’ with at least 20 vol% of organic ‘mortar’2,3, such a large fraction of organic phase limits the maximum temperature that the material can withstand. Writing in Nature Materials, Deville and colleagues now show that
NATURE MATERIALS | VOL 13 | MAY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
all-inorganic ceramics with a structure inspired by the nacreous layer of seashells can be made to exhibit high-temperature toughness, strength and stiffness4. Deville and co-authors used freeze casting — a well-established technique to obtain lamellar materials, where two-dimensional ice crystals are grown unidirectionally through an aqueous suspension loaded with ceramic particles5 (Fig. 1e). By controlling both the speed of the ice growth front and the concentration of particles in the suspension, freeze casting allows the particles to be excluded from the ice crystal, forcing them to assemble into long-range ordered lamellae of a few tens of micrometres in thickness. The ice is then sublimated under reduced pressures to generate a macroporous lamellar structure. Pressing such porous structures 433
Fast track to production
A high-throughput approach combining combinatorial deposition of materials with parallel blow-forming speeds up the discovery rate of bulk metallic glasses that can be easily formed into complex shapes.
Dan B. Miracle
H
eating and blowing oxide glasses (such as silicate glass) into a shaped cavity has been used for centuries to make bottles and jars. Blow moulding and other thermoplastic-forming methods such as injection moulding have also been used for decades to form plastic objects quickly and cheaply. For conventional metals, however, their crystalline structure rules out thermoplastic forming, so it is much more difficult — and expensive — to make metals into complex shapes. Although metallic glasses lend themselves to moulding by thermoplastic forming (Fig. 1), this has only been achieved in the past several years1. One of the reasons is that metallic glasses are less stable than oxide glasses and plastics. Indeed, metallic glasses have narrower processing windows of time, temperature and strain that require better understanding and more careful control of the forming process. Understanding is usually obtained by performing many experiments that vary process conditions for each alloy, which is a tedious effort. Hence, most often the production of metallic glasses has been limited to thin pieces and simple shapes. Now, Jan Schroers and colleagues report in Nature Materials an accelerated approach to
develop bulk metallic glasses (BMGs) that can be formed into thick, complex shapes quickly and cheaply 2. Metallic glasses have important benefits over traditional glassy materials. They are stronger, stiffer and more resistant to harsh chemicals and environments (including ultraviolet radiation and temperature) than plastics. They have better fracture resistance than oxide glasses. Their surface finish is atomically smooth, and so thermoforming of metallic glasses can be used for nanoforming and nanoimprinting 1. Parts can be made with metallic properties (thermal and electrical conductivity) and have a metallic lustre with a visual appeal that plastics sometimes try to mimic with surface treatments. The magnetic properties of metallic glasses give them an edge in a wide range of applications: they are commonly used in power electronics, telecommunications, high-efficiency (up to
99.3%) power transformers and as anti-theft devices (such as the shiny metal foil under the plastic label in DVD cases)3,4. They are also a mainstay in consumer electronics, including video-recording heads and the thin metallic layer of rewriteable compact discs. Metallic glasses also have exceptional mechanical properties, which enable farranging applications, such as surgical knives, microhinges in digital light projectors5 and golf club heads6, for example. However, the main barriers to widespread consumer applications of BMGs are the ability to shape them quickly and affordably while preserving their amorphous structure and to discover and develop new BMGs that are inexpensive and have the desirable properties. At present, glasses based on palladium and zirconium have the best glass-forming ability (that is, to retain an amorphous atomic structure in thicknesses greater than 1 mm), but these
a 7 1 2
6
3 4 5 1. Composition library 2. Adhesion layer 3. Silicon 4. Back-up layer 5. Heater 6. Gas releasing agent 7. Camera
2020K Km −1 inin −1 m
c
b 120 °C
Glass
130 °C
145 °C
Tg
160 °C
165 °C
Supercooled liquid
Onset of blow forming
Figure 1 | Metallic-glass objects formed by thermoplastic forming. Image courtesy of Yanhui Liu and Jan Schroers. 432
175 °C
185 °C
Tx Crystal End of blow forming
Figure 2 | Parallel blow-forming approach2. a,b, Gas released through 0.5-mm holes can cause the formation of bubbles out of materials from a compositional library deposited as a thin film by co-sputtering (a). Bubbles form through plastic deformation above the glass transition temperature (Tg) of the material, and stop growing at its crystallization temperature (Tx; b). c, Close-up of a region of the film where bubbles have formed. Scale bar, 1 mm. NATURE MATERIALS | VOL 13 | MAY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
news & views elements are too expensive to be practical in large volumes. Schroers and collaborators used a common combinatorial approach based on co-sputtering 7,8 to make a library of materials consisting of thousands of compositions on a single thin film (Fig. 2a). Thousands of miniature shapeforming experiments were then prepared by etching circular holes about 0.5 mm in diameter beneath the film. A compound that generates gas at a fixed temperature was placed in each cavity before the temperature was steadily increased for the entire materials library. The gas is produced at a temperature where the glass is still relatively solid, thus pressurizing the cavity. The metallic glass begins to thermoplastically deform at a slightly higher temperature (the glass transition temperature), and the process ends at the crystallization temperature, locking in the final bubble size. By simply measuring the height of each miniature metallic glass bubble (Fig. 2b,c), Schroers and co-authors were able to characterize the shape-forming ability of each metallic-glass composition with reproducible precision. The same test also gives data, such as viscosity, that is difficult to measure by other techniques,
and may give essential new insights into the fundamentals of metallic glass stability. Therefore, with a simple measurement — the bubble height after forming — this massively parallel synthesis and shapeforming approach allowed the authors to dramatically reduce sample preparation time and to determine which BMGs can most easily be shaped into complex, useful shapes. Moreover, the same combinatorial experiments that tell which BMGs can be shaped most easily also indicate which compositions are the most stable, thus overcoming another major barrier to commercialization. In fact, there is no way at present to predict which alloys will have good glass-forming ability, and so laborious trial-and-error experiments are used to discover and develop new BMGs. The approach of Schroers and colleagues establishes the glass-forming ability of new alloys more than 100 times faster than current methods, so that studies that used to take a year can now be done in a single day. To put this in perspective, over 50 years of extensive trial-and-error were required to identify all the metallic glasses known at present; the new approach has the potential to give a similar amount of information in under two months.
Still, there is more work to be done. Commercial metallic glasses can have more than six elements, and deciding which elements to add is still more art than science. The number of possible compositions for alloys with six or more elements is vast, and tackling such a compositional space would be a challenge, even with the accelerated blow-forming approach of Schroers and co-authors. And producing controlled composition gradients of four to six elements, although possible in theory, is rarely done. The authors’ highthroughput approach should thus fast-track efforts into tapping the practical benefits of metallic glasses. ❐ Dan B. Miracle is at the AF Research Laboratory, Materials and Manufacturing Directorate, Dayton, Ohio 45433, USA. e-mail: [email protected] References Schroers, J. Adv. Mater. 22, 1566–1597 (2010). Ding, S. et al. Nature Mater. 13, 494–500 (2014). Hasegawa, R. Mater. Sci. Eng. A 375–377, 90–97 (2004). Hilzinger, H. IEEE Trans. Magnetics 21, 2020–2025 (1985). Tregilgas, J. H. Adv. Mater. Proc. 162, 40–41 (2004). Salimon, A. I., Ashby, M. F., Brechet, Y. & Greer, A. L. Mater. Sci. Eng. A 375–377, 385–388 (2004). 7. Zhao, J.‑C. Prog. Mater. Sci. 51, 557–631 (2006). 8. Green, M. L., Takeuchi, I., Hattrick-Simpers, J. R. J. Appl. Phys. 113, 231101 (2013). 1. 2. 3. 4. 5. 6.
BIOINSPIRED CERAMICS
Turning brittleness into toughness Nacre-like bulk ceramics with a unique combination of high toughness, strength and stiffness can be produced from brittle constituents by an ice-templating approach.
André R. Studart
A
lthough seemingly different, the seashells of molluscs and the ceramics that are used for kitchen plates, bathroom sinks and turbine coatings have much in common. They are made primarily of brittle building blocks (95% in volume); carbonates in the case of seashells and (most often) oxides in advanced ceramics. However, whereas the latter are prone to shatter and sudden fracture, seashells are surprisingly tough. This is something that has fascinated materials scientists for decades. By means of their beautiful brick-and-mortar-like (nacreous) internal architecture (Fig. 1a) — which drives cracks through a long and tortuous path (thereby dissipating energy) before the material can be broken apart — seashells achieve a remarkable resistance to crack growth1. In fact, inorganic bridges (Fig. 1b)
present in between the submicrometre-thick bricks help distribute the externally applied forces over a larger number of bricks, thus reducing the driving force for cracking. Also, organic matter (less than 5 vol%; Fig. 1c) deforms plastically during fracture to absorb part of the energy of the propagating crack. Replicating the design principles underlying the microscopic architecture of seashells in macroscopic synthetic materials has been a long-standing challenge. Although impressive enhancements in fracture toughness combined with high strength and plasticity have been achieved by assembling strong inorganic ‘bricks’ with at least 20 vol% of organic ‘mortar’2,3, such a large fraction of organic phase limits the maximum temperature that the material can withstand. Writing in Nature Materials, Deville and colleagues now show that
NATURE MATERIALS | VOL 13 | MAY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
all-inorganic ceramics with a structure inspired by the nacreous layer of seashells can be made to exhibit high-temperature toughness, strength and stiffness4. Deville and co-authors used freeze casting — a well-established technique to obtain lamellar materials, where two-dimensional ice crystals are grown unidirectionally through an aqueous suspension loaded with ceramic particles5 (Fig. 1e). By controlling both the speed of the ice growth front and the concentration of particles in the suspension, freeze casting allows the particles to be excluded from the ice crystal, forcing them to assemble into long-range ordered lamellae of a few tens of micrometres in thickness. The ice is then sublimated under reduced pressures to generate a macroporous lamellar structure. Pressing such porous structures 433
