Processing of Bulk Metallic Glass Schroers, Jan
Advanced materials (Weinheim),
April 12, 2010, Letnik:
22, Številka:
14
Journal Article
Recenzirano
Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique ...properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)‐based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass‐transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF‐based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.
Bulk metallic glass permits a unique processing method based on thermoplastic forming. Within thermoplastic forming some bulk metallic glasses, which are high‐strength metals, can be processed like plastics. These processing methods enable to net‐shape bulk metallic glasses on multiple length scales including tens of nanometers to tens of centimeters into shapes that were previously unachievable with any metal processing method (see figure).
A method is introduced as a standard to characterize the formability, the maximum strain a bulk metallic glass (BMG) can undergo in its supercooled liquid state before it eventually crystallizes. ...When considering accuracy and practicality it was found that the maximum diameter to which a 0.1
cm
3 BMG sample can be formed during heating through the supercooled liquid temperature region under a constant load of 4500
N is best suited as a measure of formability. Among the ten different alloys considered, by far the highest formability was found for Pt
57.5Cu
14.7Ni
5.3P
22.5. More generally, the results suggest that fragile liquid behavior, large Poisson ratio, and a low glass transition temperature are attributes indicating good formability. Various parameters, as well as an analytical expression for the formability, are tested against the experimentally determined formability to assess the degree of correlation.
Effectiveness of a second phase in metallic glass heterostructures to improve mechanical properties varies widely. Unfortunately, methods to fabricate such heterostructures like foams and composites ...do not allow controlled variation of structural features. Here we report a novel strategy, which allows us to vary heterostructural features independently, thereby enabling a systematic and quantitative study. Our approach reveals the optimal microstructural architecture for metallic glass heterostructures to achieve tensile ductility. Critical design aspect is a soft second phase, which is most effective when spacing between the second phase assumes the critical crack length of the metallic glass. This spacing should coincide with the second phase's size, and beyond, the specific second phase morphology of the heterostructure is crucial. These toughening strategies are only effective in samples that are large compared with the spacing of the second phase. The identified design aspects provide guidance in designing tensile ductility into metallic glasses.
Bulk Metallic Glass: The Smaller the Better Kumar, Golden; Desai, Amish; Schroers, Jan
Advanced materials (Weinheim),
January 25, 2011, Letnik:
23, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Bulk metallic glasses (BMGs) are strong, highly elastic, and resistant to wear but still find limited utility due to their macroscopic brittle nature, high costs, and difficulty of processing, ...particularly when complex shapes are desired. These drawbacks can be mitigated when BMGs are used in miniature parts (< 1 cm), an application which takes advantage of BMGs' enhanced plasticity at small length scales as well the insignificant material cost associated with such parts. As an alternative to traditional metal processing techniques, thermoplastic forming (TPF)‐based microfabrication methods have been developed which can process some BMGs like plastics. In this article, we discuss the properties and fabrication of BMGs on minuscule length scales to explore their prospective application in small‐scale devices.
Limitations of bulk metallic glasses (BMGs) imposed by their brittle behavior and high costs are overcome by fabricating BMG parts on miniature length scales, which take advantage of their enhanced plasticity and reduced costs. This opens up new opportunities for BMGs in the fields of MEMS, bio‐implants, miniature tools, self‐assembly templates, nanoimprinting, and the study of size‐effects.
High Entropy Alloys are inherently complex and span a vast composition space, making their research and discovery challenging. Developing quantitative predictions of their phase selection requires a ...large quantity of consistently determined experimental data. Here, we use combinatorial methods to fabricate and characterize 2478 quinary alloys based on Al and transition metals. All data are publicly available at http://materialsatlasproject.org/. Phase selection can be predicted for considered alloys when combining the content of FCC/BCC elements and the constituents’ atomic size difference. Mining our data reveals that High Entropy Alloys with increasing atomic size difference prefer BCC structure over FCC. This preference is typically overshadowed by other selection motifs, which dominate during close-to-equilibrium processing. Not suggested by the Hume-Rothery rules, this preference originates from the ability of the BCC structure to accommodate a large atomic size difference with lower strain energy penalty which can be practically only realized in High Entropy Alloys.
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A wide range of behaviors, including non-monotonic rejuvenation and relaxation, and the ability to qualitatively change the effect by varying the structural state of the glass was observed during ...thermal cycling of bulk metallic glasses. For this, we considered various bulk metallic glasses, Zr44Ti11Cu10Ni10Be25, Pd43Cu27Ni10P20, Pt57.5Cu14.7Ni5.3P22.5, and La55Al25Ni20, at various fictive temperatures to study the effect of thermal cycling on structure, thermal signature, and fracture toughness. For some BMGs and conditions considered here, thermal cycling results in a looser structure and an increase in fracture toughness. We found that for certain other BMGs and conditions, thermal cycling results in relaxation, reflected in a denser structure, and a decrease in fracture toughness. All these responses are non-monotonic and reveal a pronounced extremum with fracture toughness values of ± 50% of the original value, before approaching a value similar to the original value prior to thermal cycling. Such richness in response to thermal cycling suggests incompleteness of the previous picture where monotonically decreasing local stresses resulting in a homogenization of the structure with increasing cycle number. Our finding suggests that relative comparisons of various contributions including activation barriers for α-relaxation have to be considered which are also constantly changing, to decide if further cycling results in an increase or a decrease in fracture toughness. The fracture toughness’ response to thermal cycling can be correlated with the average atomic structures’ response to thermal cycling, while the thermal response does not exhibit an obvious correlation.
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The supercooled liquid region-the temperature region where the amorphous phase first relaxes into a highly viscous metastable liquid before it eventually crystallizes-is a manifestation of the ...extraordinary stability against crystallization of BMG alloys. This unique crystallization behavior permits an alternative net-shape processing method.
Since their discovery in 1960
, metallic glasses based on a wide range of elements have been developed
. However, the theoretical prediction of glass-forming compositions is challenging and the ...discovery of alloys with specific properties has so far largely been the result of trial and error
. Bulk metallic glasses can exhibit strength and elasticity surpassing those of conventional structural alloys
, but the mechanical properties of these glasses are critically dependent on the glass transition temperature. At temperatures approaching the glass transition, bulk metallic glasses undergo plastic flow, resulting in a substantial decrease in quasi-static strength. Bulk metallic glasses with glass transition temperatures greater than 1,000 kelvin have been developed, but the supercooled liquid region (between the glass transition and the crystallization temperature) is narrow, resulting in very little thermoplastic formability, which limits their practical applicability. Here we report the design of iridium/nickel/tantalum metallic glasses (and others also containing boron) with a glass transition temperature of up to 1,162 kelvin and a supercooled liquid region of 136 kelvin that is wider than that of most existing metallic glasses
. Our Ir-Ni-Ta-(B) glasses exhibit high strength at high temperatures compared to existing alloys: 3.7 gigapascals at 1,000 kelvin
. Their glass-forming ability is characterized by a critical casting thickness of three millimetres, suggesting that small-scale components for applications at high temperatures or in harsh environments can readily be obtained by thermoplastic forming
. To identify alloys of interest, we used a simplified combinatorial approach
harnessing a previously reported correlation between glass-forming ability and electrical resistivity
. This method is non-destructive, allowing subsequent testing of a range of physical properties on the same library of samples. The practicality of our design and discovery approach, exemplified by the identification of high-strength, high-temperature bulk metallic glasses, bodes well for enabling the discovery of other glassy alloys with exciting properties.
Supercooled liquids exhibit spatial heterogeneity in the dynamics of their fluctuating atomic arrangements. The length and time scales of the heterogeneous dynamics are central to the glass ...transition and influence nucleation and growth of crystals from the liquid. Here, we report direct experimental visualization of the spatially heterogeneous dynamics as a function of temperature in the supercooled liquid state of a Pt-based metallic glass, using electron correlation microscopy with sub-nanometer resolution. An experimental four-point space-time correlation function demonstrates a growing dynamic correlation length, ξ, upon cooling of the liquid toward the glass transition temperature. ξ as a function of the relaxation time τ are in good agreement with Adam-Gibbs theory, inhomogeneous mode-coupling theory and random first-order transition theory of the glass transition. The same experiments demonstrate the existence of a nanometer thickness near-surface layer with order of magnitude shorter relaxation time than inside the bulk.
The fracture toughness of glassy materials remains poorly understood. In large part, this is due to the disordered, intrinsically non-equilibrium nature of the glass structure, which challenges its ...theoretical description and experimental determination. We show that the notch fracture toughness of metallic glasses exhibits an abrupt toughening transition as a function of a well-controlled fictive temperature (T
), which characterizes the average glass structure. The ordinary temperature, which has been previously associated with a ductile-to-brittle transition, is shown to play a secondary role. The observed transition is interpreted to result from a competition between the T
-dependent plastic relaxation rate and an applied strain rate. Consequently, a similar toughening transition as a function of strain rate is predicted and demonstrated experimentally. The observed mechanical toughening transition bears strong similarities to the ordinary glass transition and explains the previously reported large scatter in fracture toughness data and ductile-to-brittle transitions.
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